surface gauge - why flat base?

Given that a surface gauge (and indeed height gauges) need to sit securely on a surface plate, why don't they have a tripod support
(which sits secure regardless), instead of having a flat base, which is only stable if it's carefully made truly flat?
Surely a 3-point base gives equivalent functionality in perpetuity at a lower build difficulty.
BugBear (slightly confused)
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Interesting question. Acting as counsel for the defence, as it were, I suggest the following:
(1) A 3-point surface is more likely to damage a cast iron surface plate than a flat base, if set down rather heavily, and raise slight bumps which would be most detrimental. If slid about (as one has to do to score reference lines) it would be more likely to cause scratches.
(2) A 3-point base is stable only if there is room for all 3 points to rest on the reference surface. If you were short of space, a flat base can hang over the edge; a 3-pointer can't.
(3) Making flat surfaces to within a tenth or two by surface grinding is relatively easy and cheap.
David
--
David Littlewood

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Wouldn't the points be likely to be damaged or quickly lose accuracy due to wear?
AC
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AC wrote:

no - the whole point it they don't matter.
Hmm. That applies to a surface gauge, but not a height gauge.
BugBear
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if the three points were hardened ball bearings they wouldn't ...
the iron surface plate is many peaks and valleys from scrapping and meant to add up to flat when something flat is placed on it
granite surface plates ...do they have peaks and valleys ? ...i don't know, as i don't have one ....if not a 3 ball bearings would work.
all the best.markj
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Ah, OK, I was visualising points.
AC
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A kinematically correct support may be easier to make in low volume by hand; optical surface-plate instruments like spherometers are made with 3 legs, with hardened feet added. I was warned not to treat them like machinists' measuring instruments because they aren't as durable.
Large flat surfaces are easy to make in high volume if you have invested in a surface grinder. The cast iron of the base is a very good wear surface because of the hard iron carbide.
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In article

Surely - if I have understood what you mean by spherometer* - it is essential to its function to have 3 legs along with a central measurement point.
*an instrument for measuring the radius of curvature of a surface.
David
--
David Littlewood

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David Littlewood writes:

There are two-legged versions. A ring will also do. But not a plate, I agree.
http://www.truetex.com/spherometers.htm
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Interesting, thank you.
David
--
David Littlewood

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Visualize a 3-point base on a flat surface. If the lengths of the 3 points (legs) were not all exactly the same the base would not sit in a plane parallel to the surface plate. This would cause the scribe ( or meter ) to swing in an arc away from perpendicular.
Bob Swinney
Given that a surface gauge (and indeed height gauges) need to sit securely on a surface plate, why don't they have a tripod support (which sits secure regardless), instead of having a flat base, which is only stable if it's carefully made truly flat?
Surely a 3-point base gives equivalent functionality in perpetuity at a lower build difficulty.
BugBear (slightly confused)
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Robert Swinney wrote:

The tip of the scribe (the only part that matters) is at "some height", and I can't see why this height would change, under translation (sliding) or rotation.
It is perfectly normal for the *beam* of a surface gauge to not be vertical, even with a flat base.
BugBear
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bugbear wrote:

Further reading of this thread has pointed out that in the particular case of a height gauge, the beam is (and must be) vertical for the calibrations to have any meaning.
BugBear
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Well, easy to make these assumptions without too much thought, but check the numbers.
Let's take a case where the vertical post of a height gauge is 0.5 degrees off vertical - that's quite a whopping deviation. Then measured height would be true height times cos 0.5 degrees, or true height x 0.999962. Less than half a tenth out per inch. I doubt anyone outside a metrology lab would even be able to detect such a difference, let alone measure it reliably. For example, a 2 degree temperature fluctuation in your workshop would give a change in your stainless steel height gauge of about the same amount.
To put that in context, a 27 thou pip under one end of a height gauge with a 3" base would be required to give that 0.5 degrees tilt. That's about 3/4 mm, nearly enough to trip over, never mind to feel.
Even at 1 degree of tilt, it's still only 1.5 tenths per inch. Above 1 degree, it would get rapidly worse.
You would do better to check the temperature at which your height gauge is calibrated, and seeing how your workshop temperature compares.
David
--
David Littlewood

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David Littlewood wrote:

A 27 thou pip under one end of an equilateral tripod base would raise the zero point by 9 thou (if my trig is correct).
The center of the base would be raised. This effect would be far more important than the tilt.
-- Peter Fairbrother

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I may not be following you, but the cosine error that results from a slight departure from vertical is well within the tolerance of the gage. It has to be off-vertical by quite a bit before the error becomes significant.
-- Ed Huntress
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Ed Huntress wrote:

Mea culpa; while a non vertical beam does introduce an error, I didn't go as far as you and David Littlewood (correctly) did, and calculate the *magnitude* of the error.
Speaking as someone who often castigates woodworkers for talking about "dead square", as opposed to specifying a squareness *tolerance*, I am duly ashamed.
BugBear
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wrote:

Actually, since the original question was about surface gauges rather than height gauges... If the scriber of the surface gauge were to overhang the centre of the base by 3", then the half degree tilt would produce a 26 thou error due to being a sine error. So, in the case of a surface gauge, a tilt is not a good thing :-)
regards Mark Rand
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Mark Rand wrote:

I don't quite follow this. I thought the important feature was that the scriber should be at a consistent height - and it will be even if you rotate it.
Russell
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On Wed, 02 Jul 2008 10:21:43 +0100, bugbear

On a granite surface plate trying to rotate a surface gauge or height gauge is likely to cause chatter. The gauge will tend to skitter when not moved in a straight line. I worked at a place that had a height gauge with three flat feet about 3/4" diameter. It was a pain to use because of this vibration when roatating the gauge. You had to be real careful and move it slowly. Eric
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