Build your own Prone Cart.
Alot of this is Theory and if you do it differently, or better, good for you. It isn't, after all, rocket science. The most valuable tip I can give, is to get off your duff, and just do it, one small step at a time.
The dimensions of the cart work for me and can be changed to fit. The one I'm building has 16" front wheels. It's lower to the ground and makes working in the garden nice, also transferring to (and from) the ground. The hubs hold straight sealed bearings so I can use a sawed off bolt for an axle instead of messing about with bearing races. The hubs are not that easy to find but hubs from an old wheelchair will work if they have straight bearings. The rear wheels are from a push mower. About 7.5" diameter. They only mount on one side, instead of the typical fork used by casters, so they gotta be a little heavy. The first ones I made bent in use, so I straightened them and welded 1/2" rebar down the length and now they're good to go.
Step 2. Cut steel according to cut list from drawings.
Step 3. Jig. Make jig from scrap plywood according to dimensions in drawing, I used 3/4" plywood. Cut out corners with circular saw, (this allows you to weld both sides while the frame is still in the jig) Sand off splinters
Step 4. Jig. Take the corners you saved from the jig sheet and cut them into about 1" wide rectangles. Lay a piece of steel 1" from edge of jig and c-clamp into place. This is the long piece at the TOP of the jig. Blob yellow glue on rectangles and push on jig sheet so there are 2 opposing (on each side of steel) at each end. Let the glue set a while, then clamp short pieces of steel where they are supposed to be, squaring off the top piece. These will be the vertical parts of the frame. Glue and place scraps of plywood, same as with top pieces.
Step 5. Frame. Now that the jig is done you can see how it will hold the pieces "captive" while you weld all around the corners. The bottom piece is just positioned and c-clamped against the verticals, that way, if your short vertical pieces are a little too long or too short you still get a good tight joint for your weld. I try to leave jigs "open ended" like that because I don’t want to spend all day at the grinder making my pieces fit into the jig. Enough blah blah… Clamp precut steel into jig where it belongs and weld the corners front and back. I use a dc arc welder and 6013D rods 3/32" at about 55A. On this one I did not weld "all around". I skipped the fillets in the corners and just did the flats on both sides. It's strong enough.
When you have 2 sides done, join them with the 3 horizontals, 2 in the back (top and bottom) and one in the front (top only). Use whatever you got (magnets, squares…) to square it up. As you can see I tacked all three crosspieces onto one side then tacked the other side on. You gotta force the cross pieces into place for the second side. Now that you have the frame all tacked together, use pipe clamps, diagonally, to pull it into square, then tack some angle or scrap onto the frame to hold it square while you do the final welding. This is easier and faster than having your pipe clamps falling off every time you move the thing.
Step 6. Casters. As I said at beginning, the wheels are off a lawn mower, and you can see for yourself the rebar welded down the back of the upright part to strengthen it.
Notch the frame with grinder, or I like to use the drill press and a 1 inch hole saw. I should have done this before welding the sides together but forgot.
Get some 3/4" schedule 80 pipe nipples either a 6" long or 2 shorter ones. Make sure they're sch 80 as your bushings should go into these nicely. Cut off the threads with the chop saw as shown leaving 1 and 1/2" (1.5") long piece. Make 2. Chamfer the inside edges so the bushings go in easier.
Clamp them into the notch in the rear of the frame with long pipe clamp and blast them in. Use a 1/2" bolt to press the bushings in. As I did, you will have to use washers or big nuts for spacers when pressing. Doing them like this ensures they will both be lined up with each other after they're in. The bushings are 1/2"id, 3/4"od, 3/4"long with a flange. They cost about 60 cents each from MSC. The upright bolt on the caster is 1/2" dia. and a little long. When your using the cart you may notice that one back wheel is off the ground a lot more than the other. In this case simply shim that one down by putting washers between the caster and the frame until they run about even.
Step 7. Front wheels. Determine the length of bolt to use. You want the bearings to ride on the shoulder of the bolt and not on the thread, so measure the wheel width(at the bearings). Add an inch to go through the wheel mount, and a little more for a washer to keep the hub from rubbing the frame. Take a small scrap of the 1" tubing for the wheel mount and drill a half inch hole through it. Slip a half inch bolt, of the proper length, through the hole and tack weld the head of the bolt to the tubing. I use a 6010 or 6011 rod for this cause they bite into the bolt head a little better. Put washer on bolt then wheel then nut. Clamp wheel mount on frame and eye ball the wheel straight with the frame. Once it's straight blast away. Before you weld the bolt into the mount (or the mount to the frame) do a trial fit up and then you can cut off extra thread with the chop saw. You won't need all that thread hanging out catching on the wood work.
Step 8. Deck. If you hadn't already cut steel for this, do it now! Clamp pieces to frame of cart for welding. This will help to make the top lay on the frame reasonably flat. Well, if not flat at least they will be similar. Weld in tabs as shown. I use a scrap of 2x4 that has a scrap of the plywood I'm going to use for the top (1/2") attached to it. This will keep your tabs that far below the upper edge of the top. In other words, the plywood will sit inside the frame on the tabs, and will be flush with top of frame. I only weld tabs on the bottom side. It seems to work ok. Screw plywood through tabs (from underneath) with small wood screws. Oh yeah, tabs are 1/8" flat stock, 1" wide, sawed 1" long. I think the only welds I ground were on the top of the top to help keep things smooth.
Now make 2 small pieces using the 1/8" (or 14 gua, something like that) to make hinges for the top to tilt on the frame. Drill for 1/4" bolt and bolt to top. Position top on frame and weld hinges to frame..
The plywood will have to be notched on the sides for the hinges and nitrogen cylinders.
Step 9. Paint. Looks good eh? Now wire brush the whole thing, remove wheels and paint the whole thing Navy Grey.
Step 10. Cushion. I used 2" foam from sears or jc penney. I forget which. Sew the top together with heavy weight but light color fabric. I like cottony type fabric and for goodness sake don’t get black unless you like laying on a frying pan in the summer. Attach pad to deck with velcro.
I skipped some parts. The nitrogen cylinders were fished out of the garbage can at a local garage. I got a whole pile of them and these are smaller than most. You can calculate their position or just trial and error as I did.
The lift prop is 1" angle iron with 3/8" holes drilled through it for positions.
The latch pin is a 3" long 1/4" bolt with the head cut off. Cross drill for wire brad spring retaining pin. Spring is from junk can full of springs. (I knew I would need them some day)
Spring pin enclosure is a hunk of 3/4" sch 40 pipe about 1.25" long, cross drilled for spring pin.
Linkage is from 1970 ford truck carburetor. Latch handle is brake handle from old wheelchair.
Bolted sheet metal clip at top holds prop against frame. Clip at bottom keeps prop from catching on frame.
Operation is as follows: pull handle back retracting spring. Lift body weight off of chest. Nitrogen cylinders overcome remaining weight of lower extremities lifting cart. Release handle so spring pin engages hole in lift prop.
I think I will put a prop and latch on the other side too. When you are inclined there is an uneven springiness to the top with only one prop. It works ok but I think will be stronger with 2 props.