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thread problem
« on: January 14, 2009, 01:27:14 PM »
hi all

iM trying to cut a thread on the lathe with not much success. the situation is the following: I need to cut an acme thread with 12 tpi and 6 inch long on a 1.5 inch ss304 shaft. my lathe is equiped with a vfd on the 7.5 hp spindle motor and ac servo motor on x and z axis. i first tried to cut the thread with the 1 pulse/revolution index and the thread wizard. the problem is that the tool doesn't seem to hold its accuracy over a long thread. the thread is good for an inch and gets thinner as the z distance gets larger, after 6 inche there is no more thread on the shaft! the first thing i did was to change from index to timing with a 6 pulses/rev, and do spindle tunning. I then increased the speed from 400 to 700 rpm with lighter cuts. while the thread is cutting i keep an eye on the actual speed dro and it is on the spot and very stable. Any idea what cause the z axis to drift with distance? The starting point of the thread is always perfect. the infeed angle is set to 14 degrees.

one strange thing i've noticed is that on the first pass the tool is cutting air and the x axis is moving slowly in the plus direction over the 6 inchs distance!!
I'm using mach 3 ver. 3.042. it is installed on a newly formated computer with nothing else on it.

Offline Hood

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Re: thread problem
« Reply #1 on: January 14, 2009, 03:53:52 PM »
Enable the turn diagnostics plugin then restart Mach and open the plugin from the Plugin Control menu. Do a threading op (just in air) and see if there are any problems reported. If not try again actually cutting and see if its still ok.
 The threading was worked on a few months ago and it now seems perfect for nearly everyone.

Hood
Re: thread problem
« Reply #2 on: January 14, 2009, 07:19:16 PM »
thanks i will try this tomorow, but did you actually cut any long thread? because i would not have noticed this problem if the lenght of my thread had been shorter then 2 inches.

Offline Hood

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Re: thread problem
« Reply #3 on: January 14, 2009, 07:47:01 PM »
Yep, done plenty, below is a pic of four threads, think 2 were  with the parallel port and two with the SmoothStepper but it was a few months back when I was testing things out during the threading upgrades. They are 2mm pitch and about 150mm long if I remember correctly.
Hood

Offline RICH

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Re: thread problem
« Reply #4 on: January 14, 2009, 08:53:09 PM »
TUBULAR2000 & HOOD,
Haven't fooled wiith threading for a while and don't remember at what version of 3.042 it was really fixed.
I think 3.042.12 and above.
It was 3.042.012 to the now lockdown of 3.042.20
Hood, do remember?

Tubular2000 if you installed 3.042.20 you should be good to go.
RICH

MODIFIED: STRIKE THRU'S ABOVE
« Last Edit: January 14, 2009, 09:02:48 PM by RICH »
Re: thread problem
« Reply #5 on: January 14, 2009, 10:00:51 PM »
this threads sure looks better than mine. what spindle speed and first cut depth do you use? my threads cut in about 20 pass.

Offline Hood

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Re: thread problem
« Reply #6 on: January 15, 2009, 03:04:34 AM »
Think the spindle speed was 1000rpm for 3 of them and 750 for the other. I had to slow down the spindle speed as I had lowered the kernel speed due to testing and with that lower kernel speed my axis would not have been able to keep up. Not certain on the first pass depth but  it was done in 10 passes(including 1 spring)
Hood

Offline zarzul

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Re: thread problem
« Reply #7 on: January 15, 2009, 10:31:52 AM »
Tubular,

You said you watch the X axis moving slowly positive over the 6 inch length.  Is your X axis DRO actually showing it moving out during the threading pass?   It should be rock solid during the pass unless you are doing some sort of a tapered thread.

Arnie
Re: thread problem
« Reply #8 on: January 15, 2009, 10:38:27 AM »
Go back to 1 pulse per rev on the index and add a 2 second delay to the program before starting the threading cycle. This worked for me even though mach3 has a delay built in to the threading. You could also try using a G32 thread as you can then see each cut of the thread in the program.

Jim.
Re: thread problem
« Reply #9 on: January 15, 2009, 11:01:05 AM »
here is a picture of the finish acme thread. it is not very clear but you can see the tickness of the thread decreasing as the z axis gets further from z zero. in the mean time, i tried with an older version of mach 3 that used to work with no better success. for the x axis this is very strange, on the first pass the x axis is moving out about .03" over the six inch lenght, i can see the motor turning and the x dro moving on the screen. this proble realy sucks, hope someone is going to come up with a good idea