Old-Time Secrets Uncovered: Crafting Mortise and Tenon Joints

How did early builders craft flawless mortise and tenon joints?

Close-up of a blind mortise.

Close-up of a blind mortise.

Photo By Bill Friday

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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.