Shop Note

Making your own Proportional Dividers

Gerald A. Wingrove

Wingrove's Figure 11.

Figs. 11 and 12 show proportional dividers, which for those who have not met them, are instruments for directly converting a given measurement from one scale to another, used mine throughout the building of the model, setting them to reduce the measurements of the deck plan at the scale of 1/4 in. to 1 ft, to the scale of the model. I give them here as it was in the sorting out of the plans that I made use of them first, mainly for cross-checking the scale drawings produced with the aid of the pantograph but also for making scale drawings of some of the very small items such as pumps, wheel, wheel box and the hatches, etc. As will be seen from the two sketches, I have given details of an adjustable and nonadjustable instrument. We will deal with the latter first as it was the first one to be made.

It was while in the middle of an earlier model and at the time using a scale rule (this is a strip of wood or card, with the feet in the scale of the plan marked off along one edge and the feet of the scale for the model marked off along the opposite edge) that I came across the proportional dividers. After learning of the exorbitant price asked for them in the shops I decided to have a go at making a set myself. As I was in the middle of a model and did not wish to spend too much time making tools I made the first ones as simple as possible. used two pieces

* "Silver Steel" is British for what in the US is called "Drill Rod".

of round 1/8 in. diameter silver steel*, flattened out to one side of their centres and bent as in Fig. 11. With a hole drilled in the centre of each flat and a small bolt and nut made for the pivot, it needed only the trimming to length and filing of the points for me to have a very useful instrument.

The lengths "X" and "Y" depend on your scales. For our model if "X" is made half the length of "Y", when we open the points at "Y" to 1 in., which is 4 ft at the scale of 1/4 in. to 1 ft, the points at "X" will be open only 1/2 in., which is 4 ft at the 1/8 in. to 1 ft scale.

The pivot screw should be screwed down fairly tightly, so that the arms stay where you put them when you are transferring measurements between drawings.

Wingrove's Figure 12.

The adjustable porportional dividers Fig. 12, were made when I had finished the model and had a little more time. I used brass, with steel for the top and bottom slides, pivot screw and points. The body "Y" is of strip brass 1/16 in. x 1/8 in, with a small piece of 1/8 in. strip silver soldered on each end at "X". When the ends had been shaped as shown, they were drilled and tapped for the steel points to be screwed into them. The slots and shoulders down the centre of each body half were milled on the Unimat™, and are 3 inches long, which is just about the maximum length that the little machine will take in one setting. The top and bottom slides were turned from bar in the round and then half flats filed on them to make each a good fit in the milled slots of the bodies. The top and bottom locking rings are knurled and are for locking the pivot assembly in the required setting. The pivot pin has a small recess turned under the head for a spring. This holds the two body halves together under tension for tightening the pivot screw of Fig. 12.

The pattern on the end of the pivot pin was made with the aid of a knurling tool run into the slightly angled face, this provided a very useful thumbgrip.
{Gerald A. Wingrove}

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