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)
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
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
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.
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
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)
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
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
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
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
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
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
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
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
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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