Drilling and tapping in bronze

[9fingers]

Years ago I had a nightmare drilling into phosphor bronze and the experience is burned into memory

Soon I have to drill and tap some bought-in Bronze Acme nuts. http://www.trapezoidal-acme-thread.co.uk/trapezoidal-thread-nuts/round-nuts-bronze/

I am looking for any tips to do the job as I can not afford to wreck these. It may be that bronze behaves differently to phosphor bronze and I'm worrying unneccessarily? I will be doing the job on a vertical mill to take advantage of the DRO to locate the holes. Threads will be m6 x1. Should I try and avoid snatching by drilling final size at the outset or use a pilot drill first? I have hand taps as well as a spiral one in this size.

Comments welcome.

TIA

Bob

[RotarySMP]

Do you stone the rake off the drill bit?
Mark

[9fingers]

I could do if that will help - similar to brass then?

I've not used bronze before and want to prepare as best as possible for a right first time outcome.

Bob

[Arbalist]

Ive not had any problem drilling bronze but the stuff I've got is a lot redder than those nuts if it makes any difference? I've had brass snatch though so I pay particular attention to clamping the part to stop it moving. I should adjust the rake as suggested though on a few bits and put them aside for " brass only".

[ieezitin]

I would recommend really sharp taps...No cheap china stuff... top quality brands only.. Spiral flute might be better,  use a brand new one for every hole if you have to... bore out 60% thread engagement if you can get away with it.

with a brand new tap tapping the hole for the first time should be a normal thing... if you then try to tap the second hole with the same tap and it squeaks and tends to bind a little stop and use another new tap or try to stone the one you just tapped with.

Aluma-tap fluid works great or kerosene.

Best of luck.

Anthony.

[sparky961]

Spot the hole before drilling and don't move the table between that, drilling and tapping.  There's no specific reason to do this for bronze other than to eliminate one possible contributor to failure.

I'll second recommendation for a good quality, new tap.  I'd avoid, however, spiral flute because they are the weakest of all taps.  Personally I'd go with a 2-flute spiral point machine tap because they're virtually bullet proof.  If the hole is bottoming I'd start the hole on the mill to keep it square and finish by hand.  Hopefully you have enough hole depth to do this, or it's a through hole (in which case you can tap through on the mill).

This stuff does tend to "close in" on a drill bit if you aren't careful.  Use more coolant or cutting oil than you think necessary and clear chips often.  Neutral rake on cutting tools does tend to help, especially with it wanting to grab and self-feed.  Also use a slower cutting speed (SFM) than you'd expect.  You can go really slow without hurting anything but depending on the alloy some don't like it fast.  Just ballparking it, maybe 1/2 to 1/3 the speed for mild steel.... say 50 SFM or so may seem slow but also gives a nice safety margin.

[lordedmond]

Just hope it's not PB1 then your chances diminish


It's looks more coppery than brassy sometimes known as red bronze


Stuart

[9fingers]

Thanks so much for all your suggestions guys. I think I now need to ask for your prayers too!

In the meantime I might have the germ of an idea to avoid the problem altogether - needs more thought yet.

Bob

[sparky961]

Care to tell us about the past "nightmare experience"?  There might be some things to be learned from that.

[9fingers]

I was making some spring collets for a mate. These were for a CNC circuit board drilling machine and held the drills in the tool changer.
I had to turn the OD of the PB rod to about 10mm including a shoulder, then drill through about 8mm resulting in a 1mm wall tube.
Subsequently, a fine slitting saw made part of the tube length into spring fingers.

So about 30mm long, 10mm OD with 12mm shoulder and 8mm bore from memory.

I did not have any collets at the time and so in a three jaw chuck, the weak tube plus the grab of the drill wrote several off.

Even rough drilling  and boring to size caused snatch in the drill and spinning in the chuck.

I got there in the end but poor yield in material and time. He used to make circuit boards for me free of charge and then gave me a one of those horrid "can you just.....?" jobs in return.

Sorry no photos other than those burned into my memory  must have been 20 years ago at least.

Bob

[jb3cx]

I had the same problem a few years ago .spent hours machining a component made of bronz,due to the irregular shape ,and radius,all machining was don on the rotary table,6 holes had to be drilled 6 mm X 24 mm deep Center drilled ,then 3 mm drill ,then on to 6 mm ,just started to drill the 6 mm hole and the drill snatched ,the quill shot down bottoming out the drill and snapping just below the surface ,so nothing to get a purchase on to back twist it out .so off to the spark eroder,
After setting it back up on the mill .this Time I locked off the quill nice and tight and used the knee to drill the holes ,drilled all the holes ,then reamed them no problem ,after that I have always been wary drilling any brass or bronz using the quill feed ,but touch wood ,been ok since that disaster ,

[SwarfnStuff]

I just read the posts so far and was going to mention using the fine feed rather than the standard quill handle as this is what I always do with brass or bronze.

   So, I agree with jb3cx when he mentions locking the quill and using the knee to feed the drill depth.

   Same method, different (way different) size in mill. Mine is Seig X2. Don't know what your mill is so whichever suits.

    Let us know how it goes.
John B

[9fingers]

I just read the posts so far and was going to mention using the fine feed rather than the standard quill handle as this is what I always do with brass or bronze.

   So, I agree with jb3cx when he mentions locking the quill and using the knee to feed the drill depth.

   Same method, different (way different) size in mill. Mine is Seig X2. Don't know what your mill is so whichever suits.

    Let us know how it goes.
John B

Sounds like a good tip. My mill is a Myford VMC C 1986 ish - similar to the early 6x26 models but no quill fine feed only rack and pinion feed so raising the knee which weighs over 100kg should help resist the snatching effect.

I will try one as an experiment (16 tapped holes to do, 4 in each of 4 nuts). I'll try everything suggested to avoid putting any money the way of my local tame spark eroder man.
If it goes badly I do have a fallback needing no drilling but needing an extra component to machine. If I go that route it will be easier to post a photo than describe the approach.

I should get the bronze nuts in from Holland sometime this week with luck and will report back.

Everyone's support has been great.

Thanks

Bob

[awemawson]

Bob,

If you break a tap, I'll pop it out for you on my spark eroder


http://madmodder.net/index.php/topic,9319.0.html

[9fingers]

Bob,

If you break a tap, I'll pop it out for you on my spark eroder


http://madmodder.net/index.php/topic,9319.0.html

Thanks Andrew

Good to hear from you - hope all is ok

Bob

[9fingers]

On Friday the parcel arrived from Holland. see link at the top of the thread.
I've been very impressed with this company and the ease of ordering from them.

You email them with what you want, they send a pro-forma invoice price in Euros. You send funds to their bank quoting the ref no and they send the goods.
Money sent Monday, good delivered Friday the same week - result.

I decided to hold the nuts in a 4 jaw chuck clamped to the mill table.



The idea being to hold the round nut as securely as I could.

I used a sturdy (BS5) centre drill to mark the four locations for the drill. I used a 4.2mm pilot drill. A new cobalt drill (because I had some to hand) with the lips knocked off with a couple of strokes of a diamond file. I did not have a new 5mm drill but chose the best I had.

Using the knee to apply the feed with the quill locked off worked brilliantly and I soon  had four holes 20mm deep with no snags. The swarf was typical brass type - fine needles and no curls. Just little bit of a squeal as the 5mm drill bottomed but no snatching and everything stayed cold.



Tapping was a breeze with a three flute straight taper tap. First time round I used a little trefolex compound but did the rest dry as it was cleaner. Finished off with a bottoming tap - did not bother with a second tap

Usual back support to the tap wrench driven by hand - not feeling brave enough to tap under power.



I soon had all four done with no breakages - pleased and relieved.





Thanks for all the hints and I hope the thread might help others in the future.

Cheers

Bob

[awemawson]

Excellent Bob, glad it turned out well in the end 

[9fingers]

Excellent Bob, glad it turned out well in the end 

I think the secret was not not let the machine see the fear in my eyes!!

Bob

[awemawson]