Thoughts on lining up round rails..

As documented elsewhere I am building a small CNC mill (250mm x 250mm x 100mm working area). After much deliberation I decided to use MDF for this first attempt as its a 'cheap' trial activity. So far I have created a 500mm x 500mmx 50mm base and mounted the two 16mm rails on it. I have tried to ensure they are parallel and my plan to line them up was to:
- choose one as the reference and ensure its parallel to the edge of the base, which I have done within about .2mm then bolt the mounts down tight;
- bolt a piece of scrap mdf across the bearings and run this up and down the rails tightening the mounts on the second rail gradually until they are tight and the joined up bearings run freely end to end.

The problem I have is with step 2.. as soon as I tighten the bolts onto the bearings they start to make binding noises, even if the second rail is completely loose to move perpendicular to the rails they bind. Is it possible that the bearing block (SC16UU/SMA16U) distort and cause the balls to misalign internally? If so I can't find a point where all the bolts in the bearing are tight and its not binding :( when the two bearing are connected. Individually they run fine...

Any thoughts?

I'd very much doubt that the housings are deforming and causing the bearings to bind.

I would think that when you're tightening the bearings onto the MDF, they are twisting some way causing the problems.
How are you tightening the bearings onto the MDF?
Have you tried tightening the bolts up in an alternating pattern (ie. top right, bottom left, bottom right, top left), and only tightening each bolt up a fraction (say an eight of a turn) at a time?

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Originally Posted by m_c

I'd very much doubt that the housings are deforming and causing the bearings to bind.

I would think that when you're tightening the bearings onto the MDF, they are twisting some way causing the problems.
How are you tightening the bearings onto the MDF?
Have you tried tightening the bolts up in an alternating pattern (ie. top right, bottom left, bottom right, top left), and only tightening each bolt up a fraction (say an eight of a turn) at a time?

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My thoughts too... and yes I did, I know all about cross-tightening. The weird thing is i can tighten one bearing down fine - its only when i tighten the second.

At the moment i have just got 6mm holes through the MDF and 5mm x 30mm cap head socket screws with washers under the head so plenty of room to allow for movement as they tighten down... I expected them to run freely untightened but as I tighten them I run them back and forth a few times to align then tighten a little more... yet every time it starts to bind once I get past a certain point thats nowhere near tight :(

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Originally Posted by irving2008

My thoughts too... and yes I did, I know all about cross-tightening. The weird thing is i can tighten one bearing down fine - its only when i tighten the second.

At the moment i have just got 6mm holes through the MDF and 5mm x 30mm cap head socket screws with washers under the head so plenty of room to allow for movement as they tighten down... I expected them to run freely untightened but as I tighten them I run them back and forth a few times to align then tighten a little more... yet every time it starts to bind once I get past a certain point thats nowhere near tight :(

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Any chance of a photo Irving?

Your first message described exactly how I set up my rails and I can push my 100lb+ gantry with my little finger. Sounds like one of your bearings is skewed in some way.

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Originally Posted by Smiler

Any chance of a photo Irving?

Your first message described exactly how I set up my rails and I can push my 100lb+ gantry with my little finger. Sounds like one of your bearings is skewed in some way.

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Well if it is i can't figure which... here's a shot of the base with the rails mounted - no point in one with the table cos you cant see anything... not that you can glean much from this... except how messy a worker I am :( :D

I'll have another go tomorrow...

Only things that come to my mind are:

1. one of the bearings is machined out of true in some way, very, very unlikely.

2, the board you are using either as a table or an alignment tool is not perfectly flat and has a twist somewhere on the face, that would be favourite.

It's partly because of theory 2 that I decided to go with PVC plate rather than MDF as I've found it warps in the damp atmosphere here. Not had any alignment problems at all.

Does it happen regardless of which bearing is the last to be tightened i.e. can you tighten three and it locks on the fourth? It doesn't take much of a twist to lock up a bearing tight and it may not easily be visible.

Smiler... you hit the nail on the head... the bit of scrap I was using wasn't MDF but some 19mm ply offcut left over from when i made my workbench. And it had warped over the last 6mo stored on end in the garage. Not much, maybe 1.5mm curve over a 500mm length but enough.

Anyway, I took the plunge and drilled the actual table and fitted it. Works a treat, so all tightened up and runs smooth, although these linear bearings aren't as silent as I'd imagined they'd be. The table runs parallel to 0.1mm and horizontal to 0.05mm over its run, and deflects 0.15mm with 4.5kg load in the centre so I think that's pretty good.

I don't like MDF tho, drop a bit and it bruises so easily, look it it wrong and it flakes. I definitely will make machine #2 with ally or steel...

Right I'm going back to the garage to start on figuring out how the leadscrew is going to be mounted. I have an idea on that, but that's going in another thread.