Making a Dividing Plate

A dividing plate is very useful for things such as drilling holes 120° apart, milling hex flats, etc. By coincidence I had 2 hard drives fail in the past month, they had been running 7/24 for >6 years. A few minutes with a torx driver and out popped a bunch of shiny discs.

The centre hole is just under 25mm, so a piece of 1” steel was sliced off and step drilled to just under 0.25”. A reamer finished the hole to exactly 0.250. The slice was then placed on an arbor, and the circumference turned down to an exact fit into the platter’s hole.

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The Taig group on Yahoo recently had a discussion about making indexing plates. There are several ways to get the 40, 50 and 60 holes located on circles.

  • Rotary table
  • Direct via template
  • Using a pattern such as a gear, circular saw blade etc

Creating a template is straight forward. Print it out, stick on your platter with removable glue and mount on the milling attachment or a piece of angle iron bolted to the cross slide. Loosen the centre bolt, turn to next line, lock, drill, repeat many times. With a bit of care this should be accurate enough for most hobby operations.

Using a pattern such as a gear or saw blade is describe in various places. Monty Remon shows several approaches using gears. Keith Brooke shows another method using a drill press.

Lastly Nick Carter sells 60-50-40 index plates for a reasonable price.

Which brings us to the rotary table. Having just received mine from RDG Tools, I needed no excuse to start using it. I tapped the M6x1 centre hole of the table out to the full length. This enabled a locating pin to be threaded in, which I made exactly 0.250” diameter to match the adapter I made above. I set up a carriage stop so all the holes are a consistent depth, the angle on the #3 centre drill provides a nice chamfer.

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Locate the centre drill along the outer circle, lock the vertical and cross slides and start the drill-rotate-drill sequence. 60 holes later, move the cross slide to the next circle and start the 40 hole (9° apart) sequence. The centre drill I used was probably not sharp enough, lots of tear out on the reverse side. 200 twists with a small countersink cleaned up the holes!

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I still need to make the block with index pins and a bushing to fit the plate onto the drive pulley but the tedious part is done.


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