Replies

1. 
   Telemike | Jun 11, 2003 08:31am | #1

   Drill the hole a smidgen larger than the INSIDE diameter of the threads.  No more than about 1/4 of the height of the threads.

   Use a file to cut a groove in the tip of a 1/4 x 20 bolt (along the length of the bolt at an angle, not accross it) so that it forms a crude cutting edge that will machine the threads.  The cutting edge should be as close as possible to on a radius line so it cuts OK.  Then, turn it into the wood.  You'll have to press it in to start it, but then it will go OK.  Use paraffin to lubricate the "tap" if you want.

   The maple will hold threads very well, especially if it's hard maple.  The walnut won't do quite so well, but it will also hold threads pretty well.  And figure on threading the bolt in as far as possible because the hold isn't quite as good as steel.

2. 
   Andrew | Jun 11, 2003 07:11pm | #2

   The minor diameter of the thread is what you need to know. there are charts that will tell you what size drill to use. For 1/4 20 the approximate minor diameter is .200". For wood, a 3/16 bit should work fine. Standard metal taps work fine in hard woods, however, using a bolt also works well as it compresses the wood around the threads, making a nice clean thread.

3. 
   Midnight | Jun 11, 2003 11:59pm | #3

   it may be better to use helicoil inserts to partially overcome the softness of the material you're tapping into. These inserts are helical springs, having both an internal and external thread; the internal thread being matched to the bolt size you want to use.

   To fit them, you'll need the correct sized drill bit, a tap specially oversized to suit the external thread and an insertion tool.

   They're a bit fiddly to use until you gain practice, but well worth the effort. Should the thread ever become damaged, simply pull out the insert, re-tap the hole and fit another insert.

4. 
   WorkshopJon | Jun 12, 2003 12:43am | #4

   Butch,

   Have you considered using a threadsert instead? They are thin wall serrated brass inserts that are pressed into a drilled hole, usually flush with the surface. add one drop of water thin cynoacrylate after pressing (tapping) them in and they are going nowhere. 20X stronger than a hole tapped in wood.

   Jon

1. 
   simchart | Jun 12, 2003 06:44am | #5

   Yes, I have used the thread insert quite a few times.  I thought the idea od tapping wood was less than practical anyway.  I was planning to make the router fence featured in FWW#144.  After checking the rather high priced parts from the listed suppliers, I believe I'll just modifiy the plans a little.  Thanks to all that posted.

1. 
   Steinmetz | Jun 12, 2003 07:30am | #6

   Butch, Have you ever heard of 'T' nuts?

   They fill the bill for various assemblys that must be 'taken down' or, to receive thumb screws or turns.

   My home- built router table has multiple 'T'nuts because hex nuts may loosen due to vibration.

   They come in lots of standard thread sizes like:10-24 ,10-32, 1/4-20, 1/4-28, 5/16-18, 3,8-16and 1,2-13.

   They have three or four 'spurs' which when driven in with a hammer, (Or vise) will not 'spin'.

   Home 'Despot' sells them by the piece or in packs.

   Once you use them, you'll think of dozens of other uses. Stein.

2. 
   Jamie_Buxton | Jun 12, 2003 05:25pm | #8

   C'mon, folks, tapping wood works just fine if the hole goes across the grain.  I've done it a lot.  Get a standard metal tap ($3 at my local hardware store), and go for it. 

   It is true that a threaded hole in wood doesn't have the same strength against stripping out as a hole of the same length in metal.  However, it is still very strong, and if you're concerned you can make your hole deeper.   (More threaded length means more resistance to stripping.)  Furthermore, unless you're bolting down a cylinder head or something, you rarely get near the point of stripping out machine threads.   Try it.   Tap a hole three or four bolt-diameters deep into maple, run a bolt into it, and see how much force it takes to strip the threads.  With a 1/4-20 bolt, you'll find that you can't strip it with the force you'd apply with a hand knob on a fixture.  Heck, try to strip it with a wrench.  You won't be able to.

1. 
   WorkshopJon | Jun 12, 2003 07:58pm | #9

   Just for those who don't know what threadserts and t-nuts are:

   Jon

   
1. 
   SCFrankland | Jun 12, 2003 08:20pm | #10

   Here are some pics of other examples.  The ones Jon post I have not seen in my neck of the woods.  Scott C. Frankland

   "This all could have been prevented if their parents had just used birth control"

   
5. 
   patricks13 | Jun 12, 2003 10:27am | #7

   All good suggestions above. I would go with the T-nuts. Easy, fast, inexpensive and you can re-use them.

    

   You can try a regular 1/4"-20 metal cutting tap in wood but don't put much pressure on the bolt that goes in it.  
6. 
   Trees2dust | Jun 12, 2003 10:23pm | #11

   Butch,

   Jamie is right- machine threads in cross grain are very strong.  If you want to see numbers to prove it, there is a short article in the Lee Valley web site (http://www.leevalley.com), probably written by Leonard Lee.  See Articles->hardware-> Using Machine Screws in Wood.

   Rick