Screwcutting Indicator DialI've had this lathe since 1985 and the only screwcutting that I've done on it had threadcounts that were a multiple of four. That is because it doesn't have a threading indicator dial and I've never really needed one before, having access to other machines. I now need to cut some 6 TPI threads and need an indicator to fit my 4 TPI leadscrew but, as the lathe is eighty years old, finding one would have been difficult, to say he least. They are not hard to make, though, so I knocked one up over the weekend.
The first thing I had to make was the gear that engages the leadscrew and, on the basis that one makes a gear with four times the number of teeth as the leadscrew pitch, that was a 16 TPI gear with 1/8" wide teeth cut on a blank a little over 1.1/4" diameter. However, 1.1/4" will do, this doesn't need to be particularly precise. A bronze blank was machined up with 5/16" wide for the teeth and a 5/8" diameter x 5/16" wide boss for fixing onto a 3/8" diameter shaft. Because I don't have a dividing head, I checked my lathe changewheels for one that was a multiple of 16 to make up an indexer and found this 32 tooth wheel
With the gear blank made, I had to knock up a mandrel to hold both the blank and the changewheel so that I could index the thing round. I managed to cobble together a somewhat Heath-Robinson setup with a vee-block held in the vice and a detent mechanism bolted onto an old milling fixture. It looks very ramshackle but it's suprisingly sturdy and accurate. The cutting tool is a broken centre-drill which I've ground to form a 1/8" wide cutter, which is clamped into an offcut of 5/8" diameter bar and held in a collet.
I decided to try and combine my saddle/carriage stop and this thread indicator into a single unit so had to modify the carriage stop by milling away a section at the top. A 3/8" diameter hole was drilled and reamed through and the unit refitted to the lathe using the single bolt. The gear was fitted to a length of 3/8" diameter bar and offered up to the leadscrew to check both size and fit, as well as orientation of the stop block. The rotating barrel with the five adjustable stops has been temporarily removed.
Next up was the indicator dial which was made from a piece of 1.1/8" diameter brass with an angled face and parted off about 5/16" wide. The hole through was drilled and reamed 3/8" diameter, and a cross-hole drilled and tapped to take an M4 grubscrew. It was then clamped to a short length of bar, held in a square collet block and loaded to the tilt-and-turn vice to mill four indicator grooves. I don't have any engraving tools so a 1/4" centre-drill was ground to a fine point instead, the four lines being "engraved" by indexing the block in the vice.
The stop block was reassembled, a collar made for the gear spindle to stop it climbing up the block and the indicator stamped with four numbers. Although it is barely visible, I also used an end mill to create a small pocket at the top of the block to take an "O" ring. The dial rests on this and keeps the crud out plus adds a little bit of friction to the spindle.
Finally, the whole thing was fixed back onto the saddle and adjusted for fit. When not in use, The stop block can be slightly rotated to disengage the gear but without making the stop go out of line. There are sixteen positions where I can engage the screwcutting lever but I will use just the four marked positions, which will enure that all whole-number threads can be cut without creating the dreaded two-start thread.
Another worthwhile additon to the lathe and it cost precisely nothing because all materials were sourced from the scrap-box. Quite a satisfatory week-end.
Regards, Steve
