Yet another Delrin Question, Help Needed!

[bvartist]

A question for everybody that uses delrin for tenons. I've had 7 tenons fail recently. Although the tenons won't pull out of the stem material, they will twist slightly in the mortise in the stem material when I insert the tenon into the stummel. I think the problem started occurring when I changed forstner bits to drill the mortise. The one I was using drilled a mortise that I thought was a little too loose. So I got one that drills a nice, snug mortise, but am having tenon failures.

Here's my procedure, so maybe someone can make some suggestions on what I need to do to help eliminate this problem.

1) I drill a 3/8" deep mortise in the stem material
2) Use a 1/16" engraving bit in my Dremel to cut grooves on the inside of the mortise.
3) Face off and drill the airway in a length of delrin
4) cut 3 grooves approximately 1/2 the wall thickness of the delrin in the bottom 1/4 inch of the delrin with a skew chisel.
5) I use a small, flat needle file and file 3 flat surfaces in the center groove to form somewhat of a triangle
6) I spread epoxy on the inside of the mortise and the outside of the delrin and press the two pieces together in a vise, and allow the epoxy to cure usually at least overnight sometimes longer.

I originally used Devcon 5-minute epoxy, then after the first couple failures I switched to Devcon 2-ton epoxy thinking the stronger shear strength would solve my problem. After experiencing a couple more failures, I changed epoxy yet again to System III T-88 which is supposed to be one of the strongest available. I've had 3 additional failures since switching to the T-88 epoxy so I'm back to thinking its something in my procedure causeing the problem.

Possible causes.... My 5/16" forstner bit drills a mortise that is between .3115 and .3125 diameter, and most of the delrin I've measured is between .313 and .3135 diameter. Could my mortises be too tight? They are snug when inserting the stem. And it is when inserting the stem that the tenon will rotate slightly in the mortise of the stem material. However, I wasn't happy with the tenon fit in the stummel when I used a forstner that drilled a .313 mortise, seemed too loose to me. Also would filing additional, or larger flat surfaces in the delrin give me a stronger bond? Or do I need deeper grooves in the mortise to create a better epoxy “lock”? I know the epoxy won’t actually bond to the delrin, that’s why I’m looking more at how and where I cut grooves as a possible cause.

What do you guys use to cut grooves inside the mortise of the stem material? I read one post that someone used a modified saw blade.

Any help or suggestions you can give me will be greatly appreciated.

Thank you in advance for your help. Much appreciated.

David

[JSPipes]

I know it's a little unconventional, but I'm using a high grade CA glue instead of epoxy. Haven't had any trouble and all I do is rough up the surface of the delrin and inside of the mortise (in the stem) with 100 grit sandpaper. Nothing more complicated than that.

I kept the first couple I made to test. It's been 10 months now and no trouble at all. I pull those 2 pipes apart fairly often to work the joint.

Basically, I found the epoxy to be too flexible.

Hope this helps.

[smokepiper]

What´s CA glue? i use araldite from the SAAB Faichild space-center it´s epoxy that is used in air planes. 24 hours before harden and if you heat the epoxy 6 hours after application you make it even stronger. But i need to make grooves and you don´t with CA glue so you made me curious...

[bvartist]

CA cyanoacrylate (sp?) glue, or commonly referred to as "super glue".

I'm curious too, which glue do you use Joel? USA GOLD? Some CA glues have a tendency to melt some plastics so I'm curious! Sure would save some time though.

David

[smokepiper]

Super glue, i have tried a number of those with no good results. Delrin is to greasy in it self so glue doesn´t get the grip.

[jeff]

It sounds like you're experiencing this problem because of the way that you choose to groove your delrin. I assume, by your description, that you are doing so on the lathe, and the grooves are perfectly straight around the delrin. If this is the case, I believe that you need to change your grooving method. The fact that Delrin is self-lubricating prevents the epoxy from properly bonding to it, regardless of the way that you create your grooves. What you've essentially done with the method you've adopted is create a perfect mold of the delrin in which the delrin can actually move. There is little chance that it will come free, due to the hardened epoxy, but it will continue to spin. To overcome this problem, I'd suggest creating additional grooves perpendicular to the ones you are already creating in the delrin, to prevent the spinning from taking place. This will effectively create a mold again, but it will be one that prevents the delrin from actually shifting.

I hope this has been clear and that it makes sense to you.

Best,

Jeff

[smokepiper]

after making the grooves in the lathe i took a knife and cut the delring perpendicular to the grooves, i might have to work it harder to make it work...? Thanks for your advise Jeff you made yourself perfectley clear, all the way to the perpendicular word that i had to look up. So, you tought me two things, thanks!

[KurtHuhn]

(warning, lots of images)

This is not the only way, but it is what I've used for a few years now and have not had a failure yet. I was this close *pinches fingers together* to finally replying to your email, but I'll post it here for all to see.

First, start with some grooves cut in the tenon on the lathe:


Then, take a triangle file like one of these:


And cut some gouges in the grooves with a vertex of the file like this:


Here's a photo of the finished tenon:


And another:


Next mix up some epoxy and smear a dab inside the mortis in the stem:


Then. smother the tenon with the stuff:


I press it in with my drill press, which results in epoxy ending up all over the stem face:


I clean this up with a paper towel. I tcleans up pretty easily:


You use this method on rod stock or factory stems:

[bluesmk]

My only failure in this proceedure, was with CA glue. I use T88 exclusively these days. I also used a vise in the begining, and let it cure overnight. Many times I noticed a slight cracking sound when I opened the vise.......to much constant pressure. I now chuck the stem up in the headstock, and use presssure from the tailstock for and hour or two....back off, and let it cure overnight usually, but I have worked it as soon as six hours later.
My grooves on the stem I use an old mini tap, by hand, grooves on the tenon with a dremel engraver, nothing realy uniform about them.
Dan
Gabrieli Pipes

[marks]

I cut two grooves around the tenon, then connect them with three grooves going with the tenon. That way, I have grooves in both directions. I do the same in the mortise.

I use devcon two ton epoxy, and make sure that I get epoxy in the grooves with no air pockets. When inserting the tenon into the mortise of the stem, I press against my workbench with hand pressure, then let it sit. No vise involved.

I have had only one failure, and that before the pipe left the shop. I don't think I did a very good job of spreading epoxy into the grooves on this delrin and stem. I have pipes that I made up to four years ago with no longitudinal grooves, and no failures. I have yet to have one come back from a customer, and I have a few customers that smoke their pipes and take off the stems on a very frequent basis.

Surface area seems to be the key, and the rougher the better to get the epoxy to help secure the delrin. Todd Johnson posted in another post about roughing up the surface of the delrin. Next time I make a pipe, I may try that in addition to the grooves.

[bvartist]

There is little chance that it will come free, due to the hardened epoxy, but it will continue to spin.

I hope this has been clear and that it makes sense to you.

Best,

Jeff

Thanks Jeff, perfectly clear! I know the tenon won't come out easily! I removed one to see if I could figure out the reason this was happening. The only way I could get the delrin out was by clamping the stem in a vise, and grabbing the tenon with vise grips and twisting the heck out of it! By the time I got the delrin out it was so buggered up I couldn't really tell what was causing the failure.

Thankfully so far all my failures have been while I was working on the pipes. Or at least no customer complaints yet!

I use a procedure very similar to Kurt's, except that I didn't file as many flat surfaces in the grooves as he does. Which I think may be the problem! And something I was wondering about.

Thanks for the pics Kurt. Very helpful! I hope you didn't think I was getting impatient for your reply to my email. I really appreciate you taking the time to photo your procedure.

Thanks to everyone for their input! I've got a couple ideas what to try next. I appreciate the help!

David