Hole Types & How ToHole Types & How To

4 main types of holes:•Clearance Hole•Tapped/Threaded Hole•Countersunk Hole•Counterbored Hole

Clearance Hole• A clearance hole is used to allow a bolt or screw

to pass through the part – this allows the bolt to thread into another part or a hex nut

• Most clearance holes are drilled between 1/64” to 1/16” larger than the fastener diameter depending on the amount of clearance required for the application

Threaded Holes

• Threaded holes are used to fasten parts together

Creating Threaded Holes• Most commonly a three part process:

• Centre drill the hole• Drill a hole at the proper Tap Drill Size (TDS)• Tap the hole using the correct type of tap

TDS (Tap Drill Size)

• The TDS corresponds to the minor diameter of the thread size you require

• TDS can be found using a Sizing Chart or by using the following formula:

TDS = D – 1/N

Example

Find the TDS for a ½”-13 UNC thread

TDS = D – 1/N

TDS = ½” – 1/13

TDS = ½” – 0.0769

TDS = .4231”

• A full drill bit set will come with fractional drill bits as well as lettered and numbered drill bits, each having their own specific size

Countersunk Holes• A countersunk hole is used when a fastener must be

flush with the surface of the part• In this case, a Flat Head screw is used• An Imperial C-Sink is 82 degrees• A Metric C-Sink is 90 degrees

Counterbored Holes• A counterbored hole is used when the

fastener must sit beneath the part surface, but a stronger fastener is required such as a hex head bolt or a socket head cap screw

Deburring Tips

• A quick and effective way to debur a hole is to use a countersink

• Put a countersink in the drill press and bring it down until it just touches the part, you should end up with a slight chamfer on the edge or your holes, but no sharp edges or burrs!