When I was sixteen my father and I bought a Drummond “B” lathe from the swap shop in Croydon. I am now sixty seven and still using the lathe after it came to me upon my father’s death some years ago. Over the years the lathe has had a bed regrind and various modifications including, electronic three phase speed control, a new lead screw and an extended cross slide.
In amongst all the bits and pieces was an aluminium tube about an inch long with markings around it. (See fig 1) Over the years I have considered throwing it in the scrap box as I could never find what it was for.
Recently I attended a course at the BHI to learn how to make fly cutters for wheel and pinion cutting. At the course we learnt to make a form cutter and then how to use this to make the fly cutter. This involved using the form tool on the secondary slide set at an angle to cut the blank.
The secondary slide for the Drummond is normally in the cupboard as I do not use it very often. As it was stiff in operation I stripped it and cleaned the thread adjusted out the back lash and considered what set up was required for cutting the blank. During the strip down a plate fell off with a notch on it, I examined it and reconsidered the o ring on the winding shaft. (See fig2)
Suddenly the light came on and I realised what the tube was for, found it in the draw, sliding it over the shaft and the o ring. My father had made and fitted an imperial vernier for the lead screw.
Having decided that the pitch of the lead screw was ten to the inch how was I going to measure the feed in millimetres?
I turned up a length of brass to fit the shaft. A tenth of an inch is 2.54mm so using my electronic dividing machine I scribed lines at 7.0866141730 this being 0.05 of a mm. The odd 0.4mm was painted black, so one turn bar the black bit is 2.5mm. (See Fig 3)
Rotate the curser round to lose the black bit and I can use the imperial; lead screw to measure mm. With this device I can measure the feed down to 0.05 of a millimetre. (See Fig 4)