Mitutoyo "triangular straightness edge"

opscimc · Jan 8, 2014

My Mitutoyo accessory kit for gage blocks contains a component their catalog describes as a "triangular straightness edge," part no. 619122. These come in English and metric, and in a couple of different lengths, and an example of one can be seen at:

Mitutoyo 619022, 100mm Triangular Straight Edge, Rectangular Gage Block Accessory | AmazonSupply.com

Although everything else in this kit is for assembling gage blocks into various configurations, I don't understand how this particular component fits into that scheme. Is it used with the other parts of the kit and, if so, how? If it's just a standalone piece, can someone point me to specs on its "straightness" or "flatness" -- unlike the other components, my catalog only gives its nominal length without any indication of tolerances.

EDM JOE · Jan 8, 2014

I've always used mine to quickly check if a piece of stock is not flat (by approximately .0005 or more). Place the straight edge against the stock and hold both toward a light source. I figure if I see light between the two pieces, the stock may require grinding and/or lapping. I'd guess this is not the correct way to use the triangular steel.

opscimc · Jan 8, 2014

EDM JOE said:
I've always used mine to quickly check if a piece of stock is not flat (by approximately .0005 or more).

Thanks for your quick response. I can see how it can be used for that (although specifications on its actual flatness or straightness would be nice), but it doesn't explain why they put it in a box with a bunch of other components all of which are specifically designed to be used with gage blocks.

EDM JOE · Jan 8, 2014

A quick surface plate check shows that my triangular edge is flat within less than .00010 over a 4 x .635 inch surface area. As far as why it's in the box with the gage block stuff I haven't the slightest idea.

PeteM · Jan 8, 2014

As noted above, it's just a precision straightedge (three of 'em actually).

Since the accessory set is meant to turn rectangular gage blocks into a measuring system, the logic of including it is probably that you'd want to verify that a surface being gaged isn't convex or concave before attempting to measure it.

opscimc · Jan 8, 2014

PeteM said:
... the logic of including it is probably that you'd want to verify that a surface being gaged isn't convex or concave before attempting to measure it.

I just found a clue on an on-line Mitutoyo page:

http://www.mitutoyo.com/wp-content/uploads/2012/09/E-US-1002.pdf

For this item it says "Triangular straight edge: Measures parallelism."

OK, with one edge of this component held against one flat surface and the proper gage block found that just slipped between it an another surface at one end of it, if the same gage block just slipped between it and the surface at the other end, the surfaces are parallel. Although a precision square, or just the gage blocks themselves, might seem better for this, the relatively sharp edge would get into places a square block wouldn't. But, having said this, the relatively sharp edge seems problematic when combined with a gage block. Also, even if it worked this way, not having a spec. on its straightness means it can't be used for a precision measurement.

Still, the fact Mitutoyo says it is for measuring parallelism certainly must be a strong clue to how it is to be used.

The real Leigh · Jan 9, 2014

That Mitutoyo document is 58 pages long. To which page do you refer?

- Leigh

Nick Mueller · Jan 9, 2014

I have a Russian gage-block set that also includes those straight edges.
And me to, I have no clue why they are included in a set like this.
Until now, I thought that this is something specific of Russian mentality.

Why does a straight edge have to have 3 edges? All my other ones work great with just one edge.

Now, I'd like to be educated too.

Nick

The real Leigh · Jan 9, 2014

Hi Nick,

The triangular cross-section provides much greater rigidity than could be obtained with a simple flat straight-edge.

I expect that rigidity is required by the extreme accuracy of the other components in the set and the application of the straightedge.

- Leigh

opscimc · Jan 9, 2014

Nick Mueller said:
Why does a straight edge have to have 3 edges?

Yes, that's part of what puzzles me. A rectangular bar of the same "OD" as the triangle would be even more rigid. Since it's a very small component, the extra weight of the rectangle would be negligible. However, a rectangle might wring to other polished surfaces it was placed against when measuring, so maybe it's in the form of a triangle specifically to avoid wringing.

One picture of this "straight/parallel" edge in proper use would solve this mystery.

Gordon B. Clarke · Jan 9, 2014

The real Leigh said:
That Mitutoyo document is 58 pages long. To which page do you refer?

- Leigh

I don't know who the question in that post is intended for but if it's me then I just gave the "download Mitutoyo catalogue" as an example. The first page is an INDEX so ......? The catalogues I have have many types and heights of height gauges so there is enough to choose from "out there".

As I've already mentioned then I don't know if it's different in the USA but a phonecall to a distributor would get me their catalogue or relevant brochure within two days. As to why that info was found to be insulting to some I just don't know.

Gordon

The real Leigh · Jan 9, 2014

opscimc said:
A rectangular bar of the same "OD" as the triangle would be even more rigid.

If one needs a flat surface, all of the other components in the set have such.

This appears to be unique in that it has a narrow edge, eliminating one dimension when evaluating straightness.

- Leigh

The real Leigh · Jan 9, 2014

Gordon B. Clarke said:
As I've already mentioned then I don't know if it's different in the USA but a phonecall to a distributor would get me their catalogue or relevant brochure within two days. As to why that info was found to be insulting to some I just don't know.

Gordon,

You suggestion that folks over here are too stupid to know how to make a phone call is highly insulting to all our members.

You really are a jerk.

- Leigh

opscimc · Jan 9, 2014

There are at least a half-dozen variations of Mitutoyo catalogs on-line that include gage blocks, from at least four countries, but I finally found one from Japan that has some actual specifications on the components in the 'accessories' set:

http://www.mitutoyo.co.jp/eng/support/service/catalog/09/E4092.pdf

In the metric set, on p. 31, the straightness of the edge is given as 1.2 micrometers. While just saying "straightness" doesn't unambiguously define it, at least the number is small enough that for my purposes the edges are "perfectly straight." Even though we still have yet to figure out how it is to be used, at least we now know how straight it is.

The real Leigh · Jan 9, 2014

Thanks for that. Odd that it's described as a "holder", although the means for doing that is not obvious.

- Leigh

opscimc · Jan 9, 2014

The real Leigh said:
Odd that it's described as a "holder", although the means for doing that is not obvious.

I think whoever wrote that label got it wrong. The clamps in this set that hold the gage blocks are called holders so I suspect the technical writer just mixed this up. Also, some places in their catalogs they call it a "straightness edge," and on the same page they call it a "straight edge," so editorial mistakes like this do appear.

A google search just turned up something quite relevant that had not appeared in any of my previous searches. In case the link doesn't lead to it, it's on p. 277 of Chap. 11 of Handbook of Dimensional Measurement by Francis T. Farago, Mark A. Curtis.

Handbook of Dimensional Measurement - Francis T. Farago, Mark A. Curtis - Google Books

There the authors say the edges are ground to a small radius, and describe how with back illumination by diffuse white light these triangular gages can be used to discern gaps of 0.0001". Further, the color of the diffracted light in the gap (red from ~70 to 50 microinches, and blue for ~30 microinches) can be used to infer even smaller gaps.

Given this, it can be seen that the Mitutoyo specification of 1.2 micro-meters (47 microinches) for "straightness" is quite relevant. If you hold your gage with the "sharp" edge of one of the triangles against a part and you can't see any white light, but there's the hint of red, this tells you the part is parallel to the gage to within better than 70 microinches.

OK, it seems my question is now answered. We now know what it's used for, how to use it, and its specification.

The real Leigh · Jan 9, 2014

That makes sense. Thanks for the update.

- Leigh

Nick Mueller · Jan 9, 2014

The triangular cross-section provides much greater rigidity than could be obtained with a simple flat straight-edge.

Can't argue against that.
The strange thing to me is, that I have never seen (except in these sets) similar ones. The knife shaped ones are all I know. And now, they should all be of lesser quality?

Also, I think I still didn't understand what they have to do in these sets. Except ...

... except: In the link to the book, he describes a method to put the straight edge onto two blocks and check the gap with other blocks. Didn't know that with change of color (but I knew that effect).
Is it that that makes them so specific for these accessory kits?

Nick

hudson-ch · Jan 9, 2014

Hi,

I assume it should work similarily to the "Haarwinkel" of Pl Lehmann. see link:

http://pics.ricardostatic.ch/2_717198802_Big/werkzeuge/haarwinkel-pl-werkzeugmacher-haarwinkel.jpg

The triangular piece ist made from carbide and is adjustable.

Regards,
hudson-ch

opscimc · Jan 9, 2014

Nick Mueller said:
Is it that that makes them so specific for these accessory kits?

Now knowing how this component is used, it means this accessory kit really does turn a set of gage blocks into a complete precision measuring system, just as the catalog says. That is, a DTI is not needed to compare dimensions.

My 81-piece gage block set lets me build stacks to the nearest 0.0001". If I lay the triangular edge between the stack and the part to be measured and I see white light above the part I know it is shorter by at least 0.0001". If I decrease the stack by this amount and now see white light above the stack instead of above the part, I've now determined the height of the part to 0.0001" without having used a DTI.

Even though the smallest gage block increments are 0.0001", the absolute thicknesses of the blocks are calibrated to 2 micro-inches, so if I see red light it means the height of the part has been determined to even somewhat better than 100 microinches.

While usually a DTI would be easier to use, remember that this kit isn't for use just on a surface plate. There may be situations where making a precision in situ measurement only would be possible because of the small size of the triangular straight edge.

Mitutoyo "triangular straightness edge"

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