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Lathe Threading
« on: November 09, 2007, 08:47:56 AM »
Hi,
I have finally got my lathe up and running, and am having a little trouble with threading.  I am not sure if is just the way mach does threading or if I am not writing the code correctly, or have it configured correctly.
I am cutting a M24x3 thread at 1000RPM and I have a 45deg taper programmed at the end of the thread, but when cutting it gets to the end of the thread and pauses for a moment, causing a groove to be machined around the bar, and then it goes and tapers the thread out as it is supposed to.
Any ideas about what I should be looking at?  I do have it in exact stop mode, should I maybe have it in constant velocity mode???
Apart from that it is working perfectly, I had it threading at 1500rpm doing the same thread but the bar was vibrating a bit to much so I dropped the speed down a little.

Regards
Andrew

Offline DAlgie

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Re: Lathe Threading
« Reply #1 on: November 09, 2007, 08:42:14 PM »
Well, for starters, 1000rpm is very fast for threading for even a high end CNC lathe. My guess is that you have backlash enabled and the groove in the end is caused by the time it takes for the machine to reverse the X axis stepper until the mechanics can catch up and draw the tool away and out on the 45 degree taper as it's supposed to. Drop the rpm down to maybe 500 or less and possibly see what the backlash speed is set to. Exact stop should be fine for this.

Offline jimpinder

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Re: Lathe Threading
« Reply #2 on: November 11, 2007, 01:52:45 PM »
I agree this is probably the backlash - because Mach3 does pause as it adds backlash.
However - even with backlash disabled, there will still be backlash in the system and therefore you would still have a delay in the movement of the tool as it pulled out.

How do you run into your thread. Really as you set your tool, you need to move in to a point past the depth by an amount equal to the backlash, and then draw back by an amount equal to the backlash - the tool is then set to come straight out of the thread when you need it to,
Not me driving the engine - I'm better looking.
Re: Lathe Threading
« Reply #3 on: November 19, 2007, 02:38:26 PM »
This topic raises a question...........

 Quote:
  "How do you run into your thread. Really as you set your tool, you need to move in to a point past the depth by an amount equal to the backlash, and then draw back by an amount equal to the backlash - the tool is then set to come straight out of the thread when you need it to, "
 
 If the X axis is a dovetail or box way and has sufficient drag as to not move from the tool pressure, this would be OK.
I am nearing completion of my lathe which has IKO rails and trucks for the slide and very little backlash on a Thomson ball screw but moves far too freely for the above to work well, I'm fairly certain. Question is... Is it uncommon or a NO NO to use a spring or hydraulic shock/dampener to keep constant force against the slide in the retract direction to prevent the tool pressure from pushing the slide back = to the amount of backlash ? Seems like maybe a little unnecessary wear on the ballscrew but it would produce a good thread at the end.
Please bear with me, I'm new at this,
Thanks

Offline DAlgie

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Re: Lathe Threading
« Reply #4 on: November 19, 2007, 04:31:10 PM »
More often than not on this forum, guys try to make mach's backlash compensation take care of way too many machine mechanical problems, which it can't totally solve. If you have a ton of X axis backlash because you are trying to use the original leadscrew instead of a decent ballscrew, you will never really be happy. The more X axis backlash you have, the slower you will have to run the spindle while threading, and a lot of other functions as well. Note too, that there is a setting for backlash compensation speed, which may help here a little. And, when threading (screwcutting) manually, I always cut a small relief groove the depth of the thread at the end point to save the tool and to give somewhere to be able to time the end of cut, so you are not really much worse off if your backlash problems are causing this anyway. As to the use of a damper on an axis, I have never seen this used, the servo/ stepper is your damper in conjunction with acceleration/ Decell speeds set properly.
        DaveA.
Re: Lathe Threading
« Reply #5 on: November 19, 2007, 04:55:51 PM »
I do the same as you when possible.
The shock/damper I referred to was just for the constant force applied to the slide. I should have used a better example..like a gas cylinder...or just spring.
Anyway, I run into the same problem with the taper attachment on my manual lathe. Set X for the depth of cut and if the cut is heavy it pushes the slide back = to the amount of backlash. Fine cuts are OK, they don't overcome the drag of the ways.
But with bearing slides, a semi strong spring affixed as to always keep the slide forced away from the work, would keep any amount of backlash in check.
As soon as the screw reverses, the slide would follow immediately, pulse for pulse, by spring pressure.
As far as the code goes, there is no backlash to compensate for.
You're basically forcing the slide in, and letting it out with the screw under spring pressure..Zero backlash.
Just a thought to maybe overcome a common problem.
Thanks Dave

Offline DAlgie

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Re: Lathe Threading
« Reply #6 on: November 19, 2007, 06:09:25 PM »
I see what you mean, and it makes sense. The case is unusual though, most machines have some stiction as this is the large surface contact areas needed to ensure rigidity. Yours must be a small machine, I imagine. There would be one drawback to the spring/ gas strut idea though. It would work for an OD cut, but if you then wanted to use a boring bar and load the cross slide in the opposite direction, the spring would be fighting you and cause all kinds of sizing problems.
     DaveA.
Re: Lathe Threading
« Reply #7 on: November 19, 2007, 06:51:27 PM »
Yes.. it is a small machine. I plan to post a photo when I get a little more of it together.
The spring would removable, only used on certain set-ups. Mainly OD contours and thread cutting.
Some of the threads I will be cutting are 48 and 56 tpi on thin wall brass tubing.

This situation occurs naturally on any knee mill. If the gibs are not locked, the nut always rides one side of the leadscrew thread.
Up or down, no backlash.

An old bench vise with 2 turns of backlash is pain in th %#@* if it has no spring in it.

Thanks for you input,
Learning