Two noses are better than one!
Have just made my dummy lathe nose on a blank 2 MT mandrel. It will act as a gauge for the ER Collect nose I will make and also when I make a bit more stuff a handy thing to mount my 3 jaw elsewhere.
Used this a a test of the offset top slide screw cutting technique.
So here goes:
After a lot of help from the group I think I have nailed this! I am using exactly half the angle – I know some people like 25 degrees for Whitworth and less than 30 for Metric/American
1. Prepare work as normal for thread cutting.
2. Set over top slide to 27.5 degrees for Whitworth. Set tool tip square to work.
3. Infeed the top slide a bit and set to zero.
4. Using Cross slide wind in tool until it just touches the work and set to zero
5. Move carriage to right to clear work.
6. Advance Cross slide to the root diameter of thread.
7. Set Cross slide back to zero
8. Wind back the top slide to clear the work and move carriage left to where the cut will start.
9. Apply marking blue to threaded area.
10. Advance Top slide to touch the work. Move right and set first cut with top slide - say 2 thou.
11. Check: Cross slide on Zero, Top slide is whatever (Zero minus approx depth of thread root minus cut ), Carriage moving to the left (for normal thread)
12. Start cutting thread.
13. At end of cut disengage half-nuts and wind back cross slide.
14. Check the blue that its correct pitch
15. If all OK, move carriage to the right, Set cross slide to zero, and put on a bit more cut (say 5 thou). As you go deeper use smaller cuts and towards end workout the spring of the tool.
16. Repeat step 15 until within say 10% of the depth
17. Start to check the thread, apply fine cuts as required. Note some people like to cut the last 1 or 2% on the crossslide.
Before trying this, set up a piece of bar and a dial gauge on the top slide. So Step 4 is wind in cross slide and put say 5 thou on the clock. Zero the clock and cross slide, Then proceed as normal. When topslide reads zero, the clock should show depth of cut.
Attached 2 picture of the finished piece! Its a nice smooth fit and runout with the 3 jaw fitted is less than 1 thou! Not bad for a 2 mt mandrel