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I used some brass inserts like the ones pictured for a project, and incorrectly assumed that the slot was for a screwdriver(IT IS NOT)
The insert should be held with a threaded screw and a nut on the plain end, and the slotted end inserted into the hole in the wood. The slot forms two cutting edges when the insert is turned into the wood via the screw and nut.

Good point Ray--I made that assumption too, but quickly found that a screwdriver did a great job of tearing up and cracking the insert and a lousy job of getting the insert into the hole!

The steel inserts I just tried have little holes in the side that serve the same purpose as the slot in the brass ones.

Dave

[QUOTE=dsharp] I've looked into McMaster and actually tried some of their steel inserts with disastrous results (splitting). So, I'm hoping brass will be a bit better. I'm assuming I want a coarse knife-edge outer thread? Or, am I better off with a finer thread?

Thanks for the help![/QUOTE] Don,
Two things that might help, but you are probably already doing.   
1) Make sure your drill size is correct. Too small and it will crack, too big and it won't hold. Do tests on a scrap pieces to determine the proper size.
2) Use lubrication when inserting the insert. I coat the hole and insert in adhesive.
I've used fine threaded brass inserts, but have changed to a stainless steel insert from McMaster Carr with more thread diameter. Check out item number 95807A300 here. They are real nice. I make 5 piece necks so I'm not too worried about strength. I often use maple, and don't have any trouble with cracking with these inserts.
Those are 8-18 SS, so you might want to use the same type screw to avoid galvanic corrosion. I would use a little light grease to help stop galling and slow any corrosion.

_________________
"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

OK.  I did another test using the self tapping brass inserts as shown in Ray's picture--same test as I did earlier with the fine thread steel ones.  5/16" pilot hole for the brass inserts, slot side down. 

Please note that a lot of the inserts that you find have the slot cut into an un-threaded part of the insert.  These are NOT self threading--they compress the wood like a wood screw does, and don't hold very well in hardwood from what I've seen.

Anyway, the self tapping brass inserts are a little bit harder to get coaxed into entering the hole absolutely straightly (I think that John's long installation tool would make life easier), but the coarser threads definitely hold better than the fine threads.  The brass stripped out before the wood gave way (same piece of bigleaf maple endgrain).  In fact, the wood never gave way with the brass insert.

I think that either insert will be plenty strong in maple, but the coarse thread brass I believe is the better option.  For for softer woods like mahogany, I think that the coarse threaded inserts would make even more difference. 

Self tapping is the main thing though.  Those "thread forming" inserts just aren't very good in hardwood from what I can tell.  Tapping the threads and removing some wood instead of compressing the wood around the threads is just a lot stronger in hardwoods.

Dave

Sorry--one more post here on the subject.  Here's a link with a good explanation (at the bottom of the page) about the difference between thread forming and thread cutting.

[QUOTE=John Watkins] I've tried them all and the fine threaded brass, non-taper, non-locking have worked the best for me. I've probably used them on about 600-700 necks and have never had one split or pull out. They cost more than the hardware store variety, but it's worth it when you consider the costs associated with a failed neck.

My procedure is to bore the hole to the absolute minimum diameter (.338") and then tap it with a 9" long 3/8-12 tap that I made for this purpose. I tap half depth for Mahogany, Walnut etc. and full depth for Maple and Rosewoods. I then install the inserts with a 9" long installation tool. The length assures that it ends up straight.

One drop of thin CA for insurance and you're done.[/QUOTE]

John, I noticed the inserts you use have a thinner wall than those i have been purchasing. Can you recommend a source and part number for those you are using?
TIA

_________________
Dave Bland

remember...

"If it doesn't play in tune...it's just pretty wood"

HI All: Thanks for all of your input. I appreciate it!!

After much confusion about which inserts would work best for me, I ran my own tests on three different styles available from McMaster-Carr: 1) 90192A117;2) 90240A001 and; 3) 99362A700

First I must say that I wouldn't recommend insert # 1. The threads aren't truly self tapping but are thread forming. I used these first and noticed that they totally destroyed the wood falling in between the insert threads. Since these don't really cut the thread, the wood is more or less compressed exerting unnecessary stress on the surrounding wood matrix resulting in splitting. These are the one's I had problems with prompting the original post.

That said, I tested the remaining two identically. First I tested both in tapped holes. To do this, I made up several mock neck heels with tenons routed. For both inserts I first drilled 5/16" holes in the tenons and then tapped these holes to accomodate the threads on the inserts (3/8-20 for insert #2 and 3/8-16 for insert #3). Next, I counter sunk the holes slightly. Finally, I installed the inserts. No problems!

Then I wanted to see how both inserts installed into untapped holes. So, I drilled 11/32" holes for insert #2 and 0.339" (Size R drill bit) for insert #3. The first thing I noticed is that it was a bit difficult to start both inserts and keep them straight but, with a little patience and some pressure, in they went.

Finally, I wanted to see what it would take to pull them out. So I made some mahogany neck block mock ups, placed 5/8" washers between the tenon and neck block and passed a 1/4-20 bolt through a belleville washer and block and 5/8" washers into the insert and began cranking. The result was that the belleville washers flattened and dug into the mahogany neck block without pulling the insert out of either the tapped or untapped holes.

This was enough to convince me that either insert is more than adequate for this application. I would definitely recommend tapping the hole prior to installation. Also, slightly countersinking each hole allow for easire starting and allows you to install the insert below the bottom surface of the tenon.

Just a few notes, I used 5-piece heels/tenons for the tests (Mahog/Maple/Ebony/Maple/Mahog). I made my own installation tool from a bolt and nut because I didn't want to pay the 104.00 price tag from McMaster. Finally, these are just my observations and opinions. I hope some benefit from this.

Thanks, Don

I don't know why but the links to the parts aren't working. I probably didn't do something right. Sorry!!

I've used the tapered steel inserts with the large threads. They have held up just fine. I put a hard wood dowel into the neck so that the inserts are not biting into end grain.

To insert them, I use my drill press. First, I turn my drill press table so that it is parallel with the direction of the drill bit and clamp the neck to the table. After I line up the drill I drill my hole into the end of the neck. Then, without unclamping the neck, I remove the drill bit an chuck in a straight hex key that fits the hex drive of the insert into the drill press. I place the insert onto the hex key and lower the insert until it just touches the hole that I just drilled. WITHOUT TURNING THE DRILL PRESS ON, I turn the drill by hand and pull on the quill thus applying pressure while threading the insert into the hole.

This is a very controlled process and the inserts cut into the holes nicely.