Drilling Bronze?

Bit of a moot question really as I don’t have much left and don’t intend to buy any more but I had a real problem drilling some bronze bar in the lathe the other day. Even with plenty of lubricant it was really heavy going and in spite of going up in small steps the drills kept sticking. Some of the drill bits were new so sharpness isn’t an issue.

Vic,

It tends to be 'sticky' material to drill and I have found the bore also closes in as the hole progresses

I don't know of an answer except to move onto a boring bar as soon as you have a hole big enough to accept it

Regards

Brian

Edit for spelling

Edited By Brian Wood on 18/10/2018 10:20:01

Yes that seemed to be the case Brian. I did move on to a boring bar in the end but my little 6mm CCMT one is only 79mm long. I’ve now ordered another couple of bars, 6mm and 7mm at 123mm long.

phosphor bronze is the worst type of bronze to drill, to stop the drill binding when roughing out, grind the drill with the point offset so that it produces a slightly over size hole

Concur.

Can be horrible. But i also machine it dry.

We all agree then. Phosphor bronze (the red coloured stuff) is a **** to drill or ream.

So why do all our drawings and "experts" specify the stuff when SAE 660 or leaded gunmetal or cast LG2 or LG4 is just as good for bearings on most of our work and a lot easier to machine?

Confused of Boat of Garten

I am quite pleased with the response, I was worried it might be me but it seems a few of you don’t like drilling the stuff!

Good question Martin, let's see what they say!!

Brian

I imagine that historically, most people follow the sorts of materials they have seen other people use.

PB1 is tougher than SAE660 and generic 'phosphor bronze' was probably more widely available back in what many think of as the 'golden age'. The combination of habit and a tendency to over-engineer probably means most people simply think 'phosphor bronze'.

I have often suggested it would be great for someone to write a series a covering different metals and their properties and applications with a specific focus on hobby needs. An offer never comes, although perhaps when the lathework series is finished I will have a go.

Neil

Hello Vic,

Generally with drawn phosphor bronze there is a need for an accurate hole, for perhaps a bearing, then the drilling process is fairly simple, resharpen a drill slightly off centre so it cuts slightly over size but making sure it is still fractionly under the size for a reamer. The other alternative is to use a slot drill of the appropiate size, a long reach one if the hole is fairly deep. I keep a box of resharpened slightly off centred drills specificly for drawn phosphor bronze work; also I will always turn and drill drawn phosphor bronze without any lubricant but for threading and tapping work I always use Trefolex cutting paste cleaned after with white sprit.

I hope tihis is of some assistance.

Bob

Vic, drilling bronze going from a small drill to the size you want in small steps is a recipe for the drills snatching and jamming at every step. You would be better off using a sharp drill of the right size ground as mentioned above, or keep a drill as large as you can get away with, ground as above and boring the rest.

If you are making something like a fitting which needs stepped holes, drill the biggest to depth first, then drill the next size in virgin metal and so on.

Chris Gunn

And I suppose the reamer just goes through like a knife through butter? Aye right.

Martin

Re Brian Woods post of 18th October (sorry haven't worked out how to copy bits from previous posts yet).

Brians experiences with holes appearing to shrink and grip the drill when drilling bronze are exactly the same as mine when drilling Aluminium Bronze. Nightmare stuff to drill. But what is the technical explanation of this?

Surely as the material immediately around the drill gets hot from drilling, the hole would expand and give the drill an easier time?

And why are the bronzes (Phosphor and Aluminium ) alone in this behaviour?

Chas

A bit late adding to this but:

As a result of absolutely no experimentation whatsoever, how about a SHARP, carbide tipped, masonry drill, possibly even one of the spear pointed tile drills?

If you are just making a hole to be bored to size (usually the case) then accuracy of size should be no concern and the carbide tips are usually larger than the drill tip and the drill itself just there to hold the tip(s) and provide flutes to clear waste.

Will

(fire away, I've got my tin hat on)

Perhaps the O/D expands outwards & the I/D expands inwards onto the drill?

Tony

Good suggestions Will. I think you would need to modify the angles and sharpen up a masonry drill, but the tip is usually quite a bit bigger than the shank, so it should get over the "shrinking" problem.

Whatever you do with red drawn phosphor bronze, the tools must be razor sharp. Razor sharp must also include the lands down the sides - not just the point. Drills should preferably be four facet or split point. You need flood coolant. If you are trying to ream to size then you are probably going to need a new reamer (or freshly sharpened if you have that facility). Beginners would be well advised to saw off, mount to mandrel and face off rather than part off.

Of course if you are in industry and have access to top quality tools, diamond tooling, etc. etc. then the jobs a doddle, but as amatuers we have to rely on the various Chinese junk emporiums that serve the model trade - there is a huge difference - in quality and price.

The shrinking effect is similar to weld distortion - you are locally heating inside at the point of cutting, the hot metal is compressed by the cool metal outside, then the whole lot cools causing the drillled hole to "shrink".

I still remember my first loco (Rob Roy) coupling & con rod bushes. As Curly might have said "Centre drill, follow up with a pilot, poke a 15/64" drill down and follow up with the 1/4" reamer. Part off and Ditto Repeato 8 times". Well I can only say it didn't quite go like that when I was working with blunt drills, a reamer of indeterminate parentage and zippo for experience. And by the way, I was making an extra set for a friend. But that was 50 years ago now.

I come back to my first comment. Why put ourselves through the misery when SAE 660 is far easier to work and for our typical bearing loadings will work just as well?

Martin

As Martin Johnson says, sharp, Split Point drills are what works best - at least in my experience they do.

So the drill to use would be the one on the left, in the picture below, which I happened to find on tinternet.

If you try and drill pink bronze with a bit like the one on the right - even if it's a Four Facet drill, then the centre seems to rub & harden the material disastrously. I even avoided reaming this stuff after having no success at all with two new reamers.

What did work for me was to use a spot drill to establish a good centre, followed by a slightly undersized split point drill and finished with a sharp, dead-to-size drill (any style of grind) as it only cuts on the periphery.

Martin.

Good afternoon all,

Re drilling phosphor bronze:-

Perhaps the O/D expands outwards & the I/D expands inwards onto the drill?

Tony

Exactly.

Bob

Something expanding but getting smaller just blows my mind. Is this how negative interest rates work?

Martin

I don't think that Bob Youldon's explanation is the correct one -remember that physics experiment at school describing thermal expansion where a ball will not pass through a ring at room temperature but when the ring is heated, it expands and lets the ball through? The inside of the ring must be expanding outwards for this to happen.

Martin Johnson's theory sounds might explain things, but I'm still not convinced because the bronzes are such good conductors of heat and heat from the cutting zone would very soon reach the outside of say a half-inch bar. I presume that brass does not suffer from this problem because of the way it breaks into chips at the cutting edge.

My experience is that when the bronze starts to grab it quickly destroys the drill lands - after which you might as well stop and have a cup of tea while you ponder the next move

Chas.