Rotary of an Engraving Tool
©1998-2004 Antares,  Inc.

Description
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  industry.

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.


Engraving  Cutters
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 specific for certain applications  and materials. Most engraving cutters are "half-round" tools which means  the blank is split or halved on center producing a "single-lip" tool which  is one of 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 also made as "quarter-round" tools which allow 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 adjustments and changes. Top-loading  cutters are most commonly available in 1/8", 11/64", 1/4", 4mm, and 6mm  shank diameters. Cuter lengths vary to accommodate machine spindles and  accessories (burnishing attachments, vacuum chip removers,  etc.).

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. A collet is a  segmented, clamping device somewhat similar to 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 applications due to its toughness and durability.  Generally speaking, carbide cutters will outlast HSS cutters by a factor  of 5-10 times depending on the material being cut.

Cutters  manufactured from high speed steel do not have the hardness or strength of  carbide. Therefore, they become dull more quickly than carbide tools. On  the other hand, high speed steel cutters are not as brittle as carbide,  and tend to be the best choice when making deep, fine cuts in metal such  as those required for making seal  dies.



Terminology
While there is a seemingly  infinite number of cutter sizes and shapes, engraving tools fall into two  basic categories - conical and parallel. Conical cutters have an angled  cutting edge and produce a "vee" shaped, flat-bottomed cut. Parallel  cutters have a straight cutting edge that is parallel to the cutter's axis  of rotation and produce a cut with straight walls and a flat  bottom.



Cutter Geometry

The various angles on a  cutter are referred to as its geometry. Each angle plays an important role  in how well a cutter performs for 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 and  determines how fine the cutting edge is. The clearance angle is determined  by the properties of the material being engraved. Generally, softer  materials require a larger clearance angle for chip removal than that  needed for hard materials.

Most cutters fall into one of five  Antares clearance classifications: ACR (acrylic)
FLX (soft plastics -  flexible engraving stock)
PHN (rigid plastics - phenolic)
BAL (soft  metals - brass, aluminum)
SSS (harder metals - 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, it would be dulled rather quickly. Conversely, a  cutter sharpened with a smaller clearance angle for harder materials will  not produce clean, quality cuts in softer materials.

The CUTTING  ANGLE is the angle formed between the cutter's axis of rotation and its  cutting edge. This determines the shape of the cut. Higher angles produce  stronger tools and broader cuts and are recommended for harder materials.  As a generalization, the standard cutting angle for most materials and  applications is 30. For harder materials like steel and brass a 40 angle  is recommended and 20 would be choice for extremely fine or delicate work  in soft materials.

The TIP 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 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 accurately defined  and measured as the as twice the distance from the tool centerline to the  cutting edge. The width of cut is most correctly defined as the width  produced at the bottom of the cut. (Note: even though the flat at the  cutter tip is angled for clearance, the bottom of the cut will be flat -  not angled.)

Cutter width is selected based on character height and  font style. In general, single stroke characters should have a width that  is approximately 12% of the character height. For example, a quarter inch  (.250") letter should have a .030" tip (.250" .12 = .030"). It may be  desirable to decrease tip width on condensed fonts and increase it on  extended ones. On multiple line fonts, the cutter width should be such  that there will be slight overlap on each pass.

The finishes on the  cutting surfaces are also very important in terms of the quality of the  cut and the durability of the cutter. 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 grit of 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 grinding marks  produced by coarse grits or improper grinding procedures, the result will  be a cutting edge that is irregular and serrated. Depending on the  severity of the marking, it can lead to rough and burred cuts with poor  surface finishes. Additionally, each serration is a weak point that can  quickly dull or break off, exaggerating the problem further. All Antares  carbide tipped and solid carbide cutters feature our exclusive Microedge®  finish that provides optimum performance and tool life.

During the  engraving process, the cutter rotates and moves through the material. The  actual cutting is produced by a shearing action between the cutter and the  material. As the cutter engages the material, the cutting edge meets with  resistance and slices off a piece of the  material.



Speeds and Feeds
The rate of the cutter  rotation is referred to as the cutting speed, and the lateral movement is  the feed rate. Each has a profound 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  proportionally with tip size. For example, a .030" tipped cutter turning  at 10,000 rpm has a speed of approximately 75 sfpm while a .060" tipped  cutter rotating at the same speed generates about 150 sfpm. It is apparent  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. The following table and graph can be used as a  guide.

Cutter Speed in Revolutions Per Minute

Cutter Size - Measured at Tip

Material

.015"

.030"

.060"

.090"

.125"

.171"

.250"

Plastic Engravers Stock (FLX)

15,000 to 20,000rpm

12000
rpm

10000
rpm

Engravers Brass

10,000 - 15,000rpm

13500
rpm

9500
rpm

6500
rpm

5000
rpm

Free Cutting Aluminum

15,000 - 20,000rpm

7500
rpm

10000
rpm

14000
rpm

Mild Steel

15000
rpm

10000
rpm

5,000rpm

3,500rpm

2500
rpm

1500
rpm

1200
rpm

Hard Steel / Stainless Steel

12000
rpm

6000
rpm

3000
rpm

2000
rpm

1500
rpm

1000
rpm

750rpm

Wood

20,000rpm

Cutter speeds can vary greatly based on factors such as feed  rates, depth of cut, and the use of cutting fluid. The above chart is  intended to serve primarily as a comparison if cutter speeds in various  materials.

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 rate 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  harder materials, the shock created by this impact can damage the cutter  and quickly destroy its edge, thus slower feed rates are  dictated.

While the above situation not as dramatic and detrimental  when involving softer materials, a cutter still needs time to cut. Too  high a feed rate tends to tear the material rather than cut it cleanly,  resulting in rough, burred cuts. As a rule-of-thumb, the feed rate should  be adjusted to allow maximum engraving speed without sacrificing the  quality of the finished cut.

On softer, free-cutting materials like  flexible engraving stock, one pass is generally sufficient to produce a  good, smooth cut. On harder materials such as steel, brass and even  acrylic, two or more passes are recommended. The first does most of the  cutting, while the second cleans out the chips and removes the  burrs.

One problem inherent to some machines common to the awards  and engraving industry is their lack of power and torque at lower speeds.  If the cutter speed is reduced appropriately for harder materials, there  is insufficient power to produce a quality cut. Engraving machines are not  milling machines and care must be taken to not exceed their  capabilities.



Cutting Fluids
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 maintain  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 as guidelines.

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  very successfully with the use of a 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.
20,000

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