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Author Topic: Threading on lathe issues  (Read 6978 times)

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Offline Hood

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Re: Threading on lathe issues
« Reply #10 on: March 02, 2015, 05:40:28 PM »
Ok was just curious when you mentioned having issues with 16mm dia and threading and 4:1 reduction. I would have thought there would have been plenty torque for even the standard 2mm pitch on the 16mm dia.
Is your Z axis fast enough for it, just thinking if it is not then that is maybe where your problem is coming in.

Hood
Re: Threading on lathe issues
« Reply #11 on: March 03, 2015, 10:29:23 AM »
Ok was just curious when you mentioned having issues with 16mm dia and threading and 4:1 reduction. I would have thought there would have been plenty torque for even the standard 2mm pitch on the 16mm dia.

Sorry, I better set the record straight in case someone reads this thread down the road and gets the wrong impression about Mach’s ability to thread based on my feeble attempts.

My memory is apparently knackered because I found out last night that I was wrong when I said I was having trouble with ¾ hp @ 4:1 reduction doing 16mm threads.  I dredged up an old post and found my problem was with 22mm x 1.5 (ER16 size - I guess that’s where the 16 came from) and that problem actually occurred before I had built the belt reducer.

After getting the belt reducer working, I later failed at cutting 50mm x 1.5 (ER40) threads in steel…a totally different ball game!  Even then I don’t think it was so much a lack of torque issue but more to do with my errors and lack of knowledge.  The completed thread wasn’t too bad looking but was oversized.  When I tried to go back & “run from here” with a deeper cut, something went horribly wrong and I had to e-stop.  It was all over then as I never could pick the thread back up and had to finish the part on the manual lathe.

I need to gather up some scrap stock (along with my courage) and spend a day or 2 cutting practice threads until it either works consistently for me or I can clearly determine that my machine will not thread properly until further hardware improvements are made.

Until then, I'll keep me big mouth shut about threading.
Milton from Tennessee ya'll.

Offline Hood

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Re: Threading on lathe issues
« Reply #12 on: March 03, 2015, 12:43:01 PM »
Ha ha no probs Milton.

Hood

Offline RICH

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Re: Threading on lathe issues
« Reply #13 on: March 03, 2015, 02:10:33 PM »
How to Pick Up A Thread is covered in the Threading on the Lathe write up.

There are some cacluclators that will caluculate the HP required to do a thread. Results should be used as a general guideline.
Manualy calculating requires some assumptions and is not for the novice to do.

RICH

 

Offline Hood

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Re: Threading on lathe issues
« Reply #14 on: March 04, 2015, 03:46:17 AM »
There really should not be a problem picking up a thread if it has not been removed from the chuck unless you have E-Stopped and the axes is slightly out of position. Having accurate homing would cure that.

Hood
Re: Threading on lathe issues
« Reply #15 on: March 04, 2015, 09:33:51 AM »
There really should not be a problem picking up a thread if it has not been removed from the chuck unless you have E-Stopped and the axes is slightly out of position. Having accurate homing would cure that.
That's really encouraging.  The ORAC had accurate homing on the X-axis then and now has it on the Z so it should be able to recover from a faux pas.  That's assuming I learn to use run from here properly.

Also, does Dolphin generate good threading code for Mach?
Milton from Tennessee ya'll.

Offline Hood

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Re: Threading on lathe issues
« Reply #16 on: March 04, 2015, 09:59:21 AM »
I think in the standard form it produces code that is ok, I most likely altered mine to work out the depth etc without having to enter all in the CAM.

Hood
Re: Threading on lathe issues
« Reply #17 on: March 06, 2015, 06:48:42 PM »
Have a read of Threading On The Lathe which can be found in Members Docs.
http://www.machsupport.com/forum/index.php/topic,13017.msg85313.html#msg85313

May need to set your rpm manualy as that can interfere with the threading cycle.
Not sure about version .066 infulence,as i have not used it, and others have gone back to earlier version
due to problems with it.
Make sure you have 3- 5X pitch of movement before actual threading to allow for axis acceleration

I'll let others reply about .066 who have tried or used it.

RICH


I have spent the pass 6 hours messing around with this issue and I have had no success. Let me explain the things that I have done.

First: I uninstalled and installed Mach3 version 0.57 (as per Dicky)

Second: I checked to see if a Z axis is accurate and it is (including with backlash).

Third: I went to manual speed on the VFD.

Fourth: I increased the slot on the disk to 5mm

I have tried the scribing test and sometimes its accurate and other times its not.
Sometimes I notice that the true RPM readout goes out of whack for a split second. Maybe this can be an issue but haven't figured out how to fix it.
The couple of times that I noticed that it was cutting accurate I let it complete the full thread. After measuring the pitch on the thread it was off by about 0.01 inch. Spindle speed was accurate. it might of lost 1 -2 rpm at most.

Extremely frustrated. Any ideas on what else to try?

Thanks,
Raul


Offline RICH

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Re: Threading on lathe issues
« Reply #18 on: March 06, 2015, 08:21:30 PM »
A fluctuation in rpm of say 1 ie; flickers between say 99 to 100 should be ok. Idealy the rpm should be constant since the movement during threading is a
cut at a constant feedrate. If the feedrate is not constant the thread will vary in pitch. If it varies too much then the following pass is modified to adjust for the varying feed rate. That's why you set vfd  manualy to an rpm so the VFD is not fighting with the correction the program provides.

If you were to scribe a line, allowing for some time for accelration, say 0.5" ( 5 pitch diameters) and then contact with the material using a G01 Z 1.5 F... the scribed line should be constant. You would need to use a microscope to measuse the variance over a few inches. Scribe line is only as accurate as 
the ball screw / screw moving the axis.

If as you say, the rpm goes out of whack, say  it goes to zero or drops say from 100 to 20, the threading cycle will try to fix the the threading by adjusting the next cycle. I have tested it for extreme drops in rpm, almost a complete stop, and the threading recovered but the thread pitch was not in spec.

In general the more stable the rpm the better the cut screw thread pitch will be. Should you consistanty get something , say less you can always change the pitch in the wizard and it will be used, and the thread will be good. But that is just tricking the program  pathing and is self defeating.
This is all in the write-up.

I think you have an index problem, or it can be the vfd's ability to maintain rpm.

I can't speak for version 57....I use 38 and never had a problem.

If you are measuring the rpm, check to see what the +- specs are on your measuring instrument.
-------------------------
 A test that can be done to check the acceleration / indexing is to mount a thin disc, the bigger the better, say 2" diameter, and do a thread.
You should see just one tick /scribe mark in the exact same location on the disc outside diameter. Under maginification the variance of the tick marks shold be almost zero / no distance between the marks. In fact if you try to do multiple start threading you will quickly find out just what your lathe can do.

RICH
Re: Threading on lathe issues
« Reply #19 on: March 07, 2015, 08:15:30 AM »
Hey Rich, are you using the PP or an external motion controller?
Milton from Tennessee ya'll.