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Author Topic: Thread cutting on the lathe...What am I doing wrong?  (Read 5760 times)

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Thread cutting on the lathe...What am I doing wrong?
« on: September 22, 2012, 11:45:19 AM »
> I have tried about a dozen attempts to make a 5/16-18 and a 5/16-24 thread on some brass stock with no success. The first few passes look fine then the tool starts making mincemeat. My set-up is as follows:
>
> Sherline 4400 with Mach 3 Turn, using the NFS Turn Wizard (all licensed copies)
>
> PP #1 Hobby CNC Pro Controller
>
> PP #2 Homann Designs breakout board to Digispeed SL speed controller using the Sherline index sensor with one black sliver of the label that came with the sensor. I am also using this breakout board for my limit/home switches and for eStop.
>
>
> When I turn the spindle by hand and line up the black strip on the pulley with the sensor, the LED on the Diagnostics page goes out. I assume that this is telling me that I am getting an index pulse. Is this correct?
>
> I have set the Index Debounce to zero and have "Use Spindle Feedback in Sync mode" and "Spindle Speed Averaging" checked
>
> I have checked and rechecked the settings per the specs that came with the Digispeed SL and looked all over this site but nothing I have found seems to make any difference.
>
> Can anyone tell me what I am doing wrong?
> Thanks for your time,
> Doug

Offline Hood

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Re: Thread cutting on the lathe...What am I doing wrong?
« Reply #1 on: September 22, 2012, 01:44:23 PM »
Sounds like you have covered most things that could be an issue, can you post your xml and also the code just in case there is something wrong there.
Does the RPM show correctly and consistently?
Does your spindle slow down a lot when cutting the thread?
Hood

Offline RICH

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Re: Thread cutting on the lathe...What am I doing wrong?
« Reply #2 on: September 22, 2012, 10:04:54 PM »
Doug,
I have a Sherline lathe, so don't think I am bashing the product, don't use it much anymore though. Also just spent some time helping someone who was having some problems threading with his just last week. He cut 3/8-18 in 6061 AL.

So first i will make comments from experience with it.
The motor is wimpy even if you have the newer one on your lathe. You must have everything going for you, and by that I am mean razor sharp thread tool, properly set tool, all the gibs adjusted to remove play, preferably a backlash attachment mounted and adjusted, minimized overhang of the thread tool, and using the wizard to your advantage, the stock prepared by machining it to proper size ( concentric and not having say .003 runout.

24 tpi and below is reasonable to do, 18 can be iffy, 16 probably not ( but can be done if somewhat tricky about it, but the quality may be poor), forget about doing muti-start threads.

What is probably happening. The first few threads are ok and you are not getting much rpm change, but the rpm change will get progressively
worse as you cut deeper. Mach is constantly trying to provide for the rpm slowdown, but, it can only do so much. At one time one could actualy have the spindle almost stop and the thread cycle would try to fix things and the resulting thread was ,well lets say the the nut went on. The thread cycle was changed and slowdown of say  25% may mean a ruined / trashed thread. We did the 3/8-18 at 200 rpm, rpm change was around  30% max decrease for example.  Thread didn't look bad but it was just a case of a poor quality thread. Yeah the nut when on!

Now besides the above, the axis just dosen't move as accurately as it should, so if you are off on a cut by say .002", that could be enough to cause an additional slow down of the spindle.

Rather than rambling I will say watch, know and learn what is happening very carefully monitoring the threading cycle and learn from it.
Also have a read of Threading on the Lathe-Mach 3Turn.

------------------------------------------------------------------------------
Here is what you can do that will help:
1. Don't use radial cutting and instead do flank cutting. use 15 degree instead of 30 degree in the settings.
2. Make sure you allow 3-5 min pitches before threading starts
3. Use finer cuts depths, maybe .004 to .007 for the first cut and then limit to .003 or .004 there after.
    You can do even finer ....but depends on axis positioning accuracy
3. You can try alternate flank cutting but don't think the your Sherline is accurate enough, but it may avoid a plunge on some cuts.
4. Experiment with the material, brass will work harden for example.
5. I can say adjust the gibs to be on the tight side and well lubricated, but not kowing what size motors you have ...that could
    cause skipping
6. If you have backlash you will never do accurate threading, backlash compensation can help but is not the answer, due to improper
    tool location you will / may cut deeper than wanted and thus have reduced rpm during the cycle.

So don't get frustrated, you can do some nice work on the Sherline, but know it's capabilites  and that only comes from
having fun playing with it.

Have fun,
RICH
  

 

« Last Edit: September 22, 2012, 10:07:42 PM by RICH »
Re: Thread cutting on the lathe...What am I doing wrong?
« Reply #3 on: September 23, 2012, 12:42:51 PM »
Hood.....Thanks for the response. I have attached my g-code and xml file. Thanks for taking a look. 

Rich...Thanks for the detailed explanations. I understand that the Sherline is pushing the larger threads.  I will try some smaller ones and see what happens. I will also reduce the amount of cut and see what happens.

Thank You all for the responses. I am new to this and I appreciate the help

Doug

Offline Hood

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Re: Thread cutting on the lathe...What am I doing wrong?
« Reply #4 on: September 23, 2012, 01:57:11 PM »
Dont see any issues wit the xml but can you try without spindle speed averaging to see if that makes a difference. I dont think it will as I cant see the driver paying attention to averaging.
Also what about the couple of questions I posted  above?
Hood