Gunmetal

In message , davejpc writes

Gunmetal and brass are not always interchangeable so it is best practice to assume that even designers like the notorious Martin Errors has specified the correct material for the specific purpose.

I don't pretend any expertise but I have always assumed that even if phosphor-bronze and gunmetal are not physically identical they are broadly interchangeable insofar as model engineer usage is concerned. Until someone else jumps in and tells me I am wrong I shall continue to use PB as a generic term and continue to believe that gunmetal is a 'casting' grade of PB.

If you really cannot find the material specified in the drawings then it is my understanding that brass is likely to be satisfactory for cold water fittings but when used in the more extreme areas of boiler fittings it can deteriorate through a process called de-zincing. So for 'hot' boiler fittings PB is likely to be 'right' and brass is likely to be 'wrong'.

Certain grades of PB are specifically intended for use on bearings because they work harden. This same property can make for diabolical machining properties as I have recently found (but that's another story). So for wheel bearings, pistons, cylinders or anything where there is a 'sliding role' PB is likely to be 'right' and brass is likely to be 'wrong'.

So unless someone can produce strong evidence that it is total rubbish to assume "gunmetal = PB" and then take a second look at your stockists. In my experience all 'the usual suspects' both specialist model engineer suppliers and also retail metal stockists such as Mallard or College Engineering has someone on the staff who knows what he/she is talking about.

Brass exposed to high temperature water will slowly lose its zinc, leaving a porous and brittle structure behind. For that reason its not used anywhere where it's silver soldered into place and not easily replaced.

Phosphor bronze and gunmetal are equivalent for the sort of job you are doing, but some PB can be a b*** to machine

RS components sell gunmetal rod - at a price. Their 5/8" PB is £6.79 for a

13" length - leaded gunmetal is £ 5.57 (both +VAT). Their PB machines like brass - very easily. You can also get nickel aluminium bronze to admiralty standards, but a 13" length of 3/4 dia is £19.51, so unless you plan on doing a lot of steaming!!

If it calls for gunmetal DON'T use brass - try silver broze or something like that.

Gun metal is part of the bronze family. There are many different grades but any generic one of the bronze family will suit your application.

One note about Aluminium Bronze, this is one tough material, turns well with tipped tools but can blunt HSS very easily. Drilling and reaming this can be a right pig as it wants to close up on the drill. It's also requires more lubrication than the porous bronzes and can gall and seize very easily.

-- Regards,

John Stevenson Nottingham, England.

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May as well also add that sintered phosphor bronze as per small bushed etc does not like being reamed and will smear the pores over. Best turned with a sharp single point/small radius tool.

I think you can buy sintered in bar form these days, used to be in bushes only IIRC.

Peter

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I don't know what "gunmetal" is, but I _thought_ it is soft (at least compared to PB). Or do I confuse it with white metal?

Now what is it?

Nick

Well that's far from my experience, the gun metal used in castings from typical suppliers to this hobby is very different in machining properties from the phosphor bronze bar supplied by same. Try tapping a PB piston compared with the GM cylinder it's intended for, the former is a nightmare the latter a piece of cake. Also the difference in these materials is the reason PB is often used for pistons that run in a GM cylinder, they are dissimilar materials so form a fair bearing, if they were the same the piston would be supplied as a GM casting not as a more expensive piece of PB bar stock.

Anyway that's my experience, but a quick google shows that a typical GM is a

88-10-2 (copper-tin-zinc) alloy and PB is similar BUT the zinc is replaced by phosphor.

Greg