Tapping machine threads in wood

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I saw a video a while back; the Wood Whisperer, I think. He was tapping some holes in wood, for what purpose I have forgotten. He claimed that the results were pretty strong, which surprised me. Machine threads seemed too shallow for wood.
As it happens, I have a set of taps; two actually. I didn't buy either of them. A retired electrician friend of mine gave me a set when I was dealing with yet another no-longer-standard (if it ever was) electrical box buried in my plaster wall.
The other was from my Dad's garage. My Dad's set is an artifact of an earlier age ('60s, probably), complete with a wooden case with recesses for each tap (and each die). The tap recesses have a hole at one end that's a little deeper than the rest of the recess. That's to allow you press that end of the tap down in order to tilt the other end up. Nice. It looks like my Dad might have used one size of tap: 3/8". The rest appear completely untouched.
Anyway, the all that "Whispering" made me curious. I recently reconfigured my dado jig to be width-adjustable. I used a pair of Rockler star knobs and some metal threaded inserts. It works fine, but what a pain it was to insert those ... inserts.
Yes, I know there's a tool, but I don't have it. I used a the widest slot screwdriver I have, but it still slipped out of the little grooves repeatedly, especially before the insert "bit" enough into the wood to keep itself upright.
On top of that, the knobs were a few bucks each. I got to thinking that - for jigs - I might try using machine bolts and tapped threads in wood. At least for jigs that would generally be set ONCE for a project.
I decided to do a test. I bought some 5/16" x 1.5" hex-head bolts. Even at the Home Depot "single unit" price they were only 20 cents each. I could make a lot of jigs for the price of a couple of plastic knobs.
I tried putting threads in Poplar, Oak and Birch Ply. I started out with the drill size that was listed on the inside lid of the lovely wooden case. That worked, and I couldn't pull the bolt out of the wood with a claw hammer, but the crests of the threads looked flat. I assume the recommendation was for tapping metal.
I reduced the hole size a 64th at a time until the threads looked sharp. That was at 13/64". I have to say, that bit, and the hole it produced, really looked too small for that tap, but it worked fine.
I wasn't particularly careful about tapping the holes. I used a drill to drive the tap, started off slow and progressed to a medium speed. It might have taken 15 seconds to tap each hole all the way through a 3/4" thickness.
The bolts fit well, with a small amount of wiggle. I decided to try one more test, which I only did with the Oak. I threaded the bolt into the hole, but left 1/4" of room at the bottom. I put the wood right over the leg of my workbench and hit the head of the bolt repeatedly with a hammer, hard. The bolt didn't move and the threads seemed undamaged.
I'm wondering if any of you ever find a use for tapped holes. If I were building a jig that I knew I'd use a lot, and would need repeated adjustments, I'd probably use the inserts. But this was quick, efficient and cheap. I think I may give it a try for some jigs I'll need to build soon.
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On Saturday, April 4, 2015 at 10:22:18 PM UTC-4, Greg Guarino wrote:


















I can't speak to tapping wood, but I can offer another technique for using bolts in wood with no fear of ever stripping the threads in the wood.
Assuming you have access to the back/underside of the wood, use a spade or forstner bit to make a recess slightly larger than a nut that will fit onto your bolt or threaded rod. Make the recess just a little bit deeper than t he nut, leaving as much wood between the nut side and the bolt side as poss ible.
Next, drill through the wood for your bolt. Coat the threads of the bolt wi th Vaseline, insert it through the wood and into the nut. Be sure to keep a ll Vaseline off of the outside of the nut.
Now fill the nut recess with epoxy, securing the nut into the wood. The bol t will keep the epoxy out of the nut and the Vaseline will prevent the epox y from adhering to the threads of the bolt so you'll be able to unscrew it after the epoxy cures.
We used to use this technique so that we could securely attach axle mounts and weight plates to the floorboards of Soap Box Derby cars. Instead of usi ng real nuts, I made my own nuts out of 1" lengths of 3/4" steel rod. With 30+ "nuts" made from steel rod, I was able to not only secure the parts tha t needed to be secured, but I was able to get extra weight down low where I wanted it. I bolted and unbolted objects (with torques of over 120 in-lbs) hundreds of times without a single issue.
You should scuff up the nuts so that the epoxy has something to key into fo r added strength.
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"Greg Guarino" wrote:

------------------------------------------------------ I have built several jigs using 1/4"-20, 5 point star knobs, 1/4" flat washers, 1/4"-20 hex heads bolts of required length for the individual jig.
A 1/4"-20 hex head is 7/16" across the flats, 9/16" across the points, as well as a flat washer that is also about 9/16".
You drill a 5/8" hole with a forstner bit, 1/2" deep, followed by a 9/32" dia hole of the req'd length to allow the star knob to be engaged.
Stick a bolt with a flat washer into the 5/8" hole and temporially snug up hand tight on the opposite end with another flat washer and a 1/4"-20 nut.
Mix up some epoxy thickened with micro-balloons and pour into 5/8" hole covering washer and hex head of bolt.
Wait a couple of days, remove hex hut with washer and you are good to go.
Have fun.
Lew
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You might want to try the method shown in this video...

https://www.youtube.com/watch?v=tIhEqoKE8Dc&spfreload

Joe
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Don't use a screw diver, use a piece of threaded rod (or a bolt) with two nuts so you can jam them against the insert and use them with a wrench to turn in the insert.
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dadiOH
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Yeah, this is the only way to insert the kind that have a "screwdriver slot". Using a longish rod (or bolt) also makes it easier to put them in straight.
There is a better kind of insert that has a hex recess for an allen wrench for installing it. Those are hard to find, but 1000% easier to use.
John
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FWIW, the ones Lee Valley sells have the hex recess. Might be worth ordering a hundred next time they have free shipping.
Puckdropper
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On Sun, 5 Apr 2015 13:55:18 +0000 (UTC), John McCoy

It's been years since I've seen the "screw-driver slot" type offered for sale around here. Virtually all in-hex or "allen" drive.
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snipped-for-privacy@snyder.on.ca wrote in wrote:

Ah, but you're in Canada, right, where they have Robertson screws and all manner of other high-quality hardware.
The slotted kind are what's carried at Home Depot, Lowes, and such like places.
John
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Yes. I've used various, from 8-32 to 1/2-13.
Cross grain, they work well in hardwood, never tried in soft wood. They can be improved by tapping, putting cyanoacrylate glue in the hole and re-tapping when it is dry.
With the grain, they work less well because the threads are cutting cross grain and are therefore weak and break easily. The glue really helps here, same with threads on dowels.
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I'll second this. We used thin CA to reinforce the holes in balsa and plywood when I used to fly RC airplanes. The hole were amazingly durable. The key is to use the thin stuff so it soaks into the wood.
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On 4/4/2015 10:22 PM, Greg Guarino wrote:

I have done it in turned exotics but not for any of the relatively weak woods you quote. In various rosewoods and desert ironwood it is possible to tap very clean strong holes and the matching threads in ~1/4" sizes to fasten ornamental parts together. This was a very common practice in the 'olden days' when tiny parts were made of ivory and had to be fastened into wood in such a way that they could be removed or repaired. I've not had any ivory to try but it does work fine in solid bone parts.
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On 4/4/2015 10:22 PM, Greg Guarino wrote:

That slot is to cut threads into the wood. I just take a screw and put a nut on it, lock it down and drive it. Never use the brass slot to drive it.

I do this a lot. I recommend some butchers/johnsons wax on the tap. Just dip it in and it will lube the tap. Yes it helps.
Maple taps real nice. But most everything is tapable. Open pore is less desireable as they are rougher.

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Jeff

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wrote:

The slot is on the back end of the insert. How does it cut threads? I agree that using a flat-blade screwdriver isn't the best way to insert inserts. There are tools used to install these things, though I like the threaded rod idea (alignment).
http://www.woodcraft.com/Product/12K55/T-Wrench-for-14-20-Inserts.aspx

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On Sun, 05 Apr 2015 14:16:13 -0400, krw wrote:

I'm chiming in a bit late on this thread, but I was just re-reading a FWW article (#126) on exactly this subject. The author found that the threads were a lot stronger than he thought they would be. A letter in the following issue pointed out the strength increased with the diameter of the bolt. So one could get approximately the same strength as a 1/4" bolt in a threaded insert by using a bolt with the same diameter as the insert.
I've made wooden bolts and wooden screws of 3/4" and 1" diameter for homemade vises and I've never managed to break the threads no matter how tight I crank them. I have popped and/or split the 3" diameter end holding the handle. but the threads were still good.
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This is very dependent on how long the hole is. The force on the thread is taken over the whole linear length of the thread that's engaged - which when you do the math is probably quite a bit more than the length of your handle.
John
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On Sun, 05 Apr 2015 14:16:13 -0400, snipped-for-privacy@zzz.com wrote:

Lee Valley carries tools to insert the slotted inserts, using the slots: http://www.leevalley.com/en/hardware/page.aspx?pp791&cat=3,41306
They only have the inserts and drivers for 1/4-20 and 5/16-18.
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peterbb (at) telus.net
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On Mon, 06 Apr 2015 10:49:00 -0700, Peter Bennett

I think using the treads is a better idea. I like the threaded rod and jam nut idea too (for alignment), but that requires a through hole.
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snipped-for-privacy@zzz.com wrote in

Why do you need a through hole? Use the threads in the insert as one nut and install another on top of it. I usually make my inserts flush or sunk slightly low, but if you want to really sink the insert you may need to make sure the jam nut is smaller than the insert.
If you need to turn the insert from higher up, use a longer length of threaded rod and a second jam nut assembly. If you can't reach the jam nut on the insert, you may try holding the lower nut on the upper assembly with a wrench and giving it a quick twist in the loosen direction. (A slow one will remove the insert.)
I've put in a couple dozen hanger bolts (wood thread on one side, machine on the other) using jam nuts. They can be used to not only insert the bolts but remove them as well.
Puckdropper
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On 07 Apr 2015 02:56:27 GMT, Puckdropper

If you want a leader on the rod to help guide it into the piece, the hole has to be deeper than the insert (or through). If you don't want the leader, the threaded rod has no advantage over the tool. That was the point.

Not higher up. The proposal was that the treaded rod be used to guide the insert so it goes in straight.

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