INTRODUCTION: This article shows how to make a light duty collet adaptor for #2 jaws in a 4-jawed chuck. It is not as strong nor as accurate as a real collet chuck but it is good enough for holding a spindle for drilling on the lathe and turning a spindle on the lathe as long as a tailstock center is used for all but sanding the nub after parting off. The range of each collet is about 1/16" so ColletAdaptor.pdf has patterns for collet adaptors from 1/4" to 7/8" by sixteenths.
MAKING THE ADAPTOR: Download and print out ColletPatterns.pdf, being sure in the printer dialog box that "Size Options" are set to "Actual Size". Cut out the pattern for the size collet adaptor you want to make. If you're considering making all of the sizes, you probably should buy a real collet chuck instead. Fasten the pattern to nominally 3/4" plywood with 3M #77 spray adhesive. Baltic Birch plywood is generally stronger and void free compared to plywood from your local big box store.
Clamp the plywood to the table of your drill press and drill a centered hole as in Figure #1. A Fornstner bit, if you have one of the correct size, will generally yield a more accurate and smoother hole. (If you don't have a drill press, until you buy one you can drill the blank on the lathe. You'll need an undersize disc of 1/4" plywood to avoid blowing out the bottom of the hole. Hold the bandsawn pattern in the jaws and drill on the lathe. You should still mount the blank by the tailstock and true the rim as described below.) Move to your bandsaw and cut out the out the adaptor blank staying just outside the outer circle of the pattern. Then cut the exterior slots on the bandsaw as in Figure #2.
Figure #1: Drilling the collet blank on the drill press.
Figure #2: Sawing out the pattern and exterior slots on the bandsaw.
Mount your 4-jawed chuck with #2 jaws on the lathe. Measure the interior depth of the #2 jaws of your chuck if you don't know it (1/2" for the Oneway Stronghold). Close the jaws completely, else they may open when the lathe is turned on. Pin the adaptor blank, pattern out, to the faces of the jaws with a cone tailstock center as in Figure #3. Larger sizes may require a bullnose adaptor. Now turn the rim true as in Figure #4. It's done this way because it's usually more accurate to turn the rim true to a hole than to drill a hole on the lathe that's true to the rim.
Figure #3: The blank pinned by the tailstock to the faces of the #2 jaws for truing the rim.
Figure #4: After turning the rim true.
Measure from the pattern side of the adaptor and mark a little less than the depth of your jaws. Use a small bowl gouge to turn a rebate the diameter of the smaller outer circle on the pattern as deep as the mark. Clean up the face and corner of the rebate with a square scraper. The result is shown in Figure #5.
Figure #5: After turning a rebate for the jaws to grip the adaptor.
Remove the adaptor from the lathe so that you can saw the interior slots. A scroll saw with a #11 blade is excellent for this. If you don't have one, for larger sizes you can clamp the adaptor in a vise and saw the slots with a hacksaw by removing the blade, threading it through the hole, and remounting the blade as in Figure #6. You could similarly use a coping saw for smaller sizes. Inconveniently you'll probably have to re-clamp the blank in the vise to saw each slot as you may have excessive vibration if there's a sawn slot between the blade and where the vise grips the adaptor.
Figure #6: Sawing interior slots.
To use the adaptor, mount it in the 4-jawed chuck as in Figure #7. Insert a tenon turned on your spindle blank and be sure the collet adaptor is against the faces of all four jaws, the shoulders of the tenon are against the adaptor, and that four of the exterior slots are between the jaws. You'll have to tighten the jaws very securely.
Figure #7: The collet adaptor mounted for use.
If you cut a square hole with a scroll saw (use #11 blade so you get as close to vertical sides as you can) the collet adaptor works pretty well for square stock. I did several iterations of turn/extend and there was no discernable run-out.
I turned a small rebate on the bottom of the square collet adaptor. This lets the collet adaptor sit deeper in the chuck and balances the force from the metal jaws. Instead of concentrated at the bottom of the collet the force is more in the middle of the adaptor.