Threading wood - breaking

clarnibass · Aug 3, 2017

Hi

Asking for a friend.

He is trying to thread blackwood 32TPI. Diameters I think are about 20mm and 25mm. What happens is it starts fine for the first couple of cuts, then the wood breaks and crumbles. Small cuts of about 0.001"-0.002".

Thread is 55° and compound is at 27.5° (I suggested maybe try closer to 27° in case it's slightly more than 27.7° but probably not he issue anyway).

He tried various speeds, from 120RPM to 400RPM and also turning the chuck by hand.

Some suggestions were to freeze before cutting, that the wood is too dry or problematic for this, take large cuts, or cut "by hand" with a thread chaser or other tool. Another idea was to use a thread mill cutter on a rotary tool.

Any suggestions other than those? Somehow to be able to do it with the regular threading tool?

Thanks

litlerob1 · Aug 3, 2017

clarnibass said:
Hi

Asking for a friend.

He is trying to thread blackwood 32TPI. Diameters I think are about 20mm and 25mm. What happens is it starts fine for the first couple of cuts, then the wood breaks and crumbles. Small cuts of about 0.001"-0.002".

Thread is 55° and compound is at 27.5° (I suggested maybe try closer to 27° in case it's slightly more than 27.7° but probably not he issue anyway).

He tried various speeds, from 120RPM to 400RPM and also turning the chuck by hand.

Some suggestions were to freeze before cutting, that the wood is too dry or problematic for this, take large cuts, or cut "by hand" with a thread chaser or other tool. Another idea was to use a thread mill cutter on a rotary tool.

Any suggestions other than those? Somehow to be able to do it with the regular threading tool?

Thanks

If you are determined to use a threading tool, I would run it much faster, and use Beeswax between each cut. Turning it by hand is going to be inconsistent and the slightest hesitation is going to give the tool a chance to bite into the wood, and cause breakage as you explained. the angle of the compound is only relevant if you are using it to feed into the part at that angle. If you (or your friend

) are doing that then 27.5º or 27º is okay just not more.

EPAIII · Aug 4, 2017

So you are trying to cut a 32 TPI thread in wood. On a metal lathe? You say nothing about the tool being used. I would think that a tool that was designed to cut threads in metal would not do a very good job in wood. Assuming that your grain runs along the screw axis, you will be cutting across the wood fibers, leaving end grain exposed on both flanks of the thread. That means the thread will be weak and the cutting forces generated will likely tear the wood fibers apart.

Wood cutting tools usually have a very different shape from metal cutting tools. They are designed to cut the wood fibers like a knife while metal cutting tools more or less plow through the metal which is a lot more ductile than wood. Just look at the geometry of a wood chisel. Or a wood saw blade tooth.

I would think a router bit or an abrasive wheel that is dressed to the thread angle could be used to mill the thread.

Another way may be to burn them in.

Personally, I would think about using a metal cap on the areas where the threads are needed and thread the metal. 32 TPI threads in wood are just not going to last very long. You may even rub them off on the first attempt to assemble them. Or plastic.

mike 44 · Aug 4, 2017

32 TPI is quite fine for wood. I have threaded 1/4-20 with a bolt instead of a tap. Most of the wood threads I have cut were coarse, 3 TPI ,5 & 6 TPI.
These were for wood vices. Generally I did not need to do anything special with the coarse threads in beech or maple. Red or white oak I used a 3lb cut of shellac on the work before threading. Also I used a hand held die instead of the lathe. The lathe only was used to hold the shaft
Shellac may help your friend if 32 TPI is specified.

boslab · Aug 4, 2017

I read somewhere about soaking the wood with PEG,polyethylene glycol, apparently they do that for turning green wood too
Mark

gbent · Aug 4, 2017

Roll taps work fine threading holes in wood, especially when a cut tap size drill is used. Thinking the same way for your situation, I think I would make a proper sized female thread in steel and try to screw that on to form the threads. Use a small enough starting diameter there are sharp peaks on the threads. Split it so it is adjustable.

I don't think the finished threads will have much strength.

Jim Williams · Aug 4, 2017

As reinforced by others, 32tpi is very fine to try to thread wood. I suggest a 60 degree included angle wood gouge sharpened to a super quality edge, which should cut rather than scrape. Another approach would be to use something like a tool post grinder equipped with a multi tooth 30 degree angle cutter.

gwilson · Aug 4, 2017

I rigged up a mount to hold a router at 45 degrees to the wood This method will cut completely clean threads,especially in dense,hard woods like blackwood. This cuts PROPER 90 degree threads,not the 60 degree threads used in metal.Wood threads are more blunt than metal threads.

I can't find any pictures of threaded wood projects I've done. But,here are 4 wood taps I have made out of drill rod,hardened.Some times I'll make the taps at 60 degrees so they will bear on the male threads only at their roots,helping prevent breakege of the tops(crests) of the wooden thread.

rbent · Aug 4, 2017

I've had luck with supergluing the ID of the hole and letting that cure, then tapping it.

gwilson · Aug 4, 2017

Ivory can be threaded on a metal lathe,but it requires a VERY SHARP tool,and only .002" cuts at a time.

Scottl · Aug 4, 2017

gwilson said:
I rigged up a mount to hold a router at 45 degrees to the wood This method will cut completely clean threads,especially in dense,hard woods like blackwood. This cuts PROPER 90 degree threads,not the 60 degree threads used in metal.Wood threads are more blunt than metal threads.

I can't find any pictures of threaded wood projects I've done. But,here are 4 wood taps I have made out of drill rod,hardened.Some times I'll make the taps at 60 degrees so they will bear on the male threads only at their roots,helping prevent breakege of the tops(crests) of the wooden thread.

For that fine a thread, and especially if they must be 60° threads, IMO the best way is to use a rotary cutter similar to this and size the wood with ultra-thin super glue before the final passes.

Joe Rogers · Aug 4, 2017

May also want to post a similar question in the Woodworking forum here. Members there might not see this post and add to the solutions.
Joe

Tony Quiring · Aug 4, 2017

If threads need to be used more than once it also matters.

One can just make a die with a taper that will compress form the threads and it may work.

We have just threaded standard nuts on and got away with it.

One also could mix some epoxy and thin it with acetone to where it can wick Into the wood to give it some strength as well.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk

stephen thomas · Aug 4, 2017

Pool cue makers standard is a router on a metal lathe. Some are making wooden M -F matching thread for butt extensions, break cues and similar. I believe the best snooker cues, if they are more than one piece, use wood - wood joints. Some, like me, only do the F joints for a G10 or a metal pin joint.

However, most all of these are cut with a router on the lathe compound.

I also do what several have mentioned : cut the joint, then saturate with the thin superglue. For F joints I spin the lathe while the glue sets, to wring out the excess, and to hopefully add centripetal force to the penetration factor. Then I re-cut. I do use a shop made tap, but it is 10tpi for a radial joint pin. (meaning the threads are shorter than a normal 60° metal thread, and the pin bears on the cylindrical lands between crests for better alignment)

Also as other said, from a technical standpoint, 32tpi in wood does not make much sense. But with care, a router, and superglue or epoxy, I believe "adequate" threads could be made.

I do thread African blackwood with regular (metal working) gun taps for installing metal hardware directly in the wooden tapped hole, for the infill planes I used to make. The finest thread size I use on them was F 20 tpi into wood.

smt

richard newman · Aug 4, 2017

This is the sort of thing ornamental turners do all the time, and african blackwood is a favorite with them too, because of it's density, uniform consistency and lack of pores. Perhaps you could find a forum for ornamental turning, or a website with info. Just google it.

For those unfamiliar with ornamental turning, it's pretty fascinating stuff. Was a hobby for royalty and the weakthy in the 19th century, the work is very complex and the machines are fascinating.

SteveBausch · Aug 4, 2017

Find another material that won't show up in metal detectors, 32 tpi in most woods is a fool's errand.

PeteM · Aug 4, 2017

The other suggestion might be to use a metal (brass etc.) threaded insert and/or ferrule.

One assumes a threaded joint is meant to be disassembled -- and as others have said, 32 tpi is problematic. Convert the wood to plastic (e.g. the cyanoacrylate glue trick) and you have a better chance.

Even with a sharp tool and/or thread milling, it will be far too easy to damage the thread later on when dis/assembled. Changes in humidity, cross-threading, etc. could play havoc with a thread that fine.

Chip Chester · Aug 4, 2017

Beall Tool in Ohio has some process options, and tools. (Not necessarily for 32 tpi though.)
The Beall Tool Company

Chip

SteveBausch · Aug 23, 2017

OP hasn't been on site since starting this thread.

Threading wood - breaking

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