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Author Topic: Problems threading on the lathe  (Read 330139 times)

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Offline John S

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Re: Problems threading on the lathe
« Reply #200 on: April 08, 2009, 05:14:14 PM »
Got a chance to run some more tests on the lathe.

First Art asked if the averaging made any difference, with averaging off it moves between 615 and 595.
http://www.stevenson-engineers.co.uk/files/P1000649.MOV

With averaging on it's steady at 600 to 604
http://www.stevenson-engineers.co.uk/files/P1000650.MOV

Now scratch testing on alloy, fresh tool, machine cleaned and slides lubricated.
First test is 50mm long @ 2mm pitch



This is typical what I have been getting.

Next test was the other end again cut 50mm @ 1.5mm pitch which is what Steve usually tests with.



Now I have never had that before but Steve has, however mine looks cyclic in that it looks to be going on a wander at the 50mm end.

So dump the driver, reboot, install new driver, run driver test and reboot again.



Back to test 1, 50mm x 2mm and we get this.



Now that looks good so carry on and do a full thread. One thing I noticed as it was cutting was the chip load wasn't constant from pass to pass, one pass would take a sliver and the next pass would be a lot deeper. More disconcerting though was it would cut heavier in different parts of the thread.
One pss say at the start, another pass at the end and so on.

Finished thread.



Using a blued tap as a gauge, start looks good, nice crests but the end is ragged, thin crests and out of pitch. This is consistant with what I'm getting on 2mm pitch threads.

Back to test 2 with the 1.5mm pitch threads. Start again looks good.



Finished thread.



Good crests at the start but gets rough towards the end, pitch isn't bad but the tap doesn't want to sit in the threads anywhere, it rocks about as if the middle is right but the ends stop it sitting correctly.

This non compensated driver seems to be the best of the bunch so far.

Shout up if there is anything more I can do or try.

John S.

Offline ART

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Re: Problems threading on the lathe
« Reply #201 on: April 08, 2009, 06:05:58 PM »
John:

 Perfect testing, Thank you. A picture is truly worth more than 1K words.

 Very strange results.. not what I expected at all. The cyclic nature is evident in the second photo. .

 SO the special driver, ( which simply has speed correction turned off ) seems to do much better, but since its not
correcting the volume is off due to variance..

 I have to think about this one before I come up with another test, we're definitely onto something though..

Thx
Art

Offline ART

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Re: Problems threading on the lathe
« Reply #202 on: April 09, 2009, 08:18:53 AM »
Hi John:

  Here is a driver with a subtle change in the corrects.. Id be interested in the result of a thread
on this as you tried yesterday... Id like to see if the cyclic changes are still there..

Thx
Art

Offline ART

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Re: Problems threading on the lathe
« Reply #203 on: April 09, 2009, 11:48:53 AM »
Hi John:

    BTW: the end of the thread will always be ragged with thin crests, the distance of that will be equal to the deceleration of the Z axis, upping your accel for the Z axis will minimize the effect,
Id expect the last few turns to go thin if the accel isnt high enough, if your lathe can handle it, Id raise the accel up as high as possibel as long as you dont lose steps.

Thx
Art

Offline John S

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Re: Problems threading on the lathe
« Reply #204 on: April 09, 2009, 05:43:55 PM »
Pretty picture time.
Loaded the driver up Art posted and upped the acceleration as high as it would go and did the usual 50mm long 2mm pitch thread, 2 scratch cuts.



Slight tracking out from midway to the end, then cut this to full thread.



Not bad looking, crests are out even though the acceleration was right up but still off on pitch. Next run the 1.5mm pitch thread. This was also out



Didn't bother doing a full thread for this one,  so out of interest slowed the acceleration down.



Tracking better but the end is closed up due to the slowing down, went back to mid point on acceleration and started playing about with speed, so far these have all been done at 600rpm. Going up caused the pitch the get courser and it's already out, dropped down as low as 150 and started working back up.
the pitch of the thread was altering according to the speed.
This is the 2mm pitch at 300 rpm and that's the best one I have cut yet.



This is 2mm at 600 rpm, double tracking and to course on pitch.



One funny thing I have noticed is that the first cut makes one track, the second id off but subsequent passes want to follow the second pass as though the first pass is a settling path ?

John S.

« Last Edit: April 09, 2009, 05:45:29 PM by John S »

Offline RICH

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Re: Problems threading on the lathe
« Reply #205 on: April 09, 2009, 08:23:57 PM »
ART & JOHN S,
Yep, the threading is progressively changing on each cut and it's not in the
code generated by the wizard and not apparent until you do a larger diameter and longer
length threading as John mentioned.
I am using MACH3 ver 3.042.024 and a SmoothStepper and wanted to what would happen.

This thread was done with the Smoothstepper using G32 code generated by the wizard.
Stock is S.S.,  0.735" (18.65MM ) OD x 2.6" ( 66 MM) long thread x 0.0788" ( 2MM )
pitch at 300 rpm, 29 deg lead in, 25 passes @  0.0002" deep. Z clear was set at 3"
( approx 4 times the pitch ). Although not using metric this kind of mimic's a 2mm thread.

Looking at the Gcode the incremental change in the Z is 0.0001" for each pass for a
total of .0024". So if the lines were individualy scribed onto the stock, and you could see
them, then you should have 25 lines at the start of the piece and 25 lines at the end of
thread contained in a width of 0.0024" wide.

This is not what is happening with my single test. I have tried to provide meaningfull data.

Now  the first scribed line has 25 "scribe lines in it and contained in a width of 0.006".
which is located from Z=0 at approx -0.3". Now as the thread progresses and the pitch
is increasing ( it shouldn't ) and now  have 25 lines contained in a space of .028" at Z=-2.3".

I have attached some pictures:
THRD_JOHNS_HCIR123- composite picture of the next three.
THRD_JOHNS_HCIR1 - shows the thread progressing on the piece
SCALE_THRD_JOHNS_HCIR2 - shows the pass at Z=-2.3" at approx 30X.
60X_WIDTH_HCIR - this picture at 60X shows only  the 25 cuts at Z=-2.3.
                             The small black tick marks represent an even progression of 0.00113".

Hope all the info provides some measurable basis to refine threading.  ???

RICH

Offline RICH

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Re: Problems threading on the lathe
« Reply #206 on: April 09, 2009, 08:40:52 PM »
John,
Fruitless trying to come out with a good thread when the threading is in error.
If the scratch lines won't track you will never get a good thread.
Happy I tried something bigger and longer since you had me stumped.

I didn't try Art's recent file. Looks like not much of an improvement.

Don't you feel good knowing that it's not only you!  ;D
RICH

Offline John S

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Re: Problems threading on the lathe
« Reply #207 on: April 10, 2009, 04:28:15 AM »
Rich,
Can you clarify that this test was with the smooth stepper and on one index pulse or from an encoder, if so how many count encoder ?
I was of the understanding that the SS was threading OK and it was just the parallel port that was having problems.

John S.

Offline RICH

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Re: Problems threading on the lathe
« Reply #208 on: April 10, 2009, 08:23:40 AM »
John S,
I used the SmoothStepper and also just one index pulse from a slotted disc.

I also thought all was fine using the SS, but the largest threading i have done was 1/2"  
and 1" or so in length. I also demenstrated how to pick up a CNC'd thread using the slotted disc.
I deliberately did the threading this way because never had any problem. Also believe it is not a
mechanical or a diameter of stock issue.

A lot of settings / inputs even mechanical as they relate to threading can have an effect on the thread outcome, but each of those should not create a constant increase of the lead. I could probably
write a few pages on each of that thought. It's basicaly a Z distance related problem and only ART can find what it is.
 
The data given from my test says there is a 4 to 5x error in lead ( .028"/.006" ) over 2 inches. The scribed lines on the diameter just magnifies it.  Lets see what ART has to say.

Please question as ya all wish, as i have been wrong before!
RICH

Offline Hood

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Re: Problems threading on the lathe
« Reply #209 on: April 10, 2009, 08:36:23 AM »
Rich, that looks very similar to what you would get if you didnt have the "Use Spindle Feedback in Sync Mode" checked, wonder if that could be a clue for Art?
 I have not done any long threads for a while but may try and do some at the weekend if I make it to the workshop. However I have never had problems threading before, (long or short,  large or small dia, course or fine pitch have all been perfect) so my machine is probably not a good test other than to say threading does work for some.
Hood