How to Use Counterbores

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A counter-bored hole has two diameters; a large diameter that is big enough to fit the screw head and the smaller diameter houses the shaft of the screw.

Example of a counterbore drill bit.

Benefits of having a counter-bored hole are:

• A smooth, clean surface is sustained when a large fastener is needed

• A counter-bored hole allows for a part to be fit into tight clearances without needing the extra room of a bolt head


• A short-sleeved shirt, long pants, closed toed shoes and eye protection are required while operating machinery.

• An operator should not wear gloves or jewelry.

Steps to Using a Counterbore

1. Select the desired screw size.
Table 1
2. Determine the proper pilot drill bit and counterbore bit size from Table 1.
Cabinet #3
Counterbores are located in the fifth drawer down in Cabinet #3. Pilot drill bits are located on-top of Cabinet #3.
Fifth drawer Down
Counterbores are located on the left side of the drawer next to the center drills.
Damaged Drill Bit
4. Inspect drill bit and counterbore for jagged or broken edge - Seek mentor if broken or jagged.
Purchasing new bits: 
Drilling hole.
5. With proper drill bit selected, drill through hole to pilot size, oil liberally.
Speed Chart
6. Mount counterbore in mill, drill press, or lathe and adjust speed according to Speed Chart. Select head depth to screw size using Table 1.

Note: These are suggested starting speeds, not necessarily the correct speeds.

7. Cut counterbore to head depth indicated in Table 1, oil adequately.
Deburring Tool
8. Remove any chips or burrs caused from counterboring. This can be done with a deburring tool or a chamfer bit.
Insert Screw
9. Insert the screw into the hole to ensure a proper fit.