a wood thread thread

[condeesteso]

I followed Andy Ts thread on the topic with interest, and wood screws are generally excellent things so here's a bit more.
This is what it's all about:

I'll get rxh along because he enthusiastically joined me for this challenge. His Barnes lathe can cut silly pitches (down to 0.5 tpi I believe) so he undertook to make 2 taps (42mm, 4tpi) and I would then make the thread boxes. Richard was really quick doing his bit but I let the side down... ages it took, off and on. I reckon the thread box is a tricky thing - Roy Underhill makes it appear easy but the result you get depends on the details I'm certain.
Richard will be along soon to show the making of taps, then I'll share what I learned doing the boxes. I'm getting the result I wanted now, but it is not easy to get really clean threads I think!

 

[Racers]

I tried making one but couldn't get it to work, possibly because the wood I was trying to thread was too soft.

Yours looks very good.

Roy does make it look easy doesn't he!

Pete

 

[lurker]

I bought one years ago........ 3/4" with a matching tap
It has been very useful over the years for making jigs and things

 

[AndyT]

Wow Douglas, that looks like a proper result!

Sometimes these projects happen suddenly, sometimes they take time to mature. I've just checked and it was over a year ago you were talking about making some... this thread proves it.

Looking forward to seeing the rest of this one!

 

[rxh]

Yes, it has been an interesting challenge and we have learned a lot about wooden threads and making them, together with discovering some new techniques such as cutting accurate square holes in metal. I agree that the threadbox is the tricky part, particularly the cutter shape, angle of presentation and depth of engagement. The tap turned out to be a fairly straightforward machining job when the geometry had been worked out. I decided to make a smaller size prototype first and settled on 1" as I happened to have some suitable size round bright mild steel barstock (I had a hunch that BMS would work, rather than a harder grade of steel and it proved to be correct). It cut threads OK but needed a lot of effort to turn. I decided to put much more of a taper on the full size taps and four flutes instead of two, which were valuable lesson learned. I made the tap to accept 3/8" square drive tooling and double ended so that it could be reversed to finish a thread in a blind hole. Here are some photos of the prototype tap (details and photos of the full size taps to follow in a separate post):

 

[AndyT]

Now that does look smart!
I especially like that you did it the hard way - how do you make a square hole in the end like that? I would have made the square part smaller and external, but that's because I don't know any better. ;-)

 

[n0legs]

Fair play guys they work and look great =D>

Is this just an exercise or are you going vice making ?

 

[Harbo]

Clever stuff

Rod

 

[bugbear]

Fair play guys they work and look great =D>

Is this just an exercise or are you going vice making ?

Clamp making, tapestry frame making, spinning wheel band tension adjuster making, booking binding frame making, plough plane making...1

BugBear

 

[AndyT]

If I wasn't hundreds of miles away I'd be popping round to join in this one...

Meanwhile, in case it helps with the thinking and designing side of things, here's another take on the metal tap; it's a 1" size from the Beall Tool Company, designed to be used with an electric router for the external threads. These have been around for quite a while and seem to be capable of producing good clean threads, judging by the posed samples in magazines and books. I expect they would have spent quite some time and effort designing their version.

They have opted for 6 tpi, three flutes, and a thread form with a noticeably flat root.

Just for fun, here's the 6 tpi 3/4" size alongside 3/4" B. S. Whitworth (taper and bottoming)

Hope this helps, though I suspect I may be a bit late!

 

[n0legs]

Clamp making, tapestry frame making, spinning wheel band tension adjuster making, booking binding frame making, plough plane making...1

BugBear

Cheers BB.

 

[graduate_owner]

Very interesting indeed - how did you cut the square hole in the end of the tap? Also, to get the finished tap did you just start with a "cylindrical" tap and grind the taper, then grind the slots?

K

 

[rxh]

Thanks to all for your comments. Here are some shots of making and testing the full size taps. As you can see, I made them as a pair, back to back then I separated them before milling the flutes and cutting the square holes. Explanation of square hole making to follow in a separate post.

Andy, thanks for the info. and photos of the Beall system. Some details of my experiments with mechanised thread making to follow.....

 

[AndyT]

Now that's a fine looking lathe you have there! I'm particularly jealous of the original steady that you have - if anyone else happens to have a spare one knocking about that will fit on the somewhat 'individual' design of the Barnes bed, do please let me know. And you have a more practical toolpost than the American style original.

I'm almost disappointed to see what is presumably a milling machine cutting the slots rather than just you with a big file, but as shown, your way does have the benefit that it works... :wink:

 

[rxh]

Andy, yes, I milled the flutes – it wouldn’t have been easy with files. I have heard of keyways being chopped out with cold chisels but I haven’t tried this


The square holes in the full size taps are sized to accept ½” square drive tooling. The photos below show how I made them. The process has some similarity to chopping a mortice in wood.

Tools required:
- drill,
- ruler,
- scriber
- triangular file
- hammer
- small narrow cold chisel
- square “chisel” made by grinding an HSS lathe tool blank at a suitable angle (I used 76 degrees)

 

[rxh]

Here are photos of the final parts I made: the outfeed inserts for the threadboxes. I handed two of these plus one of the taps to Douglas. Now over to him to continue the story with descriptions of his adventures in making the threadboxes and their cutters

 

[Cheshirechappie]

Interesting stuff!

Pardon my poking nose in, but one thought that occurred is that the tap might cut more freely if there were more space for the chips to eject - the flutes on 'bought' taps are quite large compared to the 'made' one.

I'm sure you'll have worked this out anyway, but the 'die' part of the set-up needs to be a wee bit smaller than the 'tap' - it shouldn't be possible to screw the tap into the die. The reason is that the parts they cut need to be t'other way about to work easily - the female part needs to be a tad bigger than the male part, so that there's a working clearance between the two. Thus, the 'tap' has to cut a slightly larger part than the 'die', so the tap must be larger than the die. Not by much - just enough for a working clearance on the finished threaded parts.

 

[condeesteso]

the 'die' part of the set-up needs to be a wee bit smaller than the 'tap'

Fair point Cheshire and good practice for 'normal' tap/die sets (I assume) - but the ali female here is not cutting, it is there for 2 things: feed the pitch (maintain pitch rate) and help keep the stock on axis. I use a fairly deep infeed guide to get the stock on axis too, plus turning the blank to only just clear the infeed.
Re waste clearance, in Richard's pics there are the main deep flutes (4) and each has a secondary clearance, less deep but bigger arc. 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.
I'll get me pics together and in order - back soon.

 

[AndyT]

Just 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.

 

[rafezetter]

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.

Otherwise great work and it obviously works extremely well.