surface gauge - why flat base?

snipped-for-privacy@whidbey.com wrote:


More so than a cast iron plate? How curious (and interesting)
> The gauge will tend to skitter when

Did "flat bottomed" gauges show the same behaviour (assumiung you had some) ?
BugBear
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
wrote:

More so than a cast iron plate? How curious (and interesting)
> The gauge will tend to skitter when

BugBear
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Think of it this way:
A three footed gauge will generate error if ANY of the feet are imperfect.
A flat-bottomed gauge has more that three points. It has hundreds. If one of those has an error, it will not affect the gauge.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 02 Jul 2008 16:37:38 +0100, bugbear

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
--
Dave Baker



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

That a tripod will follow the surface waviness.
--
Dave Baker



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Ok.
-- Ed Huntress
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Charmed I'm sure.
--
Dave Baker



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

    ...sigh.....I just remembered why I left RCM behind all those years ago.
--
Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"

  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Same here. I left a few days after 9/11 when some of the people on there started calling anyone from the Middle East ragheads and camel jockeys and wanting the entire region to be bombed into dust regardless of who had actually brought the towers down.
They got their wish I suppose and look what that achieved. I hope they're all happy now as their dollar and their economy sink steadily into the toilet.
--
Dave Baker



Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Ed Huntress writes:

In other words, high-frequency spatial noise. The average height is what is accurately constant. Thus the need for an integrator (flat bearing surface) on the sampling device (height gage, etc), instead of a point detector (tripod tip).
My Mitutoyo height gage base is largely hollow, only about 1/2 inch perimeter is flat bearing surface.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Interesting puzzle.
Thinking a bit, I see it's not true in general for any surface. The surface could be like a random pile of rocks (only rolling hill-like to meet your "no step discontinuities"). Every little motion will move at least 3 feet to a new location on the surface and yet another potential imbalance. Real floors aren't commonly like that however (unless you are trying to make it balance on a rock floor :)).
However, floors made out of large flat 4x8 sheets tend to distort along a single axis forming a ridge or cylinder distortion. If you are on a ridge (aka cylinder), you can always twist the chair to straddle the ridge and keep it from rocking. Even if the 4 feet are not flat, I think you can still fix it by twisting the chair as long as the floor's ridge distortion is larger than the leg distortion (distance the two diagonals connecting the corner feet fail to meet by).
If you are located on the seam of two sub-floor sheets, and they are both twisted, but in different ways, and bowed at the seam to make a third distortion to add to the mix, I doubt there can be any gurantee that twisting the chair will fix the rock.

--
Curt Welch http://CurtWelch.Com /
snipped-for-privacy@kcwc.com http://NewsReader.Com /
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Curt Welch wrote:
<snipped>

Well, the way it was explained to me is:
The stool's four legs are located on the corners of a square.
Label the legs A, B, C, and D.
With A, B and C touching the wavy floor and D not, start rotating the stool about its central axis, constraining it so that legs A, B and C remain in contact with the floor.
If you were to rotate the stool through a full 90 degrees leg A could replace leg D as being the one not touching the floor and leg D would be in contact with the floor.
Ergo, at some point in less than 90 degrees of rotation leg D must have made contact with the floor, and if you'd stopped rotating the stool at that point all four legs would be contacting the floor.
QED
Try it yourself....
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I don't think it's quite that simple, Jeff - you've got an inconsistency there.
On the one hand, you say to constrain it while rotating, so that A, B and C remain in contact with the floor. You can do that. But you then say that after 90 degrees D will be contacting the floor and A will be off it. A can't be off it if the chair is constrained so that A remains in contact.
John Martin
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
John Martin wrote:

You're correct, John...My language was sloppy. I should have said that D could then be made to touch the floor and A would then be off it.
But, the real point is that before you get to 90 degrees of rotation you'll reach a point where all four legs will be touching the wavy floor.
It's worked OK for me every time I've tried it.
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Here's an easier way to visualize it.
Stool with legs A, B, C, D all the same length, sitting on a wavy surface. Surface is only mildly wavy with no steps or discontinuities.
Stool is unstable - it rocks on legs diagonally opposite. Let's say they are A and C. It appears that legs A and C are too long. Rotate the stool 90 degrees, so that legs B and D are where A and C were, and since they are all the same length B and D will now be solid and A and C will appear too short. They went in that 90 degree rotation from too long to too short. At one place at least in that rotation they were just right.
John Martin
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

But not necessarily at the same time.
Henry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

If it's a continuous floor surface with no steps, there's guaranteed to be at least one spot where they will all touch the floor at the same time.
Before the A or C leg can lift off the floor in the rotation, the B and D legs must first both touch the floor. It's at that point in the rotation where the B and D legs first touch the floor that they will all be touching the floor at the same time.
It's the same reason you can't draw a graph of two lines - one tiled up, and the other tilted down, that you can't get from the point on the x axis where the line A is above B, to the point where line A is below B, without going through the point where the lines intersect. It's at that point where both lines are the "same height" on the graph.

--
Curt Welch http://CurtWelch.Com /
snipped-for-privacy@kcwc.com http://NewsReader.Com /
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

That's what is to be proved.

And that does it nicely! So I was wrong.
Henry
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.