Achieving the Best Surface Finishing for CNC Machined Part

It goes without saying that you want a good finish on your CNC machined parts.  If they’re painted or powder coated, you want a nice, shiny defect-free finish.  If they’re passivated, plated, anodized, or even bare metal you want to see parts that look like care was taken and attention was paid to making them.  What you may not realize is just how much attention it takes to produce CNC-machined parts that look professionally made.

Before the Chips Fly

Planning for the surface finish starts before the bit ever touches metal.  With few exceptions, rough cutting and finish cutting are performed with different tools.  Roughing in is often done at higher speeds, with larger bits or inserts.  Like Michelangelo removing everything from a block of marble that didn’t belong in his statue, CNC machining tends to make fast work of removing the bulk of the raw stock to get down to what your finished part will actually look like.  When it comes to cutting the part down to finish dimensions, finer bits, and slower speeds are generally used to minimize tool marks and avoid deflection of the metal, which would distort the part.

Seeing It Through

That’s just the beginning of the exercise.  Bits or inserts need to be swapped out regularly to maintain fresh edges for finish cutting.  Ideally, bits are designated specifically as roughing or finishing bits, even if they’re the same size.  Operators have to watch out for anything that can cause an interruption.  During surface finishing, if a bit has to stop for any reason -even if it just starts right back up it will leave a trace on the part.

Chips have to be carefully controlled during finishing as well.  Operators need to make sure clean coolant is washing chips out of the work area to avoid scratching the part.  Using free-machining (or “free-cutting”) steel for the parts can help as well, as the chips break away cleanly and are less prone to interfere with the bit. That introduces other considerations.  For instance, free-machining steel can’t be heat treated (though it can be case-hardened).  Some parts require heat treatment after CNC machining.

The Right Finish

Our customers in mission-critical industries like defense, aerospace, and medical fields all bring special requirements with their parts.  Aerospace, for example, is particularly complicated because if two pieces of metal happen to rub against each other and rub off whatever type of finish is applied -from paint to anodizing- those parts will fuse together in a heartbeat in a process called cold welding.  More than one spacecraft has suffered the effects of losing functionality because of faulty finishes.

Macfab has a long history of working on parts with critical surface finish requirements.  We’re obsessed with checking the finish to ensure it meets tolerances, just as we double-check dimensional tolerances.  Honing, surface grinding, sanding, and polishing all have different effects on the surface.  For some critical applications, those effects lead to poor performance or early part failure.  Even polishing to a mirror surface, which would seem to be the most perfect finish, can result in microscopic metal flakes that can break off, and contaminate with polishing compounds.  We know how to handle these situations, though.

Macfab has the CNC machines to produce the most complex parts.  We have the expertise to do it efficiently.  We handle all of our own metrology and reporting in-house.  Our cleaning regimens are second to none, and our finishing services are exacting.  Producing CNC machined parts that will pass every test and perform at least as well as required doesn’t happen by accident.  It’s something we love to do.


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