[eric2011 (OP)]

Hi

I have a plastic ingot that is 3 inch deep and I want to drill holes that will have 3 inch length. But the holes I want them to be 10mm top and 8mm bottom. I hope you have understood that. What kinds of drill bits should I use?

I know I can't use a straight drill bit that has uniform size from top to bottom like a cigarette. My english is not good but I hapoe you have understood

[eric2011 (OP)]

g]This is a steel piece with 3 inch length. So after I drill the holes so steel pieces like this if you insert in the hole it stick and fit perfectly

[Bored chemist]

I think you need a tapered reamer.
https://en.wikipedia.org/wiki/Reamer#Tapered_reamer

[alancalverd]

Amazon and Ebay (other suppliers are available!) offer a range of taper drills.

[Petrochemicals]

If you know someone with a lathe just knock one out of mild steel, you are only on about plastic, or put your 8mm hole in and set an angle on the plastic on a drill press. You will struggle without access to a few bits of machinery, best would be a 8mm hole and a router with an angle on the base, you can make your own and just tape it to any router.

[alancalverd]

Or if it's a thermoplastic, drill even steps 8, 8.5, 9, 9.5, 10 mm, then heat the rod and push it in.

[Bored chemist]

If you know someone with a lathe just knock one out of mild steel, you are only on about plastic, or put your 8mm hole in and set an angle on the plastic on a drill press. You will struggle without access to a few bits of machinery, best would be a 8mm hole and a router with an angle on the base, you can make your own and just tape it to any router.

You can also cut plastic on a lathe.
But it seems safe to assume that, if the OP had access to one, he wouldn't be asking.

[alancalverd]

I used to make all sorts of vacuum, optical and gear systems on a lathe. Cutting a long, narrow tapered hole with a boring bar is definitely not a job for an amateur!

[Bored chemist]

... Cutting a long, narrow tapered hole with a boring bar is definitely not a job for an amateur!

On the contrary, I suspect it happens quite a bit when they are trying to cut a cylindrical hole.

Not my field but, if I was trying to cut a long, narrow tapered hole in plastic, and I had access to a lathe, I might start by making a tapered reamer.

[alancalverd]

I might start by making a tapered reamer.

You'd need an extremely good lathe and a lot of practice - better to take your plastic billet to an established jobbing workshop, if you don't have time for an apprenticeship. But if you do eventually qualify as a toolmaker, your son could become prime minister!

[paul cotter]

The type of plastic is also an issue: some plastics melt and foul the drill bit leading to a rough cut.