Another RANT

[Mark Rand]

I thought that I might check my advice, so I came out to the shop and drilled a few holes. In order, and the sizes I found:

Now you're just bragging Russ .

[Mark Rand]

This from someone who lives in a country where they use stones to measure weight and a "hundredweight" weighs 112 pounds.

Stones don't wear much compared with plastic, and 112lb cwts allow for a quantity discount. At least we don't use short tons to weigh things, let alone the anorexic pints of beer!

[J Tiers]

Stones don't wear much compared with plastic, and 112lb cwts allow for a quantity discount. At least we don't use short tons to weigh things, let alone the anorexic pints of beer!

There is a reason for that about the beer. Have you ever TASTED what is termed "beer" over here?

If you had, you'd be grateful that you only had to drink a short pint of it.

[rkepler]

Now you're just bragging Russ .

I hope not, I was genuinely curious (and still mostly retired). I've always maintained that the 'best' way to get a hole on position and on size at the same time was to spot, drill way under, bore under and finally ream to size (unless you really, really trust the boring operation and can set it to tenths). That's what I read, and that's what I've always done when both position and size were critical. When it's just size it's drill and ream (or drill and bore when it's not something I can ream to). I've had things that needed only clearance holes but had to match a bolt pattern so I control the position with DRO and spot before drilling.

So I was genuinely curious, and for the most part I think it is still good advice. The bit that surprised me was that the screw machine drills held size so closely, and that the carbide drill was oversize as much as it was. I think I should get a new #31 in both screw machine and jobber sizes and see where they land, single samples like this aren't very good data sets.

But anyway, poking a hole with a drill has no guarantee that the hole will be on size, about all you can say is that the holes won't be smaller than the drill.

[Frank K]

The recommended bit is a 3.1mm, just how many "local" machine suppliers are going to stock mm bits in 0.1 mm increments???
GRRRRRRRR!!!!
...lew...

Umm... McMaster, for $1.65?

[wierdscience]

Use the .120" drill and increase your feedrate.

[Paul Alciatore]

Drilled holes are not that exact anyway. If you want an accurate sized hole you are going to have to use a reamer. Reamers are available by thousandths and even smaller increments for certain sizes. Just drill is a bit small and ream to size.

BTW, if you really want a drill of an exact size, you could grind it down. Spin the drill bit and run it across the wheel. The OD of a drill bit, the margin area, is cylindrical anyway.

[PStechPaul]

Perhaps you can grind a slight taper on the drill, so you can adjust the size of the hole by how deep you drill. The thread-forming screws are tapered as well. I found some tapered drills, that match the taper of screws, but are made for woodworking:

http://www.rockler.com/rockler-insty...red-drill-bits



Or a tapered reamer:

https://www.travers.com/high-speed-s...elistname=SITE



There are many sizes of standard taper pins:

https://www.smithy.com/machining-ref...turning/page/7

Reamers for taper pins:

http://www.atlascuttingtools.com/cat...traight-Flute/

[wdtom44]

Just sharpen a one under size drill off center a little and it will drill larger. Probably if you just sharpen it it will not be perfect and will drill over size anyway, unless you luck out and it is perfect.