Homebrew Shell Drills

One of my Shop Assistant’s friends asked for help with a Science Project: building a trumpet-like musical instrument from some sort of tubing. We adjourned the meeting to the local Big Box home supply store, measured various options, and returned with a stock of CPVC pipe and fittings.

Given the budget and physical size of the valves, plus the fact that she planned to make tuning stubs from vinyl tubing, I suggested making all the connectors from fishmouthed sections of the CPVC pipe, which called for a bit of Quality Shop Time before the next build session.

A shell drill is what you use when you want a really big hole all the way through something, so the middle just falls right out. They’re handy for drilling in fragile / delicate material, because the shell supports the material until the drill reaches the far side. They’re also dead simple to make, at least when you’re drilling soft materials, which is pretty much all I do.

I always start by rummaging through the collection to find an existing shell drill that’s close enough to the right size that I can cut it down or bore it out. Here’s the assortment, some of which are obviously victims of previous modifications:

Shell drill assortment
Shell drill assortment

This one was slightly too chubby, with plenty of meat:

Original shell drill
Original shell drill

That was easy to fix:

Shell drill with reduced OD
Shell drill with reduced OD

While I was at it, I cleaned out the ID to reduce the tooth thickness. That reduces the force applied to the workpiece, which I figured would be a Good Thing considering the size of the pipe.

Fishmouthing CVPC tubing
Fishmouthing CVPC tubing

If you must start from scratch, all you need is a rod that fits inside a tube of some sort: the rod must be chuckable in your drill press and the tube must be about the right diameter for the hole-to-be-drilled. Turn them to suit on the lathe, then press / bash / braze / epoxy / pin them together, paying some attention to concentricity and alignment.

Cross-drill two holes near the business end of the tube, with diameters 1/4 to 1/2  of the tube diameter. Cut off the end to remove about 1/4 of the drilled holes. File some relief on the web between each pair of holes and you’re done.

The holes provide all the rake you could possibly want (take off more of the hole if you need less rake) and filing gives plenty of relief (what you see is grossly too much). None of this is critical for drilling soft stuff; you’ll need more attention to detail in a steel-cuttin’ shell drill.

Then clamp the pipe in the drill press and have at it! The teeth have enough rake that it’ll be grabby, so control the downward force and be sure the vise has a good grip on the pipe.

The trick is to support the pipe by ramming a dowel into its snout from one end or the other, thus preventing the sideways forces from deforming the ever-thinning end. This will take some practice, so buy a spare length of pipe.

After some of that and a bit of cleanup, we had a handful of connectors like these:

Fishmouthed tube connectors
Fishmouthed tube connectors

Which eventually became the trumpet’s valve assembly:

CPVC trumpet valve assembly
CPVC trumpet valve assembly

My Shop Assistant turned wood dowels to a slip fit in the pipe, we drilled suitable holes and Dremeled passageways to convert the dowels into pistons, and it actually worked pretty well. Not nearly as resonant as a brass trumpet, but that wasn’t the design objective.

Haven’t heard how they fared in the competition, but it was a fun project!

4 thoughts on “Homebrew Shell Drills

  1. That’s awesome. That’s also a pretty sophisticated pvc trumpet. The stuff I’ve seen previously was pretty much a bugle trombone.

    I’ve cut holes in glass the same way: use something soft, like brass or copper tubing, file some slight notches in the end, put valve grinding compound on the glass, and start peck-drilling. It worked well for producing a large number of round glass cutouts.

    1. It was a labor of love, that’s for sure. We used wood piston valves with Dremel-ed slots that were just too narrow, so there wasn’t nearly enough air flow. She wanted rotary valves, but I convinced her that building / aligning / getting all the fiddly bits working for four of those would blow the by-that-weekend schedule…

      The copper-pipe-and-grit trick works wonders on glass, but wow is it slow: my arm like to about fell off the drill press handle! The next time I attempt that, I must conjure up a lever arm with a weight; either that or turn it into a CNC project by hacking the EMC2 torch height module into accepting a force feedback input.


      1. Do you have a spindle tach? It’s possible you could tell when it’s running free and step it down until it bogs down, then wait until it runs free again.
        When I cut with valve grinding compound I have to keep adding more, I presume because it fills with comparatively soft ground glass. If I were automating this I’d be thinking about adding a carborundum slurry dripper.

        1. That’s an interesting approach: I like it!

          All of a sudden it’s turning into another CNC project. It’d actually be easy straighforward with EMC2 and HAL, because you could integrate all the weird hardware right into the code…

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