Question about tandem lead screws

[Chunky]

Is it necessary to have anti-backlash devices for both screws?
If only one screw needs the device should it be the motorized one or the other one?

[ger21]

What you need are anti-backlash nuts. An anti-backlash nut is a nut with some type if adjustment to remove any slop in the threads. There are many different types, depending on the type of screws.

Ballscrews typically use 2 nuts on each screw, joined together so that they can be adjusted apart (or together) until they fit snugly on the screw with no play. The balls in a ballnut can also be replaced with larger balls, which reduce the clearances in the nut, making a tighter fit. This method will usually still have some backlash, but it will be reduced.

Acme and allthread leadscrews typically use Delrin or other plastic nuts, with some adjusment built in. Here's an example of spring loaded nuts. http://gonebowlin.com/dumpsterCNC/ More examples here. Anti-backlash nut
http://www.cnczone.com/forums/attach...tachmentid=944
http://www.cnczone.com/forums/attach...tachmentid=945

Not plastic, but here's another.
http://www.buildyouridea.com/cnc/tk-...5/phase_5.html

Backlash can also be compensated in the control software, but the preferable way is to remove it from the screws, if possible. Software backlash comp is not perfect. The reason is that with backlash, the tool can grab the workpiece and be pulled into it because of the play in the machine. This can cause poor finish, and broken bits if using smaller tools.

To answer your question, EVERY leadscrew should have some type of antibacklash nut.

[Chunky]

Hey! I really like that Dumpster site. I knew what the backlash devices did, just didn't know if it was necessary to do both when running two on the same axis.

[Chunky]

If I can get some ACME to match up with 'em I'll probably just go with the Dumpsters and have it over with. At their prices I'd rather just get 'em and have it over with. I do like the BUILDYOUROWNIDEA version though.

[ger21]

This is what I bought. http://www1.mscdirect.com/CGI/NNSRIT...7&PMT4NO=60553

Best bang for the buck I've seen.

[spalm]

Chunky, not trying to hijack your thread (pun intended)…

Have you given thought on how to mount your threaded rod?

Gerry, do you have any hints on bearings and couplers that you could share? Did you turn the ends yourself or did you order them finished? I see that Keystone has a secondary operations division that seems like they will do this kind of work. Not sure how much it will charge. Maybe I should just order a mini lathe and be done with it.

Steve

[Chunky]

ger21]This is what I bought. http://www1.mscdirect.com/CGI/NNSRIT...7&PMT4NO=60553

Love that! What are you going to do about getting the ends machined?

[Chunky]

Chunky, not trying to hijack your thread (pun intended)….

And a fine pun indeed.

Have you given thought on how to mount your threaded rod?

I can't believe it. I just got through asking Ger the same question. I know one of the site that sells rolled screws does the machining. I've got them in my favorites. I'd assume you'd have to buy your rod there too though. One thing that bothers me about them is that they seem to suggest I should not worry about the thing feeling gritty. I'm new to CNC but I just ain't going for that. Gritty and friction- they just seem to go together. Might just be me.

[ger21]

I've got this crazy idea - I plan on using a jig with my disc sander to get them roughly to size, and then another jig for the router table, using 1/2" ID bearings to support and rotate the screws. I bought some 1/4" shank grinding stones from Mcmaster-Carr, that I'll put in the router and hopefully be able to do a decent job. (watch the max rpm of the stones)
I've read a thread or 2 here where they chucked them in a drill press, supported the other end, and sanded and filed the ends down while spinning. Same basic principle.
If it doesn't work, I'll pay someone to do it.

[Chunky]

I've got this crazy idea - I plan on using a jig ...
Is there any reason why you can't insert threaded shaft into the center of a ball bearing as is? Ditto for the motor end?

[ger21]

You can - something like this. http://www.frontiernet.net/~salterc/cnc.htm
Just not the route I wanted to take.

[Chunky]

ger21]You can - something like this. http://www.frontiernet.net/~salterc/cnc.htm
Just not the route I wanted to take.

Only one side of the bearing is free, it is sealed and has a
grease hole. Worked great.

I don't know what he's saying here. Do you?