This is the first time I have attempted to cut a thread although I have done some simple lathe work in the past on a Super7 some 30yrs ago.
An inset ceramic sink I had ordered came with an overflow device which consists of a moulded plastic elbow, one side of which fits into a corrugated pipe which connects on to the drain, the other branch is internally threaded. The tubular matching male threaded bit is a moulded bit of plastic with a bright finish which locates in a hole on the inside of the sink but unfortunately it's too short to engage the threads in the elbow. I thought originally that the thread was 3/4BSP, the thread is certainly 14TPI, but on measuring the major dia it's 1.084" and the minor dia is 1.008" which differs from the dimensions in my source of info for 3/4BSP which states 1.041" and 0.9496" respectively. I propose to use the measured major and minor diameters.
My initial reaction was to send it back, but on reflection I thought it was a nice little project to start using my recently purchased ML7. I had some white nylon bar which I've turned to the correct dimensions, I've used a parting off tool to create a groove at the end of the potential threaded part to give me time to disengage the clasp nut. I'm using a tail stock centre and the work piece projects approx 3/4" from the chuck jaws, the threaded portion will be approx 1/2"
I've set up the correct gear train to give me 14TPI, and engaged the back gear and slowest speed and tried the to cut a shallow thread in a smaller dia scrap bit of nylon and the thread seems satisfactory.
Reading Ian Bradley's book he suggests setting over the top slide to half the Whitworth 55 deg angle i.e to 27.5deg. I find this process somewhat complicated and wondered if it was really necessary with a soft material like nylon? Could I in fact just use the cross slide and cut directly in at 90deg to the work piece, 5 thou (or less?) at a time. The threading tool is a new and sharp HSS.
Any comments gratefully received.
Regards Don