Hardening Silver Steel With Spiky Bits

I'm making a Rack Cutter of the type mentioned by Neil to cut gears in his Jovilabe thread.

My version (for a different purpose) is made in 13mm diameter silver steel. You can see from the photo that the body of cutter is massive compared with the size of the teeth.

After removing a few burrs I shall be ready to heat-treat the Silver Steel.

Previous tempering efforts have all involved much simpler objects where it was easy to heat the item and judge when it was time to plunge-cool it.

I intend to use a gas blowlamp to heat the cutter and to cool with water.  I have insulating bricks.

What's the best way to bring the end of the cutter and teeth evenly to the correct temperature i.e. without overheating the teeth? Or am I worrying too much?

Thanks,

Dave

edit corrected bad English

Edited By SillyOldDuffer on 15/10/2016 10:32:10

I just dip in washing up liquid, lie on some skamolex (insulating board) blast it with a blowlamp, focusing on the shank until up to red, then I keep it bright red for a few minutes, poking it round with pliers to even out the heat.

Temper by putting in the oven at gas mark 6 for an hour

I think you may have machined away too much metal, the tips look too thin and may be brittle and you haven't any reserve for resharpening.

Neil

Clean, polish and bung it in the oven.

Regards Ian.

Pack in dry sand on a tin plate.

Hello Dave,

It looks interesting, quite a vicious looking thing!

I think you are right to worry it is definitely thinner and therefore prone to heating far quicker than the rest of the tool. The easiest way to ensure it wouldn't overheat would be to focus the torch on the lower portion of the shank so that the heat gradually radiates outward towards the cutting portion.

Neil's concern is understandable because you may find yourself confined to quite light cuts in harder steel materials or the fine edges could chip. I'm not sure what size endmill you used but maybe a 3 or 4mm end mill to cut narrow channels rather than wide ones would make the cutting edges substantial, chunkier and less prone to shock.

Michael W

Dave has seen this before, but these are the tooth proportions I used:

That said, this cutter of "Stub's" works fine on cast iron:

Old tabacco tin full of sand. place part in the sand so you can just see it and then heat from below with your blowlamp

It's going to take a helluva lot longer to get to a red heat if its in a tray of sand.

If worried about decarbonisation, wrap loosely in soft iron wire then pack with a paste made of chalk.

But I haven't ever had a problem with small items like this held red hot for <5 minutes.

It's a benefit not to get the core too hot, as that leaves it less brittle.

.

A tray of brass swarf may be preferable

... unless you plan to heat to very bright red

MichaelG.

.

Second thoughts: Being wary of Zinc fumes ...

Brass is fine for 'blueing' but Copper or Bronze might be safer.

Edited By Michael Gilligan on 15/10/2016 14:02:18

Thanks again chaps, lots of good advice.

I shall give it a go tomorrow and report back. Heating from the shank side rather than straight on to the teeth should do the trick. I have enough brass swarf to try that if I end up having to make make another cutter.

Cheers,

Dave

I didn't mention that I'm also concerned about the teeth being excessively relieved because my original question was about heat-treatment.

That said I am worried I've taken too much off.

My teeth look different from Neil's only because I made a mistake on the first row and decided carry on regardless. I also chose to cut a deeper rack than necessary simply because I was experimenting: this also may be a mistake.

But now I've made my first cutter I'm confident that I can do much better with a second.

One thing I am pleased about is how straightforward this approach to cutting gears appears to be. The teeth dimensions were calculated by me to cut 38DP. I appear to have got that part right because the cutter meshes nicely with a 38DP gear.

I shall find out later if the cutter does a proper job on a blank - or not. It's too early to break out the champagne.

Fingers crossed in anticipation,

Dave

Edited By SillyOldDuffer on 15/10/2016 17:30:16

Just a progress report. I followed Neil's advice about washing up liquid etc. and heated the shank taking care not to overheat the teeth. After plunge-cooling the cutter went in the oven for an hour whilst I starved. (Gas Mark 6 is 200℃ )

Loading a aluminium blank sized in imitation of a Meccano Part 31 (a 1" diameter 38 toothed gear), I lined up the cutter for a full depth cut and started cranking. All went well until the Man from Porlock rang my doorbell and interrupted me. Now I own a unique 37½ toothed gear wheel.

Despite the miscount I'm well pleased with the gear and the cutter's effectiveness on Aluminium. I shall try brass next and see what happens...

Thanks again,

Dave