determining drill bit sizes?

[kipling100]

Hi,

Is there some sort of rule of thumb that helps you determine what drill bit to use for what type of screws, etc.? I'm going to attempt my first ever mod (cross your fingers), and I'm not sure which size bit to use for 120mm mounting holes, and also for mounting an ac radgrillz.

Thanks for any help!

Dennis

 

[FallenMystic]

one of the easiest ways to tell would be to take your bits and slide them into a fan screw hole already there and measure it out, other than that hardware stores typically sell a little plastic sheet that has a bunch of holes in it and shows the measurements under the given hole.

 

[Michael Daly]

what drill bit to use for what type of screws, etc.?

When you say screw, are you referring to a sheet metal screw (pointy tip) or a machine screw (like a bolt, blunt tip)? Do you want the screw threads to engage the edges of the hole or clear the edges of the hole?

 

[nigerian_businessman]

http://www.efunda.com/designstandards/screws/tapdrill.cfm

Having a electronic calipers around to measure the sizes of screws, drill bits, and holes is a big help. You can pick them up on the cheap from a place like MSC. You don't need the expensive ones, even the cheap ones are accurate within .001 in. Just get a 6in electronic pair that measures inside and outside diameters.

 

[Mohonri]

It also depends on whether you want the screw to engage or bite into the material you're drilling (like screw into wood), or whether you want the screw to pass through the hole and engage something else (like a bolt and nut on either side of a sheet of metal.

 

[aggr1103]

I think bascially the OP just wants to know what size bit to use to make a hole for a fan screw in his case.

To be honest I'm interested in finding that out myself.

 

[Michael Daly]

I think bascially the OP just wants to know what size bit to use to make a hole for a fan screw in his case.

Assume a machine screw then.

If that's all, the smallest drill bit larger than the screw thread diameter. If the bit is exactly the same size, it might fit nicely if the drill bit is not sharp - an unsharp bit will result in an oversized hole. If the bit is sharp, the screw should be able to thread its way into the hole with minimal thread cutting. That will definitely not be a problem with an aluminum case.

 

[aggr1103]

Ok. I'm just going to come out and ask it then:

Instead of going through the rigor of guessing at a drill bit size based on thread diameter, can someone who has done this before just say what size drill bit they used?

 

[Lucio]

It's fairly easy, for fans you simply take a drill bit and put it in the hole on the fan until you find one that's a snug fit.

For other types of screws, drill a small hole and then make it bigger if it's too small.

 

[rsgunter]

It also depends on whether you want the screw to engage or bite into the material you're drilling (like screw into wood), or whether you want the screw to pass through the hole and engage something else (like a bolt and nut on either side of a sheet of metal.

QFT.

I drilled a smaller hole for my case fans, which let my screws fit snugly. Basically you drill a small starter hole and then drill the screws you want to use in there. I think it's the best way.

 

[aggr1103]

QFT.

I drilled a smaller hole for my case fans, which let my screws fit snugly. Basically you drill a small starter hole and then drill the screws you want to use in there. I think it's the best way.

What about replacing fans?

 

[Stevennoland]

I'm a machinist by trade, so I have a little experience in this matter.

Here is a link to determine the drill size needed for a particular tread size (tap)

http://www.widell.com/drillsiz.html

And, here is a link to determine thread clearance sizes (holes that allow screws to pass through)

http://www.smallparts.com/products/descriptions/tap & clearance drills chart.cfm

Here is a picture of the back of my case where a 120mm fan would go. The hole size used to clear the 4mm (.1575") screw used to affix the fan is roughly 5.17mm (.2035") .

Here is a picture of some 8-32 machine screws.

4mm screws and 8-32 screws are very similar in size (a difference of about .0085")

Hope this helps.

 

[Susquehannock]

Fan screw = 3/16" bit.

Otherwise follow the advice my fellow machinist above provided. The tap drill chart is your friend.