Modifying Bolts

This was prompted by a task I have just completed, and not very long after someone asked for advice on holding bolts in the lathe for turning their shanks.

I give the recipe I found effective.

'

Needed: three pivot shoulder-screws thus:

... 2-off 10mm dia X 6.5mm long + M8 thread.

... 1-off 8mm dia X 25mm + 6mm dia X 13mm + M6 threaded tail. (i.e. two plain diameters then the thread).

'

Ingredients: Suitable bolts (not machine-screws) of shank to the required bearing diameter. These were not super-critical for the application, and at least one bolt was second-hand.

.

Method:

1) Hold the bolt by its shank, with the hexagon head nestled behind the 3-jaw chuck jaws.

2) Centre-drill the tips - I completed these two steps on all three bolts in one sequence.

3) Hold the bolt by its head and a half-centre.

4) Face the underside of the head to form the end bearing surface, and to give the measurement datum.

5) Machine the diameters, leaving the shank generously over--length. Turn the excess length down to give the die something of a lead, and the intended thread part down to a little under nominal diameter.

6) Die-cut the new thread.

7) Trim to length. I used a hacksaw and file, as the end lengths and finishes were not critical. If they had been, I would have repeated Step 1) and faced the end.

'

NB: When turning the shank, unless the bolt head is backed by a mandrel stop (mine weren't), use only light cuts and gentle feed to avoid pushing the bolt back off the tailstock centre.

A larger version of John Ashton's Screw Modification fixture, (He got the John Stevenson memorial Trophy ) to take a M10 bolt would allow the job to be done without the need to centre drill, and the risk of the work moving whilst cutting the thread.

Howard