interesting project, looking forward! off the cuff I can't think of a reason why those splits couldn't just be built up into the design. e.g. spring steel and 4 screws. or heck just a vertical slot with a ballnose endmill. also can't figure out why the brass knob would need two way adjustment if you already have two settings @ the indicator. i guess I'll have to sit tight and find out!

Interest piqued John! Off to look at Tom's video...

I really like your honesty in everything you do!!!! It's so hard to find honest people these days. So much American Greed... Thanks for being one of the good guys

Looking forward to the build, just watched Tom's video I think he covered most of your questions. Also some viewers posted great comments too on improving the design.

like the collaboration I'll be jumping back and forth. Very keen to see the project devolip.

Thats really cool that you make a little build series!

Here's an idea for a variation that could let this tool do the job of an autocollimator: put two of the rear feet in line with the front one under the gauge, put the other foot halfway between the two rear ones and offset to the side (same arrangement as used on the bases of autocollimator mirrors), then the tool effectively measures deviations along a straight line. Do some Excel magic on the numbers and global flatness for the whole plate can be mapped out. If the distance from the rear to middle foot and from there to the one under the gauge is 150mm, you'd get about 0.69 arc seconds resolution (with a 0.5um or 20 millionths indicator), which is good enough for mapping a plate. Increase the foot spacing and you can measure down to whatever resolution you want...

Looking forward to the rest of the build video's on this one. Neat tool with what looks like a lot of interesting features. It will be interesting to see how precise/uniform the slot has to be in every dimension to get accurate readings.

Hi John. I think a slitting saw in your Bridgeport is a better choice. Might be easier to set up and will DEFINITELY give you a straighter and cleaner cut. My opinion. Great project. Thanks, John

The "e" just means how many places the decimal point is moved over. In this case it was 5 places. so 3.937e-5 is 0.00003937. You would typically see that where there is not enough room to write out the entire number.

I have been thinking of making a sort of Repeat o Meter. I have 2 Mahr Millimess Mechanical Comparators, a C E J Mikrokator and a few LVDT units. I was thinking of using a spring steel Flexures (if you get it wrong you can change it - unlike a saw cut in the main part). I enjoyed the video and the comment from Forrest Addy . My contribution is for fine adjustment - I have a precision stand that uses a fine screw with a gently tapering point about 1 1/2 inches long - ¼ inch OD - the screw floats and the taper acts as a fine wedge - think of taper pin but hardened. As I don't have a surface grinder I was going to use a couple of cheap 123 blocks as the main elements, many people would want to test small surface plates too. I was going to use gauge blocks or carbide inserts as feet. Only the single front foot is important for size and shape as the others are assumed to be on a good plane surface. As I understand it the practical reason for a Repeat o Meter is finding local low spots on a surface about the size of the foot on a typical measuring instrument - not tiny chips or large inaccuracies of the surface. If you put a measuring device (particularly one with 3 feet) with one foot on the low spot it will completely ignore the true plain of the rest of the surface and produce errors and confusing measurements. The link to stop the instrument from breaking itself sounds like an afterthought - I agreed that some simple restraint could be designed in just using sloppy fitting screw.

Hi, Have a look at Dan Gelbarts part on flexture design. From that I learned to drill and ream a hole to make the flexture part of the flxture accurate and bandsaw into it (I'm doing it straight . there is AFAIKT no reason to do it at an angle.)

John, for the adjustment you would want a very fine pitch since you have a very sensitive indicator. 40TPI thread is going to adjust .025"/rev so a .0001 is going to be like 1.4 degrees of rotation. If you make it a dual pitch, say the top plate at 40TPI and the bottom plate at 32 TPI the differential is going to be (.03125 -.025 = .00625 per rev) resulting in 6 degrees of rotation per .0001". 40/36 would be about 13 deg per tenth. Sounds like a fun project. Craig

cool project! I'm evaluating in my head if the 3 feet at the bottom should be made bigger in order to avoid holes in the middle, but this also might be counter intuitive if there are "raises" in the surface plate.

Nice project! The gage mechanism is inherently accurate. If it didn't have to be transported or treated roughly, it could easily be made using very light materials. I check my plate with a high precision level, which gets me within .0002-.0003.

Mahr gauges are brilliant! Used bore gages on a jig grinder!

Hi, Look up differential screw micrometer for how I think the thumbscrew on top is working. They can give a very fine feed.

That one you said, "looks rough," was probably made 50-100 years ago. Possibly during one of the World Wars when things were being done quickly. Did you see how nice the one Tom showed was? (in the picture) Also, was the one you showed a Rahn? FYI: the proper abbreviation of micrometer/micron is "mu" not "u." As in the Greek letter "μ" (pronounced mu). I love you and your channel! I like these more technical videos, and would love to see some more long videos; rather than a couple short ones. ...And less Fusion stuff. LOL.

Great project! ...I'm surely making one. A ball bearing foot would have too small of a footprint, and a tendency to dip with any small void. For the fine adjust screw, Turn a rounded groove in the top portion of the knurled screw, to drive a pin thru to make it captive it in the upper part....seems to me more of a machine tool method.

For the hardened pins take a look at misumi locating pins. Surprised you actually haven't covered misumi. Great great place to buy stuff. It's like mcmaster but customizable.

Your talking about precision and you are going to use a band saw instead of a slitting saw for the bottom part that should be precise.

Hey you and Tom are going to kick ass..... LOL....

also the adjustment knob is very likely a differential screw

Will Matthias be making the timber boxes for them?

Also, thinking about whether both "halves" of the repeat-o-meter need to be split evenly, or if the flex half could be 75% of the total area. That in theory should make 30% more sensitive right? If the static base end does need a lot of surface area, maybe having it wedge shaped than just a square footprint would allow you to make it shorter, but keep the accuracy.

I'm wondering why this tool has more than three points of contact. I'm guessing 3 points will establish planarity but may be thrown off by curvature? Either way, I'm interested to see how this develops.

I'm not certain why the flexure cut needs to be narrow all the way up, only the flexure itself needs to be short, so what about doing it with a 1mm tipped tapered endmill? Then you can do it in the horizontal plane without refixturing. The only sticking point I can see is getting debris caught in the cut, but I can't see how that would be less of a problem with an angled straight cut.

For the two slots on this part i wouldn't just a normal bandsaw. Because your cut won't be as precise as you might need it. I would rather use a circular saw blade for the mill. They are available in different sizes and width and u get a clean cut with it.

Re: the proposed case hardened feet - remember that case hardening is usually very thin ( few thousandths) and so you may loose the hardened surface in the lapping process ? Just a thought.

Btw. I would be happy to do any colaboration if you need any CAD stuff done.

G'day John, this is a great idea as I've seen a lot of interest in these over the years. I think Tom nailed all of my personal concerns with your design ideas, and I'd definitely give the bandsaw idea a wide berth. That's absolutely a job for a slitting saw. With the feet, it may make more sense to think of the back 3 as feet, and they are under the base to support it. The fourth is free to move courtesy of the flexture, and isn't actually a "foot" but rather a contact point. Do a search for Fed document "GGG-P-463c" as that was the (now redundant) spec for surface plates in the US. On page 11 it provides the critical dimensions for a repeat meter. A newer standard is "B89.3.7 - 2013" but I don't know if it's in the public domain. If anyone knows of a source I'd be grateful for a link.

Great idea... I'll see if I can make one for myself... cutting the flexture slot in the wire utter tho

After seeing you'r take on how it should work i am thinking you nailed it in one . my guess is Tom said ya . personally my input is for a six inch model as any thing smaller than a 12 inch plate wont really be helping that much .

Used Mahr gages absolutely brilliant!

If you want to reproduce original fine adjustment here are the schematics: a360.co/2qHutVI

There are 2 threads with different pitch in this adjustment screw.

LOL. SO your the guys single handedly responsible for multiplying the price of those gauges. LOL. I was looking at one of those 1 micron mahr gauges before these videos went online. I saw one for 40bux and was like ehh, next time. Now I see they are all 100+..... LOL. One thing, since you guys are collaborating, would you be willing to share those cad files?

I saw a video of plates being reconditioned - yours maybe? - with one of these in use and it never occurred to me that they'd be so simple in concept.

could you screw on some round carbide inserts? there is probably a flat zero rake version of an insert which should be pretty precise and flat.

You have an accu finish diamond lapping machine. I guess you got it for sharpening biax blades. That would work for lapping carbide couldn't it?

Would a thin kerf slitting saw work for the angled cut? Just a thought

Also instead of cutting the deep notch why not mill a recess and bolt a piece of suitable material into place?

i think, i would go with a horst link type design, vs the one point system, would be a tiny bit more accurate, and possibly easier especially if this were made via a cnc machine.

Cool video. Can you do an estimate of what this project will cost in materials and with and without labor? So we can compare with the $1500 price tag of the Rahn.

The fine adjust on the original is a differential thread!

Got my Mahr Millimess ordered!

i wonder if you could use ball bearing balls instead of those feet. or atleast for that 4th tab where the measurement takes place. i could see those feet covering small detents in surface places. not to mention are those feet flat? if they arent wont they make your measurements off?

A constant reading on the repeat-o-meter says your plate has a constant radius of curvature. That radius "could" be infinite, meaning the surface is truly flat. That radius could also be 10ft, meaning your plate is more like a ball. The repeat-o-meter can't tell the difference.

John, this video is a duplicate of the one I came here from about Toms feedback that linked to this one?

Fun project. Just watched Tom vid before yours.

You could use drill rod instead of ball bearings like Tom suggests.

John, I posted a length comment on Oxtool. If you guys are working together, maybe my pearls of wisdom will assist/addle you efforts

Why is the 'flexture' even necessary? It will resist movement, thus only showing relatively large inconsistencies in surface flatness and also it will tend to be thermally unstable as it's pre-loaded. It will also increase foot wear and plate wear compared to the indicator spring pressure fitted with button foot. The indicator will need to be re-calibrated regularly to account for the above factors and three feet only with the indicator tip in direct contact with the plate would require less re-calibration. It will also be lighter and easier to make, requiring only one base, three feet and the indicator and clamping screw and a very slightly convex button indicator attachment. What were Rahn even thinking, making it this complicated? Like you said John, $1500!

1um is 1/2.5 of a tenth

how is this better/different than some base with three feet and an indicator? what am I missing?

John! - Sent you an email too but I have a bunch of nice little carbide that might make nice feet like Tom mentioned. Yours if you want them!

I guess yours will be called a John Repeat-O-Meter. :)

Or use a EDM wire feed.

Don't throw me under the bus quite yet (new to the machining world), but just an ethical question. Is the Rahn ROM a protected design in any way? I did see the patient video but just wondering where this borders/falls on the line of "imitation being the sincerest form of flattery?"

aww shit. You give a man a surface grinder and he thinks he has no limits!! lol kick some ass! Judd deserves more bacon!

why not use ball bearings for the feet?

3.937e-5 => 3.937*10^(-5) = 0.00003937

1:10 - Wow, these Mahr dials are double the price now in 2021 at eBay...

Funny that the Rahn repeat-o-meter was originally developed in Ohio, now it is reverse engineered and again built in Ohio. :-)