Repeat-O-Meter Design Question

[Paul Alciatore]

I have been watching YouTube videos again as I am highly interested in creating and checking and improving flat surfaces. And a question that has been bothering me for a while came up again.

The classic repeat-o-meter design features a base with three feet and an arm that is attached to that base with a "hinge" that is made of a thin section of metal. That arm has a fourth foot on the bottom, near the end away from the hinge and a highly sensitive DI has it's tip resting on the top of the hinged arm, approximately above that fourth foot.

What I can not figure out is why do they use that arm? It seems to me that the arm simply transmits the movement from the foot on the bottom to the DI's tip on it's top in a one-to-one fashion. There is no amplification of the amount of movement. The foot on the bottom is very much like the tip on the DI and even if it weren't, every DI that I have from the cheapest to the most expensive and most sensitive all have changeable tips. So if a different type of tip is needed, it would be easy to just buy or make one and screw it on.

There is a fine adjustment that can raise or lower the arm (sic: it actually moves the DI) to zero the meter or zero the instrument, but it would seem that could just as well move the arm that the DI is attached to and accomplish the same thing.

Then there is usually a mechanical stop that limits the downward motion of the arm, but that is just to prevent the hinge from being bent beyond it's elastic limit.

So, can anyone tell me why they use that arm? There must be some logical reason.

[Bented]

Like so?
https://www.starrett.com/metrology/p...81320#Features

[JoeLee]

If I remember correctly that front shoe is not a separate part from the rest of the body. Somewhere I saw one that was made like that I believe it's almost completely cut through but hanging on by a thin section which you might call a "living hinge" The arms are there for safety so the front shoe doesn't break off and they limit it's movement. The arms are not tightly secured to either piece. It doesn't have to move much for what it's designed to do. If it is cut through the arms would hold the two pieces together. Starrett has one where the front shoe is a separate piece in which the arms hold the two pieces and probably prevent the edges of the two pieces from banging against each other creating dings along the edge which would interfere with the accuracy. Starrett has a couple different ones that I've seen. I would think the feet are there for ease of sliding over the granite and to prevent any suction from forming that would add resistance to it's being moved around on the plate.

JL........

[TGTool]

Paul, I think you're right that the whole arm business us just measuring movement under that forward foot A pad on the bottom of the indicator to average local irregularities should work just the same. I would just presume they figured the indicator was too fragile out there all by itself. Or there might have been some history in the development, like an indicator that didn't lend itself to that setup. If you had a test indicator, for instance, it would be sensitive to all the local roughness and divots.

[Peter.]

An indicator that sensitive would have the needle rattling about full-scale as soon as you move it. The mass of the extension acts as a damper and applies a constant weight on the front pad.

[J Tiers]

Many indicators will be affected by sideways movement, to a greater or lesser degree. It may just create needle vibration, rather than real change of indication, but is enough to be a nuisance. The separate foot, with a hinge that does not allow any movement other than minute vertical movements, separates the indicator from the sliding action, preventing any effect of the sliding from affecting the indicator.

The "living hinge" is presumably very rigid in "X" and "Y", while allowing movement in "Z". It probably has to be ridiculously thin to not affect movement. That could be duplicated/replaced with a piece of tempered shim stock.

[JoeLee]

Paul, I think you're right that the whole arm business us just measuring movement under that forward foot A pad on the bottom of the indicator to average local irregularities should work just the same. I would just presume they figured the indicator was too fragile out there all by itself. Or there might have been some history in the development, like an indicator that didn't lend itself to that setup. If you had a test indicator, for instance, it would be sensitive to all the local roughness and divots.

Not only is it too fragile but at .000020 resolution the overall travel is .002.

JL.............

[alanganes]

Many indicators will be affected by sideways movement, to a greater or lesser degree. It may just create needle vibration, rather than real change of indication, but is enough to be a nuisance. The separate foot, with a hinge that does not allow any movement other than minute vertical movements, separates the indicator from the sliding action, preventing any effect of the sliding from affecting the indicator.

The "living hinge" is presumably very rigid in "X" and "Y", while allowing movement in "Z". It probably has to be ridiculously thin to not affect movement. That could be duplicated/replaced with a piece of tempered shim stock.

I think this is the major point of it. The very thin section between the base and the arm that the indicator touches forms a flexure that is very rigid in every axis except the one you are interested in, which is "up and down" in this case.

[reggie_obe]

Why not taking scraping class and learn first hand from a master?

[J Tiers]

This is not really a scraping issue. It is a side point which is really in the "metrology" area, having to do with measuring flats and surface plates, as opposed to making them or using them.

[dian]

if you make one, make it as fed specs describe (GGG-P-463c ).

[Mark Rand]

I can't help feeling that the need for a repeat-meter reading in the standard was put there by the Rahn people that created the tool. If you are lapping a granite surface plate with a flat lap (which it is after a while), then local variations are going to disappear as you approach the overall flatness level.

[J Tiers]

I can't help feeling that the need for a repeat-meter reading in the standard was put there by the Rahn people that created the tool. If you are lapping a granite surface plate with a flat lap (which it is after a while), then local variations are going to disappear as you approach the overall flatness level.

Should do..... But then as you use the flat, you will wear it out of flat again, which can be quantified by the repeat-o-meter. I think it is more of a testing device for calibrating the flat (or surface plate).

[Paul Alciatore]

They do say that the contact points are lapped carbide. Could that be the reason? It would seem to me that a tip on a DI with a large radius would work. Or am I wrong about that?

Like so?
https://www.starrett.com/metrology/p...81320#Features

[Paul Alciatore]

A carryover from the development? Nothing else in the design seems to point to that. And I have never seen a repeat-o-meter with a DTI.

Paul, I think you're right that the whole arm business us just measuring movement under that forward foot A pad on the bottom of the indicator to average local irregularities should work just the same. I would just presume they figured the indicator was too fragile out there all by itself. Or there might have been some history in the development, like an indicator that didn't lend itself to that setup. If you had a test indicator, for instance, it would be sensitive to all the local roughness and divots.