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jeremy13
07-07-2009, 11:38 PM
This is the first time I tried threading metric and it didnít work out. I have always thought I needed to change gears for metric threading. But as I looked at the chart I didnít think I needed to. I was threading 22X1.5 the first pass looked good and the thread gauge matched up. The last pass went right through the top of the threads. In the diagram I assume the single (V) is for mm. and in. threading and the (VV) pattern is the change gear pattern.
http://i29.tinypic.com/2i23q0z.jpg
http://i31.tinypic.com/330y6o8.jpg

Carld
07-07-2009, 11:50 PM
Well, to cut metric threads with an imperial lead screw, and yours shows to be a 4 tpi lead screw, you should have had to use the 120/127 compensating gear but yours may be using the 57/66 gears to do the job.

The next thing is, you can't release the halfnuts when cutting metric threads with an imperial lead screw. That is why the second pass was right in the middle of the first pass. You have to make a pass and at the end back the tool out and stop the spindle. Then with the halfnuts still engaged you reverse the headstock back to the start of the thread and reset the tool and make another pass.

K Barton
07-08-2009, 01:04 AM
This is the first time I tried threading metric and it didnít work out. I have always thought I needed to change gears for metric threading. But as I looked at the chart I didnít think I needed to. I was threading 22X1.5 the first pass looked good and the thread gauge matched up. The last pass went right through the top of the threads. In the diagram I assume the single (V) is for mm. and in. threading and the (VV) pattern is the change gear pattern.
http://i29.tinypic.com/2i23q0z.jpg
http://i31.tinypic.com/330y6o8.jpg

On a lot of lathes that are set up to cut US thread forms the thread dial for clutch engagement only allows US profiles; in order to cut Metric profiles the lead clutch needs to be left in gear, pull crossfeed back reverse spindle to start point, stop return crossfeed to reference position, advance coumpound, spindle forward, repeate process until proper thead depth is reached, then lead clutch can be disengaged.

Paul Alciatore
07-08-2009, 02:12 AM
What Carld said, just keep the half nut CLOSED.

57:66 is not an exact ratio, but it should be very close if the makers did their job.

jeremy13
07-08-2009, 09:31 AM
I hate to beat a dead horse but do I need to change the gears. This is from the poorly translated Manuel. And it does make one mention of " not to loose half nut in cutting metric threads". And of cores no mention of changing the gears. Just a part # and description of metric gears. Looking at the drawing the single (V) applies to MM and T/1 threading and the (VV) applies to MP and DP witch I have no clue to what they mean
http://i28.tinypic.com/2qx99wg.jpg

oldtiffie
07-08-2009, 10:32 AM
Metric threads are identified by their pitch in mm.

US or "inch" threads are identified by the number of threads per inch - ie 24 tpi but the pitch = 1/tpi so a 24 tpi thread has a pitch of 1/24" = 0.0417"

I know its confusing - but that's how it is.

I'd forget about the MP and DP, but just for your interest I will explain what they are (for).

Both have a "pi" (3.1416) component in the lead of the thread being turned as they are for turning threads that need it - such as milling hobs for worm-wheels and hobbing cutters.

MP is for (metric) "modular" pitch and DP is for the familiar "diametral" pitch.

The longitudinal pitch of the screw thread must equal the circular pitch of the "gear" or "worm" with which it mates.

The circumference of the gear pitch circle is equal to the pitch circle diameter x "pi" (Pi = 3.1416)

Circular pitch is the circumference of the pitch circle of the gear divided by the number of teeth on the gear.

So we can now forget about that as the chances of having to cut a thread with a lead that is a multiple or sub-multiple of "Pi" are very slim.

I'd change the gears as directed for the metric thread/s you want to cut.

As advised by others DO NOT disengage/open the half-nuts until you have completed the thread-cutting - it is MOST important.

When screw cutting, stop the lathe spindle when you have gone as far along the part to be threaded as you want to go. Withdraw the screwing tool with the cross-slide. Then reverse the lathe and run it back to a convenient start point, move the tool in for more thread-cutting, and start the spindle in "Forward" and continue as before.

Use the slowest spindle speed you have, take your time and get used to it all.

Speeding up, tear-arsing, and "hogging it" may well come later - much later.

Good luck.

jeremy13
07-08-2009, 12:03 PM
Well I will defiantly forget about the MP and DP. I will get some in person training on that. As far as the metric threading look like I'll have to do it in reveres and on the back side I will be threading in to a shoulder.Ok so I don't add the 66T and 42T gears those are for the MP/DP threading just move the levers II CDU and 7.

Thanks for all the help!!! The machinist I visit time to time tells me I make threading a lot harder than it is.:D

oldtiffie
07-08-2009, 12:30 PM
Glad to help.

You've got it right as regards the gears. Leave them "as is" - as I suspect that the 1:25.4 = inch:mm ratio is sorted out in your head-stock gear-changes.

The "127 tooth gear" is a way of resolving the inch:mm problem in the gear-train between the spindle and the quick-change gear box on some other lathes - mine included.

127 = 25.4 x 5 so its all neat and tidy - but you don't have to bother about it.

You will see the "circular" DP and MP get mentioned in discussions on some of the more exotic gearing at times.

I have a "metric" (lead-screw) lathe so I have your problem with having to leave my half-nuts engaged when I am cutting "inch" threads.

The 66 and 57 gears will be to do with the "Pi" function only - so forget them.

Best of luck with the screw-cutting.

miker
07-08-2009, 08:02 PM
[/QUOTE]When screw cutting, stop the lathe spindle when you have gone as far along the part to be threaded as you want to go. Withdraw the screwing tool with the cross-slide. Then reverse the lathe and run it back to a convenient start point, move the tool in for more thread-cutting, and start the spindle in "Forward" and continue as before.[/QUO

What would happen if you did not withdraw the tool before reversing? In other words, just let the tool ride back at it's current setting,then add the new depth for the next cut.

Would backlash issues make the tool widen the cut on the ride back to the start? Being a bad thing!

Rgds

Carld
07-08-2009, 08:07 PM
When the backlash in the gears got taken up and the tool starts moving it would cut right through the thread you just cut and rip it to pieces.

You really need to back the tool out then stop the lathe, not stop the lathe and back the tool out. If you stop the spindle it may break the tip off the threading tool and as the thread gets deeper it's hard to stop the spindle in the same spot to keep from suddenly loading the tool and damaging something.

You can use the spindle foot break but that is not a good idea. It's best to back the tool out then stop the spindle. That makes it extremely hard to thread to a shoulder a metric thread on an imperial lathe. It's best to thread to a safe distance and finish with a thread die.

oldtiffie
07-08-2009, 08:42 PM
When screw cutting, stop the lathe spindle when you have gone as far along the part to be threaded as you want to go. Withdraw the screwing tool with the cross-slide. Then reverse the lathe and run it back to a convenient start point, move the tool in for more thread-cutting, and start the spindle in "Forward" and continue as before.[/QUO

What would happen if you did not withdraw the tool before reversing? In other words, just let the tool ride back at it's current setting,then add the new depth for the next cut.

Would backlash issues make the tool widen the cut on the ride back to the start? Being a bad thing!

Rgds


That's a very good question Michael.

Doing that will show up the total back-lash in the gearing between the head-stock spindle and the final gear drive to the lead-screw on the one hand and the lead-screw back-lash with the half nuts as well as the "end/axial float" of the lead screw in its thrust bearings. It will also include any "slop" between the half-nuts and their guides and pivots etc.

You can easily simulate that situation by marking the outer face of the chuck and putting a pointer on it and also putting a magnetic base on the lathe bed with the indicator on the carriage to indicate in the axial/longitudinal direction.

With the half-nuts engaged, move the spindle first "backward" say half a turn then "forward" to the index mark and set the indicator on the carriage to zero.

Now move/rotate the spindle "backward" (reverse) until the indicator on the carriage starts to move. Note the fraction of a turn that the spindle moved.

Now do it the other way.

The total backlash will be the fraction of a turn in the spindle X the lead of the lead-screw.

Example (I just pulled the numbers out of the air):

3/8 of a turn X 1/4" lead of lead-screw = 3/8 X 1/4 = 0.0938"

Quite a bit isn't it?

I think I'd stick with with-drawing the screwing tool!!!

And its even more surprising just how accurately a lathe can cut a thread - even with a worn lead-screw.

This is a "no-load" (minimum load) or "static" situation - ie with no driving power to drive the machine.

In a "dynamic" case it is different as regards "amounts" but the principle is the same- where the lathe and tool are "working", as well as over-coming tool/work resistance that will require a good bit of torque which in turn will cause "twist" in various components in the "spindle to tool" train. This "twist(ing)" effect is in addition to the previously discussed "back-lash and "end-float".

You can apply the same logic to any lead-screw - lathe cross and compound slides as well as milling machine lead-screw and table/slide combinations as well.

It can be used on gear trains and worm/wheel combinations as well.

It all basic stuff using basic logic and basic tools.

tattoomike68
07-08-2009, 08:44 PM
About 10 years ago I was doing alot of metric threading and got real fast at doing the work without ever disengaging the lead screw.

It became such a habit that I use the same method for US or "inch" threads for a long time. I found it was faster once you have a rhythm and are comfortable with the machine controls.

miker
07-08-2009, 10:51 PM
Thanks all, for the input, and Mick your detailed explanation is much appreciated.

I have been withdrawing the tool on the few threads that I have cut and had no problems.

Just to throw in an extra thought, on the run back to the start the spindle is in reverse. So would the tool actually cut anything if it were left engaged or would the backlash pressures described just mash the tool sideways through the already cut threads?

I have no intention of leaving it engaged :)

Rgds

tattoomike68
07-09-2009, 12:21 AM
Thanks all, for the input, and Mick your detailed explanation is much appreciated.

I have been withdrawing the tool on the few threads that I have cut and had no problems.

Just to throw in an extra thought, on the run back to the start the spindle is in reverse. So would the tool actually cut anything if it were left engaged or would the backlash pressures described just mash the tool sideways through the already cut threads?

I have no intention of leaving it engaged :)

Rgds


the tool would just burnish the side of the thread if not backed out but its undo wear and tear you dont want. It takes a half second to back out the tool, you may as well do that.

Im one of those that never use the compound to thread till the pitch is down to 2 - 3 tpi, otherwise i just stab the tool right in with the cross slide. I also thread with carbide 99% of the time at a high speed and heavy DOC.

Anyway screw the rules, do what works, if it dont work dont do it.

oldtiffie
07-09-2009, 01:36 AM
Thanks all, for the input, and Mick your detailed explanation is much appreciated.

I have been withdrawing the tool on the few threads that I have cut and had no problems.

Just to throw in an extra thought, on the run back to the start the spindle is in reverse. So would the tool actually cut anything if it were left engaged or would the backlash pressures described just mash the tool sideways through the already cut threads?

I have no intention of leaving it engaged

Rgds

Michael.

Short answer:
yes.

Long answer:
As with other circumstances, in the absence of any other comprehensive protection against poor reflexes or malfunction or mechanical failure, and if not inserting is not an option, then withdrawal is essential if the object into which it was (is?) inserted will be well and truly shagged - but it will have been screwed too none-the-less.

Needless to say, base of the thread cut, aka the root diameter, is of utmost importance.

Perhaps you should restrict your screwing activities to the "rhythm" method - as per Tattomike's post - of course.



About 10 years ago I was doing alot of metric threading and got real fast at doing the work without ever disengaging the lead screw.

It became such a habit that I use the same method for US or "inch" threads for a long time. I found it was faster once you have a rhythm and are comfortable with the machine controls.

Paul Alciatore
07-09-2009, 01:55 AM
The above assumes that your lathe will run in reverse. Mine (SB) will, but many Asian imports do not have this feature.

In spite of the fact that I can run in reverse and otherwise have all the threading features I need, I made an additional accessory, a hand crank for my spindle. I can fasten it on the back of the spindle and turn it by hand for threading. This facilitates threading up to a shoulder with either English or metric threads and backing up is also easy. I simply stop when I get to the end of the thread, back out with the cross slide, and turn the crank backwards. It is almost as fast as threading under power and you have total control at all times. It is about 16" long which provides plenty torque for cutting the threads.

oldtiffie
07-09-2009, 03:29 AM
That's a good point and a good tool Paul.

There is another problem with many of the small to medium Asian lathes as compared to larger Asian and many US/UK/European lathes (most sizes) - and that is that the slowest speed is too fast for threading and other form tooling etc.

With smaller lathes, "chatter" can be a problem. A REALLY SLOW speed will fix a lot of that problem.

The handle you describe will address and fix all of those problems.

Just one other point with screw-cutting that I missed before. Clean the lead-screw before using it for screw-cutting (includes "fine feeds" for those with no (other) power feeds) as the grit on the lead-screw will not help the lead-screw or the half-nuts at all.

oldtiffie
07-09-2009, 06:15 AM
I re-visited the gear-train set-up in the lathe as there are two trains: one for normal screw-threading (metric and inch) - denoted "V" and those for screw-threads that have a "Pi" (3.1416) component - for linear pitches as for gearing circular pitches - as discussed previously.

The "127 gear" ratio between/for metric and "inch" (25.4:1) was dealt with in the main head-stock gear-box and NOT in the external gear train as it often the case.

Here is the pic of the gear-trains.

http://i29.tinypic.com/2i23q0z.jpg

I was pretty certain that there was a Pi constant ratio between them - so I set out to find out.

The ratio of initial driver to final driven in a gear train is Driver1/Driven1 X Driver2/Driven2 etc.

The left "V" train for screw threads is 48/57 = 0.8421

The right "V V" train for "Pi" threads is 48/57 X 66/42 = 1.3233

(Note: the 48/57 is common to both trains and could be ignored but I left it in for clarity)

The ratio between the two trains is 1.3233 : 0.8421 = 1.3233/0.8421 = 1.5714 which is pretty well equal to Pi/2 = 3.1416/2 = 1.5708 so the "Pi" relationship is established.

Or re-stated, the 66 : 42 gear ratio is 66/42 = 1.5714 ~ Pi/2 (might come in handy as an "identity" one day!!).

The actual %-age error is ((3.1416/2) - 1.5714)/(3.1416/2)) X 100/1 = ((1.5708 - 1.5714)/(1.5708)) X 100/1

= 0.0382% = 0.000382 = about 3.8 "tenths" per inch.

Yeah, I enjoyed that and I just KNEW that you'd want to know!!

jeremy13
07-09-2009, 09:34 AM
Well the internal threading bar worked but not very good looking threads. Will my external threading bar work if upside down and horizontal cutting plane above the center line, and running in reverse. This is a peace of SS nonmag. I'll try the slower speeds my lathe will do 25rpm.