Geoff Byman
all pictures by the author

AS MODELLERS, we sometimes come across the need to have a special tool to do a certain job. Whilst in some cases the job can be carried out by hand, where many repetitive and reasonably accurate parts are required, then a jig or tool is almost a necessity. In engineering, I had many such jigs and tools specially made for the job in hand. In this particular case I needed to form a shaped parapet top, one that whilst quite simple in shape, would prove to be extremely difficult to cut accurately by hand. Having used scraping tools in the past, the simple paring tool approach was ideal.

1. Parapet top profile 2. Profiled cutting face

The parapet top profile can be seen in the photograph. To create this would require a profiled cutter of some sort. Rummaging around through my scrap box, a suitable piece of 19mm (¾in) square aluminium bar and an off-cut of 1 x 1 x ⅛in black angle steel were found. A shallow slot was milled centrally along the aluminium bar, 8mm wide by 0.75mm deep, to guide an 8mm wide strip of 1.5 mm, 2 mm or thicker plastic, accurately through the cutter.

The off-cut of black angle steel was slotted 8.5mm wide down one face of the angle, filed square to the slotted face and filed back at 30° to the adjacent outer face of the angle, forming a sharp edge. The angled sharp edge face was then filed to the profile required for the top face of the strip. Having created the required profile, the inside of the angle has to be tapered back to the 30° angle, retaining a sharp edge at the cutting face.

Two holes were drilled equal distance either side of the slot at 15mm centres, close to the inside edge (approximately 4mm) of the angle steel, with room for M4 Allcap bolt heads. In this case as the bolts needed to be as close to the cutting face as possible, the bolt heads were reduced in diameter to 5.5 mm.

3. Components

As the cutter is made from black steel, not being finally machined true, to allow the bolt heads to sit parallel to the mating face a flat face pocket was milled across the slot at right angles (parallel to the cutting face of the angle steel) (See milled slots visible in Photo 6.) 0.010in brass shims were made, slotted to enable them to be removed from around the bolts when slackened, in order to shim the cutting face to the various heights, allowing a thin cut to be pared off the plastic, gradually reduced in thickness by removing one at a time, to the required height for the final cutting pass. (Photos 4 and 5.)

4.Cutter angle 5. Clamped in vice

Clamping the aluminium bar in a vice (Photo 5), the bolts are slackened sufficiently to allow the strip of plastic to be threaded along the slot. Here you may elect to use sufficient brass shims to set the height for a small pared cut when the bolts are nipped down (as described above) or take the route of just a slight nip at a time on the bolts, and then do the final cut using the appropriate shim.

6. The assembled paring tool

Putting a slight angular bend up on the end of the plastic strip is a good idea, to enable small mole wrenches or the like to grip the plastic strip for pulling through the cutter. The bolts are screwed in to just hold the plastic strip with a very slight amount of pressure – applying too much pressure will cut into the strip and it will be virtually impossible to move. Pulling the strip against the flat face of the steel angle – not the filed angle – the strip is pulled right through, paring thin strips off the plastic against the cutting face. Releasing the bolts by approximately half a turn should be sufficient to allow the strip to be pushed back through. Nipping the bolts back down the half turn plus a slight pressure again, pull the strip through and another layer is pared off. Continually repeating this process until the full profile is cut into the plastic strip, using shims to obtain the correct final thickness. Virtually any profile can be cut using this method just using a variety of different angle cutters.