When to grind the point on a centre punch

[Andrew TinsleyParticipant @andrewtinsley63637]

I need to make a couple of centre punches from 10 mm silver steel. Never thought overmuch about hardening and tempering. So simple question, does one grind the point before hardening, after hardening or after tempering?

Andrew.

[Andrew CrowParticipant @andrewcrow91475]

I would turn the point first but not to a sharp point, then harden & temper then finish grind to a sharp point.

If you don’t shape the point before hardening there is a risk of over heating when grinding which will affect the hardness.

Andy

[old martParticipant @oldmart]

Get the shape close to finished requirments before heat treating, then it will only take the minimum touch to finish.

I was told light straw was the ideal colour for centre punches.

[FulmenParticipant @fulmen]

I don’t see any good reasons for _not_ finishing the tool before hardening. If you’re using a hand torch there won’t be much scale forming, just kiss it with a stone and you’re good to go. In fact I just did that with a couple of punches that turned out great.

I would temper to gold/purple, you don’t want to hit brittle steel with a hammer. And it wouldn’t hurt to temper twice, since this method doesn’t give much soak time. Actually it’s never a bad idea to temper twice.

[Andrew TinsleyParticipant @andrewtinsley63637]

The advice from Andy and Old Mart, is what I had thought the most likely way to tackle the problem. Fulmen has other ideas. The problem that I can forsee is that the tip of the punch could lose carbon (carburise?). I would do the initial heating in a gas  fired furnace, and hold at temperature for around 20 minutes. I am always wary of heating with a torch, as I am never certain about the uniformity of the results. Probably just me, as everyone else seems to get good results with a torch!

Andrew.

[noel shelleyParticipant @noelshelley55608]

temper in cooking oil ? Noel.

[Nigel Graham 2Participant @nigelgraham2]

Cooking-oil (boiling in) might not go much beyond a yellowish colour.

The purple Fulmen suggests is achievable by soaking the steel in lead kept just molten. I have used this, and seen it elsewhere, for small springs.

[Andrew TinsleyParticipant @andrewtinsley63637]

I don’t have a problem with tempering as I have a very small muffle furnace (using kanthol heaters) with PID control. So I can temper using very accurate and stable temperature control, This is separate to the larger gas fired furnace, which is good enough for the temperature required for hardening.

My real query is at what stage do you grind the point? So far we have 2 different suggestions. Does it even matter when you grind the point?

Andrew.

[Andrew Tinsley]

On Andrew Tinsley Said:

 […]  Does it even matter when you grind the point?

Assuming that the piece is fully hardened and tempered [i.e. not case-hardened], it should not matter.

MichaelG.

[FulmenParticipant @fulmen]

With a full heat treat cycle you will get some scale and probably some slight decarburization of the surface. So a final grind might be a good idea, but that doesn’t mean you can’t machine the point before HT. Chances are it will work just fine as is, and if not there will be less material to grind. That’s the only difference I can think of.

 

[bernard towersParticipant @bernardtowers37738]

The only thing I can think of is when I surface grind unhardened steel the finish is not so fine as when I grind a piece of HSS, just a thought.

[Clive Brown 1Participant @clivebrown1]

My silver steel centre punches were hardened and tempered with my propane torch. Bright red for 1-2min, quenched in water then tempered back to around med. straw and quenched again. It’s important to transfer quickly to the quenching baths to avoid under hardening and/or over-tempering. Then a final point grind. They have worked well for years.

[JasonBModerator @jasonb]

I would not be letting the hardening get near the blunt end where the hammer will make contact

Probably rough out the point leaving it a bit blunt say 1mm dia as unless you are heat treating in an oven it will be hard to get temper the same at a pointed tip and further back into the secondary taper.

[Clive Brown 1Participant @clivebrown1]

Graduated hardening along the length of the punch happens more or less automatically if using a gas torch. The pointed end is heated for initial hardening. Bright red for, say, an inch or so. Tempering will be by heating the blunt end, allowing the colours to run down to the point so the point remains the hardest part of the tool. The blunt end probably never really hardens anyway.

[FulmenParticipant @fulmen]

But in this case the OP will be using a forge. Since it’s a gas forge (just noticed that) decarburization and scale shouldn’t be a big issue, but it will be through hardened. It will then be oven tempered, so again uniformly treated. So the solution will be to give the head a final temper with a torch to soften it ever so slightly.

[Andrew TinsleyParticipant @andrewtinsley63637]

I realised after posting last night that my electric muffle furnace is NOT the way to go for tempering a centre punch! One needs differential tempering for things like chisels and centre punches! So a torch heating the struck end of the punch is the correct method and stop and quench when the tip colour is correct. I think that Fulmen’s blue / purple colour would be correct,

Andrew.

[SillyOldDufferModerator @sillyoldduffer]

I doubt it’s critical.

Pre-shaping reduces the amount of grinding needed to finish the punch after hardening.  The saving may not be worth it because not much grinding is needed to finish a centre-punch.

Not pre-shaping makes the heat treatment less risky.   Ideally Silver Steel should be heated so the metal hardens and tempers equally.    Achieving consistency is easier if the Silver Steel isn’t shaped because heating with a blowlamp risks heating a low mass point faster than a high mass body; a sharp point might reach white heat and burn long before the body reaches red.  Though the operator can manage the blowlamp so heat is applied evenly, it takes care.  Differential cooling might also be a problem due to the point cooling faster than the body on the way to the plunge, or over-hardening in the plunge – again due to lack of mass.   And tempering with a torch risks an uneven temper.  Better to soak the item in an oven and let it cool slowly.

The complications are counterbalanced by using Silver Steel.  It’s formulated to harden and temper with minimum fuss and without special equipment.  Does a good job, so, in practice, shaping versus not shaping a centre-punch may not make a difference.

Gets more interesting when a centre-punch is heat-treated throughout as Jason suggests.  The goal is a sufficiently hard point that isn’t brittle, supported by a shank where the heat treatment deliberately left the steel tough, not hard, at the hammer end.

Dave

 

 

 

 

 

[JasonBModerator @jasonb]

I did not suggest heating throughout. If you don’t heat the blunt end it won’t get hot enough to harden.

[JasonB]

On JasonB Said:

I did not suggest heating throughout. If you don’t heat the blunt end it won’t get hot enough to harden.

Whoops my bad, poor choice of words.  What I had in mind was someone new to this thinking the whole punch should be hardened.  Although not applicable to a centre-punch, other items are hardened throughout, perhaps differentially.

In #790915  Andrew mentions having a muffle furnace.  It, being a high temperature device, could heat the whole punch to hardening temperature.  Andrew decides not to temper in a muffle furnace after correctly recognising the need for differential hardening.   If a muffle furnace can be dialled down to 200°C-ish, it could be used for tempering because tempering unhardened Silver Steel doesn’t change it, only hardened steel can be tempered.  (Hardening is done fast at high temperature, tempering slow at low temperature.)

My kitchen oven is used for tempering.  Don’t need permission now I’m divorced!

Dave

 

Dave

[FulmenParticipant @fulmen]

I don’t get the panic over fully hardening a punch, if you want the head softer just give it an extra tempering with a torch. I’ll agree that it’s more work than just using a torch, but to the extent that it matters it also gives a better result. You need some holding time to dissolve all the carbides.

 

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