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drillling bronze

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Philip Burley13/09/2019 11:09:09
180 forum posts
1 photos

I have given up for this morning , broken 2 drills , trying to drill some bronze 2 1/2 mm to tap 3 mm . Where am I going wrong ?


charles hodgson13/09/2019 11:31:36
3 forum posts

Never drilled bronze, but if it snatches as badly as brass, might be worth blunting your drill and keeping that one specifically for brass/bronze.

JohnF13/09/2019 11:37:47
981 forum posts
142 photos

Which bronze ? It can make a difference but assuming Phosphor Bronze --

Grind your drill slightly incorrectly it cuts oversize by a tiny amount by removing a small amount from one side of the point this means generally that only one side of the drill is cutting thus pushing it away from the other side and this stops it grabbing !

Use plenty of cutting oil or suds, it is very different to brass - don't blunt your drill or reduce the rake


PS how deep is the tapped hole ?


Edited By JohnF on 13/09/2019 11:39:00

JA13/09/2019 11:50:33
910 forum posts
50 photos

I have drilled quite a bit of bronze from cast gun metal to phosphor bronze. I have always used a drill with zero rake, as for brass. I drill at speeds lower than brass and always with a neat cutting oil. I do occasionally get a drill that snatches and have had drills wander badly in deep holes.


Philip Burley13/09/2019 12:17:34
180 forum posts
1 photos

I was sold to me as PB1 , drilling about 1/4 inch deep , tried fast and slower and with neat cutting oil , but havnt flattened off the drill edges , Sent for some more drills Run out now !

Philip Burley13/09/2019 13:15:12
180 forum posts
1 photos

well there's a thing , Never had much success sharpening very tiny drill s , but just did it with small high speed wheel and magnifying glass , flattened the edge a bit and low they cut far better that they ever did new , lovely curly chips coming out . ! pity I spoiled the bit of bronze , have to start again now .

regards Phil

old mart13/09/2019 15:31:23
1750 forum posts
138 photos

I have drilled various grades, and treat it like leaded brass, which tries to grab the drill. With larger drills, it is easy to stroke a stone across the cutting edges to reduce the rake angle, but the small ones are difficult to do. Holding the drill in a vise and using a magnifying glass might work. The use of a mill with a slow feed on the quill is a huge advantage over the ordinary drill press. With a drill press, it helps if you can partially lock the quill, and use both hands to keep control of the feed rate, which should be as slow as you can manage. Just a drill on its own is not going to work unless the bit is really well backed off.

Neil Wyatt13/09/2019 16:25:00
17882 forum posts
705 photos
77 articles

Apparently phosphor bronze contracts around a drilled hole.


George Jervis13/09/2019 16:48:49
75 forum posts
42 photos


Would anyone volunteer to put up a picture of the drill bit that's been modified as I to need to drill through some stays on my Minnie traction engine build, I have already broken 2 drills and have had to have them spark eroded out already and don't fancy repeating


Nigel McBurney 113/09/2019 19:32:00
708 forum posts
3 photos

for PB 1 grind the drill off centre so that it cuts oversize,I find that a slower speed than that recomended for bronze works ok if was drilling 2.5 mm dia on the myford I would run at around 1000 rpm instead of the Dormer recomended speed of 3000 /4000 rpm use a drop of of Rocol tapping fluid . Drawn PB really does need some cutting rake,so I do not grind off the rake angle, cast bronze ie gunmetal will grab a drill so does need a zero rake cutting edge. I have one length of cast bronze stick ,which when drilling ,the drill wanders all over the place so I drill well under size and bore,never had this trouble before,

JA13/09/2019 19:52:01
910 forum posts
50 photos

The real problem with a wandering drill is that you do not know it is happening.


old mart13/09/2019 20:28:47
1750 forum posts
138 photos

I will try to remember to take a photo of a drill tip which has been backed off, tomorrow.

blowlamp13/09/2019 22:02:53
1344 forum posts
84 photos

If it's pink bronze then you'll probably have better luck with a drill styled like the one on the left.

It'll cut at the centre without rubbing, unlike the one on the right.



old mart14/09/2019 19:13:40
1750 forum posts
138 photos

_igp2455.jpgI have a couple of pictures of a drill bit that has been backed off, it needs sharpening.


old mart14/09/2019 19:17:00
1750 forum posts
138 photos

Blowlamp, your drill on the left is a split point, not a special brass/ bronze one. The relief angle is the same as the one on the right.

JasonB14/09/2019 19:27:05
18098 forum posts
1993 photos
1 articles

I mostly use split point and don't get a problem in bronze or brass, never stoned a drill bit.

That photo of a backed off one is not like most I have seen where the face is stoned to near vertical which is like using a zero top rake lathe tool.

Edited By JasonB on 14/09/2019 19:29:37

old mart14/09/2019 20:51:44
1750 forum posts
138 photos

Stoning the face that way has a similar effect. I think the op's problems stem from trying to feed too fast.

not done it yet15/09/2019 10:02:46
4635 forum posts
16 photos

If I have a drilling problem, I usually resort to using an endmill. Two fluters are OK for drilling, more if a pilot hole is there to enable the milling cutter to plunge. A pilot is obviously not applicable in this instance.

Andrew Johnston15/09/2019 21:17:08
5497 forum posts
647 photos

Although bronze, and gunmetal, can snatch like brass, generally I don't have a problem with drills less than 6mm or so. However, I run industrial machines. Even if the smaller drills do snatch they're not capable of pulling the tailstock along for instance. Larger drills are a different story; they can simply pull the Morse taper out. One can go up in small, say 1mm increments, but that can get tedious. I have used the slot drill trick, sometimes it works well and sometimes not; it depends upon the helix angle of the cutter.

Stoning the cutting edge has never worked for me; possibly I'm too aggresive on feedrates. Instead I've bought a small selection of slow helix drills from about 1/4" to 1/2". Above that I prefer to use a boring bar.

I haven't really noticed bronze closing up after drilling, but have definitely seen it when reaming. After reaming bronze (with a machine reamer) the reamer will not re- fit in the hole by hand. When I was grinding the rams for the water pumps on my traction engines, to fit the existing reamed bore, I had to go about 3 tenths undersize to get them to fit. So either my 17mm reamer is fudged or the gunmetal casting had closed slightly.

Assuming the thread is M3 a 2.5mm drill is slightly on the small size. I'd be using 2.7mm on bronze.


Edited By Andrew Johnston on 15/09/2019 21:18:51

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