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Continually incorrect thread pitch
« on: August 11, 2009, 10:21:48 AM »
Everytime I setup .05 pitch for 20/inch I continually get 24/inch. I haven't checked any other pitch but I have check my screw specs and pulses per inch and they are correct. 20% off would certainly show up even with my measurements. I slowed the rpm's to 70 and have tried 3 other settings. The actual spindle dro is saying 203 at 200 and nothing at 70 but the pulses are showing up correctly on the scope. The automatic setting in pins settings still show the spindle index is correct at 70 rpms too.

Come on guys. It doesn't have to be right. Just guess, I won't say anything. I'm out of ideas.
« Last Edit: August 11, 2009, 06:58:39 PM by kdoney »

Offline Hood

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Re: Continually incorrect thread pitch
« Reply #1 on: August 11, 2009, 07:17:51 PM »
Do the threads look good otherwise?
Tried running the threading plugin to see if it catches any problems?
Do you have  "Use Spindle Feedbac in Sync mode" checked?

Hood
Re: Continually incorrect thread pitch
« Reply #2 on: August 11, 2009, 08:00:36 PM »
No the threads don't look that clean. More of a U than a V. I thought that was from the tool bit. I don't have a threading plugin. I have Version R3.042.020. I am using synch with spindle speed averaging.
« Last Edit: August 11, 2009, 08:04:40 PM by kdoney »

vmax549

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Re: Continually incorrect thread pitch
« Reply #3 on: August 11, 2009, 11:12:19 PM »
I would test the z for correct setup. Use a dial guage and do the  G1 z1 test to make sure the steps per unit are correct.

Just a thought, (;-) TP

Offline Hood

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Re: Continually incorrect thread pitch
« Reply #4 on: August 12, 2009, 07:15:19 AM »
I don't have a threading plugin.

It should be there, think its call Turn Diagnostics. If not let me know and I will attach it for you.

Hood
Re: Continually incorrect thread pitch
« Reply #5 on: August 12, 2009, 08:16:41 AM »
Under config plugins it is there and I have it green checked but under plugins it isn't there.

Nevermind. I needed to reboot. It's there now.

I just checked with the scope again and it looks good. Strong 5V signal about 2mS every rpm.

When I run threading I get 86.28 rpm. Threading in effect. Rpm variation running from 0 to .04. Highest variation .06.

That's kind of strange since the spindle is suppose to be running at 70 rpm. That's my 20% variation.
« Last Edit: August 12, 2009, 08:32:40 AM by kdoney »

Offline Hood

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Re: Continually incorrect thread pitch
« Reply #6 on: August 12, 2009, 08:30:21 AM »

Hood
« Last Edit: August 12, 2009, 09:11:19 AM by Hood »

Offline RICH

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Re: Continually incorrect thread pitch
« Reply #7 on: August 12, 2009, 09:07:32 AM »
kdoney,
Some commments on threading in general:
1. LATHE - Threading requires a lathe with properly adjusted backlash compensation or better yet almost no          

backlash.The motor should not slow down during the threading pass, even though the porgram will account for it      

within reason. The lathe should be tested for accuracy before trying to do any threading.

     I would suggest that you turn down a piece to diameter, color it with say a blue permanent marker,  and then do
     a number  of fine passes ( say 10 or 20 at 0.0001" )  and inspect to see that each pass follows the same scribed
     line. The thread will only be as good as the lathe axis system is.

2. Code - I don't recall any problem with using G32 or G76( single start thread - for multi start you better have a good

system !) If you want, you can change the wizard so that it outputs G32 ( no canned cycle and all passes are coded

 individualy ). The only comment is that you should have a Z movement equal to, say 3 to 5 times the
pitch, so the axis has time to ramp to and maintian speed before the threading actually starts. So there is
a relationship here  in that your motors must be able to accelerate, but, the wizard informs you if things are not right
 for the selected spindle speed.

3. INDEX- You should have rpm display for your range of spindle speed. You may need to adjust slot width
               to get it. It can vary some 1 to 2 maybe at a particular spindle speed, the more stable the rpm reading the

better and it should be fairly close over your range.

If you don't have an rpm, I am not sure what the program is using to trigger the the start of the thread cycle,
in  fact it may never start it.

In config>ports& pins>spinde setup -  check / use -  spinde feedback in sync mode and  spindle speed averaging.
In config>ports & pins> input signal - make sure you use index for a single slot ....or  timing for a multi slot disc

4. Monitoring - You can use the turn diagnostics to monitor the threading cycle.

SO............................................
Make sure your index is working properly and then do the lathe test.

As far as making "U" instead if a sharp "V", i think it's a setup problem also, if the steps per unit are correct then it's
mechanical problem, and the lathe test will show it.

Hope this helps,
RICH

      
  
Re: Continually incorrect thread pitch
« Reply #8 on: August 12, 2009, 09:10:46 AM »
After checking my electronic system again and testing the mechanics coupling, I find the machine is rotating at the speed indicated by the electronics. It starts off about 20% fast at slow speeds and changes to about 10% at higher speeds. Since this is the case, should the threading still be correct since I have the threading movement synch'd with the feedback?

The cheap chinese lathe has been retrofitted with good ballscrews and adequate mounting and motor coupling. There is no measurable backlash greater that .001. I believe the gears in the lathe are crap and don't indicate what they should be but the electronic gearing should solve that.

It is a new cheap chinese lathe. :-)
« Last Edit: August 12, 2009, 09:14:53 AM by kdoney »

Offline RICH

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Re: Continually incorrect thread pitch
« Reply #9 on: August 12, 2009, 09:33:16 AM »
Using steppers or servos? I don't have experience on servo's, Hood or someone else will know if there are any
quirks.