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I need to drill conical holes that are narrow at the deep and and wider at the top into cylindrical bars. The measurements need to be very precise. I was thinking of using a narrow drill for the entire depth of the hole and a wider drill for the first 1-2mm, then sticking the piece into a lathe and try to make the taper to the best of my abilities with a few hand tools (round file, grinding stick).

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    What scale are we dealing with here? a 1/2" stick? a 3" stick? how accurate do you need the holes to be? What is the size of the holes needed? Not enough info for me to give a reasonable answer yet. In theory it could work, but... ?
    – bowlturner
    Dec 21, 2021 at 13:33
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    "The measurements need to be very precise." How precise? Your idea of "very precise" and that of other people are going to vary, so you need to specify.
    – Graphus
    Dec 21, 2021 at 17:55
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    Making tapered holes in wood is actually quite a common need in various areas & there are special tools for the job, which can be bought or made in the workshop. But, these tend to be for through holes and this is a stopped hole which changes the requirements a little. But since you have a lathe this is a fairly trivial turning operation. With just a little practice you could flare a drilled hole by hand with a suitable scraper and get good accuracy, but a custom-made boring tool that you simply push into the wood a fixed distance would make the job far easier and accuracy would be assured.
    – Graphus
    Dec 21, 2021 at 18:01

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I was thinking of using a narrow drill for the entire depth of the hole and a wider drill for the first 1-2mm

It'd probably be best to do the whole operation on the lathe. That'll ensure that the initial holes and subsequent work are all centered on the same spot. If you have access to a metal lathe, use that; it'll let you set the taper angle you want and reproduce it. If you only have a wood lathe, consider making a jig that mounts to the tool rest and guides a tool along the desired angle.

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  • +1. My idea to easily get the taper is with a custom tool. I'm envisaging a simple plunge operation, rather than needing to rely on the taper-cutting ability of a metalworking lathe (using the compound?)
    – Graphus
    Dec 22, 2021 at 16:20
  • @Graphus Agree... custom tool or custom tool guide amount to nearly the same thing -- the main thing is to eliminate as many sources of variability as possible. A metal late would really be perfect since it already has all that.
    – Caleb
    Dec 22, 2021 at 18:40
  • Yeah, metal lathes have a lot of in-built functionality that make them really great for a few turning operations (minus the problems with wood chips on the ways/in the lead screw or whatever).
    – Graphus
    Dec 22, 2021 at 20:57

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