How It Came To Be- The Big-Ass Billet Pulley
Once I decided to run a single stage poly-v belt reduction on the e-bike instead of a two-stage timing belt/chain reduction on the advice of Bob Schneeveis I set about figuring out how I was going to come by a foot diameter rear pulley for the bike, preferably a really sweet one, in line with my recent attempts to do things as minimally shittily as possible.
I thought about machining one out of plastic which proved to be practically as expensive or more than solid aluminum, or attaching an octogon of bar stock pieces to the edge of a sheet of aluminum, then turning it to make it round. When I described this plan to my machine shop teacher Louis he looked at me funny and said, “Why not use a single piece of aluminum?”. He then proceeded to find me a spare piece of .800 thick aluminum from the stock pile- “use this!” It started as a piece of an industrial fixture that was donated to the department and had previously been face cut and drilled in a few places- I trimmed and layed out around those spots.
So I pulled out the biggest rotary table I could find and started the longest week I have yet spent in the machine shop-
Many many more pictures after the break–
Pocketing out the face of the pulley, creating the outside wider rim. In retrospect I might have done this using a larger diameter roughing end mill. I ended up taking many passes (6?) with the octo-mill, the roughing mill could have done it easily in one. And probably scattered less confetti-chips all over the place.
The octo-mill went right thorough the fixturing washer.
It’s worth describing how I cut the spokes- as you may notice the spokes are of a constant width, the edges of the spokes are parallel, not strictly radial. I cut each spoke-opening in four steps, each edge in succession. First I cut the inside and outside arcs with the mill Y axis centered on the center of the part. I calculated the radial offset on the X axis and the start and end angles. Next I moved the rotary table so that the centerline of the spoke was parallel to the X axis and used the Y axis to offset the spoke centerline from the tool centerline, then use calculated start and end points on the X axis to cut the sides of the spoke.
Everything previous was rough cuts- I redid ever cut using a larger standard 2 flute endmill. I don’t have pictures of the finishing cuts except for the finishing cut of the center bore- it reminded me of those cacti that have the center full of soft looking fibers that turn out to be really irritating and splintery- very much like these chips.
After finishing the rough spokes I used a smaller diameter endmill to finish the edge of the pocket to its final diameter, and passing between the fixturing bolts and outside edge. This causes all the material left around the fixturing bolts to be in the reigon to be cut out for the final two spokes.
I neglected to take photos of setting up the piece to cut the final two spokes- I used bolts to clamp on the two fully cut out spokes then re-indicated the piece in. I then turned the piece over and set it up again, clamping only in the center in order to cut the piece to its final thickness and create the raised hub in the center.
I found myself having massive levels of chatter as a result of this setup- as I discovered with the very bell-like finished part the thin spokes do a good job of allowing the heavy rim to resonate independent of the hub- in this case causing the octo-mill to chatter and leave an uneven surface- I would guess at least .005 variation peak-to-peak in the finish. I wasn’t feeling particularly excited about refxituring the whole operation- so I stuffed shop towels under the rim of the part using a screwdriver. It actually did eliminate the chatter problem. It may have contributed to the general lack of absolutely perfect flatness in the finished piece, but so could a bunch of other things. I am learning that fixturing, particularly with less rigid parts is one of the hardest aspects of machining.
There I fixed it!
I also made an adapter plate to attache the larger hub of the pulley to the smaller disk-brake hub on the bike, also to move the offset of the pulley closer to the centerline of the wheel to increase clearance with the forks (a really big deal- bikes weren’t made to accommodate big pulleys like this with non-reconnect-able transmission methods like belts). It’s stainless- turned a 4x4x.25 stock into a 3.80 diameter .125 thick plate with center bore. I later drilled the bolt circles on the mill.
Bolted with a 1/2″ bolt to a custom turned arbor.
I devised a clever (I think) way to face both sides parallel- if I turned the piece over and reattached it to the arbor it would only be clamping against the small diameter and un-machined center that was under the bolt and washer in the original setup- I tried that and found the piece was running out badly. So I found the turned aluminum tube seen on the arbor and turned the arbor down to allow it to sleeve over loosely- now the piece is clamped against the machined face of the tube and the tube rests against the turned face of the arbor. The piece was visually perfectly aligned after setting it up this way.
Finally I used soft vice jaws to hold the piece in order to bore out the inner diameter and remove the material covered by the arbor bolt before. I ended up with a really pretty disc of stainless steel.
Hope you enjoyed reading this long-winded and many-pictured account of this (small bit) of this (unexpectedly huge) project! I thought my troubles were over with this pulley made- little did I expect the challenges of mounting the motor and fighting with the Plasma CAM machine, let alone getting the belt to run straight and not skip under hard torque!
In: Electric Poly-V Bike, Machining, Photo Journo