Split board with brace and jennings bit - common?

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Forum topic by ScottStewart posted 10-05-2014 09:27 PM 966 views 0 times favorited 7 replies Add to Favorites Watch
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119 posts in 1556 days

10-05-2014 09:27 PM

So the situation…

Trying to learn to become a neander, have a 5/8×2 1/2×8 in flat sawn SPF board I have crudely dovetailed as part of a box and am drilling holes in to make a chisel holder.

Drilling away with a 5/8 jennings bit and a 10” brace, and the board splits in two through the hole. Fixable, but an irritant. I assume there was wedging force perpendicular to the fibers from the scraping part of the bit.

Does this happen often? I suspect this is a sign I need to sharpen the scraping part of the bit, but I would appreciate anything else I should learn from this.



7 replies so far

View Bill White's profile

Bill White

4408 posts in 3384 days

#1 posted 10-05-2014 09:52 PM

The nickers as well as the cutting faces on the bit(s) need to be very sharp, and the lead screw helped by downward pressure on the brace should drive the bit. Excessive downward pressure on a somewhat dull bit could have been the cause of the break.
Then too, the board could have had weakened grain.


View Tim's profile (online now)


3032 posts in 1385 days

#2 posted 10-06-2014 12:50 AM

Yeah I think this happens when the spurs aren’t sharp enough, but the cutters not being sharp might do it too. Either way sharp fixes a lot.

Here’s some links to sharpening tips with a video. (Andy’s blog)

View realcowtown_eric's profile


557 posts in 1361 days

#3 posted 10-06-2014 02:52 AM

or else the spurs have been sharpened on the outside, thereby creating a wedge that is forced into the wood by the screw feed…..

-- Real_cowtown_eric

View Dan Krager's profile

Dan Krager

3231 posts in 1658 days

#4 posted 10-08-2014 12:05 AM

SPF is a group of woods that split when you look at them. The screw itself can wedge a split. It’s a good idea to make sure the bit is super sharp in the proper configuration and you are applying the correct bit to the wood. Jennings bits are made in two configurations for soft or hard wood, most of the difference being in the lead screw. Fir has some of the hardest wood right next to some of the softest wood, so a soft wood lead screw may be pulling too fast for the cutter to penetrate the hard part of the grain.
I usually put the board in a constraint when using hand brace bits just to be sure it doesn’t split. I.e. clamp across the grain at the hole being drilled, put it in a vise, or wedge it on a waste piece.
These bits also have a nasty blowout on the back if not clamped tightly to a backer piece. An alternate method is to drill until the pilot shows, then flip the board and drill from the back. Used properly, these bits offer the cleanest entry and exit you can achieve by any method.

-- Dan Krager, Olney IL There are three types of people...those who are good at math and those who aren't.

View rance's profile


4243 posts in 2584 days

#5 posted 10-08-2014 12:27 AM

> ” I suspect this is a sign I need to”

It is a sign to use power tools. :)

I have respect for those who use hand tools. I just don’t have the time for it. Carry on. Maybe I’ll learn something from looking on.

-- Backer boards, stop blocks, build oversized, and never buy a hand plane--

View Wally331's profile


340 posts in 1449 days

#6 posted 10-08-2014 01:58 AM

a wooden handscrew clamp on either side of the hole will keep it from splitting, or drill with held between the jaws of your vise. The wedging action of the lead screw will crack thinner boards pretty easily.

View ScottStewart's profile


119 posts in 1556 days

#7 posted 10-08-2014 12:24 PM

I don’t see any evidence for filing on the outside of the spurs. This bit looks to have a fairly fine screw thread. The constraint on the board is a very good idea, thank you.

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