Instructables

Fostner Bit Extension

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Boring a deep hole with a Forstner bit requires a bit extension. I made this Fostner bit extension at the TechShop San Jose, http://www.techshop.ws, using the Jet Metal Lathe and Jet Vertical Mill.It is turned from a piece of 3/4 inch diameter X 8 inch long cold rolled steel, the type available at HD, Lowes or your local hardware store.
 
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Step 1:

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Start by mounting the stock in the three jaw chuck on the Jet metal lathe and face off the end. Proceed to turn the diameter to approximately .45 inches dia. X 1.25 inches long. The jacobs chuck I will be using this extension in has a nominal .5 inch maximum opening and I want to keep the end of the shaft as large as possible, so I turned the shaft to .45 inches diameter.

Step 2:

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Reverse the stock in the chuck and face off this end. Center drill the end and follow by drilling a .375 inch diameter approximately 1.25 inches deep in the stock. The Fostner bit shank is nominally 3/8 inch diameter X about 1 inch long. The hole just drilled will accomodate it nicely.

Step 3:

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Remove the stock from the lathe and do a fit check.

Step 4:

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Move over to the Jet mill and clamp the stock in the vice using paralles to locate the stock just proude of the vice jaws. After locating the centerline of the stock, center drill at .38 inches from the end and again at 1.38 inches. Replace the center drill with a #3 drill, the tap drill for 1/4-28 threads. Drill through to the center bore at the two previously center drilled locations.

Step 5:

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Re-set the stock on parallels to locate the stock just shy of the jaw surfaces. Use a tapping block to keep the tap vertical and tap 1/4-28 two places.

Step 6:

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All it needs is a couple of 1/4-28 socket set screws and it will be ready to use.
dlewisa1 year ago
Do the set screws hold it well? I imagine since the fostner bit is round rather than having a hex shaft that the screws would allow it to slip if you were going through really hard wood.
bgerens (author)  dlewisa1 year ago
That depends on the size bit you are using and the wood you are working with. Most often I will drill progressively larger holes when deep boring, but grinding a flat on the shank is a reasonable approach, also.