Threading Wood

[IanW]

I was asked for details of my threading jig in another thread, so I'm posting it here. Because I can't attach more than one image to a message, I'll have to do this in several posts, so here goes: Section 1
All threading starts with a tap – once you have that, you can soon make a jig for your router which will thread any wood you can feed to it. There are quite a few possibilities. Metal taps would certainly do the job, but wood threads work better with fewer threads per inch than metal, for a given screw diameter. Taps specifically for wood aren’t easy to find, unfortunately. If you are a dab hand on a metal lathe, you could make one for yourself – mild steel is perfectly ok for threading wood. About 6 threads per inch is good up to one inch diameter screws, and 4TPI is better for larger diameters. Here are a couple of other solutions:
1. I saw this in an English magazine about 12 yrs ago, and had to try it. Pull the guts out of an old brass water tap – the style that didn’t use O-rings to seal the shaft. Cut a slot down the centre of the threaded part at the end, then chuck it in your drill-press or wood lathe. Spin it slowly towards you, and use a file to taper the end for starting. Because the threaded part is so short, you can’t allow much lead-in, unfortunately. Relieve the threads a bit on the side of the slot opposite the cutting edge, and you have a tap which will thread wood quite well. The only problem is that the thread is a little fine, but the clamps I made with my tap from a tap are still in regular use.
2. I wanted 2” and a 1 ½” taps, so made them by turning up a wooden blank, then chasing a suitable thread on them. After tapering off the thread, I drove in a bunch of 12G and 14G screws in a spiral pattern, which were clipped off and filed into cutters to match the profile of the thread behind them. It actually works, but it’s hard to make enough room in front of each cutter to allow chips to clear. You must back out after each half to ¾ turn to clear it. Slow going, but it does a good job in woods up to medium hardness, such as Mountain Ash.
Both taps are shown in the picture, along with two steel taps; a home-made 1 inch, and a factory made ¾ inch.

[IanW]

Section 2.
Once the tap part is solved, you need some way of making screws. The easiest way is to use a router with a simple jig as shown. Two pieces of wood are squared and screwed together. One is then drilled to the major screw diameter, the other to the minor diameter, and tapped. Cut a generous access hole in the bottom for a 60 deg. (or whatever matches the thread profile of your tap) veining bit and plane the tapped piece down until you can position the bit just to the right of the beginning of the first thread.
The jig is shown opened, with the veining bit taken out of the router, and placed in the approximate position to cut thread.

[IanW]

Section 3.
Clamp the tapped piece to your router base, and adjust the height and position of the bit, so that as it cuts a groove in your blank, this feeds onto the thread, and pulls the screw in at the correct pitch as the router cuts more thread. Now screw the piece with the guide hole back in position, and you’re ready to go. It takes a bit of practice to feel the bit bite as you feed in the blank, and start the thread cleanly – you may need to back out and trim off the daggy bit of thread, but once you have it feeding cleanly, you should be able to screw it in easily with the tips of your fingers as the thread is cut – if it binds, either your blank is too big, or you have the cutter a bit off – adjust accordingly.

[silentC]

Excellent stuff, Ian. Keep it coming!

I especially like the router jig. If you had something threaded at the right pitch to begin with, you could make the tap this way and then go from there. I suppose there must be old things around somewhere that have a decent 1" to 2" thread that could be used. I think that would be a better result for me than trying to chase the thread on the tap by hand.

[journeyman Mick]

Great stuff Ian, one of these days when I have the time (yeah, when?) I'll give this a go.

Mick

[bitingmidge]

Ian,

Great stuff...saved me asking the question!

From your experience would it be possible to get a satisfactory result by filing notches and tapering steel bolts and nuts of these sorts of dimensions?

I have often tapped smaller diameters in timber and MDF using this method, but figured the thread would be too fine at the larger scale.

I guess for the cost of a 35mm nut and bolt it won't cost much to find out!

Thanks for the info,

Regards,

P

[IanW]

Midge - yes, using a bolt or coach screw is another alternative tap solution. As you say, it's a question of getting a coarse enough thread. I've tried coach screws - right pitch, but the sharp, fine lands aren't so ideal. I think by the time you get into 2" in Whitworth, it gets down to something like 4 or 5 TPI. I don't get as mystical about it as some, but a wooden screwed tail vise is fun to make and works so well, you'll be pleased you made the effort. When well-made and waxed, wood screws run very smoothly and satisfyingly.
If you are wondering about strength and durability - I've been using one vise for close to 20 yrs, and it shows no sign of wear. I have stripped a couple of badly-made clamp jaws, and had the thread on a few screws crumble because the material was faulty or simplly unsuitable, but we have dozens of woods here that make excellent threading material.
Later tonight or early tomorrow I'll post a couple more bits on clamp-making - no tiime just now...
avagooday

[IanW]

OK - I managed to find a couple of minutes to fiddle with these, so here are the short primers on clamp-making as promised.
1. Handscrews.
Making wooden clamps is pretty straightforward. Some of the woods I’ve tried successfully for screws are: Casaurinas, Acacias, River Red gum, Red Stringy, Bush Cherry (Exocarpus), Red Penda, and others I can’t think of right now. Some exotics like Apple and Desert Ash turn and thread very well if you can get your hands on them. I turn the blank in one piece, and make the shaft a loose fit. In general, wood threads need to be a bit sloppy compared with metal, to cope with seasonal movements.
Scraps of just about anything make jaws, but softer woods will be less likely to mar work. Even Radiata pine will do, and certainly meets the criterion of being soft. None of the dimensions is critical, but each has an effect on the function. I’ve settled on three ‘standard’ sizes. The most useful size for the sort of work I do most uses ¾ inch screws with about 300 mm of thread. Jaws around 200-250 mm long and 50 mm deep give you a maximum clamping distance of around 200mm. Clamps this size can apply a surprising amount of pressure, more than enough for any adequately-made joint, but if you want more squeeze, just apply more clamps.
The 1” diameter screws are used for clamps with 300mm long jaws, and a total clamping capacity of a little over 300mm, while my little ½” jobs have jaws about 125-150mm long and open to hold about 100mm.
These clamping capacities are also pretty much the practical limit of the material as the screws are easy to break when working at maximum capacity. Well-made threads do not strip easily, but you will bend and snap the bottom screw if you tighten jaws much off parallel, but with a modicum of care and attention, you soon get used to them. About the only time I have busted clamps in the last 12 yrs is when the dog knocked over a piece of work bristling with clamps and snapped of several of the exposed ‘pusher’ screws.
Make jaws as wide as you can, not only for strength but to minimise bruising of work pieces. One jaw has both holes tapped, the other has a shallow hole at he top (inside) for the ‘pusher’ screw, and the centre hole is the full diameter of the screw.
When assembled, handscrews are opened and closed quickly by holding both handles firmly, flipping the whole thing towards or away from you, then ‘pumping’ as if you were working bicycle pedals to keep unscrewing or closing. (It’s easier to do than explain!)

[IanW]

2. Bar clamps
Sash (bar) clamps are a very useful item, but are a bit more fiddly to make than handscrews. If you want the travelling jaw to open when you unscrew it, you need some way to capture the end of the screw in the jaw. This is most easily done with a couple of tongues which mate with a groove turned in the end of the screw, held in place by the side covers. I’ve shown this with the side cover off in the second pic, and a screw end from another clamp above. The side covers also have guides which ride in grooves in the bar to keep the jaw aligned as you tighten it. Glue the guides on in-place, then take the sides of and put a few counter-sunk screws in from the inside to reinforce them.
A couple of strategically-placed dowels reinforces the headstock (next pic).
The loose side arrangement for the rear jaw keeps the face vertical to the bar as it is tightened (last pic). The threaded pin is a bit elaborate – a simple dowel would suffice.
The clamp shown has a bar of Bull Oak – a bit of overkill – any good (i.e. not too splintery!) hardwood will do fine, sized accordingly to the length. I’ve only made them up to about 600mm opening, using 1 inch screws. I’m ‘getting around’ to making some whoppers with 1 ½” screws and maybe 1800mm capacity, someday.

[AlexS]

Ian,
Thanks for that excellent information...make me want to give it a go.

[echnidna]

Great tools Ian.

What did you finish them with?

[IanW]

Echidna,
Oil 'n' wax.
The wax is the essential bit - stops glue from sticking to bits you wouldn't want it to. I give them a yearly waxing (every three years or so!), paying most attention to the threads, which I do with a toothbrush. This keeps them running nicely, and prevents any accidental glue drops from doing damage. As I said somewhere above, a drop of glue in raw threads will produce a pretty powerful joint!. All my clamps are overdue for some TLC, I'm in the middle of a move, which requires building a new w'shop. As always, it is taking much longer than anticipated.......
Cheers,

[bitingmidge]

Here is a link to the only page I have been able to find on the Web with any useful non-commercial information:Cutting Wooden Threads

The whole site has a number of areas which may be of interest, go to

ilovewood.com and check out some of the links...the above is found in the Alburnam's Archives section.

Cheers,

P

[gatiep]

Has anybody here tried the Hamlet or Sorby thread chasing tools for woodturning?

Thanks
Cya
Joe

[bitingmidge]

Has anybody here tried the Hamlet or Sorby thread chasing tools for woodturning?

I guess that's a no!

Hey IanW (or anyone else that's knows ).....what's the best method of making dowels for threading?

I have found a few options, involving setting up large scale pencil sharpeners, and belting timber through ever decreasing holes drilled in steel, but was wondering if turning was an option (Now that I have a lathe!!).

Is it feasible to build a router jig to make this quick and easy, or is there some other way?

I am talking 3/4" to 1 1/2" threads, with the bigger ones at about 600 long, the smaller probably 450 Max.

Thanks,

P