Problems with square washers (also called taper or bevel washers)
 

When I got the washers I found that they fit great on the large channel but they were a little long for the 3" channel so I had to trim the small end a little.

Thanks for the pics Greg.

It would also help if the holes in the small channels were drilled slightly off-center, towards the lip. Also if a smaller bolt and washer was used for this joint.

Suggest that folk first buy washers to match the small channels and then drill accordingly.

Glad to be of help Gerald:)

Looks like I'm rapidly becoming the poster child for how not to build a MM.:D:o:eek:

If it works at the end, you built it right! :)

It is a common thing to drill in the center of the outside face of angle and channel iron. Folk must consider that the inside face is off-center, and rather drill according to the center of the inside face.

Also, that inside face is flat for only a short distance - then you get the round fillet in the corner. I guess that the biggest washer that seats properly inside a 1.5" wide channel is a 1" square washer - that only has a 3/8" hole intended for 5/16" screws. (McMaster 91151A030) Guess I would drill them for 1/2" screws . . . .

Standard Back Marks
 

This should be a good guide to drill holes through steel sections. We used it as a standard for manufacturing of structural steel

That's from your little blue book isn't it :) I forgot that the info is in there

Note the max. recommended hole size in a 1.5" wide channel = 8mm [5/16"]. That's probably why I prefer to weld those joints.

Yes it is.
The small sections are very critical on this sizes. I few times I used non standard back marks and every time we had either washer or spanner clearance problems. Always the talk of the week when this happend. "The draugthsman stuffed up again!":(

I forgot about the spanner/wrench clearances - they are of course very important as well.

For fixing the wooden table surface to the cross-support channels, I envisaged that we do not bother with the square washers because the loads are so low (tightening wood to steel).