surface gauge - why flat base?

>>
>> 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

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

Posted by bugbear on July 3, 2008, 9:23 am

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Ed Huntress 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.
>>
>> BugBear

>
> 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.

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

Posted by Mark Rand on July 3, 2008, 1:48 pm

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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.
>>
>> BugBear

>
>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.

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

Posted by Russell on July 4, 2008, 8:12 am

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Mark Rand 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 :-)
>
>

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

Posted by on July 2, 2008, 11:30 am

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On Wed, 02 Jul 2008 10:21:43 +0100, bugbear

>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)

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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