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making a Square

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Dave S11/05/2022 21:35:48
370 forum posts
90 photos

Im building a CNC mill, and I think Ill need a chunkier square when I eventually get to the aligning stage.

I came across a webpage sometime ago (can't find it now of course) that showed how to make one from a chunk of cast iron. Not having a lump of iron the right sort of shape I poke about and found some thick walled steel box section. Hurrah, that'll do to try the process at least, and might even work for the intended job

To start the long edges are made parallel. Fortunately I have a reasonable surface grinder, so I slapped the box onto the mag chuck and started making sparks:


The weld seam was something like 0,004 lower that the edges. Not too bad.

I planned to rough grind and then finish, so once both sides had cleaned up I checked it on the surface plate.

 Basically zero movement on a 0.0001" DTI.

I was well chuffed

Now the long edges parallel It was time to rough in the ends. I hacksawed them roughly square, whacked it in a known OK grinding vice and made some more sparks:


Then with parallel ends that should be close to square it was back to the surface plate to measure up:


DTI on a surface gage rocked left and right for the high reading. Turned out to be leaning by about 0.007". For the first round of squaring up it was back to the grinder with some feeler gages:


0.003" under one side and clean up one end, then flip and make parallel (without the feelers).

At this point I left it to sit for a couple of days, partially to let it move if it was going to, and partially cos I have other things to do...

It did move, and the long sides were slightly out, something like 4 tenths IIRC. So I did a light set of finish passes on them. This is the start of one ( the beautiful dark corner):


Ended up back to basically as good as I can measure

Then another round of measure the squareness. I couldn't easily get the round part of the surface gage to touch and have the DTI at the height I wanted due to the way the angles worked out. Fortunately the surface gage has a V in the front, so one bearing later I have a round bumper again and I could measure over pretty much the full height.:


This time I needed an offset with precision in the tenths region, so I made a pair of gage block stacks with the required offset ( IIRC it was 0.2 on one side and 0.2014 on the other). The square was supported on these on the grinder, mag'd and clamped so I didn't get any ideas about flying away and then a fresh dress and a light grind and I cleaned up the ends.

The finished square has come out OK:


Its roughly 90 degrees - I calculated it to 89.996 / 90.004 based on the measurements. Ill check it again in a few weeks to see if it moves, but even if it does it'll likely be close enough for what I need.



Edited By Dave S on 11/05/2022 21:36:28

David George 111/05/2022 22:40:08
1839 forum posts
503 photos

Have you thought of a bottle square. I have three of diferant sizes and they are guaranteed square for checking many things. The base is relieved so it rests on a ring and the diamiter is parallel so it is accurate to microns.

One is 25mm diamiter, one is 35mm diamiter and the large one about 100mm diamiter.



Paul Lousick12/05/2022 00:00:36
2043 forum posts
722 photos

A big yes for a bottle square.

Very easy to make on a lathe.

Pete Rimmer12/05/2022 01:12:59
1233 forum posts
65 photos

You can buy them. Here's mine.

Kiwi Bloke12/05/2022 08:53:27
666 forum posts
1 photos

"Very easy to make on a lathe." Hmm. Well, it's easy to make something resembling such a square, but achieving the level of precision that the thing should provide is not easy. And if you really want it to be accurate, as well as precise, it's extremely difficult. That's why folk, even with well-equipped workshops, buy them...

Dave S12/05/2022 19:37:22
370 forum posts
90 photos

Surely the point is to make things?

I don’t think a cylindrical square will work for what I think I’ll need - as with the parallel faces I can square vertically and with another square horizontally at the same time.
Still if it turns out I need one I do know I can turn to under a tenth over 20” or so because not that long ago I had to…


Tony Pratt 112/05/2022 20:03:43
1963 forum posts
12 photos

Dave S, good luck to you! I'm sure you have learnt plenty whilst making your square.


bernard towers12/05/2022 21:03:33
617 forum posts
109 photos

Made mine as well, you won't make anything if you don't try!

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