I have wanted knee crutch stirrups for my box table forever and a while back I picked up a pair on eBay. This was just the stirrups, so I had to come up with a mounting system for them. This is what I came up with. Check out my profile for pictures....
I had to make 3 things for each side:
1) A piece to bolt on the table to hold the risers for the stirrups
2) The riser for the stirrup
3) A piece to interface the riser to the fitting on the stirrup
Piece #1, the piece that bolts to the table is made out of a piece of 2x4 about 6" long with a roughly 4" piece of 4x4 screwed into the center of it. The 4X4 has a through hole in it big enough for the riser (thick walled pipe) to slide through. There is also a threaded hole for a screw to lock the riser in place. I will replace this with a screw eye on my next trip to the hardware store. This will make it so I do not need a tool to adjust it and also there will be no sharp screw head sticking out. The 2x4 is countersunk from behind and 4 sheet rock screws hold the piece of 4x4 onto it. The 2x4 has 4 holes countersunk from the front for 4 14" x 20 tpi bolts and washers to slip into. Those hold the assembly to the table. All of the corners on to top face are rounded off on the disk sander. The table has space in the front for this to fit. Note if your table has an outlet on it you will need to remove it. I was going to grind the box out of the table but it was easier just drilling through it and using a longer bolt for that position. Also if that side of the table shows, yo may want to make a longer luan skirt to cover the entire hole from the outlet.
Piece #2, the riser was made from the side rails from an old treadmill that I picked up for another project. I sed a rotary pipe cutter and salvaged two pieces that are about 2 feet long each. I dressed the end up a bit with a file but they are hidden anyway.
Piece #4, the part that interfaces the stirrup to the riser. This was the tricky one. First off they need to mirror each other. These started as about a 4" piece of 4x4 that I drilled a tight fitting hole for the riser in about 1.5" deep. The pipe needs to be tapped on. The stirrups have two threaded holes in them for 1/4" 20tip bolts in a line about 1.75" apart and a spring loaded stud offset making a triangle out of the three pieces. I measured the exact hole spacing and the offset from the side of the 4x4 so the entire flange on the stirrup would be on wood, and drilled the two through holes and countersunk them on the side for the bolts heads. I also drilled one of the two holes a little larger than a proper clearance hole for the 1/4" hardware. I also took one of the bolts going into each side and put a point on it with my grinder to assist in finding the center of the blind hole. Note that you need to use a drill press as the holes go the entire width of the 4x4 and tolerances are tight. The holes can not be angled. I put both sides together and than tapped the stud a few times with a mallet and took the assembly back apart and drilled out where the dimple for the stud was. Upon re-assemble the studs fell right into place.
I also sharpened the end of another bolt and used that to mark the center of the first hole in the table for piece #1. I held it in place and gave it a good tap and I had a little prick I cold set my bit in to drill the metal. I started with a 1/16" bit and worked my way up to the clearance hole size. Once I had one hole I bolted that in place firmly and used my center to tap divots for the rest of the holes, and than too the piece back off and drilled them out, and re-assembled. There is just enough room on the side of the table if you extend the stirrups to get your hand back there with a wrench to hold the nut and use a socket on the (recessed) bolt.
These seem to work really well and overall I am very happy with the project. Finishing touches I am pondering..
1) Replacing the screw I lock the risers in place with with screw eyes. This one I will do for sure..
2) Paint the risers. I will probably do this next time I am spraying some neutral colored paint. I doubt I will make a project of it, but I suspect it will get done.
3) This is a project in itself, but spray the wooden pieces with photomount adhesive and go over them with leather. That's part of the reason I recessed all the bolt heads. I think if I did that, it would look almost stock.
So, if you have a box table and people tell you that you can not put knee crutch stirrups on it, don't listen. Step back and look at what you have, and what you need to get to where you want to be. It is not impossible. Just a little thought provoking.