Moving bed - bearing spacing?

[Snapper]

Hello chaps.

I have a 6040 style Chinese router with 20mm profile rails and ballscrews. Just having a thought here but thinking if it would be possible to squeeze an extra bit of Y travel out of reworking the frame, flipping the rails over on to the bed and having the bed move. I was also thinking this would open up the opportunity to have a bit of cross support under the bed to firm things up a little and help prevent sagging..

It all depends how far apart the bearings need to be spaced as to whether it's viable or not. I'm thinking the bearings could come quite some way inwards as the gantry would be fixed, and the far ends of the bed could be supported by some sort of roller system rather than just having a big overhang.

[Clive S]

. Just having a thought here but thinking if it would be possible to squeeze an extra bit of Y travel out of reworking the frame, flipping the rails over on to the bed and having the bed move. I was also thinking this would open up the opportunity to have a bit of cross support under the bed to firm things up a little and help prevent sagging..

I think I remember you saying you was going to draw it out in CAD !! As that would be the best way for a feasibility study

[Snapper]

Not me, not this! I only had this little brainwave last night. It's a short lived one too, because I failed to take into account that I would need to subtract the bearing spacement off each end of the bed. If I kept the bearings sandwiched together I could still gain 50mm travel, but viability aside that's a lot of faff for 50mm!

It was late when I wrote the post and still early when I unlocked my phone and realised I'd fell asleep before I'd finished writing it. A well excused brain fart I'd say.

[Clive S]

Not me, not this! I only had this little brainwave last night.

Was this not tthe same machine:-

http://www.mycncuk.com/threads/10618...8261#post88261

[Snapper]

That's a long memory. I've now had umpteen ideas for this, so I'm not sure which particular one I was talking about in that thread.

But for the record, I'm thinking of trying what zee is doing on his build, I saw the idea on some US CNC parts site. If anyone has tried that technique with the tramming plate I'd be most interested to hear how well it works. I'm starting to toy with the idea of building a new frame and gantry for my machine (which was the basis of the idea in this thread), which I'd probably incorporate it into.

[Snapper]

So... after a bit of thought I'm going to revisit this idea, come up with a design, look at the advantages it gives me then put a cost on it and see how I feel about it.

I'm going to work around all the stuff I already have, as I have a full machine with 20mm profile rails, 1605 ballscrews, and try and come up with a better frame and gantry assembly to add rigidity.

I mapped out rebuilding the machine in its current format, just with using far beefier extrusions than the floppy thin things it's currently made of (the gantry beam is 20mm thick t-slot extrusion, 15mm gantry arms). I realised I'm going to lose work area anyway and like the idea of having a moving bed and the freedom to add all sorts of bracing to the gantry so I'm going to look at it properly.

I already have a solid 725 x 480 x 20mm bed made from aluminium tooling plate. The rails I have for it are 700mm long 20mm hiwin rails, and 4 HSR20A blocks. I don't know whether to bolt the rails directly to the bed or build a frame under the bed with the rails bolted under the bed frame. Bearing spacing is something I have no idea about. Should they sit at the edges in the width or maybe a third of the way in? Then in the length have a 150/200mm gap between the carriages which are 74mm long, giving a travel of 350/400mm. This is obviously where moving the gantry or the bed will be decided.

The other thing I have to decide before thinking more deeply about it and coming up with a plan is construction material. The tools I have at my disposal are

6x4 bandsaw
Angle grinder
Drill press
Nuts and bolts
Current CNC router

I can get a mate to do any welding but he reckons it will be difficult to get everything square and straight and will likely twist a bit during welding. How do you folks who have built steel frames manage to get everything properly square, and how do you manage to get a flat and coplanar surface to mount the rails on? General lack of experience working with steel is also going to make things more challenging. I can foresee costs escalating sharply and rapidly as I end up having to send stuff out for professional machining/correction.

Aluminium profile, whilst weaker and the base material more expensive, is easy, I have plenty of experience using it, I have my current machine that I can use to mill flat keys in for the linear rails. If I go with the fixed gantry I have the freedoms to belt and brace it to my hearts content. The cost would be purely material cost and any processing I can do by myself. I would probably go with heavy gauge 90x90 Bosch or 80x80 ITM profile.

The other option is just rebuild the current design using 80x80 and 25mm end plates/gantry arms etc which I priced up coming in at under a very reasonable £300 including fasteners and any extra tooling I would need (drill jigs).

The main goal is to get a better surface finish from cutting aluminium. Faster feeds and bigger cut engagement would be more of a (much welcome) added bonus.