Design
The knob is 3.50 in long and 1.50 in across at its widest, with grip grooves and a raised diamond knurl along the body. It threads onto the shifter through a hole drilled with a Z drill and tapped M12 × 1.5. I designed it mainly for weight and feel in the hand.
Drawing
The drawing went through four revisions before it was ready for the lathe. The main change was which end I would machine from, and that choice drove how I dimensioned it: I reworked the scheme so it was easy to machine from and easy to read, with every dimension needed at the lathe and nothing cluttering the sheet.
Machining
I made the knob from aluminum round bar on a manual lathe, in this order:
- Turned the stock down to size
- Plunged a parting tool to cut each groove
- Drilled and tapped the blind M12 × 1.5 hole
- Knurled the grip
- Turned the taper and chamfered every edge
- Parted the knob off the stock
The lathe's DRO reads to a ten-thousandth of an inch, which let me hold every dimension within 0.005 in.
Result
The finished part matches the drawing: four knurled bands split by grooves, a tapered base, and a faced top. On the rig it feels great. The size fits my hand well, and shifting with it is really satisfying.
One lesson for next time: I should have chased the tapped hole with a small countersink, so the live center used for knurling would sit perfectly in line with the part. Without it, the first few knurls came out slightly uneven. You wouldn't notice at first glance, and over time I've come to like the pattern it made.
Gallery
