Single point tool to cut an internal 5/8-10 LH ACME thread

+1 for Jasons suggestion of moving the topslide to trim a bit more out of the thread before applying the tap.

If you have access to a decent CAD program its well worth spending some time drawing things out to help your visualisation of what's going on. Something thats saved my bacon a time or two before embarking on a tricky "can't see inside" job by demonstrating I'd not actually thought it through. Even if I have thought it through and visualised things correctly confirmation is good for confidence.

I'd draw the ACME, overlay the 60° thread and shift it around to get the envelope it will cut after moving the topslide. Then draw the tap and shift it underneath that envelope so you can see what will be left for it to cut. Important to leave enough material for it to bite on. (I have some large Whitworth first cut taps that are basically reamers because the taper is so shallow that its almost impossible to make them bite cleanly and follow the thread.)

Hefty coarse thread taps are notorious for pulling out of line when they start to cut due to the unbalanced load so put some serious thought into how it can be held in line. If it weren't stupidly wasteful of material I'd make the nut blank long enough to have a plain section in front bored to the major diameter of the thread and long enough to support against the full depth section of the tap.

Get some delrin and practice. Given confidence, a robust technique and stable set-up this is a straightforward job. Try to wing it and make things up as you go and it will turn round and bite you.

Clive

Free-hand grind a piece of 1/8" or 3/6" HSS. Take a piece of 1/2" silver steel. Mill or saw & file a slot in the end to fit the tool bit so the top of the bit is on centerline. Silver braze the bit into the slot. Use a cutoff wheel to remove any of the bit sticking out the back of the shank. Contour the back of the shank so there are no projections.

It is easier to grind a long bit than a short one. Keep the bit full length until it is ground to the right contour.

You will probably have to wind the tool back to the beginning as there may not be enough clearance to simply withdraw the tool. It depends on the amount of bit stickout.

You can simplify tool grinding if you angle the the trailing edge of the bit to the flank angle. Then only the leading edge of the bit need be ground. Of course that requires the slot in the silver steel be at the same angle.