Workshop Tools – Adam @ Hilltop Cottage

I’m planning on cutting some gear wheels and, having struggled with using a hole-saw to cut blanks, decided a trepanning tool was probably the answer.

The tool is only meant for cutting 1/8″ thick stock. Tested on brass and aluminium (may not be up to the job with steel). Use low speeds and if you don’t know what you are doing, don’t do it!

The Finished Trepanning Tool

The dark colour of the main body is due to a (failed) attempt to case harden the pilot; it is just uncleaned scale from from heating. I actually found the setup to be sufficiently rigid that the pilot didn’t take any load with 1/8″ brass and the narrow cutting tool shown below in the bottom right of the picture.

The Plans

These are not proper dimensioned drawings in an engineering sense, and were produced as part of the design process, but they should be sufficient. Stock used was steel 1″ dia bar and 3/8″ square.

Trepanning Tool Assembly (click to open full-size image)

Also available as a DXF File.

Brief Construction Notes

Arbor

Cut 1″ dia stock to slightly over length

3 jaw chuck :-

face off (light cuts), centre drill and support with tailstock centre
turn down 1/2″ shank
brighten up about 1/4″ of 1″ dia body (used later for concentricity setting with DTI)
cross-drill, widening progressively to 7/16″ (ideally finish to size with reamer)
drill and tap M5 (and shorten M5 cap screw to match)

4-jaw chuck :-

hold using 1/2″ shank, adjust to near zero runout with DTI
turn pilot (should be concentric with shank)

Cutter Arm

3/8″ square stock

4-jaw chuck :-

face off both ends (low overhang from chuck)
centre-drill second end, loosen 2 adjacent jaws, withdraw from chuck and support on centre, retightening the jaws to same setting
turn to 7/16″ dia to fit arbor

Pillar drill :-

drill and ream 3/16″ for cutting tool
drill 1/8″ for end of slot
drill for clamp bolt (M4 tapping drill size)
cut 1/32″ slot
drill clear for clamp bolt (up to slot)
thread M4 for clamp bolt

The Cutting Tool

Grind a 3/32″ or 1/16″ wide cutting point. Give it plenty of side clearance because the tool will be making an arc. Tighter arcs => more side clearance will be needed at the expense of a weaker tool.

Since round tool steel is used, the cutter can be rotated to adjust the in-use clearance a little to compensate for slightly uneven grinding, and changes in cutting radius, but I suppose the cutting face should be fairly close to lying on a radial line.

Having recently faced some cylinder drilling, I finally got around to making a depth gauge. It took about a morning. The sizing is very-much determined by available parts, starting from the springs and having some 3/8″ OD brass tube with nice thick walls (ID approx 1/4″) to hand. Two springs gave me about the 1 1/2″ of gauge travel I wanted.

No dimensioned drawings this time, but see below for a photograph of the disassembled gauge from which you should be able to work it out. In lieu of drawings, here are a few notes…

The end cap was made first, drilled to about 1/64″ less than the OD of the brass tube. You can see where I then turned down the tube to get a nice press fit. I’m very fond of press fits: most satisfying and clean. If you over-do it then you can always braze/solder but it never looks so nice. I have a small arbor press.
The other end of the tube was threaded using ME 3/8″ x 32 TPI since this matches the tube OD and is a fairly fine thread.
The 1/4″ spring-stopper on the 1/16″ central “spike” was another press fit thanks to some metal forming as I parted it off. Don’t bend the spike!
The thumb screw was made by tapping (5BA) a shaped and parted-off piece of hex stock, screwing a 5BA cheese-head screw firmly into place with a bit of thread-lock and then turning away most of the head. NB: face, chamfer, drill, tap, insert screw and turn-off head are all done without removing the work from the chuck.

The images above are links to larger photographs.

Category: Workshop Tools

Making a Trepanning Tool – Notes and Plans