a wood thread thread

[Cheshirechappie]

In practice I don't find clogging an issue at all (usually 2" stock) - but friction is big. I did a tweak on the tap which has helped - cut 2 nuts yesterday quite quickly but it's a Weetabix job, basically. I am sure I can tune this further.

Normal practice in making metal-cutting taps is to relieve the thread profile behind the cutting edge, so there is still some guiding but not full (frictional) contact. That's not easy to do without some pretty fancy toolgrinding kit, but a good approximation could be obtained by cutting more of the thread away from the outside diameter of the tap behind the cutting edges. Filing is probably the easiest way to control things. It's usually done on the 'cutting' threads (the first few, where the tap is tapered) but not on the 'non-cutting' threads further up the tap. That might reduce the weetabix count a bit.

Another thing that may be worth experimenting with is varying the rake angle of the cutting lips by angling the side of the flute. Most metal-cutting taps have a definite positive rake; I'm not sure whether that would be advantageous in wood, or what angle would work best. At the moment, they're effectively cutting at 90 degrees to the stock - analagous to the action of a scraper plane. A slightly more positive rake might cut a bit more freely.

 

[rxh]

Forgive me if this is a foolish suggestion (you can blame the TV show "how it's made") but for the square hole in the taps would it not be possible to make some form of square tapered reamer you simply force down the hole? They use something very similar to get the final shape on plane mouths at the Clifton plane factory with a hydraulic press - theirs is flat obviously, but still square.

If it was made with just 2 opposite cutting sides it would be easier to knock through with less resistance than 4 sides.

Thanks Raf. I didn't see that programme but I imagine some sort of broaching tool was being used. Sadly, I don't have such equipment but my method, although rather slow, cuts a good square hole (even a blind one) and is cheap

Normal practice in making metal-cutting taps is to relieve the thread profile behind the cutting edge, so there is still some guiding but not full (frictional) contact. That's not easy to do without some pretty fancy toolgrinding kit, but a good approximation could be obtained by cutting more of the thread away from the outside diameter of the tap behind the cutting edges. Filing is probably the easiest way to control things. It's usually done on the 'cutting' threads (the first few, where the tap is tapered) but not on the 'non-cutting' threads further up the tap. That might reduce the weetabix count a bit.

Another thing that may be worth experimenting with is varying the rake angle of the cutting lips by angling the side of the flute. Most metal-cutting taps have a definite positive rake; I'm not sure whether that would be advantageous in wood, or what angle would work best. At the moment, they're effectively cutting at 90 degrees to the stock - analagous to the action of a scraper plane. A slightly more positive rake might cut a bit more freely.

Thanks CC. No doubt further improvements could be made to the tap design but I'd rather not tinker with the existing ones too much as they do cut good threads and provide some exercise :wink: . However, if I make any more I would be tempted to try some modifications to the design.

ust to extend the options... you will know this I expect... but for bigger sizes there is a different sort of tap, shown in some of the French sources. A guide thread on the far end of the tap, which can be just a thin bit of steel in a saw kerf. For the bit that cuts the female thread you have a single tooth, like a metal turner's boring bar.
The advantage is that there is less resistance (only one cutter not a group of cutters) and also you can take more than one pass, advancing the tooth each time.
Not so many Weetabix needed.

Yes Andy, I think we've approached the limit of what is reasonable to make an all-metal tap from. Here of some photos of a very big thread on a press that I saw in Italy last year. I imagine that it was made using the technique you mention. The steps give an idea of the size of the screw. I was interested to see the thread angle used.

 

[condeesteso]

Here are pics of the thread box, cutter etc. For this thread I used 8mm sq O1. Roy Underhill's program about this is reasonably handy but he does skim over some bits (due to program length no doubt) - it's here: http://www.youtube.com/watch?v=VIveDZVxTD4
Things to note:
- the vee is not 60 degrees, more like 50 - due to the tangential location into the stock. In fact it hasn't got straight sides either, they should actually flare outawards to follow the thread form off the tap accurately - but that is hard to get accurate, the lower angle is important to get the surfaces bearing well.
- The edges rake forwards - a sort of negative rake in that the vee cuts last. The outer waste obviously has to be freed first. I raked mine about 30 degrees, doubt it is critical, a 'just in time' approach should be ok, and as the cutter is into a tangent, even if flat it will cut outers first, but the rake seems 'standard' and maybe helps slice the cut.
- I think the clues come from the woodturners and carvers, get the bevel almost rubbing. The filed outer vee is taken until the 2 faces meet and the back bevel angle appears. I did all possible shaping then heat treated with a final hone after that.

I kep the box simple, adding handles later. The bed for the cutter slopes about 4 degrees (depends on pitch) so routing that was easier without handles. The clamp is a coach screw (M10) and the cutter must be very solid into its bed - as you would expect.

The blanks are turned to just fit the infeed guide (just) - then oil soaked. I use boiled linseed thinned about 50/50 with spirit. I reckon thinning is essential to allow the oil to actually penetrate deep enough. I use plumbers pipe and soak for about 2 days. It helps a lot to have the oil to cut depth. The blank swells of course but should just fit the infeed. I see oil squeezing out of the endgrain as I feed in.
Cutter depth is really critical so micro-tapping adjust til; right. If it's tight in the outfeed it's too far back of course. Tiny adjustments make big differences, a delicate tough is necessary but once right keep it.

I made Richard's with a depth screw adjuster, I just tap mine but mine's a bit rough of course.

In the pics I've shown clean cuttings coming off, a really good sign.
Wood has to be close even grain, and straight / true. I sort the straight grain stock to one side, and will split a board with the axe then plane the edge, if necessary.

I have pics of cutting the nuts, but that's the next episode

 

[AndyT]

That looks wonderful!
As ever, your attention to detail is making this so much more than just a workaday tool.
Is this all part of a grand plan?
Are you going to be flooding the market with adjustable music stools?

 

[xy mosian]

Another impressive display of the variety of talent lurking here abouts. Well done to both of you.
xy

 

[condeesteso]

thanks xy and Andy. We all do that... dissappear off into a cold workshop for days on end and come out with a thing
I think, Andy, that rx is head of music stools. I choose a life of vice.

 

[rxh]

thanks xy and Andy. We all do that... dissappear off into a cold workshop for days on end and come out with a thing
I think, Andy, that rx is head of music stools. I choose a life of vice.

:lol: Yes, I do have have an idea for a music stool design.

Here is a footnote. Readers of this thread may recall that I started off by making a 1" x 4 TPI prototype tap. It seemed a pity not to use it for anything so I set about trying to make external threads without using a conventional threadbox. I tried using AndyT's method as shown in his thread:

an-alternative-way-to-cut-big-threads-in-wood-t78726.html?hilit=alternative%20thread

The results were not good, I think mainly because the cutter I made did not have quite the right shape (which is very important, as noted by Douglas above). I tried a different way, using a very old small router and a 60 degree bit, as shown in the photos below. It took a bit of trial and error to get the correct depth of cut right to obtain good threads. I haven't seen the Beall system but I imagine it works in a similar way. Now I can make internal and external threads in this size.

 

[AndyT]

Very nice!
Functionally that is like the Beall system. (There are videos on their website. )

One refinement they suggest is a simple crank clamped onto the dowel so you can turn it at a consistent speed, but you seem to have done just fine without.

 

[xy mosian]

The dodgy bit about some music stools, in their simplest form, is the grain direction.
I made two in the '70's. The design was basically a tripod with central column to take the threaded stem which supported the seat.
Initially I was tapping the end grain, of the column, and found that the wood, Central American Mahogany, would not hold the thread.
My solution was to add an inset cross-grain plug the the top of the stem.
Then the tap was being used across the grain and was successful.
There are examples of screw height adjustable stools about which don't have a column, the legs being connected to a lower horizontal disc. This in turn takes the threaded stem. A lower shelf is added for sideways support.
HTH
xy

 

[rxh]

One refinement they suggest is a simple crank clamped onto the dowel so you can turn it at a consistent speed, but you seem to have done just fine without.

Thanks Andy, that's a good idea - I'll give it a try.

Initially I was tapping the end grain, of the column, and found that the wood, Central American Mahogany, would not hold the thread.

Thanks Xy - you are correct. I have discovered the difficulty in tapping end grain too, so my stool design avoids this.
However, I have had success in tapping M10 threads in end grain ash using an ordinary metalworking tap. I did this to make a hill walking stick that would separate into three pieces to go into my bag for air travel. I made this several years ago and it has had a lot of use, with multiple connections / disconnections.

 

[bugbear]

Things to note:
- the vee is not 60 degrees, more like 50 - due to the tangential location into the stock. In fact it hasn't got straight sides either, they should actually flare outawards to follow the thread form off the tap accurately - but that is hard to get accurate, the lower angle is important to get the surfaces bearing well.
- The edges rake forwards - a sort of negative rake in that the vee cuts last. The outer waste obviously has to be freed first. I raked mine about 30 degrees, doubt it is critical, a 'just in time' approach should be ok, and as the cutter is into a tangent, even if flat it will cut outers first, but the rake seems 'standard' and maybe helps slice the cut.

Wow - that's cutting really cleanly. I hadn't really twigged the "outer[AKA upper] waste obviously has to be freed first".

There's something similar going on in the video in this (old) thread:

best-v-tool-for-relief-carving-what-is-the-other-tool-t80677.html

But I also own an old 'V' plane (used for fitting out pigeon holes etc) where the blade is flat and pointed. I've never tried it cross grain

BugBear

 

[condeesteso]

yes, BB, I'm vee pleased (haha) with results now. I was half-expecting to be picked up (maybe by your good self :wink: ) on my use of 'tangent' which is misguided. The cutter actually aligns to a shallow cord of the circle, tangent is wrong and misleading.
I've been using rxh's tap a bit recently, with a little tuning (I think Cheshire mentioned above somewhere) - a bit easier now but I am still working on getting shallow thread blocks/nuts (50mm say) bang-on axis. I have a plan
Pics of that bit soon, promise.

p.s. yes, Homer Dangler's vee tool is exactly what is needed here from the look of it, bevel rubbing and all. Sods to sharpen well too (though I'm no carver).

 

[rxh]

I spotted this threaded stem on a bar stool in a hotel in France recently and wondered if it had previously been used for something else. I was interested to note that the thread is left handed - the French like to do things their own way - vive la difference?

 

[lanemaux]

What do you know, a memorable screw you found in a bar , a true rarity. :lol: :lol: :lol:

 

[xy mosian]

Initially I was tapping the end grain, of the column, and found that the wood, Central American Mahogany, would not hold the thread.

Thanks Xy - you are correct. I have discovered the difficulty in tapping end grain too, so my stool design avoids this.
However, I have had success in tapping M10 threads in end grain ash using an ordinary metalworking tap. I did this to make a hill walking stick that would separate into three pieces to go into my bag for air travel. I made this several years ago and it has had a lot of use, with multiple connections / disconnections.

Sorry for the tardy reply. I have been mulling over the thread in end grain ash. I am suprised that ash held the thread as it obviously has.
Then I began to think about the thread in end grain. This of course is a thread cut across the grain, which can be done as seen in the male parts. My Sergeants tap, as most others I have seen images of, scrape the thread. Of course in the Box great care is taken to cut the thread in order that it forms correctly.
In the November 1950 edition of Popular Mechanics Sam Brown shows a Tap which achieves the same cutting action.
http://books.google.co.uk/books?id=r9gD ... 18&f=false page 218 is the beginning of the arcticle, which is available in full. The Tap described would be more difficult to make but may give more reliable results in end grain.
xy

 

[condeesteso]

Here are some pics of the makings:

my tube for soaking the thinned oil - it's 1 1/2 plumbing pipe with an offcut of gutter welded on (solvent cement). Cut the tube of a length so the stock sticks out of the top, or otherwise be prepared for a mess. I am surprised the volume of thinned oil I get through with these. I now buy BLO in 5 litre cans.

Here's the making of a cutter. 8mm O1. I file the outer bevels first getting the angle where I want it (happens to be about 50 degrees) and also meeting along c'line so the actual back bevel appears where the 2 planes meet

I get as close as I dare to a finished form before heat treating. The inner vee is got out mainly with a saw file, but the final part needs a thin tapered file (given the 50 degree internal). The apex need not be a tight angle, the tap cuts a rad at the bottom (edit: top I meant) of the profile, so rounding the cutter here a little is fine, and a lot easier to achieve

A little trick, I get the outer faces flat and centred etc, then file a very shallow hollow behind the cutting edge (like with a Japanese blade) - it makes honing a lot easier after heat treatment.

I'll get pics of the tap in use soon.

 

[AndyT]

Great stuff! While enjoying watching your account of the design and build process, and not wanting to deflect you from making what is no doubt a superior item, I thought I had better mention, for the sake of anyone else wanting to have a go but at a less ambitious level, that Axminster sell the cutters as spare parts for £9.96 - http://www.axminster.co.uk/axminster-wood-thread-cutting-kit-replacement-cutter

 

[condeesteso]

Quite right Andy - I have a couple of sizes (well, 3 actually at the moment but one's a borrow ) and they work really well. The cutter alone is worth a tenner as the work involved making them is considerable. Beware the steel is a bit soft - keep a watch on the edge and re-hone when necessary. I found once the edge goes it can fold over or break away badly and effectively write the cutter off. Also note the max depth of thread they will cut. The Axi sets go to 1/16"though the actual cutter will go deeper (with the Axi set, I/D is always 1/8th less than O/D regardless of screw diameter) - I've been needing to make bigger cutters here. But the Axi set (I got a 1" recently about £38) are really excellent provided you are OK with 6tpi and quite shallow threads (obviously 'shallow' is relative to diameter, on the 1" it's quite deep enough, not so on the 1 1/2" really). Mine worked very well indeed out of the box.

 

[midnightlunchbox]

I have a wee query on this subject. I have been given a thread box and tap set (like the Axminster one) from a friend and I have prepared some boxwood into hexagonal lengths for making nuts and bolts. The kit is brand new, never been opened and the cutter is very sharp. My question is, is the boxwood liable to thread easily or will I have to make several, shallow passes with the thread box until a deep enough thread is made? Will I need to lubricate the boxwood? Just trying to get to grips with the job before I start.

Cheers
Craig

 

[AndyT]

With a thread box, you have to cut the whole depth in one go. The back of the box has a threaded hole in it, which is necessary to guide the work through at the right rate, to give the designed thread pitch. If your new thread is not full depth, the work just jams tight in this threaded guide.