Sunday, December 10, 2023

A flat work table for making cargo bike frames

Cargo bike frames don't fit very well on a standard bike frame building jig.  I've gotten around this by placing a sheet of plywood on a flat section of the floor and lofting the plan on to it.  This has been very solid and square, but my knees and back are starting to complain.  When I decided last spring that it was time to start building the two wheel recumbent frame I've been dreaming about, it was also time to build a worktable.

A machinist's metal worktable is usually a carefully machined cast iron or steel surface that weighs a ton or two, is damaged after the first few projects, is not portable, cost more than my budget, and doesn't play well in small spaces.  The next most stable bench would be a woodworker's 2" to 3" thick laminated hardwood top, which is still too substantial for my space.  My projects weigh only 20 pounds and this bench is for layout and fitting, not hammering, so the surface mainly needs to be flat and long.  This rules out most welding tables.  Last summer a neighbor put out a flat panel hollow core door free for the taking at the end of his driveway and I thought it would be perfect, but after washing it and putting it in the sun to dry I found that hollow core doors will sag and warp.  This left only plywood or medium density fiberboard (MDF) for a top.  Because MDF sometimes snaps in two when bent, I used plywood.  However I used 3/4" CDX and then spent hours gluing veneer and adding wood filler to make a smooth surface.  If I build another table I'll use a better grade. or even a hardwood plywood.

The top is 30" wide by 80" long.  Measuring the bikes I have, the tallest frame was just under 24", and the longest frame (not counting the forks) was 82".  I wanted to be able to move the table outside while set up and my exterior door has a clear opening of 33" wide, however any clamps around the edge would take up 1-3/8" on each side, so I rounded to 30" wide for easy math while measuring from either side.
Similarly I decided to use 80" long for easy math, although it could leave a few inches of a frame hanging over the edge.  This isn't a big concern because the frame jig can extend past the table.  I took some time to make the edges of the top very straight, square, and parallel to use for references during measuring (less than 1/64" deviation over the length).

The table is 34" tall.  While standing with my arms bent at the elbow so that my forearms were horizontal, I drooped my hand as though I was working on tubing, and measured the height of my fingertips.  (This placed the tabletop 3-1/2" below the bottom of my elbows.)  This turned out to be very comfortable, close enough to see well when coping notches but not so high that I had to splay my arms.out sideways when reaching across the table.

The top and frame
Plywood warps, and the top is secured to a strong frame with thumb knobs so that it can easily be adjusted flat with shims stuck between the top and the frame.  
A 2x4 frame.assembled with 6" screws and construction adhesive provides a rigid base for the plywood top.  Allowing for a 4" wide lip around the top to fit my F clamps, the frame is 22"w x 72"l.

Attaching the Top to the Frame  Eight blocks glued and screwed to the underside of the table fit into the corners of the frame to locate the top.  After painting everything with a one part epoxy paint, the sides of the blocks were covered with clear plastic shipping tape to prevent the blocks from sticking to the frame.  I originally thought I'd use fixtures that clamp through the table (similar to a machinist's metal table), and drilled three rows of holes.  However the jig makes this unnecessary, and only the center row is needed for anchoring the jig to the table.

Underside view of securing the top to the frame.  The blocks fit snug into the corners of the frame, and a thumb knob is used to pull the top tight.  I used 5mm thick corner braces designed for metal shelving to resist bending as the knobs were tightened, but the brace tolerances were terrible.  After cutting the corner weld apart and bending the braces square on a press, I drilled 1/4" holes in the sides and mounted them on the frame with a 1/8" gap to the blocks.  In this picture I've drilled a hole in the brace and the block that fits a 5/16" stair handrail bolt, (which has a wood thread on one end and machine screw thread on the other), and have tightened double nuts on the bolts and screwed them into the blocks.  In the center are a couple of thumb knobs, and in the can are handrail bolts that I've shortened to fit the knobs.

A Typical Frame Jig  Since the bike frames were less than 24" tall, a thin sheet of plywood can be ripped in half lengthwise and used as the base for building a jig.  The table has a row of 1/4" holes down the center and the jig base is fastened to the table with several carriage bolts to keep it from moving around.  Then the design is easily drawn onto this base with tape measures and tee squares using the accurate edges of the table for reference.  Next shim blocks are glued onto the base drawing to hold the frame parts in alignment - the widest part of this bike frame is the 5.5" rear rack, and all blocks were cut to hold parts at a 2.75" centerline.  I used a tablesaw to cut the blocks out of scrap 2x4, but a miter box should work as well.  Tabs were glued to the sides of the blocks to help position the parts, but as the frame was assembled the warpage from welding gradually accumulated to create a tight fit, and it might be better to screw these tabs on so that they can be repositioned.  I ended up using a sheetrock knife or chisel on a couple of them.  I used 1/4" thick plywood for the base which flexed enough to let me finish building the frame, but this picture taken towards the end of building shows three F clamps holding everything down flat to the table while a pair of sticks clamped to the seat mounts are used to line them up parallel with the table.

A multitude of holes is needed in the center row to allow several positions for jig bolts to leave spaces for squares or other marking devices on the drawing while fitting up the frame.  A dial gauge holder with a sheet metal strip can be held against a part such as a bottom bracket, and then the bracket removed and a long tube slipped under the strip to be marked for length.  I've found that bottom brackets vary in outside diameters, so I cut discs that fit inside the brackets that were glued on the blocks to locate the brackets.

Legs  Folding shelf brackets attach the legs to the frame, which reduces the rocking to about 1/4 that of standard folding table leg hinges.  The disadvantages are the legs fold an inch wider and there is almost no sideways racking resistance. but this table only needs to fold enough to be out of the way against a wall, and the legs are paired together by a sturdy cross brace that prevents sideways movement.  Since the legs butt directly against the frame side rails and a cross member, the table is quite strong.

End Results  After shimming, the top is very flat with no light showing through underneath a 72" straight edge.  The table was also adjusted flat across the width using a yardstick, and a pair of winding sticks were used at several locations to remove twist.  There are a few small dips but the jig helps to smooth them out, and this is more than flat enough for a bike frame.

For comparison, a jig for a regular bike frame would be about half as large.

The rear rack clamp set up was much easier than propping up the rails any other way.  A 4" wide board between the two rails kept them parallel, and a 90 degree angle plate (made of plywood) lined up on the drawing kept the rails both straight and perpendicular.  The tail end of the rack does extend off the edge of the table by 4", but there are enough jig props that it doesn't matter.

Another example using the plywood 90 degree angle plates is holding side rails vertical on a recumbent seat frame.  I've made a couple of these seat frames on a smaller surface with a tight fit and the rail ends have turned out slightly misaligned.  For the next seat I'll draw out a rectangle on plywood fastened to the table and glue on stop blocks in the corners to hold the rails square and parallel, and then use the angle plates to keep them vertical while I tack weld the cross supports.  Note: only tack welds are done on the table, and then the assemblies are taken over to my metal cutting cabinet where I can rotate the assembly during welding to minimize distortion.   I also use scraps of sheet metal under the tack welds to reduce weld spatter burning the jig.

If you would like to build a recumbent seat, a basic pattern with instructions can be found at Recycled Recumbents:
I've found that his pattern has about 3/4" too much lumbar support and is about 3" too tall for me, and I've combined his pattern with angles from a couple other seats to suit me.

The Breeze  The first project on this table was the frame for The Breeze, my two wheel recumbent cargo bike, built this fall and nicely straight with well aligned angles.  It still needs battery and solar panel mounts, and a steering linkage, but I have to install components and check the clearances on them first.  I've spent well over a year sketching this frame in Nanocad Free ( to be sure everything fit, and this winter the project will finally be building the rear swingarm, the part that I started the design with 2 years ago.  I've been spending a lot of time on advocacy for better bike/ped infrastructure, (such as a bike corridor along Route 5 from Massachusetts to Canada), but hopefully next summer you'll see some blog posts about the complete bike.

All in all the table has been very comfortable, reducing bending when test fitting tubes and going back and forth to a workbench, and is a vast improvement in jig holding and accuracy.  If I did need to use it for rougher work I could cover it with a plywood or sheet metal layer and still have the flatness.  As it is though it saves a lot of measuring, straight edge work, and string layout, and is straight for 80". Very nice!