This is a proof of concept model. I was not so keen on the performance of the last Z axis I made, so I decided to try this. So far this seems to slide well, and is rigid. Instead of those plastic end caps I will make the final version with steel angle ends. The plastic was just to check my dimensions, and hold it together temporarily.
Nice design! Do you have (or need) any form of adjustment on the bearing positions?
@Barnaby_Relph
I made these temporary end caps to check the fit of everything. I also took care making the truck plates to make sure they were very precise. If you look at the top plate you can see there is a pattern in the material. That pattern has to run perpendicular to the tubes with both plates for everything to fit together right.
Or at least that’s the theory. I never actually tried the plates perpendicular to each other. All of that has to do with how I fixtured the plates to drill them out. I used a corner fixture on my mill. Then I drilled the diagonal holes, and flipped the plate over, and drilled the other diagonal pair, on each plate. That maintained alignment of the plates.
There’s a good chance I did not have the hole the same offset from each leg of the corner fixture when I set it up. But by drilling opposite corners, and flipping it over I canceled that error out. Or distributed the error symmetrically, depending on how you want to look at it.
I don’t know if that made any sense. Here, I’ll draw a picture. http://i.imgur.com/u69HpgU.jpg
Hole 1 gets drilled, then 2 gets drilled. Then the plate gets flipped over and the other diagonal pair of holes gets drilled. That way parallel sides of the plate are always stopped by the same fixture legs. X edges to holes, and Y edges to holes don’t get mixed up with each other. As X edges are always against the green stop. and Y is against the blue stop.
I just know that no matter now hard I try I can never put a hole exactly where I want it. I can get it damn close, but it is invariably a couple thousands out one way, or another. This diagonal flipping eliminates that.
I got to say, it appears to have worked too.
Wow! Thanks for the write-up and diagram! I had to have a think about what you meant, but I’ve got it now. That’s a very nice solution! My holes are never where I want them either, so I’ll remember that for the future.
@Barnaby_Relph
yeah I punched the first hole, and lined it up with a centering spud. Basically a pointy metal rod I chuck up in my mill. Then I went ahead and center drill spotted everything. Just to make sure no twist drill wandered on me. I put a little pilot hole in everything, then drilled it all to the final size. Nothing to it really.
Cool drilling trick.
@marmil
Yes I have to come up with a different trick for drilling the metal end caps. I think I know how I want to do it. The plastic ends were just to work out the dimensions. There’s some weird geometry going on with how the tubes nest in the bearings. So I measured it as best as I could and made the plastic ends just to check the fit. I think the plastic is a bit tight, so in the metal I am going to make it slightly looser. Just a couple thousandths of an inch. With the plastic I cheated. I drilled the pair of holes, then ripped the plastic in half on a table saw. So the holes were exactly the same distance apart. With metal I don’t have that option. I just have to get it right.
I guess in this design, it’s either the spacing between the rods, or the spacing between the bearings that determines the fit. Would it be possible to use something with a little give as the spacer between bearings, to allow adjustment? Perhaps make the spacer from car fuel pipe, so a largeish chunk of rubber. As you tighten the bolts, the rubber compresses slightly and allows the bearings closer to the rods they run on. definitely not as good as getting the dimensions dead on, but might allow for wear compensation?
@Barnaby_Relph
I can just spin the tubes if there is any wear. Plus I have more of these tubes. So I am not too concerned about wear. There are mechanisms for making adjustments. But I’d rather keep the build simple. If I oversize the holes there will be enough slop to adjust things anyways.