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 Post subject: Removing a broken drill bit
PostPosted: Jan Sat 14, 2017 9:43 pm 
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I was drilling a pilot hole into the voice coil frame of a speaker, using a 1/16" bit in my benchtop drill press. (The Detrola 448 mounts it's speaker on a bracket, using tapped holes in the frame around the voice coil and field coil.) I had the speaker mounted in a vise, but not bolted down to the table. You know what comes next - the vise walked a bit from the vibration, the drill was now off-center, and the bit broke.

The frame is about 3/16" thick steel. I think I got most of the way through before the drill bit broke. It broke just a bit below the surface. How can I remove the broken bit? The final hole size needs to be #29, for an 8-32 tap.

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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sat 14, 2017 10:39 pm 
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You will probably have to start a new hole somewhere else.

If you had a small carbide drill bit, you might be able to drill thru the stuck piece of bit. A regular HSS bit will just dull trying to disintegrate the broken portion.

If you want to spend some money an EDM shop might be able to zap it out for you.

Rich


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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sat 14, 2017 11:01 pm 
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Rich, W3HWJ wrote:
If you want to spend some money an EDM shop might be able to zap it out for you.
That's how they do it in industry.

There ain't no cheap solution to this problem, other than re-locating the hole.

BTW... You should ALWAYS bolt any vise to the drill press table before using it.
If that thing gets caught, flies off the table and hits you in the chest, your heirs will sell it.

- Leigh

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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sat 14, 2017 11:04 pm 
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I agree with Rich. A carbide bit is your only option to drill out HSS. You'll want something larger, maybe 7/64" or 1/8" although that may be too close to your tap drill size. The only problem you might encounter is keeping the drill on center, it may want to wander to the softer material of the frame.


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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sat 14, 2017 11:29 pm 
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radiomania wrote:
A carbide bit is your only option to drill out HSS.
Won't work.

Given that the broken end of the bit is guaranteed to not be flat, any new bit will deflect.
Any regular bit of similar size will have very little lateral strength.

The only possible solution would be this:

Image

These are marketed variously as "starter drills" or "drilling countersinks" or just "countersinks".
They have a massive body compared to the drill diameter, so deflection is minimized.

Here's a link to an appropriate size at McMaster-Carr:
https://www.mcmaster.com/#2925a54/=15wvdlr

One common solution for this problem if the broken bit is above the surface (it never is):
Grind the broken end flat, then use the countersink or a larger carbide bit to start a hole.
Use a drill bushing in a fixture to guide a carbide bit into the broken bit.
Obviously you must build the fixture with the drill bushing.

- Leigh

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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sat 14, 2017 11:44 pm 
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Another way to do it would be to clamp the speaker frame in a milling machine and mill it oversize with a solid carbide end drill or mill. Since this will inevitably bigger than the diameter you want, a plug would then be pressed into the speaker frame and the correct hole drilled.

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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sun 15, 2017 12:00 am 
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Thanks for the suggestions. I think I'll start over with a new speaker.

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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sun 15, 2017 12:27 am 
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If you can, drill a hole in from the back side of part then use a hammer and punch to force the broken bit back out the top.

-Matthew

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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sun 15, 2017 4:35 am 
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The other side is next to the magnet. There's no access to it :-(

It's not a big deal. The original dynamic speaker was distorting badly, so I'm replacing it with a modern permanent magnet speaker. It should be easy to find another new speaker.

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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sun 15, 2017 5:21 am 
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Hi Steve,

Are you also replacing the field coil with a suitable filter choke.
That's usually the main problem when replacing an ED speaker.

Field coils typically have very high inductance.
I've measured over 80 Henries on large console speakers.

- Leigh

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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sun 15, 2017 6:46 am 
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Leigh wrote:
Are you also replacing the field coil with a suitable filter choke.
That's usually the main problem when replacing an ED speaker.

It's just an AA5 design with a 4" speaker. I think replacing the field coil with the resistance listed on the schematic will work well enough. If not, I guess I'll have to pull the field coil off the old speaker and use it as a choke.

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 Post subject: Re: Removing a broken drill bit
PostPosted: Jan Sun 15, 2017 6:55 am 
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stevebyan wrote:
If not, I guess I'll have to pull the field coil off the old speaker and use it as a choke.
That would work.

Alternatively you can use a smaller choke and increase the size of the filter capacitor at its output to achieve an acceptable hum level.

You do need to match the DC resistance of the original field coil.
That can be done with a resistor (having suitable power rating) in series with the new choke.

- Leigh

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