# Machining phosphor bronze and threading stainless steel



## tmuir (Aug 23, 2008)

Hi all,

I'm in the process of making another new cylinder and piston set for another steam toy I'm restoring.
I've completed the cylinder and now I'm onto making the piston.

This is what I need to make.






As you can see it's pretty straight forward but rather than make an exact copy of the original I've decided to make the piston from phosphor bronze rather than brass as it is running in a brass cylinder.
I've never machined phosphor bronze, does it machine much different to brass?
Also the rod I'm thinking of making from stainless steel rather than 1/8 brass as I find on a lot of these engines the brass rod has slightly bent.
I've got a small amount of K&S (hobby shop) 1/8 inch stainless steel so I'm unsure of the grade but will it be much harder to thread than brass and should I do anything different?
I'm guess a cutting compound on the die will be in order unlike brass.
I will cut the thread over size and then make a second pass to cut to correct size.

Fairly simple questions I know but hopefully a couple of points may save me from having to do it again.

Thanks


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## Bogstandard (Aug 23, 2008)

Tony,

I personally think you are making a mistake using phos bronze for your piston. You will do just as well using brass.

Phos bronze has much harder wearing properties than brass, so if your reasoning behind the move was to make it last longer, it will do that, but the down side will be your brass cylinder will wear quicker as it is not as hard. Which one is easier to replace? Usually it is the other way around, bronze cylinder, with a sacrificial piston in brass.

If you still want to go ahead, then bronze sometimes causes a few problems with machining.

It all depends on the type of phos bronze, there are hundreds, if not thousands of them, some machine better than others.
PB1 is the standard sort of grade, and I have found that I can machine it very well with a razor sharp ground up HSS tool. Anywhere near dull and you can find you can get into trouble, if the surface gets 'rubbed' by a blunt tool and forms an almost impossible skin to get thru. 

I hope that this has helped, but also it has maybe thrown a spanner in the works.

John


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## tmuir (Aug 23, 2008)

Thanks for that John.
I just thought I remembered reading somewhere that brass in brass wears quicker that bronze in brass, must have got it the wrong way around. Will leave the piston brass as I have a lot of brass and not much bronze.
Yes a replacement piston would only take me about 20 or 30 minutes whilst a new cylinder considerably longer.


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## Dhow Nunda wallah (Aug 24, 2008)

I have some experience with stainless in the 300 series.
Lovely to machine, pig to thread.
I suggest you get the OD down to the minimum which will hold, then use your die.
About 60% thread engagement is plenty.
Yes, you will need lube, the more, the merrier.
More coolant than lube. May not seem like it, but at the tip of the tool, it's getting HOT and will work harden.
That applies even if drill, tap or die.


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## Mcgyver (Aug 24, 2008)

> I've got a small amount of K&S (hobby shop) 1/8 inch stainless steel so I'm unsure of the grade but will it be much harder to thread than brass and should I do anything different?



the machinability of all stainless is less than a 360 brass but within stainless, there are many kinds that will work very differently. Knowing what you have is everything. 303 is free machining and will cut beautifully, 304 is miserable to work with because of work hardening. Get a piece of each from a service centre and compare. usually, if its not specified, its not free machining - 304 is common for example. I'd guess you would have hard time threading that as i doubt it would be free machining

on the piston, i'd go with brass as John says. Do you know what the original piston is made of? as its a restoration, authenticity is important so i can see why you;d want to use something at least the original colour. On wear though, since its a toy, there's a good chance both were originally brass. Not being an industrial product, duty cycles are low and for consumers low cost is important - brass would have been easy and quick to machine. just a guess. If you do go with bronze i use the same tool grinds as for brass, zero rake but i think it would be fine in brass - how much is it going to be run?


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## SmoggyTurnip (Aug 25, 2008)

This was made with 316 stainless steel - Machines pretty good had has better corrosion resistance than 303.


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## tmuir (Aug 25, 2008)

Mcgyver  said:
			
		

> t
> on the piston, i'd go with brass as John says. Do you know what the original piston is made of? as its a restoration, authenticity is important so i can see why you;d want to use something at least the original colour. On wear though, since its a toy, there's a good chance both were originally brass. Not being an industrial product, duty cycles are low and for consumers low cost is important - brass would have been easy and quick to machine. just a guess. If you do go with bronze i use the same tool grinds as for brass, zero rake but i think it would be fine in brass - how much is it going to be run?



The original part would of been brass, it was made just after the war so we had a lot of brass available.
It really won't be run that much so brass will be fine. I was really just looking at an excuse to try to machine a different metal.

Probably will just stay with the original metals. It is a restoration project but as about 30 to 40% of the model was missing I will be just happy if it runs and looks like it should doesn't need to be completely authentically reproduced. My new cylinder will probably have higher tolerances than the original parts ever had.


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