Tuesday, July 20, 2021

Today's Obscure Machining Question

You want a 32mm inside (or perhaps a bit larger, you do not need a close fit) square aluminum tube.  The closest you can get is a 1.5" exterior .125" wall tube.  The inside is about 31.5 mm.  I tried filing it out.  This is a shockingly slow method.  

So buy a larger tube. The available choices of commercial square tube are 1.75" .25" wall or 2" .5" wall.  None of these get you to 1.375" interior which is close enough for what I need.  Wall thickness is pretty much irrelevant; it carries very little load.

I tried to mill the inside of the tube using an end mill; in this case, 2" long .375" diameter.   That goes deep enough for my needs but not into the corners.  A .125" mill would do the corners, buy I doubt anyone makes a end mill that length and diameter; it would be too fragile.  There are tools for cutting square holes on a lathe (which seemed impossible until I saw it on This Old Tony's YouTube channel).  But that seems perilously close to buying a well rig to install a fencepost.

Any suggestions?


  1. Is 1.5" interior too excessive? https://www.grainger.com/product/GRAINGER-APPROVED-Square-Tube-6ALP5

  2. You could use a square graver to clean out the corners. If you don't know an engraver in the area, just take some 1/8" square key stock, cut an appropriate length and then put a 45 degree face across one end, oriented to give you a nice, sharp point.

    It will be a *terrible* graver but should be good enough for your use. Use a very small hammer to gently tap it down along the corner.

    The EngraversCafe.com forums have a lot of folks who can give you more info - and you might even find one in your area who can fix you up with a proper graver and tips.

    1. Unknown: Yes, 1.5" is too big. KCSteve: So rather like a planer for metal. I may have a different way to solve this problem, slower to machine but a known type of solution.

  3. Do you need the 1.375" all the way down the tube?

    Or only a small portion of the ends? If the latter, build up with either weld or epoxy or something like that....Or use some setscrews and a plate.

  4. You could take it to a wire edm shop and get much sharper corners.

  5. You could take it to a wire edm shop and get much sharper corners.

  6. You said you needed 32mm and then later you said 0.375". The 1.5" OD tube is 1.25" ID which is really close to 32mm (31.75mm). If you really need 0.375" that is a different kettle of fish. Apparently it only needs to be around 2" long and the interior finish is relatively unimportant since you tried filing it. Try a powered reciprocating file. You can get them made to fit a recip saw or air tool. You probably don't want the little complete tools with the needle files.