Balsa Hardness Gauge (oz15864)

About this Plan

Balsa Hardness Gauge.

Quote: "A Balsa Hardness Gauge. By JH Maxwell.

BALSA, as every model builder knows, varies considerably in strength and density. Thus, when describing the construction of a model, one must specify the grade of balsa used, and usually this is done by stating 'hard,' 'medium,' or 'soft.' This somewhat rough and ready method of grading works fairly well, but for a long time keen aeromodellers have been acutely aware of its limitations. Chief among these is the fact that the terms 'hard,' etc are purely arbitrary, and what one man calls 'soft' balsa another may regard as 'medium.'

Colour Coded Balsa At various times, attempts have been made to grade balsa in a rather more scientific manner, and in particular the colour coding system introchiced by a number of supply companies (notably JASCO of New York) in the late 1930's is worth mentioning. With this system, each sheet and strip was coloured on one end in accordance with a density code. For example, yellow indicated a density of 6 lb per cubic foot, light green 7 lb, and so on up to 15 lb per cubic foot.

Colour coding was certainly a step in the right direction, but it too had its limitations. The weighing and colouring process was carried out when the balsa was in the billet stage; that is, before it was sawn up into sheets and strips. But balsa can vary quite a lot even within a relatively small billet, and so it often happened that the colour on the end of the strip or sheet bole little relationship to the actual density. We can personally remember strips which, according to their colour, should have had a density of 9 lb per cubic foot, but in actual fact their densities ranged from 6 to just over 12 lb per cubic foot. This was no fault of the suppliers; it is just that balsa is naturally inconsistent.

Hardness Gauge: Experience with colour coding taught us that balsa grading, if it is to be effective, must be carried out in the final sheet and strip stage. This was obviously too big a job for the supply companies, or even the local shopkeepers, and therefore we felt that what was needed was a simple method or device whereby a model builder could quickly grade each piece of balsa as he selected it from his stock.

There are three basis on which balsa could be graded ; namely, Density, Strength, and Hardness. Density we rejected because it requires accurate measuring (in thousandths of an inch) and accurate weighing. Likewise, Strength is impractical because it requires accurate measuring and special test apparatus. That leaves Hardness, and it seemed to us that it should be possible to devise some sort of suitable hardness gauge.

After a good deal of experimenting with a variety of devices, we eventually evolved the hardness gauge illustrated in the accompanying photograph, and drawing.

The principle of this gauge is that a serrated ring, standing on edge, is pressed on the surface of the balsa with a certain fixed force. Each of the serrations in contact with the balsa leaves a little impression, and the softer the balsa the farther the gauge sinks into it, and the greater the number of impressions. Thus, by counting the number of impressions, one can ascertain the grade of the balsa.

In practice, the head of the gauge (ie the serrated ring) consists of a loop of 20 swg (0.036 in) steel wire, tightly bound with 26 swg (0.018 in) copper wire. The outside diameter of the loop is exactly 3/4 inch and the coils of copper wire must be touching one another on the inside of the loop. There are, of course, tiny gaps between the coils on the outside of the loop, and these are made equal by pressing the edge of a penknife blade into each.

The head is fitted into the end of a handle, consisting of a length of inch square pine or similar wood, and secured with a liberal application of balsa cement. The handle in turn is a smooth sliding fit in a sleeve which is made from inch sheet balsa..."

Direct submission to Outerzone.

Quote: "I discovered this gadget while I was scanning a Super Tigre report in the April 1951 Aeromodeller and I reckon it's worth sharing. I would advise that it not be used on foamies ;) Regards Danny MZ"

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