MT 2 Dowel Collet
Design by Jeff Willaford
One method for making bottle stoppers uses a 3/8” dowel glued into the blank to hold it during the turning process. A cork bushing is eventually placed over the dowel to form the sealing surface for the completed stopper.
This method assumes you have way to grip the dowel. Some scroll chucks have jaw sets available that will grip a 3/8” dowel. Metal collets are available that are ideal for this task, but they are expensive. This article describes a shop-made collet that works surprisingly well, and it costs practically nothing.
The Morse taper of the headstock spindle is used for the body. A drawbar runs from the handwheel to the collet and is used to pull the collet into the taper. This closes the collet around the dowel and holds it securely.
If you have a 1/4-20 drawbar that you use to hold a Jacobs chuck in the headstock spindle, and if you make the length of the collet approximately the same length as the Morse taper on the Jacobs chuck, you can use the same drawbar for the collet. Otherwise, you will have to make a drawbar. Instructions for making a drawbar are given farther down in this article.
1. Mount a piece of spindle stock about 3.5” long between centers and turn it to a diameter of about 1.25”. Turn a shallow tenon on the tailstock end so that it can be gripped by a scroll chuck using #1 jaws.
2. Install the blank in a scroll chuck using a point center at the tailstock to align it in the chuck. Turn 2.75” of the blank at the tailstock end down to about 0.85”. Use a Jacobs chuck to drill the pilot hole for the 1/4 - 20 thread that will accept the drawbar. Drill the hole to a depth of about 1”.
3. Change the drill bit to something like 5/32” and extend the hole about 1” deeper into the blank so that, eventually, a small knockout rod can be used to eject a part that might get stuck. Then coat the inside of the pilot hole with thin CA and let it cure for several minutes.
4. Cut the threads for the drawbar using a 1/4 - 20 tap.
5. Bring the tailstock back and use a point center to engage the threaded hole. This will support the blank while the taper is being formed.
Notes: (1) The large end of the taper is slightly larger than a standard taper because you must allow for compression when the collet is pulled into the headstock spindle.
(2) The part of the taper that is not slotted (and which will not compress) must be slightly undersize to allow the collet to be drawn into the bore of the spindle.
6. Shape the taper. Use a dial caliper and a parting tool to establish the diameter at the ends and the midpoint of the taper. As you get close to the final surface, you can check the fit by removing the live center from the tailstock and bringing the Morse taper of the tailstock ram up over the taper being turned. Rotating the piece (by hand) will make burnish marks where contact is made and thereby reveal the high spots.
7. After the taper is formed, remove the collet from the chuck and remove the chuck from the lathe. Screw the collet onto the end of the drawbar, and then insert the drawbar through the headstock spindle. Put the nut on the drawbar and seat the collet securely in the Morse taper. Turn off the excess length and square the face of the collet.
8. Drill the hole for the dowel. The nominal size is 3/8” but it doesn’t hurt to check the actual diameter of the dowels you plan to use. The ones I have are slightly oversize. They will still go into the collet but the fit is tight. You may wish to drill the hole a fraction larger than 3/8”, such as 25/64 or 13/32”. If in doubt, drill a test hole or two in scrap stock and check the fit of the dowel.
Be sure the drill bit centers on the squared face of the collet. Use a center drill to mark the center if you have one, or turn a small dimple to help center the bit. Drill the hole to a depth of about 1.5”.
9. Switch bits again to the one used in step 3 above and finish drilling the hole for the knockout rod. Make sure the knockout rod you intend to use will go all the way through the blank.
10. Turn the square face of the collet down to the diameter you desire.
11. Remove the collet from the lathe. The next step is to use a bandsaw to cut the slots, but in order to do this safely, you must make a fixture to hold the collet while the cut is being made.
12. Make the fixture. Take a section of a “2 x 4” about 6” long and mark a line on one of the sides at the center of the piece. Continue the line across one edge, and locate the midpoint of the line on the edge. Drill a 3/4” hole almost all the way through the piece. It is not a bad idea to make a hole for a knockout rod, in case the collet gets stuck in the fixture.
13. Wrap painters tape around the large end of the taper on the collet so that it fits snugly in the hole in the fixture. Then use a bandsaw (or other thin-kerf saw) to cut the slots, following the line drawn down the face of the fixture. Rotate the collet 90º and cut the other two slots.
Make a Drawbar
A drawbar consists of an appropriate length of 1/4-20 threaded rod and a “nut” to tighten it against the handwheel. Many variations are possible, but the one thing each must do, in addition to pulling on the collet, is to keep the rod centered at the handwheel end.
Perhaps the simplest method is to turn a bushing that fits into the bore of the spindle. The threaded rod passes through the center hole. A wing nut screws onto the end of the rod and tightens against the bushing.
If you have a 1/4–20 tap, you can turn and thread a “nut” that combines the tightening and centering functions into one piece. This is the approach I took when I made the drawbar for my lathe. It is shown in the drawing, and a photograph of the cross section is included below.