Re: torch table followup From: Gene Heskett (firstname.lastname@example.org) Medium risk You may not know this sender. Mark as safe | Mark as unsafe Sent: Sat 2/09/08 7:17 PM To: marcin_ose (email@example.com)
On Saturday 09 February 2008, marcin_ose wrote: >Gene, > >Thanks for the hints. > >On the drive system, then, the 3/8" 'resolution' of the belt is >sufficient to get accuracy about 10 times better than 3/8"? That was >my main concern. I would want to use microstepping. I was concerned >that the 3/8" pitch makes the belt too sloppy for 3/100 accuracy.
I think it would do that, I'm using a similar but finer toothed belt on the z axis drive for my mill, and as long as I have the thing tensioned up so the belt can't throw some slack on the push side, the backlash is zip. Its the finest pitched 3/8 belt McMaster-Carr had. Belt tooth pitch is relative, the finest resolution is the motors step size, with Jeffs drivers, that is 1600 steps per turn of the motor. I suppose one could find a cyclic error with a fine enough resolver, caused by the shape of the teeth, but for driving a cutting torch, I'd think any pattern you could see would be well swamped by the turbulence of the cutting flame. As a friend of mine would say, good enough for the girls I go with. :-)
>Regarding rack and pinion type drive - I was planning to use not a >belt loop, but a belt attached firmly to a surface. The pinion would >drive on top of that. Do you think that is a feasible solution, or I >wouldn't get the traction that I want? Could gravity make a sufficient >hold, or would I need extra clamping down onto the track?
Fastening the belt to a solid surface is the first problem, I wouldn't trust a superglue like substance to do that since the belt has sufficient flexibility that all the forces must be absorbed by the glue joint within a few thou of the actual tooth contact. Then the gravity or helper springs implied to maintain contact would, in the bearings of the swing mount, represent another source of backlash unless on very good swing bearings in order for it to follow the wibbles of the rack holder. I'd much rather use a continuous loop, over an adjustable for tension idler pulley on the far end, with the end of the carriage attached to the belt by clamping a short section of rack to the belt, no holes punched. Short pieces of rack can be had at McMaster-Carr I believe. As long as the clamp is the same elevation as a sight line from pulley to pulley, then the major precision worry is the sag of the carriage guide rods and the sag and whip of the belt over that length unless suspended in a track so it cannot sag or whip. The track will need a few holes to allow the dirt to be swept on through though.
Here also I think I'd take a lesson from the panel saw at your local Lowes and use a larger pipe with skate wheel bearings, maybe arranged on jack screws above the main frame to allow the pipe to be jacked as true as you can measure. Spot weld (resistance weld if you can, less warpage of the pipe) the bolt to the side of the pipe and drop it into the main frame with a nut above and below the frame. 4 or so per 8 foot length as long as we're playing "what if" here should do it nicely. A 1" solid rod is fairly rigid, but a 2" piece of 4340 pipe with .032 walls is even more rigid, and weighs a heck of a lot less. Not to mention its a bunch cheaper than the same length of drill rod and more than adequate dimensional tolerance.
Like I said, I wish I had the room for such a tool. OTOH, then I'd have to hit the road and find work for it too but I'm too old to undertake another career. :( So I'm semi-happy with my modified micromill I've tried to make an X3 out of, should have bought one in the first place. Hindsight as always, is 20-10. See my mess at <http://gene.homelinux.net:85/gene/emc> (vz blocks port 80) 8-)
I do hope you find my comments useful.
>PS. I like your boxes quote.
Its been there for quite a few years now Marcin, and there are times when I think its time for the last box to come off the shelf, the jury box isn't working that well anymore. And then they do manage to stuff somebody that needs it, putting off the inevitable. For a little while.
-- Cheers, Gene "There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author) It is easier to get forgiveness than permission.