Lathe electronic lead screw project

[Kram]

Ive wanted to play with this stuff for a while. Ive got a couple of arduinos and steppers but not had much time. I was going to start with wire feed on mig welder as it doesnt need much accuracy from the software side.. compound slide has a 1mm pitch thread... it could use some electrical assistance

 

[DanielW]

compound slide has a 1mm pitch thread... it could use some electrical assistance

Fire away.

 

[Kram]

one of these might work well?
http://www.ebay.co.uk/itm/371320448933

 

[Henrik]

Was mildly pondering this earlier...

@Henrik - how many pickup points does your spindle sensor have, just a single magnet?

Just the one for now.

The github repo now has code to recalculate the feed rate if the the rpm changes (i'm currently using a simulated number of milliseconds between revs).

The problem with using an encoder with many points per rev is that there's no easy way to determine the start of a revolution, i.e. making a multiple pass cut ends up depending entirely on not missing a single interrupt from the encoder.

At 600 pulses per rev, you end up with 3600 interrupts per second, and whilst that's not that many at all on its own, it might be enough to delay the stepper ISR (i.e. the clock interrupt).

I was thinking about this earlier as well, and one solution (if the one pickup per rev is not enough) might be to have the hall sensor to signify the start of a revolution, and use a rotary encoder to calculate the speed only, and recalculate the feed rate if the rpm changes too much.

Having said that, I believe a lot of the PC based CNC lathe packages use a single pulse per rev, so the problem may not be *that* bad for normal threading anyway.

However, I see there's 100ppr encoders - less may be more, as the little arduino is only 16MHz single threaded

 

[MattH]

Plenty of fully cnc lathes just use an index pulse for the rpm feedback. This works fine for mid speed threading so I can't see it being an issue for federate calcs.

 

[Kram]

I would avoid magnet/hall sensors. They can have quite a wide width to the pulse? good enough for rpm but not for index.

There are encoders/optical wheels with an index pulse... Id guess a larger wheel would fit the OD of spindle better than a small encoder..

http://7xcnc.com/hardware/encoder/
Mine has something like this as standard inside the spindle, guessing no index pulse as its just for rpm display.

 

[Henrik]

Quick update to the project.

I've got some steel and started fabricating the bearing housings for the lead screw.


First up, I used some 65mm x 12mm bar to create a rudimentary bearing housing

This was then welded onto another piece of 65x12mm bar in the right'ish place. Sadly I had to bastardize the bar with the angle grinder so that I would be able to get the tail stock off with the new bit mounted... My kingdom for a band saw It looks totally rubbish, but the mill is broken, so it will have to be cleaned up once that is up and running again.

The little 135 amp welder clearly struggles on this thickness:

It's not exactly structural, so it will probably be OK, I'm hoping.

Once I had the bits welded together, I drilled two holes to mount it to the lathe, but then I unfortunately broke a drill in the hole in the lathe, so had to make a third hole.. Tapped two holes to m6.

Bits mounted:

And roughly what it will look like with the bearing / screw:

The screw will sit in the bearing with a plastic sleeve to centre it.

 

[ukracer]

Eh! fun ya say? there ain't no fun in using Mach3...anyway are yer going to use a cnc manual controller? £27 from china,you won't pay import tax on that just vat.

You got a link to that mate?

 

[rtbcomp]

A Gray code encoder would give you accurate positional indexing, we used them on line printers where the drum was rotating at a few hundred RPM and the hammers had to strike at exactly the correct time.

Another way is to count clock pulses between marks on a encoder wheel, 'instantaneous' speed can be obtained between any two marks that way.

 

[Henrik]

Made some progress on this project this morning after a long hiatus.

I've machine one end of the screw on the lathe. Had to go slow in back gear due to the length of screw, so finish isn't great, but it will do its job.

The other end needs to be machined also, but that should be a simpler form (basically just a 10mm shank and a bit of m8 thread.

Used some masking tape around the screw showed into the headstock as a support, because I lack a steady




Any good easy suggestions for cutting a key slot into a pulley that has an 8mm through hole?

 

[Henrik]

Lead screw done (except for keyway)



Hoping to get away with locking the pulley with a screw instead of a key.

 

[Henrik]

Some more progress.


First up a milled keyway in the lead screw (402 size, IIRC).
View media item 9399
Used a bit of tool steel to cut a keyway in the pulley, by fastening the tool steel in the tool holder and moving it in and out into the pulley, which was fixed in the chuck (without the chuck spinning obviously )
View media item 9396
Spacer for the lead screw to move it slightly to the left and also center it in the bearing:
View media item 9390View media item 9389View media item 9391

Now, for something interesting... We've got ourselves a 3d printer at work, and I've finally started printing some little bits on it.
First up, a bearing support for the stepper motor. The idea is that a 10mm shaft runs through the part, supported by bearings on both sides, and the stepper motor connects to the shaft. Should hopefully save wear on the bearings in the stepper:

As it came out of the printer:
View media item 9394
After removing supports and pushing the bearings in:
View media item 9398

Next up, a resettable dial with 200 steps to go with the 2mm/rev lead screw for when I want to use the lathe manually. Chuffed with this
View media item 9393View media item 9392View media item 9395

Next steps is to make a bearing support for the other end of the lead screw and then somehow mount the stepper motor (might try a printed mount if I can design one).

If anyone wants the STL fies for the 3d parts, let me know.

 

[Henrik]

Hmm, not sure why the images aren't displaying - any ideas? They're from the gallery

 

[Henrik]

Left hand side lead screw mount..

3d printed plastic - hopefully it will be strong enough to not fall apart in normal use, but weak enough to break if I run the tool or saddle into the chuck.

 

[Pete.]

I met a guy who has done this. I had a look at the threads he was cutting and they were very nice, also cuts very tidy tapers. He has since junked the leadscrew however in favour of ballscrews.

 

[Henrik]

I met a guy who has done this. I had a look at the threads he was cutting and they were very nice, also cuts very tidy tapers. He has since junked the leadscrew however in favour of ballscrews.

Yeah ball screw seems to be the way to go, and if this experiment works out well I think I might upgrade to them too

Mine will not cut tapers initially (z axis only), but the goal is definitely to add the x axis also. Not sure if I'll write my own software for that, or just use e.g. LinuxCNC - guess it comes down to if I want to use the lathe to actually make something, or write some software, hmm.

 

[Henrik]

Sadly the adaptor on its own had way too much flex, so had to fab up a holder for it so all its doing now is locating the bearing.

Excuse the atrocious welding. It's literally the first joint I'd ever done with tig (even earlier than my test pieces...). Turns out it's not as simple as MIG

At least now the screw is mounted, and I can use a drill to move the saddle left and right quite successfully.

Next step - make hole through driven pulley, create a shaft for it to sit on, and mount stepper to the red bearing housing.

 

[eLuSiVeMiTe]

Been toying with the idea of an e leadscrew for the last 12 months.

Did it ever get finished.

 

[jack-daniels]

This guy has a series on making one (not watched them all yet as I don't own a lathe )

Apologies if its already been posted.

 

[eLuSiVeMiTe]

This guy has a series on making one (not watched them all yet as I don't own a lathe )

Apologies if its already been posted.

Watched the first few parts a little while back.
Saved for future reference
Thanks for sharing though.
For the time being I'm just gonna drive my leadscrew for feed speeds. My drive chain is a bit noisy for running in the evenings