Tag: design

Prototyping ain’t easy, but it’s necessary

As the saying goes, everyone has a plan until they get punched in the mouth. The same goes for taking a design and turning it into a physical object. You may not have the tools or material you need, you may realize that in reality, things don’t operate the way that you want, or the way you designed something just isn’t feasible to build. The other side of the coin is that maybe you come up with a better design while you build.

Building the DIY Repeat-O-Meter, most of the above happened. This doesn’t lead to a pretty finished product. Extra holes drilled throughout the part to try one one idea or another. Slots roughly cut, then a design change halfway through. It’s not the best work I’ve ever done. But I’ve built enough prototypes at this point, that I at least expect this sort of thing to happen.

The most difficult part was determining how much of a slot, and how thin of a web to leave on the arm to get reliable flexing, that would not lead to permanent deformation, but be “soft” enough to provide good readings. It took a bit of experimentation, but I finally arrived at something that seemed like it would work.

Next up: Finally Finished.

The Repeat-O-Meter continues. The Design Phase

Having bought a small, cheap, granite surface plate (12″ by 8″), that (I think) was Grade B (as compared to the AA and A which are better), it totally makes sense to check it’s relative flatness. Because the work I do in my bedroom requires high precision. Or at least that’s what I told myself.

a Rahn Repeat-O-Meter at a hefty 10″ long would barely have fit on my tiny surface plate. Forget about trying to take any type of systematic measurements with it. Instead of building a full size Repeat-O-meter, I’d have to create a smaller version in order to have it work on my small surface plate.

So let’s do this. Let’s spend hours of our life designing, developing, and building a small DIY repeat-o-meter, that I’m going to be unsure of it’s actually accuracy and repeatability. And let’s not mention the fact that you need a millionth’s (0.00005″) level indicator to actually do this, which are not cheap.

Taking the blueprints from the BarZ DIY Repeat-O-Meter, I started in Fusion360 and built a model almost exactly to the ShadonHKW prints. Immediately changes were made to the design. Partly because of available stock, partly because of available hardware (I was doing this on the cheap, I had to drop some money on a good indicator… which would cost more than the surface plate.) But also because I didn’t know what I was doing.

The basic design is simple, as referenced in an earlier post, you have a base with 3 flat feet that holds the dial indicator out on a cantilever post. This post sits above the lever arm. The lever has one foot at the front. As you move the device across the surface plate, the lever arm will influence the dial indicator, giving you a reading of the variations of the flatness of the surface plate.

For the Pivot, the BarZ design uses two center drilled holes on the lever, and two ball bearings (held in by set screws) on the base. While this should work just fine, I never built this version. The original Rahn Repeat-O-Meter uses a flexure hinge, and that design appealed to me, and as I’ve never made a flexure hinge before, I really wanted to try to see how a flexure hinge worked. At the time, I’m sure I was reading about compliant mechanisms. (And for more flexures and compliant mechanisms, check out this excellent Veritaseum video.)

One of the benefits of a flexure hinge is that I can worry less about the stiction/friction between the ball bearings and the lever arm itself. While this would normally not be an issue at all, when dealing with measuring millionths of an inch, and being moved across a plate, any little friction/stiction could possibly add error to my measurements.

For the simple flexure hinge, I just designed a slot into the arm, taking material off the bottom where the actual flexing will occur until there would be a good “hinge” but it wasn’t so weak that I had to worry about permanent deformation or failure from metal fatigue. I read up a bit on the topic and found some numbers of around 0.050″ for my use case. That said, it wasn’t a scientific process at all. “Okay, that looks right” and “we’ll see how it works in metal” was the best I could come up with.

And with that, the design process was done. And all that was left was to make the thing. Of course it never seems to work that way.

(Next: “Wow, you call yourself a machinist?”)

While learning how to machine using the lathe and mill, I became interested in what seemed to me a chicken/egg problem; “How do you make a precision instrument if you don’t have a precision instrument to make it with?” The answer: it starts with a flat surface, and you build up from there. (For more about this answer, I recommend “Foundations of Mechanical Accuracy” by Wayne Moore.)

Precision starts with a flat plane. Without a smooth, regular surface, it would be nearly impossible to create square objects, and from there, impossible to build all the machines and technologies to create the world around us. (Also, I can’t help but mention that as I wrote this, I was reminded of the flat plane of Central Place Theory by Christaller, for the geography nerds out there.)

And thus my unhealthy obsession with flatness began.

I started with reading about Surface plates, the granite plates that have been ground to millionths of an inch flatness. A typical surface plate you may see in a shop would be 24″ by 36″ and 4″ thick, cut from granite and ground flat with diamonds. (For a very bad copy of the Federal Specification from the GSA, which almost all surface plate manufacturers base their tolerances on, because you can’t easily find the new spec, see here: the older spec, GGG-P-463c, now superseded by ASME B89.3.7.) In order to meet these standards, you need to have your surface plate calibrated, so how do we go about doing that?

And this is where I came across the weird looking thing, with the name straight from the 1950’s: the Rahn Repeat-O-Meter.

A Repeat-O-Meter measures the variation in relative flatness across the surface plate. It’s a rather simple device, consisting of essentially a floating lever attached to a base. The base has an arm above the lever that holds a high precision dial indicator (0.00002″usually.) The lever lifts the plunger on the dial indicator as you sweep the whole thing across your surface plate. And as you move the Repeat-O-Meter around your plate, you see the variation in the flatness of your plate (relative, because it shows the variance from one point on the plate compared to another point.) Obviously, the less variance, the better the plate for high precision work.

When I googled the Repeat-O-Meter, I came across one of my favorite machinist Youtube Channels, Oxtoolco (Tom Lipton) and his videos on the Repeat-O-Meter and re-surfacing his granite plates, and his collaboration with NYCCNC on re-creating their own DIY Repeat-O-Meter. (And by the way, you’re going to see a lot of references to Tom Lipton on this blog when it comes to metrology, flatness, and all things machining. He’s my spirit animal.) And just as an aside, I’d be remiss not to mention this amazing design by Robin Renzetti.)

However, my biggest influence was the guys at Bar Z. This video (and their blueprints found in the description of the video) of their own DIY mini Repeat-O-Meter.

Now, I didn’t learn all this all at once. I had been reading and watching these videos over a period of a few months. And in that time, I had enough use for a surface plate that I bought a small (12″ x 8″) import granite surface plate from Ebay. And not knowing how flat this rather inexpensive and small plate was, I wanted a wear to investigate it’s properties. So I set out to make my own Repeat-O-Meter.

And this is where the trouble began….

(Next Up: “A slight change.”)