surface gauge - why flat base?

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)

** Posted from http://www.teranews.com **

Posted by bugbear on July 2, 2008, 11:36 am

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Robert Swinney wrote:

> 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.

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

Posted by bugbear on July 3, 2008, 7:21 am

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bugbear 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.

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

Posted by David Littlewood on July 3, 2008, 8:15 am

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>bugbear 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.

>
>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.
>

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

Posted by Peter Fairbrother on July 3, 2008, 2:18 pm

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David Littlewood wrote:

>> bugbear 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.

>>
>> 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.
>>

> 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.

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

>
> 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

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