Broken planes is a subject that comes up often in the Facebook Unplugged Woodworking group. Stripped threads are common on wooden planes that use threaded arms to position a fence. They usually break next to the arm’s foot as that is where the fence is most often needed.
This is my example, it will be my repair experiment. Years ago I demoted it to a kerfing plane by replacing both skates with a blade cut from an old rip saw. I screwed an inch and a half spacer block on the fence to skip over the defective threads, which worked, but is awkward and heavy. I’m going to simply cut out the defective section, which will shorten the range of the plane, but who plows grooves six inches out anyway.
These are the two threaded arms. Each was made from a single piece of wood with a 3/4″ O.D. threaded section. The challenge is to securely and accurately splice the amputated threads back on the foot.
So my plan removes the stripped part, then makes a half inch round tenon on the end of the good threaded rod, with a matching half inch mortise in the foot. The two parts are reassembled with a 1/4-20 threaded steel rod pulling them together, I think it will be at least as strong as the original solid wood part.
Most of the work was done on my Delta DP-300 drill press on which I have carefully aligned the press table square to the quill.
The first task was to make a fixture to hold the threaded arm accurately aligned with my drill chuck. I had to file the hole in the drill press table a bit to get the threaded arm to pass up through easily from the bottom.
To make the alignment fixture, I screwed a bit of 2×4 to a piece of scrap, clamped that to the press table, then ran a 3/4 inch Forstner bit down as far as it would go, I had to finish the bore with a longer spade bit. I removed the drilled 2×4, cut a slot on the table saw, then installed two screws to help clamp the threaded arm in place. It did take a small amount of sanding to get the threaded arm to pass through.
This is the bottom of the fixture. Two screws hold the drilled 2×4, they are placed so they will not interfere with the clamping slot on the top side. It’s easy to align the fixture on the drill press table, insert the threaded arm from underneath through the hole in the table about half way into the fixture. Lower a 3/4 forstner bit into the top of the hole, lock the table, and set the clamps.
I sawed the stripped arm off about an eighth inch from the foot. That left an inch or so of threadless wood on the shaft to practice on. In fact, I used a piece of 3/4 dowel up in the fixture to make the first practice tenons.
The first operation is to drill down on the sawn face with the 3/4 Forstner bit. That leaves a center dimple and faces the end off square.
The mortise will be drilled with a Forstner bit so I made a half inch hole in a piece of hardwood scrap to test the size of the tenon. I believe this is called a Mullet.
I considered a few alternatives to make a tenon. Maybe a hole saw (too sloppy). I looked at a half inch plug cutter (would have to regrind the tip to get a shoulder). I decided to use a cheap circle cutter, which can be tuned and has an angled bit that would make a nice tapered seat. The inside of the bit is ground flat so it was easy to sharpen with diamond paddles, and the pilot drill is smaller than the #7 size needed to tap the hole. I also ground a relief angle on the inside of the cutter. It was not designed to make a clean cut on the inside, making an angle of 15-20 degrees away from the cutting edge helps a lot. You only need to grind the cutter up about a half inch from the bevel, leave it flat where the set screw clamps.
It was very difficult to set the diameter accurately. I hit on using feeler gauges to measure the gap between cutter and pilot drill. I would hold the cutter against the feelers and tighten the set screw, which allowed me to add or subtract a few thousandths from the tenon diameter in a controlled manner.
You have to lower the circle cutter onto the wooden shaft slowly, it’s difficult to see where the cutter is when the whole thing is spinning. After a half dozen practice cuts in the 3/4 dowel, I had a tenon that fit well in the test mortise. I set the bit depth so that the tenon is a quarter inch long when the body of the tool contacts the wood. And with the fixture, I’m sure the tenon is axially aligned with the chuck and the dowel.
I made one test tenon on the end of the threaded arm, then took a deep breath and sawed the bad part off, leaving about 3/8″ of the stripped area to make the final tenon.
Again, faced off the freshly sawn end with a Forstner bit. Then made the tenon with the circle cutter. It looked good.
The final operations on the truncated arm were to drill and tap a hole about an inch and a quarter deep. I had a couple of 3 inch machine screws to use, but threaded rod would be good also. I used a tapered tap and ground the end of the sawn off bolt to match, to allow a bit more wood where the bolt ends. The bolt was screwed in by tightening a couple of nuts on the protruding end so I could turn it with a wrench. I also cut small grooves in the tenon for possible glue squeeze out.
That completes the preparation of the tenon.
Now to create an accurately aligned matching mortise in the foot. The first step is to secure the separated foot in a good sized wooden clamp for machining. Don’t want fingers near that router bit. I used an engineers square to check that the surface that contacts the fence is exactly perpendicular to the drill press table.
Now lower and lock the quill, run the drill press to maximum RPM, and carefully rout the sawn surface flat. I did this in three shallow passes leaving about a sixteenth inch of the original shaft.
When this arm was originally made, the outside diameter of the threaded part was even with the sides and top of the foot. That made it fairly easy to find the center of the cut off with a marking gauge.
I center punched the foot and drilled an eighth inch pilot hole
Followed by a half inch Forstner bit in about 3/8 inch. The pilot hole was enlarged in three stages finishing with a #7 bit, appropriate for a 1/4-20 tap. I wanted to engage an inch of thread under the mortise so the hole was run in about 1 1/2″.
I had to create a tapered seat to match the tenon. I did this in the drill press with a counter sink bit.
The countersink chattered if it wasn’t fed very slowly but did a decent job. Actually I found it worked better to remove the drill press drive belt and turn the countersink by hand.
Next, the hole was tapped to a depth of about an inch and an eighth. I went through the full set of tapered, plug, and bottoming taps. To ensure the threads were accurate I make the first pass with the tap in the drill press chuck turned by hand. The plug and bottom taps were run in with a tap wrench.
The long threaded bolt was cut to have about an inch and an eighth protruding from the end of the arm.
The final test – will it go together? It did fit a little tight but the arm is parallel to the fence face as best as I can tell. The real test will be is the fence parallel to the plow skates after it’s put back together. That might be the subject of another web log post.
I brought the two parts into the house where it’s warm enough to apply liquid hide glue and screwed the arm home snug but not tight. Here is the repaired fence arm next to the unfixed second arm. You can hardly see where the two pieces are seamed together. The small piece is what was cut out of the bad threads.
The process was successfully repeated on the second arm.
Now the plane could be reassembled. I found the fence would no longer clear the body, so I make a couple of thin spacers to get clearance. I’ve added leather washers to the thin inside fence nuts so the minimum space between blade and fence is about 3/16″. I may tune the fence further at a later date but for now it appears to be parallel to the blade so is very usable.
I tried it out, set the fence to a quarter inch and it kerfs beautifully.