Drill diameters

[burnrider]

Most drills mike .001 to .002 smaller than their size. Never paid attention till I was checking for number/letter drills after buying on ebay. Same for Titex as US made drills in mike size. Trying to build a small inventory for tap threading.
If you buy drills on flea bay claiming to be number & letter, it's true- for about 30% of the total lot. The rest are fractional as stamped on the drill. It's a good buy (95 bits w / 30-40 sizes for $40-45), but only contain 25% of claimed Titex non-fraction stock.

 

[Tom Griffin]

Hi,

Not really sure what you're driving at here. But drills, as a rule, are not high precision cutting tools no matter who or where they got made. And when you start comparing fractional, letter, and number drills, it isn't surprising to find seemingly mis-sized drills. Manufacturing tolerances and expectations being what they are. Of course, cheap drills are unsurprisingly cheaply made and QC'ed.

As I got told by an old drillman way back when, "What do you want? It's a #@$%!&^* hole ain't it?":biggrin::biggrin:

dalee

Don,

Dalee hit the nail on the head. If you are sweating one to two thousandths, you need to be talking about reamers, not drills. Not only are drills incapable of drilling a hole to a precise diameter, they don't produce a hole that is round either. If you need a close fit hole then drill it a few thousandths under and ream to size.

It is also very difficult to accurately measure a drill because the drill is only contacting the micrometer at two points. That makes it very likely you'll get an undersize reading.

Tom

 

[Tony Wells]

Plus the fact that the geometry changes as you leave the cutting edges. The web gets thicker and heavier, and the measured diameter is in fact, tapered smaller as you go. The idea is to allow a little clearance behind the actual hole size. Of course, as mentioned above, twist drills are not generally used where precision is required, so it's not unusual to measure them and find variations of all factors.

 

[burnrider]

I figured they should be within one thou. Lesson learned, use the NSK micrometer & reamers for thread tapping I think important. Thanks for posting.

TX

 

[burnrider]

Thread repair for engine work can be useful information. Another good use is setting up multiple bolt patterns on a project. Good for moving parts, really miserable if nothing lines up, and fitting means over size drilling. This isn't production work, for that I just buy Chinese.