Shrinking the bearings

The first task I have given myself is to figure out how to shrink the carriages, and therefore there is a need to shrink the internals.

Bearings
The standard bearing is a skateboard bearing, 8mm inside bore, 22mm outside diameter, 7mm width. The code is 608zz.

6 is the bearing type (single row deep groove)
0 is the material strength (extra light),
8 is the inside bore, and
zz means both sides are enclosed.

By the looks of it, these skateboard bearings are as small as they can get without compromising on strength. The best I can see is a 7mm internal bore, but because M7 is then a peculiar size for the bolts it would make a huge mess for the sake of a 1mm saving. They're also a narrower bearing which probably wouldn't be good. So the bearings cannot be shrunk.

Bolts
The standard bolts here are M8. This seems to basically line up with the bearing internal diameter. Unfortunately as described above there is no option of reducing the size of the internal diameter, so the bolts also need to stay at M8. I think this is a good thing though, the next size down would be M6 and I wasn't comfortable with an M6 bolt. So no shrinking here either.

Motor
This is an interesting one. Basically the motor can be any size as it sits outside of the carriage. Therefore I might as well stay with the NEMA 17 or 23 motors.

Belt
Erm, no. I don't want this shrunk. It's only 10mm already, it would have to be reduced quite considerably to actually make a saving.

Frame
At last, something that can be shrunk!! The standard frame is 40mm x 40mm. The biggest steel I have is 25mm by 25mm. Therefore it makes sense to shrink it down to 25mm.... or does it?

I believe one of the reasons for the frame size is to reduce rotational errors. This comes back to an earlier post where I said the bearings ride near the edges of the frame. If they were in the middle, there wouldn't be anything stopping the whole carriage rotating on the frame. The same applies here, if I bring the bearings in to ride on 25mm square section, I'm losing 15mm of twist resistance. OK, I'm simplifying things but it makes sense in my head.

But... there can't be rotational forces on the Y movement, because the gantry and the two motors prevent it. So in this case I could have the bearings centrally mounted and nothing would happen. Therefore I can use 25mm square section and it should be fine. I'll have to revisit the X movement maybe, but I can do that after.

Conclusion
Well, the conclusion is that it's not going to come down by much!! Each direction inside the carriage needs to accommodate a 22mm bearing, 25mm steel, then another 22mm bearing. 1mm either side for clearance, and 3mm thick steel. This gives a total of 77mm. I could use 80mm steel if that was it, but it's not. I need to add 13mm for the belt drive (10mm belt, 1mm shoulders, and 1mm clearance). So the one direction would need to be 90mm. And of course, it would have to be 90mm square as it's almost impossible to get 80 x 90.

I think this ties in with what I expected, I'm reducing the frame by 15mm so I wasn't going to gain much more on the carriages. I'll go for 90mm square section.

Then all it leaves is the length of the carriage, currently 130mm. I'll have a bit more of a think about that one...