AFP/ATL Glossary of Terms 

AFP-Automated Fiber Placement The process by which a machine lays narrow tows on a form.
Angled Cuts

If a machine's head is equipped with rotatable cutters then the ends of tows can be cut to match ply boundaries.

ATL-Automated Tape Layup The process by which a machine lays wide tapes on a form.
Barber Pole Barber pole is a synonym for spiral.
Bat's Ears See Flag.
Bird's Beak See Flag.
Butt Splice

A full course butt splice is used to interrupt a very long course, like a spiral, which requires more material than can be loaded onto a machine's head at once. All of the course's tows are terminated at the butt splice, and restarted in another tow-packet at the same location. The tows of the two packets do not overlap. For structural integrity, a butt splice should only be placed in a cutout. A butt splice can also be used in a 90 degree tow packet which loops back on itself, if there is a suitably located cutout. This practice will avoid numerous staggered tow splices.

CATPart CATIA V5, from Dassault Systèmes, is a CAD (Computer Aided Design) system. It stores points, curves, surfaces and solid models that describe a design in the CATPart file format, which is proprietary to Dassault Systèmes.

A course consists of a head path and all the tows which are laid as a machine's head traverses the path. In the picture below there are 5 courses, labeled A thru E. Note that course C has a head path which is interrupted, but when combined with all the tows, is still considered just one course. But it contains two tow-packets, while the other four courses have one tow-packet each.

Course Spread Ideally the maximum width of a Course would simply be the sum of the widths of the maximum number of tows laid by a head. In practice it is often spread wider by the pressure of the roller. This extra width is called "course spread" and is typically less than 1%.
Course Stagger

When many layers are placed on a flat panel, it is desirable to ensure that layers with the same ply angle do not have their course edges exactly aligned. This can be achieved by offsetting the courses perpendicular to the ply direction, by a distance which is not a multiple of the tow width. With 12 or 16 tows, and plies in 4 directions, offsetting successive plies with the same angle by 1/5th of the course width ensures that edges will not align until the 21st layer. This precaution is of dubious value on a curved surface, because the additional thickness of four extra layers will cause the fifth layer's course edges to be differently distributed when compared to the those in the first layer.

Cutout A cutout is an area on a form where tows will be laid, but it will later be cut from the composite structure, and either scrapped or used in a non-load-bearing capacity. Any number of splices can be programmed in this area, without regard for their length or proximity. Safe restarts are also often programmed in these areas. Access hatches and small windows are typical cutouts.
Dead Zone Dead zone is a synonym for a cutout.
EEOP Engineering Edge Of Part. Typically the manufacturing edge of part is larger, and the material will be trimmed after curing, to the design or engineering edges.

A flag is formed by a small number of minimum length tows at a sharp external corner of a ply boundary. The excess material in these tows can all be on one side of the flag, as shown at "A" in this picture, it can be placed on either side of the flag, as at "B", or alternate tows could have their excess in opposite directions. The configuration at "A" is sometimes called a "bird's beak", while that at "B" is referred to as "bat's ears".


A form is the surface on which composite material is laid. It may be mounted on a rotisserie, as in this picture, which enables it to be indexed so that different areas of the surface are presented to a head for painting, or the rotisserie can spin while tows are laid. Mold, mandrel and tool are alternative terms for a form.

Guide Curve A curve which is on, or close to, a form can be used to specify the head paths in a ply. This is an alternative to using a rosette rule or a spiral. The first head path will coincide with the projection of the guide curve on the form, and the other head paths will parallel the first one. Typically this technique is used to lay a test coupon which explores or verifies the capabilities of the machine.
Geodesic Path

A geodesic path is one of minimum length between its ends. On a plane it would be a straight line, and on a sphere it would be part of a "great circle". Geodesic paths are commonly used in filament winding of pressure vessels.


A head is the part of a composite fiber placement machine that brings together a set of tows and feeds them to a roller which presses them onto the form. The spools of tow material may be mounted on the head, or the tows may be transported from a creel area to the head, as needed. In this picture, the 12 large cylinders around the outside represent spools of material, and the smaller cylinder in the center represents the roller.

Head Path

 A head path is the curve that the centerline of a head's roller needs to traverse to lay tows on a form. In this picture there are 5 green head paths, labeled A thru E. Note that head path C is interrupted, but still considered to be a single path. A head path may comply with a rosette rule, trace a spiral around a form mounted on a rotisserie, be parallel to a guide curve, or be parallel to a prior head path.

Hoop Ply A hoop ply is one laid on a spinning form where the desired ply direction is perpendicular to the axis of rotation. If there is sufficient tolerance in the ply direction, a spiral technique can be used. Otherwise the ply will contain numerous splices.
Layer A layer, also often called a sequence, is the set of plies that are at the same depth in a composite structure's lay-up. Typically each layer would be inspected prior to covering all or part of it with the next layer's material.

 A lead-in is the portion of a head path, where no tows are being laid, that precedes the application of any tows. A lead-in is typically longer than a run-out. In this picture, both lead-ins and run-outs are colored red.

Mandrel Mandrel is a synonym for form.
MEOP Manufacturing Edge Of Part. Usually larger than the engineering edge of part so that the material can be trimmed after curing, to the final size.
Minimum Length Tow

A typical head design involves a roller which presses tows onto the form, a powered feed mechanism that moves tows until they are pinched between the roller and the form, and a set of knives that can cut the tows individually. The distance between the roller/form contact point and the knives determines the minimum length of tow that can be laid. If the trailing edge of the tow is cut before the leading edge has been pinched by the roller, it will fall to the floor. It is impossible to lay the short tows that would be called for by a ply boundary with a sharp external corner. Minimum length tows are laid instead.

Minimum Steering Radius Unless a tow is allowed to follow its natural path on a form, it is "steered" by the head along the required path as it is laid. But there is a lower limit to the radius that it can be persuaded to follow, without causing wrinkles or puckers. The steering radius is determined by comparing the stretch required on the outside of the bend with the compression required on the inside.
Mold Mold is a synonym for form.
Natural Path

Typically tows wider than 1.5" (38mm) cannot be "steered" as they are laid onto a form. A natural path is the shape that such a tow will assume if it is hand laid and allowed to "drape", with no guidance. A machine can be used to lay wide tows only if it is programmed to follow the desired natural paths. This picture illustrates a single tow conforming to a natural path on a conical form.

NX NX, from Siemens AG, is a CAD (Computer Aided Design) system. It stores points, curves, surfaces and solid models that describe a design in the ".prt" file format, which is proprietary to Siemens AG.
Packet There are two types of packet, tow-packets and work-packets.
Pad Some machines require the roller to be in contact with the form, or previously laid material, for some distance prior to the start of a new tow. Typically, ply boundaries for such a machine are offset to create ramps so that the roller can run over progressively more prior layers before introducing tows for the next layer. When there isn't enough space for this technique to be practical, or when the cost of the extra material which will be cut off later is prohibitive, an alternative is to insert inexpensive pads. After some small number of layers are placed, perhaps 10 or 20 that have coincident ply boundaries, a small step is formed. An adhesive pad is butted up to this step, so that the roller won't bump into it just as tows need to be started for the next layer.
Painting When tows are laid on a stationary form, the machine's head is said to be painting. If tow-laying can be reversed quickly, permitting bi-directional application of material, painting can generate significantly better lay-down rates than matching the head motion to a spinning form.
Paint Zone A paint zone is an area of a form which can be accessed by a machine's head without indexing or spinning the rotisserie.
Path There are two types of path, head paths and tow paths.

A ply is a single contiguous area of composite material to be laid on a form. There can be one or more plies in each layer of material. Two plies in the same layer do not overlap.

Ply Angle Ply angle is a term used in conjunction with a rosette rule to specify the desired direction of all the head paths and tow paths in a ply. If the form is mounted on a rotisserie, then by convention a 90 degree ply is one where the desired path direction is perpendicular to the axis of rotation.
Ply Boundary

A ply boundary is the set of curves on a form which determine the extent of a ply's material. There may be external and internal portions of the ply boundary, as illustrated by the pink lines in this picture.

Post-processor A post-processor, often shortened to "post", is a piece of software that converts the internal format of, or an external neutral format from, a CAM (Computer Aided Manufacturing) system to the G-Code format required by a particular NC (Numerically Controlled) machine.

A ramp is formed when ply boundaries for consecutive layers are offset. If the offset distance between boundaries is 20 times the material thickness, the result is called a 20:1 ramp.

Roller A roller is the component of a machine's head which presses tows onto a form. It is typically quite "spongy" so that it can apply pressure to tows across almost its full width, while conforming to the contours of a double-curved surface. There can be a single roller for all the tows laid by a head, individually articulated rollers for each tow, or an intermediate arrangement with several tows per roller.
Rosette A rosette is a coordinate system, associated with a set of rules, which ensure that all the plies maintain their correct relative angles across the double-curved surface of a form.
Rotatable Cutters

Some machines are equipped with tow cutters that can be programmed to produce angled tow ends. Typically the head must be lifted from the form if the angle at the start of one segment of tow differs from the angle at the end of the prior segment. When only some of the tows in tow-packet are interrupted, it is not possible to lift the head to adjust the angles. In this case, the ends may be cut at the mean of the two "ideal" angles.

Rotisserie A rotisserie is a mechanism for rotating a form. It may be used to index the form's location, presenting different areas of the form to a machine head. Or it can be spun, probably at a constant rotational speed, while one or more heads lay tows.
Restart "Restart" is a contraction of safe restart.

A run-out is the portion of a head path, where no tows are being laid, that follows the application of all tows. A run-out is typically shorter than a lead-in. In this picture, both lead-ins and run-outs are colored red.

Safe Restart

A machine malfunction, a power failure, or a break within a spool of tow material can make it impossible to complete a towpacket as required. It may be necessary to strip off all the material laid in the incomplete packet, then restart the machine from the beginning of the packet. For a long tow-packet, such as a spiral, this can be a time consuming and costly issue. To alleviate the beginning of the packet. For a long tow-packet, such as a spiral, this can be a time consuming and costly issue. To alleviate the effects of a malfunction, the program can include numerous safe restart points. At each of these points the program contains all the instructions required to complete the tow-packet from that location onward. These instructions include a new lead-in. The incomplete tows would need to be cut at the safe restart point, and only the "forward" portions would be stripped off the form. When the remainder of the packet is laid, the result is a butt splice at the safe restart point, which is why these points are usually placed in cutouts.

Another possible location for a safe restart is within a segment of a tow-packet where no tows are laid. There can be segments like this, often shorter than the sum of the lead-in and run-out, where it is inefficient to raise the head from the form.

Safe Restart Zone "Safe restart zone" is a synonym for a cutout.
SAT File ACIS, from Spatial Corporation, is a solid modeling system used as the kernel of many CAD (Computer Aided Design) systems. The SAT file format is proprietary to Spatial, and a SAT file contains the points, curves, surfaces and solid models that describe a design.
Sequence Sequence is frequently used as a synonym for layer. However it is also used to describe the order in which tow-packets are linked to form a work-packet.

A spiral is a technique used to lay a ply on a spinning form when the desired direction of lay is perpendicular to the axis of rotation. By allowing the head paths and tow paths to deviate slightly from the desired direction, the tows can be laid in a single course which spirals around the form. This has two benefits; it eliminates splices from the ply, and it is quicker because there is much less off-part motion. A possible limitation of this technique is that the head may not be able to access enough material for the entire course.


There are two types of splices, a full course butt splice and the more common tow splice. A tow splice is where the start of a laid tow is overlapped by the end of the same or another tow. There may be joined tows in a spool of raw material, and it is very difficult to predict where these may end up as splices in a ply. Splices may also be programmed into a ply, the most common case being when the ply direction is perpendicular to the rotation axis of a spinning form. Each uninterrupted course in such a ply will loop around the form and back on itself. For strength, the splices are typically staggered, as shown in magenta in this picture.

Stagger "Stagger" is a contraction of course stagger.
Steering Radius See Minimum Steering Radius.
STL File An STL file is a commonly used to communicate a faceted three dimensional surface model. The STL format originated with STereo Lithography, and such a file usually has the ".stl" extension to its name.
Tape A tape is a continuous strip of composite material which is typically purchased wound on a spool. Common tape widths are 6" and 12", with thickness of a few thousands of an inch.

A tow is a continuous narrow strip of composite material which is typically purchased wound on a spool. The material is sometimes called "slit-tape tow" because it is manufactured by slitting a much wider roll. Common tow widths are ½" and ¼", with thickness of a few thousands of an inch. In this picture, there are 12 yellow strips representing 12 tows laid on the gray form.


A tow-packet is the portion of a course from the start of a lead-in to the end of a run-out. In this picture, there are 6 tow-packets labeled A thru F. Note that packet E has a central segment where no tows are laid, but because the head is not retracted from the form, the course is not split into two tow-packets. In contrast, packets C and D are derived from a single course.

Tow Path A tow path is the centerline of a tow on a form. VCP VCP, short for "VERICUT Composite Programming", is software from CGTech and is used to generate G-Code programs for a composite fiber placement machine.
VCP VCP, short for "VERICUT Composite Programming", is software from CGTech and is used composite fiber placement machine. to generate G-Code programs for a composite fiber placement machine.
VCS VCS, short for "VERICUT Composite Simulation", is software from CGTech for simulation of the action of a composite fiber placement machine and analysis of the resulting plies and layers.
Virtual Spin Axis  If a portion of a mandrel is distinctly eccentric when spun on a machine's rotisserie, it can result in rapidly varying surface velocities relative to the roller. To mitigate this problem, some machines are able to make the tow laying head follow this "cam action". If you attached a video camera to the spinning mandrel, the head would appear to rotate around the center of the mandrel's cross-section instead of about the rotisserie's axle.
Work-Packet A work-packet is the set of tow-packets passed to a composite fiber placement machine in a single G-Code file, and includes any required linking motion between tow-packets. A work-packet can contain anywhere from one to hundreds of tow-packets. Typically all the tow-packets in a work-packet will be on the same layer, because of the need to perform inspection between layers.
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