Thursday, February 5, 2009

Bearing mounts

Last night I made the two new bearing mounts. These are brackets that hold the angular contact bearings for the ACME rod. The task was to bore a 16mm hole for the rod, and then a blind 30mm hole for the bearing. The 16mm through hole had no real accuracy considerations, but the 30mm needed to be 29.99mm so as to ensure that the 30.00mm bearing were a press fit.

I had originally planned to do this using the boring bar, but on reflection realized that that would make it difficult to do the accurate flat base required to support the bearing.

So I got out the rotary table. Now normally the rotary table is a pain in the butt to use. Partly because it's difficult to clamp things on my relatively small table, but mostly because I'm terribly slow at indicating the part to be aligned with the table center. I'm just awful; it normally takes me more than an hour to get terrible accuracy. And thus I avoid it like the plague.

My brain wave was to use the lathe to made a trivial jig; The table has a 17.50mm hole in the center, and my part already had a milled 16mm hole in the center. So I made a piece of delrin (plastic) that was just a short section of 17.5mm dia, stepping down to 16mm dia.

A few taps with the mallet inserted it into the table, and then the blanks were a press fit onto the 16mm section. Instant alignment. Yay!

Even better, this made clamping much easier as I just needed to clamp at one point. Much win all around.

Actually milling the blind 30mm hole was then pretty simple. Just offset a 12mm end mill 8.950mm from the center, mill 5mm into the blank, and run the table around a full 360 degrees. Easy.

The slow bit was getting the hole to be a solid press fit for the bearings, and 29.950mm is much too small. So I increased the hole size in 10 micron increment thru to 29.990. At which point the bearings went in with about 200 newtons of force. Perfect!

I think this is some of the more accurate milling I've done. I'm pretty happy with how it turned out.

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