Using Delrin Tenons

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The Kurt Huhn Method

Intro

The basis for this article originally appeared as a sticky on the Pipe Makers Forum, and written by Kurt Huhn:

First, decide on your tenon diameter. Most folks will use 5/16", but 3/8" and 1/4" are not uncommon.

Next, find a drill bit that's going to give you the right size hole for your delrin. As any machinist will tell you, 5/16" is not 5/16" from one drill bit or piece of stock to the next. In this case, you can count on the delrin being the same size, as these are manufactured to rather close tolerances. Like it or not, you're going to need to experiment on briar. other woods will not react the same as briar to drilling, and will not give you an accurate gauge of hole size. Scraps of briar are fine, as long as you can drill them using the same methods you intend to use when you make a pipe.

Have a selection of drill bits handy. Use bits from different manufacturers, as one manufacturer may have different tolerances than another. The design of the drill bits may also play a part in making a hole that's final size is different from other bits. Drill with these bits using the same methods of pipe manufacturing - the methods you use will be important in determining final hole size.

It's important to note that you MUST clear the wood chips frequently while drilling the mortis. If the wood chips build up, it will generate excess heat and pressure which will change the final hole size as those wood chips press against the sides of the mortis.

Once you have a drill bit that you know will provide a consistent hole size, you're ready to start using it. Be aware, that the bit that provides the right hole size for briar, may be to small for drilling a hole in vulcanite or acrylic. You'll need to be sure of your drill bits before you start.

My procedure is as follows:

Drill the briar:

- chuck the block in and line everything up along my construction lines for airway, mortis, and  
  tobacco chamber. I drill the mortis/airway first.
- set the spindle speed for 500RPM or less.
- face the shank with a 1-1/4" precision Forstner bit. I take care not to use to heavy or 
  aggressive a hand in this step. You want to avoid grain tearout.
- drill the mortis, about 3/4 of a turn of the tailstock at a time. This works out to about 1/12 
  or so of an inch for each advancement. After each advancement, I remove the bit completely 
  from the hole, and clear all the dust and chips. The depth is roughly 1/2" inch when the 
  mortis is completely drilled.
  (at this point I continue on with the airway and tobacco chamber, that's not covered here)

Drill the stem:

- I chuck the rod stock and face with a 1-1/4" precision Forstner bit
- I drill the stock somewhat more aggressively than the briar. Vulcanite, Cumberland, and 
  acrylic are more stable than briar under heat and pressure while drilling. Don't go too nuts 
  here, because you can burn the mortis in the stem if you use too much pressure or speed.
- total hole depth in the stem material is about 1/4" to 1/2" depending on stem shape.
- I also use a bit that gives me a flat bottom in the stem mortis. This way the amount of epoxy 
  in contact with the smoke is minimal, and the opportunity for a void at the end of the delrin 
  is minimal.
- continue to drill the airway in the stem

Drill the airway in the delrin:

- Slice off a length of delrin approximately the length I need. I usually go about an 1/8" 
  longer just to be sure I can bottom out the tenon in the mortis in the stummel.
- I set the spindle speed to about 1200 RPM to drill delrin. Delrin is extremely stable under 
  heat and pressure, and is self-lubricating. This also mitigates any risk you might have of   
  using too much forward pressure and setting the drill bit off-course.
- I do not face the delrin
- I use a short drill bit, or a jobber drill bit with most of it's length swallowed up in the 
  chuck. This helps prevent the bit from wandering.
- after drilling, I use the tip of a skew chisel to cut 3 or 4 grooves in the delrin where it 
  will be epoxied to the stem material. This is necessary, since delrin will not take adhesives 
  very well.

Put the stem and delrin together:

- first, I take a triangle file and slightly deepen the grooves on at three points around the 
  circumference. Do not depen the grooves all the way around. The goal is to get a slight   
  triangle shape in the grooves so that the delrin will not rotate in the stem once the epoxy 
  has cured. Just a couple swipes of the file in each groove at 120-degree intervals should be 
  quite sufficient to hold it.
- mix up some epoxy (5-minute, 15-minute, 24-hour, whatever) with a toothpick
- use the toothpick to put a dollop of epoxy in the stem mortis
- spread epoxy all over the outside of the delrin. Be sure to get epoxy in the grooves and get 
  all the big air bubbles out.
- insert the delrin tenon into the mortis. Epoxy will go everywhere, this is normal.
- at this point you can either wipe up the excess epoxy with a rag or paper towel, or you can 
  wait a couple minutes and just peel it off when it becomes rubbery. Be sure to get it all, 
  otherwise your stem won't fit right.
- after the epoxy has fully cured, you can remove the epoxy that got into the airway by  
  carefully re-drilling the airway. I do this by hand, holding the stem and with a tapered drill 
  bit turning on the lathe at 500RPM. BE CAREFUL!!
- now you can sand or file off whatever excess delrin there is so that you get a nice tight 
  stem/shank meeting.

====Conclusion====: That's it in a nutshell. Sounds like a lot of work, but it goes rather quickly for me as I tend to batch most of that by doing 3-4 stems at a time. Way less time than trying to hand turn tenons on a wood lathe. I also prefer the buttery feel of delrin as a tenon over vulcanite - but that's primarily a personal preference.

Revised method

Intro

The following outlines a method used allowing the stem and Delrin to be drilled together after the two are assembled: As with any aspect of pipe making, there are many ways to achieve good results with Delrin. The following method is constantly evolving, but may prove helpful to some in developing their own:

====Preparing the stem====: Note: If convenient, consider leaving the stem material in the lathe for the entire process (glue up and drilling). This prevents having to re-center it for drilling operations. Multiple sessions where the stem is in and out of the lathe can introduce problems, even with self-centering chucks.

Prepare the stem material in the lathe by first roughing in the diameter of the stem rod by turning it down. Then face the end as you would normally. The following pics show that being done with a Forstner bit:

Inlayed stem forstner.JPG Inlayed stem facing.JPG

Drilling

The diameter of the Delrin used for the tenon will largely depend on the pipe's final stem and shank diameter. It is critical that your drills match the Delrin you're using, especially with regard to the fit of the tenon in the stummel mortise. Both drills and Delrin very slightly, so some experimentation with speed, feed rate, and different drills, even of the same marked size will result it the best fit. Once you find a good drill, make sure you keep track of it. It may be advantageous to use a different drill for the stummel than the one used for the stem material.

Inlayed stem drillingforDelrin.JPG

Preping the Delrin

Next measure and cut the section of Delrin to be used. Make sure you leave extra length to allow for trimming it to perfectly match the depth of the mortise. Here we are also gluing up insert material, so that is also accounted for in the length:

Inlayed stem measuring delrin blank.JPG

Round the end of the Delrin to match the bottom of the hole drilled in the stem material. This will minimise any voids and pockets of epoxy:

Inlayed stem rounding Delrin.JPG

====Glue up====: First dry fit everything to ensure your fit is good:

Inlayed stem dryfitting.JPG

Roughing up the Delrin, and the hole in your stem material will greatly improve the bond between the two materials:

Inlayed stem roughing hole.JPG Inlayed stem roughing delrin.JPG Inlayed stem Delrin ready.JPG

Then mix up epoxy and coat both surfaces of both the hole of the stem and the end of the Delrin (as well as both surfaces of any insert material being used):

Inlayed stem epoxy.JPG Inlayed stem applying epoxy.JPG

Use a thin even film of epoxy. Rotate and press all the materials against each other to release any air bubbles and squeeze out any extra epoxy:

Inlayed stem glueup.JPG Inlayed stem glueup2.JPG

Carefully wipe away any squeeze out off of the tenon (the squeeze out around and insert materials can be easily trimmed when it's dry, but the tenon should stay as clean as possible). Then use the trimming tool in the cross carriage of the lathe to press the insert materials against the main stem and to each other and lock it into position. Use something chucked into the tail stock to press the Delrin firmly into the stem while the epoxy sets, and lock it into position.

Inlayed stem clamp.JPG

These serve as excellent clamps. If possible, let all this sit overnight, or until well cured (5 minute epoxy will speed the set time in terms of clamping, but will still not be fully cured for some time. It is risky to drill and fit the stem assembly until it is fully cured). When ever possible, I leave the assembly in the lathe through the entire process. During cold or damp weather I leave the light down close over the assembly (as shown above) while the epoxy is curing.

to be continued