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Straight, square & parallel (DIY)


Nov 29, 2023

New here, professionally I’m in software and digital electronics. Tinkerer by blood, recently inspired by the three plates method.

Looking for information on how/whether it could be extended to have square and parallel edges. I see parallel straight edges are quite expensive per length, see examples Busch Precision Parallel Straight Edges.

Thank You!
Yes, there is a pretty direct extension to generating squares using the same three item technique. Start by making the bottom face flat. You can then do the three item rotation with the vertical faces, keeping the bottoms on your surface plate throughout.

For parallels, not so much, but if you have a surface plate and a surface gage (aka dial indicator stand), you can test and correct the upper face without much trouble.
There is a fascinating paper called "The Whitworth Measuring Machine" written in 1877 which explains how to achieve precision surfaces (including 90 degree and parallel) and precision measurements from scratch. Fairly easy to read and I thoroughly enjoyed it. Been a couple years, I should go through it again. I've attached a PDF; it's also available free on Google.


  • The_Whitworth_Measuring_Machine.pdf
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I haven't looked at the Whitworth yet, but as a quick hit, you should be able to produce two parallels using a second planar surface (small surface plate or maybe parallel) You can think it through but it's fairly logical. After the first sides of two parallels mark up correctly, scrape the opposite sides flat - not parallel yet, but flat to the surface plate. Now lay them side by side and use the second surface plate to mark them up. It will be immediately obvious which sides and ends are high and they can be scraped. Swapping side to side and swapping one parallel end for end will again show discrepancies from flat and parallel. When they both print satisfactorily no matter how they switched or rotated, you can be sure you now have true parallels.

Good luck.
As mentioned, Wayne Moore's book, _Foundations of Mechanical Accuracy_
Also _Machine Tool Reconditioning_ for in depth in the field "how to" mostly with simple fundamental tools, along with theory.

Plus, if you are a hands-on guy, your Q would probably be better in the rebuilding (scraping) section - at least one world trotting Guru who has trained techs everywhere including Asia and Europe, posts there; or in the metrology section for theory.

There are a number of us on the forum who cast and sell straight edges and parallels, or have in the past.
DG Foster retired as a doctor to take on the challenging profession of cast iron founding for lightweight SE's, and there are others. I used to, but 4 in a row foundries went bust after casting mine in sequence (just bad luck, i know) and the last lost/destroyed/stole the patterns.

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I agree with all of the above, and one thing to add:

This idea of "how my much do you DIY" comes up on here with some frequency. If you are approaching this from a scholarly advancement direction and gaining skills, truly generating flat or square references can be interesting and fun. If you just need the tools and can't afford them, it will be cheaper to buy the tools than try to build them. Most of us here have a foot in both worlds though such that we do this work to some capacity more than just a hobby where our time and resources have a figure attached, but we also enjoy doing it, so finding that balance will be a little different for everyone.

For me, the balance is to find a deal on an old casting and scrap it myself. I've commissioned my own straight edge castings before and while fun, I wouldn't do it if a job was waiting on them.
The whitworth machine, the original was in the London science museum was capable of resolving 1/000000 of an inch according to the curator who was kindly describing it to us, amazing guy whitworth, hexagonal bullets broke him aparently, when rejected he rented an office across the way from Whitehall so he could glare at the guys who rejected him, didn’t take it too well I guess, though his rifle was terrifyingly accurate.