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HBP Rod Alignment

Having salvaged the sliding bearings from the ABP, I built up the HBP, stuck it into the Thing-O-Matic, and wasn’t in the least surprised to discover that it was mechanically jammed solid on the rods. This time, however, I wanted to measure the actual rod (mis)alignment to see what was going on.

Remember that, as described there, the X stage overconstrains the rods by forcing them through four bearings. It would be much better to use a pair of sliding bearings on one rod and a set of ball bearings rolling on the other, much as MBI did with the Y stage. Something involving 603 or 693 bearings, perhaps… there’s a scant 12 mm clearance from the top of the rod to the bottom of the HBP.

So I laid it upside down on the surface plate and tickled it with a scribe mounted in a surface gage. Of course, I’m doing it all wrong, but the results are close enough. What you can’t see are the two half-inch chrome-steel lathe bits supporting the platform; it may be warped a bit, but that’s part of what’s being measured.

Measuring HBP rod misalignment

Measuring HBP rod misalignment

I adjusted the scribe to just kiss the slip of waxed paper (0.02 mm) atop each end of each rod, which turns out to be surprisingly easy to do by feel.

Measuring rod height

Measuring rod height

Then eyeball the result on a scale.

Rod height on scale

Rod height on scale

One should tweak the surface gage until a dial test indicator reads zero, then stack up gage blocks to the same height. I actually hauled out my box o’ blocks before I came to my senses.

Anyhow.

The far rod was spot-on level and the front rod was off by 1 mm from one end to the other. They were within 0.2 mm of equal spacing horizontally, which was somewhat surprising given the amount of side sanding required to fit the ABP into Y stage.

Height Right Center Left
Back 42.6 42.7 42.7
Front 43.5 43 42.5
Separation 39.93 39.72

I sanded out one of the holes, laid a bead of expanding urethane adhesive around the bearing housing, slid it into place, and then held the rod level with the tip of the scribe. The two random cylinders held the rods in the proper horizontal alignment.

Setting HBP rod alignment

Setting HBP rod alignment

When the glue cured, the rods were basically dead parallel in both planes.

Note the nuts epoxied on the bottom surface. That’s part of the cough precision platform alignment system…

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