Taps for Wood -- Which kind?

[John L. Poole]

I'm slowly working on Pat Warner's Router Table Fence, aka "Precision Fence" (see Fine Woodworking #144 (Oct. 2000) - "Micro Adjustable Router Fence" and

[soon to be revised]) and have some questions about taps and tapping in wood. Pat specifies tapping for some of the assemblies and refers to his article in Fine Woodworking #126 (Oct. 1997) "Threading Wood for Machine Screws", p. 63, where he features four sizes for thread holes: 9/64",

13/64", 1/4", 5/16".

I recall from metal working days that taps come in sequence, e.g. "taper", "plug", and "bottom". For an informative and concise summary of tap and tapping, I found the following site helpful:

questions are, all in connection with tapping wood:

1) Do you use a sequence of taps, e.g. a taper and plug, of just use a plug? I was reading at the web site referenced above about the compression of the threads and wonder if using a sequence of taps produces a better thread because each tap helps compress the threads. (The tap photographed in Pat's article looks like a taper tap.)

2) Do you use the recommended drill size for the tap, or try something more or less since we're tapping into wood?

3) Should one strive for American National and Unified Screw Thread system's "Coarse UNC" or "Fine UNF"?

4) What about using linseed oil both as a lubricant and something to help harden the threads -- you'd let the holes dry for a day or two before inserting the screws?

Are entertaining these question making my progress slow?? Answer: yes.

John

[Darrell Feltmate]

[dadiOH]

No reason to do so...just use a taper for easy entry. __________

That would depend on the hardness of the wood. _____________

I'd use coarse, doubt fine would work well in wood. ____________

You don't need a lubricant and linseed - or any other oil - won't make the threads any harder (the wood is harder than dried oil). What

*will* firm up the threads is cyanoacrylate glue (Super glue)...do your threads, run in super glue to saturate threads, let dry, retap.

[Bruce Barnett]

Some people use a router. See Beall Tools:

[David Merrill]

Plug and bottomming taps are normally applicable only to the tapping of blind holes when it is neccessary, for some reason, to extend the depth of threads to nearly the full depth of the hole. An example would be sight mounting screws on rifle barrels. Where blind holes are not essential for some reason (same example) they are generally to be avoided. Use thru holes instead and stick to taper taps. You might also consider the various type of threaded metal inserts designed to be used in wood, especially if the screw is to be loosened and retightened more than a few times.

David Merrill

[mac davis]

I ordered their spindle tap several months ago and it's still on back order... and we're moving out of the country in a month.. *sigh*

Mac

[fredfighter]

I'd stick with coarse or taps and dies made for wood.

The instructions that came with the wood taps I have say to use a lubricant like linseed oil.

Every antique wooden hand screw clamp I have ever seen in the "as found" condition was coal black, indicating to me that the old-timers used linseed oil on them. If you clean the black off, there is wonderfully patinated wood, typically yellow birch, underneath the gunk.

I would not use a drying oil like _boiled_ linseed oil for fear that it would glue the male and female threads together. Even if it was thoroughly cured, there is a lot of heat generated by friction between the threads so it would not surprise me to see some of the cured oil soften and then reset, like the hot-melt glue used on nails.

I lubricate wood threads with a mixture of light and heavy mineral oil (baby oil and laxative) with parraffin dissolved in mineral spirits.

Since I haven't tried anything else, I can't say how it compares to anything else.

Changes in humidity still cause the threads to bind from time to time. A wooden dowel (or hole) that is circular at one ambient humidity is not circular at any other.

[Roger Shoaf]

I second this advice. They make some real nice threaded inserts for wood. Since you are making a machine tool part, the ability of the screws to cinch down tightly makes this desirable.

Why scrap a part because the quaint wooden threads fail while you are working?

[dingbat]

plug is the same as bottom (at least in the UK) The sequence of three is "taper, second, plug"

Second should be used for starting small holes in soft materials, such as small holes in aluminium sheet or wood of almost any size. Taper works fine if you do it right, but the small amount cut with each turn makes it all too easy to strip the thread out while you're cutting it.

A coarse thread is essential, and some thread forms are mrginally better than others. Whitworth (UK) or UNC (US) are good choices in hard maple and the like. BSF, UNF and the typical metric threads are no use. Dedicated wood threads are even better and important if you're using softer timbers, such as beech. OTOH, Whitworth / UNC have much easier availability, particularly if you're trying to make one component in metal.

Other thread forms like Acme or buttress work great (especially for clamps), but they're hard to cut with a tap or die, because of their sharp corners. You might do well with a pair of taps (the first taper/second tap also has the corners radiused) and cutting the male thread with a single point tool instead of a die-set.

Some hard timbers, like hickory, are poor for thread cutting. They're strong in bulk but they're too "stringy" to give a good thread form. Others, like softer but fine-grained tropicals, are surprisingly good - as any turner can tell you. It's worth reading Holtzapffel for advice here.

Large threads in hard maple can usefully be lubricated on cutting, just to keep temperatures down and stop the tap sticking (beeswax). It doesn't seem to help cutting much, but it certainly reduces the turning force.

Cyano afterwards is worth it too, particularly if you have vacuum available (run the tap through again afterwards). Any thread in wood is a bit of a "dog on its hind legs" situation and any help is worth having.