I Need A CNC Threading for Dummies Book

[JimDawson]

what puzzles me is that my cuts look worse the closer I get to the chuck.

It appears that the Z is losing position as the cut progresses. But I can't tell if it's over feeding (pitch getting longer) or under feeding (pitch getting shorter) or just variable. By the end of the thread, the peaks have moved about 1/2 thread pitch.

Let's assume, for the moment, that the Z feed speed is constant. Then the only possibility is that there is variation in the spindle speed.

 

[MontanaAardvark]

Let's assume, for the moment, that the Z feed speed is constant. Then the only possibility is that there is variation in the spindle speed.

I have the luxury of being able to swap out the entire headstock, since I have three Sherline systems. This one is actually the oldest one and when I did the CNC conversion, I set it up to allow me to pull the headstock just by undoing the one screw Sherline uses.

Maybe that's the first experiment tomorrow.

 

[Boswell]

I wonder if you are loosing steps with your stepper motors. Does the issue show on every pass? If you just do a few scratch passes do you get the same issue? You can clearly see an out of sync cutting starting about half way down the threads. running a minimum depth scratch pass would help isolate the issue between hardware and software. If you run several scatch passes with no issue then I would think there is likely a hardware issue when the cutter is under load. If you do get the issue with a scratch pass then more likely it is a software (computer) issue.

[Edit: I wrote this several hours ago, so a little out of date, not sure why it did not show up right away. most likely the Keyboard to Floor Interface]

 

[MontanaAardvark]

I wonder if you are loosing steps with your stepper motors.

That's one of the few things I'm pretty sure is good. While I was testing to see if my speeding up the rapid movements worked I put a 1" DTI indicator on the bench (.001" resolution) and ran both axes back and forth the full inch (one axis at a time). I wrote a simple Gcode file that ran it 15 full cycles - 15 times in each direction - and then ran that file 10 times. 150 cycles. Actually I did that more times on the Z axis than the X.

I have had electrical noise problems with the Xylotex hardware (stepper drivers) before. I need to take a look at that. First, though, I'm going to try to thread a half inch rod at 1/2-13.

 

[jumps4]

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?

That is the same problem I chased for months.
It is probably the mach3 version....
locate your mach3 directory and make copies of all of the files with the xml extension (so you don't loose all of your settings, pin numbers, motor tuning )
download Mach3Version3.043.057.exe and run from the site I gave you above
recopy the xml files you saved to the mach3 direcrory if your xml files were replaced.
and that is all there is to changing versions, (you may have to reinstall smooth stepper i'm not sure ) as long as you make copies of the xml files there is no setup. just install and retest
there are better threading wizards in the mach3 forum
good purchase for lathe: http://forum.machsupport.com/forum/index.php/topic,20001.0.html
Steve
Steve

 

[MontanaAardvark]

That is the same problem I chased for months.
It is probably the mach3 version....
locate your mach3 directory and make copies of all of the files with the xml extension (so you don't loose all of your settings, pin numbers, motor tuning )
download Mach3Version3.043.057.exe and run from the site I gave you above
recopy the xml files you saved to the mach3 direcrory if your xml files were replaced.
and that is all there is to changing versions, (you may have to reinstall smooth stepper i'm not sure ) as long as you make copies of the xml files there is no setup. just install and retest
there are better threading wizards in the mach3 forum
good purchase for lathe: http://forum.machsupport.com/forum/index.php/topic,20001.0.html
Steve
Steve

Thanks for the tips. Downloaded the new wizard, and was going to look for one anyway.

One thing I'm concerned about is that Mach makes a lot of decisions on how many passes to make, and other parameters. Since the Sherline is a light duty lathe, I'd prefer to make gentle cuts. In the Wizard I'm using, there's no way to limit the cuts. I'm not doing this for business, so it's fine by me if it takes a bit longer because it makes more passes.

 

[MontanaAardvark]

I swapped the lathe headstock. Long story, but I believe this is the headstock that came with the lathe when I bought it.

The jitter in the RPMs dropped in half, or more. At 300 RPM, I see 299 to 301 on the RPM indicator on Mach 3.

Then I tried the 1/2-13 thread in aluminum. This is slightly undercut - the valleys aren't deep enough. I recut it and it was still a little tough to get a nut on, but I eventually did without using a tap.



There's about .70 to .75 sticking out of the chuck and it's threaded 0.60".

I didn't try the new threading wizard or "downgrade" to the older Mach3. Just limits on time available.

 

[MontanaAardvark]

I started a thread about this on the Yahoo! Sherline CNC group because it eventually might become a Sherline Chucker lathe and wanted to talk with anyone who has been down that road. As luck would have it, Ron Ginger, the guy who wrote the Mach3 threading wizard is on that group and gave me some advice. Ron advocates for Mach4, primarily because threading works much, much better and secondarily because it's the only mach version being supported now.

I rearranged the shop a bit so that I can see the RPM display while threading. I setup to cut another 10-32 screw about 3/4" long and did all of that to keep an eye on the spindle RPMs when it's cutting a pass. The speed didn't change - which seems like the display stopped updating. While retracting the cutter and getting ready for the next pass, it shows a + 1 (no more than + 2 for sure) RPM difference.

When I cut the 1/2-13 test piece last Sunday, I could hear the spindle motor bogging down a bit. Yesterday, when I cut the 10-32 test piece (in brass; 360, I think) I didn't hear the motor bogging down at all. It's the best 10-32 I've cut but still has an issue.



The issue in this 10-32 screw has moved from the end closest to the chuck to the end farthest from it. On the far end, the tops of the threads appear wider - and flatter - and the cuts don't look as deep. On the chuck end, they look more properly formed. A nut will thread onto the wider looking end, but requires either a wrench or nut driver. Once it goes on past that first quarter inch or so, then it's easy enough to thread it on by hand.

I have not downloaded and started working with Mach4 yet. That will mean redoing my pulse generator and (I guess) going over to a quadrature encoder. I don't understand what quad encoding does for you; AFAIK, the only advantage is that it tells you the direction. From the standpoint of measuring speed, they don't do more for you than the same number of pulses/rev that a single pulse would give. At least, that's what I've been told over the years.

 

[JimDawson]

It really looks like the part might be springing away from the tool on the start of the thread. Just by eyeball it looks like the pitch is consistent. Maybe program in a few spring passes and see how it looks.

From the standpoint of measuring speed, they don't do more for you than the same number of pulses/rev that a single pulse would give. At least, that's what I've been told over the years.

It depends on the encoder as to how many pulses/rev you get. In quadrature, you get 4x pulses as the number of lines. So a 1000 line encoder would give you 4000 pulses/rev. Your electronic hardware would have to allow for quadrature input to take advantage of that.

If you are only reading the index pulse, then yes, you would only see 1 pulse/rev.

 

[MontanaAardvark]

I'm pretty sure I bumped up the number of spring passes from 1 to 2, so still not "several". That's an easy fix, though.

It depends on the encoder as to how many pulses/rev you get. In quadrature, you get 4x pulses as the number of lines. So a 1000 line encoder would give you 4000 pulses/rev. Your electronic hardware would have to allow for quadrature input to take advantage of that.

If you are only reading the index pulse, then yes, you would only see 1 pulse/rev.

I wrote that poorly. A quadrature encoder gives pulses that are 90 degrees to each other (think sine and cosine). The timing between those two wires tells you the direction the shaft is going and the period of either signal, either sine or cosine, can give RPMs.

If you have 1000 pulses per rev, it's possible to sense the speed every third of a degree (0.36 degree) from either wire, but the only information quadrature pulses give that you don't get from either sine or cosine is direction. Except for the direction information, you're getting nothing out of the two wires that you wouldn't get out of one wire. I think it's always safe to assume our motors are running in the right direction. Or look at the quadrature information before starting to cut to make sure the spindle isn't running backwards, if it can.

I don't believe our systems are fast enough to tweak the feed rates thousands of times per spindle rotation and have the mechanical parts respond. Mach3 apparently only makes the feed rate calculation once between passes. In my 10-32 example, it used the same Z-axis feed rate for 24 turns.

Some interval between adjusting Z-axis feed once in 24 turns and "several times per turn" seems like a better thing. Your encoder may be giving your 4000 PPR, but you're very likely using very little of that.