Anyone have a Repeat-o-Meter?

Spencer in NH · Jan 27, 2016

Does anyone have a Repeat-o-Meter and is willing to shoot some quality pictures of it. I am thinking of making a shop-built version as a project. I will be happy to share my design.

Thanks!

TDegenhart · Jan 27, 2016

What's a Repeat-o-Meter?

Tom

Spencer in NH · Jan 27, 2016

TDegenhart said:
What's a Repeat-o-Meter?

Tom

A time-honored device for measuring the flatness of a surface plate using a sensitive indicator and a hinged arrangement of two blocks.

Spencer in NH · Jan 27, 2016

http://www.practicalmachinist.com/v...inspection/how-does-repeatometer-work-288275/

Milland · Jan 27, 2016

Spencer in NH said:
A time-honored device for measuring the flatness of a surface plate using a sensitive indicator and a hinged arrangement of two blocks.

Going to use actual hinges? I'd use flexures if I was making one (which I want to for one of my 20 millionths indicators).

Edit: I see the forum software is wonky again...

Spencer in NH · Jan 27, 2016

Screen Shot 2016-01-27 at 7.34.36 PM.jpg

Yep, they're flexures.

(From Fed Standard GGG-P-463c)

Milland · Jan 27, 2016

Spencer in NH said:
View attachment 161443

Yep, they're flexures.

(From Fed Standard GGG-P-463c)

From that image, I wonder if there's a hardened roller bar between the two blocks and it's being retained by two "flexure springs" in order to force a single axis of rotation. Actual flexure elements wouldn't (I think) be as shown in the image.

Edit: Eh, maybe if they're acting as parallelograms they'd have enough torsional stiffness to prevent rotation, but it would be nice to see a closeup of the real thing.

Spencer in NH · Jan 27, 2016

Milland said:
From that image, I wonder if there's a hardened roller bar between the two blocks and it's being retained by two "flexure springs" in order to force a single axis of rotation. Actual flexure elements wouldn't (I think) be as shown in the image.

They're not... from the images I found via Google (rather low resolution) there is a single flexure plate, which makes sense.

The real Leigh · Jan 27, 2016

Milland said:
From that image, I wonder if there's a hardened roller bar between the two blocks and it's being retained by two "flexure springs" in order to force a single axis of rotation. Actual flexure elements wouldn't (I think) be as shown in the image.

There's no roller bar.
The flexure element is as shown in the image.

Understand what this is...
It compares the highest point under the comparator stud with the plane defined by the highest point under each of the three support studs.

It's normally used on a surface plate or similar finely-finished surface.
On any supporting surface, the highest points are what support the load.

- Leigh

Tlaw · Feb 22, 2016

I've been looking for the same. I don't see how two flexure springs would work. Seems like they would make a rigid connection. The Rahn photos I have looked at on line look like the two bottom pieces are cut into two pcs. at an angle and are held in alignment with two straps on the sides. The cut almost looks like it doesn't go all the way through, leaving a thin section unsawn to serve as the hinge. Sure would like to see a better photo. The new standards show a different design in a little better detail. I may follow the iso model.
Walt B

js412000 · Feb 22, 2016

It has a single flexure. The base is made from a piece of steel flat bar 1" x 2 x 10 1/2 with a simple saw cut to within .050" of breaking through the thickness. I don't know why the saw cut is angled.

The side plates are loose fitted with a dowel pin at the top and an undersize shoulder screw at the bottom. They serve to prevent the flex hinge from bending too far when the instrument is lifted from the table.

The fine adjustment uses a differential screw mechanism. I believe that the knurled adjusting wheel should be staked permanently to the screw. On my example, the wheel has come off and the screw shows evidence of having been turned with a pair of Vise-Grip pliers.

The 4 carbide pads on the bottom are 3/8" diameter and look like they are glued into shallow recesses.

Spencer in NH · Feb 22, 2016

Thank you for the pics and taking the time to post them, JS!

dcsipo · Feb 22, 2016

js412000 said:
It has a single flexure. The base is made from a piece of steel flat bar 1" x 2 x 10 1/2 with a simple saw cut to within .050" of breaking through the thickness. I don't know why the saw cut is angled.

The side plates are loose fitted with a dowel pin at the top and an undersize shoulder screw at the bottom. They serve to prevent the flex hinge from bending too far when the instrument is lifted from the table.

The fine adjustment uses a differential screw mechanism. I believe that the knurled adjusting wheel should be staked permanently to the screw. On my example, the wheel has come off and the screw shows evidence of having been turned with a pair of Vise-Grip pliers.

The 4 carbide pads on the bottom are 3/8" diameter and look like they are glued into shallow recesses.

View attachment 163913 View attachment 163917 View attachment 163914 View attachment 163916 View attachment 163915

does the differential screw mechanism work like a Norris adjustor? Does it also spread the plates or just squeezes them together? Sorry for asking too many questions.

dee
;-D

js412000 · Feb 23, 2016

The adjusting screw can both squeeze the plates together or spread them apart.

The upper end of the screw has a 5/16-24 thread and the lower end has a 1/2-20 thread.
Both threads are right handed.

The indicator holding arm is made from one piece of steel and is split into two "leaves" by a saw cut.
The upper leaf is tapped with 5/16-24 and the lower leaf 1/2-20 threads.

The "leaves" cannot be disassembled.

The differential adjusting screw is made in two pieces. The 1/2-20 screw is drilled through with an approx. 1/4" dia. hole.

After assembly the two pieces of the adjusting screw are pinned together with a 1/16" diameter cross pin that can be seen in the photo.

dcsipo · Feb 23, 2016

js412000 said:
The adjusting screw can both squeeze the plates together or spread them apart.

The upper end of the screw has a 5/16-24 thread and the lower end has a 1/2-20 thread.
Both threads are right handed.

The indicator holding arm is made from one piece of steel and is split into two "leaves" by a saw cut.
The upper leaf is tapped with 5/16-24 and the lower leaf 1/2-20 threads.

The "leaves" cannot be disassembled.

The differential adjusting screw is made in two pieces. The 1/2-20 screw is drilled through with an approx. 1/4" dia. hole.

After assembly the two pieces of the adjusting screw are pinned together with a 1/16" diameter cross pin that can be seen in the photo.

View attachment 163979 View attachment 163980

Is it something like this?

dee
;-D

TGTool · Feb 23, 2016

Dee,

Nice drawing but there needs to be clearance under the head of the thumb wheel. The 5/16 screw will just move up and down through it's tapped part so there needs to be some over and under clearance. Since the two threads move together the 1/2 is doing the same but at a different rate from the difference in thread leads.

Spencer in NH · Feb 23, 2016

TGTool said:
Dee,

Nice drawing but there needs to be clearance under the head of the thumb wheel. The 5/16 screw will just move up and down through it's tapped part so there needs to be some over and under clearance. Since the two threads move together the 1/2 is doing the same but at a different rate from the difference in thread leads.

...resulting in about 0.008" plate separation per turn.

Demon69 · Jun 3, 2017

Tom Lipton has done a walk through on the Rahn, I liked some of his ideas.

Mike Henry · Jun 3, 2017

And Tom will be collaborating with NYCNC to make one or two of them (both on YouTube) for any that want to play along.

ewlsey · Jun 3, 2017

Mike Henry said:
And Tom will be collaborating with NYCNC to make one or two of them (both on YouTube) for any that want to play along.

Why? There is almost no practical application for such a tool. It cannot be used to qualify a surface without some way to measure flatness.

Build a repeat-o-meter. Build an autocollimator or set of differential levels. Build a lapping plate. Then what? Send that equipment to a lab for calibration to NIST standards?

Depending on your location and the size and condition of your plate, calibration from an outside service is probably less than $500. I used to work in a foundry with around 20 plates and 2 CMM machines. One guy could calibrate all of them in a day, including lapping a few that needed it.

He puts a sticker on it that is traceable to NIST and makes the ISO people happy. We call him again the next year.

Anyone have a Repeat-o-Meter?

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