Old-Time Secrets Uncovered: Crafting Mortise and Tenon Joints

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Inset photo showing a mortise and tenon joint.
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Blind mortise from the Star gin was hand-drilled.
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Close-up of a blind mortise.
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Blind and through-hole mortises.
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Another mortise and tenon joint.
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Tenon: A projection on a piece of wood shaped for insertion into a mortise to make a joint.

Today’s question: How did
the old guys cut perfectly square mortises in wood, including through-holes and
blind-pockets?

I’d not really thought about
it until I started restoring a cotton gin stand dating to the 1840s (for more
on the Star gin, see Restoring a Pre-Civil War Cotton Gin). The entire structure was built with
poplar (called cottonwood around here). There were four 8-foot runners the
length of the frame and eight upright posts about 3 feet high. Each pair of
posts on each end of the top runners was attached using the same very long
tenon of 20 inches, one set to the base and the other at the tip. Each mortise
and tenon was perfectly fitted. The bottom ends of the eight posts were
blind-mortised into the bottom runner (or support rail). Lower cross braces
were through-holed. Cross caps on the tops of the posts were set with blind
mortise/tenon; they were also perfect.

Since one of the bottom
rails had extensive damage from dry rot and the other one was completely rotted
away, those had to be replaced. I helped a friend cut a large, mostly dead
cottonwood tree from his yard. New runners and rails were cut from the trunk by
another friend using a big portable band saw. As I got into the project, it was
clear that several of the cross pieces and one upright post were also too
damaged to serve further, so I used all of the cottonwood from that tree.

So now, how to cut the
mortises? I purchased a square-hole drill rig, feeling pretty smug about having
a project that I could fully justify and thereby amortize the expense. I set
about cutting the blind mortises for the bottom rails using the jig, with poor
results. The jig was a cheap Chinese unit, and not at all up to this task. The
hole edges were quite ragged and the walls not always vertical. Still puzzled,
I lucked onto taking off one of the caps and peering down inside the blind
mortise.

I’ll fully grant that the
old carpenters possessed great skill and sharp tools, but they had another
secret and it wasn’t a square-hole drill. Clearly visible across the bottom of
the pit were the tracks of an ordinary circular wood bit, with the bit diameter
equal to the hole’s width. The hole centers were spaced such that the edges
were scalloped evenly across and along the hole. A very sharp chisel was then
used to remove the scallop lands left by the round bits. This method had been
used on every mortise in the gin stand: 32 in all, half of them blind holes.
Using the soft-grained poplar wood was a big help as well.

Each tenon was pegged in
place after fitting into its respective mortise with a long oak dowel, drilled
all the way through, most of which were still in place and were reused. These
had been hand-carved, judging by the whittle marks. My new replacements for the
lost ones were initially shaped by a sander, but it turned out that a good,
sharp box blade could make them very serviceable in less time and they fit very
well.

So now, do any of you
carpenters out there need a slightly used square-hole drill jig? Comes with
1/4-inch, 3/8-inch, 7/16-inch and 1/2-inch drills, each with its square-corner
cutter. FC

Retired scientist Bill Friday, Huntsville, Ala.,
collects and restores old tractors, grist mills, cotton gins, engines, pumps
and tools. He is a member of the Southland Flywheelers Antique Tractor & Engine Club. Contact him at
bill.friday@earthlink.net.

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