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Author Topic: Lathe Threading Troubles  (Read 2724 times)

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Offline Katoh

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Lathe Threading Troubles
« on: October 31, 2015, 08:14:13 PM »
Friends
I have completed my retrofit of a Halfco AL54B lathe same thing as an American Grizzly602. Just a quick run-down on the lathe, it uses  570oz/in steppers on each axis, spindle is VFD driven and it is all retrofitted with ball screws and the steppers are controlled by Gecko G203's. The lathe works very well indeed and has other little bells and whistles I have added, it turns accurately to 0.05mm with backlash enabled and behaves it self for 99% of the time. From the beginning I noticed with some trial runs threading was an issue, I ended replacing the VFD as I noticed every now and then a skip in the spindle motor, sort of like a sporadic misfire in a car. The new VFD is now rock solid but still threading is not what it should be.
I tried cutting a M32x1.5 thread last evening, using the wizard and taking light cuts in mild steel at 200rpm the thread was actually decent for a change, realising I need to take a little more to have the nut fit properly I ran the code again, it run well to the last pass where you could here the Z axis motor ramp up higher than before and totally destroy the thread? This can be a random thing it could be the 4th pass or the second or it may not happening, but either way it destroys threads and the job!
Any thoughts?
Cheers
Katoh

Offline RICH

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Re: Lathe Threading Troubles
« Reply #1 on: October 31, 2015, 11:20:11 PM »
Set the rpm manually.

What Mach 3 version are you using?

RICH

Offline Katoh

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Re: Lathe Threading Troubles
« Reply #2 on: November 01, 2015, 04:35:35 AM »
Hi Rich,
How can you set RPM manually? Do you simply edit the code and remove any reference to S and then input speed on the manual screen?

I am using the latest Mach 3 version only downloaded and upgraded a week ago 3.043.066.

I forgot to add I’m using a PP set-up its not run by smooth stepper.
Cheers
Katoh

Offline RICH

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Re: Lathe Threading Troubles
« Reply #3 on: November 01, 2015, 05:04:33 AM »
I read that 3.043.066 has problems and currently I am using / testing .062 for the lathe.

Just set the VFD manually to an rpm, however you do that, and not via Mach.
The way it is / was and don't think anything has changed, if the VFD us trying to maintain rpm and Mach is monitoring
the rpm and adjusting for rpm variations the two systems are fighting each other.

Single index / input for the rpm to Mach and manually set the rpm.

RICH
 

Offline Katoh

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Re: Lathe Threading Troubles
« Reply #4 on: November 01, 2015, 06:04:46 AM »
Rich
I have a copy of 3.042.035 on file, don’t know where to get .062 from.

Sorry confusion is starting to set in, if I set the speed on the VFD manually, I don’t see how it interacts with Mach after that? More to point I’m trying to work out how can do this.
Would it work if you delete all reference to any spindle speed in the code, be it either canned or longhand , start the spindle with a desired speed set on the manual page then run the code. Sorry I might not be making sense but the darn machine is set to work of Mach only and I’m trying to get my head around this.
Do you mean actually running the vfd of itself, this means reprogramming so it does not read any of the mach commands?
I must try one other thing that is running a thread from an independent cam package in longhand, this is the only successful thread of I made in the past.
Cheers
Katoh

Offline Katoh

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Re: Lathe Threading Troubles
« Reply #5 on: November 01, 2015, 05:19:56 PM »
Would going to a Smoothstepper setup help?
Cheers
Katoh

Offline RICH

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Re: Lathe Threading Troubles
« Reply #6 on: November 01, 2015, 07:42:14 PM »
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don’t know where to get .062 from

ftp://machsupport.com/Mach3/

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Would going to a Smoothstepper setup help?

I don't follow info on the Smooth stepper anymore and no longer use it so you need to go to that site to find out what is working these days.
You need not purchase a Smooth stepper to do threading. PP works fine.

Quote
Do you mean actually running the vfd of itself, this means reprogramming so it does not read any of the mach commands?
Yes, you set the spindle speed on the vfd manually ( I know nothing of your vfd so can't help you).

You can use the Simple Threading wizard to generate code for threading.
Also you may want to have a read of Threading on the Lathe MACH3 which can be found in Members Docs.

I don’t see how it interacts with Mach after that
You set it manually but I don't know what is telling your vfd what the actual spindle speed is, so if the rpm varies then the vfd is trying to keep the rpm constant
and while it's doing that Mach is reading the rpm via an index and thus is planning the next pathing based on what it read, so one system is fighting the other.
That is not good. Threading was changed a long time ago to work but it only uses  simple single index. External motion devices such as the SS take care of threading instead of it being done by Mach via the PP.

RICH

Offline Katoh

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Re: Lathe Threading Troubles
« Reply #7 on: November 05, 2015, 09:02:35 PM »
Hi Rich

I firstly did not want to confuse one topic with another, which I think I did, my apologies for that.

I did a little testing of my own, after reading countless threads, on this this subject, and I do understand your meaning of running the spindle independent of mah. My problem is I want my cake and eat it too, I want to have it the run fully from the PC.

OK I started again from scratch and setup everything as you have written in the lathe threading DOC. I must say I have read through it in past but for some reason my mach setup was less then advisable. After altering quite a few different config settings I noticed a huge improvement in the way the lathe behaves while it tries to cut thread. I cut a thread using the wizard M12x1.5 using 16 passes in a mild steel rod, 400rpm and a 0 as the lead in, I’m surprised the thread actually turned out as good as it did , pic shows thread compared to the same size tap.

One other thing that I tried on another scrap piece was turning a thread at different speeds, I turned a thread at 300rpm then left it in the chuck and returned it at 200 rpm then 400rpm, what I noticed all three turned the thread properly but the start point was not in the same position on each of them, the slower turn started the thread earlier on the same piece and the faster later, no doubts the thread was ruined but interesting test non the less.

I will have a go at SS to see if it makes any improvements or not, if not its not a waste as I can simply use it on a Bridgeport mill I’m currently converting.

Thanks muchly
Katoh

Cheers
Katoh
Re: Lathe Threading Troubles
« Reply #8 on: November 06, 2015, 04:38:56 AM »

One other thing that I tried on another scrap piece was turning a thread at different speeds, I turned a thread at 300rpm then left it in the chuck and returned it at 200 rpm then 400rpm, what I noticed all three turned the thread properly but the start point was not in the same position on each of them, the slower turn started the thread earlier on the same piece and the faster later, no doubts the thread was ruined but interesting test non the less.


No surprise once you consider that you are using a feedback loop with a single position sensor and consistent signal timings, it is logical that start points will vary and will get later for faster rpm, it also explains why constant speed is critical to avoid pitch variations.

 - Nick

Offline RICH

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Re: Lathe Threading Troubles
« Reply #9 on: November 06, 2015, 07:32:04 AM »
You should have 3 to 5x pitch lead as that will provide time for the acceleration of the Z axis when threading.
Always run the Calc number of passes as that will tell you if you are exceeding Accel of the axis.

Scribing soft material is recommended as a simple test since there would be no rpm loss.

Threading a simple thin round disc ( say 1" or 2" dia  and 1/16" thick  ) can  show  repeatability of threading start point.
Actually you use the  G32 command  from some common point for testing.

NOTE:
Threading results are only as good as:
- YOUR   "Lathe System" , you have backlash and that is not good.......
- Set up
- etc.

No device is going to  cure any bad of the above . If you think a system is good then test by doing a 0-80 thread. Then try multi start threads and see if you can do a 2 or 3  start thread.

When I talk about threading it IS NOT about the nut goes on ....... rather, it's what class of thread / standard can one achieve.

RICH