johnsmachines

I purchased some bronze disks for use in the model Trevithick dredger engine.  The disks 204mm diameter had been bandsawn off rod.  I had specified minimum thicknesses of 7mm and 12mm.  One disk was 9.2-9.7mm thick and the other was 12-15mm thick.

The bandsawn blanks of LG2 bronze.

In preparing the disks for machining I filed off a few protrusions, and using a straight edge, identified the valleys and ridges.

The thicker disk was held in the 3 jaw chuck and both faces were turned flat with no problems except avoiding the needles which were thrown off in a wide arc around my lathe.  Final thickness 12.5mm.  A persisting divot should be able to be avoided in the final part.

The thinner disk needed to be packed out from the jaws of the chuck by 4-5mm in order that the lathe tool  cleared the jaws during machining.  In the past I have used machined packing pieces, but it is always a fiddle to hold the workpiece, the 3 packing pieces and the chuck key in only 2 hands.  Today I had a brainwave.

I used rare earth magnets!

I tried to measure the thickness of the magnets, but they are so powerful that I was not confident that I was getting accurate readings.  So I just used them and measured the thickness of the finished workpiece.

I am sure that this idea is not original.  But it is to me.

Here is the thin workpiece held in the 3 jaw, and packed away from the chuck by rare earth magnets.  Of course the magnets are attracted only to the jaws, and not the bronze workpiece, which helps, but I will try this on steel later.  Should work for magnetic metals also.

After machining both faces I took various measurments of the workpiece thickness.  The measurements in mm were 8.73, 8.68, 8.69, 8.72, 8.70, 8.72.   Not perfect, but not too bad at all.   I wonder if I might have improved the measurements by surface grinding the magnets.  I wonder if the chuck and its jaws are contributing to the variation.  It was certainly an easy method.

If the workpiece had been thinner I could have increased the thickness of the packing by doubling up the magnets.

For those who are following the Trevithick Dedger Engine build, the bronze was not cheap.  The 12-15mm disk was $AUD90 and the 9mm disk was about $AUD80.  From George White P/L, Melbourne.  I will be nervously trying to not muck up the machining.