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5671

General Mach Discussion / Re: Problems threading on the lathe

« on: September 29, 2009, 02:14:34 PM »

Thanks Art,
Depth is not even a concern for me, it's the lead / longer threading, as shown by what i just posted.
The x axis is very accurate in it's moves and the Z is almost neglagible.

I am going to guess that i can cut threads out to about 3/4" to 1" all day long and they will be good.
Hope i don't seem anal about the threading. For what i do this lathe will be fantastic.
 
So over time i will try a different PC and have a friend look at the sensor triggering / timing since he has
the equipment to do it and is extremely knowledgable.

What fooled me was the distinct cuts, and you got to watch each and ever one of them to get the story as i just
posted.

By the way, a good test of the sensor triggering  would be to do multiple start threads.
Another would be to pick the thread up and recut it ( it's somewhat of a bitch to do in CNC). I posted something in members docs on how pick the thread up.

I guess the thought here is to eliminate the problem such that no correction will be required. Maybe wishfull thinking with the computer and sensor currently in use, but, wouldn't know unless some testing was done.

As always appreciate the comments,

RICH

5672

General Mach Discussion / Re: Problems threading on the lathe

« on: September 29, 2009, 01:33:58 PM »

Here's another scribing test.
Attached are two pictures of a threading scribe test which was done at 115rpm and 60
( 0.0001" deep passes ). I used a 30 degree angle so it was radialy cutting thus no Z offsets in the Gcode. Picture 1 shows finished scribing and there was a definite improvement. Picture 2 shows
the end of the passes and the total width of all the cuts was cut in half as compared to the same at
402 rpm and there are five distinct cuts.  

Now i watched each cut under the microscope since i can see the actual tip point and what material is being removed. For the first 20 passes it tracked the first pass. Then there was a slight shift at pass # 30 about 0.002" increasing the lead.  Then at pass #46 the cuts started oscillating, such the one would be on the first pass line, then maybe the middle of the width, and finaly further down the lead. Pass #53 opened up the width some 1 or 2 thou ( lead increase ) and then pass #54 opened the width up around 1 thou ( lead decrease ). Passes after that kind of tracked the middle of the of all the passes.

So it seems that the sensor is not constantly stable, maybe.......since that's what the o'scope's telling me, but i don't trust it. But then it could be the PC or combination of both.

If anything, i can say it dosen't take much to screw up a thread.

And for those following this, when you post and ask a threading question, and maybe you get so many different thoughts on the matter, some generalized comment, now you know there may not be a simple answer to a system of mechanical, electrical and software all trying to work in harmony!

RICH

5673

General Mach Discussion / Re: Problems threading on the lathe

« on: September 29, 2009, 12:07:05 PM »

Russ,
I looked at the sensor signal and it's has a nice crisp on-off / high low of same duration and no ringing to it. My o'scope is not the best in the world and it does seem like the signal is floating some, it should be stable such that it triggers and appears on the scope in the exact same spot, but again i don't trust my scope.

The Schmidtt trigger is good because you can refine / deal with the hysteresis ( however you spell that). So i won't discount using it.

Art,
I tend to agree with you and it may be a combination of both the sensor a little out of time and the computer. There are four of us here that have the same problem. Now it's difficult to put it all into perspective since every system has their owne quirks which could play into the threading problem
differently.So i will stay with my system since the mechanical end is known not to be an issue.
I think i can get my friend  to lend me his computer to try as it has a very stable pps in the driver test.

One common problem observed is that somewhere during  the passes the Z makes a deeper cut.
Say at pass 10 out of 30 passes. I am thinking that it's when the % variance is accounted for.
I don't have a signal generator that i can use for a pulse. The one i borrowed had the range but can't vary the width of the pulse.

Would it be of any value to add to the Turn diagnostics the pass at which the change in % variance occurs or is implemented?

I will post shortly with a picture and more info since i have tried another scribing cut.

Thanks for the commnets,
RICH

5674

General Mach Discussion / Re: Problems threading on the lathe

« on: September 28, 2009, 10:01:11 PM »

Hi All,
Took a look at the Turn Diagnostics for 402  and 115 rpm.
I just did a dry run of  the threading code at the 60 passes for scribing of the tube at both rpms.
There was a definite difference between the 115 and 432 rpm info:

During the higher rpm the Highest % variation during threading changed four times up to a high ending value of 18.4% while the low rpm only varied by up to an ending value of 1.37%. Additionaly,  the Mega Clock variation was greater for the higher rpm. Rotation real time rpm increased by 5 rpm for the higher and only 0.5 for the lower rpm. So there is a difference on what's happening with index pulse and also how the threading cycle is applying correction based on that info.

Now what is also interesting is that the % variation during threading was somewhat stepped for the 402 rpm, such that the first step change went from 10 to 18% after 15 passes, and gradualy creeped to the final 18.4% , which would kind of confirm what was observed when doing the actual scribing,.... at some point during the threading cycle there is a definite change in the Z axis movement.

So as posted before, that the 3/4-13 actualy cut at 115 rpm had no lead problems while the scribed one at the 402 rpm did,  this may just provide the explaination as to why.

The difference of when the five scribed lines "ended" is about  10 degrees from each other.
If i did the math correctly, that's o.oo3" of Z travel and that is quite close to what you can measure in the picture.

So i guess it's time to try a scribed line at the lower rpm and see if what i am thinking is just rubish
or not. Or i'm reading to much and incorrectly into the diagnostics info.

RICH

5675

Show"N"Tell ( Your Machines) / Re: CNC CONVERSION - 6" ATLAS LATHE

« on: September 28, 2009, 01:59:22 PM »

BarryB,
Just the threading from what i know of.
RICH

5676

General Mach Discussion / Re: Problems threading on the lathe

« on: September 28, 2009, 12:19:03 PM »

Simpson,
"It would be interesting to calculate the number of pulses effected during the duration of the disturbance"

Yep, same goes for the variance in pulses per second, but may not be as simple or relative as you would first think.
RICH

5677

General Mach Discussion / Re: Problems threading on the lathe

« on: September 28, 2009, 12:01:05 PM »

Simpson,
One of things that Art did was to provide for spindle slowdown and a lot of time was spent for the hobbiest.
A lot of those punny lathes ( i got one ) will have problems when trying to do anything over say a 40 or 32 thread.
You get spindle slowdown, and at say 24 TPI, it may be some 10% or more, yet the program adjusts , and you can accomplish the thread. I have even stopped the spindle for a second and recovered the threading cycle.
I have cut 1/2-13 in a steel rod with that punny lathe,but , you really need to experiment.
It does work! Trust me i have spent a lot of hours testing it. Just need to keep things in perspective though.
RICH

BTW,I agree with you on that if you have adequate torque / HP  ( not only the spindle but the axis system also ), accuracy and repeatability, to begin with ,then there will be  threading success. There is a whole community of users that don't have that. That's why i did the lathe conversion. But it is more complex than just the machine part. Computers can just suck at times. 

5678

General Mach Discussion / Re: Problems threading on the lathe

« on: September 28, 2009, 11:47:16 AM »

Hood,
 It's an Intel P4 and all the the others are Intel's also.
 I need to look at Threading Diagnostics and compare some data it gives. Was interesting to find that the PP wont read the 53 rpm while the SS would.
I noticed that the variance in the Mega clock rpms varied as the rpm went up, which relates to the locked RPM.

HMM......... the scribed lines i did were at 402 RPM while the actual threading ws done at 115 RPM, but fewer passes .....so if the variance is smaller at the lower rpms i wonder how the scribed lines would turn out at the lower rpms.
RICH

5679

General Mach Discussion / Re: Problems threading on the lathe

« on: September 28, 2009, 11:07:47 AM »

here is a screen shot of the driver test
RICH

5680

General Mach Discussion / Re: Problems threading on the lathe

« on: September 28, 2009, 10:57:17 AM »

Art,
The dirver test shows  a straight line, a small spike every now and then.
RICH