# surface gauge - why flat base?

• posted

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

In article , bugbear writes

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

• posted

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

AC

• posted

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

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

• posted

no - the whole point it they don't matter.

Hmm. That applies to a surface gauge, but not a height gauge.

BugBear

• posted

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

• posted

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.

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

BugBear (slightly confused)

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• posted

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

• posted

(legs) were not all

plate. This would cause

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

More so than a cast iron plate? How curious (and interesting)

Did "flat bottomed" gauges show the same behaviour (assumiung you had some) ?

BugBear

• posted

bugbear, waxing trollish, sez: "Did "flat bottomed" gauges show the same behaviour (assumiung you had some) ?

Fuggegitabbit already! Go back and visualize the arc the "vertical" member would swing in if the 3 points weren't all equal.

Bob Swinney

More so than a cast iron plate? How curious (and interesting)

BugBear

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• posted

1) If both surfaces are flat then depressions or damage to either doesn't matter. A tripod would follow such depressions. 2) For a surface gauge perpendicularity doesn't really matter but for a height gauge it does. How do you know if a tripod is set up perpendicular? The answer is you don't. 3) Wear. Three points of contact would wear much faster than a flat base.
• posted

Because they ARE carefully made truly flat, often by hand scraping. If yours isn't, you can scrape it in quite easily once you have a surface plate.

If you have a big square base, you can carve away some of the middle and 2 corners to achieve a 3-point base. But, make each spot at least a cm square so it won't wear too quickly.

Jon

• posted

I don't know about cast iron plates. We did also have a round based height gauge with the vertical beam mounted close to one edge and it chattered the worst. Rectangular based height gauges did not have this behaviour. Eric

• posted

That a tripod will follow the surface waviness.

• posted

Ok.

-- Ed Huntress

• posted

He's a yank from RCM - you can't expect him to speak the same language :-)

Yeah but: Ed has already forgot more of that language than you 2 clowns will ever know.

Bob (never built racing engines) Swinney

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