Why can’t you get square corners on CNC?

Can you get square corners using CNC?

Square corners are about as rare as hen’s teeth when it comes to CNC machining. While CNC bits are circular by design, they can produce a perfectly straight line cutting through all kinds of materials. And every machinist knows the intersection of two straight lines is a perfect 90-degree corner, right? It ought to be but it isn’t; at least not when it comes to CNC machine straight lines for inside corners.

It’s about engineering and physics.

Simply put, the bit on the CNC router is round and requires a certain amount of clearance to operate properly as it moves across surfaces. When the router bit stops or turns, that tiny amount of clearance, resulting from the previous cut, That takes the sharpness out of the end cut leaving a rounded corner. There’s no such thing as a square router bit and getting a router bit to stop perfectly in line with another cut still won’t make the corner square.

So what happens when you try to make a square corner on CNC?

Even the smallest diameter bit will still leave material inside an interior corner. That can cause problems further downstream if the remaining material interferes with items using the corner for placement or connection to another part. The larger the diameter of the router bit the more pronounced the problem becomes. CNC routers are also dependent upon the connecting joints, ball bearings, and other equipment that can become worn or loose, causing inaccurate cuts and missed specifications.

Are there workarounds for getting square corners on CNC?

There are only a couple of options for squaring the corner that CNC machining can’t cut. The first option is to use a manual solution like a chisel or other tool to remove the unwanted material. This will slow down production cycles significantly but should achieve the goal.

The other option is to manipulate the operating application so that the router bit overcuts the corner. It allows for any follow-on manufacturing processes. By adjusting the cutting radius to a larger diameter, more material will be removed from the inside corner preventing interference further down the line. It’s called making a “dogbone” cut.

Types of CNC overcuts that make square corners.

Some different cuts can be used with internal corners to reduce the impact of CNC router machining:

  • Short Side Mortise Dogbone – cut that rounds the shortest side of a cut while keeping the intersecting long cut line straight.
  • Long Side Mortise Dogbone – cut that rounds the longer side of a cut while keeping the intersecting short cut line straight.

 The two examples recommended still won’t perfectly square an inside corner on a CNC router bit. It’s just not possible to put a round peg into a square hole exactly. Depending upon the CNC machine’s operating system, the tools available, the specification to be met, and the time allowed for machine handling, the best method for achieving success depends upon several factors. Machine maintenance, materials variations, and other considerations should be included in the programming of the cutting codes to allow for the highest specification adherence.

Solutions exist!

Operator’s manuals are always useful when dealing with a conundrum like squaring a circle. It also helps to speak with other CNC machinists as well as with the manufacturer’s service representative or dealer. The more ideas and experience you can pour into the solution, the better a solution you’ll achieve. When it comes to inside corners on a CNC machine, solutions are worth their weight in gold.

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