Newby Height gauge query

Using a surface plate, I've just checked my 12" high height gauge against
gauge blocks, with similar results for different gauge blocks.
What I've noticed is that the machined measurement surface of the height
gauge arm isn't parallel to the surface plate. (It's around 0.010" higher at
the tip as compared to its height at a position 1.5" closer to the column.)
The micrometer has (standard?) 0.0005" increments, so am I right in
expecting something better than this?
If the height gauge column is tilted with reference to the base, would I be
able to check for this by sighting against a precision engineer's square?
(Which I don't have, as yet.)
I can't see any signs of damage to the height gauge - it's an APE Microball
type, and was supplied in its fitted case.
Many thanks for any advice.
Reply to
Malcolm Stewart
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It doesn't take much to ding the base of the gauge, and make it sit crooked. You may need to stone down a ding on the base, or rescrape the base. You might be able to use a good square to check how vertical the column is. I have a set of machinist squares, and was able to check them when I hand scraped in a set of 3 precision squares. Well, those machinist squares are not all that precise, apparently. I don't have a reading on the error, but it looks like a substantial part of a degree, ie. easily detected by backlighting the gap between the ground square and the scraped one.
Jon
Reply to
Jon Elson
Jon Thanks for your reply. Very early this morning, I tried laying the height gauge column horizontally on the surface plate, and found it was bowed. The bow was such that it might account for some of the tilt I'd found. Close to the base, I found using my gauge blocks as squares (?) that the column appeared to be square to its base. I'm hoping to purchase a couple of squares later this morning. I assume that with two, I can check them against each other. Will then have to decide how to straighten the column!
Reply to
Malcolm Stewart
You can't. The way metal bends, it stretches more easily than it compresses, so that things get longer when they are bent. Straightening them leaves them stretched, in most cases. It wouldn't be a big deal except in a measuring instrument. It might be possible to get it straight, although this can be real difficult. But, the accuracy has almost certainly been badly affected, and I doubt you can fix that.
A sad case, for sure.
Jon
Reply to
Jon Elson
Hi Jon, All may not be lost. The height gauge is an APE Microball type in which the coarse height is set, every 0.5", by clamping a shaped probe between the gaps formed by a column of 0.5" steel balls contained in what looks like an aluminium outer casing. There's a photo of one, same size and very similar, here
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coarse settings are on the other side of the column. (Mine cost very much less than the one in the advert - which could explain the problems; but I suspect the vendor sold in good faith.)
So, if I do manage to remove the bow, the length will be set by the steel balls rather than the aluminium casing. I assume that the balls must be under spring pressure as steel and aluminium expand at different rates. Near the base, the column does appear to be at 90deg to the base checked against a pair of engineer's precision squares. At the moment, using shims I've managed to adjust the slope on the underside of the gauging arm and it's now essentially parallel to my gauge blocks from 0 to 80mm, with ~ 0.25mm zero offset. Above that there's obvious tilt, and it gets worse as I stack the gauge blocks. So lots to keep me occupied after the holiday. Thanks for your advice.
Reply to
Malcolm Stewart

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