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How to drill a square hole?

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Ian Abbott26/08/2010 19:00:38
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Apparently, the constant diameter triangle used on the cutter in the photograph is achieved by mounting three pieces of hexagonal bar in the three jaw chuck, then rotating the bars by two flats each to bring the next face into line.
 
This procedure gives one three triangular lengths of bar after three rotations.
 
This is how it was explained to me by the maker.
 
Interesting thought here, with a constant diameter triangle, it shouldn't matter how you position the bar in a three jaw chuck to machine a diameter on the end.  
 
Ian 
Anthony Knights27/08/2010 18:07:58
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Just a thought. If the item has a constant diameter, why would you need to turn a cicular end on it?
 
Anthony Knights27/08/2010 18:10:21
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Sorry. I have a dyslexix keyboard. I meant to type ciRcular.
 
Ian Abbott27/08/2010 20:06:56
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It's not so easy to fasten a triangle into a bearing or onto a driving shaft.
 
However, on that subject, the geometric centre of the triangle isn't in the middle, if that makes sense.  Or, while the upper part of the outer surface will maintain a constant distance from the surface that it is rolling on, the centre of the triangle goes up and down relative to the surface.
 
That's as much as my mind can manage after a day in the noise of the Dartmouth Royal Regatta, which ended the day with a free air show, featuring the Red Arrows and the Tornado on reheat. 
 
Ian
Who's hearing wasn't deficient. 
Anthony Knights28/08/2010 00:38:14
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As a mere beginner at this game I wasn't aware one could obtain hexagon bar which could be hardened enoungh to use as a cutting tool. The idea of using hexagon bar to make the cutter is amazing and just goes to show the ingenuity of the people involved in engineering. Is there any information available regarding the size of cutter in relation to the hole required, or is it all down to the drilling guide (jig or whatever is the correct terminology).
Ian Abbott28/08/2010 18:23:50
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Can't remember what I was told about the cutting tips.  One could braze on a harder material I guess, then harden & temper.  The one in the photo looked like the bar cut away on the mill, with no additions.  I remember that he couldn't get hexagon in anything but mild steel.
 
Ian 
jacques maurel02/09/2010 22:24:13
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I've turned the solid of equal width between centers, the process is thoroughly discribed in ME N°4285.
Anthony Knights04/09/2010 01:02:09
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Bonjour Jacques,
                                unfortunately. having only recently become involved  in mechanical engineering (having spent most of my working life in electronics) I do not have access to a copy of M.E.No.4285.
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