Introduction

Center drill
Center Drill

When turning a workpiece that is longer than three times its diameter, support from a live center in the tailstock is required to prevent spring. Without support, the workpiece will deflect away from the cutting tool, resulting in a larger diameter at the unsupported end.

For the workpiece to receive a 60° center, a matching 60° conical hole must be machined into its end. This is an application of the cone-in-cone kinematic constraint.

The center drill is used to quickly and conveniently create the receiving 60° hole feature: its taper matches the taper of the center and its pilot provides clearance for the center's tip.

When turning between centers, with the workpiece pinched between a dead center in the chuck and a live center in the tailstock, the stock will need to be prepared by center drilling both ends.

Learning Goals

Understand

  • Distinguish a center drill from similar-looking tools.
  • Choose the correct center drill size for your workpiece.

Perform

  • Perform center drilling with good technique.
  • Judge the proper center drill depth/diameter by eye.

What is a center drill?

The center drill is a "2-in-1" tool

The center drill prepares a workpiece to receive a live center.

It has two cutting features:

  1. Pilot Drill: The small tip that creates a starting hole for the countersink and provides relief for the tip of the live center.
  2. Countersink: The 60° conical section that matches the 60° taper of the live center.

Avoid confusing these similar tools for a center drill...

Spot drills, chamfer mills, and countersinks have different purposes and angles. Don't substitute them for a true center drill.

Spot drill
Spot Drill

A spot drill is short and rigid, used to make a small hole that stabilizes a drill as it begins to cut. Its tip angle is 90° so it cannot be used to make the hole for a 60° live center.

TO CLARIFY:A center drill has a "spot drill" section, but the center drill also includes other features.

Chamfer mill
Chamfer Mill

The chamfer mill is used to mill chamfers on the edges of a part. It's not used to start a hole, and it's not used on the lathe.

Countersink
Countersink

A countersink is not used to start a hole. It cuts a chamfer at the edge of a hole, usually at 82° to receive a flat-head screw.

TO CLARIFY: a center drill does have a "countersink" section, but its angle and purpose are different.

Procedure: Setup and Technique

Center Drilling: The Animated Version

Oil
Between each cutting action, retract the center drill to apply cutting oil and clear chips.
Spot
Use the tip of the center drill to create a starting point for the pilot.
Pilot
Make the hole for the countersink to expand.
Countersink
Make the 60° conical hole for the live center.
Conical Contact
We want cone-in-cone contact (matching tapers) between the workpiece and the live center for full kinematic constraint.
Ring Contact
Over-drilling will result in ring contact, which will not fully constrain the orientation of the workpiece.

Workflow: Center Drill

  1. Face the workpiece before drilling.
    • Drilling into a rough, saw-cut face will likely break the tip of the drill.
    • If you have features to cut on this end, hold the workpiece with enough material extended to make those features.
    • Otherwise, choke up the stock in the chuck.
  2. Pause to consider...
    • Has anyone been cutting tapers with an offset tailstock?
  3. Calculating RPM:
    • What diameter to use? → This is a good discussion... I'll use half the tool's diameter as a fair compromise.
  4. Use cutting oil at each step.
  5. Do not drill deeper than countersink.

Checklist

  1. Face the workpiece before drilling.
    • Drilling into a rough, saw-cut face will likely break the tip of the drill.
    • If features are needed on this end, extend enough material to make those features; otherwise, choke up in the chuck.
  2. Pause to consider: Has anyone been cutting tapers with an offset tailstock?
  3. Calculate RPM. As a rule of thumb, use half the tool's diameter as an effective diameter for SFM-based RPM.
  4. Use cutting oil at each step.
  5. Do not drill deeper than the 60° countersink.

Principles: What makes a good center?

What does the print say?

If the print calls out a center:

Make your center as the blueprint specifies... it's a part feature and an aid to manufacture.

If the print doesn't call out a center:

Leave extra material to remove the center later, or ask the customer if leaving a center would be acceptable.

If the part has a bore or other ID feature:

Make the center deep enough for its diameter to chamfer that ID feature, unless the print specifies otherwise. For example, if the print specifically calls out a 45° chamfer, you can't leave the 60° surface so you'll have to make the center smaller than the bore.