Chasing rainbows, er, threads

And so, driveway out of the way, I can return to chasing threads in wood. I am making progress, I am glad to say. It may not look like it:
But we have a distinct thread to the left of the picture. The right side looks like a porcupine. So here is what I have learned. One, you need to turn down the right side of the cylinder, maybe 2-5mm. The shoulder gives you a reference point to start. Two, take your trusty lead pencil turning tool and use it to establish a passable pencil thread. If you mess it up, just sand it off and try again. Three, you need a tool rest with a square top. The rounded things supplied for wood turning work very badly, if at all, and this is because the chase must be horizontal. Four, any hesitation is fatal. If you stop you will make the chase into a form tool. Pretty sometimes, but useless. Five, when you have successfully done a pencil thread you have a very good idea of how fast to go. For my 2mm pitch, the whole thing takes one second to do a few cm of thread. Six, again a virtue of the pencil, just follow the pencil for the first try -- striking the thread, the chasers say. Seven, on subsequent passes it is easier, but you have to catch the thread just right. The turned-down section is a great help. Otherwise you will cut another thread and that is not easy to fix. Once the thread is deep enough the chase pulls itself along.

On my next try I munged up the start, but the last bit came out very nicely.
The pencil strikes again. I have a highly improvised rest -- a hex key (Allen) wrench clamped in the (metal turning) tool post. But it has a flat top. Mandatory, I think. Got that trick from the late Tubal Cain, Simple Workshop Devices. Mr Cain (T.D. Walshaw) did chasing on his trusty Myford screwcutting lathe, and started the thread by the simple expedient of setting the Myford up to screwcut. All he had to do was hold the chase! But, he said, it is much faster than doing it in the standard way. On subsequent passes he would chase in the usual manner, and the whole thing was done in a few minutes.

I am simultaneously debugging both my technique and the chase. Not advisable, but then nothing ventured, nothing cut. Also the material is far from satisfactory. It is lilac branchwood and far too green. So it chips out. I am also certain now that this can be done on the pole lathe. In fact it may be easier because the pole lathe does not rotate quite so fast. I have the Taig slowed down to the max. Or to the min, to be more exact. Another lesson is that the smaller the pitch, the easier it is to do the thread. That is because on a 2mm pitch the whole thing is done (or munged up) in one second, two at most. Slow speed good, small pitch better than coarse.

I need to try the pole lathe. This summer I will consider uprights so I can bring the lathe in come Fall. Meantime... My shop is a mess; all the outdoor tools are in it and I really have no room. But any day now the snow will melt. There are now bare ground patches on my driveway.

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.

Chasing threads, adventures and misadventures

It has been a hard day. I spent an hour and a half blowing snow and am not much further ahead. It would be great snow for making an igloo. It froze last night, and you can cut blocks from the stuff. But trying to blow it away is another matter. Well, such is life in Alaska; let us return to screw cutting. In this episode we have my adventures in thread chasing. We saw the chases under construction. Look like a saw and a chisel got together. Now, in principle, this is simple. Let's suppose you want to cut an outside thread. So you move the chase along the work so that it scratches the surface of the cylinder that you have previously turned. Ah, but at what speed do you move it? Why, at the speed that gives you the proper pitch. My learned books (and this article) say this takes some practice. It sure does. Very hard to find the right speed. It is fatal to stop or hesitate. It is also hard to cut to the right depth. This is called "striking the thread."

My first attempts at this operation were disastrous. In one morning, I converted half a dozen cylinders into porcupine look-alikes. Nothing even remotely like a thread. Eventually, I got an idea. I took an ordinary lead pencil and used it on the tool rest; and yes, I'm using the Taig. About 30 cm of snow between me and the shop right now. However, this would work on any lathe. In fact it would work better on a pole lathe because it doesen't rotate quite so fast.
Picture a bit blurry, sorry. But you move the pencil along the rest until you get a uniform pitch -- and the pitch you are looking for, in my case 4mm. When a thread varies in pitch along its way, or crosses over, it is said to be drunk. My first attempts weren't just drunk, they had the D.T.s! But if you, er, munge it up, no problem: take a piece of sandpaper and get rid of the pencil marks. Much better than making porcupines. Eventually I started getting decent pitches:
Now we have an idea of the traverse speed. The smaller the pitch the slower the traverse. You can verify this with the pencil trick. A 4mm pitch is really too large, but having filed my chases I must use them! So I turned up a bigger cylinder, did a few "pencil tries" and screwed, so to speak, my courage to the sticking point. Lady MacBeth would be proud of me.
And, by George and Lady MacB. we have struck a thread. The next job is to chase it. One must put the chase in exactly the set of grooves you have stuck, and traverse accordingly. At a 4mm pitch, it is a matter of seconds. I suspect you are looking at an apprentice porcupine. But we persevere. It's fun, too. The depth is crucial. You want to scratch at this point, not cut. Unfortunately at this point the work leaped out of the 4-jaw chuck so I have to recenter it. This is easy on metal but harder on wood, because wood compresses. Moral: when you chuck an irregular piece of wood, label it with the jaw of the lathe. I always label my jaws, but sometimes forget to label the wood with the corresponding jaw. Pole lathe users can disregard this counsel, on account of there are no chucks.

Stay tuned for the further adventures and misadventures of an apprentice screw chaser.

Nuts!

In the last episode, we had made a screw. Now we have to make a nut, in the greek manner. According to Hero, the greeks split the nut -- but I am getting ahead of my narrative. The nut should be bored to the inside diameter of the thread. Let me see, 16mm outside (OD), 2 mm depth of thread, so that's 14mm. I do not own a 14mm drill, so the first job was to bore the 14 mm hole on the Taig.
You may say this is cheating. Use of power tools! But if I hadn't done it I would still be waiting for the 14mm bit to arrive in the mail. Boring is a very accurate way to get a given diameter hole. You put a boring bar in the tool post, and you can adjust it to the nearest thou -- .001" or .025mm. The nut is held in the 4-jaw chuck. OK, we have a nut blank. The greeks then split the nut in half. They rubbed lampblack on the crest of the bolt, and then made an impression on the nut. Fortunately my kerosene lanterns needed cleaning, so I had an ample supply of lampblack.Because the nut is 14mm and the OD is 16, we can't get an impression anywhere except the edges. But that's OK, we can pencil in the threads. Now we have to chisel out the valleys in the nut. This proved to be very difficult. It's all end grain, notoriously difficult to chisel. I was reluctant to call mission control in Houston.
So I decided to take a break and look ahead to the next step, using a chase to cut threads. Using some old files, duly annealed, I filed up a pair of chases, one inside and one outside. For the inside one, I drew up a paper pattern, pasted it to the chase, and filed away. 4mm pitch, 2mm depth. For the outside chase I used the inside one as a pattern.
All my learned books recommend a much smaller pitch than 4mm. But it is very difficult to file say a 1mm pitch without a microscope and a set of very small files -- actually, I have some; jeweller's files or swiss pattern files. Still difficult. The old file, horizontal in the picture, is the inside chase. It is used to cut the inside thread, as in a nut. The vertical old file is the outside chase. Now, chasing is in the future, but as it turns out, the inside chase came in handy.

I finally broke down and called mission control. Houston said "use your burins, dummy! Out." Oh. Thank you, Houston. Last year I was experimenting with carving stuff on endgrain, and I made two or three burins for the purpose. A burin is just a very narrow chisel with a lozenge edge, used to cut endgrain! Brilliant, Houston. So I dug out the burins and used one to cut the beginning of the thread. Once the thread was established I could use the inside chase to widen the grooves.
In the still life above, the burin at the top and the chase next to it. You can see the half-nut scooped out with the chase, and all we have to do is the other half of the nut. Once you get a good fit, you have to glue the hut halves together. I suppose the greeks used a band as well, since I very much doubt they had any Elmer's glue.

In another episode we will look into my adventures in chasing threads. This looks difficult, and it it is. But I have hopes. Stay tuned.

The Taming of the Screw

After making the Doctor Buggy, I was at a loose end for a day or so. It may be March, but there is plenty of snow on the ground. I observe signs; for example the parking lots in Willow are melting. Liquid water seen! However, if you try walking out at my place you will be up to your knees in snow. Can't ski on it. Soo-- what to do next? The answer was provided by a Swedish blog -- all about slojd, a Swedish word, usually translated as "craft." In the blog, the cutting of threads is discussed. Now, cutting threads in wood is a fascinating subject. There are all sorts of ways to cut threads. This is because the lowly screw, available for pennies at the hardware store, is a supremely useful object. My idea is to try all of them, except for a screw-cutting lathe, which is rather expensive. Let's go back in time, as far as we can.

We start with the Greeks. As we all know, without olive oil the entire greek civilization would have never gotten out of caves. Modern Greece would come to a crashing halt as well, if deprived of good old olive oil. But to make OO, as well as wine, you need a press. The press is best operated by a screw, because, you see, a screw is an inclined plane wrapped around a cylinder. This gives you both mechanical advantage and control. Hero (of Alexandria), year 100 AD or so, described the screw-making process very well. And Bergeron, in his 18th Century Manuel du Tourneur, does the same thing. The Chinese, who seem to have invented almost everything, did not get around to the screw till the time of Bergeron. So how did the greeks (and I) make a screw with only primitive tools? Well, we start with a wooden cylinder and a piece of paper.

The wooden cylinder is a piece of lilac 16 mm diameter. The piece of paper has been cut 4 mm wide. This is the pitch I picked for my screw-to-be. The pitch of a screw is the distance the screw travels in one complete (360 degree) turn. I pulled 4mm out of a hat, but I was aware that if it was too much finer I would have trouble with the next step. And I used the Taig lathe to turn it down -- I can adjust the diameter down to better than 0.1mm in wood. But I could have done it on a pole lathe or used a rounder plane, so this is no big deal. If you try this at home, and have no lathe, go buy a dowel at the hardware store. For a first attempt 25mm would be better than my 16. When cutting paper I found that a knife and a steel ruler are much better than scissors.

OK, now wrap your piece of paper around the cylinder. Hold it down with tape.
Since the ancients had neither paper nor tape, I conjecture that they used leather and thongs. Now this wrapping business is critical to the future of your screw. (A posteriori wisdom). If you leave gaps or if you wrap too tightly, the pitch will be irregular. Bad news. The ideal is to leave one saw width between the wraps, because the next step...
...is to take the thinnest saw you have and cut down between the wraps. Above, I have put a piece of tape on my faithful miniature Dozuki to make the depth of cut. The tape has come adrift. Tape does not stick to wood very well. Masking tape works better than "scotch" tape. If we were careful, we have a spiral groove running down the cylinder. How deep? Well, that's another parameter in the screw business. I used a nominal 4mm which came down to about 2 mm when I got through. I will explore this aspect of screw lore anon.

Now, your saw cut is the valley, or the trough, of your thread. The middle of the unsawed portions is the peak, or ridge, of the thread. So you take the appropriate chisel and cut down from ridge to trough. In my 16 mm screw, I have to use a microforged miniature chisel.
More after-the-fact wisdom: turn a stub (about the depth of thread) on the front of the screw. It holds the tape, and is a guide of how deep to chisel. This phase of screw cutting is not at all difficult, but it demands care and can get a bit tedious. At the end:
We have a respectable screw, if not a perfect one. This screw is a "demo" since I stopped to take pictures as I went. If this screw was for decorative purposes we'd be through. But it isn't. So the next stage is to make a nut for the thing to turn on. Making nuts the greek way is quite a production, so I will leave it for the next post. Again, if you were trying this at home, a larger diameter would be helpful. Not so much eyestrain, and small chisel errors don't hurt so much.

But I think that this screw-cutting business is fascinating. We have to look at chasing, taps and dies, screw-copiers, screw-cutting lathes, -- almost endless! Until next time, happy screw cutting.