Messages

6471

General Mach Discussion / Re: Problems threading on the lathe

« on: March 01, 2009, 10:37:59 AM »

Art,
Thanks for the reply. I am usng the SS wiith the latest plugin. I'll get some info off to Greg and see what he needs,
says, and work with him. Threading on my end has never been better, but, no joy for some others, quite obviously. There are two issues in my mind, namely:
1. Clarity on how the basic threading is "designed" to work ( program reaction to spindle slowdown ).
   ( Not conjecture ie; based on what i found / posted in reply #96 ) and maybe some flavor of anticipated program
    limtations.
2. With one above known, the user or the advice given can better deal with the mechanical system influences
    causing a threading problem in the first place. There will be varing degree's of users lathe systems and this is only
    the beginning as more users start threading.

I will be bold enough to say it again, SOFTWARE  solution to a mechanicaly created one is a patch only and can only do so much. In that light the punny guys got a lot to thank Art for.

RICH

6472

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 10:17:17 PM »

Trevor,
Did you read my reply #96. Your probably correct but i didn't spend a whole lot of time trying to find every single thing. I will waite for a reply from
the writers of the software. What more can i say.

I didn't watch the video yet. I suggest for now just do lighter cuts, reduce your velocity a tad, use backlash compensation. It is the way it is.

RICH

6473

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 08:27:58 PM »

Rieks,
The compensation was deliberately built in. It was done to take care of Punny lathes, does work, but there is a limit on anything. The better your
spindle speed is to start with and maintain that speed when cutting,  and if you can minimise the mechanical influences ( backlash, uneven cutting, poor set-up etc. ) that may cause it to slow down, and if you use good machining practice, the axis components can do their job as it realtes to speed ,then you should do fine.

Threading is a "SYSTEM" of all the components. Not one thing. It's like a sterio, one part of the system not up to par and  the music you will hear has been downgraded.

RICH

  

6474

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 06:30:20 PM »

OK FOLKS,
Here it is and if you can manualy vary spindle speed the you can duplicate it.
The threading cycle has been changed from before in how it compensates for spindle
slow down. It's also a matter of degree of slowdown. 
 
Generate the Gcode with  M76 and have all the cuts .001" deep, 1 to  2" of thread,  and run at 100 rpm  so you can see what is happening. My dry test had 80 passes.This makes for easier monitoring.

As it should each time a new pass is made the X will move .001" for the next cut. All is good. At pass
#10  I varied the speed  70%. I am using the SS so you can't see speed change but you can see feed rate change. The DRO's "seem" to go bonkers......but i don't think so......
The program see's that thread pass as out of the norm in which it can correct, so it takes you back to starting X DRO and does an air cut down to the X value at the bad pass, so it is doing a "tapered" move. 
Maybe what it has done or is doing is recalibrating / reseting the buffer for the next pass. But it needs some good data to work with.
The next axis move will now go to the next X pass value and that value is correct, just .001 deepr than the past bad one. Let it continue for  10 passes and each one will be correct. 

I duplicated this 8 times in the 80 passes.

Now if you stop the spindle, and stop time seems a little time dependant  on how it will behave, the Z will just finish the pass ....
the thread would be history in real life .......and the threading cycles will stop. Mach has lost the index...
it has no basis on how to continue.

So either the threading cycle is ******************x'd up or the way it deals with spindle slow down has been changed.

RICH
Now I have been convinced, but the mechanical stuff still induces it.  

....... ART, Brian please clarify what the  proper interpretation of the threading cycle is........!

6475

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 02:02:36 PM »

Think your x  stepper is loosing a step and then Mach is able to still control the steppers but every thing is out of whack. So mechanical problem leads to a software problem. I may be very, very, wrong! I would suggest that you try bringing your velocity down some. I think your step miss is x axis during the rapid before the next index, thus the the pitch is still ok. Then along with backlash , a screwy / upset program ,
it just repeats. Odd i must say. But I still want my friend to listen to the video and see if he hears what i hear.

I have had steppers miss steps, not in a long time, as i am conservative on velocity & accel settings.
Once i was milling  a circle and the stepper missed steps but the " second circle came out all right
just a 1/4" of center from the first Z pass cut circle. Mach values were wrong but it maintained control. Sort of like "the driver lost control of the car but was able to bring it back onto the road with what was in his head only the car ended up in the wrong lane". In another case the axis just went totaly out of control at  rapid setting. Post estop MACH's DRO values were nowwhere near the dro' on the mill.

12v @1.5a delived to the 195 Oz/in stepper,.....you have speed but i don't think you have much torque.
You would think that if you dry run / skim cut and all is well that you shouldn't have any problems.

RICH
 

6476

General Mach Discussion / Re: Problems threading on the lathe

« on: February 28, 2009, 06:43:13 AM »

TrevorH,
Just a whole bunch of basic comments which make a difference in thread cutting and final dimensions:
1.As stated by Chip, try to get the cutter on the centerline of the stock.
2.Use a cutting fluid when cutting threads and idealy it should be constantly applied.
3.When setting the axis for the cut remove the backlash.
4.The more rigid the setup the better.If poor, fix it. Use the tail for additional support.
5.Take a finish cut on the stock, so you know where to set the cutter, it provides for
   a more even cut load especialy if the first cut is deep.
6.When you set the cutter account for the tip radius. ( look at how a sharp v depth is defined )
7. Flank cutting can provide for more equal cutting / loading ( attempts to maintain equal area of
   chip / shearing of metal ). I usaully use radial ( 29 deg ) for small threading.
8.Use sharp properly ground cutters. Carbide thread inserts are great but use ones with a small
   nose radius on the smaller lathes. I have some that just don't have sharp edges and thus require more
   power to do cutting.They sit in the box. Don't have any chasing cutters but they certainly add to the quality
   of the thread form.
So be maticulous as any of the above can mean the difference between a good or bad thread.

For a 1/2-13 thread you have a "mean" tolerance range for OD of .012" and for pitch diameter .005"
if you want to fall "somewhere" within a 1,2,3A Class fit. Lets forget about uniformity of pitch over the
thread for the point i want to make.So no great shakes on the diameter if the pitch is ok. Lets say you
do a finish cut on the stock and then adjust the cutter placement to get the pitch right ( or adjust accordingly in the
gcode). Lets say you lost a thou based on the above 8 comments, that leaves you .004" range for the pitch.
You have backlash in the X and Z and .005" one or both. The chance of throwing a pitch over the plate for a
strike is...... well...........hope i made my point.
 So yes, you should use backlash compensation as a software fix for just one part of the mechanical system ills.
Even though it does work great ( SS dosn't have it yet), you will see an improvement in threading but it's a patch.
Ideally you need to get rid of it or minimize it.

Sorry for being longwinded and didn't even get to your video, but, all of the above are in my thoughts as i watched
it.
RICH

PS: Yes i will be posting additional comments but i want a friend to watch the video first.

6477

General Mach Discussion / Re: Problems threading on the lathe

« on: February 27, 2009, 09:12:56 PM »

Trevoh,
Are you using backlash compensation?
What is your stepper rated torque?
RICH

6478

General Mach Discussion / Re: Problems threading on the lathe

« on: February 27, 2009, 07:58:30 PM »

Thanks for the video post. Now will listen to it very carefully.
Trevoh, your not crazy in anyway.
But me.......well that's another story! 
RICH
 

6479

General Mach Discussion / Re: Problems threading on the lathe

« on: February 27, 2009, 07:18:26 PM »

TrevorH,
I used your Gcode that you posted to cut  a 1/2-13 thread in 6061 on my Punny little lathe. ( 400 rpm spindle speed)The posted pic is of the G32. Also cut one useing M76. The results were the same. In the pic you can see a difference of 2 thou at the thread beginning and that is due to the actual diameter dfference as i didn't start off with a finished stock diameter.
The spindle did slow down. Never experienced an X runaway.  During the spring cuts there were a few times when more than a hair shaving was taken off. I have about .002" backlash in the X and a little under the same in the Z. So during the spring cut is it's possible for me to get more than just .0001" cut. i can adjust / take out more of the backlash but I leave it as is for now.
The flank cutting ( your code had 15 deg infeed ) worked as it should, just a nice forward curl. Note there are no
steps in the backside of the threads. Your's has them ( that's backlash and i can duplicate it by increasing mine).
I believe your problem is mechanical. Note no waviness in my threads. Software dosen't do that. Mach reacts rather
quickly, but not instantaneously to a slow down, when that accurs the next pass will try and fix it. Yours almost looks like a gouge. On the next cut if it happens again, because of some other / prior pass, then again it tries to fiix, but
then who knows what it's fxing ( just my thought). You may be better off not taking as deep a first pass cut ( .014").
This way you get a v started which can help guide the tool. Just some thoughts for you.

Now i will admit that there MAY be some quirky thing from the software point of view, but not convinced yet.
Or maybe i just haven't stepped in poop lately ( i do have the knack of finding it).
RICH

6480

General Mach Discussion / Re: Problems threading on the lathe

« on: February 26, 2009, 09:30:48 PM »

Hood
May be all wet here, maybe someone can chime in,  but here's goes.
You have the following:
1-supplied voltage- say 60 hz
2-spindle sensor frequency to mach - 1 pulse per rev
3-VFD generated dominant freg- i can measure but most would not be able to do
4-the stepper pulses per unit-
Now you want to be away as far as you can go from any fundamenetal or harmonic....thus in terms
of suppled voltage  60,120,180, ...etc you want the spindle sensor being at 30, 90, etc.
So your input / output MACH pulses have less chance of having an intereference. Now if it's brute noise,
that's a diifferent story,. If that's the case then it would not be so intermitant and the grounding, shieldnig etc will have more effect.
A little far fetched but if the field builds up i can see where you can get some "goofy" pulse or interference
that could make the X axis not behave yet contnue. But like i said, a little farfetched.
Just some out of the box thinking,
RICH