ballscrew repair and some general questions

[jimbo]

Hello,

I have been reading the board for the past week, and have been re-inspired to get going on my cnc project. I am in process of building a cnc router with servos. It will have cutting capacity of approx 24"x24". It will be a fixed gantry, moving table design. I have nearly all parts except for z axis assembly. I am much stronger in woodworking than metalworking. Since there are some very knowledgable people on this board, I'd like to pick a few brains.

1. I purchased a complete assembly to use for the fixed gantry off ebay. When the seller disassembled it for shipping the balls fell out of the ballscrew. He gave it to me for half of what I bid. Is it possible to repack the ballscrew? Any tips? I took the two circuits off the ballscrew cleaned all the grease off, then use a piece of wire to keep the balls from falling out the opposite hole as i filled it. Wasnt able to fit as many as I thought should go in, taking into account a certain amount of balls to reside in the circuits. There are no markings for brand on the ballscrew. It has 1/8 balls.

2. I am using linear rails, the round type for the moving table. How do I ensure parallism and that they lie in same plane when I assemble the table. The rails include standoffs. I am trying to think of how to use a dial indicator when the two rails will be set 16-18" apart. I would think I would have to set these up before fastening the table surface to them.

3. The linear bearings that I have do not have holders, ie a way to mount a table to them. Since I only have woodworking tools available, I was thinking of making these from blocks of maple or ash, then when the machine was running I would make some proper ones from aluminum. Should I just do it right the first time. I never see these come up on ebay.

4. How much backlash is tolerable in a cnc router. How much can be compensated by software? I realize this related to what parts I am going to cut and how much accuracy I demand. I have a lot of things planned, like engravings, inlays, signs, model parts, etc. Mostly using wood with some plastic and aluminum.

Thanks, Jim.

[balsaman]

Hi Jim,

1) Most ball nuts use 2 different size balls, alternating big, small, big small etc. This is to allow the smaller ball to roll between the bigger balls, without rubbing the screw, to reduce friction. It will be hard to tell the difference in size. I think its just a few thousands. Usually you need to buy a new nut if the balls come out. You could try measuring a few to see if thats the case for you ball nut. How does it work on your screw since you reasembled it?

2) As for the table rails, on my last machine I mounted everything as close as I could but made sure the bolts I used allowed me to loosen, tweak, and retighten. I also mounted the bearings to the table as parallel as I could, inserted the rails etc. move the table to the extreme end of travel, bolt down that side, then move the table to the other extreme, and bolt the other side.

3) Make them from wood for now. What's the rest of the machine going to be made from?

4) the software can compensate for tons of backlash, but it's better to build in as little as posible. I had .006" on the y on my last machine and it was annoying because I knew it was there but it didn't affect the parts after I dialed in the backlash comp.

Eric

[chuckknigh]

http://makeashorterlink.com/?C55F63745

This machine uses just about every "clever trick" that I've found on the net. My rails are fully supported, too -- lots of pictures for inspiration.

I don't yet know how well it works in practice (haven't made it move, yet) but it works great with a cordless drill attached to the lead screws...

-- Chuck Knight

[HuFlungDung]

Hi Jimbo,

You may have to purchase a micrometer to measure the difference between the balls. They could be within .0005" of each other. That would be 1/2 a graduation on the micrometer spindle. Like Balsaman said, insert them alternately in the raceway, large one, small one. etc.

If this is not a precision ballscrew, then you may have nothing much to worry about, anyway.

The best way to get them all back in is to roll them in with the screw. This is a painstaking process, because in a precision ballscrew, you may have a lot of difficulty trying to close up a gap in the balls. However, in the initial stages of loading it, you can use something soft for a "ball pusher", like the plastic straw off your can of WD40

[kong]

So if you buy a length of ballscrew-rod and ball-nuts seperately, how do they arrive so you can assemble them without this happening?

[wjbzone]

Kong,
My ball nuts came with a cardboard tube holding the ball in. It is fairly easy to slide the tube over a turned down end of the ballscrew and thread it on. I've had mine off and on several times with no problem. (save the cardboard tube)
Bill

[jimbo]

Wow, thanks for responses.

I did not realize that the balls alternated by a few thousands, makes sense. When I last attempted, I just put the balls in one by one, and rotated the ballnut forward to pack the balls together. if i recall I was able to get 20 or so balls in the nut before I thought it was full. I did not try to fill the circuits, because I thought I should have been to fit more in the nut. There are approx 50 balls total. I will give it another shot this weekend. I attached a pic to show the gantry that was $75. I really would like use this because alot of work is already done. I looked online and found a couple of companies that repair ballscrews but the are the big cnc mills/lathes type. I can see them laughing at a puny 1/2" ballscrew that I send them. I'll ask anyway.

I assume a ballscrew is considered precision if the screw is not of a rolled type. if that is the definition then I think have a precsion ballscrew.

I'd like to use as little wood for the construction of the router, as you can see the gantry is all aluminum. Havent decided what I will use for the moving table, would like to use 1/2" aluminum plate with a 3/4" mdf surface. Bearing blocks for y axis from maple or ash for short term or longer if they work well. Really havent decided about the structure to support the whole thing yet. I could build one from wood pretty quickly but I am concerned about warpage and settling. Ideally Id like to make a 2" square steel tubing structure but my welding skills are subpar due to inexperience, i do have both a stick and mig welder. The legs will be removable and the gantry will be able to be unbolted for moving.

Jim.

[jimbo]

Since everyone likes pictures...

These are the y axis linear slides. They are only supported on the ends. They are 1.25" x 42" L and quite heavy. One set is extra for now since I bought them before the gantry.

I am wondering if anyone on this board is familiar with the SST/Teknic servo controllers. I purchased several off ebay for cheap, $30 avg. They look like they will work well for me, 12 amp continuous, drives both brushed and brushless servos, and take a step and dir ttl signal.

Jim.

[jimbo]

Also are the ballscrews typically assembled dry or with grease? I am not sure if using grease would help with assembly or make it worse. Would a wheel bearing type grease be suitable?

Jim.

[HuFlungDung]

Hi Jimbo,

I would simply lubricate the screw with oil. However, if it will assist you in assembling the ballnut, there is no harm in using grease. Use a modern synthetic grease, very lightweight, so it doesn't dry into a hard cake.

[lsfoils]

There is an excellent article in the Home Shop Machinist magazine for May/June that describes taking a ball screw nut completely apart, cleaning it and putting it back together. My posting is a little late here but you may want to review this article to make sure youv'e covered all your bases. I don't recall the name of the article but its about converting a G1005 Grizzly mill/drill to cnc.

[jimbo]

Sorry to bring an old thread back. But just an update. This afternoon I reassembled the ballscrew nut successfully. I am happy to be able to reuse it.

I used a micrometer that was good for .00005 (5 hundred thousandths!), every ball measured .12455". I did a dry assembly with the original balls, I was able to fit 24 balls in the nut and 10 in the circuit tube. This was a dual circuit nut and I only had 67 balls. I am assuming same number of balls for both circuits, so I was missing one ball. Found some chrome steel balls at Enco. These measured slightly larger at .12460 to .12465. I went ahead and used all new balls, I figured since the ballscrew was well used and the new balls would tighten it up. It was easier than I thought. Assembled one circuit at a time, with 24 in nut, and 10 in tube, using a dab of wheel bearing grease to keep the balls from falling out of the tube while placing it back on the nut.

It seems to work pretty well. The nut travels smoothly and under gravity will descend the ballscrew. One step closer to getting the machine together.

Later, Jim.