Bike Rack, for Grizzly Peak Century

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How they did it

Making the bike racks involved fabricating three components: the cross-bar, the legs, and the end caps. The cross-bars start with 10' length of 2" EMT (Electrical Metallic Tubing, more commonly known as conduit). Eight holes, four at each end, are drilled through the conduit to hold the legs. The holes are an "odd" size, 15/16", to fit the conduit (so the bi-metal hole saw is not easily available locally). The holes are not drilled straight through the 2" conduit, rather they are at 10° from the horizontal, and 35° from each other.

The Crossbars - the prototype


For the prototype, the pilot bit in the hole saw was replaced with a 6" piece of smooth 1/4" rod. A carefully computed paper template was taped to each end of the 2" conduit, and a center punch marked the center of each of the 8 holes. A hand-held electric drill was used to drill 9/32" holes at each center punch. Then the special pilot was slid through the pair of holes (which gave the 10° angle), and the hole was partially drilled. The 2" conduit was rotated 180°, and the hole was drilled all the way through. Then the conduit was rotated back, and the partially drilled hole was finished. (If you drilled the first hole all the way through, there would be no locating hole to align the pilot for the hole from the other side.)

This process was repeated for each of the four pairs of holes. Doable for the prototype, far too labor intensive for 30 racks. (also, drilling by hand is very hard on the drill bit. We wanted to minimize the number of bits needed for the more than 250 holes drilled.)

The Crossbars - production


Jig, set up for first hole. Mark on conduit is set
to the jig-line closer to the operator.

Jig, with 1" spacer set up for second hole. Conduit is
rotated to line up with farther line.

Jig in coolant tub.

James Rosenau, in GHI Plant #1

To make more than about 5 bike racks (the initial order was for an additional 30), it was necessary to develop a process that would be repeatable, speedy, and would prolong the life of the tool bits. Tool bits (like the hole saw) work best when held perfectly rigidly with respect to the work piece, are operated at the correct speed and feed rate, and are cooled. A hand-held drill had none of these characteristics.

Jeff decided to use his 1hp drill press. The correct speed for the 15/16" hole saw was 320rpm, and the drill press could be set for that speed. (While the hand-held drill had variable speed, it was not variable constant speed, and so was very dependent on tool load.) A jig would be necessary to hold the work securely, and make the holding repeatable.

The jig shown was made from construction lumber and bolted to the drill press table. The drill press table itself was tilted to 10°, and the far-end of the 10' conduit was set on a ladder step. The jig has two 90° "v-blocks" to hold the conduit, with a stop block at the (downhill) end.

The stop block has a pair of lines, 35° apart. In use, the "top" of the conduit is marked with a Sharpie on each end. (The top is arbitrary as long as it is the same on each end; we marked them by eye in the stack.) For the first hole, the mark is aligned with the stop-block line closest to the operator. The first hole is drilled all the way through. It was not necessary to set the quill stop; the total travel of the quill was just enough to get the bit all the way through the conduit.

To drill the second pair of holes, a 1" spacer block is inserted next to the stop block, and the conduit rotated away from the operator 35° to the far line.

Toward the end of production this method turned out to be insufficiently accurate, as quite a bit of metal cuttings lodged in the wooden jig. This caused the 1" spacer to sit slightly crooked in the V-block, causing the second hole to be drilled in the wrong place. A small error in the 35° separation of the legs was not a problem by itself, but 6 of the cross bars had to be re-done because the error was not the same on each end (resulting in a "wind" of the legs). These corrected legs were aligned on the "second" end by putting a piece of 3/4" conduit in the existing hole in the other end of the 2" conduit, and aligning that conduit to vertical with a level.

The hole saw created an enormous amount of heat (especially as it became worn), so coolant was necessary from the beginning. Coolant is readily available from industrial sources in 5gallon quantities. We didn't need that much, so Jeff made some "homemade" coolant with 1 gallon water, 8 ounces motor oil, and 1 ounce dishwasher detergent. This was applied directly to the bit while drilling with a spray bottle. To contain all the coolant (and to keep as much as possible off the rust-able drill press), we fabricated a dishtub to hold the jig, and fitted it with a drain line to recycle the coolant into a gallon container. The gallon of coolant was recirculated about 10 times over this project.

Jim Rosenau is shown in the "coolant pump" position, in Grizzly Heavy Industries plant #1. (Plant #1 was selected because of the need for a Bridge Crane with sufficient capacity for this job.) On the first day, Jim and Jeff perfected the setup (including carrying Jeff's drill press up 5 flights of stairs) and drilled 16 cross-beams. Destroying the first drill bit. (We eventually used three 15/16" hole saw bits and two pilots. Drilling steel is not for the light-hearted.)

Drilling in the holes in the cross-beams produced a fairly ragged holed (more so as the bits wore). These had to be deburred, and we found that a 1/2 round file, while very tedious, did the best job. All of the deburring was done by Micky and Chris on days 2 and 3.

The Legs


Cutting the 48" legs, using Jim's small vise.

Cut and deburred legs, waiting to be drilled.

Jig to drill legs.

The legs are cut to 48" (1 inch shorter than the prototype tested by Phil Morton). We experimented with cutting the legs with a tubing cutter and with a reciprocating saw. The tubing cutter produces a fairly nice "rolled" edge, but is slow. On our initial tests with a reciprocating saw, we got such a ragged cut that the time savings were mostly lost with filing cleanup.

For day two, Jim loaned his pipe vise, a large tubing cutter, and a conduit reamer (as used by electricians). With the conduit held steadily in the pipe vise, the reciprocating saw produced a much cleaner cut, and quite quickly. Small touch up with a file followed by a twist with the reamer produced a very good edge. The 132 legs produced were all cut by Micky and Chris on days 2 and 3.

Once the legs were cut, they needed a small stop screw 3" from the end. The prototype was drilled by first using a center punch. But another jig was built, holding the conduit level, to drill a 7/64" hole 3" from the end. With this jig Jeff was able to drill all 132 legs.

Then a #8x1/2" sheet metal screw was driven into the end.

Each pair of legs got a "rubber band" (cut from a bicycle inner tube) to put around the protuding ends of the legs. These will keep the legs from dropping out if the end of the rack is lifted. (Dragging the racks is likely to bend the legs, so I hope they will be lifted if any moving is necessary.) Jeff cut all the rubber bands.

The Endcaps


Cutting 2" ABS into 4" pieces.

Another jig, to cut the 60° slice from each cap.

Heat forming the caps; the one on the right is not done.
The jig in back is a pusher to get the formed caps out.

Finished caps.

The plan for the Bike Racks always included storing the legs inside the cross-beam. But some sort of cap was needed; otherwise you had to hold the cross-beam perfectly level to carry it. I originally considered using a regular 2" sewer cap; a rubber cap with a large hose clamp that fit over 2" sewer pipe and sealed tightly. They are a) expensive, b) might not fit conduit (which has a smaller outside diameter than does the thicker-walled sewer pipe), c) will not stack particularly well, d) require tools to remove and install, and e) were likely to get lost.

Instead, I tried cutting a slice out of a 4" piece of 2" ABS sewer pipe, and squeezing it closed to spring-fit inside the end of the cross-beam. Perfect! They have enough (plenty!) of friction to stay in the tube, and can be put back into the end of the tube when assembled, so they won't be lost. (the 4" length also allows end-end ganging of the Bike Racks, but that is not likely to be useful.) Note that there is still a large hole through the end cap when in the conduit; that hole is just the right size to keep the pair of legs from getting out.

But making the caps is not easy. First, I cut up 3 10' lengths of ABS with a Miter saw. Note the end mark on the saw outfeed is just a piece of tape. I tried my usual method of clamping a block of wood as an end mark, but I found that as the blade lifted it would catch the ABS, push it into the block, and jam it against the blade. That would cause the ABS to explode; not good.

I cut the 60° slice out of each four inch piece on my table saw. You cannot just lay the piece against the fence and slide it through; as the blade exits, the force holding the ABS down causes it to "pinch" on the blade. This grabs the blade and throws the ABS with some force. While your hand is poised above the blade pushing down....

So another jig was called for. The four inch ABS is slid over the wooden disc and held with a spring clamp to the dowel above the blade. The jig is cut with two faces 60° apart. Feed the piece through to make the first cut, then rotate the jig to the other face to make the second cut. Over only three 10' lengths of ABS there was quite a bit of variability. Some had so much "spring" that they squeezed the blade anyway; on the first cut there would be enough heat generated to melt the ABS and set off the smoke detectors! Fortunately, I was wearing hearing protection anyway, but my cat was nonplussed.

Once cut, the ABS is still too stiff to be squeezed by hand small enough to fit inside the 2" conduit. So I had to cook it. Thirty seconds in simmering (100°C) water would soften it enough that I could squeeze it into an uncut 4" piece of ABS. I dropped it into cold water, and a few seconds later pushed it out of the 4" ABS with the pusher jig shown. Note that boiling water may be too hot; if left in the water for only 40 seconds (instead of 30), it would get so soft that it had hardly any spring-back. (In an experiment, I completely flattened and extra piece of ABS.)

I made about 75 caps so there are some extras.

The Final Product


33 racks, 8 extra caps, and a box of rubber bands.
Jeff Oh might consider the design of the carrier.

The order for Bike racks started at 24, then went to 26, then to 31. So we made 33 additional racks to add to the prototype (which is currently with Phil Morton for stress testing.) The cost of the additional 33 was about $27.10 each (including tax), and a whole lot of work. It would not have been possible without the tireless and thoughtful assistance of Jim Rosenau, Micky Bloom and Chris Witt of Grizzly Heavy Industries, as well as Phil Morton's off-site testing.

For more information, contact Jeff Kurtock.