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Old 03-16-2014, 03:41 PM   #1
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Default Broken Center Drill Fix

I had about a dozen years experience as a machinist when I took a new job at a company that manufactured resistance welding equipment. Up to that point, the only copper I ever dealt with was from a beryllium copper foundry which machined similar to CF steel. Imagine my surprise and consternation the first time I set up a 3" diameter bar of 110 grade (commercially pure) copper in a steady and the center drill "self fed" and snapped off the pilot drill section quicker than I could blink. I now was looking at having killed an expensive (several hundred dollar) piece of stock that was cut to length.

Luckily an old hand came to my rescue, explaining the tool to fix the problem was already half made. The "trick" was to use a die grinder with a cut-off disk to grind a slot right down the center of the bit, right where it snapped off to about the same depth as the pilot drill length. I then stoned a negative edge on the lips of the 60 degree flute edges. This means to stone a land perpendicular drill axis, essentially removing the helical relief for a short distance. This prevents self feeding.

The modified center drill was returned to the lathe and the bit was fed very slowly into the stock. It ended up going a bit deeper than the countersink area but left the broken tip exposed in a thin veneer of copper. A small punch and hammer plopped it right out. A larger center drill with a stoned negative edged on both the pilot and countersink saved the part (and perhaps my job).

I had never broken a center drill before this. I learned quickly to have an entire set of drill bits (center and twist) on hand for soft copper alloys with negative edges stoned or ground at the ready. I hope this is a help to someone.


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Old 03-16-2014, 05:02 PM   #2
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Thanks for the tip. It is all easy when you know the solution to the problem.

Like tell folks this stuff is easy any five year old can do it with ten years experience.
Tin


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Old 03-16-2014, 07:27 PM   #3
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Useful tip - I have broken several centre drills of late , this has never been a problem before , I believe that the drills are of "foreign" manufacture and of poor quality or badly heat treated HSS.
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Old 03-16-2014, 08:28 PM   #4
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Default Center drill

I've never went wrong with KEO. Some low-cost imports grind too much material away.
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Old 03-17-2014, 12:39 AM   #5
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I think some of you missed the message!

In soft materials, such as brasses, coppers and some other materials,
your tool will get sucked, or drawn into the material during machining operations. OOPS! If you haven't experienced it , this is a little Hint for you.
A fresh new or just sharpened tool will grab the material, dubbing helps to prevent this phenomena, or Oh- --it moment.

Excellent suggestion!!
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Old 03-17-2014, 01:09 AM   #6
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So true brass, copper and most plastics benefit from a zero rake tip. does not have to be much .
Some plastics even suggest grinding off all but a fraction of an inch of flutes.
Plastic seems so easy to machine until it sucks in a drill bit and holds on for dear life. DAMHIKT.
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Old 03-17-2014, 09:22 AM   #7
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TN -- nice to be on a job where the old hands are so helpful. Makes life so much easier!
Good little trick he taught you there for removing the old point from the job.

I remember years ago, machining 12" diameter copper "spot" welding wheels in the car factory that were used to weld two halves of the petrol tanks etc together. It was tricky stuff to deal with all round, not just centre drilling. Much more grabby and stringy than brass or bronze.
All I remember from it was acute angles for rake on turning tools and sharp, sharp HSS toolbits stoned to an even sharper finish.

Have fun on your new job!
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Old 03-17-2014, 09:58 AM   #8
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I was re-reading some gems from the pen of Professor Dennis H Chaddock who amongst other things was involved in the design of the Atom Bomb, made ball bearings on a home lathe, was the Principal of a prestigious engineering college in the UK and inventor of the Quorn tool and cutter grinder which could be made in the home workshop- on a lathe only and could tackle almost any tool problem. I've got one- not an atom bomb but a Quorn!

Chaddock in his wisdom suggested that instead of fiddling about with blunt twist drills and centre drills and other things which seem to pre-occupy our incursions with things that date from the time of the ancient Greeks that 'we dispense with a lot of the bally hoo' and use 'four facet' grinding on twist drills- which gets rid of the centre drill and all its mystical problems of making little dams in children's modelling clay and filling it with a mixture of nitric and sulphuric acid and fiddling with a stainless steel bit of wire. After all, four facet drills are nothing more than end mills and slot drills- which seem to work.

Regards

Norman
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Old 04-07-2015, 05:18 AM   #9
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While we where trying to free the frozen tailstock ram the Idea Fairly whispered in my ear. Just a little heat would do it. Why dig out the propane torch when the foundry furnace is handy and hooked up. I swear I only put it in for a few minutes, long enough for the handle to melt off.
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Old 04-07-2015, 07:33 AM   #10
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To drill the center hole in copper: Use kerosene or soluble oil as lubricant. Brass and copper is unlike with lathe tools: Brass need 0 degree rake and copper need 16 degree rake. Also copper is softer than brass. Copper tends to get stuck in the cutting tool if it is cut dry. Best to use kerosene, soluble oil as lubricant under work with copper.


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