Deep hole drilling is a marvel of modern machining, capable of creating precise, deep bores in shafts for hydraulic systems, gun barrels, and drive assemblies. However, the operation often leaves behind a surface that’s functional but not finished—characterized by micro-peaks, valleys, and potential stress risers. To transform this into a smooth, durable, high-performance surface, a precise finishing operation is critical. Here’s a step-by-step guide to perfecting your shaft finish post-drilling.
1. Setting Up the Job: The Foundation of Precision
A flawless finish begins long before the burnishing tool touches the metal. Proper setup is non-negotiable.
Secure & Align: The shaft must be impeccably secured in a steady rest or between precision centers. Any residual runout or vibration from the deep drilling process must be corrected. The goal is absolute concentricity; the finishing tool will follow the existing bore's path, so misalignment here will only polish imperfections, not remove them.
Cleanliness is Key: Thoroughly clean the bore to remove all chips, coolant residue, and microscopic debris from drilling. Any particles left behind will be rolled into the surface, creating scratches and defects. Use high-pressure coolant flushing and specialized bore-cleaning tools.
Inspect the Baseline: Perform an initial surface roughness measurement. This gives you a quantifiable starting point (e.g., Ra 125 μin) and a goal for post-finish quality (e.g., Ra 16 μin or better).
2. Selecting & Setting the Right Burnishing Tool
For achieving a smooth finish on an internal bore, roller burnishing is often the preferred method. It cold-works the surface without removing material.
Choosing the Tool: Select a roller burnishing tool with a diameter adjustable to your specific bore size. The number and material of the rollers (often hardened steel or carbide) matter for the final texture and tool life. Ensure the tool’s reach is sufficient for the depth of your hole.
The Critical Setting: Tool diameter adjustment is the most crucial step. The tool must be set to a size 0.001-0.002 inches (0.025-0.05 mm) over the pre-finished bore diameter. This creates the necessary interference fit for the rollers to apply uniform pressure.
Too loose: The rollers skate, producing no effect.
Too tight: Excessive pressure causes galling, seizing, or work hardening that can lead to micro-cracks. Follow the tool manufacturer’s specifications precisely.
3. Mastering Coolant Pressure and Process Parameters
Coolant is not just for cooling here; it’s a vital component of surface formation and tool protection.
High-Pressure, High-Volume Coolant: Use a high-pressure through-coolant system. Aim for a minimum of 1000 PSI, directing the flow ahead of and through the burnishing tool. This serves three purposes:
Evacuates Micro-chips: Washes away any microscopic material displaced during cold working.
Lubricates & Cools: Prevents built-up edge on the rollers and dissipates heat generated from plastic deformation of the surface.
Ensures Consistency: Maintains a clean, consistent interface between rollers and metal for a uniform finish.
Process Execution:
Speed: Use a low to moderate spindle speed (50-200 SFM). This is not a cutting operation; speed controls heat and feed.
Feed: Employ a slow, steady, and continuous feed rate. A typical range is 2-10 inches per minute (IPM). Do not stop within the bore. A single, uninterrupted pass is ideal.
Direction: One pass is usually sufficient. Do not reverse and re-burnish the same section without resetting, as this can over-work the material.
The Result: More Than Just a Pretty Surface
A properly finished shaft bore isn't just smooth—it's superior. The roller burnishing process compresses the surface peaks into the valleys, creating a denser, harder, and more wear-resistant layer. This also induces beneficial compressive residual stresses, which greatly improve fatigue life and corrosion resistance. The final, mirror-like finish minimizes friction for fluid flow or moving parts, enhancing the efficiency and longevity of the entire assembly.
By meticulously following these three conditions—rigorous setup, precise tool setting, and aggressive coolant management—you move beyond simple machining to true metallurgical enhancement, turning a deeply drilled hole into a high-performance engineering asset.
