Turning Phosphor Bronze

[Sub MandrelParticipant @submandrel]

Brass is copper + zinc. Bronzes are essentially copper + tin or other metals – normally tin when not specified, but it can contain large amounts of aluminium, phosphorus and other metals (often several at the same time) as well which hugely affect its properties. Gunmetal is a bronze with zinc as well as tin which has the hard wearing properties of a bronze with the easy maching of a brass. That said there are different grades of gunmetal, but expect 'standard' gunmetal to be a nice metal easily machined to a good finish straight from the tool.

Wikipedia is very good on brass, bronze and gunmetal.

Neil

[Chris GunnParticipant @chrisgunn36534]

Wolfie, my tip concerns drilling, try and drill the hole you need in one pass, do not try and put a small drill through, and then follow up with a bigger one, as the bigger drill will dive in, and jam up. If you can use an offset point drill as suggested, do so. If the job has a stepped bore, drill the big hole first, then drill the small one second, if the job allows. If you are drilling the job in a bench press later, clamp it in the vice, and clamp the vice, and be careful the drill does not snatch. Use new drills if you can, if you have a miller, drill the hole in that, and use the table raising screw to put the cut on, not the drill press lever feed which you can lock up. Sharp tools are a must.

Chris Gunn

[nigel jones 5Participant @nigeljones5]

Hi Wolfe…just a thought, are you turning it too fast?

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

(1) Ordinary drilling of phosphor bronze and other difficult materials becomes a much more controlled and easy operation if flood cooling is used . This requires getting coolant right to the cutting edges and in relatively large quantities so that there is no heat build up at all , the cutting action is well lubricated and chips are flushed clear .

To do this properly coolant needs to be forced down the spiral flutes of the drill . This can be done sometimes just by pointing a copious flow of coolant into one of the flutes but better to have a properly directed flow via a pipe . On small scale operations an ordinary garden spray gun fitted with a small diameter extension pipe is quite satisfactory .

Commercial drills are made in larger sizes with internal coolant channels .

(2) There are far better ways of opening up a pilot hole to a relatively large size than using ordinary twist drills :

(a) By boring single point with a very rigid boring bar . Ideally a properly made boring bar with inserted tip .

(b) By using variations of the gun drill principle with ideally three or more inserted tips .

(c) By using a final sizing drill consisting of ideally three or more inserted tips preset for diameter in holder .

(a) (b) and (c) are available commercially but easy enough to make DIY versions .

nb : A final sizing drill is sometimes called a bore finisher .

: (b) and (c) are essentially the same except for orientation of the tips – (a) cuts on the end like a drill whereas (c) cuts more on the periphery like a boring bar .

(3) In cases where you are already in difficulties with drilling Phosphor Bronze and similar materials the situation can usually be recovered by using an end mill to cut cleanly through the bore . Ideally but not essentialy a ball ended one .

(4) In any kind of drilling of difficult materials only a short section of the drill at the cutting end should be full diameter – the rest should ideally be relieved on diameter . Basically if most of drill is not touching bore it can't get hot and jam .

Michael Williams .

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

More :

Bore finishers are not commonly used outside of the precision engineering industries but they would have many applications in model engineering – especially for those apparently troublesome steam cylinder bores . Single pass , dead size , repeatable and mirror finish .

On a related topic :

Flimsy boring tools don't just flex – they are completely unstable and can flex into or out of cut continuously and randomly leaving a poorly finished , out of parallel and out of round bore .

Boring of model size cylinders can often be done using either saddle or top slide ( if length of cut is not a problem ) . Each has merit but note that if topslide is set to cut truly parallel and all other slides are locked the set up is much more rigid than using saddle traverse .

Edited By MICHAEL WILLIAMS on 05/02/2013 23:43:31

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

More :

The bore finisher mentioned above has some resemblance to a type of tooling once in common use but now largely forgotten – the balanced multi cutter boring head .

This consisted of a drum end on the usual tooling shank with at least three and often five or more inserted individual radial cutters . Preset for size off the machine they were used to give a final high accuracy and high finish final cut to many types of larger size bore .

These more complicated types of boring tool are really intended for use in milling machines and purpose built boring machines rather than simple lathes .

Many types of ordinary boring can in any case be done better in milling machine than in a lathe . A well designed and made adjustable boring head for use in a milling machine is a great asset to any workshop .

Michael Williams .

[Will RobertsonParticipant @willrobertson16447]

Hi Woolfie,

I'm not sure if it'll help you with Phosphor Bronze but some folk reccomend a zero back rake angle for turning tools used on bronze – that works very well on bronze for me but I've never tried it on Phosphor Bronze.

Many thanks to everyone and Michael in particular – a lot of very aluable information for me to think over. I'll try to do more reading on bore finishers – how were all the insterted radial cutters adjusted to precisely the same radius?

Will

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hello again Will ,

I'll try to do more reading on bore finishers – how were all the insterted radial cutters adjusted to precisely the same radius?

One of two ways :

(1) On a between centres jig – quite a common bit of equipment in tool or gauge rooms .

(2) By finish grinding all the tool bits in place in the head as if it was one cutter .

Regards ,

Michael Williams .

PS: A balanced multi tool boring system doesn't have any large forces pushing it off line and very true bores result .

Modern day versions for low / medium precision work are specially made for one specific size and arranged so that when ceramic tips are changed and set properly the tool stays within original diameter tolerance .

Edited By MICHAEL WILLIAMS on 07/02/2013 12:57:49

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