Drill size and stand-offs

Hi everyone,
Been working on a PCB design since after my first one (a success, now pressure on my second one).

My first PCB was basically a shield with pre-existing pieces in place, including drilled holes for stand-offs.

Now I'm making my own board with bare minimal elements to run atmega 328 with arduino. I need help with drill holes.

Now I'm looking at the arduino serial 3v3. The holes are 0.125984 according to Eagle. This is probably a clearance hole for #4 screws. According to an engineering website, a loose fit for #4 needs 0.1285 diameter hole and a tight fit needs 0.1160 diameter hole.

My question is: since drill bits are all standard in size, there is only one drill bit that will make a loose-fit hole, how large should I put for a loose-fit hole for #4 screws? How do I indicate, like in a mechanical drawing, that this is a loose-fit hole for #4 so the CNC machine won't go crazy if my number is 0.001 too big or too small compared with the drill bit it has?

Plus, the arduino serial 3v3 board has holes with three concentric circles, a large one, white (silk layer?) and two small ones gray (one drill one else for what?). If I create one hole from Eagle, it's got one white and one gray, no second gray circle. Did I do something wrong?

Thank you very much!

In a mechanical drawing I would specify a hole size and a tolerance.
You need a minimum size of 0.1285 to get a loose fit. I would
specify the size as 0.1285 +MAX-0 where MAX is the maximum
value you can tolerate.

(* jcl *)


Thanks jluciani. Unfortunately I couldn't do that in Eagle PCB program. I've converted 0.125984 into mm and it's 3.2mm. Flipping through sparkfun's catalog gave me an idea. They have a PCB with different font sizes and thickness, drill holes etc on the board as a sample of what you may get for certain settings. There is a 3.2mm hole on the sample board. I suppose that IS a valid size in ISO standard drill bits, right? I've changed my thru holes to this size. Hope it works.

This is probably a clearance hole for #4 screws.

I always assumed that the holes were designed for M3 (metric, 3mm 9?)) screws. (The hole is 3.2mm) They weren't designed in the US, you know...

Right, if the fab house can drill a 3.2mm hole, #4 should be ok since 3.2mm is between loose and tight fit size for #4. My question is: if I indicate 0.125984, will the fab house machine get confused.

When you export your EAGLE project to an actual CNC drill file, it will be using a list of the drills that are (supposed to be) actually available (the "rack".) Or the board shop will do the same if they accept EAGLE files. EAGLE then picks the nearest actual drill, and complains if it is not within some tolerance (that you also specify in the CAM dialog.)
For example, if I run a "launchPad-like" board with a drill set that only includes 1/32, 1/16, 1/8, and 1/4 inch drills, I get complaints:

Generated by EAGLE CAM Processor 5.7.0

Drill Station Info File: /Users/billw/Documents/EagleProjects/--CCAM-Export/Launchpad-runtime-ss.dri

 Date              : 7/31/10 1:58 AM
 Drills            : /Users/billw/Documents/EagleProjects/cam/foo-test.drl
 Device            : Excellon drill station

Parameter settings:

 Tolerance Drill + :  2.50 %
 Tolerance Drill - :  2.50 %
 Rotate            : no
 Mirror            : no
 Optimize          : yes
 Auto fit          : yes
 OffsetX           : 0inch
 OffsetY           : 0inch
 Layers            : Drills Holes

Drill File Info:

 Data Mode         : Absolute
 Units             : 1/10000 Inch

Drill Tolerances:

 -- Requested --  -- Used Drill --

 Size       used  Code  Size

 0.0315inch     7  T10   0.0310inch

Missing Drills:

 -- Requested --

 Size       used

 0.0400inch    10
 0.0360inch    20
 0.0320inch    44
 0.0394inch     4

Drills used:

 Code  Size       used

 T10   0.0310inch     7

Total number of drills: 7


.1285 is a #30 drill bit size. 3.2MM is also a standard drill bit size. It might be worth your time to ask the house you plan to have drill your boards what sizes they can supply or recommend in their experience.

Just for completeness sake here is a link to common drill sizes in metric and US (Fractional, Numbered and lettered sizes)

Best of luck.

Thanks a lot Mr. Swarf and westfw. I will ask about the fab house. I'll be using batchpcb.com. I had luck last time I used it, got two boards for the price of one. They send their orders to China. I suppost it's going to be metric. I wonder who else is still using English units besides US. maybe no other country.

I wonder who else is still using English units besides US. maybe no other country.

Probably not, the imperial system is the product of a demented mind set. Australia changed over in the mid 70s, it was painful at the time but worth it. But a lot of stuff is still imperial at heart, for example a sheet of ply can be called a 1200x2400 but in fact be 1220x2440 (4x8 feet).

Then just when you build something with studs at 1220 centers you forget to check and buy 1200 (metric) sheets.

Thanks everybody, my pcb is ready for the fab house. Here is it's pretty look. I did auto routing and used some tweaking to get it the way it is right now. Wish me luck.

Hard to tell but many of those traces look too close to other pads,etc...
What did you use for clearance (trace/trace, trace/pad,etc...)

I'm actually new to PCB designs. I'm using eagle. I can't find where to set clearance. I only found trace grid, which is 8. I changed it from 50 to 25 and then 8. Any help is highly appreciated.

OK, I found the clearance to be 8mil for all. What is your suggestion? Thanks.

BTW, do you think I will need a ground plate? I saw it somewhere on a tutorial.

Um, no caps for the crystal?

I agree with other people that the clearances should be bigger.
Use the "DRC" button and fiddle with the assorted tabs.
(see http://www.instructables.com/id/Make-hobbyist-PCBs-with-professional-CAD-tools-by-/ (though the actual values used there are aimed at home-etching, the principles remain the same.)

Also you should probably change the autorouting grid. It should be an even factor of both the placement grid and the smallest component pin spacing. The eagle autorouter is pretty sucky...

Thanks westfw. There are caps for the crystal. They're on the bigger circular pads near the crystals. I'm using 8mil grid per the fab house requirement. My placement grid is 0.05 or 50mil. I think I will change that to something like 40mil next time I make a circuit. There are metric components like the 16x2 character LCD on the top, and then English components like the rest. Then the fab house probably uses metric but somehow requires 8mil grid, maybe it's just 0.2mm converted into 8mil. I hope all these difference in units go away someday.