Precision Machining Operations: Addressing Poor Part Surface Quality

For outsiders unfamiliar with CNC machine shop operations, once the chips stop flying away, the milling machine goes offline, and another precision machine part rolls out, they are likely to assume that the final product is accurate and to specification. Unfortunately, this is not always the case. In any CNC shop for that matter, the finished product is not delivered to the customer until it has passed the inspection service process. . The dimensions of the part must meet the tolerances set by its design, which includes a thorough inspection of the finish of all machined surfaces. Often, a part may meet a specific dimensional specification, but must be scrapped because the overall appearance and surface finish are substandard. Not only does this lead to aesthetic issues, but more importantly, it negatively affects the functionality of the part.

On top of that, in the final stages of the milling and machining process, it takes less work than the desired result to have to send the part to the scrap bin. CNC machinists are naturally detail-oriented, take pride in their work, and want to see a complete project. Scrapping parts due to surface finish issues can also be a real profit killer for a machine shop. Poor quality leads to increased costs, increased downtime, delayed deliveries, and in worst cases, lost orders. The question becomes; how do mechanics and shops solve this problem? What to do when the final result is not as expected? Let's take a quick look at some tips on how to get the best surface finish with a CNC mill.

1. Increase the cutting tool speed

Faster speeds, measured in surface feet per minute (SFM), mean less time the material is in contact with the cutting tool. This reduces edge build-up, which can lead to poor surface finish, and also extends the life of the cutting tool. Tool failure is the leading cause of damage to precision machined parts. Never estimate or guess the speed required for your work.

2. Always clear/control the chip

Removing excess chips or build-up edges (BUE) and not allowing them to touch the workpiece during machining is a critical task and key to producing a high-quality surface finish. Chips and BUE tend to build up and, if not removed, can easily damage the surface finish the cutting tool comes in contact with. Chip breakers, compressed air and the best solution, flooded coolant can be used to remove chips and BUE for better control and less scraping/damage of parts.

3. Different tools should be used for roughing and finishing

Don't use the same tools used for roughing to finish the part. Instead, roughing tools that are used sparingly should be used to remove most of the material, and new, sharper tools should be kept and used only for finishing passes. When combined with a roughing or finishing tool, the tool insert radius, rake angle, feed rate and insert material itself can lead to higher quality.

To the uninitiated, once the milling and machining is over and the delivery date is approaching, simply checking the aesthetic appearance of the precision machined part seems to be a necessity. Regardless of the material, a thorough inspection of the work done is a key factor in the function of the part. Especially suitable for industries such as aerospace and medical equipment manufacturing. Excessive chipping and/or BUE, slow running speeds, and improper tool use are the main causes of poor surface finish. Lack of machine maintenance, low coolant levels, poor settings, and CNC programming errors are also contributing factors. We provide tips for a variety of machining operations, including milling, turning, and grinding, and if used consistently, should greatly improve the surface finish of the job in question, resulting in satisfied customers and commercial reuse.