Material Preparation

This material is recommended for people new to milling, for surface milling jobs, and for initial geometric prototypes. It comes in two forms: expanded and extruded. Expanded foam generally consists of a number of small spheres that are pressed together to form larger blocks which are then cut into smaller blocks and sheets. Extruded foam generally is generally available in board formats and has two faces that have skins on them, which can cause the board to change bow and change shape when milled. Both forms area available in a limited range of densities. The most commonly used density of expanded foam is 2 pound.

• Advantages: inexpensive, lightweight and transportable, mills quickly with inexpensive tools, paintable with gesso and water-based paints, forgiving as related to errors in the toolpath strategy, requires minimal preparation, available in a wide range of sizes, allows relatively deep cuts as longer cutting tools can be used
  
• Disadvantages: not very rigid, so may require additional support, some forms have strong colors that may desire painting, will not withstand significant wear and tear, stock available may not be of consistent width or precisely prepared
• Sources: available in 2’ and 4' widths, up to 8’ length, and 2”, 3”, 4", and 6" thicknesses from the GSD Lab Store; Also available locally at Insulation Technologies who can cut to requested sizes
• Machines: This material is most suitable for the two CNC routers (AXYZ and C.R.Onsrud), the Roland CNC mill, and the robots

This material was designed to be milled and so performs beautifully, producing better results than polystyrene foam. A wide variety of densities are available from manufacturers, from very soft materials to very hard and dense types. For most models and parts made at the GSD, the low-to-medium density range is sufficient, between 15 and 20 lb. densities.

• Advantages: dimensional stability, uniform color, homogenous material, mills relatively quickly, capable of being milled into relatively thin surfaces, requires minimal preparation, able to be sanded and painted, allows relatively deep cuts as longer cutting tools can be used
  
• Disadvantages: relatively expensive, moderately heavy when compared to other foam, specific densities are readily available in specific colors, which are not often attractive on their own
• Sources: Sold in GSD Lab Store in a limited range of sizes (4" thickness in blocks of 2'x4', 4'x4', or 4'x8' size) of grey 20 lb foam. Other densities, colors, and sizes are available online from General Plastics, Freeman Supply
• Machines: This material is most suitable for the two CNC routers (AXYZ and C.R.Onsrud), the Roland CNC mill, and the robots
  

This type of material comes in sheets ranging in thickness from about 1/8" to 1". There are many different types of plywood and other laminated wood products. These types of products may have different grades of finished faces on one side only, on two sides, or on no sides. The inner core layers can also differ significantly. Some products have fewer, thicker layers that contain knots and voids, whereas other products will have equal thickness layers with no voids for all laminations. Some products are not laminated sheets, but laminated chips or strands of wood, instead. The adhesives used in all of these products are abrasive to the cutting tools.

Over time or during particularly long-running surface models, these tools will become dull, generating heat and degrading in their ability to cut well. In extreme cases, dull tools will cause the material to burn and potentially catch fire. Like wood, greater attention must be paid to the cutting tools and cutting parameters. Unlike foam, these materials have a greater likelihood of violently reacting to the milling process, so are less forgiving to work with. Incorrect settings are more likely to lead to broken tools, damage to the model, damage to the machine, and fire. For surface milling, we recommend marine grade Baltic birch plywood as it is made of many thin, higher quality layers than other grades of plywood. To achieve sufficient thickness, it may be necessary to glue multiple sheets of plywood together to form a block.

Material will vary slightly +/- in thickness, affecting the fit of joints and pockets. 3/4" plywood is a nominal size and may be closer to 0.715", so double check measurements with your drawings where it will affect fit and finish. Measure several locations along the edge and use the largest dimension. Additional room (~0.005") may be needed in pockets and joints to allow for expansion and adhesives.

Please review the information on compression spirals for best results when performing 2D contour and pocket cuts.

• Advantages: withstands wear and tear, layers create interesting effect (but often distracting), dimensionally stable, can be sanded and painted
• Disadvantages: potentially expensive, depending on grade chosen, may require specialized cutting tools, layers may be of different quality and contain knots, heavy, cannot make deep narrow cuts as cutting tools cannot be as long as those that can be used with foam, requires preparation to form thicker blocks from sheets, milling time is longer than that for foam
• Sources: available locally at Boulter Plywood and Home Depot
• Machines: This material is most suitable for the two CNC routers (AXYZ and C.R.Onsrud), and in limited applications, for the Roland CNC mill and the robots

Medium Density Fiberboard (MDF) and Low Density Fiberboard (LDF) are most commonly available in sheet-based form similar to plywood, but it is possible to buy blocks from specialty vendors (Freeman Supply). These products are made of wood pulp that is pressed and bonded with adhesives. The adhesives used in all of these products are abrasive to the cutting tools. Over time or during particularly long-running surface models, these tools will become dull, generating heat and degrading in their ability to cut well. In extreme cases, dull tools will cause the material to burn and potentially catch fire.

When milled these materials create a fine dust, which is a nuisance to lab staff as it makes equipment maintenance, room cleaning, and managing a healthy work environment more difficult. Compared to foams, plywood, and most hardwoods, MDF is also more demanding on generic cutting tools. Specialty endmills are better in terms of maintaining a sharp tool and extending tool life. When available, LDF that is made with non-formaldehyde resins is preferred over other types.

If you are gluing up a large multi-ply stock, consider generating a contour model and roughing out your blanks on the bandsaw before assembly. This can save huge amounts of time when milling and the contour stock geometry can be imported into MasterCAM and used to create more optimized tool paths. Use plenty of glue, spread evenly to avoid dry spots and poor adhesion between layers which will impact the end result.

• Advantages: uniform color, homogenous material, can be milled to be very smooth, more dimensionally stable than hardwood, generally less expensive than plywood, hardwood, or HD foam, can be sanded and painted
• Disadvantages: requires expensive cuting tools, creates fine dust, heavy, cannot make deep narrow cuts as cutting tools cannot be as long as those that can be used with foam, requires preparation to form thicker blocks from sheets, milling time is longer than that for foam
• Sources: available locally at Boulter Plywood and Home Depot, specialty sizes available online through Freeman supply
• Machines: This material is most suitable for the C.R. Onsrud CNC router, and in limited applications for the AXYZ CNC Router, the Roland CNC mill, and the robots

Solid wood materials come in a range range of hardnesses with varying grain qualities, making some varieties easier to work with than others. Generally, we work with hardwoods, rather than softwoods. Commonly used hardwoods include Maple, Walnut, Ash, Basswood, Poplar, Mahogany, and Cherry. As this material is literally a slice of a tree, there are limits to the size of stock that is available. Sold in boards at fine lumber purveyors, this material may be rough sawn or finished or 2 or all 4 faces. To prepare this material for milling, a substantial amount of work will need to take place in the woodshop first, using equipment that requires advanced training.

Not all pieces of lumber are suitable to being made into relatively flat and stable workpieces. Material may have environmental or drying stresses built in that will react badly when it is machined. Time to acclimate to the shop and surfacing (to flatten and stress relieve) in the wood shop may help, but it is not a guarantee. It is best if you know what you are looking for when you go to purchase this material as you could end up with a piece that turns into a pretzel when you start to cut into it on the router. When a piece of hardwood wants to start moving or changing shape, there is very little that you can do to stop it. Trying to mount it to a more stable sacrificial board to hold it flat will simply not work. There is no good way to hold a non-flat piece of material to the vacuum bed of the large CNC router. Talk to Fabrication Lab staff before you proceed down this route and while you prepare your file and material.

• Advantages: looks nice, will withstand wear and tear
• Disadvantages: can be expensive, not dimensionally stable, limited sizes, may require expensive cutting tools, milling time is longer than that for foam
• Sources: available locally at Boulter Plywood, Anderson McQuaid, Longleaf Lumber, Rockler and Home Depot.
• Machines: This material is most suitable for the C.R. Onsrud CNC router, and in limited applications for the AXYZ CNC Router, the Roland CNC mill, and the robots.

Machinable Wax is a special wax designed to be cut by mills for a variety of purposes. It is able to hold high resolution and be cut with small diameter endmills. It is dimensionally stable and relatively easy on cutting tools. Machinable waxes allow parts to be prototyped and checked for size and fit before then machining them from metal. Some types have melting temperatures that allow the material to be used in the lost wax casting process. It is a suitable material for making molds for other casting and forming operations as it releases well without the need for additional sealants and release agents. Multiple blocks can be glued together with cyanoacrylate to form a larger block. For many of the above reasons, it is a popular choice for engineering school machine shops. It comes in block and rod form in a variety of colors, and can be ordered online from a number of vendors. The shavings from this material can be recycled by re-melting them and forming them into new blocks.

• Advantages: Very versatile material for prototyping, casting, and mold-making. Dimensionally stable. Comes in block form.
• Disadvantages: Colors are particularly strong and the material does not receive paint well. May be more expensive than foam if making prototypes where precision is not important. Limited stock volume (under 2 square feet).
• Sources: This material is available online from a number of vendors, one of them being MachinableWax.com, another is freeman supply.
• Machines: This material is most suitable for the Prototrak CNC Knee mill and the Roland CNC mill due to typical part size, but it could also be used on any of the other routers provided there is a provision to secure it to the machine.

M

A wide variety of machinable plastics are available, including cast and extruded acrylic, polypropylene, HDPE, and PVC. Plastics themselves can vary significantly in terms of how well they are cut with an endmill. Specialty tools are made for different hardness of plastic and particular toolpath settings are needed for different types of plastics. Plastics can generally be obtained in sheet or block form from a variety of vendors. These materials are often best cut with the addition of compressed air to aid in chip removal, and sometimes with the addition of lubricant. Neither of these additions are supported adequately on the CNC routers, making the use of specialty tools more important to aid in proper chip removal.

• Advantages: unique qualities are achievable (transparency, flexibility), may not require any material preparation
• Disadvantages: requires specialty tools, milling time is longer than that for foam, difficult to make deep narrow cuts as cutting tools cannot be as long as those that can be used with foam, file preparation requires more attention to detail
• Sources: some types of plastic are available locally from Altec Plastics, online from McMaster-Carr and others
• Machines: CNC Routers, Vertical Mill, limited applications for Roland, Robots

There are different types of aluminum, alloys 2007, 2011, and 6020 are the most machinable, being more brittle than other types of aluminum. Aluminum can be purchased in sheets, plates, or blocks. Like plastics, it is important to use specialty tools when machining this material without the aid of compressed air and lubricant/coolant. When routing thin sheets on the router, there is a great potential for the sheet to be lifted up by the machine and cause damage to the part, machine, tool, or you. Special tools that help to hold the material down while being cut may be necessary. Small parts from block material are best done on the knee mill where a mist of coolant can be applied and the operator can freely add lubricant. This material presents its own challenges that are much different than those encountered when working with wood. A considerable amount of attention will need to be placed in preparing a Mastercam file. Chips produced when cutting this material cannot be collected in the dust collector attached to the CNC routers. Talk to Fabrication Lab staff if this is a material that you need to use.

• Advantages: best for use where the material properties of aluminum are required for the project (conductivity, strength)
• Disadvantages: specialized tooling, chip removal/collection, heat dissipation/chip welding
• Sources:
• Machines: Knee Mill (limited sizes), limited applications on CNC routers, machining thin stock is possible on the Zund which has lubrication and chip collection built into it's router spindle

There are a few more considerations when milling Graphite. The material must be fully dried before milling–any moisture that remains will damage the tools used when milling. Because grahite dust is electrically conductive, the material cannot be milled on the larger routers. Chips produced when cutting this material cannot be collected in the dust collector attached to the CNC routers. Considerable attention must be paid to the order and direction of toolpaths when using graphite to avoid chip out and dust getting caught in the material. Talk to Fabrication Lab staff if this is a material that you need to use.

• Advantages: Great for mold making where heat is necessary in the molding process
• Disadvantages: Difficult to mill, requires custom tools
• Sources:
• Machines: Knee Mill (limited sizes)