Here we go again . . . MK4

[routercnc]

Second beam cut to length using the first beam as a guide:


Decided to use the CNC machine to drill the holes. Here is one of the beams clamped down and being dialled in. Obviously the edge is not very straight but I wanted to get it fairly close:


6 holes at each end drilled and then milled to 6.8mm, plus a 6mm dowel pin hole at the far end:


Then onto the tapping to M8:

Yes I know, ended up with holes a bit close to the edge - they were covered by another part in CAD and did not realised I put them so close to the edge. Will still work, but could have been better.

Then a trial fit. Note that only the front beam has holes in, and these are only in one side. Still lots to do on these bits. It all fits and aligns well but as I suspected would happen when the bolts are tightened it lifts/rotates one end of the gantry about 0.5mm from the flat plane. This is because the RHS is not perfectly flat along the lengths and so the ends are forced to align with the sides of the RHS.


I intend to machine just enough on each edge of each face to get it all square. This could be just the outer 3-6mm or so as I'm hoping the raised bits are on the edge / corners so just knocking these back a bit will create surfaces which are in plane with each other and square. I think that when I tighten the bolts the membrane flat parts will pull up tight to these new surfaces on each edge and everything will be square again.
I don't want to machine to whole surface as this will make the part quite a bit smaller. We'll see how it goes and then I'll repeat on the other one. Maybe should have done this before the holes . . .
but until it was pulled up tight I didnt know how much it would move.

Anyway then I'll take off the triangular corner plate at the far end and machine in a matching dowel pin hole which will allow me to get the ends the right distance apart from each other.

Actually on this note I noticed that the 6mm end mill I used to drill the 6mm dowel pin hole in the RHS actually made a slightly larger hole. Putting a 6mm shank in there showed it to be a loose fit. I suspect the runout in the chinese spindle may be adding 0.05mm all round so 0.1mm larger diameter? If so I'll use a smaller end mill and interpolate the hole but creep up on the fit.