What is the best way to mark out and drill holes for bearing blocks and linear rails. I was thinking about printing a drawing from cad to scale and then transfering the hole centres with a centre punch and then go from there?
Great got a new set of starrett punches coming in the mail can't wait to try them out. Do you normally drill and tap the mounting holes or use a nut plate/ standard nut and washer from behind. I guess it depends a lot on the thickness of the steel you are mounting too?
Just a word of warning - double check and measure off the print before you punch... I eventually had my plates drilled at a shop but used a 1:1 print out to check the work against. Fair enough - the prints had been kicking about in the damp-ish boot of my car, but that was enough to warp/stretch the paper and introduce some alarming deviations - I thought the machine shop had ruined my plates before I realised what had happened... In theory and as Clive says, the method should work perfectly, just don't take it for granted that the paper's going to remain stable over time. Oh, and don't forget to tape it down..!
+1 on Mike's suggestion of using the rail as a jig and using a transfer punch through the existing mounting holes.
I used M6 threads into 60x60x5 square section to hold my plates.
A fair few of 'em, but I found no need to use backing - as Clive says, it would have been hugely impractical to do so...
Hiwin type rails have one side ground as a datum side, same with the bearing blocks. So I laid a straight edge along where the rails would sit and clamped it at each end, then the rail was clamped to the straight edge using the datum side. I used a drill the same diameter as the holes in the rail to spot drill the underlying metal giving a dimple the same as you would get from a centre punch, then I drilled the dimpled holes with a tapping size drill right through. Becaue my X rails were mounted on 3mm thick box section I glued a piece of 30x6mm flat bar inside using Gorilla glue, the Y rails were mounted on aluminium that was 6mm thick so was okay on it's own. Bearing blocks were marked out as you say with a printout, I counterbored the aluminium to make the cap head screws flush, you need a bit of adjustment for the bearing blocks so the mounting holes don't need to be a very neat fit. I finally tightened the bearing blocks down once they were mounted on the rail but first made sure the datum faces were on the same side so I could also use a straight edge along them, i.e. 2 blocks per rail.
Last edited by EddyCurrent; 14-05-2014 at 06:45 PM.Spelling mistakes are not intentional, I only seem to see them some time after I've posted
For mounting my supported rails I clamped them in place accurately, then used a centre punch to very lightly mark the centre as a best guess. You can't see if it is central until you remove it, so I then looked straight down the hole, and made subtle adjustments with the punch to 'push' the hole around a bit. When happy I gave it a big hit ensuring that the punch was very vertical. Then double checked it and moved on. Finished by drilling the pilot and tapping the hole. Took along time but when it's done it's done.
In terms of thread engagement I've always used up to 1.5D as maximum depth as you get rapidly diminishing returns in terms of thread load for the deepest threads. So M8 has about 12mm of thread. Not detrimental to go over, but not helping either.
In terms of minimum engagement I must admit that I've always used judgement, but out of interest I had a look at Roy's site (DIYCNC) and found the following. Lots of maths to get stuck into but the upshot in minimum thread length appears to be:
M3 = 2.4mm
M5 = 4.0mm
M8 = 6.5mm
These are for bolts and holes of the same material. Steel bolts into aluminium holes are supposed to be deeper.
Link : Screw thread Calculations
Scroll right to the bottom and find the row 'Length of thread' and the symbol Le. There's an approximate calc, and a more accurate calc.
My only experience of doing this was a 3d printer but it seemed to work well.
Basically, get one rail mounted square to the frame (clamp firmly and drill using a bush as Mike suggested) and then loose clamp the other rail and fit the gantry (or whatever the bearings are attached to). If anything moves as you slide the gantry up and down then somethings out of square and needs adjusting. When it's all sorted, clamp down properly and drill using a bush as before (with the gantry near the place you're drilling to stop it moving on the clamps).
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