all-thread as rack gear?

[ws5z]

Has anyone tried to use a all-thread rod as a rack gear. I custom built some concave helical spur gears to run on the all-thread out of Delrin , and secured the rods to the frame of my x and y axis carriages. Spur gears connected to the steppers directly. Using this on a cnc plasma cutter, as screw drives using standard all-thread was going to be too slow. I'm interested in anyone's experiences or thoughts on this type of setup. ????

[BillTodd]

Sounds like a clever low cost idea to me All-thread will not have the optimum tooth profile for a rack, but that wont matter for a low/medium accuracy device like a plasma cutter.

It'd be worth keeping an eye on the Delrin pinions to make sure they don't wear out too quickly.

Post a couple of pictures. I'm sure there are plenty of people here who will find it interesting

Bill

[dertsap]

you'd probably create a better discussion on the topic if you only post the question once
http://www.cnczone.com/forums/showthread.php?t=101922




.

[Mike Everman]

Oh yes, it's a great idea. I've been working on it for some time and now about ready to prototype.
For making the pinions, the first thing that leaps to everyone's mind is to put a tap in your lathe spindle and feed it radially inward on the edge of a blank that is free to rotate. It does work, but not great, and does not create the perfect (30deg pressure angle) involute tooth form.

The finer point to it is to drive both blank and tap, and know what pitch diameter is going to be a valid one for a given number of teeth.

It occurred to me recently that if one were making a scratch built nc, that he should start by getting all the nc gear working first, then make up some simple fixturing and bushings to go on your stepper motors, one to hold the tap on the end of one motor shaft, one to hold the blank on the other, using two of your motors to make a quick gear hobbing rig. Quick and dirty like.

The control only needs to be driving them at the proper ratio and feed it in slow.

Think I'll start another thread on what I've done on it so far.

[Mike Everman]

Oh, yes, please post pics of your pinions!

[Mike Everman]

Here's part of a spreadsheet showing some values of pitch diameter for various tooth numbers when considering both metric and English thread pitches.

[dertsap]

edit !!!!!!!!!!!1
patent what patent
I know nothing

[Mike Everman]

Here's what the pinion looks like, within the limits of solidworks modeling. I'll also attach a patent I'd found a while back that I'm sure wouldn't stand up in court.

Edit: ha ha, crossing posts. I like pics of real hardware instead of solid models for sure!

[Mike Everman]

Huh, the pdf didn't attach. I'll try again. OK, it's too big even zipped. It's US5664457.

[Mike Everman]

...and if you have a gear pinion on your motor, then picking a screw thread that's a good linear pitch for it gave some interesting results, that is, there's no good solutions in English threads for English gear pitches, but very close fits with several metric threads. (I haven't added metric modules)

Of course, we've got a 30 deg pressure angle on the rack meeting a 20 degree p.a. for the gear teeth, or do we?
I can't do the math off the cuff, but the helix angle of the screw thread makes matters worse, that is, the effective pressure angle at the rack goes larger and farther away, unfortunately.

Making the pinion is definitely the way to go.

[RomanLini]

That pinion in the picture won't work very well, it would have massive climb in 2 axes.

The closest thing you could get to a decent working system would be to have 2 identical pinions, (only one needs to be driven) on either side of the allthread "rack" which is a design I came up with in about 2001. That stops the climb away from the rack (converts to a force trying to push the pinions apart) and the climb problem in the second axis perpendicular to the rack is converted to a rotary force trying to rotate the 2 pinions around the rack, which is much easier to counter than a perpendicular deflection.

Feel free to patent it, US patents are not worth toilet paper.

[Karl_T]

If the goal is a low cost rack type system that performs well, I think there are two better concepts.

1. Use a sprocket on your drive and roller chain. real cheap at any farm and fleet store.
2. Use a timing belt pulley on your drive and a length of timing belt. An improvement on #1 with a bit more cost. Will work quite a bit better with small diameter pulleys

I used idea 1 on my 7' by 12' plasma table. i got no pics but found plenty here on the Zone when I looked into it.

I don't mean to rain on your parade, just check out all the concepts first. My 2 cents.

Karl