Wednesday, April 20, 2011

Tap tap and die die

All right, time to get back to cutting threads in wood. My chasing efforts are at a standstill; I need to get back to them but today, I took the snow shovel and managed to open the shop door enough to get in. I retrieved my tap-and-die box set that I bought some time ago from Garret Wade and had a go at cutting threads via commercial means. I must say, if you have but occasional threads to cut and some spare change, this is the way to go. It is not cheap. I had a look at Garret Wade yesterday; they are actually cheaper than they were a few years back; a set (tap and die) will set you back about $50 US. The kit comes in two parts, a tap and a die box. Let's look at the die box first; this is used to cut the bolt -- the male thread. I happen to have the 3/4" set, 19mm in reasonable units. The die box is a large piece of hardwood. It comes apart by loosening some screws, and we behold its innards:
So this guy consists of a threaded insert -- Aluminum by the looks of it -- and a cutter. The cutter is a square section bar about 5mm thick, filed (or more likely, milled) into a 60 deg triangular section. A V-gouge, in fact. It is sharp (although like all commercial gadgets it can use some attention) and it overhangs the threaded insert by an adjustable amount. It is held in by a brass fixture which can be loosened to allow adjustment. In the foreground a commercial 3/4" dowel with the first attempt to cut threads on it. It worked, sort of, but it wasn't very neat. It required inhuman amounts of force to get the cutter to cut. Oh, yes, there is another piece to this thing; a wooden block with a hole in it. It is used to guide the dowel for the first cut. Good idea. But once I have some thread cut, like two turns, I prefer to take the block off so I can see how the cutter is doing. The adjustment of the cutter is crucial. Took me a morning to learn, but eventually I got the hang of it:
Really it's just like any other tool. If it cuts a long, uninterrupted shaving, it is properly adjusted. If it's too far back, it won't cut at all. If it's too far forward, it will jam. So you adjust and re-adjust. Extra factors are the exact diameter of the wood , the wood itself, and no doubt the aspect of the Zodiac. No wooden cylinder is an exact cylinder. Metal, maybe. Above a lilac branch. I turned it to about 18mm ( a tad undersize) and it worked much better than at the nominal 19mm. But we have a screw, Houston. How about a nut, Houston?

Well, to make a nut we drill a hole and run the tap through it. But how big a hole? Can't drill it nominal (19mm) -- tap would just slide through. Enter the tapping equation:

Tap hole diameter = Nominal Diameter - Pitch.

Needless to say, the only information with the kit is the "Made in Taiwan" label. Boo to Garret Wade, who usually do better than this. A little work with calipers says the pitch is about 2mm so 19mm - 2mm = 17mm which is as close as makes no nevermind to 3/8". And I do have a 3/8" Forstner bit, so to the drill press we went. And then we ran the tap in.
As with any kind of tapping operation, the hard part is to get the tap square to the nut. Once you do about two turns, you're in (literally). Another view shows the tap handle and the whole setup:
The nut is held in a vise clamped to the table. When you are turning even a measly 19mm nut, the amount of torque required (note tommy bar on tap) is considerable. I actually split several nuts before coming up with the above setup.

And grand finale (music, Maestro, please): does it fit? Well...
... yes, it does. I admit to having done this before. The bigger the diameter the harder it is. So before you build your Olive Oil (or Grape) press, perhaps you should do a 12mm jobbie first, and work your way up. But it was a pleasant way to spend a breakup day. Too cold to go out, too muddy to walk! Cut threads instead.

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