Creating a 12.7mm – 1/2″ hole….

[flatlineParticipant @flatline]

Hi, I want to drill a 12.7mm – 1/2″ hole approx 1″ deep through mild steel.

 

For accuracy do I understand that I should drill it slightly undersize, and then ream with a 12.7mm tool….??

 

Would a 12.5mm drill be correct before I ream?? I will be holding the drill in a Jacobs chuck in the tailstock on my ML-7.

 

Many thanks in advance!!

[egaParticipant @ega]

A half inch reamer will probably cut slightly oversize; for accuracy bore the hole.

It would help to know the application.

[Anonymous]

Ideally you drill undersize, say 12mm, true the hole position with a boring tool, and then ream to size. You need a machine reamer, not a hand reamer. For a 12.7 to 25.4mm machine reamer the hole should be 0.3mm to 0.5mm undersize. So drill 12.2mm to 12.4mm. I’d be nearer the 12.4mm diameter.

If positional accuracy is not paramount then the boring stage can be missed out.

Andrew

[duncan webster 1Participant @duncanwebster1]

If it’s already in the lathe I’d just bore it to size, I certainly wouldn’t buy a reamer for a one off

[roy entwistleParticipant @royentwistle24699]

Holding a 1/2 inch drill in a Jacobs chuck on a Myford. I think you’re pushing your luck

At that size it wants boring

Roy

[HopperParticipant @hopper]

It depends on what accuracy you want, and for what application. More information please.

Drilling 12.5 then finishing with a 12.7 drill will give an accurate hole by drilled hole standards. (or drill 31/64 then 1/2″) But that is not accurate enough for some precision work. Drills usually drill a bit oversized and wander about a bit.

Drill to 12.5 then ream to 12.7 will give you a more accurately sized hole. Reamers can be bought to make the hole different “fits” on a standard 1/2″ pin etc. Or you can ease it out with an adjustable reamer.

Drill to about 12 then bore to 12.5 then ream gives the most accurate result as far as hole size, finish, hole location and hole orientation goes.

What is it that you are making and what function will the half inch hole serve?

And you can drill a half inch hole in a Myford with no worries at all. I use a 1″ drill with the shank turned down to fit the tailstock chuck on mine. For a half inch hole I would drill a 1/4″ pilot first, then proceed as above.

 

[noel shelleyParticipant @noelshelley55608]

You have not given the reason for the hole or the need for any special accuracy and boring will depend on the accuracy of the lathe, you may well get a tapered hole. Hopper has given a good method and a 1/2″ in a myford is no problem, made easier by stage drilling, start with a 1/4″. Sometimes people get carried away by accuracy and the need for it, as a lad I had to make parts to .00025,  but it was a special part for RR. Noel.

[flatlineParticipant @flatline]

Many thanks for all the input to my original post.

Lots of food-for-thought & options as to how I’ll proceed with my first little project!!

Many thanks once again, Rich D.

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