Making Plectra

by admin on May 1, 2015

Tools to convert Delrin

Tools to convert Delrin

This photo shows all the tools I use to convert sheet Delrin, or in my case acetyl into harpsichord plectra. Delrin is just a bi-product of acetyl, a brand name given by DuPont, who I believe developed Delrin from the basic plastic to give certain industrial qualities. This advice was given to me by a plastic engineering company following inquires I made about purchasing some Delrin sheets. I was advised to experiment with pure acetyl sheet as the engineer thought it might be more suitable for my needs. That proved to be the case, since I found it hardened less quickly and was there for more durable.

Acetyl sheets

Acetyl sheets

The sheets the material comes in are quite large so I reduce them to manageable pieces of 10 inches by 8 inches as shown in the above photograph. I have heard – and indeed been told by other harpsichord makers – that there is a grain to this material, and it is important to get that correct and in line with the plectra. There was and is no sign of grain at all on the sheet material I purchased. My supplier implied that the grain shown on other materials was probably rolling marks from making the sheets; the specification of the material I bought was very high, producing a surface so fine that there is no visual evidence of the material being rolled at all. I have made plectra in both directions and not witnessed any plectra failure from my material at all so the jury is out about is there grain or not.

From my working sheets I cut 18mm strips onto which I will engineer a taper for the underside of the plectra.

18mm cut strips

18mm cut strips

You will note I have lined one side of the sheet before converting it into strips; this is purely so that I can be consistent with the face side I will use for machining. You will also note that the strips have been mounted onto blocks of wood, (lime) with double sided sticky tape. The face of the block has been accurately cut with a 2-degree angle in order to transfer that angle onto the plectra strip when being cut by a fixed right angle blade; accuracy is essential in order for the final plectra to be consistent.

The small model maker’s circular saw now comes into action: the table saw bench could be used and the cut angle set on machine, but this little saw is gentler for this sort of work, and the accurate control of the wooden fence so easy and with the tap of a light hammer! I was told by my plastic engineer that plastics machine very well, providing you control heat. He advised only the front edge of a cutter be used, giving a fly cutter action. This is why I use a wooden fence set at an angle making sure the back of the blade never comes into contact with cutting area of the work.

The small model maker’s circular

The small model maker’s circular

The work needs to progress carefully in several stages until the correct amount of material is removed, finally, carefully-cleaning the machined angle with facing pads of abrasives 204 grit – 400 grit and finally 800 grit.

The strips are then mounted with double sided sticky tape onto slats of wood in order to control the conversion into a comb.

Mounted strips

Mounted strips

You will note I have covered the edge that will face the fine circular blade used to cut the comb of individual plectra.

I have shown in the first photograph the blade I use for this job, it has very fine hand saw teeth with no set at all. The sledge controlling the strip being cut has been custom made for this job enables plectra to cut at any width from adjusting the fence of the circular saw table.

Cutting the plectra

Cutting the plectra

The strips produce approximately 100 plectra which are separated with sharp top cutters. I like to separate the strips keeping the individual plectra in rotating order when working with a new instrument but of course they can be separated and packed onto containers. I mark the underside on each strip with different coloured pens so they are kept in strip batch order.

The finished product

The finished product

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