Silver steel expanding on hardening

[Glyn DaviesParticipant @glyndavies49417]

[Glyn DaviesParticipant @glyndavies49417]

I recently made the wobble broach described in MEW issue 157. It is designed to take 8mm diameter broaches and I carefully bored the broach holder so that some 8mm silver steel was a close sliding fit.

I spent some time today making a selection of broaches from my 8mm silver steel and then hardened them by heating to red and quenching in water. I then tried to fit them in the broach holder and they will not go in. They mike .3145".

So I need to reduce the diameter of the broaches by about two thou. I tried some 240 grit wet and dry in the lathe and it's awfully slow going. Other than using a centreless grinder (which I don't have in my shed), can anyone suggest a method of getting these back to .312" diameter?

Thanks

[VersabossParticipant @versaboss]

Get a Cermet insert and turn it down, with high rpm and fine feed.

Greetings, HansR

[WindyParticipant @windy30762]

Have you tempered them as straight hardening increases the size I found this when making small end pins.

Tempering reduces the size from just hardening from red heat.

Paul

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Depends on what you have to hold them by .

If they can be mounted true in collet or between centres then grinding is best for sure .

Ideally a tool post grinder but for this job one of those hand held battery grinders from Lidl mounted in a bracket will do the job reasonably well with several light cuts .

Bit of care with safety glasses etc .

If nothing accurate to hold by then use a hand lapping method . Easiest is split block of aluminium with a couple of tap wrench handles fitted .

Rotate while running up and down a few dozen times .

MikeW

[Sub MandrelParticipant @submandrel]

I would try Paul's suggestion first, if you haven't already tempered the broaches. They will snap on first use if you don't anyway.

if they still don't fit,. isn't it easier to take a skim off the inside of the holder?

Neil

[Ian S CParticipant @iansc]

To grind it, it does not take more than a few minutes to make a bracket to take a Dremel(or similar) tool. Cove the lathe bed, and just take it lightly. Ian S C

[Glyn DaviesParticipant @glyndavies49417]

Many thanks for replying to my question. I polished all the broaches and heated the base until the straw colour just got to the tip and quenched. And they still won't go into the holder. I then tried the raw silver steel and it also won't fit!

The 8mm silver steel (not 5/16" ) that I used when boring the broach holder is smaller than the 8mm silver steel that I made the broaches from. I never though to check because I thought silver steel was ground to fine tolerances, but the two lengths of 8mm that I have vary in size by .002". Unless I was sold some 5/16" when I asked for 8mm.

I'll make some more broaches from 5/16" and temper them as suggested.

Thanks again.

Edited By Otley on 24/01/2014 10:39:52

Edited By Otley on 24/01/2014 10:40:35

[Mark RandParticipant @markrand96270]

Yes, the .3145 measured after hardening is still half a thou less than 8mm (excuse the mized units).

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