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PostPosted: 15 Jul 2010, 20:53 
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My partner & I have just built a prototype thread-cutting jig for use on a wood lathe. This prototype assumes you have access to woodworking equipment only (ie no metal working lathe to turn bits & bobs) and have to make do with easily available chuck adpaters and threaded rods etc. You will also need some scraps of MDF and box wood (or similar).

Currently it cuts 16tpi threads using a length of 3/4" UNF mild steel rod as the leadscrew. I chose this thread size for a number of reasons - firstly, it is suitable for use on turned boxes, secondly I was able to easily source a cheap chuck adapter for my chuck to fit this size rod (it's the same size as the common 3/4" x 16tpi lathe spindle), and thirdly I found a source of the threaded rod & nuts at a reasonable cost at www.ironmongeryonline.co.uk

The most expensive thing I had to buy was the cutter. This is a 3/4" 60 degree double angle milling cutter with 3/8" shank. The only UK supplier I could find of double angle milling cutters was MSC/J&L Industrial Supply www.mscjlindustrial.co.uk
They stock two sizes of cutter, a 1" and a 3/4". The 3/4" will let you thread the inside of smaller items than the 1". They supply the cutters in a variety of materials, the cheapest are the carbon steel variety and even this was £30.00 inc delivery.

I fit the cutter into a 3/8" collet that goes directly into the MT2 taper in my lathe spindle and is secured by a drawbar. Suitable collets can be had from www.chestermachinetools.com at around £6.00 each. You could hold the cutter in a jacobs chuck, by this will stick out further and may introduce "waggle" to the cutter.

We mounted the whole thing on an X-Y cross slide which gives very precise side to side and back & front adjustments. It also made it easy to set the jig up so the cutter was cutting at centre height. However, it is a nice to have feature, and not essential. You could build your own sidewas cross-slide mechanism using another bit of threaded rod!

Here's an annoted photo of the jig as it stands at present

Image

And one from the other side

Image

And a close up of the cutter

Image

If you are dead keen, you can find a PDF diagram (not to scale) here www.teknomage.co.uk/photos/threads/diagram.pdf

Now all we have to do is make a minor adjustment to take some sloppiness out of it (you have to turn the handwheel rather carefully at the moment) and learn to use the thing properly!

From reading Richard Raffan's book on box-making he suggests that 16tpi threads are cut 1.5mm deep, and my first rough attempts on boxwood look ok to me - and just about fitted together!!! The thread in the photo was cut in holly, and was ok until we reversed the thing back over the cutter - which cracked off the crests of the threads :-(

Will have some more practise tomorrow. I am hoping I'll be able to cut threads in somewhat softer woods than boxwood with this system... we shall see.

tekno.mage

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PostPosted: 15 Jul 2010, 21:10 
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T.M

Thanks for showing your project - essentially the same approach, but I needed to meet a limited budget (less than £10) :!: :(

Dave

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PostPosted: 15 Jul 2010, 21:29 
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Bloody hell Kym that's excellent! Well done indeed. Very interested in seeing the rest of the adventure.


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PostPosted: 15 Jul 2010, 21:42 
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It's something I've been thinking about ever since I saw the Richard Raffan box making DVD where he uses the (expensive and unobtainable in the UK) Bonnie Klein jig.

I've had a go at hand chasing a few times, but found I wasn't very reliable at it (and oddly much better at chasing very coarse 6tpi threads which are a bit too big for box lids!) and wanted a method that would reliably work every time so all I'd have to do was get the measurements of lid & base flange right (much easier than hand chasing threads!)

It took me ages to find a supplier of a suitable cutter, mainly because I didn't know what they were called - and to be honest, it is cutting a form of thread more suited to metal than wood, so you do need to remove the very tips of the crests of the threads very carefully with fine grit abrasive to prevent them breaking off. I'd really prefer a cutter that made a rounded or square crest thread form which is more suited to threads in wood. My partner has already suggest a small slitting saw might do the job for a square thread form - so I'll be looking out for something suitable (hopefully at a much cheaper price than the double-angle cutter I had to buy from a real full price industrial engineering supplier!)

tekno.mage

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PostPosted: 17 Jul 2010, 08:05 
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A quick update on our progress with the thread-cutting jig. No more pics as yet I'm afraid, as it was quite late last night we were playing around with it and the weather was foul (which meant we stayed put in the workshop rather than walk to the house to get the camera in the rain!)

My partner made & fitted some nylon spacers to the clamps which hold down the leadscrew nuts and that has taken all the sloppiness out of the mechanism.

It appears the infomation on thread depth for 3/4" UNF given by Mr Raffan in his box-making book is incorrect (could be a typo?). He suggests cutting a depth of 1.5mm, which although making a working thread on boxwood is actually cut far too deep leaving sharp crests which crumble away on other woods.

Having now done some interweb research on thread forms and a quick calculation, it appears that the correct thread depth is actually 0.859mm - pretty close to the 0.8mm which we worked out last night by trial and error!

Initially setting up the jig to cut each thread was a bit fiddly and we had to think hard about which way to turn which knob or handwheel, but some strategic labelling has helped with that and I'm sure it's something that will become familair with use. An improvement would be to make the jig stand-alone (ie without the lathe to drive the cutter) with an old router, motor or something driving the cutter at reasonably high speed.

So far, we have successfully threaded Yew, Holly, Apple and Hazel (from my pile of pre-roughed and dry box blanks we used the slightly defective (ie slightly split or massively distorted) ones as test subjects for threading.

It's certainly a very different kind of process making boxes with screw-fit rather than push fit lids. Accurate measurements then cutting to hard numbers (rather than to a burnish mark!) is a must if you want to cut both threads to fit correctly first time. All rather alien to me, as most of my turning is about making one thing fit another without ever needing to know the actual size of anything in mm or inches!

Slightly flattening off the crests of the threads once cut with 400 grit abrasive makes a better looking job and doesn't affect thread function, and the minor shoulder trimming needed to make the grain match is a lot easier than I expected.

The best part of making a box with a threaded lid is not having to worry about a jammed on lid popping off when finish turning once you need to have to remove the tailstock support :-)

tekno.mage

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PostPosted: 17 Jul 2010, 14:32 
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Great bit of engineering, Kym. Thats fantastic.
Thanks for posting up... its excellent. I'm sure that'll get a fair bit of admiration !
Really like that ! .. excellent ! :lol: 8)

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PostPosted: 17 Jul 2010, 16:17 
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Thanks, Jenx :-) I'm currently rather hurriedly preparing enough "items to be threaded" to take along to our club "hands on" meet tomorrow where I'll have the jig up at running. I need to have plenty of suitably sized tenons and matching holow pieces available so I don't have to keep taking the jig on & off the lathe (it it seriously heavy!), or bother folk using one of the other lathes to prepare things for me!

So far I have pieces of dry apple, hawthorn, damson, yew, holly ready - as well as some small 3/4" sticks to thread and then thread 3/4" UNF nuts on to! Next task (don't laugh) is some of the not so easy to thread woods - ash, birch & maybe sycamore. I'll be soaking the areas to be threaded with sanding sealer tonight though to harden them up!

tekno.mage

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PostPosted: 17 Jul 2010, 19:07 
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ah Kym so this is what you were talking about. Great result, looks brill.

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PostPosted: 17 Jul 2010, 21:41 
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Yes Mark, and it does work quite well - but I did have to research more than I really would have liked about 3/4" UNF threads!

Expect to see some experimental boxes with screw top lids in the near future :-)

tekno.mage

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