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kimoyo
11-06-2006, 07:06 PM
Aaron gives you a choice for your z-axis ballnut, you can get single circuit preloaded or double circuit. He recommends the double circuit eventhough it has a backlash of around 0.002 because the forces on the z-axis can exceed the preload and cause issues.

Are these forces from the weight of the head?

If the forces are from the head weight, could we add gas springs like on this syil

http://www.cnczone.com/gallery/data/500/medium/a8.jpg

in a similar way to the z-slide of the ih mill?

http://www.industrialhobbies.com/Products/mill_2_images/mill4.JPG

Then put preloaded ballnuts on the z-axis without any issues? Thanks.

Adobe Machine
11-06-2006, 10:13 PM
Or a third option.Re-load the primary ball nut ( the one with the flange), I believe IH uses Rockford double ball nuts ..I used .0002 and .0003 oversize, every other one. This means that you are not tensioning the following nut so tight to eliminate backlash that it causes u..m..p ... on acceleration.You really can get down to .0003-.0005 back lash with out a lot of ball screw friction.

Have you bought a mill yet ?

Adobe (old as dirt )

wildcat
11-06-2006, 10:30 PM
Don't the forces that are being considered in ballnut choice have to deal with machining forces?

I went the recommended route figuring the .002 backlash is going to be pretty small compared to the contours encountered during 3D milling.

JRouche
11-07-2006, 10:18 PM
The pre-loaded ballnuts use a belleville washer spring pack to load the nut. The spring is rated at a certain load.

Once the load rating is passed (weight of the head or accelerating/decelerating forces) the nut halves (spring pack between the two halves) move closer together (or apart) causing backlash. JRouche

kimoyo
11-08-2006, 12:38 PM
Adobe - I decided to go ahead with the IH mill and kit.

Wildcat - I wasn't even thinking about 3D milling which would be harder than the flat sheet milling I was planning on doing. I'm just trying to get the best performance from the kit.

JROuch - Thanks.

I'm still not sure what the best thing to do is.

If the 250lb weight of the head could be compensated (and cantilevering helped) for by the gas springs could the preloaded ballnuts be an good option?

If I get the double circuit and backlash was an issue, I believe Adobe is saying I could reload the nut with oversize balls and push the backlash down even further.

Thanks.

kimoyo
11-08-2006, 07:32 PM
I had a very good conversation with Aaron today where he explained to me whats going on.

The issue with preloaded nuts isn't the weight of the head. The reason people install the gas springs is when their motors are too weak to lift the head which is not an issue with the IH kit. The issue is when machining thick amounts of material you may WANT more than 600lbs (the load limit of the spring) of force pushing down. So if you want more than 600lbs pushing down the heavy head actually helps, lol.

He also explained that the backlash could be anywhere from 0 to 0.002 but that he always puts the outer rating on his website. So my backlash might be much less.

Adobe - Thanks for you help and suggestion. Aaron actually does use oversized balls :) but I'm not sure if its in your configuration.