Each 3.5" hard disk will contain one or more of these disks which is 5.25" in diameter, with a 1" hole in the middle. The nominal thickness of the disks is around 1mm (less than 1/16") - I say nominal because I've found the more modern disks typically are thinner (less rotational inertia), whereas the older drives are thicker (up to 1/16")
The only thing you can rely on is that all disks in the same platter (collection of disks) will be the same thickness.
Figure 1 - Collection of platters destined to become division plates
I've been collecting hard disks for salvage for quite a while. The magnets are useful (see shed tip #1 ), and I also salvage bearings from them. The disks get shuffled into the pile for making division plates, and the casings go into the scrap aluminium bin (for foundry supplies) - only the boards, screws and little plastic doo-hickies get tossed. I learn a fair bit about mechanical design from looking inside the harddrive as well - there's some really clever braking mechanisms used to return the head, lock it, and so forth simply driven by ground effects from the spinning disk platter.
To make up a division plate:
I insert this mandrel into the back of the spindle of the dividing head. (see figure 2)
Figure 2 - Direct indexing mandrel
The mandrel is made to expand and grip the inside of the spindle once the 1/4" nut is tightened up. A gear is placed between the 2 large washers which is an exact match (or multiple of) the desired index count. When I generated the 40 hole plate, I used a 40 tooth gear, but I could have used a 80 tooth gear if I had one.
Figure 3 - Mandrel in place
The mandrel is sized to match the collection of C218 changewheels I purchased a few years ago. Those changewheels are the basis for the leadscrew of the Taig lathe and are the same metric mod 1, 20 degree PA changewheels used in the myriad of 7x12 lathes available in the US and other locations.
I place a chuck on the dividing head, and use a holder to grip the blank division plate in the chuck.
A centre drill is gripped in the lathe chuck using an arbor supporting a normal drill chuck.
Figure 4 - Dividing head spindle nose
A detent plunger mechanism is attached to the dividing head body which engages the gap between the teeth of the gear wheel. Turning and locking the spindle turns the blank plate, and all I need to do is feed the head into the drill to make the holes in the plate. The bracket supporting the detent plunger system is made from an offcut of an aluminium angle extrusion. The bracket is bolted to the body with two socket-head screws, and has holes to pass over the heads of the body bolts. Figure 5 (below) illustrates how it is fitted, with one securing bolt removed for demonstration purposes.
Figure 5 - Detent plunger system fitted
The detent plunger has a wedge shape when viewed from the side, but an inverted V shape when viewed from the front - this is to allow the indexing of the tooth tip instead of the gap between the teeth. This means I can index in the gaps of a 20 tooth gear, then rotate the plunger 90 degrees, and then index off the tooth tip and obtain another 20 positions - allowing me to generate a 40 hole plate from a 20 tooth gear.
The mount block for the detent plunger is bolted to the vertical arm of the bracket in one of 3 positions, allowing for a wide range of gear diameters. The large hole drilled through the body (from it's previous scrap origin) is used to secure the plunger with an elastic band if the internal spring is needing a little help.
The last article will cover (albeit briefly) the sector arms, plunger arm, and retaining mechanism.
Next project for documentation will be the field desk if I get it completed on schedule.
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