high-speed turning for pseudo-lathe work

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Re: high-speed turning for pseudo-lathe work

Postby PaulRowntree » Sun May 11, 2014 4:07 am

I may have to listen more to the suggestions offered here on the forum (or perhaps listen better ...)
Another test, this time with a 2"x2"x6" piece of Yellowheart. I rounded off the corners with some custom gcode (like Gary did, but using the edge of the cutter) then ran a roughing + finish toolpath to make this shape. It took 25 left-to-right-to-left passes to do, about 15 minutes. During the cutting in 'lathe mode', the router sounded good and steady, no evidence of abrupt digging etc. The tool was moving up/down slowly to distribute the wear across 0.5" of cutter edge.
When stopped, there was a lot of tear out on the extremities. The finish on one side of each 'bump' was very good (the top side of each in the photo), the lower side was rougher, not good enough in my view. But the tear out was clearly unacceptable.
So again, I should compare this to std wrapped machining, but so far the combination of moving cutter+moving work is ... underwhelming.
As others have suggested :oops: .
Attachments
Roller Toolpaths.jpg
roller mk1.jpg
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Re: high-speed turning for pseudo-lathe work

Postby PaulRowntree » Tue May 13, 2014 1:26 am

Not giving up yet ... I ran another piece of yellowheart, same design, but with a 1/4" spiral cutter instead of the 3/8" straight tool. Slowed down the feeds, raised router rpms to ~13K, and got much better finish, only 2 points of tear out on entire piece. Surface finish was much better, but still the surfaces towards the tailstock were smoother by eye than the ones towards the headstock; very similar to the touch. The wood had reversed grain direction for last test, so this is probably machine related.
Not perfect yet, but still has promise.
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Re: high-speed turning for pseudo-lathe work

Postby IslaWW » Tue May 13, 2014 1:16 pm

Paul...
You mentioned above that you can run for 10 hrs @ 1000 rpm and not loose position. So, lets say you did. 360* x 1000 x 60 x 10 = 216,000,000*. Does mach have a command to "mod" the number and get the position back to "the real world" like WinCNC? I know I couldn't believe how "far away" my rotary could be from zero, yet appeared to be in the same spot. :) I installed a limit switch to rezero the B when swapping back to indexing with the stepper from spinning with the servo.

This is one of the last tests that I'm doing with a top positioned cutter. Due to your feedback, I will postpone side cutting til last. Parts will be in for my back knife today and I will test how that works with the spin motor. Here is a vid of my last nites test. Please ignore the smoking dull bit! :)

https://www.youtube.com/watch?v=QPp1itlsbnI
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Re: high-speed turning for pseudo-lathe work

Postby PaulRowntree » Tue May 13, 2014 2:51 pm

Gary, that is so much more powerful than what I have going here, both the spindle and the drive. Well done! The bolt is very cool, with all the right embellishments. What kind of wood is that?

I keep position count inside the external embedded processor, and every time it stops the rotation, it then reads my pulse counter, does the math

steps_past_zero = total_pulses MOD Steps_per_rotation
steps_to_go = Steps_per_rotation - steps_past_zero

then outputs steps_to_go.

This puts it back to the starting position, without involving Mach3. So far, the only time this has failed has been when I stalled the motor. When control moves back to Mach3, it doesn't know that it was disconnected.

Are you hand-coding your gcode, or do you create it with scripts?
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Re: high-speed turning for pseudo-lathe work

Postby IslaWW » Tue May 13, 2014 3:51 pm

Paul...
That is expensive "imported" white walnut. AKA pallet poplar. :) Actually cut from pallet skids.

That makes sense on the script, as I cant see "winding it back to zero"!

That was all hand coded. After I watched the vid last nite I had to remove the clunky stuff. Looked like my Grandmother coded some of it!

Hopefully tonite I will get to test the back knife operation.
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Re: high-speed turning for pseudo-lathe work

Postby PaulRowntree » Tue May 13, 2014 6:37 pm

The perfect source of wood. Bravo!

I am building up a library of Lua scripts for 4th axis work. Each building block does some unit operation to generate gcode, and they can be strung together to do more interesting jobs. It adds in a header and footer pirated from Vectric post-processor generated gcode file. The plus is that this can be easily run from the controller without having to have VCP or Aspire out in the garage. I could send this if it is interesting (but it is a work in progress) ...
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Re: high-speed turning for pseudo-lathe work

Postby IslaWW » Wed May 14, 2014 2:01 am

Paul...
Finally got to the point where I can reply to your original title. I managed to get the back knife slide and tool bolted up and operational. Things are starting to come together. I need more practice, but for a first test, shows promise. Plenty of torque in that servo. Toolpathed in VCPro.

https://www.youtube.com/watch?v=_yznJrzd730
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Re: high-speed turning for pseudo-lathe work

Postby PaulRowntree » Wed May 14, 2014 2:16 am

Now that is how it should be done! What would the material speed be in the video? How was the surface finish?

Time to make a fixed tool holder ...

BTW : if you get an interest in turning something just a bit bigger ...

http://ptsail.org/2011/01/15/lathe-turn ... diac-mast/
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Re: high-speed turning for pseudo-lathe work

Postby IslaWW » Wed May 14, 2014 3:29 am

Paul..
The rpm was either 1800 or 2000 rpm. I wont have the tach installed for a while.

1050 sf/m @ 2" diameter.

I don't think I want to try and turn anything as big as that log!
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