Friday, 18 August 2023

Velocette KTTs 55 and 305: Patterns for Castings


During the preparation of all the parts for the hubs I found that I only had one rear brake arm (part # KS-17/2) -  specifically, one of these:

I needed 2 more. The arm has the distinctive “kink” to avoid the brake arm and rod interfering with the frame. It is unlikely to be able to find even one of these, let alone 2, and so I immediately set about making patterns for castings. As I would soon be making a visit to the foundry, I made a quick audit of other small parts for which I would need castings.

The first was handlebar clamps, part # F-37/26

and girder fork top links (part # 25F and 26F)

and bottom links (part # 23F and 24F)

Most of these parts were needed for the Velo cammy special as I had most of the parts for the 2 KTTs.

I have described in some detail the process that I have evolved for making castings from original parts here and here so only a brief description is given below for these specific components. In essence, the process is to use the original part to make a silicone rubber mould and to then use that mould to cast facsimiles in resin which can be given to the foundry to make sand moulds for casting the metal parts.

The first step is to prepare the original part by blocking off any holes or other features that would prevent the part being extracted from the mould, either from the RTV silicone rubber mould that will be used to make the resin pattern or by the foundryman making the sand mould. These small aluminium additions are held in place with a couple of dabs of epoxy resin.

Pieces of aluminium rod are then attached with epoxy resin to form a sprue and a riser and body filler is used to smooth over any irregularities.

The flat surface of the handlebar clamps will be machined and so a thick piece of gasket paper has been added to provide a machining allowance of the final casting. All of these additions can be easily removed when the silicone rubber mould has been made.

A small wooden box is then constructed and multiple 2BA screws inserted into the base of the box around the perimeter of the part and the part is suspended approximately 1.5 cm over the base. To aid separation of the silicone rubber from the wood and metal parts all surfaces that will be in contact with the silicone are coated with Vaseline prior to pouring.

RTV silicone is then poured up to (but not above!) the top of the part or, in the case of the rear brake arm, halfway up.

The box is then disassembled and the silicone rubber surface that has already been poured and set is coated with Vaseline. This is very important as the silicone rubber that will be poured to form the upper part of the mould will readily adhere to the existing silicone rubber on the lower part, making it impossible to separate the 2 parts of the mould.

The silicone rubber for the upper part of the mould is poured up to the bottom of the screw heads

and, after setting, the box and mould can be disassembled to extract the original part

and the mould can be reassembled for casting resin copies of the original.

An identical process was followed for the girder fork links (noting that washers have been added to the bosses and a thin aluminium sleeve has been put over the threaded portion to provide a machining allowance).

Resin copies were cast for the 4 components and after removing the resin sprue and riser and using body filler to smooth over small surface imperfections (caused by air bubbles that come from either the silicone rubber or resin casting processes) the resin patterns, 2 of each, are ready to go to the foundry.

The red paint is to indicate to the foundryman that these surfaces will be machined which can help them to position their feed sprue and riser.

Some of these parts, particularly the fork links, need to be of high strength with the ability to withstand shock loading and all parts will be cast in AB2 Ni-Al Bronze.

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