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I want to drill a cone-shaped hole in some thick wood. The hole needs to be about 75mm deep and 45mm diameter at the surface. The accuracy of the dimensions isn't so important, but I do want the finish to be very smooth inside the walls.

I tried drilling with hole saws and forstner bits to produce a series of "steps" to the bottom, this worked well but I'm now struggling to grind away the steps into a smooth cone.

I was thinking of using a step-drill to finish the hole - but they don't seem to make them this big - and they are very expensive.

So I'm looking for either a way to drill a cone from scratch - or a way to grind the "steps" to make the walls smooth.

I have a wide array of power tools and hand tools but no lathe or CNC machine!

  • A little help for those who are not familiar with mm or who need to search in inches: 45 mm is 1.77 inch or nearest standard size 1 3/4 inch. – Willeke Feb 5 '17 at 11:33
  • @Willeke Thanks for that. I should have allowed for Americans in my question. – Lefty Feb 5 '17 at 11:42
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    This does work here, but might work better at Woodworking SE. – cobaltduck Feb 7 '17 at 18:20
  • Do you have a drill press? – Brad Feb 7 '17 at 21:13
  • @Brad Yes, but not a very good one. What's your plan...? – Lefty Feb 7 '17 at 22:55
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A tapered reamer would produce a smooth finish, though I've not seen one of ~50 mm diameter.

You might make your own tool from a piece of metal. Form a cone from some sheet metal, e.g. iron or brass (not aluminum) and solder the overlap, leaving an abrupt edge along the cone. Drill or punch two diametrically-opposed holes at the wide end and slip a rod stock or cross-point screw-driver though the holes and you've made a light-duty reamer, suitable for smoothing soft woods (not hornbeam or lignum vitae, though).

  • I like the idea of your "conical drill-bit" - but I think even easier would be just to cut a triangle of the right dimensions, mount on a shaft and let the sharp edge of the metal grind the hole out to the final size - a bit like a tile drill. Still trying to work out how to centre a triangle of metal on a shaft though....?! – Lefty Feb 6 '17 at 22:08
  • A triangle should also work, but if the metal is not thick, it will twist. A standard wood bit has a triangular-shaped tip, after all. – DrMoishe Pippik Feb 7 '17 at 18:59
  • Good point. If I don't have thick enough metal, I might try bracing with some wood either side in some way. – Lefty Feb 7 '17 at 20:48
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Grinding the steps should be easy enough. Use a round file to get rid of the steps, then sand to the finish you want. If you want a smoother finish than you can get using sanding alone, apply a few coats of varnish and sand that.

  • I do have a round file and haven't tried that so will give it a go and report back. I suspect it may be a little too laborious. Sanding is the problem - there's not enough room to get paper in there properly. I REALLY like the idea of the varnish though. I think it will solve a lot of problems, so thank you very much! – Lefty Feb 6 '17 at 22:03
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One possible soloution if you don't mind a coupld of small screwholes in the back of your panel.

Fix a peice of scrap wood to the back of your main panel.

Drill through both your panel and the scrap wood with a holesaw.

use a router to cut the angle on the edge, the scrap wood is used to guide the router.

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