# Drilling Bronze



## petertha (Dec 26, 2017)

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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## EmilioG (Dec 26, 2017)

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.


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## francist (Dec 26, 2017)

I've always regarded my zero rake modified drills to be for any of the red metals, not just brass alone. 

-frank


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## petertha (Dec 26, 2017)

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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## kd4gij (Dec 26, 2017)

I never step drill bronze or any of the softer metals. Just push the size you need through and ream or bore to size.


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## Technical Ted (Dec 26, 2017)

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


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## EmilioG (Dec 26, 2017)

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.


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## Tony Pisano (Dec 26, 2017)

Agree with above, we always put a slight flat on drills for drilling bronze. No oil


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## petertha (Dec 26, 2017)

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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## Tony Pisano (Dec 26, 2017)

No oil needed. Don't grind the spiral edge.


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## petertha (Dec 26, 2017)

EmilioG said:


> I just use a few passes on a stone, usually not as severe as in the above attached images.



I'd have to go back to the video, I thought he said 0.2mm (~0.008"). I blued mine so I could see the shine of new metal as I stoned the flat. I suspect I'm right around that width, maybe a tad less. Kind of curious to try now.

The other thing I heard is: don't use drills on brass that were used on steel. I'm not clear on why exactly. Not sure if its a play on words that really means 'steel lip geometry' vs. 'ideal brass lip geometry' ie. just keep some separate drills for soft alloys, or it has to do with something else. I only have one set so violated that. But I expect to do more brass work so may well keep them in a dedicated box because grabbing is no fun.

Sidenote. I have a few HSS drills in .375-.625 range, uncoated, very sharp. I just bought them as loose stock. They did the grab/dig on 6061 aluminum where I have never had that issue on my normal silver/deming ?sp? drills. That particular lip profile had a funky grind, looked like a shallow positive arc vs. a straight flat. They work great on steel but could feel they wanted to suck the drill in. I'll try & rig up my camera on a magnifying glass to show.


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## Technical Ted (Dec 26, 2017)

petertha said:


> 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 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.



If the drill is pulling in then I would do like others have suggested and dub the cutting edge. 

The sides of the drill don't (or shouldn't anyways) do any cutting. On a properly sharpened drill, all cutting is done on the end.

Ted


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## Technical Ted (Dec 26, 2017)

petertha said:


> The other thing I heard is: don't use drills on brass that were used on steel. I'm not clear on why exactly.



I think this is going on the same line as where I suggested only using very sharp drills when drilling materials that close in around the sides of the drill and seize up on the sides. I have seen this with some types of bronze (there are a lot of different flavors). Some are really wicked to drill. Drilling steel first would dull them up some and cause issues. I've heard some suggest to only use brand new drills on materials that do this for the same reasons.

Ted


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## francist (Dec 26, 2017)

I agree with the "no oil" others have mentioned. With something that makes fine chips like cast or bronzes the oil just tends to combine with those and make a slurry which is hard to evacuate. 

Having said that, I was working on a job once where I was hand-tapping about thirty 3/8"-16 holes in 360 brass blocks. It was a new tap, and a good one, but about halfway through each hole the tap would start to squeak. Eeeh-Eeeh-Eeeh-Eeeh ..... as I wound the tap in. Drove the mechanic in the next room to madness, so much so that he got a radio from his truck to drown out my squeaking. I let him suffer through it, but afterwards it was starting to get on my nerves a bit too. A very light wisp of cutting oil from an almost-dry acid brush onto the tap worked miracles. No more squeak, the tap wound in smoother, but there was not enough oil to gum things up. I do it all the time now (except when the mechanic guy comes around next... 

-frank


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## EmilioG (Dec 26, 2017)

You could use wax to tap and that squeek is normal when tapping brass.  You can power tap on a mill at low speed if you have reverse.
I don't see a reason for not using drills that have been used on steel to drill brass, as long as the lips are dubbed and the drill hasn't seen a lot of use..  
I keep a set of non-ferrous/plastics drills in a separate index and they're all hss bright un-coated.  BoeLube is great for tapping.  There are wax stick lubricants as well. Drilling/tapping brass dry works too.  I also make sure I buy ground taps, mostly Guhring, and make sure they're sharp.  A dull tap is worthless.  Plastics: I mainly use 60° DP angles and carbide dagger drills., but you can use dubbed twist drills with no problem.


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## kd4gij (Dec 26, 2017)

I do not dub my drills. Good sharp drill and feed it as fast as it will cut all the way through in one shot. And I have done that with a 2 1/2" drill.


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## Silverbullet (Dec 26, 2017)

I use to run lots of brass and bronze , first never step drill the holes , start with just a dimple then right to your largest diameter for drilling . Reaming is fine or boring after. Had a very good old machinist teach me that during an apprenticeship learning turret lathes and the set ups.


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## EmilioG (Dec 27, 2017)

kd4gij said:


> I do not dub my drills. Good sharp drill and feed it as fast as it will cut all the way through in one shot. And I have done that with a 2 1/2" drill.



I've drilled brass w/o dubbed lips feeding fast and hard, peck drilling. It was hot or miss, so I just started dubbing the lips. Thin material may present a problem with sharp drills. I didn't want to chance it.


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## mikey (Dec 27, 2017)

Another vote for going to the main drill after spotting to avoid grabbing. I don't "dub" my brass drills, either; I just don't pilot drill and that makes all the difference. You just need to use enough pressure to keep the drill cutting and it goes right through. I use a tiny bit of DoAll wax stick lubricant like Emilio said when tapping brass; I actually just touch the tap to the stick and no sticking or noise.


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