Part 6 by Jason Ballamy


I machined this on the other end of the bit of 40mm dia steel that the crank came out of. Start by facing the end and then turn the 12mm dia x 6mm long spigot followed by drilling and reaming 6.0mm.

The cam can be formed in a number of ways: milling, turning in the 4-jaw or even filing to the template included on the drawing. I opted to mill it using the rotary table as follows:

- centre the rotary table under the spindle and zero the dials or DRO, lock y-axis

- hold work in chuck , put a 6mm rod in the collet chuck and bring down into the hole

- move table 11mm in the x-plane allowing the chuck to slide along the rotary table

- Bolt chuck to the rotary table

  1. -Fit a cutter into the collet chuck and work your way in from the edge rotating the work on the table until the x-axis is 25mm plus half your cutter dia.

Saw off from the rest of the bar and then hold by the spigot in the 3-jaw, face back to 3.5mm thick and then take a further 0.5mm cut but stop to leave the 8mm raised area in the middle which will make sure the cam only contacts the bearing inner race.

You could mill the two 10mm radius corners that blend the arc into the main diameter but I just opted to file it to shape.

Final job is to drill and tap for a M3 grub screw.



Start by milling up a block of aluminium to the overall 16x22x17mm. Then hold in the mill vice with the top facing upwards and locate it centrally below the spindle followed by drilling the two M2.5 clearance holes.

Change to a 6mm dia cutter, touch off on the top of the block and then lower so you are taking a 3mm high cut, cut in 4.29mm from each side and while at that height bring the cutter in from each side to form the counterbore centrally over the stud holes.

Now holding the block on it side drill and then bore for the bearings aiming for a light press fit, if you over do it then a high temperature Loctite such as 648 can be used to retain the bearings. I opted to use number 686 unshielded bearings which are 6x13x3.5 from ARC Eurotrade but 12mm OD ones would work just as well. As the flame can lick around the bearings, avoid ones with rubber seals though, if you have some to hand, the seal can be flicked out with a pin or small jewellers screwdriver.

The last job is to round over the top, I did it by holding the work on a mandrel and then taking cuts at 5degree intervals on the spin indexer and then finished with a file. Could also be done on a rotary table or just by filing.

You can now fit the bearings, slip in the crankshaft and check all turns over nice and smoothly.

Included on the drawing are the four simple turned ‘washers’ that fit into the counterbored holes in the sub base. Once bonded in with JBWeld or similar they will look like cast bosses. Turn and drill sufficient material for the four parts and an allowance for parting cuts and then part them off and deburr.

Part one here  Part two  Part three Part four  Part five  Part six Part seven Part eight

Part nine -

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