Workholding for Irregular and Non-Round Parts in Deep Hole Drilling

Deep hole drilling assumes the workpiece is round — held in a chuck, between centers, or in a collet. When the workpiece is a hex bar, a square casting, a forging, or a flat plate, the standard workholding does not apply. The challenge is the same: hold the workpiece rigidly and align the hole axis with the drill. The solution is custom workholding that adapts the workpiece shape to the drilling machine's round-centric clamping system.

Workholding Challenges ​

Common Challenges ​

Risk Assessment ​

Workholding Methods ​

Method Comparison ​

Soft Jaws ​

Collet Blocks for Hex and Square ​

Custom Fixture Design ​

Alignment Methods ​

Alignment Procedure for Non-Round Parts ​

Alignment Tolerances ​

Clamping Strategy ​

Balanced vs Unbalanced Parts ​

Clamping Force Guidelines ​

Vibration Damping ​

FAQ ​

How do I hold a hex bar for deep hole drilling? ​

The best methods are: hex collet block (fastest — standard tooling, insert hex bar through block, tighten, mount in chuck), hex soft jaws (machine hex profile into soft jaw blanks — holds the full hex length), or pot chuck (a cup that surrounds the hex OD). For short runs, a collet block is the most practical. For production, custom soft jaws machined to the hex profile provide the best grip and alignment.

How do I drill a deep hole in a square bar? ​

Use a square collet block or machine soft jaws with a square pocket. The square bar must be aligned so that the hole axis is concentric to the bar center (or at the correct offset if off-center). For long square bars, use a steady rest with modified pads that contact the square faces — standard V-rollers will not work on square cross-sections. Set up a dial indicator to verify alignment before drilling.

Can I drill a deep hole in a flat plate? ​

Yes — but the plate must be held securely and the hole must be positioned correctly. For thin plates, use a vise with wide soft jaws that support the full plate width. For thick plates, use a fixture that clamps the plate edge-on with the drill entering the narrow face. The challenge is maintaining alignment — the plate's entry face must be perpendicular to the drill axis. Gun drilling is preferred over BTA for non-round workpieces, as the workpiece does not rotate.

What is the best way to hold an irregular casting for deep hole drilling? ​

Design a custom fixture that locates off machined datum surfaces (not off the as-cast surfaces). Use the 3-2-1 locating principle: three points on the primary datum, two on the secondary, one on the tertiary. Clamp directly over solid support sections of the casting — never clamp over thin sections or unsupported areas. Include a drill bushing at the entry face to guide the drill through the irregular surface. Test the fixture with a sample part and verify hole position and alignment.

How do I prevent vibration when drilling non-round parts? ​

Vibration in non-round parts is caused by interrupted cutting (if the drill entry surface is not perpendicular) or by part resonance (if the part has thin sections or is unbalanced). Solutions: use full-contact soft jaws that support the part across a large area, support the part near the drill entry point with a steady rest or bushing, fill hollow sections with damping material, reduce spindle speed, or add mass to the fixture to change the resonant frequency.

Non-round parts require adapted workholding for deep hole drilling, but the principles are the same as round parts — hold the workpiece rigidly, align the hole axis with the drill, and support thin sections. Soft jaws machined to the part profile are the most versatile solution for most shapes. Custom fixtures are needed for complex castings and forgings. Take the time to set up alignment carefully — the first article will confirm whether the workholding is adequate. This article reflects industry practice as of 2026.