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Peter.
12-25-2010, 06:10 PM
Can you identify the pressure angle of an involute cutter from the DP and depth as marked?

A No2 cutter, 12DP, depth marked as .1798 - I need a 20PA cutter.

oldtiffie
12-25-2010, 06:23 PM
Probably not. Thet are usually either 14 1/2* or 20*

This is how it is marked on the cutter:

http://en.wikipedia.org/wiki/Milling_cutter#Involute_gear_cutter

The Fixer
12-25-2010, 06:24 PM
The DP (Diametral Pitch)is based on the diameter of the pitch circle and the No. identifies the range of gear teeth that the cutter is able to cut.
if u do a search here for cutting gears u will see there's been considerable discussion on the topic.
The PA determines the profile of the teeth based on the angle of contact of the meshing teeth. it usually says on the cutter what the PA, DP and No. is.

Peter.
12-25-2010, 06:28 PM
Bugger :(

The PA is not marked on the cutter. Is there no other way to tell?

John Stevenson
12-25-2010, 06:40 PM
Usually if they are not marked they are 14.5 degrees, but that's not 100%

Optics Curmudgeon
12-25-2010, 06:43 PM
That one sounds like a 14.5 degree one, the full depth of a 14.5 degree tooth is 2.157/pitch, for 20 degrees it's 1.800/pitch.

Joe

Peter.
12-25-2010, 06:54 PM
Right-o, thanks. Saved me finding out the hard way :)

oldtiffie
12-25-2010, 07:04 PM
Can you identify the pressure angle of an involute cutter from the DP and depth as marked?

A No2 cutter, 12DP, depth marked as .1798 - I need a 20PA cutter.

Depth of cut for 14 1/2* PA = 2.157/DP = 2.157/12 = 0.1795"

Depth of cut for 20* PA = 2.25/DP = 2.25/12 = 0.1875"

The cutter PA is 14 1/2*

Terry L
12-25-2010, 07:24 PM
Okay, so who's right ? Optics Curmudgeon or Old Tiffie ?

John Stevenson
12-25-2010, 07:25 PM
Both,
Works out the same.
tiffie has just repeated what Joe said earlier.

Terry L
12-25-2010, 07:41 PM
Okay, color me stupid here, but 1.800/12 = .150 Not .1875. What am I missing here ?

Optics Curmudgeon
12-25-2010, 08:21 PM
Tiffie's got the right 20 degree number, I was looking at the stub tooth table on that one.

Joe

oldtiffie
12-25-2010, 08:33 PM
I can vouch that its (too?) easily done Joe.

I've caught myself nearly doing the same as you did several times.

Terry L
12-25-2010, 09:05 PM
Thanks to both of you. I really appreciate it. I have cut a few gears, but not many, using an involute cutter and a dividing head. I've even repaired a metric gear using an American cutter. My set-up probably wasn't correct, but it worked. So I was happy.

Another thing I was wondering is which is the most commonly used.....the 14.5 or the 20 degree PA ?

lazlo
12-25-2010, 09:11 PM
Another thing I was wondering is which is the most commonly used.....the 14.5 or the 20 degree PA ?

Up until the 60's, 14.5° was most common. Now, 20° is most common.

14.5° pressure angle will mesh quieter and with less vibration, but 20° PA has a stronger tooth.

Black_Moons
12-25-2010, 09:42 PM
Can't you just roughly measure the angle of the tooth on the cutter? 14.5 and 20 degree is pertty different.

oldtiffie
12-25-2010, 09:48 PM
BM.

Its hard enough to measure (guess?) the "angle" on a gear with convex faces and its a lot harder on a gear cutter with ony small gaps between the teeth and concave curves/sides.

The maths provided all that was needed as the OP's cutter had enough information to work out the Pressure Angle (PA).

oldtiffie
12-25-2010, 10:10 PM
Thanks to both of you. I really appreciate it. I have cut a few gears, but not many, using an involute cutter and a dividing head. I've even repaired a metric gear using an American cutter. My set-up probably wasn't correct, but it worked. So I was happy.

Another thing I was wondering is which is the most commonly used.....the 14.5 or the 20 degree PA ?

Lazlo is correct regarding the use if 14 1/2* and 20* PA gears.

I prefer 20* as it is stronger, matches nearly all my Chinese tool gears (metric) and does not undercut so much with gears with less than (I think) 32 teeth (I need to look that up).

These tables should set you right as regards gear tooth proportions/sizes:
http://i200.photobucket.com/albums/aa294/oldtiffie/Black_book/BB_50-51.jpg

The DP (USA and UK) gear system is the inverse of the metric modular system:

DP - 1/Mod

Mod = 1/DP

The gear proportions are the same.

Here is a table of both DP and Modular fit together to that with a bit of luck and care a Mod cutter can repair a DO gear and vice versa.

http://i200.photobucket.com/albums/aa294/oldtiffie/Black_book/BB_52.jpg

You can always use a narrower cutter, set it to the required tooth depth of cut and take three cuts - one centred and one each left and right. Its easy enough to measure the teeth - several ways - with shop vernier calipers or a disc micrometer.

It may not be perfect but it will get you going.

I hope this helps.

Terry L
12-26-2010, 03:09 PM
Yes, it does help. Again, thank you and I appreciate it.

Terry

Peter.
12-28-2010, 02:08 PM
Okay I've found a No1 cutter of correct DP and pressure angle, but for 65 teeth I need a No2 cutter.
How wrong would it be to cut a 65-tooth gear with the No1? would it mean more backlash, binding, or just a totally incorrect tooth profile?

John Stevenson
12-28-2010, 02:56 PM
Peter,

Take a look at this.

http://www.stevenson-engineers.co.uk/files/12DP65_135.jpg

Green is the 65 tooth gear.
Brown is a 135 tooth gear i.e. cut with a No1 cutter, the 5 thou dimension is the error per side.

BUT if you make the blank 12 thou on diameter smaller and cut with a No1 cutter to normal depth you get the tooth shape in red.

I think that's about the best you can do, for a non critical application it will work fine.

.

Peter.
12-28-2010, 03:10 PM
Thanks John - I think I'll go with that then. £25 material and £20 cutter is all it's going to cost me, and I can share the cutter cost with another guy. I have an old 40:1 dividing head so I'll make a suitable plate and cut it my mill.

Peter.
12-31-2010, 07:58 AM
Need more help. Just waiting for the guy with the No1 cutter to get back so I thought I'd take another look around.

Can someone work their magic on this one - it doesn't list the depth or the PA.

http://cgi.ebay.com/B-S-MFG-Gear-Tooth-Cutter-2-12-DP-55-134T-D-F-134-/200526505458?

B&S MFG Gear Tooth Cutter 2-12 DP 55 To 134T D+F .134

What is D+F?

John Stevenson
12-31-2010, 08:25 AM
Total cutting depth.
D = depth
f = clearance.

edit.
That depth doesn't tie up with a standard cutter, sounds like an American stub tooth.

Peter.
12-31-2010, 10:19 AM
*makes rude comment*

Thanks again, John. I have everything I need now bar the actual cutter itself. If I had the material I'd even have a bash at making a cutter. It sucks having a whole 5 days off work and not being able to do something interesting coz the shops are all shut!

I'll console myself by machining the internal features on the blank and making a mandrel to mount it on my dividing head.