There are plenty of reasons to add letters and numbers to a part. Text can serve to identify or brand the item, aid in assembly, or convey instructions to the end-user. In many processes, adding text to a part is a secondary operation; letters and numbers are written, printed, stamped, decaled, or otherwise appended to an existing item. In machining, alphanumeric text—either raised above or recessed into the surface—can be an integrated feature of the part. Properly designed, this can eliminate a step in production while producing clear copy that is as durable as the part itself.
In most cases, while it conveys information, text is a cosmetic rather than functional feature of the machined part. But while it serves a different purpose than other features, it is subject to the same milling standards. Because text tends to be smaller than most features, includes lots of narrow lines, points, and curves, all in tight spaces, and must be legible to serve its function, it must be designed with an understanding of the machining process (see Figure 1).
Figure 1: The “o” (left image) is too small to mill, because of the narrow line and curves. The “L” (right image) is a larger font and allows room for the tool to mill.
In a past Protomold Design Tip, we discussed text on injection moulded parts. Many of the same criteria apply to machined text, though in reverse; raised text on a moulded part is produced by recessed features in a mould and vice versa. On a machined part, however, what you cut is what you get.
In designing parts for Firstcut’s automated machining process, you need to consider three factors. The first is the minimum size of our cutters, 0.5mm. The second is the round shape of our cutters. And the third is the directions in which our 3-axis, six-sided cutting process can approach a work piece.
Cutter size determines the minimum width that can be cut. In recessed text, this will be the minimum width of a line. In raised text, it will be the minimum space between lines, either within a letter or number or between characters. Obviously this determines the minimum size of text that can be cut, but it also affects the style of text. Sans serif fonts (those with plain ends on the main strokes) are a better choice than serif fonts (the ones with short lines at the ends of their main strokes). The reason is that, in some fonts, serifs are thinner than the main strokes and, thus, harder to machine. (Arial is a typical sans serif face; Times is a common serif face.) In Firstcut’s process, areas that are too narrow for our cutters will be left uncut. Any area in your 3D CAD model that cannot be cut will be indicated by coloured areas in your FirstQuote (see Figure 2, where the blue colour is indicating material that we are unable to remove).
Figure 2: FirstQuote as-milled view shows recessed lettering on a part. The blue coloured patches indicate material that will remain on the part.
Rounded cutters will affect the shapes of corners. Outside corners can be cut square, but narrower inside corners will be rounded to the radius of the cutter. So, for example, a raised letter “N” will have sharp corners at the tops and bottoms of the uprights, but the narrow angles where the sloping line meet the uprights will be rounded. A recessed letter “N” will be exactly the opposite. (These are the opposite of the Protomold moulding process in which the mould, not the part, is machined.)
Typically, text on an orthogonal face (one of the six sides from which our cutters work) will be cut with a square-end mill. Text on any other plane is cut with a ball-end mill. The angle at which the cutter approaches a feature will affect the size and sharpness of features that can be cut. As the angle of approach moves off perpendicular, the minimum width of the cut that mill can make will increase. Also, the sides of the line being cut will be off-vertical, potentially reducing the sharpness of the character. For this reason, text should be placed as close to one of the orthogonal planes as possible.
Text on a curved surface can be challenging. For maximum clarity, do not position such text perpendicular to the surface; instead, position it perpendicular to a virtual plane (ideally one parallel to an orthogonal surface). Consider, for example, text on a vertical cylinder. If the text radiates outward from the centre of the cylinder, most of it will be at off-angles to the orthogonal plane in which the cutter operates (see Figure 3). Instead, you can use your modelling software to create a virtual plane within the cylinder that is parallel to an orthogonal face, create the text on that plane, and “extrude” it toward the surface of the cylinder. The text can be left recessed below the surface or raised above it, but in either case it will be cut as sharply as possible for maximum readability.
Figure 3: For recessed lettering on a cylindrical part, the coloured polygons indicate material that will remain on the lettering because these areas are inaccessible from any milling direction.
In summary, when it comes to text, larger is better, and location on an orthogonal plane is preferred. Check your FirstQuote for indications of problems, and if you still have questions, don’t hesitate to call +44 (0) 1952 683047.