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Drilling Bronze?

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Will Noble26/10/2018 13:46:26
39 forum posts
7 photos
Posted by blowlamp on 26/10/2018 13:17:40:
Posted by Bob Youldon on 26/10/2018 12:57:00:

Good afternoon all,

Re drilling phosphor bronze:-

Perhaps the O/D expands outwards & the I/D expands inwards onto the drill?

Tony

Exactly.

Bob

Something expanding but getting smaller just blows my mind. Is this how negative interest rates work? disgust smiley

Martin

Going back to my apprenticeship - which was certainly not yesterday:

The material heats up and therefore expands. With bronzes, that's quite a bit.

If you consider the item in this instance as a tube - forget the fact that you are trying to make a hole in a solid bar - when it expands, it gets bigger in every direction.

The outside diameter and length get larger, that's easy to see, but since the material expands in every direction, the metal on the inside face of the bore has to move and it does so inwards, ergo the hole gets smaller.

Simples?

Add to above the material is 'sticky', the drill heats and expands as well and you have all the makings of a *** of a job.

That's my twopennerth - which I hope has reassembled Martin's mind.

Will

CHARLES lipscombe26/10/2018 13:54:12
119 forum posts
8 photos
Posted by Will Noble on 26/10/2018 13:46:26:
Posted by blowlamp on 26/10/2018 13:17:40:
Posted by Bob Youldon on 26/10/2018 12:57:00:

Good afternoon all,

Re drilling phosphor bronze:-

Perhaps the O/D expands outwards & the I/D expands inwards onto the drill?

Tony

Exactly.

Bob

Something expanding but getting smaller just blows my mind. Is this how negative interest rates work? disgust smiley

Martin

.

The outside diameter and length get larger, that's easy to see, but since the material expands in every direction, the metal on the inside face of the bore has to move and it does so inwards, ergo the hole gets smaller.

Add to above the material is 'sticky', the drill heats and expands as well and you have all the makings of a *** of a job.

Will

Inwards expansion does not happen in the school ball and ring experiment

Chas

larry phelan 126/10/2018 14:00:11
806 forum posts
14 photos

Did not Sparey mention something along these lines on page 88 of his book,when talking about drilling large dia holes in the lathe ?. He refered to them as "Flat drills" which never jam. Might be worth trying.

ega26/10/2018 14:04:36
1790 forum posts
153 photos

larry phelan 1:

Spot on!

Sparey and other classic titles should be in every ME's library.

Edited By ega on 26/10/2018 14:05:40

Chris Trice26/10/2018 16:12:26
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1362 forum posts
9 photos

If tubes expanded inwards when heated, you'd never be able to shrink fit a tube on a shaft or heat a track rod ball joint taper socket to release a stubborn one.

Edited By Chris Trice on 26/10/2018 16:13:26

Neil Wyatt26/10/2018 17:37:10
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Moderator
18139 forum posts
713 photos
77 articles

I think it's more likely to be a process related to the release of locked in stresses.

Imagine a bock of foam, squash it into a rather smaller cardboard box.

Now cut a hole though the foam and it will 'expand inwards' to reduce the size of the hole.

Obviously a lot less distortion can be locked into a metal, but you only need a small reduction to grab a tool.

Neil

CHARLES lipscombe26/10/2018 21:32:52
119 forum posts
8 photos
Posted by Chris Trice on 26/10/2018 16:12:26:

If tubes expanded inwards when heated, you'd never be able to shrink fit a tube on a shaft or heat a track rod ball joint taper socket to release a stubborn one.

It's pretty hard to get past Chris's logic here!

Neil's suggestion might have merit but it does not explain why this behaviour is unique(?) to phosphor and aluminium bronze.

Best wishes, Chas

Mick B126/10/2018 21:33:38
1661 forum posts
88 photos

Well, I don't know I'm sure. I've turned, drilled tapped and died the stuff (PB1) without noticing much difference from good BDMS. The tools need to be sharp, a bit of extra top-rake seems to help and the swarf comes off in ductile, helical ribbons, but I've not met any of the grief some seem to be describing. Maybe it's just that the components I've made in it are small? For example the acorn cups in my album pics.

I have seen some very high-copper bronze (or perhaps it is effectively just copper) that does exhibit all the difficulties described - to me it seems pretty much unmachineable. Taps stuck in it almost irretrievably, even with a hole 15 thou over tapping size. But it's the wrong colour for anything I've ever thought of as phosphor bronze.

Edited By Mick B1 on 26/10/2018 21:34:49

SillyOldDuffer26/10/2018 22:08:25
Moderator
6191 forum posts
1345 photos
Posted by Neil Wyatt on 26/10/2018 17:37:10:

I think it's more likely to be a process related to the release of locked in stresses.

...

Or is it that the properties of bronze allow the end of the drill to wedge into the metal (rather than cutting chips as it should) causing the drill's helix to simultaneously screw into the metal while being forced open to jam at the sides? Behaving a bit like an expanding masonry bolt. I image it's the same effect that causes drills to snatch as they break through sheet metal.

Dave

not done it yet26/10/2018 22:23:40
4890 forum posts
20 photos

It is more likely temperature at the cutting tip and just behind it. Differential cooling and no cutting on the flutes of the drill, so hot tip=bigger hole and cooler after the cut=smaller hole. Difference =drag or jam.

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