Thread Cutting

Anyone who has ever attempted to make a properly fitting box lid will tell you that in general it is not easy. I wanted to make face turned boxes with a secure lids that work over a reasonable range of humidity and temperature without jamming or slopping so badly that they appear to be poorly made. It is difficult enough to get this right using very stable timber on small end grain boxes (I have made many of them), let alone a 7" face turned, Iroko, lidded box, which I plan to do as a test.

My theory is that using a large pitch thread, in this case 8 TPI, it might be possible to make lids that work properly on projects that would not normally be considered suitable, using timber that I would not expect to be suitable either.

Threads are generally cut in wood using thread chasing tools; there is a knack to be acquired and those who do so make some very nice work. However, threads cut in this way are usually fine pitched, typically 18 or 20 TPI, and work successfully only on the limited range of timber that will take fine detail without crumbling. This is generally the sort of material one might use for ornamental turning, by that I mean dense, fine grained, limited in availability and mostly expensive. Materials like Cocobolo, Lignum Vitae, Mopane, African Blackwood and Box would all come under that heading. And with such a fine pitch it is probably only going to be suitable for smallish projects. I’m thinking threaded parts of say 3 inches diameter at most.

I was looking for a way of cutting a coarser pitch thread, say 8 TPI so that it could be used on larger projects, larger grained timber and with a correspondingly greater thread depth to accommodate more variation. I chose the ISO thread from Fig 1 as it is easier to cut than forms with a rounded top and bottom, and 8 TPI, which is the lead-screw pitch on my engineering lathe. This is not all that scientific, but it also looks as though it might be about right.

Fig 1

I could not imagine attempting to chase threads of this size, even if I could make the tool to do it. However, I don’t doubt that somebody somewhere does it beautifully. Anyway, I decided to design something based on a single point (about 60 degrees) rotating tool mounted in the lathe with the work mounted on a hand driven lead screw secured in the vice of my compound cross-slide. This ‘v’ shaped tool had side clearance (about 10 degrees) but no top rake and the ‘v’ point was slightly rounded by barely touching it to the grindstone.

Fig 2 is a picture of the tooling I made to do the job.

Fig 2

This cutting tool, made from ³⁄₁₆” square section HSS, is held in a 6½” long holder made from 1” diameter steel rod on which I turned a No. 2 Morse taper to suit my lathe, and a draw bar to secure it. In operation there were no balance problems running at around 1500rpm.

The lead screw has a 1⅛” x 12TPI threaded adapter to suit my chucks and faceplates, modified to screw into the end of 6” x 0.975” diameter shaft threaded as Fig 1, 8 TPI. This was screwed into a threaded 1.375” outside diameter collar with a flat milled to aid location in the compound cross-slide vice. The overall run-out as measured at the chuck body was about 10 thou, certainly good enough for the job.

Figures 3, 4 and 5 show the tooling in operation.

Fig 3 Fig 4

Fig 5

I wanted the try-out to be a challenge for the process so I chose to make a 7” diameter face-turned Iroko box (Fig 6). Normally the three threads on the box would be more than enough (two would probably do it) and the five on the lid makes securing it tedious, but I cut them to prove the process.

Fig 6, 7” Iroko box face turned box with 8 TPI threaded lid.

Fig 7 and Fig 8 are close-ups of the thread formed on the outside of the box and the inside of the lid respectively. Both are of end grain showing that there is no crumbling and the tool has taken quite a clean cut.

Fig 7 Box Fig 8 Lid

5 September 2021

I made this tool in January 2014 and this is the first and last time I used it, but as I recall it was an interesting project, and you never know.