# surface gauge - why flat base?

In article
wrote:

Doesn't work for me with a pub table, all that happens is I get the "what's he up to now?" look from the landlord. So back to the tried and tested folded beer mat that the bloody cleaners throw away each night. Grrrrrr!
Regards,
David P.
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On Sun, 06 Jul 2008 18:24:08 +0100, David Powell

So do the landlord a favour and install leveling feet on your table - different table every visit, the people will really wonder what's up with you. Gerry :-)} London, Canada
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So you have a non standard wavy floor. It's what I believe the mathematicians would call 'ill conditioned' Get the landlord to replace it with a better one.
Henry
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On Mon, 07 Jul 2008 15:47:11 +0100, Dragon wrote:

Any continuous-surface-floor (no jumps, steps, holes) would be ok, if the ends of the legs are coplanar. Most likely one of the pub table legs is shorter than the others, by about the thickness of a folded beer mat.
-jiw
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Because someone took the leveling foot home for their own use. Gerry :-)} London, Canada
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John Martin wrote:

My curious mind got to wondering when I first got involved with this whole silly musing about four legged stools and wavy floors.
Looks like it was over 12 years ago, and this link provides a grandios explanation of why what I said really does work:
http://mathforum.org/kb/message.jspa?messageID 83862&tstart=0
Happy Independance Day to all.
Jeff (Who can't recall any other US holidays referred to by their dates instead of their given names. He doesn't think that the shipment of mayonaise on the Titantic <Cinco de Mayo> is a US holiday yet.)
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
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Very cool. I like your logic much better than mine. :)
Even without the constraint of the rotation, it's (now) easy to see that if you keep three legs on a wavy floor and move it around randomly, you will always be able to find a spot where the the forth leg would hit the floor making all 4 touch at that point. The rotation idea is simple proof that there there must always be at least one location where that was true (assuming the chair legs were correctly aligned on a plane).
--
Curt Welch http://CurtWelch.Com /
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On 03 Jul 2008 20:19:38 GMT, snipped-for-privacy@kcwc.com (Curt Welch) wrote:

There is of course a "flaw" in this proof (pun intended)...it assumes that the available floor space is big enough for all 4 legs of a stool to be on it at the same time. You'd have a hard time proving it in my workshop <G>
Regards, Tony
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bugbear wrote:

Because they ARE carefully made truly flat, often by hand scraping. If yours isn't, you can scrape it in quite easily once you have a surface plate.

If you have a big square base, you can carve away some of the middle and 2 corners to achieve a 3-point base. But, make each spot at least a cm square so it won't wear too quickly.
Jon
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Jon Elson wrote:

Indeed.
I'm not questioning wether a (truly)flat base works, or can be made to work.
I was trying to explore whether an alternative design, with lower production costs, can also be made to work
BugBear
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wrote:>

...
Surface gauges also have to work on milling machine tables with tee slots.
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I would guess that making a block with a single flat side is in fact far easier and cheaper to mass produce than any type of tripod. The cheapest way to make a tripod with precision flat feet might even be to make a block with a flat side and them remove the extra material to turn it into a tripod.
Making a tripod with pointed feet so that you don't have to deal with the flatness issue ends up with something that will damage your surface plate so that's not much of an option. Making hard rounded feet on the tripod is again, only harder than just making a block with a flat surface and also risks damage or excess wear to the surface plate. So though it might seem like an interesting approach, I think the answer is that in practice, there is no type of tripod that makes more sense than a block with a flat side for use with a surface plate.
BTW, my new surface plate just showed up today and I just spent the last hour making a flat piece of aluminum for the fun of it.
--
Curt Welch http://CurtWelch.Com /
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Curt Welch wrote:

I did that, although I was making a small aluminium sanding block (I'm now sure my sanding block needed to quite THAT flat, but it was, as you say, fun).
BugBear
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I think you`re overlooking the fact that height gauges have other uses other than sitting on a surface plate and if it was three legged it would not be as useful. Mark.
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snipped-for-privacy@ems-fife.co.uk wrote:

I use my height gauge to scribe my workpiece. Most of the time this means sliding the height gauge on the surface plate. Much less damage to the surface plate using a full periphery contact surface than if that pressure were concentrated on three points, yes?
--Winston
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Well ... a 3-point base has less surface area, so it is more likely to wear out of spec more quickly for a given amount of use. Remember that it is important that the height gauge's beam be truly vertical. I guess that you *could* fit the base with a high precision bullseye level and make two of the three contact points adjustable so you could set it truly horizontal (and the beam truly vertical) before each use. But, of course, this requires the surface plate to be truly level before you start. :-)
Enjoy,         DoN.
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Email: < snipped-for-privacy@d-and-d.com> | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
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wrote:

The height-gage base is flat, and covers some area, considerably more than three small "feet", (think *pounds-per-square-inch* of contact area, here) so it can be moved around on the surface plate and still remain true, plumb, lever - whatever - CONSISTENT. To minimize wear (which translates to *inaccuracy* ). Dimensional stability at temperature-change is better on a flat base, too.
Good granite surface plates can easily be lapped to .00001, if the operator is savvy, and the big metrology companies have some people who travel around and do just that - by hand! Small dings in the surface will not affect the result of a good height gage with a big base, whereas a three-legged thing could easily be rendered utterly useless if one leg got into a small ding.. That is another reason that granite plates are preferred for inspection-lab and calibration-lab purposes.
Iron, no matter how good a job of scraping is done - and I have seen a number of really good ones - get dinged and they raise a spot. Maybe good enough for work-cell spot-checking, but not the most durable nor precise.
Flash
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JS's favourite vehicle has three wheels and tips over easily by all accounts! Would a three legged surface gauge not suffer the same fate and thus be awkward to use?
Henry