Computer-aided manufacturing

Industrial manufacturing methods are generally classified as subtractive or additive

Subtractive manufacturing methods involved cutting material away from the blank. This is how lathes, milling machines and grinding machines operate.

Additive manufacturing methods involved adding material to the part. The simplest example is 3D printing. The part is constructed via adding material until the final shapes is achieved. Welding a structure together is also considered additive manufacturing.

All traditional manufacturing machines began life manually operated. People were required to operate the controls to move the operating head in the right path to create the desired shape. As design and technology progressed, the skills required to achieve high precision in fields such as aviation required operators with very high skills.

In the quest to produce “more” and “faster” for “less money”, computer machine controls began to appear. These systems literally took over the role of an operator by moving the operating head on the desired path. The person spinning the handwheels were replaced with electric motors, with computer programs used to control the motors and thus drive the position of the operating head.

Numerical control

Numerical Control (NC), or Computer Numerical Control (CNC), is the umbrella term for the technology of controlling manufacturing machines.

The motors that move the material - or the operating head - need to be told precisely where to move from, and to.

The term G-code is commonly used in these discussions, and G-code is a language specifically for controlling movement of machines. M-Code is the language used for miscellaneous controls.

G-code comprises of the letter “G” followed by a two- or three-digit number, and then a cartesian co-ordinate (X, Y and Z) specifying a precise location is space (remember your high school math?!).

Each G code refers to a specific movement. An example of a single line of code is:

G00 X30 Y20 Z60 M03


  1. G00 is the code for Rapid Tool Movement
  2. X30 is the x-coordinate or position.
  3. Y20 is the y-coordinate or position.
  4. Z60 is the z-coordinate or position.
  5. M03 is the code for Spindle On, Clockwise

So, in plain English, this line of code says: turn the spindle on in the clockwise direction, and undertake rapid movement to position (30, 20, 60).

Prior to the availability of computers on the manufacturing floor, a part programmer would inspect the drawing, make decisions about the sequence of machining tasks, then select and key appropriate G-codes into a machine to generate a punched paper reel. The paper reel would then be loaded into the lathe or mill, and the punched paper reel would provide the instructions to the machine to control motion. Obviously, any human error would require this entire process to be repeated until the part was produced exactly.

Nowadays, the part planner’s role is mostly redundant, as software can take the 3D model and constraints entered by the user to automatically generate the G-code needed to create the part. This part of the process is often called Computer-Aided Manufacturing, or CAM.

The G-code file is then loaded or transmitted to the machine’s computer controller which simply obeys the commands it is given. Depending on the complexity of the part being created, the file could contain many thousands of lines of G-code.

As with any process, if the user puts garbage in, they’ll get garbage out, so the use of these tools takes some education, but they are well within the realm of the enthusiast custom motorcycle builder.

Numerical control via G- and M-code underpins all the manufacturing equipment you might find in industry and in the garage of a custom motorcycle builder. Milling machines, lathes, machining centers, 3D printers, laser cutters and routers all share common G-code to create desired parts.

Additive manufacturing

One common branch of additive manufacturing involves a machine dispensing raw, and often liquified material from a nozzle into precise locations to build up a three-dimensional shape. It is common to refer to this type of additive manufacturing as "3D printing".

As with any manufacturing technology, several types of 3D printing have evolved over time. Of most interest to bike builders would be cheaper plastic filament 3D printers and metal 3D printers.

A 3D printer deposits thin layers of material in multiple passes. Special software called slicers are required to cut the 3D model into layers that can be printed. Each layer has its own G-code. Once the layer is complete, the printer head raises slightly – by the amount of the layer thickness, and the printer deposits the next layer. This process is repeated until the part has been constructed, layer by layer.


The precursor to metal 3D printing is older technology called powder metallurgy and sintering. Powder metallurgy is a blanket term for manufacturing processes where metals in powder form are pressed into precise shapes. This is distinct from subtractive manufacturing like machining, as there is little material wasted and this helps reduce manufacturing costs.

In some powder metallurgy applications, the part must be heated to create the bond between the powder particles (in addition to compression). This heating process is called sintering and is done with special ovens. Making gears from powder metal and sintering is a common and cost-effective manufacturing process compared to a more expensive approach of machining gears from chunks of metal.

In the 1980s, Selective Laser Sintering (SLS) was developed. The SLS process uses a laser to provide the targeted heat to sinter specific locations in a layer of powder. Once the sintering is complete, the build surface lowers and a *recoater* spreads another thin layer of powder across the bed, and the next layer is sintered. The process continually repeats to build up the final shape.

A more recent development is Selective Laser Melting (SLM). The difference between SLS and SLM lies in the temperatures. Sintering uses heat below the powder melting point to create a chemical bond between the powder particles. SLM uses a heat source to raise the powder above its melting point. This liquifies the powder being heated. However, both processes require expensive machines.

The latest generation metal 3D printers now emerging use Fused Deposition Modeling (FDM), or Fused Filament Fabrication (FFF) (this is the technology also used in consumer-grade plastic 3D printers) For metal 3D printers, powdered metals are mixed with a polymer-based binder, which is then made into the filament.


Basic plastic 3D printers are now very affordable, and seem to be available from every electronics, computing and big box store you visit, plus a myriad of online vendors.

Today’s 3D printer evolved from industrial processes for depositing glue and gasket materials. A company called Stratasys pioneered early development in the 1990’s and commercialized the first 3D printers. Once original patents expired, the open-source community collaborated with the UK’s University of Bath on the RepRap (Rapid Replicator) 3D printer project. A “rapid replicator” is intended to manufacture (or, in this case, print) the parts needed to make another identical machine. This project significantly decreased the cost of 3D printers and laid the foundations for the consumer-grade 3D printers available today.

As mentioned above, common terms attached to 3D printing with plastics are Fused Deposition Modeling (FDM) and Fused Filament Fabrication (FFF). The FDM acronym was trademarked by Stratasys, so other vendors of printers and the 3D printing community will often refer to “Fused Filament Fabrication” to prevent infringements.

Industrial 3D printing techniques developed by Stratasys also included heated printing chambers and build platforms, which improve the properties of the printed part. In our consumer grade printers, the hot nozzle deposits material into a cooler atmosphere, and on very cheap printers, a cold print bed.

The printing process is straightforward in theory: a motor drives plastic filament into a heated nozzle (or hot end). The nozzle melts the filament, and the control system in the printer moves the print head, build platform - or both - to the precise location needed. The nozzle deposits the molten plastic onto the bed. Once the layer is complete, the control system moves either the print head or build platform apart by one layer thickness. Then the process repeats layer upon layer until the part is constructed.

Consumer-grade 3D printers are limited by the filaments they can extrude. The capabilities of the printer hardware – such as the hot end temperature limit, and the heated bed temperature limits – dictate the usable filaments. Common materials include polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polyethylene terephthalate glycol (PETG) and nylon. Different materials have different properties.

While most printers ship with PLA filament, it’s very low melting point means it weakens above 50 degrees Celsius. In this respect, PLA prints are risky to use in a motorcycle application where the engine and exhaust will exceed these limits. Even a stationary motorcycle in the summer sunlight will absorb enough heat at times to reach these temperatures.

ABS filament has higher impact resistance, is lighter, and has higher heat resistance than PLA. While this makes it more suitable for a prototype motorcycle part, ABS requires a heated printer bed, and the parts are more susceptible to warping and dimensional inaccuracy. These factors may increase problem of your parts fitting correctly.

PETG combines some of the desirable properties of both PLA and ABS. It has good moisture and temperature resistance, dimensional stability and low shrinkage. PETG requires higher extrusion temperatures so the printer hot end will need the capability of melting this filament.

Each filament material performs best with specific printing parameters, such as the temperatures mentioned above – so specific printing profiles can be configured and saved to suit different filaments.

Before the printer can start, another processing step is required. The printer deposits the molten filament in layers, so the 3D model must be analyzed by a slicer to create the G-code for each layer.

In addition to slicing the model into printable layers, other parameters can be configured, including the wall thickness, and infill percentage. While it is possible to print a completely solid part, the printing time would be astronomical. As a result, the slicer software creates an internal infill pattern – such as a honeycomb – at the desired infill percentage.

Print settings also impact the surface finish of the part. Visible print layer lines are almost inevitable but setting the printer to print thinner layers reduces this effect if you don’t mind dramatically increasing print time. Print layer lines might be acceptable if you plan to sand, fill and paint the part.

To print a model faster, the slicer settings might be adjusted to print thinner walls, and low infill percentage, say 15%. To print a stronger model, the walls might be made thicker, and the infill raised to 30%. The tradeoff of strength or appearance versus printing time is a decision required for every part sent to the printer.

3D print strength and durability

When an engineer assesses the strength of a part, a common consideration is the cross-sectional area of the part in question. Expected loads must be distributed across this area, leading to a certain level of stress in the material.

A 3D-printed part with a honeycomb infill pattern has an inconsistent cross section. At any location where a cross section is taken, the infill pattern will mean some sections contain plastic, and some sections are air spaces within the infill pattern. The area available to withstand the loads is greatly reduced compared to a solid part, and proportional to the infill percentage selected in the slicer.

Often a 3D printed part will need to be a bigger overall to provide enough cross-section and strength, but if your part already has a large area inherent in its design, this won’t be an issue.

This might be acceptable if the load experienced by the part is very low, but you could design a much smaller solid part to withstand that same load, rather than a larger part filled with a honeycomb infill pattern.

The ability of a part to withstand repeated load cycles is called durability. Load cycles come from the rider, the quality of the road, the ability of the tires and suspension to dampen those impacts from the road surface and wind or aerodynamic loads from travelling at higher speeds.

None of these variables can be properly quantified without testing. Motorcycle manufacturers perform laboratory testing and road testing on pre-production bikes. On a custom motorcycle, the only real testing is only on the road once the build is finished.

Considering these factors, 3D-printed parts in plastic have limited application on a custom motorcycle and careful consideration needs to be given to their use. Use as load bearing critical parts should be absolutely avoided, as should use in critical systems like suspension and brakes. Environmental factors such as heat also need to be considered. 3D printed plastic parts will be most useful in such applications as spacers, complex geometry brackets such as for a speedometer, covers and brackets for lightweight parts.


In addition to making low load parts, 3D printing has perhaps some of its greatest benefits as a very dimensionally accurate and function prototyping process.

Perhaps you want to install upgraded front forks from a later model motorcycle. To keep the fork geometry the same as the factory bike, this might require custom yokes or triple clamps machined from aluminum in a milling machine.

Prior to laying out the Benjamins for an expensive milling process and part, the triple clamp could be modelled and printed cheaply to check the fit on the motorcycle. Design changes could be incorporated and tested through successive prints until the part is perfect for the application. This approach is far cheaper than having the custom yokes machined then finding your design has problems.

Any part that will be expensive to manufacture could be considered a candidate for prototyping with a 3D printer.

Bucks and manufacturing aids

A buck is a skeleton or template for a three-dimensional part used in manufacturing low volume or one-off parts. For a custom motorcycle, a buck might be constructed for a fender or a tank. In extreme cases, bucks can be made for an entire vehicle body. Sheet metal or fiberglass formed accurately over the buck should result in a correctly fitting part.

If your custom fender or fairing has been modelled in 3D, 3D printing it might be an alternative method of manufacturing a buck. Larger parts may need be constructed from multiple prints, but if your printer can run handle long prints and run unattended (such as overnight) then bucks could be created for a few dollars of filament.

Similarly, if you need a specific assembly tool, clamp/holder or aid that you don’t have, 3D printing is an option to manufacture your own workshop accessories. Search online for websites that host existing 3D-print-ready models that you can download.

Stereolithography or Resin 3D printing

Stereolithography (abbreviated SLA) is a form of additive manufacturing. When special light-reactive resins are exposed to particular wavelengths of light, the resin solidifies. A sister technology is a Digital Light Processing (DLP) printers, with SLA using a laser and DLP using a digital light projector screen to cure the resin.

Different resin formulations produce parts with specific optical, mechanical and thermal properties.
An SLA/DLP printer contains a resin tank with a transparent base. The laser (or light) shines through the tank and solidifies the resin on the build plate.

Compared to fused filament fabrication, SLA/DLP prints have a very fine surface finish and are dimensionally very accurate. This allows them to be used in more demanding applications, such as medical and engineering parts.

Whilst manufacturers are bringing out higher performing machines at lower prices, FFF printers are very popular and tend to be the first choice. Expanding your workshop to include an SLA/DLP printer might be outside your budget, both monetary and time investment to learn the technology.

Consider the type of parts you might want to produce and stack up the capabilities of an SLA/DLP printer for your purposes. It might be a better choice than a conventional 3D printer.

Subtractive manufacturing

Subtractive manufacturing refers to removing or cutting material from a piece of stock to realize the desired part.

Traditional hand tool techniques such as sawing, filing or using a grinder are considered subtractive, as is the latest generation of multi-axis CNC machining centers and waterjet cutters.

While you can build a custom motorcycle predominately with a grinder and a file, the accuracy and speed of a machine based subtractive process can be very useful at times to the home builder.

Cutting (plasma, waterjet, laser)

Cutting parts out of flat plate can be done with an angle grinder, bandsaw or a handheld gas cutting torch.

CNC machines control the precise location of the same gas cutting torch, a plasma torch or even a suitable laser to cut parts out of a plate. These tools work well with ferrous materials (anything based on iron, such as mild steel).

For non-ferrous applications, like aluminum, a waterjet cutter is a good choice. This machine uses high pressure jets of water containing an abrasive (such as garnet) to scrape away material to form the desired shape. This technology is also suited to materials such as ceramic and glass.

While smaller workshops could feasibly purchase a small 1.2 meter square CNC plasma cutting table, the home builder wouldn’t likely justify such a purchase. You’ll need to find specialists in your nearest industrial area who can perform laser and waterjet cutting tasks for a fee. In many cases for a one-off part, the machine set-up costs can’t be recovered over multiple parts, so it may be more economical to carve the part out with an angle grinder and sanding discs.

Routing and milling

The action of routing or milling both involve a spindle rotating a cutting bit. The generally accepted definition of routing is a machine where the spindle moves over the stationary stock, and milling is the opposite, where the material moves under the stationary spindle.

While a “regular” CNC milling machine has three axes (X, Y and Z), a multi-axis machining center expands to allow rotation of the part along these axes. Rotation along X and Y axes results in 5-axis machining and adding rotation to the Z-axis gives the sixth axis. The predominant benefit of five or six axes machining is the reduced set up time in moving the part in the vice to allow the cutting tool to reach all required locations, but these are industrial level machines.

Manually-operated 3 axis milling machines are great additions to the advanced builder’s workshop for their ability to drill bigger holes than common drill presses and flatten and smooth surfaces and edge with milling cutters. For the first time builder, this type of work is likely to be out of scope or skillset.

Small desktop routers are also certainly within reach of the home builder; however, their spindles are generally underpowered for cutting metals. Very light cuts in aluminum may be possible but check reviews of the machine beforehand.

As is the case with laser and waterjet cutting, you’ll need to find a local engineering shop to do your milling if you want that custom set of triple clamps. If you aren’t able to draw the part in the format they need for conversion to milling G-code, many engineering shops can also do this for a fee. However, with high costs due to the one-off nature of the job, the more you can do yourself, the less you'll have to outlay from your wallet.

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