I kind of thought that, but those of us who care enough can see your full detail pictures and you look to be a bit busy with something more important than how big your forum images look!Quote:
Originally Posted by devmonkey
Kit
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I kind of thought that, but those of us who care enough can see your full detail pictures and you look to be a bit busy with something more important than how big your forum images look!Quote:
Originally Posted by devmonkey
Kit
Gantry extrusion has arrived from KJN, it is a real beast. KJN cutting appears very accurate.
Attachment 26902 Attachment 26903
So now I need to decide how to mount 15mm rail to the extrusion. I was considering 3 options:
1. Mount rail directly to extrusion between the two slots, drilling and tapping.
2. Mount rail on aluminium plate which is in turn bolted to the slots.
3. Mount rail directly to slot.
This shows that the profile is not flat across the width which I believe is by design so that the slots pull up to meet whatever you are clamping to them. The slot edges are 0.15mm lower than the centre, the centre and the 2 corners are perfectly aligned within the resolution I can measure with finest feeler gauge.
Attachment 26905
Now I have the extrusion (1) is a non-starter as there is a very slight peak in the centre and I would need to mill it flat which is hard given the gantry is longer than my mill table, although not impossible.
(2) is what many people seem to do and so obviously works but is more work for me.
Can I get away with (3), see picture below, what do you think?
Attachment 26904
I have a 50mm fly cutter, it would be possible given the extrusion is already flat and parallel to fly cut between the high points, would have to tram the mill carefully first.
That's serious looking profile! What total length did you get for your £340.24?
Kit
I wasn't that expensive, think it was about £130 plus vat for 1100mm.
Looking at your pic with the straight edge it looks to me that the slope on the inside of the slot is slightly less steep than the slope from the slot to the outer edge. This would mean that your rail would be slightly canted if you go for fixing option 3).
You have very keen eyes! Yes at the slot the extrusion is 0.15mm low both sides, since the outside flat is shorter than the centre flat it must be at a slightly steeper angle. I have measured and calculated the cant angle the 15mm profile would have as 0.055 degrees or 198 arcseconds, does this matter (it will be mirrored on the second rail).
Or should I just man up and hit it with this:
Attachment 26908
That's not a lot of angle to be true, and when tightened up it might straighten it a bit more. On the other hand facing off the central flat and drilling holes would give a firmer fix I suspect as you're going into continuous metal, rather than a slot - and as Jazz pointed out with HGR15 there's not a lot of contact either side of an 8mm slot to give friction.
I temporarily bolted a length of rail to the slot, it seems to mount extremely firmly and doesn't move (maybe helped by the very slight V angle it is sitting in) but must be slightly canted as discussed above. Will decide whether to face mill it tomorrow, another option is just to mill a few mm each side of the slot so the rail can sit flat, this would also preserve the 3 planar points (both corners and centre) in case I mess up.
I initially wanted to fix the rail to the centre however this does increase the moment between the Y bearing and the Z plate by 20mm which may counter the increased stiffness from the firmer rail mounting.Then again the centre of this extrusion will be stiffer than either slot as it i supported equally whereas the slots are supported on one side by the unsupported corner. No idea really.
In the mean time I spent at least an hour tapping the ends:
Attachment 26915 Attachment 26916
If I was living in a densely populated part of the UK I'd be asking if any fellow forum member had a nice flat machine that could mill a shallow slot just wide enough for the rails to fit along the full length of the centre of the profile. Possibly in exchange for HobNobs. That looks to be by far the most rigid place to put them.
One of my earlier machines used a heavy gauge extrusion with 2 slots and I used the slot closest to the spindle rather than go into the centre. I think it was slightly crowned across the width as well.
I used a full length steel insert inside the slot (a long thin bar with the edges chamfered) to thread into rather than lots of small t- nuts and this helped pull it flat at least locally.
There probably was an overall angle to the rail but I also had a similar rail on the underside surface which would slope the other way. When these were connected to Y axis assembly it must have preloaded the bearings slightly trying to make them parallel to each other. I didn’t notice any issues and the machine ran for years. You can check on the Hiwin site what misalignment is allowed and compare to your angle.