One Pass Tenons:
Making Pickett or Bradding Tenons on a Lathe
Machine Tools in the Blacksmith ShopOne Pass Tenons:
First, you need a lathe with the capacity for the job. Both my 13" and 14" lathes have 1.5 HP motors and this is enough to put a 1/2" tenon on a 3/4" square bar in one pass. Note that having suficient horsepower DOES NOT mean putting an oversize motor on a small machine. Use the machine with its design horespower.
Second, to be efficient on square stock you need either a square collet, a 4 jaw combination chuck or a square work holder.
Third, in production you want to setup work stops and a carriage stop so that every tenon is the same length. If you are only making a few a silver pencil mark may do.
Before starting adjust a left hand shouldering tool to the diameter of the tenon (standard centering adjustments apply). This should be a sharp HSS tool with a radiused tip. Then just clamp the work, start the lathe, bring the cutter to bear and gently lean on the carriage crank. . . cut to the stopping point.
Note that heavy semi-interrupted cuts like this are easier to make than full interrupted cuts just taking off corners. Hand feeding will easily let you take full cutter width passes (fine feed rate, deep cut depth).
Taking the cut takes less time than changing work in the chuck. A brake on the lathe to stop the spindle quickly will nearly double the production rate.
On my small 6" lathe I made square work holders by taking a nut that fit the spindle and welding on a thick end plate. I drilled the plate for the square size and filed to fit. A hole was drilled and taped from the side for a set screw.
On a larger lathe a square work adapter could be chucked in whatever type of chuck you have rather than changing the chuck. My lathe has a 4 jaw combination chuck (has a scroll to close).
I know a few folks that can forge a tenon nearly this precise in about the same time. But it is a high skill forging job. This method can be turned over to an employee with less skills and the parts made quickly, efficiently and accurately. I also find it very handy for attaching candle drip pans where I want a VERY short tenon that just holds the pan on long enough to braze it. Slightly longer would be enough to brad them on.
- guru Monday, 02/09/09 01:23:25 EST
TURNING TENNONS IN A LATHE
I agree completly with the Guru. One trick I learned was using a simple split bushing to hold square bar. Say, for half inch square bar, you make a round bushing about 1-1/4 OD with about a .700 hole. Split it down one side, so it can close a little when you tighten down the chuck. This will hold a half inch square "on the corners" which works great and saves making a square hole.
- Grant Sarver Monday, 02/09/09 21:05:12 EST
You are doing it all wrong! ;-)
I have done a few million of these, not as tennons but as a bearing surface on the end of a shaft, we used two different tools depending on the size. For the smaller ones we used a piloted counter sink with the pilot removed, and for the larger ones a hollow end endmill.
Mounted in a sliding toolholder on the tailstock with a 2 foot handle kinda like a handchucker without the turret, and depending on the size of the tool 4 or 6 or 8 teeth, the chips would fly.
The right way to to a job is on time and under budget, how you do it doesn't matter.
- Hudson Tuesday, 02/10/09 10:34:32 EST
This can also be done using a "box-tool". This is a screw machine tool with two or more simple cutter bits and pilot holes. The outer pilot fits across the corners of the square bar and is a seperate top plate on the box, the inner "pilot" clears the tennon and is part of the box. One to four lathe bits are held in place by set screws and have back side adjustments. A back stop in the tool will produce consistent lengths.
In operation more edges remove more material faster and with longer edge life. They also balance the load on the part so that it does not try to run off center. The optional external pilot also supports the work preventing it from running off center. This is more useful if the work most extend a significant distance from the chuck. Box tools can be mounted in the chuck, on the carriage or in the tailstock depending the type work being done
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