Cutting out the Corby Starlet undercarriage legs.

In Western Australia the cutting out of your undercarriage legs is seen as "the apprenticeship", an arduous task that must be done and one that identifies you as a serious builder once you have completed them. It takes about 4 days of slog and can only be done in a manner that does not heat the spring steel, which by the way is unformed truck spring material.

In cutting out my legs I used just 2 hacksaw blades. yeah, just 2. The first one was a Stanley yellow blade and lasted just 6 inches before it was blunt and had teeth that were actually rounded. It was such hard work that I was applying coolant to the blade for most of the way to try to get it to cut. When the blade finally threw in the towell at 6 inches I sat down to have a serious think. There is at least 116 inches of cutting and it would take 20 blades and was going to be expensive. It was actually harder than that since you cant put a new blade into the slot left by a blunt blade without destroying it since the blunt blade makes a narrower cut.

I'm a self taught Model Engineer and I went back to basics to sort out the problem. If you cut high tensile steel at too fast a pace in a lathe it will heat up the tool bit and heat up the job at the point of the cut. The cycle of heating and cooling at the location of the cut results in the gradual hardening of the job. The resultant increase in hardness will actually make it harder and harder to machine the work. The end result is that the very tip of your best piece of high speed cutting tool will actually melt away. The solution to this problem is to actually turn the job in the lathe at a far slower rate. I reasoned that what I was experiencing with the saw blade was something similar.

I put in blade number two, the best quality Eclipse blade I could find and cut away the waste to get access to the front of where I had reached and duely set about the 110 inches that remained. I periodically felt the saw blade with my hand and if I could feel any heat I slowed the speed of the stroke. I found in the end that the stroke speed that I needed to keep the blade from heating was quite slow. It was actually less than half the speed I would normally saw away at.

As I got settled into the rhythm of cutting I began noticing things. My arms never felt as though they'd drop off, I found that my circulatory system kept pace with the demands my muscles were making. I used to measure out 6 inches and put a pencil mark. When I reached the pencil mark I would have a spell and a cuppa. I found that the slower I stroked the saw, the faster I was actually cutting. In 110 inches of cutting you have a long time to reason this out. An old machinist I knew actually solved the problem. If you cut too fast you actually heat the surface of the steel in the base of the cut. On the return stroke the thermal mass of the rest of the steel absorbs the heat and cools the area, but importantly, leaves the surface slightly hardened. What is effectively happening at the surface of the cut is a heating and quenching cycle that in spring steel will leave it incredibly hard. If you dont harden the steel with each cut stroke, the subsequent cut stroke is that much better because the steel is softer.

In spring steel, sawing very slowly will actually have you cutting faster. You will find it a hell of a lot more pleasant task as well. In all the cutting with the second blade I never applied coolant. I reasoned that it was actually making the problem worse by quenching the steel I was cutting.

When I had the legs roughed up I used a Shaper to plane the steel down to the markout line. This the shaper could do quite slowly without heating the steel and by timing the cut rate I could leave the machine to it while I did other things. With all the legs planed out to shape I then set to with a new file and draw filed the oval edge profile into the legs. The guys in the spring works advised making the edges a "D" profile to prevent cracking during the heating and bending to shape. (something I have yet to have done)

Apprenticeship completed, that is where my project sits until my restoration of a 1951 Auster

J1B is completed.

Howard Jones
National Secretary
Sport Aircraft Association of Australia.