Rotary engraving is the term used to describe engraving done with a rotating cutting tool in a motorized spindle. The tool, or cutter, cuts into the surface of the material to a predetermined depth and produces a groove of the same shape and dimension of the cutter. Rotary engraving can be performed on a wide variety of materials with plastic, brass and aluminum being the most common in the awards and engraving industry.
Standard Conical Cutter
Rotary engraving can be done using the simplest pantographs to the most complex computerized engraving machines. The principles are the same on all. On a pantograph, the operator lowers the cutter into the material and then forms the character by tracing a master (copy type, template, etc.) On a computerized machine, the cutter spindle (Z-axis) is lowered mechanically and then is moved laterally (X-axis / Y-axis) by stepper motors to form the characters.
The tools used for rotary engraving are generally referred to as ‘cutters’. Cutters are manufactured from different materials and are produced in a variety of configurations for specific applications and materials. Most engraving cutters are ‘half round’ tools which means that a full round blank is split or halved on center producing a ‘single-lip’ tool which is one having only one cutting edge. This configuration affords a significant amount of clearance and allows the tool to run at relatively high speeds to maximize material removal and produce good finishes. Some cutters are made as ‘quarter-round’ tools which allows even greater clearance, but they are inherently weaker and are recommended for specific applications.
The majority of the engraving machines used in the awards and engraving industry have spindles that use ‘top-loading’ cutters. These are cutters that are inserted into the spindle from the top and are typically held in place by means of a threaded knob. This arrangement allows for easy cutter changes and adjustments. Top-loading cutters are most commonly available in 1/8″, 11/64″, 1/4″, 4mm and 6mm shank diameters and in a variety of lengths to accommodate machine spindles and accessories (burnishing attachments, vacuum chip
Some machines, particularly industrial ones, utilize collet spindles. The cutter is inserted into the top or the bottom (usually the bottom) of the spindle and is held in place by a collet which is a segmented, clamping device somewhat on the order of a drill chuck. By means of a ‘drawbar’, the collet segments are tightened against the shank of the tool holding it securely in place. This arrangement is more rigid and precise than the top-loading spindle, but does not offer the ease of cutter change and adjustment.
Most engraving cutters are manufactured from carbide or high speed steel (HSS). Carbide is an extremely hard and abrasion resistant material and is recommended for the majority of engraving cutter applications due to its toughness and long life characteristics.
Cutters manufactured from high speed steel do not have the hardness or the toughness and, therefore, become dull more quickly than those made of carbide. They are not as brittle, however, and tend to be the best choice when making deep, fine cuts in metal as in the engraving of seal dies and the like.
The various angles on a cutter are referred to as its geometry. Each plays an important part in how well a cutter performs in a particular application.
The CLEARANCE ANGLE refers to the angle of the cutting edge with respect to the face of the cutter. This angle allows for chip clearance, determines how fine the cutting edge is and is selected based on material properties. Generally, cutters for soft materials have greater clearance than those for hard materials. Most cutters fall into one of four Engravers Express clearance categories: FLX (soft plastics, i.e. flexible engraving stock); PHN (rigid plastic, i.e. phenolic); BAIL (softer metals, i.e. brass and aluminum) and SSS (harder metals, i.e. steel, stainless steel)
A cutter for flexible engraving stock has a high degree of clearance and a correspondingly fine edge. If this cutter were used to engrave hard steel, its edge would be ruined rather quickly. Conversely, a cutter sharpened with less clearance for harder materials will not produce as clean a cut in soft ones.
The CUTTING ANGLE is the angle formed between the cutter’s axis of rotation and its cutting edge and determines the shape of the cut. Higher angles produce stronger tools and broader cuts and are recommended for hard materials. Tools with lesser cutting angles are weaker, but are best for fine work in softer materials.
The TIP-OFF is the flat at the tip of the cutter which determines the width of the cut. Since an engraving cutter needs to be .end-cutting’ as well as ‘side-cutting’, the cutter tip is actually a cutting edge. It is formed by two angles that provide clearance and are selected based on the material being engraved. Tip width is most correctly defined as being the width produced at the bottom of the cut.
Cutter width is selected based on character height and font style, As a rule of thumb, single stroke characters should have a width that is approximately 12% of the character height. As an example, a half inch (.500′) letter should have a width of .060″(.500″X.12=.060′). lt. may be desirable to decrease width with condensed fonts and increase it on extended ones. On multiple line fonts, the cutter width should be such that there is a slight overlap on each pass.
In addition to the various angles, the finishes on the cutting surfaces are very important in terms of the quality of the cut and how well a cutter holds up. A grinding wheel contains abrasive particles (grit) that act like miniature cutting tools and produce a series of grooves in the surface of the part. The finer the wheel, the smaller the grooves and the better the finish.
The cutting edge on an engraving cutter is the junction of the face and the back of the cutter. If either of these surfaces have grind marks produced by coarse grits or improper grinding procedures, the result will be a cutting edge that is irregular and serrated. Depending upon the severity of the condition, it can lead to rough and burred cuts with poor surface finishes. Additionally, each serration is a fragile point that can quickly dull or break off further exaggerating the problem. Engravers Express Micro-edge cutters are manufactured with extremely fine finishes that allow for optimum performance and tool life.
During the engraving process, the cutter rotates and moves forward through the material. The actual cutting is produced by a shearing action between the cutter and the material. As the cutter moves forward, the cutting edge engages the material, meets with resistance and shears or slices off a small piece of the material.
SPEEDS AND FEEDS
The cutter rotation is referred to as the cutting speed and the lateral movement is the feed rate. Each has a pronounced effect on the quality of the finished cut. The cutting speed is actually the measure of the distance traveled in surface feet per minute (sfpm) by the cutting edge and varies proportionately with the tip size. For example, an.030″ cutter turning at I 0,000 rpm has a speed of approximately 75 sfpm while an .060″ cutter rotating at the same speed generates about 150 sfpm. It is obvious then, that small cutters need to turn faster to achieve the same results as larger ones and vice versa. Cutter speed is determined primarily by the material being engraved and the following table and graph can serve as a useful guide.
Feed rate should be proportionate to cutter speed and is dictated by material properties, horsepower and torque. At a given cutter speed, a slow feed will produce more, smaller cuts and finer finishes. A higher feed will produce fewer, larger cuts and rougher finishes. Due to its single-lip design, an engraving cutter makes an ‘interrupted cut’ which means the cutting edge is not continually engaged in the material. At each rotation, the cutting edge hits the material as it starts the cut. On hard materials, the shock created by this intermittent action can damage the cutter and quickly destroy its edge, therefore dictating slower feeds.
While the above situation is not nearly as dramatic on softer materials, a cutter still needs time to cut. Too high of a feed rate tends to cause the cutter to tear the material rather than cut cleanly resulting in rough, burred cuts. As a rule-of-thumb, the feed rate should be adjusted to allow maximum engraving speed without sacrificing quality of the finished cut.
One problem inherent to many machines common to the awards and engraving industry is their lack of power and torque at low speeds. If the cutter speed is reduced appropriately for hard materials, there is insufficient power to produce the force required to make a good cut. Engraving machines are not milling machines and care should be taken not to exceed machine capabilities,
Many of the materials common to the awards and engraving industry can be cut effectively without the use of cutting oils or lubricants. Flexible engraving stock, phenolic, engravers brass and aluminum all fall into this category. There are many other materials, however, that must be cut with a cutting fluid to achieve satisfactory results and reasonable cutter life. Cutting fluids keep the cutter cool and prevent chips from adhering to the cutting edge.
The subject of cutting oils is very specific and complex, but the following are generalizations that may be helpful in a broad sense.
All steels should be engraved using an appropriate cutting fluid to improve the cut and extend tool life. Soft aluminum that is not ‘free-machining’ can usually be engraved effectively using kerosene or a tapping fluid specifically formulated for aluminum. Plastics that tend to melt when engraved can often be engraved with the use of water soluble cutting oil. Engraving acrylic is a good example of this.
The use of cutting fluids even on materials that can be cut dry will often improve the finish of the cut and extend tool life.
ENGRAVERS EXPRESS has a complete line of diamond gravers, rotating diamonds, burnishers and rotary cutters for all popular engraving machines to meet all your needs and applications