To an outsider unfamiliar with CNC machine shop operations, once the chips stop flying, the milling machine winds down and another precision machine part rolls out, they may well think the end product is accurate and up to specification. Unfortunately, this is not always the case. At Advanced Precision Machining, and any CNC shop for that matter, no finished part is delivered to a customer until it passes an inspection reporting process. A part’s dimensions must conform to the tolerances set in place for its design, and this includes thoroughly inspecting the finishes of all machined surfaces. As is often the case, a part may meet specific dimensional specifications, but must be scrapped because its overall appearance and surface finish is not up to standards. This not only results in an aesthetic issue, but more importantly, can negatively impact a part’s functionality.

The bottom line is that during the finishing stages of the milling and machining process, having to send parts to the scrap bin is less than a desirable outcome. CNC machinists by nature are detail-oriented, take great pride in their work and want to see a project through to completion. Scrapping parts as a result of surface finish issues can also be a real profit killer for machine shops. Poor quality leads to increased costs, more downtime, delayed deliveries, and in the worst case scenario, lost orders. The question becomes; how do machinists and shops address this issue? What can be done when the end results aren’t up to par? Let’s take a quick look at just a few simple tips and tricks on how to get the best surface finish possible using a CNC milling machine. Bear in mind that this is only the tip of the iceberg! 

1. Increasing cutting tool speed

Faster velocities, measured in surface feet per minute (SFM) mean material is in contact with the cutting tool for less time. This works to reduce edge buildup resulting in poor surface finishes, and also extends the life of the cutting tool. Tool failure is a leading cause of damage to a precision machined part. Never estimate or guess at the required speeds for your job. 

2. Always clear away/control chips

Clearing excess chips, or built up edge (BUE), and not allowing them to contact the workpiece during machining operations is a vital task and key to producing a high-quality surface finish. Chips and BUE have a tendency to build up, and if not cleared, can easily damage surface finishes as the cutting tool makes contact with them. Chip breakers, compressed air, and the best solution, flood coolant can be used to clear away chips and BUE for better control and less scratching/damage to parts.

3. Different tools should be used for roughing vs. finishing applications

Don’t finish a part with the same tool used for roughing. Instead, slightly used roughing tools should be utilized for the bulk of material removal, while brand new, sharper tools should be saved and only used for finishing passes. Tooling insert radius, rake angle, feed rates and insert material itself, when combined with roughing vs. finishing tools can lead to a higher quality finish.

To the layperson, simply examining a precision machined part for its aesthetic appearance may seem all that’s necessary once milling and machining has ended and delivery is near. Thoroughly inspecting completed work, regardless of material, is instead a critical factor for a part’s functionality; especially for applications in industries such as aerospace and medical device manufacturing. Excess chips and/or BUE, slow operating speeds, and improper tool use are leading causes of poor surface finishes. Lack of machine maintenance, low levels of cooling fluids, poor setups, and CNC programming errors are also contributing factors. The tips we’ve provided are applicable across a variety of machine shop operations, including milling, turning and grinding, and if adhered to, should go a long way to improving the surface finishes on your problem jobs, resulting in satisfied customers and repeat business.

Do you have questions? Want to learn more about surface finishing applications? APM’s Colorado machine shop is available for all of your inquiries. We’re dedicated to manufacturing the highest quality precision parts while providing the best customer service experience in the machine shop business. For additional information or to request a quote, please visit https://advancedprecisionmachine.com or call 303-776-1910.

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About the Author

Gerry Dillon is a co-founder, current owner and certified CNC machinist at Advanced Precision Machining (APM), a full-service machine shop located in Longmont, Colorado. Before making his home in the United States in 2000, Gerry was born and raised in Ireland and took an interest in milling and machining from an early age, ranking #1 in the Irish National Apprenticeship Program. In 2005, he began what’s grown into a leading Colorado machine shop. Gerry brings over 30 years of machining experience to the shop floor, and is certified in all aspects of geometric dimensioning and tolerancing.