DAC90A Perspex backing discs

[DON THOMSON]

Hi

Having punted around in a fairly half-hearted way to find a pair of perspex backing plates for the knobs on my DAC90A I decided to have a go at making a pair. I don’t have access to a small lathe but from time to time I will turn up a small object using power drills. As a result I have acquired some technique and a small collection of bits and pieces that are very helpful.

The bottom right item in the picture is the assembly I made up ready to start shaping the outside of the new backing plate. All of it runs truly except the perspex which is roughly cut. All of the items on the bolt are in close contact with it, there is no side movement possible. The large brass washer was the same size as the example top left until I turned about 1/16" off it to be the same diameter at the wide end of the original piece I had left. The washer at the other side of the perspex block is turned down to the diameter of the original’s narrow end. These are guide plates to stop me cutting too deeply and the brass washer also stabilises the thing.

Reading along the bolt we have the head, a small gap, a rather special nut, a thin flat washer, the large guide plate which has inserted snugly in its centre a washer threaded to the bolt’s thread, the perspex block, the small guide plate and finally a nut to hold it all together.

The perspex block was made by cutting two discs out of 4mm perspex using a woodworker’s hole saw, sizes: a smidgen over 1 and 5/8" and 1 and 1/8". They were cemented together using superglue being careful to align the holes and pressing them firmly together. When this hardened a drill was run through to clear any leaked superglue or misalignment. The hole was too large for the bolt, of course, and I used a 7mm length of aluminium tube which was a press fit in the perspex and a sliding fit on the bolt to ensure a close fit.

In the past I have had the greatest difficulty in getting rigs like this to run truly using various combinations of nuts and bolts until I found the rather special nut. It had extremely sharp edges and appeared to have been machined, a precision item, unlike the stamped out things we normally buy.

Here I made a big mistake. I spent ages making out of steel a tool to the profile of the original backing block: the idea being that I would merely have to hold this against the perspex and it would automatically almost cut the right shape. When I tried it wouldn’t cut for toffee. I thought I could use it to check the shape as I went along but I soon got fed up with that: it was easier just to put the original alongside the block I was turning, the differences stood out a mile.

Using a variety of knives, files and sandpaper I arrived at the final shape. I removed the small guide piece and cut a small bevel at about 10º and 3/64" wide on the front edge.

Finally I polished it using bakelite polish

Then I got to work on the hole in the block using the countersink, top right of the picture. Working from both sides(which allowed me to see if I was wandering I was able finally to pass the bit right through the block. As I have no ready means of drilling hole above1/2" I used a tool of my own devising. It comprises a short length of copper plumbing tube with teeth snipped on at one end and given a slight offset, a nut soldered inside near each end with a screwed rod through and a wing nut to lock it all together. Normally I use it like a drill but I felt that would be risky so I used it as shown. As the teeth wear and the offset is straightened out the hole cut is slightly tapered, exactly what I wanted. I had to dismantle the tool to get the perspex block on the first time but for the second and the final passes I was able to slip it on from the front and then give the teeth a greater set using pliers. A quite rub of the inside of the hole with sandpaper, a touch of adhesive(not superglue, I might want to take it of at a later time) and job done.

Don

[GJR 11L]

Good bit of work there Don, I must admit to getting quite a satisfying feeling from making a part that cannot be bought, be it for a radio or an old motor.
I'm glad that my '90As are both early examples though, it's easier to cut a circle of felt!
As a matter of interest, what speed did you use to turn the job and did you have any attempts that melted before producing the final piece?

[DON THOMSON]

Hi JPB,

Thanks for your comments.

I've got about 8 drills and use whichever seems sensible, eg for the final work on the hole I used a battery powered infinitely variable speed drill.To reduce the brass washer I started with two normal speed drills, one holding the disc, the other a sanding disc.(I finishing it off with sandpaper when I got close to the size. No problems with heat.

Regards


Don