Drilling Bronze

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petertha

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I made some bronze bushings from C932 and I don't have a lot of experience with this material. Initial turning was straightforward enough, it came off not as curly swarf but kind of cast iron looking spray of fine shavings with a hard sound. As I progressed up the drill sizes I experienced 'the grab' at ~5/8" diameter which caught be off guard. The preceding hole size was probably trying to warn me of that but I wasn't paying attention. I was using cutting oil & backing out often to clear chips. It took some persuasion to back the drill out manually in reverse. Fortunately that was the last drill size so I could carry on with boring bar which went very well.

The dig in or grab kind of reminded me of brass or sticky aluminum. I wasn't expecting this issue & don't want to again, so want to learn from this experience. I have since re-dressed some drills with appropriate reduced rake angle & set them aside dedicated for brass (and maybe now also bronze use) but haven't tried them yet?

OLM lists these bronze alloys in round. I used C932, aka SAE 660 which they say "comparably easy to machine".
http://www.onlinemetals.com/productguides/bronzeguide.cfm

I've heard similar experience to watch out for 'phosphor bronze'. Of the 3, only the C932 has phosphorus content & its only <0.15% according to specs.
But C655 (silicon) bronze is indicated as "more difficult to machine". I usually take this to mean surface finish as opposed to other problems like auguring in. So is problem more related to C932 being softer than the other bronze alloys & phosphorus just so happens to be a constituent related to this particular alloy & jus gets fingered as the culprit? Strangely, C932 has high lead % which usually makes for a good 'free machining' constituent.

Any tips appreciated.

btw - I need to go back & check my boring bar insert. It might be neutral or even negative rake but don't hold me to it. It was a sharp one & cut/inished beautifully, no issues there. If so, does that indicate that 'brass drills' would be appropriate for bronze?

material references
C932 http://www.matweb.com/search/DataSheet.aspx?MatGUID=b673f55f412f40ae9ee03e9986747016
C954 http://www.matweb.com/search/DataSheet.aspx?MatGUID=b950d0d72b5b467689f2d9c5d9030ae8
C655 http://www.matweb.com/search/DataSheet.aspx?MatGUID=97fc9cb4ed1f45319793fb018b38e645

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Dub the drills cutting edges. Run both edges against a wet stone. I have a small set of these drills for bronze, brass, copper.
There is a YouTube video from Tom Griffin showing this with plastics also.
 
I've always regarded my zero rake modified drills to be for any of the red metals, not just brass alone.

-frank
 
Good, hopefully I'm on the right track now. My camera wont get close enough to show my edge, but I blued the drills so I could see the witness & did this operation with a fine stone. Pics from Clickspring YouTube video.
And yes, saw that same Tom Griffin video on plastics, thanks. Very visual wake up call.

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When you say "the grab" do you mean the drill pulled/screwed into the material or were the sides of the drilled hole closing in on the drill diameter squeezing/grabbing it? These are two different problems when dealing with certain materials. Dulling the cutting edge with help with the pulling in/screwing in, but for the side pinching you will need to do something else to help.

One thing I've done in the past with some materials is to drill with just one drill bit (no step drilling) and sharpen the drill making one lip longer than the other... this causes the drill to cut over size giving some clearance for the drill sides as you drill in deeper.

If you can't use just one bit, make sure your drill bit is extremely sharp. A dull bit will heat the material as you cut and then as it cools it will shrink down on the drill's diameter pinching it and making it bind. A very sharp bit helps to reduce the heat while drilling.

YMMV,
Ted
 
I've never had a problem with grab or pinch after I dubbed the drills cutting lips. I just use a few passes on a stone, usually not as severe as in the above attached images.
 
Agree with above, we always put a slight flat on drills for drilling bronze. No oil
 
Thanks guys. These are the real world techniques & issues you cant read about in a book. My next try I was going to go 'middle road' & try & drill with 0.5" diameter in one shot with my new dub drill weapon. I think I have sufficient power & rigidity to do .750 or .875, but was more concerned by chuck jaw grip & and heat & chip plugging. Although backing in & out on the same diameter drill is probably different than progressive opening up from a smaller hole.

On the stock sharp drill, yes by grab I mean the drill pulled & screwed itself into the material & kind of stuck there within the bore. I was maybe 1" depth on the 0.625" drill size. I did not have any grab issue entering a progressive hole which is usually the nail biter time on the 90-deg lip. The stock was cold, so no friction heat.

I haven't tried my dub drills yet but was almost wondering if the first say 0.2" of the spiral cutting part of the drill should be similarly treated? Nobody mentions that, only the conical cutting part, so I assume no.

I was also wondering about cutting oil or not. On the previous smaller size it made the (slight) squeal go away so assumed it was doing some good.
 
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