metric drill set explained?

Topics include, Machine Tools & Tooling, Precision Measuring, Materials and their Properties, Electrical discussions related to machine tools, setups, fixtures and jigs and other general discussion related to amateur machining.

Moderators: Harold_V, websterz, GlennW

TRX
Posts: 147
Joined: Wed Aug 20, 2008 7:30 pm
Location: Central Arkansas

metric drill set explained?

Post by TRX » Sun Sep 30, 2018 12:27 pm

So now that I'm using metric drills more often now I thought I'd look into a metric drill set. Perhaps I'm missing something obvious...

In a complete "Imperial" drill set there are fractional, number, and letter drills. These are actually three separate standards, lumped together because enough of the off-sizes are useful for selecting different tap drill sizes, press fits, and clearances for through holes, with some weirdo left-over sizes with no apparent use other than "put a hole in it." Packaged together, these are ye olde "115 piece" drill set.

The "metric" sets I've seen seem to make big jumps between sizes. Which I can easily get around by using Imperial sizes when necessary, but what do all-metric shops do when they need over- and under-sizes? If there's, say, an 8mm hole, what kind of metric set do I need to get a 7.7 and an 8.1, or whatever, along with the 8mm so I can do tap drills and clearance holes too?

Magicniner
Posts: 518
Joined: Thu May 30, 2013 4:40 am

Re: metric drill set explained?

Post by Magicniner » Sun Sep 30, 2018 1:04 pm

I have a Dormer metric set covering 1mm to 6mm in 0.1mm steps and another cheaper set covering 6mm to 10mm in 0.1mm steps.

TRX
Posts: 147
Joined: Wed Aug 20, 2008 7:30 pm
Location: Central Arkansas

Re: metric drill set explained?

Post by TRX » Sun Sep 30, 2018 1:19 pm

So that'd be 60 drills for the 1-6mm set and 40 for the other one? I'll look for Dormer, then.

I'll see if anything turns up for "100 piece set." What I keep getting from web searches are 13pc 1 to 13mm sets with 1mm steps and some 20pc sets with a few random x.5mm sizes tossed in.

Harold_V
Posts: 17769
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: metric drill set explained?

Post by Harold_V » Sun Sep 30, 2018 3:42 pm

TRX wrote:
Sun Sep 30, 2018 12:27 pm
The "metric" sets I've seen seem to make big jumps between sizes. Which I can easily get around by using Imperial sizes when necessary, but what do all-metric shops do when they need over- and under-sizes? If there's, say, an 8mm hole, what kind of metric set do I need to get a 7.7 and an 8.1, or whatever, along with the 8mm so I can do tap drills and clearance holes too?
One thing you should explore is the concept of attempting proper fits by using twist drills. It's just not sound thinking. There's a few things you really need to understand, otherwise you've going to struggle, and may not understand why.

Twist drills are NOT known for drilling holes on size, nor are they known for drilling round or straight holes. If a drill happens to drill size, I think you'll find that a drill blank of he same size won't fit the hole. It won't be dead round, nor will it be straight, but the properly ground drill blank will be, thus it won't fit in the hole.

For the typical application, sure, they do an acceptable job, often far better than required, but when it gets down to fits, they don't come close to being satisfactory, and if they happen to do so, it's the luck of the draw.

Here's an example of where they do not do well. If you work with dowel pins, you'll be hard pressed to find a drill that yields the desired amount of press fit, or the desired amount of slip fit. The difference you're looking for is less than a thou. In such a case, one drills undersized, then opens the holes with a set of reamers. Much as I don't care for them, they serve a real purpose. For critical fits, you may choose to bore, or even hone, each of which offers the opportunity to improve the created hole. Even reamers can yield less than round holes, or bell mouthed holes, neither of which are desirable when you're trying for proper fits. Reamers tend to be more precise than drills, but they, too, have their issues.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

User avatar
tornitore45
Posts: 1726
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Re: metric drill set explained?

Post by tornitore45 » Sun Sep 30, 2018 5:27 pm

As stated before getting hang up on precise size of drills is nor worth the chucking time. However...

The 115 imperial set is an irrational concoction, if you list the size in ascending order and the differences from one size to the next the gaps are all over the map. The gaps range from 4 tenths to 0.016". Naturally one would expect the gaps to be closer among small drill and wider between larger drill but is not so. Percentage wise the gaps run between 0.1% and 11%
#28 and 9/64 differ by 0.1%
The gap is 10% to 11% around #77 #76 range

In the metric set the gaps between successive sizes rise in discrete ranges from 0.05mm for the small drills to 0.2mm up to 12mm drill the gap keeps increasing as the size get larger. Makes sense.

This must be seen in the historic contest. American early industry was disperse and uncoordinated the copper wire drawers had their Gauge Standard, the Sheet Metal rollers had their own Gauge standards, each industrial segment was a metrology island.
In Europe, when the metric system was adopted it was a good opportunity to start international standards and the ISO was born which gave us the drill bit sequence, the threads forms, screw preferred sizes, the taps dimensions, the Reynard series and a host of rational standardization's.
Mauro Gaetano
in Austin TX

User avatar
liveaboard
Posts: 893
Joined: Sun Dec 08, 2013 1:40 pm
Location: southern Portugal
Contact:

Re: metric drill set explained?

Post by liveaboard » Sun Sep 30, 2018 5:46 pm

I work in metric only; I do not currently own a single imperial drill bit, or much of anything else for that matter.

I have drill bits mostly in 0.5 mm divisions, and mostly only whole mm above 12. For some reason, I have a 14.5, no idea why.
And sizes for tapping, which I keep separate to avoid using them for dumb work by mistake, because they're hard for me to get and cost more. 4.2mm, 6.8, 10.2
As Harold said, they just aren't that accurate, and I would add; especially when I use them.

Harold_V
Posts: 17769
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: metric drill set explained?

Post by Harold_V » Mon Oct 01, 2018 1:43 am

liveaboard wrote:
Sun Sep 30, 2018 5:46 pm
they just aren't that accurate, and I would add; especially when I use them.
I'm not convinced you have any less talent than most of us. However, all it takes for a drill to not cut size is a slight ding on one (cutting) edge, or to sharpen it unevenly. If the angle changes, side to side, that, too, will make the drill cut oversized, assuming there's no pilot hole. Twist drills rotate on the web, which does not cut (the web of a twist drill has drastic positive rake which displaces the metal so the lip can make the cut. That's why a pilot hole reduces drilling pressure so much). That being the case, the drill revolves around its own perceived center, due in part to the length of the typical twist drill. For lack of better description, the drill "wings", with one lip doing all the cutting (oversized). If the hole is deep enough, it will often slowly move towards center, yielding a tapered hole.

If you shorten the drill substantially, it will be far more rigid, but in that process you also enlarge the web, as the flutes on twist drill are tapered, they are not straight. Flute depth diminishes as you move towards the shank.

By their nature, drills want to pilot on the OD, which is cylindrically ground, but with a very narrow land, which reduces friction in the hole, yet still provides a modicum of guidance. Also, twist drills are not straight, each tapering a thou or two towards the shank. That, too, helps reduce friction when the drill is in a hole.

If one must use a twist drill for an onsize hole, I can think of no better way to achieve the end than to double drill. Many subscribe to the myth that that isn't a good idea, but it was mandatory for us when I worked in the missile industry. By opening the hole undersized, leaving a few thou per side for the next drill, the drill will self center on the margin (the web no longer makes contact) so the diameter of the drill in use, more or less, dictates the size of the resulting hole. That's particularly true if the drill runs true, and is jobber length. Shorter drills have the potential to overcome a drill's ability to self center, often exacerbated by a poor running drill chuck.

Note that double drilling, as I suggested, when working in copper alloys and gray iron, it's a real good idea to reduce the rake angle of the drill to 0°. Not doing so often results in a tragedy, with the drill picking up the part. This one I know first hand.

If one is stuck with a chuck that runs poorly, that issue can be overcome by gripping the drill by a very short portion--so it can oscillate in the jaws and seek center.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

John Hasler
Posts: 1280
Joined: Tue Dec 06, 2016 4:05 pm
Location: Elmwood, Wisconsin

Re: metric drill set explained?

Post by John Hasler » Mon Oct 01, 2018 3:19 pm

Harold writes:
Note that double drilling, as I suggested, when working in copper alloys and gray iron, it's a real good idea to
reduce the rake angle of the drill to 0°. Not doing so often results in a tragedy, with the drill picking up the part.

Or pulling the taper out of the socket.

User avatar
SteveHGraham
Posts: 7787
Joined: Sat Jan 17, 2009 7:55 pm
Location: Florida

Re: metric drill set explained?

Post by SteveHGraham » Mon Oct 01, 2018 6:36 pm

It would be nice to live in a metric world.
Every hard-fried egg began life sunny-side up.

Harold_V
Posts: 17769
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: metric drill set explained?

Post by Harold_V » Tue Oct 02, 2018 1:44 am

SteveHGraham wrote:
Mon Oct 01, 2018 6:36 pm
It would be nice to live in a metric world.
I dunno! My world, aside from the refining of precious metals, has always been Imperial. I can do, with Imperial, anything and everything one can do with the metric system, and, to me, it's no more confusing. It has served mankind for several generations. WW II was fought (at least the US part) by the Imperial system. That's how the Sergeant Missile was built, too.

Yes, I do understand the convenience of the metric system, but in order for it to make sense to me, I have to convert to Imperial units. Had I been raised with metrics, that most likely wouldn't be the case, but I wasn't.

Lets here (*hear) it for the Imperial system, metrics be damned! :P
*Thanks, Marv. I knew better.
H
Wise people talk because they have something to say. Fools talk because they have to say something.

User avatar
tornitore45
Posts: 1726
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Re: metric drill set explained?

Post by tornitore45 » Tue Oct 02, 2018 6:30 am

What is the next size up from a 6 mm wrench? The 7 mm of course.
What is the next size up from a 1/4" wrench? Mmmm let's see 1/4 = 2/8 = 4/16 = 8/32 , dammit where is that 9/32 wrench?

I need to build a transformer, how many 0.4 mm wires fit into a 22 mm layer? 22/0.4 = 55 I can do that without pencil and paper.
I need to build a transformer, how many #26 wires fit into 55/64? Where is the damn table of wire diameters? I can't find my slide ruler right now.

This is not your typical equivalence but how many inches in a mile?
How many ounce-inch for a pound-foot of torque?

We lost a satellite for that stupid metric system. :roll:

Yeeeee for the Imperial system :lol:

:P :P
Mauro Gaetano
in Austin TX

User avatar
tornitore45
Posts: 1726
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Re: metric drill set explained?

Post by tornitore45 » Tue Oct 02, 2018 6:49 am

They built the pyramids measuring cubits, just to say that is not a matter of accuracy or capability is simply a matter of practicality.
You can divide the cubit by 360 for a smaller unit and have a longer unit of 3631 cubits and still send man to the moon with such a system.
Mauro Gaetano
in Austin TX

Post Reply