My mini repeat-o-meter

[Peter.]

I'm building a smaller version of the Rahn repeat-o-meter so I thought I'd show some progress pics. It's made from a slab of cast iron about 7/8" wide and is 8" overall in length.

For the hinge I cut two slits 30 thou wide 300 thou deep and 80 thou apart. The hinge itself is just a ground-down stanley blade which is clamped by four small grub screws with small ball bearings in the bottom of the hole pressing on the clamp. I set the gap between the body and the shoe using 5 thou shims and it gives 65 thou total swing on the shoe. The feet are HSS rounds set into holes using loctite then surface ground together. 3/8" diameter at the ends and a pair of 1/4" in the middle. There's no need for supporting bars because the tiny gap controls the swing of the shoe and the hinge is plenty strong enough to support it. If it does break, well, I'll just grind another stanley blade.

Here are the pics of progress so far.










This week I will slot the top bar for adjustment and fit the dial indicator then give it a test drive. I might make a handle or I might just leave it how it is since it's easy to store like that.

[BCRider]

That is very... no... VERY clever. Especially the hinge design which incorporates so many features with just a few simple design thoughts.

If it were mine I think I might get a feel for how I would most hold it and then use something like a 1" ball end mill to cut two finger grip flutes along the upper beam where it will be lifted and moved the most. Or possibly just a series of three flutes on each side cut with a 3/16 or 1/4" ball end mill to give me a lot of crisp edges for my fingers and thumb to dig into. Like with a 1/16 to 1/8 wide flat between each smaller flute.

Actually if such smaller size grooves were cut to different lenghts it could even become a nice stylish visual feature too?

I gather you've tested it and the weight of the riding bar is enough to flex the hinge and it closes the whole .005 gap and rests fairly firmly on the corners?

[Peter.]

Well thank you.

Yes the shoe falls under it's own weight until it closes the gap both directions. It actually has a little 'down' spring caused I think by the clamping of the blade.

I think I will put in those flutes just to save it slipping from my hand, thanks for the good idea.

I forgot to say that I had to surface grind a spacer for the gap in order to grind the four feet all on the same plane. With the spacer in there and the part inverted on the mag chuck, the weight of the shoe was enough to keep the whole lot flat.

[TGTool]

Another feature to think about might be some heat insulation like micrometers have. In theory, the reading could change from heat expansion by handling it at the top. If you're going for style, I think wood inset on each side and patterned with gunstock checkering files would look really handsome. Or they could just go on the outside.

[BCRider]

Another feature to think about might be some heat insulation like micrometers have. In theory, the reading could change from heat expansion by handling it at the top. If you're going for style, I think wood inset on each side and patterned with gunstock checkering files would look really handsome. Or they could just go on the outside.

That's a very valid point. And yes some nice wood scales would work like a treat for that.

Inletting them is certainly doable but then you're reducing the amount of metal and thus the stiffness of the arm. I'd go the easy route and apply them as surface mount scales both to make the job easier and to further reduce contact with your hands directly on metal. And they could be shaped with the top edge a slight bit wider than the lower to aid with easy and more sure grip.

[Paul Alciatore]

Looks like a nice build. I hope to make one some day.

I believe the Rahn and Starrett designs both have stud mounted handles at both the top and rear.

https://www.starrett.com/category/pr...&sortBy=wp/asc

https://duckduckgo.com/?t=ffab&q=Rah...e%2Fsmp_17.jpg

[eKretz]

The heat transfer issue is the reason for the sort of out-of-place-looking t- handles on the Rahn version of this tool. With those, the only connection between the handle and the Repeat-O-Meter is the small shaft/bolt connecting them. Minimal heat transfer that way but you can still grip the tool solidly.

[Peter.]

Well guys (and any gals), it's all together and works exactly as expected.



I reamed the hole for the dial gauge to be a firm sliding fit. I will improve that later with a proper clamp and also put a stop on the stem to hold the depth consistent that way I won't have to re-set the pointer if I rotate the body for better vision. I put a thumbwheel on the top of the shoe for coarse adjustment. M10 x 1mm pitch thread on it and I surface ground the top centre.

Testing it out on my old iron plate it reads nicely though a little jumpy on the scraped plate. I will put it on a granite later and I'm sure it'll work very smoothly on that. The needle pointer isn't affected by direction of travel so the hinge is doing a good job.

As for heat input - I wrapped both hands around the top of the rear body getting as much skin contact as possible. Held it for 3 minutes and the needle didn't move. I'm happy that it's not going to be affected by holding it. TBH even if it did move a division or two over sevral miutes it would still return good comparative measurements.

Anyway, I'm calling this done

[nickel-city-fab]

ooooooo that's a nice indicator....

[Paul Alciatore]

Exactly.

The heat transfer issue is the reason for the sort of out-of-place-looking t- handles on the Rahn version of this tool. With those, the only connection between the handle and the Repeat-O-Meter is the small shaft/bolt connecting them. Minimal heat transfer that way but you can still grip the tool solidly.

[Peter.]

Repeat-o-meter in action.

https://www.dropbox.com/s/i5l51b8qjg...meter.mp4?dl=0

The comments about hand heat afffecting the reading made me wonder about it so I made this 3 minute video - speeded up x8:

https://www.dropbox.com/s/zbiymvzmqm...0heat.mp4?dl=0

[BCRider]

Fantastic results all around. Interesting that the heat into the material isn't affecting the reading by any sort of amount we can see. And also interesting to see your Repeatometer in action.

[Peter.]

Thank you!

I think the dial moved half a division over the 3 minutes, but it was down, not up. Might have just been my arm getting tired.

A 1 second blast with a mapp torch up where the hand hold is had it climb 10 divisions, but that's 3000degF not 100

[eKretz]

Looked like more than half a division to me, but the camera was pretty far away. Generally, a ½ micron or .000020" resolution indicator is used on a Repeat-O-Meter also - yours is double that so it will move half as much. Anyway, if attempting to measure to this sort of level, it can certainly be agreed that zero divisions deviation from handling is better than half a division. You should add at least some sort of insulating measure.

[dian]

so what about the angular movement? is the forward foot flat?