surface gauge - why flat base?

Three points of contact is a lot more fun to level. I spent a bit of time today to level

4 points of contact on a balancer system.

As far as your question goes, range. A flat surface gage can be used in many more situations. I've used 1/2" of a machinist square often to square up a part vertically in a Kurt vise.

Wes

The flat base averages the error if I'm thinking right. It is late so I might not be.

Wes

Well ... a 3-point base has less surface area, so it is more likely to wear out of spec more quickly for a given amount of use. Remember that it is important that the height gauge's beam be truly vertical. I guess that you *could* fit the base with a high precision bullseye level and make two of the three contact points adjustable so you could set it truly horizontal (and the beam truly vertical) before each use. But, of course, this requires the surface plate to be truly level before you start. :-)

Enjoy, DoN.

The height-gage base is flat, and covers some area, considerably more than three small "feet", (think *pounds-per-square-inch* of contact area, here) so it can be moved around on the surface plate and still remain true, plumb, lever - whatever - CONSISTENT. To minimize wear (which translates to

*inaccuracy* ). Dimensional stability at temperature-change is better on a flat base, too.

Good granite surface plates can easily be lapped to .00001, if the operator is savvy, and the big metrology companies have some people who travel around and do just that - by hand! Small dings in the surface will not affect the result of a good height gage with a big base, whereas a three-legged thing could easily be rendered utterly useless if one leg got into a small ding.. That is another reason that granite plates are preferred for inspection-lab and calibration-lab purposes.

Iron, no matter how good a job of scraping is done - and I have seen a number of really good ones - get dinged and they raise a spot. Maybe good enough for work-cell spot-checking, but not the most durable nor precise.

Flash

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

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.

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.

Interesting, thank you.

David

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

Indeed.

I'm not questioning wether a (truly)flat base works, or can be made to work.

I was trying to explore whether an alternative design, with lower production costs, can also be made to work

BugBear

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

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

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

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

Ah, OK, I was visualising points.

AC

=2E..

Surface gauges also have to work on milling machine tables with tee slots.

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

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

JS's favourite vehicle has three wheels and tips over easily by all accounts! Would a three legged surface gauge not suffer the same fate and thus be awkward to use?

Henry