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)

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

>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

Ah, OK, I was visualising points.

AC

Posted by Jim Wilkins on July 2, 2008, 7:05 am

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>
> Surely a 3-point base gives equivalent functionality
> in perpetuity at a lower build difficulty.
>
> =A0 =A0 BugBear

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.

Posted by David Littlewood on July 2, 2008, 7:32 am

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In article

>>
>> Surely a 3-point base gives equivalent functionality
>> in perpetuity at a lower build difficulty.
>>
>> BugBear

>
>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.
>

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

Posted by Richard J Kinch on July 3, 2008, 12:10 am

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David Littlewood writes:

> 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.

There are two-legged versions. A ring will also do. But not a plate, I
agree.

http://www.truetex.com/spherometers.htm

Posted by David Littlewood on July 3, 2008, 5:20 am

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>David Littlewood writes:
>

>> 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.

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

Interesting, thank you.

David
--
David Littlewood

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