Cant find "loose" linear rail for floating head

[Beefy]

Hello Everyone,

I've wasted nearly $300 on two different types of linear rail for the floating head part of my Z-axis (torch height control).

First linear bearing was IGUS which is a hard anodized aluminium rail and the bearing block has plastic sliding bearings which are adjustable. Problem is when you adjust the bearings enough to take up any play they are way too tight.

So cut my losses and ordered the typical steel rail and rectangular block. No play in this one but the bearing pre-load makes this one stiff too. Now I'm cutting my losses for a 2nd time - ouch.

Went to a linear bearing supplier to physically check before buying this time. However the seller could not find any type where the bearing would fall under its own weight. All were quite "sticky" due mainly to bearing pre-load, not the wiper seals or the grease.

When touching off on thin metal I need a bearing which will basically will drop under its own weight (plus the weight of the cutting torch). Tom Caudle from Candcnc suggests a counteracting spring to almost pull the torch and bearing back up thereby making the touch of mechanism almost weightless, and so not pushing thin metal down when touching off. This is what I want to do but this requires a bearing which doesn't stick. There are the round bar bearings which seem to fall freely but these require 2 round bar rails to be set up with associated frame.

Anybody had any success with what I am trying to achieve. Is there a particular brand and model of block which is free enough to drop under its own weight.

Thanks,

Keith.

[mactec54]

Beefy

You can get the linear bearing with no preload, You could also take some of the balls out of the bearings you have, & they will slide free,if you need to take out a lot of balls to get them free, then put a smaller ball every other one

[Beefy]

Mactec that is good information. The guy at the linear bearing shop either doesn't know that or he didn't think to mention it. I'll give them a call to see if they know about that one.

Cheers, Keith.

[mactec54]

Beefy

They most likely never heard of that being done before, we did this for a plasma machine to,but was not on the Z axes, the X axes had some heavy preload on the linear rail bearings,which was way to much, for the small motor they had driving it, so did it on that, It worked very well & they were able to still use there small motor to drive the gantry

[Beefy]

I think you are correct, they have never heard of that. I called the shop back after I read your post and the guy said that the bearings start off with pre-load as standard then only go up in pre-load at the customers request.

I never know what to believe here in Australia. When it comes to technical matters there seems to be so much "you can't do that" thrown at me, yet in many cases I may have already done it or know for a fact it can be done.

A fellow CNC enthusiast who works in a bearing shop here said that they can't find ball bearings outside the standard size increments, yet he told me that on cnczone it has been discussed about re-balling bearing blocks with balls which are a tiny bit smaller. Apparently it does get done.

I also asked the question of re-balling to the linear bearing shop here and again all I got was, "Oh you wouldn't want to try that yourself".

Thank God for Cnczone and the knowledgeable member like yourself helping out.

Anyway I've picked up a 2nd hand smaller linear bearing on Ebay. The seller reckons it does drop under its own weight so if that is true it should be OK when I mount the torch and holder on it - fingers crossed.

A member of the Candcnc Yahoo group suggest an air slide / bearing too. Said there's almost no drag. I have no idea of the construction, weight or whatever of one of those but it's definitely another thing I'll keep in my construction parts list.

Cheers,

Keith.

[mactec54]

Beefy

It's done everyday, this is how they load the ball-nuts on lots of ballscrews, some of the THK linear rail bearings, have a plastic spacer/cage between the balls in there linear bearings

[flyinwilly]

Beefy:

I had similar problems when I built my table so I designed my own. It works great with very little side play and is very inexpensive and easy to build. See my build log. It is on post #32.

New Machine Build- A Different Machine - Downdraft/water table

Good luck.

Willy