Machining phosphor bronzes

Some of them are nice and others are pig.

I got one difficult sample.

It turns well to a nice finish, will part off and drill with small (up to 5-8 mm drills).

However drilling with larger drills or slot drills ends up in failure, regardless how sharp they are. Regrinding doesnt help either.

I predrilled 8mm and tried to open up bore up to 12 mm and this was of most horrible drilling experiences I ever had, even if successful after long struggling.

Drill just doesnt want to cut and that's it.

It is not a very hard material either but now I better understand why it is so good for bearings.

Do you have any ideas why it behaves so or know tricks around it?

Try a pilot hole no larger than the web on the next drill , it will reduce the tendancy to grab and pull in, and don't spare the cutting fluid. I've also heard that stopping occasionally and cooling the job helps reduce the grabbing. Cutwell do drills with a modified profile for bronze can't say I've used one though.

I modify the drills I use on brass and bronze. I grind the front cutting edge to give a flat cutting face which stops the drill grabbing and use a cutting oil or coolant.

David

Fairly recently I had an issue with a piece of PB when using a brand new slitting saw. It was quite thin (about 1mm) and I ran it pretty much as I would for brass. It contstantly struggled, jammed ( tried with and without lubricant) but finally siezed and stripped several teeth off the saw.

In desperation I stuck it in a vise and attacked it with a hacksaw (32 tpi blade) and it cut like butter!

So if its heat or 'springiness' causing the seizures, slow and steady seems to be the byword.

regards Mike

If the bronze used by Martin is the drawn pinkish colour hard bronze I think it may be, the one thing you do not want to do is back a drill off as you would for brass to prevent snatching.

We've dealt with this issue recently somewhere - drawn bronze is notoriously difficult to drill due to heat build up and gripping the tool - either drill or reamer. It's self 'promoting' too in that it work hardens very quickly if a tool is not razor sharp so increasing friction and increasing heat extremely rapidly. If flood coolant is available then that does help but for those without this can be one machining op fraught with problems

Step drilling is an issue unless as Lee say the previous hole is no wider that the web of the next drill. Very sharp edges are required and it can help to grind one side of the drill slightly wider than the other.

This material always presents challenges when drilling - I have found it is much better to get as large a hole in as possible with one drilling from a very small pilot hole then bore it to size. Boring for some reason does not create any issues - providing the tool is sharp.

Tug

One trick is to make the drill, cut oversize. Just slightly offset sharpen the drill. Some do work harden, and is a time when you do not want to be cutting dry. Water bases coolants are great for these sort of situations.The modern synthetic coolants have something in them that help in high pressure situations like drilling. A split point drill to the nearest you have and then bore from there. Step drilling does not seem to be as effective as drilling from solid. For a 12mm hole, use a 11.5mm drill straight in from the centre or starting drill. Pre drilling even at 4mm will make the material often work harden.

Neil

There is a comment in the initial post that phos bronze is good for bearings,phos bronze is not a god bearing material,the only time phos bronze can be used a bearing for shafts,is when the shaft is very hard and ground and there is a lot of lubricant,leaded bronze is a far superior bearing material. One use is where steel or iron pivoting levers with limited motion are bushed with phos bronze to make the motion easier and to prevent seizure from corrosion in exposed situations,

Will "slow helix" drill bits work better than "standard" drill bits in phosphor bronze?

Have a look at Neil Wyatt's web site ( Stub Mandrel ), in the model engineering section there's an excellent overview of materials and their working properties - including PB. Hope that helps

Edited By Mike Hurley on 04/03/2021 09:58:57

They work fine for me in bronze and brass.

Andrew

Many thanks for all your comments.

@David,

Will try your mehod of drill grinding next time.

@Ramon,

Yes, that was my phosphor bronze - very much like material you describe.

Interestingly turning and parting off, very much like boring is rouble free.

It is drilling what is problematic.

Did you or anyone else have tried to tap this material btw?

Also many thanks for comments of some of you that "small pilot hole is a good pilot hole" and Neil's remarks about usefulness of predrilling in such situation in general.

Martin

This thread brings up a question I've had: Can silicon bronze be used for boiler bushings, etc. ???

It machines very well.

Thanks,

Pete

Possibly running the drill at lower rpm would help? Could explain why the smaller drill worked ok but the larger one was spinning too fast if the spindle speed was not reduced to suit?

Low speed and keeping the feed up to it so its taking a decent cut and not rubbing will in most cases reduce work hardening.

It's impossible to draw hard and fast conclusions about machining "bronze" because even among the phosphor bronze variants there are so many different alloys that all machine differently. You can get leaded phosphor bronze such as Colphos that is made to be machined easily. Others are just plain cows to deal with. Trial and error with any unidentified piece is the only way to really know.

Interestingly turning and parting off, very much like boring is rouble free.

You may not have noticed that these cutters all have back-clearance?

Posted by Martin Dowing on 04/03/2021 22:36:06:

Many thanks for all your comments.

@David,

Will try your mehod of drill grinding next time.

@Ramon,

Yes, that was my phosphor bronze - very much like material you describe.

Interestingly turning and parting off, very much like boring is rouble free.

It is drilling what is problematic.

Did you or anyone else have tried to tap this material btw?

Also many thanks for comments of some of you that "small pilot hole is a good pilot hole" and Neil's remarks about usefulness of predrilling in such situation in general.

Martin

Martin,

I have made bushes using this material and had no issues with tapping but like any other cutting tool on bronze it does need a good sharp tap for best approach. However as already noted it is the drilling and reaming that brings out the worst in this particular material.

I would echo Nigels comment that this material is best suited as a bearing on hardened steel shafts for it can (not neccesarily will, gall and sieze) That said it is what I have made all the bottom end con rod bushes on my diesels and they are running on tough but unhardened EN24T steel without any issues so far.

Though theres quite a few pieces of it lying in the drawer, sourced from various shows over the years, it's a material I try to avoid if possible due to the propensity it to seize a drill. It was not that long ago that I had a drill (about 3mm) seize in a piece and snap the drill quicker 'than the eye could see'

Several years ago on a similar theme over bronzes I did this to show the differences in basic bronzes likely to be found in a home workshop.

As already said by someone else there are many different 'brasses' - gunmetal, not shown above probably the most used by ME's

Where possible I prefer to use manganse bronze for bearings in steam engines. The cast bronze is also an excellent bearing material in combination with leaded steel for slower running bearings .

Hope that's of a little further interest - Ramon (Tug)

Edited By Ramon Wilson on 05/03/2021 08:28:12