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I am trying to bore out a red brass pipe from 3/8" to 10mm using my drill press and a standard twist HSS bit. Running the drill at 600rpm I can't get more than half an inch in before the drill starts binding. I tried running first with cutting oil, and then, thinking maybe the oil wasn't sticky/viscous enough, tried with Permatex silver, which only got me a few mm further before binding.

My guess is that I need some sort of metal-cutting bit that has a narrower shank so that there isn't friction except where it's actively cutting. If so, what are such bits known as?

Is there any other practice I might be missing to make this work?

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What you are looking for is called a reamer.

Some 10mm reamers:
http://www.aliexpress.com/item/Free-shipping-cnc-nc-2015-Reamer-10-0-X30X75L-High-speed-reamer-high-degree-of-finish/32244988192.html

http://www.aliexpress.com/item/Square-End-10mm-Cutting-Diameter-6-Flutes-HSS-Hand-Reamer-Milling-Cutter/32240625696.html

http://www.aliexpress.com/item/Free-Shipping-New-1-pcs-Solid-Carbide-Rotary-File-Burr-Cylindrical-half-circular-nose-10mm-W1018M06/650695215.html

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  • $\begingroup$ Excellent! Since it looks like most reamers have full-diameter flutes and shanks, can you explain why those wouldn't bind when a standard twist drill bit would? $\endgroup$
    – feetwet
    Nov 25 '15 at 2:31
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    $\begingroup$ I am not an expert in reamer design, but the shallower screw angle (or absence of screw angle) will give it less tenancy to "bite in" and bind. This allows you plunge at a much slower rate, and the bit will not try to outrun the down force. Also a drill bits primary cutting surface is on the tip; the side edges basically clean and transport material out. On a reamer those side edges are designed for cutting. When the side edges come into contact with a fixed irregularity in the sidewall the more expensive reamer will cut it off and reduce the likeliness of binding. $\endgroup$
    – ericnutsch
    Nov 25 '15 at 7:04
  • $\begingroup$ Great explanation -- I think worth adding to the answer itself! $\endgroup$
    – feetwet
    Nov 25 '15 at 14:58
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Reamers mentioned in an ericnutsch's answer are good solution (remember that these reamers are designed to increase diameter of existing aperture up to 0,5mm), but may be the better solution is to use senker. But seems your problem is not in using wrong reamer type. I would suggest you to check the straightness of your bit and drill. May be a drill or a bit have some wobbling due to overheating or overloading in past, also your drill motor must have enough power (rpm is not a major parameter). Lack of power may cause the same effect with using senker or drill bit.

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