Stepover, cusp height and determining CNC quality.
One of the questions we’re often asked is “what standard, or quality of finish will I get at this price?”
The ultimate quality of a jobs’ finish depends on several factors, the first of which can be described using two interrelated terms, namely cusp height and stepover.
A good way to think of each of these is that stepover is relative to the effect the tool itself will have on the surface being cut, while cusp height is relative to (and directly influences) the final quality of the job. There is a considerable difference between the two.
Basically, stepover is the distance between each pass of the machines tool head. The image (above) shows a tool head on a job. The tool path is directly to and away from you, in and out of the image. It is not the main factor influencing the final surface finish, as the tool used will largely determine the cusp height. Cusp height refers to the small ridge that’s left behind on a job after each pass over the job (as per the image below, which details both cusp height and stepover). Ultimately, cusp height (the cusp height is the small ridge between each 'dish' left by the pass of the tool) is determined by a combination of the type and diameter of tool and stepover used; basically, the smaller the stepover, the smaller, or lower, the cusp height is. But this is not always the only determinant, as the tool that’s being used will affect the cusp height, irrespective of the size of the stepover.
Therefore, cusp height is much more relevant to the ultimate finish quality as it combines all of the variables that affect a jobs final quality.
As a simple example let’s assume that the job in progress comprises a simple shape featuring a sloped surface, and requires only one tool – say a 10mm ball cutter (a ball cutter is a type of tool featuring a taper at the sharp end leading to a point). If we set the machine to deliver a cusp height of 0.1mm, the job will take 60 minutes to cut (excluding setup time), and will have a very basic standard of finish. The job will require a lot of handwork (i.e. sanding) to achieve a smooth surface and be called ‘finished’. This is not only time consuming (which costs money), but will reduce the original machined accuracy of the CNC machined surface, which sort of defeats the purpose of using CNC to do the job in the first place…
If, using the same tool to cut the same object a higher standard of finish was required, an option would be to set a smaller cusp height. If we reduced the cusp height to 0.05mm, the quality of finish would improve, however the downside is that the machining time would approximately double from 60 to 120 minutes. This would influence the time the job is on the machine, and hence the cost.
If the goal is to achieve a really high standard of finish, the cusp height might be set at 0.01mm. This would result in a surface quality that would require minimal manual work to achieve the desired ‘finished’ quality. However, the downside would again be that the job is on the machine longer, and hence will theoretically cost more. Irrespective, this is the standard setting that Styrotech CNC uses for all machining jobs – unless otherwise requested – and at a price that’s totally competitive with contemporary CNC machining companies.
So, when it comes to determining the ultimate quality of your CNC job, and how it comes off of the machine, make sure that you ask about and understand stepover and, more importantly, cusp height. Remember, final quality depends on several factors combining, namely cusp height, stepover and the tool that’s being used, and none of these act independently; they each require the other to determine the true measure of finish quality, which is going to be cusp height.
Of course, we’re always looking for feedback in how we might do things better. So, if you do have any comments, please feel free to get in touch and let us know what you think!