Reaming question

[Anonymous]

Posted by julian down on 25/04/2015 16:55:53:

One thing I've often wondered – if you heat up a piece with a hole in, does the hole get bigger or smaller? I can imagine the material as a whole expanding, but doesn't the hole get smaller as the material around it expands?

Definitely not – think about it.

Andrew

[julian downParticipant @juliandown56088]

I suppose if I consider the circumference or rim of the hole then it must get larger.

[Roy MParticipant @roym]

Micheal G, there could be logic in your reasoning, but also if there is any initial resistance to cutting (and cutting oil alleviates this to some extent), this may compress the reamer and result in a smaller hole. If you have ever tried to ream aluminium-bronze or really tough stainless, you will know there is a danger of the material gripping the reamer (or drill),and either seizing up, or resulting in an undersized hole. There is a good reason why there is such a wide selection of taps, reamers, drills etc.,in industry. The resourcefulness of the model engineer is to be applauded! Roy M.

[Anonymous]

Posted by julian down on 25/04/2015 17:05:11:

I suppose if I consider the circumference or rim of the hole then it must get larger.

Eaxctly. From a practical viewpoint we know that a shrink fit can be obtained by heating the outer part and cooling the inner part prior to assembly. If the hole in the outer part got smaller as it got hotter the technique wouldn't work, but we know it does. QED.

Andrew

[Robin GrahamParticipant @robingraham42208]

The thing about the hole expanding or contracting used to be a standard question asked to students at uni physics course entrance interviews. Imagine you have a magic zero-kerf holesaw, you cut out the core, heat everything up to the same temperature, does the core still fit in the hole? Well yes, you could have just heated the whole thing!

Robin.

[John McNamaraParticipant @johnmcnamara74883]

One little trick worth remembering, Particularly if the pin is a loose fit in the hole is to Knurl the end of the rod. Preferably with a straight knurl. I like to stop the knurling so that it is a couple of mm below the surface of the hole it is being driven into, it helps it hold.

Knurling positively stops any tendency for the pin to rotate.

The process works superbly well (permanently) attaching small aluminium gears to a steel shaft too.

Regards
John

[julian downParticipant @juliandown56088]

Thanks for that tip, John. This is all good stuff.

Julian

[Neil WyattModerator @neilwyatt]

> Imagine you have a magic zero-kerf holesaw, you cut out the core,

.,.. then you can't get it out because its stuck in there by molecular adhesion. Damn.

Neil

[Robin GrahamParticipant @robingraham42208]

It's a magic holesaw Neil. Being zero thickness, it doesn't have any molucule thingies obviously.

Robin

[Danny M2ZParticipant @dannym2z]

To prevent reamer chatter (when chambering a rifle for example), some gunsmiths advocate wrapping the flutes with waxed paper.

I am not sure why this works, maybe somebody has an explanation.

* Danny M *

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