An alternative way to cut big threads in wood

[AndyT]

I'm interested in ways to cut threads in wood. I've got - and have used - one of the cheapish threading kits like this one

but they get pricey in the larger sizes. Old ones are hard to find and can be even more expensive than new ones. And of course you need a new set for each size you want to work.

Some of you may remember I have an old Barnes treadle metalworking lathe which is set up to do thread cutting. I thought I'd do an experiment to see what it could do on wood.

I just used bits of scrap for this to see how it went.

Here is a bit of old dry holly branch being turned on my electric woodworking lathe. (You may have read that holly is difficult to dry without splitting. I can confirm that's true. I accepted most of a neighbour's tree years ago, when I had no idea about drying wood except to put it in a corner and forget about it.)
At least in this case I was able to turn beyond the split and add some superglue in case it got worse.

That gave me a piece about 1 3/4" in diameter. I transferred it to the Barnes lathe, held in a three jaw chuck at one end and a revolving centre at the other.

Here I was lining up a thread cutting tool as you would for work on steel. You'll notice that I've added a woodscrew as an extra stop against one jaw - it's important to keep the work in the same position relative to everything else when cutting threads, or else successive passes won't line up with each other. I put the right gears in place for 6 threads per inch, engaged drive and started to cut.

Hopeless!

Cutting this with the metal tool was not a good idea. Even on nice hard dry wood like the holly, and taking a very shallow cut, the result was a fuzzy mess of broken fibres. I took my blank back to the wood lathe and turned away the mess, leaving it at about 1 1/2" diameter, to try again.

Luckily I had a better idea. The tool holder on this lathe takes 3/16" square HSS blanks. By one of those rare bits of good luck the cutting tool inside my cheap 3/4" thread cutter is on a 3/16" square body. It was pretty simple to remove it, clamp it in the tool holder and shim it up with some bits of steel so it hits the work at something like the right angle. Success!

You can see the nice curly shavings and the smooth cut surface this leaves behind.

I could even advance just a tad on the cross slide so as to deepen the cut on one side - you can see how this makes a thinner, straighter shaving

So external threads work quite well. Time to try a matching internal one. My 1 1/2" piece will need a hole 1/8" less. I don't have a 1 3/8 auger bit so I used an expansive bit, which worked ok in a bit of old beech.

I mounted the wood on the faceplate, took a skim round the hole to smooth the inside and make sure it was properly centred, and then swapped over to the only tool I have that I thought might work, an old internal thread cutting tool that came with the lathe.

(Sorry that's a bit out of focus.)

This sort of worked:

It's a bit rough - more torn than cut I suppose. I did take two passes but the beech is not really very tight grained and will inevitably behave differently on side and end grain. Nevertheless, it was good enough to be able to screw the other piece into it and get it out again, so I am calling it a success.

I don't have any plans for using this technique just yet, but it's nice to know that I could choose to make something that needs big threads if I wanted to. (I don't need to make a Moxon vice!) I expect choosing better wood than the beech would help, as would oiling the wood before cutting it. If anyone has a cunning design of internal cutter that would perform better on wood, please say. I don't really want to start making and hardening steel taps in big sizes if I can get away with just one tool for any size hole.

I will look out for a 6 tpi internal thread chaser, which I expect might be the answer.

 

[custard]

Thanks for posting, I enjoyed reading that.

 

[rafezetter]

Andy - for cutting the inside thread - how about soaking the wood with CA? This would greatly harden the fibres and make them act more like metal than wood and should give you a much cleaner cut, it would also have the added benefit of much less wear. You could use the expansive bit 1mm or so smaller to cut the round hole, then soak in CA and cut again to get the final hole size. Another soak to harden up, then cut the thread. Cheap pound shop thin CA would be fine for this, it soaks in just like water and I use it to harden up edges for chamfering on cheap wood /plywood so I don't get break out.

(and I'm hopefully making a moxon vise soonish just out of interest what would you charge for such a service? Threaded bar is fine, but just as a what if..?)

 

[Woodchips2]

Well done Andy, good bit of innovation =D>

Regards Keith

 

[graduate_owner]

I tried that with a metal turning tool and got a rubbish result as well. I was thinking to give it another go with a v-shaped wood turning tool, or something similar, but haven't done so yet. The pictures you've posted give me some hope for success so I'm going to have another bash. I haven't tried cutting internal threads though.

The trouble with having metal working stuff as well as woodworking is the amount of space taken up. It's great to have the use of the metalworking kit, but my shed is really cluttered now, (metal lathe, milling machine, surface grinder ...) and it's about 40ft x 18ft. I seriously need a tidy up. (How do people manage in 8 x 10 sheds, or even smaller?)

I'm thinking one improvement would be to remove the dust extraction outside the main workshop, which will reduce space taken up, dust escaping into the air and NOISE. Triple whammy.

K

 

[AndyT]

Rafezetter, thanks for the CA suggestion, it sounds worth a try. So if I'm doing some more, you'd better get in touch and we can discuss details. I do owe you a favour after your help with my secondary glazing.

And it's nice to know I'm not the only one playing about doing this stuff!

 

[xy mosian]

Hi Andy, add me to the list of those with an interest in thread cutting in wood. I'm just not as good at getting off my backside to do anything as some of you guys.

I was particularly taken by the use of a 'single tooth' half-nut by Derek in this thread.bench-in-a-weekend-t78328.html. Images 10 and 11 show the part. This of course neatly gets around the need for a Tap of any sort. It also throws up ideas about quick release jaws on Moxon type vices.
This link, from Derek's post, http://www.youtube.com/watch?v=4ZmnGLpqAGI , deals with making and using a Tap. At smaller diameters, say less than 2", 50mm, there may not be sufficient material to successfully hold the cutter. Of course the cutter could be set at an angle to the axis giving more support. For the tooth I had thought that an old twist drill might be a good starting point, the blunt end of course, suitably ground.

One other thought I had came after making, and using, my tapered reamer for chair mortices.

After being initialy doubtful I was suprised by the ease of use and quality of cut, scrape really. How about a saw blade off-cut inserted into the end of a threaded piece to act as the teeth. I think the cutter would need to be double- edged, to keep it central, and it would need making with some degree of accuraccy for best results. Basically it would be a plate with teeth filed on both edges. A plain part to the body of the Tap, for alignment, and clearance for cuttings would be needed.
Most metal taps cut the thread gradually, this can be seen by the taper on the tap. The single tooth tap shown in the link above manages this too. Some other taps use a one pass method. With a saw plate cutter I think I would tend to a two, or three thread taper. More than one tooth is needed for ease of making and any more than three gets just plane boring to make, to me anyway when trying to prove a process.

graduate_owner, The male thread is cut across the grain. The metal thread cutter, used on wood, will tear this. Wood threading devices generally use a V-cutter arranged to cut the outer fibres first, this gives clearance for when the inner fibres are cut. Think of the tooth profile on a cross-cut saw. One of Andy's images shows this well.

Sorry about the ramble. Keep up the good work Andy.

xy

 

[condeesteso]

Brilliant work Andy - as you know these things fascinate me! I think you are very close indeed - the screw is sorted and if it were me I would stick with this approach to the nut given some benefits of pitch matching for example. Noting the clean screw thread came when you used the vee cutter which is approaching tangential, if you can replicate that on the internal I suspect that would make the big difference. An 'L' shaped cutter to fit into an existing toolholder, get it so the bevel just rubs (as in good woodturning practice). May need to play around with a lump of O1 and some files but the cutter in my Axi is quite crude and rather soft steel - I discovered that when I came to sharpen it a while ago. So I don't think the cutter is too critical although sharp will always be good as we all know.
I assume your Barnes will do coarse pitches, the problem I have faced is my Myford will only go to 8tpi, and on these threads I think you want very coarse: 2 - 4 tpi depending on diameter (I think my 2" bench vice screws are about 2tpi). The CA idea sounds interesting too, but generally I still think beech soaked in thinned BLO will give a great result.
I shall have to revisit this topic as I plan a new bench in a while (timber drying at the moment, so no rush) and I quite fancy all wood, including a wood wagon vice like Derek's.
Keep us posted, but I would seriously stick with the lathe approach, I'm sure you are very close to an excellent result and once cracked, you can easily do different diameters - that's quite an advantage over any tap-based solution.

 

[bugbear]

Nice photos!

* it's fairly clear that the scraping cut was doomed; if we stop considering the lathe to be "magic", no one would
try to cut a 'V' across the grain with a vertical 'V' shaped cutter.

* your 'V' cutter works well; if you don't have a thread cutting 'V' cutter, perhaps a carver's V tool could be adapted to fit the tool holder?

* the obvious tool, although rather out of keeping with the Barnes, is a small router, mounted on the toolpost, with a 'V' (AKA lettering) cutter

One last thing - are you driving the cut using the treadle; I suspect a mandrel handle might be better, given the VERY fast pitch.

http://www.instructables.com/id/Lathe-S ... ford-ML10/

BugBear

 

[AndyT]

Thanks all, that's good food for thought. A few notes:

The most basic thing to try is sharpening the internal cutting tool I used. Having had a closer look at it, there was clear room for improvement. I've now touched it up lightly on the grinder and with a diamond hone and it has a better scraping action when tested by hand on scrap.

I think I will stick with the simple scraping cutter for internal threads. For one thing, it's what is used on taps made to cut wood, whatever the design details. For another, making any sort of internal slicing cutter is pretty challenging! There is a picture of something which might be better in Holtzapffel's book on Simple Turning, figure 517, but it's not easy to copy. In theory I like the idea of a single point cutter held on a boring bar. That could give adjustment of bore size and angle - but it's a whole other project to make one, so I will try simpler ways first.

So as well as sharpening the cutter I will try treating the inside of the hole with linseed oil or with CA glue. I will also try other woods. I want to stick with the lathe rather than make taps as it gives me any size and also means I can swap gears and choose 2, 3, 4, 5 or 6 tpi. In practice, when cutting the internal thread I just pulled the flywheel round by hand. There is a limitation in that I can only make a nut smaller than the faceplate. I can't make a threaded hole directly in a long piece of wood such as you'd have on a Moxon vice.

I do actually have an electric router and a 60° cutter but for now I will stick to the slicing cut from the screwbox cutter for the external threads, unless I want to make a really coarse thread where it won't be broad enough. I did briefly try clamping a carver's v tool in place, but I decided against it - it was too delicate to treat that way and I didn't want to break it.

 

[bugbear]

I did briefly try clamping a carver's v tool in place, but I decided against it - it was too delicate to treat that way and I didn't want to break it.

We've found a use for the Aldi V gouge!!

BugBear

 

[AndyT]

Ok here's a quick update. I popped in to Bristol Design to see if they had any 6 tpi thread chasers - they didn't. But they did have some coarse metric ones that looked interesting:

5.25mm pitch is nearly the same as 5 tpi - 5x5.25 = 26.25mm ≈ 25.4mm

and 4mm pitch is nearly the same as 6 tpi - 6x4 =24mm ≈ 25.4mm

After all, I don't want engineering tolerances on these threads. So far, all I've done is used the 4mm chaser, turned round by hand inside the beech 'nut' to scrape off a bit more wood from the threads. This had the pleasing result of making it look a bit tidier

and also means that I now have a nice 'running' fit, suitable for a clamp, where before it was actually a bit too tight.

I like this; it means I can make something nearly the right size and adjust it till it fits - that is an approach I am comfortable with.

These chasers, being for metric threads, have a 60° angle, rather than 55°, which is better for this. They seem fairly common on ebay and I think they could be useful for fitting wooden threads made by any method.

I did notice that the external thread is closer to 90° than 60° - I think this must be a function of the tangent angle of the cutter.

Nevertheless, it fits smoothly, which is what I am aiming for, not some theoretically perfect thread.

I've also bought some cheap CA Glue and will do some more experiments with that and linseed oil and report back when I get the time.

 

[rxh]

Andy,
That’s a very interesting set of experiments. I intend to try your method of making external threads and I have had a go at making internal threads your way using my Barnes lathe. The thread I made was in 1 ½” thick beech, 4 TPI with a tapping hole diameter of 35 mm. Before starting threading I soaked the wood in thinned oil and I made a chamfer on each end of the hole so as to give the cut a less abrupt start and finish. I did the threading by “hand cranking”, lubricating with 20/50 motor oil and advancing the cutter using the topslide set at an angle of 29 degrees (in textbook fashion – the thread angle is 60 degrees). The result is not too bad, as you can see in the photos below – there is some raggedness towards the exit side but I think a cleaner thread could be obtained by taking lighter cuts.

For comparison, I made a thread of the same size using a tap. This turned out better as you can see but I believe that your way can produce very good threads with a bit more tinkering. The CA hardening idea is worth a try too, I think.

 

[Bod]

Andy - for cutting the inside thread - how about soaking the wood with CA? This would greatly harden the fibres and make them act more like metal than wood and should give you a much cleaner cut, it would also have the added benefit of much less wear. You could use the expansive bit 1mm or so smaller to cut the round hole, then soak in CA and cut again to get the final hole size. Another soak to harden up, then cut the thread. Cheap pound shop thin CA would be fine for this, it soaks in just like water and I use it to harden up edges for chamfering on cheap wood /plywood so I don't get break out.

(and I'm hopefully making a moxon vise soonish just out of interest what would you charge for such a service? Threaded bar is fine, but just as a what if..?)

CA from a £shop, sounds as if I am missing out on something.
What is it?
I am about to start a plywood job, not getting breakout would be a great bonus.

Thanks Bod

 

[YouSpinMeRightRound]

CA from a £shop, sounds as if I am missing out on something.
What is it?
I am about to start a plywood job, not getting breakout would be a great bonus.

Thanks Bod

Superglue (contact adhesive)

Really interesting stuff, AndyT!

 

[nev]

CA from a £shop, sounds as if I am missing out on something.
What is it?
I am about to start a plywood job, not getting breakout would be a great bonus.

Thanks Bod

Superglue (contact adhesive)

Really interesting stuff, AndyT!

superglue = =D>
contact adhesive = [-X
CA = CyanoAcrylate
http://en.wikipedia.org/wiki/Cyanoacrylate :wink:

 

[YouSpinMeRightRound]

superglue = =D>
contact adhesive = [-X
CA = CyanoAcrylate
http://en.wikipedia.org/wiki/Cyanoacrylate :wink:

Well, I learned something today. Thanks!

 

[Bod]

CA from a £shop, sounds as if I am missing out on something.
What is it?
I am about to start a plywood job, not getting breakout would be a great bonus.

Thanks Bod

Superglue (contact adhesive)

Really interesting stuff, AndyT!

Well I never!
I presume it soaks in, then hardens, just don't touch before dry.
Works the same as Ronseal Wood Hardener, just cheaper for small quantities.

Thanks
Bod

 

[AndyT]

Another quick update. RXH - it's nice to see someone else trying this too - your internal threads look much cleaner than mine - what is your cutting tool like? And what wood is that? I also see that you have made a more normal tool post for your Barnes - I only have the orginal which is a bit limiting.

This was how I lashed up a wide bodied thread chaser to see how well it would cut

not a good method - it still tended to rotate away from the work. (I should stress that this job was cut very slowly, by pulling the flywheel round by hand - I would not use an electric lathe like this!)

I should have mentioned that for anyone wanting to cut external wood threads on the lathe, Axminster do sell just the cutting inserts, as used on the thread boxes, for £9.14 each:



which you can fit into your modern tool holder:

As for the CA soaking, I think it does help. I made another nut, from the same beech. This glue was 5 x 3g tubes for a pound. It soaked in nicely.

For cutting the main thread though, pretty much all the hardened wood was removed, so it was not the whole solution. However, I then put some on the completed thread, before scraping round with the chaser tool to get the right fit, and for this, I think it was helpful, with less fuzziness especially on the end grain. It's surprising how little wood needs to be removed to go from 'too tight' to 'looser than planned.' A little ordinary dry soap rubbed round the internal thread helps smooth running.

More later.

 

[rxh]

Andy,

In answer to your questions:
- My cutting tool is a Soba HSS metric (60 deg.) internal threading tool, bought from Chronos. Here is the link:

http://www.chronos.ltd.uk/cgi-bin/sh000 ... ml#a136330

- The wood is beech (kitchen worktop offcuts).
- I didn’t make the tool post – it came with the lathe and doesn’t have any maker’s mark. It is very easy to adjust for height by screwing or unscrewing the cylindrical parts together/apart.

I look forward to your next report