Wooden threads re-visited, part 1

[IanW]

.......Do you soak oil into any timbers to cut threads? I did that with English Oak on Sue's spinning wheel .....

Ray, I don't use oil with the primitive tap. I have tested it a few times & although oil does soften the fibres & ease the passage of the cutter, it makes the swarf pack up hard, so if the wood is a tough beast like Blue-gum, it's better to take lighter passes than use oil. You'll still get a nice, clean result. I do use oil (boiled Linseed), with the larger metal taps, it usually helps a lot, but in a few woods it seemed to make little difference. I just brush a good coating into the hole with a toothbrush before starting, and if it's a deep thread, you may need to back out & re-oil any dry patches a couple of times. I don't usually use oil with the smaller sizes pf taps, driving these is easy & while oil would make it a bit easier still, it's usually not worth the extra bother.

I don't have enough experience with the old-style threadboxes to comment (I've only used someone else's, maybe twice), but I've read a few articles saying oiling the blank screw before cutting the thread is a help. It's not a good idea if threading with the router jig, you'll end up with sticky sawdust everywhere (damhik!). For large-size screws, I often put a bit of paste-wax on the shaft & polish it up a little, it makes it easier to turn them smoothly through the jig.

That's my mileage...
Cheers,

[IanW]

I have to recant a little - you can go smaller than 1 inch!

When I was making the 1 inch tap, the first shaft I turned I cut a bit under-sized, and so I tossed it aside & made a new one. I was about to consign the under-sized tap to the fire-starter box this morning when I began to wonder - could I go smaller than 1 inch? Curiosity got the better of me, & I decided to try for a 7/8" screw. I popped it back in the lathe & planed the shank to 3/4". Cutting the mortise & fitting the cutter was more fiddly than doing the slightly larger one, but eventually, I got the cutter installed & the tap was ready to go: a.jpg

Using the tap, I made a threading jig for the router. This part was a bit tricky, because the best way to test whether your nut is tapped properly is to screw in a previously-made screw. Of course I didn't yet have a screw to test the tapped nut, so I had to rely on my eyeball, & stop when the thread looked "right". The jig was then mounted on the router, I turned up a short length of 7/8 dowel and after a couple of adjustments it was cutting a nice thread. b.jpg

Encouraged by this, I turned up a couple of clamp-screw blanks & threaded these, made some jaws and tapped the necessary holes, and voila - another pair of clamps is born: c.jpg

The tapping itself was easy, but I did have trouble with the wedge slipping - even more so than with the 1 inch. I made a new wedge to tap the clamp jaws and it was a bit better, but I need to investigate more & try & figure out just what the problem is.

So, for someone wanting to make a few clamps, but reluctant to get into buying a lot of gear that will only be used a few times, this is a no-cost option. Of the handscrews I've made, the size that I find most useful use 3/4" screws, but 7/8 is only a bit larger, so the set I made would be just as useful, I think. I did make the jaws a bit longer than the 3/4" size (275mm vs 250 for the 3/4 screws), and a bit fatter to accomodate the larger screw holes: d.jpg

But they are still quite manageable, and certainly less bulky than the thumping great 1"-screw size I made when I began making these things. I hardly ever use them, because if I need to clamp something that the 3/4 clamps can't manage, it's usually better to switch to bar clamps.

I think my curiosity is satisfied, I won't try going any smaller - it might be possible to make a tap for 3/4" threads this way, but I don't think I'll try & find out. The tap was more fiddly to make than the 1 inch. It's only 1/8" less in diameter, but it made more difference than I expected: e.jpg

I wouldn't recommend you start your wood-threading career with one of these - make a larger one first & get the hang of that, then go for a smaller size if you are keen to try it..
Cheers,

[tony_A]

Another great thread Ian, thanks for taking the time to write up and post

[IanW]

No-one noticed, or if you did, you were too polite to mention it!

I had a bit of bother with one of the screws for the clamps I made yesterday. I don't know why, but when I started backing the screw out of the threader after cutting the thread, it jammed solidly in the jig and refused to unscrew. I could not see what was causing it to jam, I jiggled & joggled it to get it to move, & in the end I just forced it. After a few turns, it loosened up & unscrewed as it should, but the last 6 turns of thread that were inside the jig at the end were stripped. You can see it in the pic above, but here is a close-up: Damaged thread.jpg

This is the first time that's happened to me. My theory is that a large piece of thread broke off because of a fault in the wood, & jammed it, but I'll probably never know for sure exactly what happened. So this morning I turned up a few more screw blanks to replace the damaged one & make a matching clamp. They all threaded smoothly with no dramas, so now I have a pair: Pr 7_8 clamps.jpg

One other thing I meant to show last night is this: Poor thread.jpg

This is what can happen if you take too large a bite at one time. I was having trouble with the wedge holding the cutter as I mentioned, and on this test nut, ended up advancing it far too much on the second or third pass. It was hard to turn the tap & I could feel the cutter struggling each time it ran into sloping grain - I should've backed out and adjusted the cutter back a bit, but instead just muscled it through. The result is this very furry thread, which is far from ideal. Most woods tap cleanly, it's unusual to get anything like this, even if you force the tap too quickly, but something to watch out for. I did a second test in the same wood, taking more sensible bites, & the thread was fine.

Cheers,

[M Williams]

I know this is an older thread but I had a quick question.

How do you know how deep or tall the threads should be?

For example. 2 tpi. 90 degrees. Do you measure from tip to tip or root to root and that is also the height measurement?

If tip to tip of threads is .25 inches. Is the thread depth also .25 inch?


I ask because I also wonder how you know what the root diameter will be for a new screw when you drill out and make the threadbox.

[IanW]

While root diameters and truncation of threads is important with metal threads, it is not such a precise matter with wooden threads. You could calculate these if you wish (allowing for a bit more truncation & a bit more clearance due to the instability of wood vis a vis steel, for e.g.). Unless you use an expansion bit to drill your pilot holes, you are usually limited to a fixed set of incremental bit sizes,and you are unlikely to be able to match your calculated root diameter precisely - so err on the loose side rather than the tight side.

In practice, it tends to work itself out, you make screws to fit your tapped nut. I always advocate starting with a tap, whether it be a full metal job like this 2" 4tpi beast:
2 inch tap.jpg

or the "ancient" kind. which you can easily make for yourself:
a.jpg

Because our hardwoods are just too darned hard to be reasonable, I long ago gave up on the traditional matching threadbox in favour of a router-driven method. So you make the tap and produce a nut that looks right to you, then this nut becomes the 'driver' for the router threading jig:
Screw threading.jpg

So all future screws will be made from this "standard nut" and all future nuts are tapped to fit the screws.

That may be rather unsatisfactory if you are an engineer who likes to work with precise measurements, but it actually works just fine. I've made an absurd number of wooden screws over the last 40 years and never needed to measure anything other than the major diameter of the screw shafts. The rest falls into place of its own accord.....

Cheers,
Ian

[M Williams]

It makes sense that you don't need to have precise numbers like with metal. But how do you come up with rough numbers?

You decide on a major diameter and a tpi, but then how do you know what size tap to make? Or how big of a guide block hole to drill?

I mean obviously you don't want a, for example, 1 inch sized tap with a cutter in it and try to make a 2.5 inch major diameter nut. But how do you establish your starting point?

2 inch hole for 2.5 OD? Or 1.75 inch hole for 2.5 OD? Again though, this depends on tpi correct? It'll differ some?



Does a 2 inch 2 tpi screw have roughly same thread height as a 2.5 inch 2 tpi screw?




Thanks for replying so fast Ian, just trying to keep myself from drilling guide holes or making a tap way to small or big. I'm excited to try this.

[IanW]

MW, these are very reasonable questions you raise, but I have no straightforward answers! Yes, tpi will affect thread depth and does need to be allowed for when choosing pilot hole diameters.

If I dusted my trigonometry skills off (supposing I can find any after more than 60 years since high-school days!) I should be able to calculate thread depths. All you need do is calculate the height of an isosceles triangle with an apex angle of 60 degrees (which seems to have become the de-facto standard) and a base of whatever fraction of an inch your desired tpi yields. That will give you the thread-depth. You need a fair amount of clearance though (i.e., a somewhat smaller root diameter that the above calculation would give), & I have no exact formulae for that, it depends where you live & your local climate, which a certain company discovered to their cost when their maple screws ended up in countries where the annual moisture cycles can be pretty extreme. So despite the maths, we get back to a bit of guessing & a certain amount of trial & error is involved for your locality & your choice of woods whatever you do.

Fortunately, it's not as difficult as it might seem because there is a fair bit of leeway, the threads don't have to be perfect to work well (in fact perfect would probably NOT work well). As I said, if you start with a tap, that will set the main parameters and everything else follows. If you are using a ready-made tap, a few tests with different sized bits in a piece of scrap will soon sort it out for you and you'll have a workable minor diameter to go on for your threadbox. If making your own "primitive" tap, which I strongly advise if you are contemplating 2-3tpi at major diameters of 2" or more because driving a 2" tap through a couple of inches of hardwood to cut a thread in one pass takes a manly effort and cutting a thread in several passes is far easier. So if going this route, decide on what major diameter you want first, and for anything from 2-2.5" diameter, a pilot about .25" smaller should work. You would need to allow a bit more thread depth for a 3" screw at 2-3tpi, less for a 1" screw, and so on. As long as thread-depth is reasonable, it'll work and adjustments can be made when threading the matching screw if using the router threading method.

If you want to make a traditional thread-box, I'm no help whatever. I have only made one of those & it was a complete flop, due to my inability to get the cutter sorted. I probably could do that easily enough now, but the router method is just so much quicker & more certain with our very hard woods that I have no incentive to re-visit that experience. I was following a set of instructions for my miserable attempt so didn't have to give that aspect any thought, I just drilled the hole to what the diagram said.

By the time I started fooling with the "primitive" tap described above, I knew from experience roughly what size difference works for large diameter screws and knew that if I used a pilot that was a bit less than exact root diameter, or cut the threads a bit shallow in the nut, this can be adjusted for by turning the screw blank a smidgin under size. If you do calculate root diameters geometrically, you'll likely get a figure that is too tight for practical purposes - it may work on the day if your wood is bone-dry, but be far too tight when warm humid weather sets in (my advice for wood threads is "loose but not sloppy"). Even sloppy can work, the screw in my tail vise was the first large screw I made & came out looser than I wanted because I'd set the cutter too deep. I used it anyway, because I'd used up the only suitable dry wood I had at the time & intended to replace it later if it stripped (as I fully expected it would do). But it's still there, doing yeoman service every day, at least 35 years later! It sometimes annoys me a bit because it "rattles" when I'm closing it up quickly in dry weather, but not enough to prompt me to change it out for a better-fitting screw (I've had one on standby for at least 25 years!).

Which is all a long-winded way of saying I can't answer your questions with precision but you have certainly made me think about it & I will give it some more thought over the next little while. I'm sure it would be possible to produce a table for major & minor diameters of different tpis but it would come with a caveat that local experimentation & confirmation is required.....

Cheers,
ian

[M Williams]

Thank you! I think you explained and summed that up pretty well. I will try to think of this in looser terms lol.

If you ever put together a chart of tpi's, minor to major diameters and what not, I think it would be an invaluable starting point for a lot of us crazy enough to go down this rabbit hole of wooden threads.

Do you by chance have a pdf of your article from awhile back by chance? I'd like to read it if I can find it.