Grades of metal

[Alan]

Hi Everyone

I am in the process of building a Sparey 5cc, however I have knackered the material supplied for the crankshaft. I have other bits of steel lying around but I do not know if they will have the strenght to be made into a crankshaft, is there a way one can "test" metal to see if qualities are suitable for, in this case a crankshaft ?

cheers

Alan

[44767]

Testing materials would be more expensive than buying another piece. If you had a piece of material of a known grade you can compare the sparks generated off a grinding wheel. But it sounds like you don't have any identifiable bits! Different kinds of steels give quite different sparks but it is not a guaranteed methd of identifying a particular grade.

For a crank shaft, I'd be looking for a piece of 4140.

[Alan]

Thanks for that Mike, I do have the original peice that was supplied from Hemmingways. I will try this on the grinder and compare the "sparks" with other bits of steel i have.
Regards

Alan

[sapper]

If you do not have a copy of Tubal Cain's book Model Engineers Handbook treat yourself, it is indispensable. Page 92 has a plenty of info on identifying workshop materials including the spark test.
I understand the idea of the test originated in the US at the end of WW2. The boffins were trying to find a way of identifying the metals recovered from clapped out Sherman Tanks. They visited a scrap yard only to find a guy holding lumps of metal against a large grinding wheel, watching the sparks then throwing the metal into the appropriate skip. Probem solved.
Sapper

[tomol409]

There is a Corus steel depot near me. I approached the chaps in the warehouse for some scrap ends and they were quite satisfied to get rid of some from their scrap bins. They saw the ends off all the bars because they are deformed after coming off the rollers in the steel works. Importantly, these ends are all colour coded with paint, and therefore perfectly indentifiable, if you know these codes. Charts of these grades are obtainable by surfing the net. I obtained a bag full of these ends, almost too heavy to carry, and they only cost me the price of a couple of pints to cross the blokes palm with, and have kept me going for years. Tomol.

[classicsteve]

I wouldn't mind some of those cut-offs.

Grades of metal is a big topic, and on top of it you can add the heat treatment, which as we all know can transform some grades of metal from Jekyll to Hyde. If you just stick to steel, you can spend months just getting a basic grounding, especially due to the specs differing from country to country. If you are using aluminium, then one grade will have superior strength, but rotten corrosion resistance. Its all very tricky.

The good news is that scale models rarely put more stress on than their full-size counterparts. For example the mass of a loco scales with the cube of the scale, while the bearing area goes as the square of the scale. This is good. The modest strength of mild carbon steel will often do the job of higher spec steels in real life - of course hardness is a different issue. Part of metal heat treatment relates to hardness.

I did a 4 year degree course on metallurgy about 20 years ago and I can tell you it is a really big subject ! Trust me that in modelling you want to keep the metallurgy as simple as possible. You are blessed with working at small scale, and that makes life easier.

All of this assume you are ordering the metal up front. When faced with something unidentified, then it can be really hard to tell what it is. I worked under a works metallurgist for a while, who taught me how to tell the difference between spheroidal graphite cast iron and flake graphite cast iron by smell when you file it. The only other way is by polishing, etching and a microscope - and otherwise the composition is identical. Good old blacksmith's tests have their place for the modeller.

Steve

[alanstepney]

For a small-ish IC engine crankshaft it is probable that the original spec called for something harder than EN8. probably EN16 or 24.
(Or whatever they are in the new designations)

Most of the harder steels give a shorter red spark rather than white/yellow ones.
The other simple test (albeit a very rough one) is to try filing two pieces, and comparing the effort needed-depth of file marks etc.

From memory, the Sparey designs were not particularly critical nor high performance, so almost anything would do the job.

[classicsteve]

I would have thought the the yield stress would be more important than hardness for steel for a crankshaft.

En24 (817M40 or SAE 4340) is really good stuff and from a metal stockists it is normally in 'T' condition - which is machinable but with a high yield stress (650-680N/mm2) - just about twice that of EN8 in its softest condition - and about three times the yield stress of EN1. But alloy steels are expensive, which is why people use the plain carbon steels if they will do the job.

If the design had plenty of strength in hand, then they may have selected their steel for easy machinability and good surface finish.

Steve

[Deleted]

I once had to make a number of pins for a prototype rotary hydraulic arrestor gear, designed to stop Phantom jets in about 100 yards! I made them out of 2' x 2" EN26Y, just about the hardest and toughest material I ever had to work. The Clerk of Works who specified this material didn't understand 'machinability'!
Avoid.....
JB

[kneedeepinswarf]

According to my drawing and copy of the original Aeromodeller write-up for this engine, Sparey himself specified "3 per cent chrome-nickel steel, unhardened" for the crankshaft. This might help in finding a suitable material.

Lionel