Hardening for vice jaws.

[jimmer]

i have a decent vice where one of the original jaws is snapped in two and has a corner missing.
after hunting around for replacements it occured to me i could make my own and it'd be a good first project to learn about hardening and tempering.

i have a great book on the subject, which i haven't fully digested so need to read again, but i understand the theory and practice to some extent.

so my questions are: how are vice jaws usually treated? do they tend to be hardened to the harder/more brittle end of the spectrum? what temperature/colour would i heat them to? and what would be good to quench them in?

i could come to my own decisions i guess, with reference to my book, for the latter 2 questions, if i had an answer to the first.

thanks
jimmer

 

[MusicMan]

I don't think they need to be too hard, as then they damage the work more easily. I usually use softer liners (e.g. aluminium) anyway. Though it is true that harder is more durable, until it gets brittle.

I would suggest making them of tool steel ground stock (from eg Cromwells). This is stronger 'as is' than mild steel, which is perhaps a bit soft (and can't be hardened much). Small quantities are not too expensive. If you want to try hardening, heat to bright red heat for about 10 minutes, then quench into oil (less violent than water and less liable to crack, but do have a fire extinguisher handy). For tempering heat gently till the surface film is a dark straw colour; you don't want it too hard for this application. Pretty much any oil will do, e.g. ordinary engine oil.

Remember that all this is repeatable and reversible (unless cracked). To get back to the original condition, heat to bright red for 10 min and allow to cool in the air. Then you can reheat, requench and retemper till you are satisfied.

 

[jimmer]

excellent, thanks MusicMan, just the info i was after : )

 

[ED65]

what temperature/colour would i heat them to?

You don't have a sure-fire way of measuring this and judging by colour is something that takes both controlled conditions and a very good eye (not just experienced, some people don't have the colour discrimination necessary to be able to judge accurately). So instead just do it empirically: when the steel has lots its attraction to a magnet you're at critical temp and ready to quench.

and what would be good to quench them in?

Depends on the type of steel. But even water-quenching steel can be quenched in oil; won't go as hard, but it'll harden so it's the safe bet if working with mystery steel. The converse, quenching an oil-hardening steel in plain water at room temp, can lead to excessive brittleness or directly to cracks or shattering.

Final hardness is not normally determined by the quench, it's now almost always done post-quench, by tempering.

You pick your tempering temp – based on the hardness you want and the steel type – then heat the fully hardened or "glass hard" steel to that temp, hold for a short while, then allow it to cool slowly. If doing this in the oven you can just turn the oven off and leave it to cool down, alternatively you can pack in dry sand or, better, ashes. Tempering is sometimes repeated once or twice but I'm not sure if that would be necessary for parts like this.

Now all that said, from the forums I know many machinists both professional and amateur prefer to make jaws from mild steel, some even from brass or solid copper. The jaws will take more wear and tear of course but they're effectively disposable while the workpiece is not. On the other hand if you'll be fitting soft jaws (copper, aluminium, fibre, wood) over them use tool steel and harden away.

 

[--Tom--]

Id remake some in mild and see how they get on before going ahead and hardening a set. If you do harden remember to drill and countersink your holes first.
It’s useful to have a vertical v groove at one end and a horizontal one the whole width to help holding round stock. Easy to add to a mild jaw, if hardening it can create a stress riser and cracking so would add to the gamble.

 

[jimmer]

thanks everyone for the replies.
hardening is somthing i want to start doing, which is why i was considering having a go.
making in mild is a good idea, but i think must of my use tends to demand a bit harder. plus i slip other jaws over .

so what are the pros and cons of using mild verses tool or higher carbon steel?

 

[chaoticbob]

Jimmer, for what it's worth, out curiosity I tested my engineering vice (it's a big old 'Made in England' job) and the jaw face pieces seem to be somewhere between 40-45 Rockwell C. That's not very hard and well above dark straw temperature if tempering down from quenched gauge plate. Hardening steel is satisfying, but this may not be the best place to start!
Robin