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Author Topic: Mach turn Thread pitch is off?  (Read 1983 times)

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Mach turn Thread pitch is off?
« on: July 02, 2016, 05:49:55 PM »
I have an old dyna metronics dm3000 lathe which over the past couple months i've been converting over to mach 3. I've come so far but today i hit a snag.

I wrote a quick program to machine a .5 aluminum rod down and thread a 3/8-16 external thread. After completing the part my diameters are dead on along with turn length. However i couldn't get a 3/8 nut to thread onto it. I checked the thread pitch on the piece and it measures .050 instead of .0625. I looked through the mach threading manual and also did research on this page but was unsuccessful. As the machine is threading , the dro shows the spindle at the program speed of 525rpm. I even used a laser tachometer and received the exact same number. My spindle pulley ratio is set to 1. I have both "use spindle feedback" "spindle speed averaging" checked under spindle setup. I ran multiple parts at different depths thinking it was struggling but that didn't change anything.

What could be causing this? Im using an index pulse card and slotted disc. Could having too big of a slot in the disc be a possible problem? It seems weird since everything else is dead on, including the spindle speed.

Also i am using the latest version of mach 3
Re: Mach turn Thread pitch is off?
« Reply #1 on: July 02, 2016, 06:33:49 PM »
I'm no expert by any means but these 3 things come to mind:

1)  Disc size has no effect.  The slot passes the sensor once per rev regardless of disc size.

2) Go back to version .062.  .066 has problems.

3) Check z-axis travel with a D.I.  If you command 1", does the axis travel damn near 1" on the nose?

I'm sure there will be someone soon that can help if it's more than a simple issue.

Good luck, us lathe people got to stick together!:)
Milton from Tennessee ya'll.
Re: Mach turn Thread pitch is off?
« Reply #2 on: July 02, 2016, 06:37:01 PM »
Thanks for the quick reply. I will check again with a dial indicator but from what I recall it was dead nuts. Where can I download the older version?
Re: Mach turn Thread pitch is off?
« Reply #3 on: July 02, 2016, 08:37:45 PM »
Milton from Tennessee ya'll.

Offline RICH

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Re: Mach turn Thread pitch is off?
« Reply #4 on: July 03, 2016, 07:26:55 PM »
Quote
Could having too big of a slot in the disc be a possible problem

Better to error on a larger slot and should not be a problem. The Mach manual gives instructions on to calculate how wide it should be.

What are you using to generate the code?
Did you check to see that you have adequate acceleration / velocity?
Check that your index is working ( see the Threading on the Lathe Manual).
Did you take into account the threading tool tip radius?

Don't use the latest version ie; 066, 062 seems to work fine here.

RICH
Re: Mach turn Thread pitch is off?
« Reply #5 on: July 04, 2016, 06:06:11 PM »
I am using the simple threading to generate the code using g76.

I tuned the z axis motor for accel/velocity when first putting the machine together to prevent stall. It seems to work fine even through normal cutting. Also I have the thread cutter starting .300 away from the part for adequate acceleration .

Only way I tested the index was manually turning the chuck and noticing the led on the index board go on and off when passing over the slot. I then checked the calibration with the use of a laser tachometer and its within 1rpm.

I didn't take into any account of the tool tip radius. The tool table is left blank for any tip radius on the threading insert. The radius setting can throw off the pitch?

Also I will download the 062 version tonight and see what the outcome is with that
Re: Mach turn Thread pitch is off?
« Reply #6 on: July 04, 2016, 07:01:53 PM »
Just changed the mach 3 version back to 062. Upon reinstalling this older version It didn't change any of my motor settings or port numbers. Is that normal?

Ran a part on this older version and still the same results. I can't understand this. Everything is dead on as far as sizes and length but the pitch is closer to an 18tpi rather than what I have programmed at 16tpi.

I guess ill start going step by step with mach threading manual and see If I can find any issues
Re: Mach turn Thread pitch is off?
« Reply #7 on: July 04, 2016, 08:53:57 PM »
Got it! after trying and trying I began to notice something a little weird. I tried to speed up the turning process to break a better chip rather than a curl but no matter how much I kept upping the ipr it didn't seem to get any faster. At one point I had it set at .06 per revolution but it was only going .01

I also noticed that if I turn the chuck on to say 500rpm I can slow the chuck down with the drag of my hand to about 450 which wasn't good at all. I started to tweek some dials on my spindle board and got the spindle to have a full amount of power and for some odd reason the feedrate was now working properly. Very weird indeed but I decided to thread a part and the pitch is right on! Very excited

Thank you both for taking time out of your day to shoot me some suggestions on this. Thank you

Offline RICH

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Re: Mach turn Thread pitch is off?
« Reply #8 on: July 05, 2016, 06:45:29 AM »
Mach monitors the spindle rpm. The spindle needs to have adequate spindle torque to be able to cut the thread. Changes in spindle rpm while
cutting can cause problems, Mach alters the feed rate based on rpm but can only do so much to fix the thread. I talk about this in the write up.
One should know by trial / testing of cutting threads  what their lathe can do. For smaller lathes  a different type of cutting method ie; alternate flank
and pass depth  settings should be used to assist in cutting deeper thread depths.
 
Initial od of the stock, proper total depth of cut, accounting for tip radius etc. will go a long way to achieve proper thread pitch and pitch diameter.
Your threads will only be a accurate as your lathe. Be meticulous in setting up for threading as in some cases just a small change in pitch diameter
is  an out of spec thread.

All of the above is in the write up.

If your the  "the nut goes on" type of machinists and happy so be it, but, there are reasons for thread spec tolerances.

Have fun,

RICH