Suitability of metal? updated

[rafezetter]

I bought one of those rotary expandable cutters and sharpened the blades that came with it, but after using it to cut just 2 holes in 16mm ply the blades are just not up to it, the tips have bent under the stress.

I've no idea what type of steel they are, but I can bend the blades with pliers without too much effort, so I was wondering if anyone could advise whether making a new pair from an old (possibly hand forged) plane blade would be worth doing?

Also if I did would it be worth hardening / tempering to make them tougher or would the temper used for a plane blade be enough?

 

[rafezetter]

No takers on this so far - my main concern in using a (probably hand forged) plane blade to make a new pair of cutters is the friction heat they will experience and what effect that will have on the metal.

 

[blackrodd]

Does the hole drilled have to be an odd size?
If not forstner bits are good but this answer seems too simple.
Do you have a pic? Rodders

 

[sdjp]

No takers on this so far - my main concern in using a (probably hand forged) plane blade to make a new pair of cutters is the friction heat they will experience and what effect that will have on the metal.

A plane blade is likely to be a carbon steel - the exact type is alway tricky to determine, but if we work on the assumption that it's somewhere between 1080 and O1 steel, that's probably 'close enough'.

That is to say - it is entirely possible for a powered tool to produce enough heat by friction when cutting wood to overheat that steel, and soften it off. (It is possible that this is what happened to the original cutter bit, in fact). In general, that would need to be somewhere in the range 300 - 400 C, at the cutting tip, and you'd expect to see blueing at / near the tip [0]; that means that you'd expect to see some scorch marks if that's happening (but not always). If you're careful in use, there's no reason that such a cutter can't work - but you might find you have to use them slower than you ideally want.

I don't think you'd get a huge advantage in annealing, shaping, hardening and tempering again - you might be able to go very slightly harder, but the heat production in use might well remove that small advantage. And for use on a power tool, you want it tempered somewhat to give some toughness, lest the cutter fly apart at speed. It would be easier to shape if softened, but other than that, I don't think it would net you much. If hardening and tempering is easier for you than grinding it (keeping it cool), then that's what's probably best to determine the route.

If you can stomach the quantity of grinding involved, then High Speed Steel blanks are cheap on eBay [1], and they won't soften until red hot (at which point your ply is on fire; so I think we can call that 'good enough'). Supplied hardened, it's pure stock removal to shape them, but HSS will outlast anything else in use, and you don't have to keep it cool for grinding.

[0] Assuming that the steel estimate is correct, then it'll still cut well enough at blue'd temperatures; but it's unlikely to stop there.

[1] 3 - 4 quid for a 200mm x 3mm x something blank; probably enough to make a few sets of cutters out of.

 

[rafezetter]

@ SDJP - Many many thanks, really - this is exactly the sort of answer I needed. The cutter speed can be reduced now that I have a pillar drill to as low as 500rpm, which would probably suffice given that I would sharpen the cutters almost as much as my plane blades, but also as a HSS blank is so cheap and would give me more options (and be simpler to process) I think it would be a shame to "waste" an old forged plane blade.

I'm not bothered about the grinding, for me it's all part of the process, and I currently have the time anyway!

 

[chuckey]

Did you mean a fly cutter? I would buy (from a market stall) a pack of cheap diamond files and touch the "blades" up to give them a proper cutting edge and side relief so they do not rub and over heat. I don''t the term blade, its like a knife blade , something which is flat and tapered in all directions one of which gives a sharp edge. The sort of shape you need is like the blade (cutter) on a plough plane, but tapers towards the back so the rear corners do not rub.
Frank