What drill bit size to use?

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******* hell!!!
An M3 bolt/thread is 3mm nominal outside diameter. But the tips of the male thread are rounded. I can't recall the radius, but it will be in the data.
The M3 bolt thread will pass through a clean exactly 3mm diameter hole.
The shank of a M3 bolt will be 3mm +-tolerance. That tolerance may make for a very tight fit, or a very slack fit in an exactly 3mm diameter hole. But the hole also has tolerances. Not least, that a 3mm drill measures smaller than 3.000mm.

0.126" = 3.2004mm
0.125" = 3.175mm
You could throw an M3 at these hole diameters and there's a good chance that the thread will fall through.

Wake up and get your calculators out, if you can't visualise 3mm.
 
Thanks, but by how much exactly. I'm sitting here with the 23rd edition of Machinery's Handbook. I know the answer is probably in there somewhere but I cannot find it.

There is no single correct answer for this one - in fastener engineering (my 1st job in the auto industry) this decision is driven by two factors. The larger the clearance hole the easier assembly will go (assuming there are multiple fasteners) as the larger clearance allows for slight misalignment in hole locations between the mating pieces while still passing the fastener. The smaller clearance hole provides more load bearing area (the area between the OD of the fastener head and the ID of the clearance hole) which is critical in highly stressed attachments to avoid brinnelling of the surface under the fastener head, which can compromise the attachment.

Generally in our hobby we rarely encounter highly stressed attachments carrying large cyclic loads...perhaps to some extent in some speaker structures. So you want to basically decide with what precision you are able to locate the holes in the two (or more) mating pieces. If you can work to +/- 0.1 mm in locating and drilling all of the holes relative to design location then you will need 0.2 mm of clearance to allow all of the fasteners to pass. When finished this would provide a workmanlike appearance.

One way to get nice results while sort of cheating this process is to drill all of the holes in one of the mating parts, clamp them together in design relationship, and use a transfer punch to locate the hole centers in the mating part. A transfer punch has an OD matched to the clearance hole size and a point in the center. This way the small errors in hole location on the 1st part get duplicated on the second and you can use a tight clearance hole since you are not trying to allow for the two errors to stack at each hole.
 
******* hell!!!
An M3 bolt/thread is 3mm nominal outside diameter. But the tips of the male thread are rounded. I can't recall the radius, but it will be in the data.
The M3 bolt thread will pass through a clean exactly 3mm diameter hole.
The shank of a M3 bolt will be 3mm +-tolerance. That tolerance may make for a very tight fit, or a very slack fit in an exactly 3mm diameter hole. But the hole also has tolerances. Not least, that a 3mm drill measures smaller than 3.000mm.

0.126" = 3.2004mm
0.125" = 3.175mm
You could throw an M3 at these hole diameters and there's a good chance that the thread will fall through.

Wake up and get your calculators out, if you can't visualise 3mm.


I used to be a scientist, I can visualize 3 mm. If not I have my Mitutoyo machinist's ruler sitting here. Three millimeters is approximately a third of a barleycorn for what it's worth.

My plan is to make a reamed hole. Reamers are expensive and I was hoping to get the correct size the first time rather than going for trial and error.

What 3 mm bit measures smaller than 3.000mm? Where I have seen a diameter tolerance listed it is usually +something, -zero.
 
M3 needs a 2.5mm drill.

Ask any of us in Europe to buy you a metric drill and post it to you.

Unfortunately the USA seems to reside with imperial measurements that dont quite meet the requirements.

A 2.5mm drill bit is so light that it can almost come AirMail.

Heatsink alluminium is not very strong. If you overdrill the hole you stand a VERY REAL risk of stripping the thread as soon as you put a bolt in it.

Just be patient and order the correct drill size.
 
I used to be a scientist, I can visualize 3 mm. If not I have my Mitutoyo machinist's ruler sitting here. Three millimeters is approximately a third of a barleycorn for what it's worth.

My plan is to make a reamed hole. Reamers are expensive and I was hoping to get the correct size the first time rather than going for trial and error.

What 3 mm bit measures smaller than 3.000mm? Where I have seen a diameter tolerance listed it is usually +something, -zero.

Drill bits usually measure very slightly smaller than their nominal size because they are not perfectly symmetrical. If a slightly asymmetrical bit is nominal diameter it will produce an oversize hole. Even with this a drill bit will usually produce a slightly oversized hole as you note in your tolerance example. Reamers are a much more accurately machined tool than a drill bit and should measure very near net. To determione the size reamer you want you will need to consider the issues discussed in my prior post.
 
I used to be a scientist, I can visualize 3 mm. If not I have my Mitutoyo machinist's ruler sitting here. Three millimeters is approximately a third of a barleycorn for what it's worth.

My plan is to make a reamed hole. Reamers are expensive and I was hoping to get the correct size the first time rather than going for trial and error.

What 3 mm bit measures smaller than 3.000mm? Where I have seen a diameter tolerance listed it is usually +something, -zero.
A ruler is (usually) not the appropriate tool to measure a hole.

The tolerance of a twist drill bit is usually of little concern since any application which dictates the use of only a drill bit is not going to require a highly-precise hole diameter, circularity, or cylindricity. If it is, it will require a reamer or hone afterwards.
 
Metric threads have been made easy for us since they adopted a 60degree thread face angle.

Take the screw thread size and subtract the pitch.

eg M3 0.5mm pitch needs a 2.5mm drill.
the drill size is sometimes stated on the tap.
If it is not stated explicitly then read off the size and pitch and subtract pitch from Msize. That is the drill size to use for a full engagement thread.
 
@Lo_tse
Pm me your address ill post you a couple of 2.5mm drill bits

Madtecchy,

Thank you for your kind offer. Your generosity is much appreciated! But it would not be right for you to pay for it. I have to respectfully decline. Anyway, I will check out the Newark link that you sent. Thanks!

I thought I was asking a simple question:rolleyes:!! Just want to drill a hole and put a screw in it :p. I am a little overwhelm with all the information posted in this thread. Looks like there is much to learn. I would like to thank everyone that had repsonded to my thread. I love this forum!!
 
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