Hello Guest it is July 21, 2019, 03:40:59 AM

Author Topic: Questions on Threading With Turn on Sherline - From Yahoo SherlineCNC Group  (Read 28062 times)

0 Members and 1 Guest are viewing this topic.


Wow!  So many good replies to answer to.  So let me do the simplest one first: 

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.

Nope, fully registered.  I've owned it for quite a while, although I use it most often on my mill.


Thanks,
Bob

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

I tried a couple of times, but have not been able to get any results from that, and (blush) to be honest, I have not tried to read up on it and learn how to use it. 


Bob
"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.








Thanks.  I am the pocket protector/engineer type myself, but my field is radio design.  I design high performance receivers for a living.  So math is second nature, and I have gone through some of the calculations you talk about since those postings.  The only advantage I see to the stepper spindle is that the spindle speed will be under actual closed loop control, but the disadvantage is that the Sherline's motor by itself has way more torque than the steppers I was looking at.  Being a designer of control loops (PLL frequency synthesizers), I really like closed loop control, but when open loop works, it's cheaper, simpler, and "good enough".


Bob


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


We need video!

Dave

Hey Dave, Video is not allowed in the Blue Room of the Club but I might sneak a photo.
I'm soon moving to the Red Light District.
RC
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.
   

The spindle sensor shines an IR LED at the spindle shaft and uses a phototransistor to sense it.  The shiny shaft is interrupted by a piece of black tape.

The design is from Dave Hyland's website,  http://davehylands.com/Machinist/CNC/CNC-Spindle-Encoder/index.html#01-Encoder.jpg  and I know Dave has successfully used it.  I have had one screw of many turn out acceptably. 

As I told Hood, I did not spend enough time trying to use that tool but tried a couple of times and didn't see how to get info out of it. 

I am also suspecting my feedback pulse, although I want to go check that the settings that Rich pointed out are correct. 

Here's where I think I am: 

1)  I think I've ruled out the Z-axis stepper loosing steps.  I've run repetitive back and forth motions (20 round trips one time) cutting a rod down and the lathe returns to the same point every time.  I've done this a couple of times and different ways.  The cut pieces are the right shape without stairsteps from being cut in the wrong place.   

2)  Since I'm running the spindle at full speed while I'm doing this, I think that rules out EMI from the motor into the Xylotex controller.  I had that problem on my mill, although it took hundreds of back and forth moves to really show up. 

3)  I think I've ruled out the Z axis motor being too wimpy by doing everything at slow speed.  The Sherline motor should have around 450 in-oz of torque at the low speeds I'm running.  My Z axis is running at 50% of top speed, or less.  

4)  I've actually watched the threading progress under a 10X magnifier, and I see the cutter not following the thread.  It's like it's starting out of sync by some "phase error" (30 degrees, 60...90... it varies).  It could be starting in the wrong place in Z, but I think I trust the Z axis (as I said in (1) ).

I think what might be happening is that the spindle pulse doesn't "look" right to the interface.  I think it's possible that the spindle sensor might be putting out false or extra edges - perhaps caused by EMI to the sensor from the stepper motors, or the Sherline motor.  That is what I wanted to try and test tonight. 



Bob

 

Offline Hood

*
  •  25,849 25,849
  • Carnoustie, Scotland
    • View Profile
I have never used the turn diags myself  as it only works with the parallel port and I have a SmoothStepper now. I think the important thing in it is whether it reports problems with the index pulse.
 Just out of curiousity can you post the code that you are using for the thread, just in case there is a problem there. Also have you tried cutting a larger dia without the tailstock so that you can get a longer start at the thread.
Hood
Another thing Bob,
  Is your black tape as wide as this illustration ?
I should only need to be about 3/8" wide for that dia., I would guess.
Also, most other similar applications show a flat black wheel with a shiny spot for the index.
I would try it with a narrow tape first.
Juast a thought,
RC

Offline RICH

*
  • *
  •  7,342 7,342
    • View Profile
BOB,
I am attaching a file's in two posts which you can use to test your lathe out with.
I still suggest you just do a clean up cut and then color the end of the shaft as shown in
shaft setup pic. Use your 1/4" stock and note that I have the threading tool point at the end of 
and on centerline of the stock. Set your cutter the same ( you don't have to have the diameter
the same as mine ) and then set the "X" value to 0.246 in the DRO and also set Z to 0.0.
Set the spindle to 100 rpm.

Note that you can use the diagnostics pulldown to see what is happening as your threading.
You will see the variation of the speeds and also a percent which MACH compensates for.
Additionaly there will be status and also show triggering of the thread cycles. This is a nice feature.

Since the depth of cut is only 0.0001" and 78 passes your should only get a small scribed
line around the shaft.I have attached pictures of the thread path at 20, 40 , 78 passes.
You should get the same results if the all is set up. The last pic is at 30x showing pitch
and width of the cut. Sorry about the picbut it was off hand thru the microscope and the scale turned.

Doing this will enable you to save your brass and check out your system.
If you can't get these kind of results you will never cut a thread worth a darn.


Hope this helps,
RICH      POST#1

Offline RICH

*
  • *
  •  7,342 7,342
    • View Profile
BOB,
POST#2 MORE PIC'S

RICH

Another thing Bob,
  Is your black tape as wide as this illustration ?
I should only need to be about 3/8" wide for that dia., I would guess.
Also, most other similar applications show a flat black wheel with a shiny spot for the index.
I would try it with a narrow tape first.
Juast a thought,
RC

Mine is actually on the spindle, not the pulley.  Here's a shot of mine.

http://www.pbase.com/montana_aardvark/image/108459041

The tape is aright around 5/16" wide.  


Bob