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John Stevenson
06-20-2010, 06:16 PM
New thread.

From reading Brian's post it seems as if he's gone about making a flycutter type form tool because the button method is too over whelming or not understood. Personally I can't see why if someone is able to make a model engine they can't turn two buttons to a specified diameter a specified distance apart.

Anyway it seems to me that the button method can be bypassed and the relieving also done if you can accurately machine the correct radii onto a form tool as opposed to grinding it on by eye.

We still need the button specifications though and these can be found In Ivan Laws book on page 115, for a given size we need diameter, distance apart, and infeed written down on a piece of paper, no CAD drawing needed.

Now you can buy off the shelf tapered end mills in various tapers, J&L sell then in 1, 3, 5, 7,10 and 15 degrees per side in various diameters.

http://www.mscjlindustrial.co.uk/CGI/INPDFF?PMCTLG=00&PMPAGE=221&PMITEM=TEM-10030B



Here's a 10 degree one on a 3/8" shank.

http://www.stevenson-engineers.co.uk/files/formtool1.jpg

I keep these in for milling insert pockets for lathe and boring tools, 10 degrees is too much for what we need but it will do as an example.

We put a piece of gauge plate / tool steel [ soft ] in the vise and using a washer bored / drill to the button diameter the hight is set, note we are working on the tool upside down.

http://www.stevenson-engineers.co.uk/files/formtool2.jpg

Now centralise the cutter on the blank and then take a cleanup pass across the end to give front rake. Set the dials or DRO to zero.

Now move over half the centre distance and feed in the amount of infeed, that's one side of the cutter done.
Repeat on the other side, literally a 5 minute job.

That's the cutter done, we now have this.

http://www.stevenson-engineers.co.uk/files/formtool3.jpg

Top view.


http://www.stevenson-engineers.co.uk/files/formtool4.jpg


This clearly shows side rake and front rake of the radii, because of the 10 degree angle it removed a lot of the tooth form at the lower end which has weakened to form but as it was the only tapered cutter I have in the crib it was pressed into service to prove the concept.

At this stage [ with a correctly formed cutter ] it needs hardening and tempering and is ready to cut.

This can be used for fly cutting in a holder in the mill or as a shaper tool in a shaper or slotter. It's exactly the same as the button method but without the step of making buttons etc.

[EDIT] Instead of holding a blank in the vise there is no reason that you could not do these two operations to a rotary cutter with 4 or 5 pre gashed portions on it doing each arm in turn at one side offset and then repeating on the other side.

.

leesr
06-20-2010, 07:06 PM
Sir John

I believe you did an excellant eplanation of making a form cutter.
it was a brilliant way of making a gear or spline with limited equipment.
I learned an economical way of making a form cutter.

Because of the chance I BBS site it has kindled the Idea of starting a home work shop in my garage. I like to tinker.
I would like to obtain Ivans Law Book but the price jumped ten fold.

Cheers
Leesr

914Wilhelm
06-21-2010, 12:24 AM
[QUote] Instead of holding a blank in the vise there is no reason that you could not do these two operations to a rotary cutter with 4 or 5 pre gashed portions on it doing each arm in turn at one side offset and then repeating on the other side.

I'd like to see the setup for this. Otherwise 2 thumbs up for this way of cutting a single flycutter with the conical mill.

John Stevenson
06-21-2010, 04:08 AM
Wilhelm,
If I get chance tonight I'll do a dummy setup with a piece of steel but I am busy in the shop.

It is only a proof of concept in that all the machining and necessary angle can be put on accurately using either the dials or a DRO.

Takes the uncertainty out of it and it's actually very quick. That whole setup took literally 10 minutes and this included pictures, a rotary cutter will take longer as you have the blank to make.

Circlip
06-21-2010, 04:09 AM
Well, Ya done a Hob, a form cutter now a flycutting "Cleaver", what's next?? Hacksaw and file??:D

Regards Ian.

oldtiffie
06-21-2010, 05:28 AM
A very good exercise in lateral and creative thinking John.

A 5* "tilt" - as per Ivan Laws book - will not only allow a top/back rake if needed - as well as providing a 5* front clearance but will not materially affect either the cutter or the gear tooth profile. That 5* back rake will make cutting a lot easier - and faster - with a lot less tool wear.

It would not be difficult to finish the profile with a good die grinder.

jackary
06-21-2010, 05:33 AM
Brilliant John you've even simplified Ivan Law's method.
Alan

bob ward
06-21-2010, 05:52 AM
One of those forehead slapping "why didn't I think of that" moments.

John Stevenson
06-21-2010, 07:19 AM
There is one serious disadvantage with this method is that it's fine for gears for small engines etc but once you get up in the gear size, as in DP and number of teeth the buttons increase in size.

Take a 60 tooth 16 DP gear.
Ivan states you need 15.07 / DP so 15.07 / 16 = 0.941"
So you are looking for a 1" tapered cutter. Well they make them but at a price, however it's often possible to get cutters at auctions very cheap BECAUSE they are not standard / normal.

If fact looking in a drawer this morning for something I spotted a No4 morse taper reamer - that will make some big gear cutters :D

Like most things it's not the answer to everything it's just one part of a puzzle.

.

J Tiers
06-21-2010, 08:13 AM
if I have this correctly figured, the only reason the cutter is tapered is that it allows setting the proper size to equate with the button. it has a side benefit of providing relief, but that might be provided another way by tilting head or workpiece.

It would be unlikely for a standard end mill to be the proper size. But in case one was, it seems that there would be no objection to using it, so long as it was set off-tram, or the workpiece was, in order to provide relief.

because it is not tapered, it would not cut away so much of the material below the edge.

So....

When you get to larger sizes, it seems that you might be able to use a boring head to generate the circular form. You 23.9mm would be of a suitable size for that.

Black_Moons
06-21-2010, 11:43 AM
I have a material question:

What kinda cutters and toolbit are you using here?

You mention tool steel, but what alloy is that exactly?

Is there any way to mill hardened HSS? Maybe with carbide? Cermet?
Everyone seems to say its impossable(Read as: Very hard) to harden HSS yourself due to the exact temp requirements, but is there anyone that will do it cheaply for one offs?

dp
06-21-2010, 12:10 PM
Wrong thread...

leesr
06-21-2010, 03:28 PM
There is one serious disadvantage with this method is that it's fine for gears for small engines etc but once you get up in the gear size, as in DP and number of teeth the buttons increase in size.

Take a 60 tooth 16 DP gear.
Ivan states you need 15.07 / DP so 15.07 / 16 = 0.941"
So you are looking for a 1" tapered cutter. Well they make them but at a price, however it's often possible to get cutters at auctions very cheap BECAUSE they are not standard / normal.

If fact looking in a drawer this morning for something I spotted a No4 morse taper reamer - that will make some big gear cutters :D

Like most things it's not the answer to everything it's just one part of a puzzle.

.

Hey JS

for a fly cutter
why not use a radius dresser to obtain the approx radii required for the Involute form, dress the grinding wheel?

Cheers
Leesr

djc
06-21-2010, 03:36 PM
...the only reason the cutter is tapered is that it allows setting the proper size to equate with the button. It has a side benefit of providing relief, but that might be provided another way by tilting head or workpiece.

But by doing so, you are not now cutting a circular arc, rather a portion of an ellipse.

If you don't have a sufficiently large tapered cutter, it might be possible to set up the blank form tool on a rotary table and "circular-mill" it.

leesr
06-21-2010, 04:38 PM
we do pencil grinding using small sank type grinding wheels. the profile is dress with a dresser that approximates the involute.
the same can be used with small center cutting ball nose, end or profile mill cutter.

Cheers
Leesr

Lew Hartswick
06-21-2010, 06:03 PM
FORGET I screwed up and I can't seem to delete the whole thing. :-(
...lew...

leesr
06-21-2010, 09:09 PM
FORGET I screwed up and I can't seem to delete the whole thing. :-(
...lew...


delete what?

Leesr:confused:

J Tiers
06-21-2010, 09:27 PM
But by doing so, you are not now cutting a circular arc, rather a portion of an ellipse.

If you don't have a sufficiently large tapered cutter, it might be possible to set up the blank form tool on a rotary table and "circular-mill" it.

How bad could that be?

First, the angle, in the low # of degrees, is not large, and the 'ellipsification" consequently small.

Second, the circular cutter is itself an approximation to the proper shape, so that "objection" amounts to questioning the added (or possibly reduced) amount of error due to the very slight change to a shape that isn't ideal anyway.

oldtiffie
06-21-2010, 10:30 PM
Originally Posted by djc
But by doing so, you are not now cutting a circular arc, rather a portion of an ellipse.

If you don't have a sufficiently large tapered cutter, it might be possible to set up the blank form tool on a rotary table and "circular-mill" it.

How bad could that be?

First, the angle, in the low # of degrees, is not large, and the 'ellipsification" consequently small.

Second, the circular cutter is itself an approximation to the proper shape, so that "objection" amounts to questioning the added (or possibly reduced) amount of error due to the very slight change to a shape that isn't ideal anyway.

You are quite correct JT - precisely so.

If a circle is tilted vertically about its horizontal axis its shape when projected back to the horizontal plane will change. When horizontal (zero tilt) the horizontal shape will be a circle. When it is tilted 90* the horizontal projected shape will be straight line the length of which will be equal to the diameter of the circle it was projected from.

At all other angle of tilt the projected shape in the horizontal plane will be an ellipse.

The concern seems to be about the perceived fore-shortening of the circle as it becomes an ellipse.

It is very little in fact at low angles of tilt. It varies as the cosine of the tilt angle.

That's all fine or confusing perhaps, but to put some figures to it. The greatest/largest difference will be at the centre of the arc.

At 1 degree the ellipse is 0.9998 x the circle radius (or diameter)

At 2 degrees the ellipse is 0.9994 x the circle radius (or diameter)

At 3 degrees the ellipse is 0.9986 x the circle radius (or diameter)

At 4 degrees the ellipse is 0.9976 x the circle radius (or diameter)

At 5 degrees the ellipse is 0.9962 x the circle radius (or diameter)

At 7 degrees the ellipse is 0.9925 x the circle radius (or diameter)

At 10 degrees the ellipse is 0.9848 x the circle radius (or diameter)

So its all pretty small. Ivan Laws 7 degree tilt will cause the ellipse to be a maximum(at the centre) of 0.0075" per inch - ie 7 1/2 "thou" per inch of tilted radius or diameter.

That is pretty insignificant here in this instance and for all practical purposes can be disregarded.

Paul Alciatore
06-21-2010, 10:31 PM
I have two thoughts on this.

First, neither Ivan Law nor any of the other advocates of his button method adequately explain and emphasize that it produces only an APPROXIMATION of the true involute gear tooth form. And yes I have and have read his book. Round buttons produce ROUND faces on the gear teeth, not involutes. You are actually better off using a form cutter of the wrong tooth count range in many cases (like if it is only one size number off) instead of using the round button method. THE ROUND BUTTON METHOD IS ONLY AN APPROXIMATION! It is just an imperfect answer to the question, how does a home shop machinist make a gear cutter.

Second thought is I see absolutely no reason to resort to a tapered cutter. I would rough it out with a straight cutter of a smaller, standard diameter than called for and then use a stone that has been dressed to the calculated diameter to finish it. You will get a much better finish and you can just increase the down feed to get a fresh area of the stone at the cutting edge of the form tool. You can harden it between the milling cutter and the stone work. with both English and metric sized milling cutters to choose from, you can probably get fairly close to the final size. As for the clearance angle, simply doing these operations at an angle (5-10 degrees) will suffice for that.

oldtiffie
06-21-2010, 10:45 PM
Originally Posted by J Tiers
...the only reason the cutter is tapered is that it allows setting the proper size to equate with the button. It has a side benefit of providing relief, but that might be provided another way by tilting head or workpiece.


But by doing so, you are not now cutting a circular arc, rather a portion of an ellipse.

If you don't have a sufficiently large tapered cutter, it might be possible to set up the blank form tool on a rotary table and "circular-mill" it.

JT is correct.

John Stevenson's tapered cutter is set vertically and is a cone and if driven/fed directly into the job will cut a cone - as it did.

A cone is really just an infinitely large number or disks ("washers"?) that reduce incrementally at a consistent rate/diameter over a common centre.

John Stevenson has very creatively and very effectively selected and used the/his required "disk or "washer" to set the radius at the face of the tool.

John's method cuts circles - not ellipses - and has the required clearance/rake angles.

A very clever and elegant solution indeed.

leesr
06-22-2010, 12:05 AM
the trick is to have software that can draw an actual involute, old tiffie don't shoot me, use something like Les Harris used to calculate the x & y coordinates of the involute, use cheap cad software or better yet buy the software that sir john uses, which is about $100 bucks, which I believe is a deal.

I use software from SES I purchased for $400 bucks but it strictly in dos.
however I create geometry & send it to the EDM guys to cut 1-5 pcs of internal splines or Internal gears. & it's being very accurate.

but I make sure the EDM guy's have gages to verify they have the right
geometric splines, & it will assemble.

OK the moral here is to make a mating part to check your splines.
some one mention to me in an other post that fancy software cannot produce exact splines. this not so I will tell you why.

lets say I have external splines to cut I need to make a dozen. OK I need to make a functional gage. I will blank the gage by turning it in my "grind the faces & the id". then I will send this blank to the EDM house, the will take my geometry & post it to a usable
G code program, edit to make it functional.

locate and cut the internal spline. now I get the gage back, I take MBW checks between wires to verify at the true involute form diameter, at the pitch diameter, then near the minor diameter.
all is good, I then proceed to make my external spline, as I am making splines I verify the external splines with internal spline to make sure It fits. Important !!!!

that is why it is a valuable tool to have software that can generate the involute one way or an other.

for form grinding gears my company as well as other companies have an ancient mechanical dresser that is called a base circle dresser. which is a mathematical approximation of the Involute gear. however that said the operator can nail the exact Involute, how do we know, the once again we can check it with the gear involute checker.

now most gear shops have fancy involute dressers that can form the involute on the form whell right on the machine. software guy's. yes that how it's done.

The small shop can use a 3 wire check which will be fine for the Home Work Shop.

for a gear shop Inspection machines are are required that are calibrated to the bureau of standards

Cheers
Leesr

TGTool
06-22-2010, 12:33 AM
So your shop is doing the same thing John is doing using your base circle dresser, making a close-enough approximation? It sounds like that backs his assertion of how gears are often done in production. Yeah, a gauge can be cut with EDM if you have access to the machine or the money to spend. That doesn't guarantee yet that the gear or splines you cut are correct, just that they will fit into the mating gauge. The gauge can't distinguish between a circular form that's not the theoretically correct involute and interfering with the gauge, or whether the splines just need to be cut a little deeper. In the grossest example you can show that a square shaft is round, just by making a hole big enough for the square to fit in. I know, that's not what you're doing, you're checking for a GOOD fit, but to really verify that the cut part is truly the correct involute you need expensive equipment, not just software.

leesr
06-22-2010, 02:07 AM
So your shop is doing the same thing John is doing using your base circle dresser, making a close-enough approximation? It sounds like that backs his assertion of how gears are often done in production. Yeah, a gauge can be cut with EDM if you have access to the machine or the money to spend. That doesn't guarantee yet that the gear or splines you cut are correct, just that they will fit into the mating gauge. The gauge can't distinguish between a circular form that's not the theoretically correct involute and interfering with the gauge, or whether the splines just need to be cut a little deeper. In the grossest example you can show that a square shaft is round, just by making a hole big enough for the square to fit in. I know, that's not what you're doing, you're checking for a GOOD fit, but to really verify that the cut part is truly the correct involute you need expensive equipment, not just software.

TGTool

the difference is my shop has a CNC Involute gear
& spline checker, we can calibrate gages to to the bureau of standards.
Production gages are normally ground & calibrated., not just EDM.
secondly your assumption that gages can not check involute is incorrect,
gages & are made to the max effective for externals , & go effective for internals, the gages incorporate all the following errors, Involute, lead, total index error. all are required by the specification. Gages have to be inspect & calibrated. if no gages are available then an independent inspection check has to be completed for all the attributes.

Yes expensive equipment is used, CNC Gear Shapers, CNC Gear Hobbers, CNC gear Grinders, CNC gear checker which all are for high production, for gear hobbing & gear shaper cutters are also manufactured to specifications and are also are inspected for the correct Involute, total index & tooth to tooth error. & so on.

the gears are cut & ground & are inspected for the correct geometry. includes Involute error, total index error, Tooth to Tooth error, or total composite error with a master gear. now for the small quantity of gears &,or splines is an easy way of making a small number of gears with out ordering or having to have the exact tooling. I gave that as an example.

making simple gears the are soft or core harden to 40 HRc is simple. making gears that having very complex geometry that require complex heat treating like Gas Carburize, Nirtride or other fancy case harden surfaces is an other issue. stuff can distort through heat treat & becomes scrap.
I make stuff that no one Else want to mess with no less manufacture it.
These parts have have very tight tolerances, and have to pass the customers drawings. those that do not have experience with such gears will lose their ass. due to scrap.

so the tips I give here is just that tips. & are subject to hashing out.
I am not use to making gears or splines to the OP method.
which for Home work Shop is acceptable method and is out of the box thinking.
What I was trying to recommend is for a small shop with out the expensive equipment like we have.
I am not to ego, am I still humble to say I can still learn something new.

Cheers
Leesr

John Stevenson
06-22-2010, 04:27 AM
Ladies, ladies, lets not get our knickers in a twist [ pause here for some asinine Tiffiepedia link ]

As Paul says it is ONLY an APPROXIMATION of an involute.

Just as the factory made gear cutter, made on equipment we only dream of is only accurate for the lowest number in the range.

The rest are ONLY APPROXIMATIONS.

Then we have to take into account work holding, is it concentric to a nano second on the arbor, how accurate is the machine, more, how rigid is the machine.
Do we know that if we wind in 0.108" it will be 0.108" deep on EVERY tooth ?

Industry with far better and accurate machinery know this problem exists and have gear depth gauges that map imperfections out on graph paper, whether shaped or hobbed.

We are talking home / hobby shop here, not production or lab spec setups.

Do we even have the equipment to even measure any imperfections in our gears ? Some of us may have some of the equipment but how many know how to use it correctly ?

.

Timleech
06-22-2010, 04:35 AM
Hey JS

for a fly cutter
why not use a radius dresser to obtain the approx radii required for the Involute form, dress the grinding wheel?

Cheers
Leesr


I've done this to make a form D-bit type of gear cutter, it worked passably well though despite a lot of care I had trouble getting the cutter spot on the correct size.

Tim

J Tiers
06-22-2010, 08:38 AM
Ladies, ladies, lets not get our knickers in a twist

As Paul says it is ONLY an APPROXIMATION of an involute.



So, JS, what do you think about tilting a boring head for approximating the larger cutters without conical EM?

Approximating an approximation??

drmico60
07-16-2010, 12:58 PM
I have followed this thread with interest.
The use of tapered milling cutters is fine but one would need several to cover the full range od gear cutters - and they are quite expensive. I went out and bought a set of cone drills from Screwfix (www.screwfix.com) order number 45562 at around 22 (circa 35 US$). This set of tapered drills cover the range 3-30 mm. I have tried these out to make tooth form cutters exactly using John Stevenson's method and they work very well.
When I get time I will write up the technique, with photos on my website.
Mike

drmico60
07-16-2010, 05:23 PM
Further to my earlier post I have written up some details on making gear cutters using cone drills on my website. Use the following link:
http://mikesworkshop.weebly.com/making-gear-cutters.html
Mike

Tony Ennis
07-16-2010, 05:53 PM
Very nice. How sure are you that you're producing proper involute gears?

John Stevenson
07-16-2010, 06:09 PM
Mike,
Nice page and well presented.

Tony,
They are not true involute but just arcs. Having said that the contact faces are so small that it would be very hard to measure any actual errors.

Home shop gears usually have errors given that we are limited by equipment and tooling.

.

brian Rupnow
07-16-2010, 07:34 PM
Very nicely done Mike. Thank you for the information.---brian

NzOldun
07-16-2010, 08:51 PM
the trick is to have software that can draw an actual involute, old tiffie don't shoot me, use something like Les Harris used to calculate the x & y coordinates of the involute, use cheap cad software or better yet buy the software that sir john uses, which is about $100 bucks, which I believe is a deal.

I use software from SES I purchased for $400 bucks but it strictly in dos.
however I create geometry & send it to the EDM guys to cut 1-5 pcs of internal splines or Internal gears. & it's being very accurate.

but I make sure the EDM guy's have gages to verify they have the right
geometric splines, & it will assemble.

OK the moral here is to make a mating part to check your splines.
some one mention to me in an other post that fancy software cannot produce exact splines. this not so I will tell you why.

lets say I have external splines to cut I need to make a dozen. OK I need to make a functional gage. I will blank the gage by turning it in my "grind the faces & the id". then I will send this blank to the EDM house, the will take my geometry & post it to a usable
G code program, edit to make it functional.

locate and cut the internal spline. now I get the gage back, I take MBW checks between wires to verify at the true involute form diameter, at the pitch diameter, then near the minor diameter.
all is good, I then proceed to make my external spline, as I am making splines I verify the external splines with internal spline to make sure It fits. Important !!!!

that is why it is a valuable tool to have software that can generate the involute one way or an other.

for form grinding gears my company as well as other companies have an ancient mechanical dresser that is called a base circle dresser. which is a mathematical approximation of the Involute gear. however that said the operator can nail the exact Involute, how do we know, the once again we can check it with the gear involute checker.

now most gear shops have fancy involute dressers that can form the involute on the form whell right on the machine. software guy's. yes that how it's done.

The small shop can use a 3 wire check which will be fine for the Home Work Shop.

for a gear shop Inspection machines are are required that are calibrated to the bureau of standards

Cheers
Leesr


Another method is to look up the mathematical expression for an involute and then feed it into a spreadsheet. Set up a table with small increments, (can be as small as 0.0001), in the A column and feed in the equation in the B column, as far down as you require. For gear cutting you are only intersested in the first bit of the curve. Then get the spreadsheet to graph your table and you can generate a very close approximation, depending on how small you make the steps. Print it as smalll or as large as you like;)

drmico60
09-13-2010, 01:29 PM
I have made a slight modification to John Stevensons method that puts some from rake on the tool and this greatly reduces the cutting forces during gear cutting. There is a full description here:
http://mikesworkshop.weebly.com/improved-gear-cutters.html
Mike

rockrat
09-13-2010, 09:15 PM
Interesting. Mike, would you upload one of the cutter finished (2167183.jpg) photos from your blog to this thread but make it a link to a larger photo? I would like to take a closer look.

Thanks, and interesting thought by the way.

rock~

RLWP
02-01-2013, 06:26 PM
I have made a slight modification to John Stevensons method that puts some from rake on the tool and this greatly reduces the cutting forces during gear cutting. There is a full description here:
http://mikesworkshop.weebly.com/improved-gear-cutters.html
Mike

First, apologies for gravedigging, I was searching for stuff on gear cutters and found this

Mike, I haven't thought this through yet...

By having the tool at an angle to the axis of the cone, you will be generating an ellipse rather than a circle. Without engaging both brain cells at once (and risk falling off this chair) it seems to me that your ellipse is closer to the involute than a circle would be

Richard

J. R. Williams
02-01-2013, 07:17 PM
Are end mills sold to produce small gears? I recently made a small 18 pitch , 8 tooth gear for a Hardinge Index unit and a small end mill would have been nice. I free hand ground a fly cutter to make the gear. The gear retracts an index plunger.

darryl
02-01-2013, 07:36 PM
I'm pretty sure that testing the gears produced in the home shop arena would be done by actually running them in mesh. The quieter they are, the better job you done.

Kenny G
02-03-2013, 04:07 PM
It would be great to hear if anyone has experience or knows anyone who cut their teeth on these gears??
Not a chain in sight!

https://lh5.googleusercontent.com/-0N4kFAHAihk/UQ7PT0Er2uI/AAAAAAAAApM/Tow7kQ6ZFcg/s800/merlingearsmodded.jpg

The Artful Bodger
02-03-2013, 05:25 PM
I think Sir John made Merlins? Or was that Meteors?

oldtiffie
02-03-2013, 10:02 PM
I think it was Ivan Laws book that made the point that a small part of a large circle very closely approximates a straight line (ie chord).

Given the way involute curves are drawn or plotted a small section of a larger involute curve not only approximates a straight line but it also has its small curve approximating Laws curves (ie "buttons") very closely indeed - a matter of "10ths" (or less).

With Laws buttons the flanks of the gear tooth get very close to the involute the larger the pitch circle, the larger the number of teeth and the larger the DP and the shorter the gear flank and the larger the pressure angle.

http://i200.photobucket.com/albums/aa294/oldtiffie/Sketches/PA_sketch1.jpg

It is not hard to calculate the diameters and distances apart of Laws buttons.

michigan doug
02-04-2013, 12:57 PM
Let's say one had to make a gear set in the home shop environment, i.e. no fancy edms or cad/cam available.

Let's further say we make these gears using some "reasonable approximations", as described by several posts in this thread.

What happens if I set these mildly non-involute form gears up in a jig that allows me to vary the distance between the gears and I mesh them and slowly drive them while I introduce a fine abrasive paste or slurry. As they slowly gently wear each other, I can push them a little closer together. Will this make the gears better or worse? Quieter or noisier? Closer to a true involute form?

What says the brain trust?

Finest regards,

doug

RLWP
02-04-2013, 01:12 PM
I believe that's how they used to bed in traction engine gears about a hundred years ago Doug

I wouldn't bother if I were you

Richard

michigan doug
02-04-2013, 04:50 PM
It's not that I need to do this at the moment...

But I want to know the outcome. Will it help, or will it not?

Finest regards,

doug

The Artful Bodger
02-04-2013, 04:59 PM
It's not that I need to do this at the moment...

But I want to know the outcome. Will it help, or will it not?

Finest regards,

doug

Certainly, if you press a soft gear blank against a hard gear, for example plastic against a steel gear, the plastic gear eventually takes on the perfect shape of a gear. But I suspect that if two hard gears were run together they would 'wear in' until they began to 'wear out'!

Spin Doctor
02-04-2013, 06:17 PM
Let's say one had to make a gear set in the home shop environment, i.e. no fancy edms or cad/cam available.

Let's further say we make these gears using some "reasonable approximations", as described by several posts in this thread.

What happens if I set these mildly non-involute form gears up in a jig that allows me to vary the distance between the gears and I mesh them and slowly drive them while I introduce a fine abrasive paste or slurry. As they slowly gently wear each other, I can push them a little closer together. Will this make the gears better or worse? Quieter or noisier? Closer to a true involute form?

What says the brain trust?

Finest regards,

doug


Well I can't say for Ford*, GM or Chrysler but I do know that AMC would run every ring and pinion set in a Gleason Lapper with an abrasive slurry running over them. The sets would then be matched to each other. You would not believe just how quiet the gear sets were when checked for noise level. But when run in reverse they were always noisy with that typical reverse gear whine. That's because the back side of the teeth are not lapped plus you are running them on the "wrong" side of the tooth their being an offset hypoid

*For a while AMC was making gear sets for Ford and were they noisy. The Ford engineer on site made the comment "We'll just add more sound deading" :confused:

michigan doug
02-05-2013, 04:24 PM
I suspected that it would produce good matched efficient sets, but maybe not the perfect generic gear that would run well with any other generic well made gear.

It may be analogous to making an optical flat. You can never achieve your goal with just two pieces of glass grinding against each other. But if you use three, and interchange them, you can end up with excellent results.

That was informative...

doug

willmac
02-05-2013, 06:23 PM
Well I can't say for Ford*, GM or Chrysler but I do know that AMC would run every ring and pinion set in a Gleason Lapper with an abrasive slurry running over them. The sets would then be matched to each other. You would not believe just how quiet the gear sets were when checked for noise level. But when run in reverse they were always noisy with that typical reverse gear whine. That's because the back side of the teeth are not lapped plus you are running them on the "wrong" side of the tooth their being an offset hypoid

*For a while AMC was making gear sets for Ford and were they noisy. The Ford engineer on site made the comment "We'll just add more sound deading" :confused:

lapping was a standard part of the Gleason hypoid production process and as far as I know all major automotive companies did this. I have worked on these machines in UK, so not just USA automakers.

Your comment about whine when running in reverse is of course correct, but did you know that there was one one popular and innovative small British car that was designed from the outset to drive on the coast side of the hypoid set? I'll leave the identity open for a while to see if anyone knows the answer.

TGTool
02-05-2013, 08:23 PM
Your comment about whine when running in reverse is of course correct, but did you know that there was one one popular and innovative small British car that was designed from the outset to drive on the coast side of the hypoid set? I'll leave the identity open for a while to see if anyone knows the answer.

Well that must have been the Reliant Robin, just rife with innovation.

Paul Alciatore
02-06-2013, 01:30 AM
I am quite interested in gear making and have just reread this entire thread. The reread has given me some new thoughts on this subject.

But first, the question below. I am just spit-balling this in my head, but if the conical cutter is tilted in a plane that is parallel to a radius of the gear to be cut (to the long axis of the tool bit), then the ellipse generated on the face of that tool would have a radius that is larger than the circle of the cone at that point on the side of the ellipse (at the point which would correspond to the root of the gear tooth to be cut) and it would be smaller than that circle radius at the point where that gear radius would intersect it (or at the end of the long axis of the ellipse). So, as you go higher on the gear tooth, the radius would decrease. This is the exact opposite of what an involute curve does. As the involute goes up the gear tooth, from the root to the top, the point by point radius increases. So, the error from this method would actually be larger than with Law's circular disk method of cutting the tool. I have not calculated how much so and it would of course vary with different sized gears. So, the approximation becomes worse, not better.

Now, my other thought upon rereading this whole thread. First, the conical cutter method really is a touch of genius. I do like it. But, as I said above, the error is theoretically worse than with the button method. However, with this method you have more control over the generation of the actual curve and it should be possible to generate a point by point path (imagine this path on the face of the tool bit being cut) that can be traversed by the conical cutter to generate the correct form on the cutter. This correction could be applied manually with a series of small moves or perhaps via CNC with the proper program. There is no theoretical reason why a completely accurate involute can not be generated in this fashion. You would start at much the same position as described in the original post and then move sideways by a small increment and into the tool by another small increment according to a calculated path. A dozen or two of these moves should be capable of generating an involute for the length of the gear tooth that is accurate to the limits of the machine you are using to do this. The more accurate your moves, the more accurate the involute.

A bit of math is needed to work this out and I do not have the time to attempt it at this time. Other coals on the fire. If anybody else wants to try, I invite you to do so and please share the results.



First, apologies for gravedigging, I was searching for stuff on gear cutters and found this

Mike, I haven't thought this through yet...

By having the tool at an angle to the axis of the cone, you will be generating an ellipse rather than a circle. Without engaging both brain cells at once (and risk falling off this chair) it seems to me that your ellipse is closer to the involute than a circle would be

Richard

John Stevenson
02-06-2013, 04:08 AM
A bit of math is needed to work this out and I do not have the time to attempt it at this time. Other coals on the fire. If anybody else wants to try, I invite you to do so and please share the results.

End of the day it's all an approximation of an involute and at the scale we are talking for home shop use, possibly 16DP being the largest needed to be cut ?? then we are talking tens of thous.

Probably a lot less than the accuracy of the machines involved bearing in mind many of the operations will be interrupted cutting on machines just not sturdy enough to do this and hit a target size.

There has to be a point where armchair maths is left behind and seat of the pants, dirty hands takes over .

RLWP
02-06-2013, 04:08 AM
{snip}

if the conical cutter is tilted in a plane that is parallel to a radius of the gear to be cut (to the long axis of the tool bit), then the ellipse generated on the face of that tool would have a radius that is larger than the circle of the cone at that point on the side of the ellipse (at the point which would correspond to the root of the gear tooth to be cut) and it would be smaller than that circle radius at the point where that gear radius would intersect it (or at the end of the long axis of the ellipse). So, as you go higher on the gear tooth, the radius would decrease. This is the exact opposite of what an involute curve does.

{snip}

Hi Paul,

Yes, you are right. To generate an ellipse the 'correct' way, the tapered milling cutter would have to be tilted perpendicular to the fly cutter, not parallel

Hmm, a bit more to mull over yet

Richard

RLWP
02-06-2013, 04:09 AM
{snip} seat of the pants, dirty hands takes over .

My wife complains about the state of my clothes too

Richard

drmico60
02-06-2013, 05:14 AM
But first, the question below. I am just spit-balling this in my head, but if the conical cutter is tilted in a plane that is parallel to a radius of the gear to be cut (to the long axis of the tool bit), then the ellipse generated on the face of that tool would have a radius that is larger than the circle of the cone at that point on the side of the ellipse (at the point which would correspond to the root of the gear tooth to be cut) and it would be smaller than that circle radius at the point where that gear radius would intersect it (or at the end of the long axis of the ellipse). So, as you go higher on the gear tooth, the radius would decrease. This is the exact opposite of what an involute curve does. As the involute goes up the gear tooth, from the root to the top, the point by point radius increases. So, the error from this method would actually be larger than with Law's circular disk method of cutting the tool. I have not calculated how much so and it would of course vary with different sized gears. So, the approximation becomes worse, not better.

The above quote is by Paul Alicatore. In response:

If you cut a hole, radius r, in an inclined sheet then the axis in one direction will increase to r/cos A where A is the angle that the sheet is tilted. For small angles this increase in length of the axis is very small. I make my cutters with a tilt angle of 5 degrees and cos 5 = 0.996. Thus the long axis of the ellipse is 4 parts in a thousand longer than the short axis of the ellipse. This small difference makes very little difference the the geometry of the gear teeth cut.

However, my way of making the cutters using a tilted blank does make a big difference to the performance of the cutter since with some positive rake the cutting forces are greatly reduced.

Mike

brian Rupnow
02-06-2013, 08:02 AM
This issue of building single point gear cutters is covered quite nicely with pictures and illustrations in "The Shop Wisdom of Philip Duclos" available thru this website.--Brian

willmac
02-06-2013, 03:17 PM
Well that must have been the Reliant Robin, just rife with innovation.

No - Hillman Imp - lots of problems with lack of proper initial development. but the engine/gearbox/transaxle was a very innovative design/ The engine was fairly closely related to a Coventry Climax engine.

TGTool
02-06-2013, 10:00 PM
No - Hillman Imp - lots of problems with lack of proper initial development. but the engine/gearbox/transaxle was a very innovative design/ The engine was fairly closely related to a Coventry Climax engine.

Don't leave me hanging. Tell me more about the design decisions and how and why they got what they did.

That Reliant suggestion was really a joke - more ways than one.

Paul Alciatore
02-07-2013, 03:14 AM
Richard, I thought of that, but the purpose of tilting the cutter was to create the clearance angle, wasn't it? So it seems that it should be tilted parallel.

Or, perhaps not. If the cone angle is enough to create the clearance, then you could tilt it perpendicular and still keep the clearance. Hummm!




Hi Paul,

Yes, you are right. To generate an ellipse the 'correct' way, the tapered milling cutter would have to be tilted perpendicular to the fly cutter, not parallel

Hmm, a bit more to mull over yet

Richard

drmico60
02-07-2013, 04:52 AM
Richard, I thought of that, but the purpose of tilting the cutter was to create the clearance angle, wasn't it? So it seems that it should be tilted parallel.

Or, perhaps not. If the cone angle is enough to create the clearance, then you could tilt it perpendicular and still keep the clearance. Hummm!

Paul, see my previous posting. A tilt of 5 degrees makes very little elliptical distortion , less than 4 parts in 1000.

When I make my cutters using this method, see http://mikesworkshop.weebly.com/improved-gear-cutters.html, then I use a conical cutter with a half angle of 10 degrees and a blank tilted at 5 degrees. This results in the long axis of the cutter having a front rake of 5 degrees and a relief of 5 degrees when used in my tool holder.

Mike

EVguru
02-07-2013, 04:52 AM
Well that must have been the Reliant Robin, just rife with innovation.

The reliant diff isn't a hypoid, it's a plain spiral bevel and was derived from the Austin 7 (as was the engine).

willmac
02-07-2013, 08:34 AM
Don't leave me hanging. Tell me more about the design decisions and how and why they got what they did.


I wish I could tell you more, but unfortunately I can't. Years ago one of my neighbours where I lived in Coventry was a gearbox guru, who worked on the Imp design. He was retired, but we had worked for the same company in some of the same areas. He told me a few stories about the design of the IMP powertrain, amongst which was the point about driving on the coast side of the transaxle hypoid diff gear. As I am sure you know, the Imp was released with insufficient testing and development and a few faults caused a loss of reputation. The problems were fixed, but it was too late to rescue it entirely. The gearbox was very good for its time and the whole engine/gearbox/transaxle unit was very innovative. See this http://www.imps4ever.info/algemeen/history.html for more background, particularly the page about the gearbox. Also take a look at the pages about Tim Fry and Mike Parkes. Tim fry was 20 years old and just out of his apprenticeship when he persuaded the owner of what was then a volume car manufacturer to take the risk of letting him design a new small car. Amazing and not possible these days.

Now I should apologise to the OP for taking this thread a bit off topic.

EVguru
02-07-2013, 08:57 AM
A Hypoid arrangement is used to offset the centreline of the pinion from the centreline of the ring gear.

Normally this is done to lower the propshaft and allow the floor to be lowered to give more passenger or load space.

In the Imp transaxle, the centreline is raised, so I'm guessing this would be where the 'coast side' comment comes from. It is of course by definition the drive side.

I don't know why they used a Hypoid (with quite a small offset). Usually longditudinal transaxles use spiral bevels, which are a little more efficient.

Some off-road vehicles use an upward offset to make the propshaft less vunerable.