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Author Topic: Problems threading on the lathe  (Read 330166 times)

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Offline Rieks

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Re: Problems threading on the lathe
« Reply #30 on: February 25, 2009, 03:33:59 PM »
This is the code:

G0 G40 G18 G80 G50 G90
G00 G53 X0 Z0
T101M5
G00 X10.5
G00 Z3
G00 X10
M03 S200
M08
G76 X8.12 Z-20 Q1 P1.5 J0.2 L45 H0.4 I29.5 C0.5 B0.1 T0
M9
M5
M30

rIEKS

Offline Hood

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Re: Problems threading on the lathe
« Reply #31 on: February 25, 2009, 04:09:10 PM »
certainly looks ok, shouldnt have any problems with that. What version of Mach are you using and if you look in C:\Mach3 for m1076.m1s, if you open in notepad does it say 12/25/08 Rem G95  at the start?
Hood

Offline Rieks

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Re: Problems threading on the lathe
« Reply #32 on: February 25, 2009, 05:32:56 PM »
Indeed there is  12/25/09 Rem 95

Offline Rieks

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Re: Problems threading on the lathe
« Reply #33 on: February 25, 2009, 05:34:22 PM »
Small misser  12/25/08 Rem 95

Offline Hood

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Re: Problems threading on the lathe
« Reply #34 on: February 25, 2009, 05:37:39 PM »
Can you attach your xml and I will see if I can replicate the problem.
Hood

Offline Rieks

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Re: Problems threading on the lathe
« Reply #35 on: February 25, 2009, 05:42:03 PM »
Can You gif me Your E-mail, then I can mail it to You tomorrow, its almost midnight.

Offline Hood

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Re: Problems threading on the lathe
« Reply #36 on: February 25, 2009, 05:43:52 PM »
sent my email in a personal message.
Hood
Re: Problems threading on the lathe
« Reply #37 on: February 26, 2009, 10:40:05 AM »
I am having the same or a similar problem.  I have posted In a another thread, but it seems to be a bit slower than this one.  Here is what I wrote:


Has a solution to this problem ever been found?  I am using .020, and it is giving me very similar problems.  If during a cut the spindle slows down very much at all the z speed compensates just fine, but the next pass is cut very unusually.  The next pass will be cut at what appears to be a constant taper with the z axis very slowly moving from a depth that is outside of the thread to the final depth that the pass should have been cut at.  This movement can be seen both physically, and on the DRO.  I have also had a problem where the next pass after a slowed down pass will be completely outside of the thread in the air.  I am not losing steps, and the pitch is always spot on, but the depth is very messed up.  I have run a test piece of code posted elsewhere in this forum that is a g-32 code that has a very shallow thread cut in 80 some passes.  I have run it three times, and the results are perfect, and repeatable. 

I am very confused as to what is going on here.  I don't have a very good understanding of what the g76 is commanding while it cuts at all the different depths to achieve a thread, but something seems to be amiss.  I am pleased to hear that you can change the output from the threading wizard to a g32, or what I would understand to be a longhand version of the code, I will try that as soon as possible to see what happens.  Thanks



I did do some more playing around yesterday, and I found that I was finally able to cut a 1/4-20 thread in brass and have it come out perfectly, but I had to do it in 40 passes.

I found that if when using the turn diag. screen I can watch the spindle speed deviation and watch the problem when it happens.  It seems that when the speed differs by about 10-12% the following pass will be messed up.  The pitch, or Z movement always stays correct, however the depth of the thread is cut at a taper, starting outside of the thread, and then tapering in so that by the end of the thread it is at the correct depth.  This then has a snowball effect, because as it tries to cut the next pass it is effectively cutting too deep since the material that should have been removed on the previous pass is still there, thus the spindle slows down, and the cycle starts over again. 

I only have a 1hp motor, and I realize of course that it is a limiting factor, but the cutting on a taper is very strange to watch.  Let me know if there are any other suggestions, and I will put them to the test.

Thanks.

Offline RICH

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Re: Problems threading on the lathe
« Reply #38 on: February 26, 2009, 01:24:33 PM »
TrevorH,
Only expect software to compensate so much for the mechanics of the lathe. What the program does to compensate for the mechanical ill's is very complex. It may take you additional passes but if in the end you get a good thread
that's what's important. My understanding of what happens is that the threading program does adjust to rectify a bad pass in the next pass. It can only do so much. Since threading is very machine dependent, i would suggest to anyone, that they do some experimentation.  

1. As you have done, try air cutting and see if timing is working as it should  and that the axis's have the
    accelleration / velocity to do the required moves. If you are using the wizard to generate the code , it will calculate
    and see  if you are within your motor tuning,
    and if not, you should change passes and spindle speed accordingly. I also must remark that if your max velocity
    and acceleration settings don't leave any headroom for cutting, then your not being very conservative and will
    surely run into a problem sooner or later.

2. Next confirm the quality of cutting by doing doing manny very small passes onto a marked shaft. You should get a
    single nice clean spiral. You will see if all is not well. I have posted pictures of this in other threads on threading.

3. Next comes the actual cutting. All the things associated with threading come into play. How well the "system"
    actuall does  may require you to adjust accordinly for your "system". Small lathes need to have as much going
    for them as possible. Sharp cutting tool nicely ground for the thread, properly placed for cutting, and adjusted
    accurately ( taking into account tip radius , etc.). So find out where things will go sour by doing some
    experimentation at different speeds, feeds , and .....material.  That experience will go a long way to being
    successfull.

I wish someone could say that at "x%" variation of speed threading will fault but just don't find that to be practical.
You shouldn't be there to begin with!

We just tried my Punny Lathe out the other day cutting a 1/4-20 @ 900 RPM / .001" first pass, .006" other passes with 2 spring passes in Al. Thread was perfect. BTW, I only have a fourth of your HP.

RICH


 
 
Re: Problems threading on the lathe
« Reply #39 on: February 26, 2009, 02:02:15 PM »
Rich, Thanks for the reply, Here is a little more about my setup.

1HP dc variable motor-0-1700 spindle rpm, most threading done at 3-500 rpm
Linear rails, and rolled pre-loaded ballscrews, with a couple thou backlash, but no compensation currently enabled in Mach
my velocity is limited to about 70 ipm, so I have set the motor tuning to allow for 50ipm max where there is plenty of power aviliable
The threading tool is a commercial carbide indexable insert with a very sharp point, no radius.  It is set as close to exactly on centerline as I can get.

I agree that  a smaller lathe will take many passes to achieve a good thread, and that not too much can be expected from a 1hp 200lb machine, but what really confuses me is what exactlly is going on when mach does the strange tapered cut that several of us have experienced.  Unfortunately it doesn't allow much room for error since once you "overload" the spindle and the software has to correct beyond a certian ammount(some say 5-6%, I say 10-12%) Then the operation is pretty much doomed since the next cut is going to be cut at a taper, leaving the next pass cutting at basically twice the programmed depth.    As I have stated before the interesting thing is that the pitch of the thread always seems to be perfect for me, with a incorrect depth caused by the taper being my concern.

I will continue to play around as time allows, and for now I will err on the side of lots of shallow cuts.

Thanks again for the input.