|Will Noble||26/10/2018 13:46:26|
|39 forum posts|
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.
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.
|CHARLES lipscombe||26/10/2018 13:54:12|
|119 forum posts|
Inwards expansion does not happen in the school ball and ring experiment
|larry phelan 1||26/10/2018 14:00:11|
|806 forum posts|
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.
|1790 forum posts|
larry phelan 1:
Sparey and other classic titles should be in every ME's library.
Edited By ega on 26/10/2018 14:05:40
|Chris Trice||26/10/2018 16:12:26|
1362 forum posts
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 Wyatt||26/10/2018 17:37:10|
18139 forum posts
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.
|CHARLES lipscombe||26/10/2018 21:32:52|
|119 forum posts|
|Mick B1||26/10/2018 21:33:38|
|1661 forum posts|
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
6191 forum posts
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.
|not done it yet||26/10/2018 22:23:40|
|4890 forum posts|
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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