Saturday, December 26, 2009

First Shawm - Really Boring now


As well as a hollow centre for the lathe, I needed a long drill or auger to actually do the boring.

Trevor Robinson reckons that the ultimate for this purpose is to use shell augers, and even gives some instructions on how to make a shell auger if you don't want to buy them. Now that I have some experience in the matter, I see these instructions as valid enough but not really giving much guidance with the key aspect, which would be the careful and central sharpening and honing of the cutting end.

My approach was more direct: I went to a local hardware shop and bought the longest 6mm drill I could find, which was an auger about 300mm long.  This was still too short, but was a good start.



I made this auger longer by the simple expedient of fixing a 6mm steel rod to the end of it.  The joint consists of some brass shim wrapped around the shaft, with the whole assembly held together by soft soldering.

My welding plant was out on loan, otherwise I would have made a steel sleeve and brazed the auger and extension together.  In retrospect, this would not have been as good as the brass and solder joint, since it would have been too stiff to adjust for best straightness, without possibly bending the extension or the auger.  However, the brass/solder joint was a nice balance between stiffness and adjustability.



Here is the initial stock length, ripped down to a 50mm x 50mm section, being drilled.  I clamped a basic guide on the end of the lathe to assist with keeping the drill in the right line.

Aligning the bit by eye, I started the bore with a 15mm bit, and turned it with a brace since the lathe did not have enough torque or gearing to turn against this.

When this maxed out at about 150mm, I changed to a 10mm spade bit on a 400mm shank and bored that to its full depth, before changing to the auger.



Looking into the hollow centre.

The initial hole is 15mm diameter, bored with a brace and bit.  Then it comes down to 10mm diameter, drilled with a spade bit with a 400mm long shank.

This provides clearance for the joint in the 6mm auger seen here to pass inside the workpiece.



Things seemed to going along quite well, and I was becoming quite hopeful, when Pop!, the end of the auger appeared out of the side on the workpiece about 70mm from the end.

Bother!




To see where I had gone wrong, I cut the workpiece into sections , and here they are.  On the left is the tailstock end with a 10mm bore, moving toward the headstock  as you look across to the rightmost end.

It is quite apparant that the hole was deviating right from the outset, and that it actually ran in a fairly straight line until it emerged from the side.  This was sort of good news, since it suggested that better aiming of the drill would greatly improve the result.

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