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I often make fences on my work surface by clamping down levels and squareslike so:

Fench from squares

I only ever use my speed square to overlap another square because that is thick and its accuracy is rough enough that I'm not worried about it bending.

My question is am I risking damaging the accuracy of my squares and levels by clamping down the following:

  • Triangule shaped speed squares
  • Metal Carpenters squares
  • Aluminum I-beam styly levels.
  • Combination squares
  • Machinist squares

I'm not concened about scratching the surfaces with the clamps, just making them less true.

  • 1
    It's OK to clamp down squares etc. However, I wouldn't use a c-clamp or another clamp that can exert lots of force (especially on the I-beam levels, aluminium is deformable). If you must use a very strong clamp use a scrap piece of wood to spread the load. – Graphus Sep 3 '17 at 8:40
  • I believe Speed Squares are generally aluminum, but don't forget, if you're exerting a lot of pressure on it, it may not bend, but it might make a dent in whatever it's overlapping. Take that into consideration when doing your clamping. – FreeMan Sep 8 '17 at 12:26
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It's certainly possible to damage squares and/or levels by clamping. So if you're looking for a yes or no answer, it would have to be yes.

However I can think of plenty of situations where I would clamp either of these and do so in a way that does not degrade their accuracy. I would say the risks are these:

  • Clamping directly (without a pad of some sort) risks scratching, denting (much worse) or otherwise deforming the surface. This sort of damage can change the angle of a carpenters square and similar thin metal squares. It also could damage the scale causing you to mis-read it. I would follow Graphus's advice above and always either use a pad or use clamps with padded (e.g. rubber) jaws.

  • If the surface you're clamping to isn't straight, you can bend a square and definitely an aluminum level. This can also happen by clamping over an edge (like the pad is half on and half off the board or bench. I would avoid clamping an aluminum level to anything that isn't perfectly straight and liable to stay that way. Even then I would clamp it lightly; enough to keep it from moving but no tighter. Definitely use pads here or rubber jaws.

  • I would be very leery of clamping any of my machinist squares, except to something that was known flat and smooth to within .001", like a 1-2-3 block. A small divot would be all it took to bend a rigid blade like that. Also be careful there is nothing under it like chips; wipe both sides with your hand immediately before clamping. Don't even think of skipping use of a brass or aluminum pad under the clamp (with my machinist squares anyway :).

  • Same goes for a combination square if it's a good one (PEC, Starrett). If it's a Johnson from Home Depot I wouldn't be so careful :)

In short, my advice is:

  • Know the risks

  • Clamp these when it speeds your work, taking care to avoid damage.

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