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I Need A CNC Threading for Dummies Book

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For the last couple of months, my main project around the shop has been moving threading onto a CNC Sherline lathe. Sort of by definition, anything that gets a small thread on it (say anything from 4-40 up to the biggest threads I've ever used) is going to be small enough for the Sherline's envelope. I have a Sherline long bed lathe that I bought back around '07 and converted to CNC. I threaded a couple of things back around 2008 or '09 but other than proving I could do it, I hardly ever touched that lathe.

This can be long a story and I don't know quite how long to make it. Not sure how much background is needed.

I'm running Mach3 and Windows 7 on the PC. Because I have three machines on the same PC and for the generally better performance you get this way, I'm running an Ethernet Smooth Stepper. Because the shop has moved around a bunch of times in the intervening 10 years, the lathe has been moved. The last time I tried to just move the axes on the lathe, it didn't respond at all and I had to resurrect all the hardware. When I went to hook up my home made optical sensor, it blew up. I had grabbed the wrong power pin.

I rebuilt it based on an optical interrupter (instead of reflective) from CNC4PC C3 card and made the hardware to mount it.

HardmountRunning.JPG

I have an oscilloscope and can watch the voltage pulses out of this. When I look at them, I see that the pulse is about 2-1/2 milliseconds long (2500 microseconds) but jitter about 200 microseconds. Call it +/- 100 usec. That's +/- 4%. Is that a problem? Don't know. Nothing to compare to.

I tried the scratch test and the first time it came out messy.

FirstCutsNewLathe.JPG

The threads look less like a"V" and more like a "W". Like it's missing the index or calculating something wrong. Plus the threads on the left look different. The peak of their W isn't as pronounced. I don't think it's a lighting trick or other photographic thing.

After Jim posted this update: https://www.hobby-machinist.com/threads/fanuc-ot-to-dc_cnc-conversion.66432/page-4#post-630786 One of the things it got me thinking of was just to run the motor faster. Plus I was suspicious the chuck might be loose.

Yesterday, I increased the motor speed from 200 to 300 and scratched a blank again. To be honest, I wasted a couple of days to get a successful scratch to this because I had Mach3 set in diameter mode and was entering the cutter positions in radius mode. Took me a while to realize the scratches weren't deep enough. Anyway, yesterday the overlapping 10 scratches looked like single scratch, so I decided to go for it and thread a test piece. 1/4-20.

FirstSuccessCNCThreading-12-19-18.JPG

It was a little tight - I had to thread on that nut with a wrench, but it worked fine after that. This is a 1/2" long stretch of 1/4-20 threads.

Figuring that with CNC, if I can thread 1/4-20, I should be able to thread anything, I went for 10-32 in aluminum. Did the prep work on the manual lathe, and then set it up. First pass success. Again, I had to tighten the nut with a wrench, but aluminum would surely shear off if the threading was too far wrong.

10-32-1st-try-12-19-18.JPG

Although the nut screws on well, if you look closely at the left end of the thread, the shape isn't good. It looks like the cut varying from the right end to the left end is still there. It doesn't cut exactly the same path every time.

So I'm not quite done, but these threads are usable in a pinch. I want to make it better. My questions of the day are why the threads don't cut the same from end to end, and why do they seem to have more variation the farther into the cut I'm getting.

Can anyone tell me if the 4% variation in the pulse width/timing is normal? I see the RPM counter on the Mach3 Turn screen wobble around, too. Maybe from 295 to 302 when doing the 1/4-20 at 300 RPM and maybe a bit more variation when at 450 RPM for the 10-32. They don't seem to vary the full +/-4%, but maybe the display doesn't update quickly enough to see that. Could this variation all be from not using a tailstock? I didn't think so because I thought the right end would be worse.
 

Comments

#2
First let me say that I'm no expert in Mach3. But as I understand it, Mach3 reads the RPM before the start of the cut then calculates the feed speed based on that RPM. Once the cut starts, if the RPM varies then so will the threads because the feed rate is constant. The spindle/axis are not really geared together.

I wouldn't think a 100 microsecond variation would make a lot of difference in the start position at 300 RPM. You might try adding a low pass filter (maybe 0.01 mF?) to the pulse output to reduce the spike and smooth out the pulse timing. I don't know what the input scan timing is in Mach3, but there might be some variation there.
 
#3
Try starting your thread cycle at least 4Xs the pitch minimum in the z axis . :) For a 1/4-20 thread in aluminum . you should be pushing 1500 rpm minimum . At the rpm you're running , you are tearing the material off .
 
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#4
Hi aardvark
I fought mach3 for years with this issue and I can give you a few tips
the latest version of mach3 ending in 66 won't work for threading and that is the reason they made mach4
066 is junk so no matter what you should start there.
I'm using Version R3.043.057 without issue.
I'm in a rush to leave but if you search this site for mach3 and my name "jumps4" you should find the website to download the older versions.
the Sherline is not really ridged enough for most carbide and a nice sharp high speed steel tool will greatly improve the results.
you will probably find your depth of cut cannot exceed .001 or .002 after the first pass of maybe .010 or less and a lot of spring passes.
the metal rod your threading makes a big difference. I purchased rod from home-depot that will not thread it pulls like bubblegum.
hot roll and a lot of cold rolls are fussy so before blaming the lathe try brass it is the best for minimizing the issues with material.
This in my 9x20 making threaded inserts in cold roll with HSS tool...
https://www.hobby-machinist.com/threads/9x20-lathe-cnc-conversion.8692/page-4#post-288577
good luck
Steve
 
#5
If what Jim said is about Mach 3 setting the feed rate based on an initial value is true, then there would be no reason to expect consistency in your threads. The instantaneous spindle speed on a small lathe will vary with the cutting load. Almost certainly, there will be some slow down from no load speed. The fact that you are seeing jitter would indicate to me that your spindle speed is varying slightly from one rotation to the next. On a large lathe, I would expect spindle speed to be much more uniform. The motor would be fully capable of dealing with any threading load and the mass of the spindle and chuck would tend to smooth out any variations

To accurately thread, the spindle speed should be constantly monitored preferably with multiple pickup points encoder wheel and that information fed into the controller which would change the feed rate accordingly.

My experience with Mach 3 was on a mill. Mach 3 would be sending information to the driver motors to synchronize their speed to each other. I am not aware that it would be able to adjust a G code program being executed to compensate for a change spindle speed.
 
#6
Try starting your thread cycle at least 4Xs the pitch minimum in the z axis . :) For a 1/4-20 thread in aluminum . you should be pushing 1500 rpm minimum . At the rpm you're running , you are tearing the material off .
Mach3 said it couldn't go faster than 300 RPM on 1/4-20 (I'm not sure I remember that right - it might have been 200 RPM) I got up to 450 RPM on the 10-32 thread. I may have the axes constrained too slowly - rapids are set to 15 IPM with the standard leadscrews - but I don't think the machine would be capable of threading at 1500 RPM under any circumstances.

The Sherline headstock is about 1/8 HP and the machine isn't terribly rigid. In general, the whole system has to balance; speed of rapids, RPMs of motor, horsepower (torque) in the cut, everything. There are certainly tradeoffs in using the Sherline instead of a more powerful machine, but I'm working with what I have.
 
#7
Hi aardvark
I fought mach3 for years with this issue and I can give you a few tips
the latest version of mach3 ending in 66 won't work for threading and that is the reason they made mach4
066 is junk so no matter what you should start there.
I'm using Version R3.043.057 without issue.
I'm in a rush to leave but if you search this site for mach3 and my name "jumps4" you should find the website to download the older versions.
the Sherline is not really ridged enough for most carbide and a nice sharp high speed steel tool will greatly improve the results.
you will probably find your depth of cut cannot exceed .001 or .002 after the first pass of maybe .010 or less and a lot of spring passes.
the metal rod your threading makes a big difference. I purchased rod from home-depot that will not thread it pulls like bubblegum.
hot roll and a lot of cold rolls are fussy so before blaming the lathe try brass it is the best for minimizing the issues with material.
This in my 9x20 making threaded inserts in cold roll with HSS tool...
https://www.hobby-machinist.com/threads/9x20-lathe-cnc-conversion.8692/page-4#post-288577
good luck
Steve
Thanks, Steve. In real life, I'm Bob. I keep thinking it says that in my profile, but that's another forum, I guess.

I will check the version. This sounds vaguely familiar and I think I might have changed back to 57, but that's easy to check.

I don't see how I control the depth of cut in the threading wizard. When I threaded by hand on the Sherline, I never advanced the cutter more than .002 - but that's in radius so .004 in diameter. Maybe a better CAM program than the easy one that came with Mach3 Turn is called for.
 
#8
No ideas on the cnc stuff but that isn't a threading tool.
 
#10
While that insert is a 60° cutter, the nose radius is way too large, looks like about 0.016 radius, threading inserts for small threads come to a pretty sharp point

1545326816452.png

or
 
#11
I don't have a good way to measure that. To the eye, it looks like a point, unlike my carbide insert threading tool, which has a very rounded tip to my naked eye. In the full size crop from my original I can see a radius, but I don't see how to measure that.

Cutter_FullSize.jpg
 
#12
Ahhh, OK, I looked at the wrong picture. Yes, the brazed carbide tool will work for threading.
 
#13
Ahhh, OK, I looked at the wrong picture. Yes, the brazed carbide tool will work for threading.
D'oh! I forgot I had that other picture in the mix. I used that for the scratch piece that it was in the picture with. At best, the threads would have been U shaped. Maybe OK for 3/8-16 or bigger, but I don't know why they sell those things.

I need to order some more of those sharp point tools.
 
#14
the latest version of mach3 ending in 66 won't work for threading and that is the reason they made mach4
066 is junk so no matter what you should start there.
I'm using Version R3.043.057 without issue.
I'm using R3.043.062. I think that was because .066 didn't play nice with the smooth stepper.

Is there any reason to suspect this version?
 
#15
Hi Bob
I asked about the threading problem in the Yahoo mach2/3 user group where the original beta testers hang out and they sent me to the 57 version I did try others but I don't remember what versions. I fought this for months blaming my machine and breakout boards.
This pdf may help if you don't have it and there may be a rev:3
As far as the tool the less hp you have the sharper that tool better be.
I thread from 300 to 600 rpm on a standard breakout board a smooth stepper may work better but 600 works pretty good on aluminum
I do all threading with coolant.
check the pdf file, your window in the disk may need to be longer to see a defined pulse. my pulse width is optical but had to be wider
here are all the mach3 versions

ftp://ftp.machsupport.com/Mach3/

here are all files related to mach3/4 from their ftp site at Newfangled solutions

ftp://ftp.machsupport.com/../

there are more threading wizards on the mach3 forum and from newfangled solutions there is a conversational lathe program the will do most everything plus threading (purchase item)
Good luck
Steve
 

Attachments

#16
Ah , yep . After seeing what your machine was , I was way off on my info . I figured it was a full blown cnc . One other idea if needed would be a geometric diehead which would work in your toolholder . Either way , Go with a dead sharp point .
 
#17
Hi Bob
I asked about the threading problem in the Yahoo mach2/3 user group where the original beta testers hang out and they sent me to the 57 version I did try others but I don't remember what versions. I fought this for months blaming my machine and breakout boards.
This pdf may help if you don't have it and there may be a rev:3
As far as the tool the less hp you have the sharper that tool better be.
I thread from 300 to 600 rpm on a standard breakout board a smooth stepper may work better but 600 works pretty good on aluminum
I do all threading with coolant.
check the pdf file, your window in the disk may need to be longer to see a defined pulse. my pulse width is optical but had to be wider
here are all the mach3 versions

ftp://ftp.machsupport.com/Mach3/

here are all files related to mach3/4 from their ftp site at Newfangled solutions

ftp://ftp.machsupport.com/../

there are more threading wizards on the mach3 forum and from newfangled solutions there is a conversational lathe program the will do most everything plus threading (purchase item)
Good luck
Steve
Thanks much for that pdf. Started reading it already.

I'm wondering if it's time to switch over the Mach4. I bought Mach3 around 2005, so I've certainly gotten my money's worth out of it. I'm running my Grizzly G0704, my Sherline 4-axis mill and lathe on it.
 
#18
Interesting day. I've been running maximum rapids on my Sherline motors limited to 15 IPM. Why? I honestly didn't remember. So I put my 1" by .001" indicator on a stand and moved Z back and forth for an inch to see if I'd loose any steps by doing faster moves. I went from 15 to 40 IPM by fives and only after several minutes of back and forth tests at 40 IPM did it drop some steps and lose position. I backed off to 30 IPM for rapids. Then I did the cross slide at 30 IPM (same size motor) and ran it for several minutes (at least 10, maybe 15) and it never dropped steps either.

That means I doubled the speed on both axes, which will allow more ability to pick proper feed rates.

Then I tried to re-create a piece I had done by hand for my last engine model, about 5 turns of 4-40 on the end of a quarter inch diameter rod. On this one, I ran the speed up to 1000 RPMs and it went very well. Probably overcut it a little as the nut wobbles a bit too much. Most everything else I've done I've undercut.

FiveTurns-4-40.JPG

I did another 10-32 thread before this, only longer than yesterday's and it looked like the other one above. The threads look worse the farther to the left they extend.
 
#19
Does this machine not have a canned thread cycle?
 
#20
Does this machine not have a canned thread cycle?
Edit to add: I think the answer is yes.

In the sense that Mach3 has a "threading wizard" that writes a GCode snippet that cuts the thread using a G84 canned cycle. It doesn't do something like let me say, "4-40 external threads 0.150 long", but I enter starting and stopping diameters, thread pitch, end point, and a few other things.
 
#21
That sounds like a canned cycle , although threading used to be G76 . That's inflation for ya . ;)
 
#22
Edit to add: I think the answer is yes.

In the sense that Mach3 has a "threading wizard" that writes a GCode snippet that cuts the thread using a G84 canned cycle. It doesn't do something like let me say, "4-40 external threads 0.150 long", but I enter starting and stopping diameters, thread pitch, end point, and a few other things.
Excellent, this is pretty much what most conversational controls do, this is a Bridgeport control from the 90's.
Easy as could be, fill in the blanks.
It has a good deal of safeguards that will not allow certain moves in conversational mode but will do whatever you program in G code manually.
Anyone that knows how to run a manual lathe could be running this machine in hours.
 
#23
P . To me , that looks confusing . I used a Funuc 6mb ( conversational programming ) on a Makino Count 15 back in the 90s . My codes were something like this .

G76 P1020060 X Z F
G76 P Q U W R

Been a long while ago , so I know this is not perfect :grin:

I miss that lathe . :frown:
 
#24
G76 P1020060 X Z F
G76 P Q U W R

What could be less confusing then that (-:
1=External thread
2= Internal thread
Lead/TPI
Thread height/Full depth of finished cut
First pass DOC
Following passes DOC
Finish pass DOC
Spring cuts, just as one does on a manual machine, same finish diameter just dusting it off.
Withdraw is the distance that the tool will retract in order to clear chips
Engage Angle is like using the compound on a manual lathe, in the case of this thread it is a 1 1/4-4 Acme thread or 29 Deg. included
Start Z (.200") from the beginning of the part, same as a manual machine
End Z (13 3/8") from zero, same as a manual machine
Start diameter and end diameter are the same unless you are making a tapered thread, same as a manual machine

Anyone with a small amount of experience with a manual machine can run this.

It is fill in the blanks, no code and no math.
Simple manual operation done for you simply by filling in the empty text fields.
If one can not operate a keypad all bets are off
 
#25
Yep , similar to the Fanuc , just fill in the blanks . :encourage:
 
#26
If I can ask, then, what puzzles me is that my cuts look worse the closer I get to the chuck. I'm trying to figure out how to fix that.

It's not like they're the wrong pitch, it's like sometimes the cutter ends up in the wrong place. Look at the left end of this picture.

Long-10-32.JPG

(the bad focus will hurt your eyes if you look to the right - sorry)
See how there appears to be two different cuts? There's like a wrong number of peaks and they're shaped wrong. This is about a 3/4" long piece of 10-32 thread. It gets about the first half of the threads right, but the farther it advances to the left, the worse it does. I did this after I fixed the backlash but before I adjusted the G00 rapid moves to go faster

This seems like it could only be that it's calculating really wrong values for Z. I can't see how it could be the index pulse because they don't booger up the threads on the right and if they were wrong by a half or a quarter turn, I'd guess they'd mess up those threads but it would stay the same end to end.

Any ideas of what to look for?
 
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#27
Have you tried cutting a larger thread , say a 1/2-13 or so ? If so , same results ? I would put an undercut in the back btw . I think I said it in an above post also , sometimes these small threads can be a PITA and end up looking like a plowed field . You get down to where the material can't support itself and trouble arises . 10s ? Should be no problem , but I did have to use dieheads on alot of projects just because of problems like this .

Try that 1/2" with an undercut and see what you come up with . :)
 
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#28
Have you tried cutting a larger thread , say a 1/2-13 or so ? If so , same results ? I would put an undercut in the back btw .
I haven't cut a 1/2-13, but I've cut 1/4-20. Not as long, just half an inch. I think it would have showed up though.

First_qtr-20-12-19-18.JPG

I can try the 1/2-13 tomorrow morning.
 
#29
The 1/4 looks better than the 10 . You have alot of excess stock sticking out of that chuck which multiplies your problems .
 
#30
If I can ask, then, what puzzles me is that my cuts look worse the closer I get to the chuck. I'm trying to figure out how to fix that.

It's not like they're the wrong pitch, it's like sometimes the cutter ends up in the wrong place. Look at the left end of this picture.

View attachment 283133

(the bad focus will hurt your eyes if you look to the right - sorry)
See how there appears to be two different cuts? There's like a wrong number of peaks and they're shaped wrong. This is about a 3/4" long piece of 10-32 thread. It gets about the first half of the threads right, but the farther it advances to the left, the worse it does. I did this after I fixed the backlash but before I adjusted the G00 rapid moves to go faster

This seems like it could only be that it's calculating really wrong values for Z. I can't see how it could be the index pulse because they don't booger up the threads on the right and if they were wrong by a half or a quarter turn, I'd guess they'd mess up those threads but it would stay the same end to end.

Any ideas of what to look for?
I can only make some observations on this.
Whenever a machining process develops a change over a short distance such as above there are machine issues at play, otherwise the thread would be awful the entire length and not just at one position, speed an feed must remain constant in order to single point thread as opposed to facing a large part without CSS, the cut speed changes drastically in this case.
That being said there may well be a problem near the chuck where most of the work has been done in the past.
 
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