Wooden threads re-visited, part 1

[IanW]

Recently, I was tapping a couple of 1½” nuts in some moderately hard wood, using the metal tap a friend & I made some years ago, and found I was struggling to turn the thing once it was engaged & cutting a full thread. I gave up trying to use my monster 2 inch tap on my own, several years ago! Despite the metre-long bar I use, it defeats me once the tap is fully engaged.

There is no such problem with the “primitive” tap I described in my AWR article (issue #92) a few years back. These use a single cutter which cuts the thread in successive small steps, each requiring only a small effort. I decided the solution to my declining muscle-power might be to make a couple of these to match the large metal taps. The question was, could I make the primitive style accurately enough to be interchangeable with my metal tap?
To explain a little, these taps are just a simple jig for driving a cutter spirally through the hole to be tapped. The cutter (which is actually a scraper) is wedged in a mortise in the leading end of the tap, and is advanced by tapping it through a little more after each pass. To drive the cutter at the desired pitch, a thin plate engages a spiral kerf cut in the shaft of the tap. As the shaft is turned, it is forced forward or back at the same pitch as the kerf in the shaft. The cutter is advanced in steps of about 0.5mm, which means it takes 6 or 7 passes to cut the full thread for a 2 inch screw. To make a tap that matches the metal version, all I have to do is set out & cut the driver kerf with sufficient accuracy & consistency. 1.jpg

The depth of the cut for each pass is limited to some extent by the toughness of the wood, but probably moreso because there’s no really efficient way to clear the swarf. About the best you can do is to scoop out a generous relief around the cutter tip. Note the little pile of accumulated sawdust in front of the cutter as it exits the hole: 2.jpg

Despite the swarf clearance limitations, it’s not a major issue if you take small bites with each pass. That also gives a cleaner thread with little breakout at the exit side. The toughest woods I’ve tapped required no more than 8 passes to cut the full thread depth, and since most of us would only ever tap a couple of holes at one time, the extra few minutes it takes to proceed carefully is neither here nor there.
Nothing ventured, nothing won, so I made a 1 ½” Tap, taking great care with my measuring & setting out, & a little to my surprise, the first nut I tapped with it fitted perfectly on a screw made with the old system. Encouraged by that success, I made a two-inch version, to complete the set. 3.jpg

Both taps cut threads that fit screws made for the metal taps. These two bench screws were cut on threading jigs made using the metal taps. The nuts were made with my “primitive” taps & screwed on easily, with no slop or binding. 4.jpg

All good!

I was in a bit of a hurry making those large taps because I wanted to use one of them immediately, & also, because I wasn’t 100% sure they were going to work as well as they did, I didn’t bother to take any WIP shots.
However, I decided that since I was screwing around, so to speak, I would make at least one more, and attempt to answer a question I’ve had since the first time I saw one of these taps. What I wanted to know is, how small a diameter can be threaded using one of these gadgets? Large diameter screws are no problem, you have lots of wood to put the mortise for the cutter in, but things get pretty tight as the diameter of that wooden shaft decreases.
Now most sensible folks will never need more than a couple of large bench screws in an average lifetime (the first one I made is still going strong in my tail vise 35 years later), but how about an unlimited supply of clamps? To make these we need much smaller diameter screws, something like 25 or 20mm at 6tpi. The shaft for the tap needs to be smaller still (the root diameter of the thread) - is this feasible?

There are two potential limitations. One is the mortise through the shaft for the cutter. There are practical limitations on how small you can make the cutter & wedge, which means the hole for them will be proportionately larger in the smaller diameter. There is less wood supporting the cutter, & it becomes even less when you scoop out the depression in front of the cutting tip for shavings. At some point, there won’t be enough to take the forces involved. A second limitation could be the ‘driver’ kerf. As the diameter of the shaft decreases, the arc of wood engaged by the guide plate decreases, & at some point, the pressure on that smaller area will exceed its tensile strength.
Based a little on experience, but mostly on guesswork, I reckoned that a tap for a one inch thread would be pretty much the limit, so I decided to give that a try. This time I took a few pics along the way.

Part 2 to follow:

[IanW]

Essentials of a tap:

The shank of the tap should fit neatly in the tapping hole to keep the cutter firmly & evenly applied as it is rotated through. It should therefore be the same diameter as the root diameter of the thread you wish to make, which is the nominal diameter of the screw less 2x the depth of the thread itself. Here, we have to make some compromises because drill bits are only available in a limited number of sizes. For a one inch screw, a root diameter of 3/4” is too small, while 7/8” is a bit large. However, wood threads should be heavily truncated, so 7/8” will work ok, and is the closer fit in this instance.

I made a tap blank of 7/8” diameter from some Pecan I happen to have. This is a tough wood (a close cousin of Hickory), but turns nicely and takes threads quite well. It’s also much loved by some wood-munching insect that chews up both sapwood and heartwood with equal eagerness, so needs careful storage while drying!
To set out the ‘driver’ kerf on the larger taps, I measured the diameter of the turned shaft with calipers, calculated the circumference, and set out my kerf lines on a piece of printer paper as wide as the circumference. This was wrapped tightly around the shaft, forming a continuous spiral at the pitch of the thread. But the smaller the diameter, the more effect any measuring error will have, so I used a “no maths” approach for this one. I wrapped a piece of printer paper around the shaft, stuck it down firmly with sticky-tape, & carefully cut a straight line along the shaft. That gave me a piece of the exact circumference of the shaft.

The tap I want to match is 6tpi, & I don’t have any ruler marked in twelfths of an inch, so instead I found a manky old 1 inch screw that had chipped badly when it was being threaded, bandsawed it down the middle, & used that to mark off the intervals: 5.jpg

When setting out the kerf lines, be careful to draw them so they slope from a mark on right side to the next mark down on the left side– if you draw them the other way, you’ll set out a left-hand spiral like I did the first time I made one of these taps!

The paper template was wrapped tightly back around the shaft and secured with sticky-tape, being careful to ensure all the lines met nicely & formed a continuous spiral. I then put the tap back in the lathe and ran a knife along the line, working from right to left as I slowly rotated the tap towards me with my left hand:
I then used a fine saw to cut a shallow kerf along the knife-line. The pic shows the uncut lines at left, the knife cut in the centre and the shallow saw-cut at right: 6.jpg

To deepen the kerf left by the fine saw, I used a small saw with “reversed” teeth, which cut a slightly wider kerf, the same thickness, as the scrap of saw plate I intend using for the guide. I held the saw in the kerf, starting at the left, & ran the lathe backwards at its slowest speed. After a couple of light passes, I had a kerf about 2mm deep. I try to get it as even as possible, but it doesn’t have to be perfect, the guide plate will smooth things out a bit when you run it through the jig.

To make the jig which drives the tap, you can use any old scrap of wood that’s big enough. It doesn’t have to be especially hard wood unless you are planning on tapping hundreds of holes. I’m using a scrap of Jacaranda in this instance (I used Radiata pine successfully for one jig). Bore a hole through the centre, of the same diameter as the tap shaft (the root diameter of your thread), then counter-bore a larger diameter hole on the opposite side, about 15mm deep. This is to allow the cutter to be backed out of the nut you are threading back into the jig : 7.jpg

The next task is to fit the guide plate. This has to sit in the jig at an angle which matches the helix angle of the thread, and the no-maths way to set this out is to put the tap in the jig, place the driver- plate in the kerf on the entry side of the jig. The plate will sit at the correct angle in the kerf so all that’s necessary is to snug a ruler up to the plate and draw a line across the top of the jig. 8.jpg

Drill and place two screws through the front so the two parts can be re-attached accurately, remove the screws, and saw along the line marked on top. If your cut is clean & straight, there is no need to do anything other than cut a very shallow rebate on one side to sit the guide plate in. File a small arc in the guide plate so it can sit well down in the kerf of the tap: 9.jpg

Screw the two parts of the jig back together and check that the plate engages the kerf firmly & drives the shaft back & forth without backlash. A bit of paste-wax brushed into the kerf & on all mating parts will help it to run smoothly. In this case, all seems to be well: 10.jpg

Part 3 to follow:

[IanW]

Now for the fiddly bit.
The next job is to chop out the mortise for the cutter & wedge. This needs to be done with care so that the front face of the cutter aligns with the diameter (or fairly close to that). The cutter should fit snugly & sit flat against the “bed’’ side, so it can’t move when being driven through the nut. I didn’t have too much trouble with the larger sizes, but I had a bit of a fiddle with this one, particularly getting the wedge angle right. I put a slight taper on the cutter, to prevent it slipping back under the pressure of cutting. Eventually, I had them fitting nicely (or so I thought): 11.jpg

I drilled a ½” hole through the boss & fitted a short tommy-bar (~125mm). Keeping this bar short is a safety measure so I can’t put too much force on the cutter. If it is too hard to drive with the short bar, it’s taking too big a bite & needs to be backed off a little.

All clamped in position & ready for a trial run: 12.jpg

This shot illustrates the one drawback of these taps – it gets very awkward when the piece you are tapping is small, the clamps can get in your way!
The first bit of wood I grabbed to tap was a heavily spalted piece of Jacaranda – although it did take a thread which screwed onto a previously made screw: 13.jpg

The thread was a crumbly mess. 14.jpg

I tried again with a piece of Blue-gum which is one of the toughest woods around, and got a far better result: 16.jpg 15.jpg

At this point I reckoned I had it nailed, so I went for broke and tapped a couple of clamp heads. Apart from being awkward to hold, the Blackwood tapped cleanly and I soon had my clamp-heads working nicely: 17.jpg

I did strike a bit of bother, the cutter slipped back a bit during a couple of passes. I think it was because the face of my tiny mortise is a little rough, so before I use it again in earnest, I need to investigate & get things fitting better.

So there you have it – it’s quite feasible to make a 1 inch wooden tap that cuts a very clean thread, and all from scrap bits that cost me nothing.

And one last tip: I cut a kerf in a piece of wood & tapped it on to the top of the guide-plate, which makes it far more comfortable when pressing it down as you rotate the tap…. 18.jpg

Cheers,

[orraloon]

So well explained Ian. Really left me no questions to ask.
Regards
John

[woodPixel]

I have an idea to clear the swarf.

It is essentially a hole drilled in the end of the top, at a diagonal, up towards the cutter - exiting in front of it. A vacuum with a sufficiently small pointy sucker attachment is used to suck it out. Apply it periodically.... perhaps a second hole is drilled exiting in front of the first to allow ingress of air (ingress --> egress --> cutter face)

[IanW]

So well explained Ian. Really left me no questions to ask....

Thanks John, you'll probably find a few when you actually do it, but it is a pretty simple principle, so I'm sure you'd work them out...
Cheers,

[IanW]

...... a hole drilled in the end of the top, at a diagonal, up towards the cutter - exiting in front of it. A vacuum with a sufficiently small pointy sucker attachment is used to suck it out. Apply it periodically.... perhaps a second hole is drilled exiting in front of the first to allow ingress of air (ingress --> egress --> cutter face)

I think you could do something like that with the 1 1/2 inch & larger taps, WP., but drilling a hole big enough to suck swarf through would probably weaken the smaller tap too much. Perhaps I'm just being lazy, but I don't see a need to add any complications to this very basic & simple gadget. As I said, it's a relatively minor problem as long as you take small bites. If the nut you are making is more than about 45mm deep, you may find the last few turns leave a more shallow groove because the build-up of swarf forces the shaft hard against the opposite wall. This is easily fixed by running the cutter through twice for each setting, it evens out the groove nicely. The threads on the clamp heads above are 60mm deep, and I managed those with little trouble. You'd seldom need to tap much deeper than that.

As an easy, no-cost way for anyone to make a couple of bench-screws, or a half-dozen clamps, it will do the job well, but if you want/need to get more serious than that, it would make sense to invest in a metal tap. They will certainly do the job quicker, but the thread made by the primitive gadget is usually just as clean & regular

However, don't let me discourage anyone from experimenting & improving the system...

Cheers,

[woodPixel]

An outstanding writeup. After reading it a second time I think I understand enough

The other side of the equation, the threads! Have you improved your methods from when you detailed them last time? (I'll find the... thread!!)

[IanW]

........ Have you improved your methods from when you detailed them last time? (I'll find the... thread!!) ....

I don't think I've changed anything in the method I use to cut the screws with the router jig since the very first thread I made, W.P. In fact I used the jig I made for 3/4 inch screws over 30 years ago, up until a few months ago. But a friend asked me to show him how to set it up a couple weeks back, & I had a heck of a time getting it to work properly. Usually, I get it close to spot-on first try, but not this ime! And because I had someone looking over my shoulder, I just kept trying for the "quick fix", tapping it this way or that, instead of doing a bit of thinking & trying to analyse the cause. After chewing up a couple of hundred mm of dowel, I got it working, but the thread was still not perfect.

When my friend left (thinking it was all too much bother, I'm sure!), I had a proper look at what was going on. The answer was pretty clear, the first turn or two of thread was chewed-up & worn, after its many years of use. This is due to the occasional 'bad starts' where I don't twist the dowel smoothly into the first thread as it's cut, and it creates a crooked first land. That usually causes it to jam after about a half-turn, & the sensible procedure is to stop, back it out & cut away the crooked bit of thread with a knife. Leave any straight thread, which you can then feed in over the (stationary) cutter, switch on, & away you go. When all is right, you can easily turn the dowel with your fingers, as fast as you like, and a perfect thread should just keep rolling out from the other side.

The chewed-up starting threads on my worn jig made it almost impossible to align by eye, so the first bit of thread cut is not the right pitch, it's either too tight or too wide (hence my tapping back & forth to try to correct it). But if the wonky bit is just a fraction of the first turn, it may continue to feed. I can feel it's tight & not right, but in my impatience, I sometimes just force it through until the wonky bit exits and it starts cutting smoothly - those screws get trimmed off & become the "pusher" screw at the top of the clamp, so the bit of bad thread doesn't matter. Not best practice, though, and I reckon that is what has caused the wear & damage to the jig.

The solution was very simple- I spent about 15 minutes making a new jig. The fresh, clean thread was easy to align with he cutter & I got it set up first go and cutting nicely. The beauty of this system is that once it's set up and cutting properly, you can feed screw-blank after screw-blank through as fast as you can turn them. It's hard to stop before you run out of wood!

The Beal threader uses threaded aluminium inserts, which would be far more durable than wood, but most people would only want to thread a few dozen clamps-worth of screws, and a few odd bits & pieces and a wooden jig will do that easily. My worn-out jig has cut literally hundreds of screws, being used for numerous demonstrations at wood clubs, wood shows, etc. The new one is made from tougher material (some nice tight-grained River Oak I happened to have on hand), so it should see me out very easily...

Cheers,

[wheelinround]

Ian an excellent look at threads.

I agree re-swarf/chip removal a hole on the opposite side of the cutter would decrease build up.

I have need of a 3/4 tap & die as well as a 1 1/2 (I've got 2 commercial 1in). I do have a couple of 3/4in bolts I consid erred making the tap out of but thread tpi far from 6 which seems ideal for wood use threads.? Guess the ML7 needs a workout.

As for driving force insteadbof 2ft bar have you tried drill attached for steady speed and reversability? Just a thought us old blokes need qll the supoort we can get.

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[wheelinround]

Ian I have some Jarrah if you'd prefer that to River Oak?

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[IanW]

..... I agree re-swarf/chip removal a hole on the opposite side of the cutter would decrease build up....

Ray, I just don't think it's practical to do much more than I've shown for a small-diameter wooden tap, you'll run out of structural material very quickly. It's very different with metal, where you can machine out very generous chip clearance: 1 & 2 inch taps.jpg

The 2 inch shown has a hollow core and slots in front of the three rows of cutters - it has no problems at all!

...I have need of a 3/4 tap & die as well as a 1 1/2 (I've got 2 commercial 1in). I do have a couple of 3/4in bolts I consid erred making the tap out of but thread tpi far from 6 which seems ideal for wood use threads.? Guess the ML7 needs a workout....

Definitely no more than 6 tpi for up to one inch. I have made finer threads (8tpi) for 5/8" screws, but you have to be very selective in he wood you choose to thread at that pitch.

For 2 inch screws, no more than 4tpi, three or even two is better, but my mate's lathe only goes to 4tpi, so we settled on that for the 2" & 1 1/2" taps he made for me.

The beauty of the "primitive" tap is that you can make it any pitch you like - your not limited by the cogs on your lead-screw drive.

......As for driving force instead of 2ft bar have you tried drill attached for steady speed and reversability? Just a thought us old blokes need all the support we can get...

Ray, old cobber, if your DP can take the shank of my 2 inch tap, let alone turn it through a lump of Spotty, it must be a monster!

Seriously, taps up to 1 inch diameter present no problems, I can still manage them easily, despite my advancing enfeeblement, so I think you'll be fine with those. A decent-sized DP might manage to turn a 1 inch tap through wood, however, although I usually start taps on the DP or lathe in metal, I don't find the need to start wood taps that way. They usually have a very long nose compared with taps for metal, and easily follow the pilot hole. I have a bottoming tap for 1" 6tpi, and it's virtually impossible to start straight, but I have only used it once or twice in all the years I've had it.

The large taps from 1 1/2" up were a problem, but I reckon I've solved that now with my matching "primitive" versions. They are very easy to use & I should be ok with them for some time yet - they take less effort than pushing a saw or a plane, so I'll probably quit before they become too hard for me...

Cheers,

[Mountain Ash]

Thanks Ian. You are very generous in sharing your knowledge. Those of us following in your foot steps are extremely fortunate.

[IanW]

Ian I have some Jarrah if you'd prefer that to River Oak? .....

Ray, I have quite a bit of Jarrah, thanks. It's ok as a threading wood (you can thread almost anything with a sharp router cutter) but like most Eucalypts & close relatives, tends to chip a bit. That doesn't affect the function of the screw significantly, it just looks a bit daggy. Good River Oak usually takes a very clean thread.

Lots of woods thread nicely - Crows Ash is my current favourite, but there are many other non-commercial woods that are as good or better. Here are some freshly-made 3/4" clamp screws: 3_4 screws.jpg

From the top they are "Bush cherry" (Exocarpos cupressiformis), Crows Ash (young tree), She-oak, and Crows Ash from an old mature tree. All have taken an excellent thread (the bit of fuzz in the She-oak threads brushes out easily with a brass-wire brush). Trying out different woods whenever I come across them has been a bit of a fetish of mine, but there are still dozens of good woods that I'll never get to try, so I recommend you try whatever you think is suitable. Anything fine-grained that peels smoothly on the lathe usually threads well....

Cheers,

[wheelinround]

Sadly I've discovered an allergy to Crows Ash causing me respiratory problems [emoji25]

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

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