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Author Topic: Questions on Threading With Turn on Sherline - From Yahoo SherlineCNC Group  (Read 28065 times)

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This is what I got tonight.  I set the motor speed down to 100 RPM and used the threading wizard (as usual).  The threading seemed to go alright for a few passes out of the 12 it was going to take, but afterwards, I could not get a nut to thread onto this. 

Then I did something really unusual.  Without having changed anything, I started the cycle over again.  If Mach always picks up at the start, times and does all the calculations, it should start right again, only this time, it shouldn't cut anything.  I've done this manually threading, and that's how it works.  Only I heard it cutting a few times.  This is how it came out.  Clearly it looks like the cutter was never in the right place twice.  The name on this picture is well-deserved! 

http://www.pbase.com/montana_aardvark/image/108407099 

In light of what I got, I turned this piece around to work on the clean end, and wrote a little G-code to go back and forth 20 times along the threaded rod, advance the cutter into the work to make a mark, then pull it back out, go to the other end and repeat.  It did this at both ends of a 0.5" gap.  Afterwards, I find no places where the mark is not where it's supposed to be, so it was repeatable doing this.  No missing steps, and the spindle was running. 

What else?  I cleaned the chuck with a good degreaser to make sure the blank wouldn't slip, and I always use a live center touching the center of the blank. 

If the Sherline motor has enough torque, my Z axis is not missing steps, and I'm running at slow speeds on Z, doesn't this say my wimpy Z-axis stepper is not the problem?  Then what is? 


Bob

Offline RICH

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Bob,
Wimpy is yours and punny is mine. ;)
I just posted 1 min before you.
Take a cleanup cut for say .75", and use the wizard to generate gode based on the cleaned up diameter,
use the 100 RPM, .....BUT......set the pass depth for .0001" in the wizard. This way you are checking the threading cycle. You won't do any deep cutting so slipage of belts or motor slow down should not influence the following of the tool along the thread. It should repeatedly just give you a fine scroll around the rod at pitch.

BTW make sure the Z  starts some distance , .1 or .2" before threading starts as this will give some time
for the stepper to accelerate.

RICH

This is getting really old. 

Another couple of wasted pieces of brass.  This time, trying to cut a 6-32 screw.  Turned down a piece of .250 bar until it matched the outside diameter of a #6 screw.  Set the motor to 150 rpm and the "first cut" depth in the wizard to .0001".  Made the number of passes it set, and the the thing I have is not a screw. 



The one on the left is the second try for the night, after the first one failed.  The first one was a different screw up.  I cut the outside diameter too large, using my machinery handbook.  Then I measured that cheap, zinc-plated, $1 per pound screw at the top, re-cut it to the right diameter, and ran the threading wizard's output again.  It looks more like a screw, but you can see each thread peak is quite wrong. 

Questions:  I'm using a live center - seems like I have to, because these things are well beyond the 2 or 3:1 aspect ratio that you need a center for.  When I do that, it's hard to get much of a running start into the thread (letting Z accelerate).  I have used about 0.1" beyond the screw end, but it's crowded there. 

Oh, before I cut the thing, I sized the quarter inch rod down to .130 with my CNC lathe, running the motor at full speed, and the Z axis as fast as I dare.  No problem.  Every pass ended in the right place.  So it's not the motor causing EMI getting into the controller box (seen that before). 

How fast should the feedback pulse to Mach from the spindle sensor be?  Should it be a tiny pulse (thin piece of tape) or is it just looking for the leading edge?  What problems should I look for there?  The motor speed display looks stable.  It drops about 2 to 4 RPM when using 150 RPM, but Mach is supposed to handle that, right? 


Bob

 

For some reason, the picture link didn't show up: 

http://www.pbase.com/montana_aardvark/image/108434363

 

Bob
« Last Edit: January 22, 2009, 11:07:37 PM by Overloaded »

Offline Hood

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Bob
 No need to turn the piece down to the dimension first for testing, just cut the thread you want on a lrger dia to see if the pitch is right, that may make it easier for you to try without the tailstock to get a run up. Also it will allow you to use a larger Dia then you can cut off the thread and thread again at the new dia thus not wasting material.  I would say you should be fine with 5x dia out of the chuck as long as you have a solid enough grip on it.
 Have you enabled the turn diagnostics plugin and then monitored to see if there are any problems with your index pulse?
Hood

Offline simpson36

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I don;t think anyone has asked if you are running the 'evaluation' version of the software.

If so, single point threading is disabled and you will simply waste more time and materials until you eventually discover that the feature is crippled  . . . as I did.

I have evaluated and then purchased a lot of software, but this is the first time I've come across a 'demo' version that does not make clear exactly what is crippled, leaving the evaluator wasting many valuable hours chasing unsolvable problems. Very annoying. Unconscionable actually, in my opinion.

Offline RICH

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bOB,
Just a few program setting checks here:
1.In config>input signals> index is checked and port & pin iis assigned along with proper pulse setting
            "  >spindle setup> in special functions box, the spindle feedback in sync is checked and  also
                                       spindle speed averaging
2.in config>general config>  I use .005 for shuttle accel, 500 for debounce interval / 100 for index debounce
3. in plug control>turn diagnostics - you can use this to monitor the threading as it shows info on rpm, index etc

Don't get frustrated. There is more to just putting in a piece into the stock and running a program.
Why i say this is that, to do threading, a number of things all need to be in place or else poor results.
More so for a punny lathe.
For now all we want to know is the machine is set within it's capabilities, the program settings are correct,
basic threading movements are working correctly. Accurate threads is another story in itself  which is greating
influenced by the machine and tool .
So, with just doing a skim cut numerous times we can eliminate some influences which can screw up a thread and check things out plus you waste no material.
Sorry, i walked along your same path and there is no short cut to do a thread.
RICH

    
« Last Edit: January 23, 2009, 06:38:20 AM by RICH »

Offline simpson36

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"When the impossibility has been ELIMINATED, whatever remains, no matter how improbable... is possible."
- Sir Arthur Conan Doyle

We geeky pocket protector types love to quote that . . . but it works . . . so to identify what is NOT causing the problem is useful:

Steppers cannot 'catch up' with lost steps, so eliminating the stepper, lead screw and controllers should be as easy as seeing if the axis returns to their start point after the thread cutting. If they do, then unless I am missing something about how steppers operate, you simply can eliminate the steppers as a suspect.

If your pulse counter is on the spindle with the chuck, then you can eliminate belt slippage as a cause since that would show up immediately in the chuck speed.

The way I read it, you are considering replacing the spindle motor with a stepper. Is that correct? If so, forget about that idea. You can't compare  the power available from a stepper and from your spindle motor in terms of torque.

I'll try to explain, but most people have a lot of trouble with these concepts:

How much you can cut is going to be determined by how much POWER you have available.

Torque is not POWER.

Torque is a STATIC measurement of twisting force.  POWER (often expressed as horse power), is a measurement of work done over TIME.

The term 'work' involves movement, the force is given as torque and the distance is RPM.

A stepper producing 200 oz in of torque at 400RPM is not producing anywhere near the power of the spindle producing the same 200oz-in at 6,000RPM.

A formula:  RPM * TORQUE (in ft-ibs)/ 5252 = HP   

so . . power at the chuck:   Stepper  .0793   vs   Spindle motor 1.19   

Same torque spec, yet 15 times more power.

Sorry this info won't solve your threading problem, but I think it is relevant to a proposed solution that's been contemplated. Hope it helps.









« Last Edit: January 23, 2009, 10:41:59 AM by simpson36 »
Hey Bob,
   This from your first post:
Quote
I currently have an opto-coupled feedback sensor that appears to give me stable RPM numbers.
What type of sensor is this ? Later you mention reflective target or such.
Is it sensing on the actual spindle and not the motor or other shaft ?

I for some reason suspect the index signal to Mach to be the problem.
And did you follow Hood's suggestion ?
Quote
Have you enabled the turn diagnostics plugin and then monitored to see if there are any problems with your index pulse?
Hood

Threading is working flawlessly here. Actuall threaded 900 pieces today and yesterday, 48 TPI. My best run yet.
   

Offline budman68

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Threading is working flawlessly here. Actuall threaded 900 pieces today and yesterday, 48 TPI. My best run yet.

We need video!

Dave
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Just because I'm a Global Moderator, don't assume that I know anything !

Dave->    ;)