Contents
Cal Poly Pomona

USING THE CNC ROUTER

The ShopBot CNC router consists of an electronic variable speed router mounted to a three-axis system of step motor driven gantries. Computer input to the ShopBot control box can move the router to any location in a 44 inch by 44 inch by 6 inch area. A block of material that fits within those dimensions can be milled by the ShopBot into a three dimensional object. The shape of the final product is restricted only by the properties of the material being milled and the ShopBot’s ability to maneuver the router and cutter around the model or design.

The two major factors which affect the ShopBot’s ability to maneuver are the gearing of the step motors which control the gantries and the shape of the cutter(s) used to mill the model. Types of cutters and their use will be discussed in a later section.

The gear increment of the step motors is approximately .002 inches. This means that when the control box obeys a command to move the router to a specific location, it will move the router to within .002 inches of that location on each axis. This level of accuracy is more than sufficient for most woodworking and model making purposes. Industrial applications require greater accuracy.

The ShopBot router mills a model or design by plunging the rotating cutter to a specific range of depth on the Z-axis and then traveling horizontally along the X-axis and Y-axis, removing any material in the path of the cutter. Only a small quantity of material is removed in a single pass. Progressively deeper passes are made to achieve the desired depth of cut.

The appropriate plunge depth per pass depends on the density of the material being milled and the size of the cutter being used. Less material should be removed at each pass when milling higher density materials such as hardwood or plastic. More material may be removed at each pass when milling low density materials such as foam.

The speed at which the router should travel along the axes and the speed at which the cutter should rotate depend on the physical and chemical properties of the material being milled. The combined affects of material density, structure, grain, homogeneity, and resonance must be taken into account to ensure clean cuts and reduce tool wear. The best cuts come from minimizing vibration between the material and the cutter.

What kinds of materials can the ShopBot mill?

Materials that are suitable for milling with the ShopBot CNC router include wood, foam, and plastic. The router does not currently have metalworking capabilities. Choosing the right material is important. Various materials possess properties which make them better suited to specific types of model or design. It is up to the student to evaluate the cost effectiveness, availability, and practicality of a material for a project.

Wood is a good material choice for a wide variety of projects. The rich diversity that exists among species of wood makes it possible to choose a wood that possesses the specific structural and aesthetic characteristics required for the model or design to be milled. Three-dimensional models milled from wood will most likely require some hand sanding.

Plywood must be selected carefully for milling purposes. The regular shop grade plywood sold at most hardware stores will have voids and spaces between the layers which make it a bad choice for milling. Higher grade plywood such as Baltic birch is a better choice because it is manufactured to be void free. Plywood is an acceptable material for two-dimensional designs and cutout models. Plywood is not recommended for three-dimensional models, because of the excessive number of laminations required to achieve adequate depth. The glue that binds the material is unhealthful and excessively damaging to cutters.

Just as with other machines in the shop, the use of materials such as particle board and MDF is not encouraged. Although highly homogenous and relatively economical, these products contain unhealthful chemicals and are excessively damaging to cutters.

Foam can be an excellent choice for three-dimensional modeling. Large models can be milled from foam quickly because of its low density and exceptional homogeneity, which allow sizeable quantities of material to be removed at each pass of the cutter.

Plastic cutout models and other two-dimensional designs can be easily and efficiently produced by the CNC router. Due to the material’s high density and abrasive nature, milling plastic generates a considerable amount of heat. It is essential to choose a high carbon or Lexan plastic that is resistant to heat so that it will not melt and warp when milled.

As mentioned above, the density of a material is in direct proportion to the volume of material that can be removed at a pass. Choosing a less dense material can speed up the milling process by reducing the number of passes that must be made. Fewer passes also means less wear on the machine and the cutter.

A material’s structure affects how the cutter and the material interact. Some materials have grain structures that sheer quickly and cleanly when encountered by the cutter. Other materials have more fibrous structures that tear and resist the cutter, and these must be milled more slowly to yield good results. Fibrous materials will dull a cutter more quickly and generate more heat during milling than non fibrous materials.

The chemical composition of a material can affect milling as well. Materials which contain resins can ignite if excessive heat is generated when milling. Burn marks can be avoided in such materials by slowing cutter rotation and increasing router travel speed. Materials which contain silicates are abrasive and damaging to cutters, and also generate a significant amount of heat.

Vibration can result from inconsistency in any of the above mentioned properties. If a material’s density, structure, or makeup varies from one place to another, router travel speed must be reduced to compensate for the instabilities that will result. Otherwise if these properties are relatively constant, a material is considered homogenous.

Instability can also occur as a result of resonance. Some materials may naturally develop standing waves of vibration at specific frequencies due to feedback in the interaction between the material and the cutter. A slight adjustment of the cutter’s rotational speed or of the router’s travel speed will stabilize the material. Failure to correct resonance problems will increase cutter wear.

When selecting a material, be sure to consider how the piece will be secured to the table as it is milled. Common methods for securing materials include clamps, screws, and double-sided tape. These methods may be used in conjunction with one another to prevent lifting and to dampen vibration.

Screws are generally preferred due to their inconspicuous size and their effectiveness at firmly securing the material. Depending on the model or design being milled and the material that is chosen, it is usually best to leave a few inches of extra material along two or more edges to place screws.

If it is not desirable or not possible to leave extra material for screws, such as with very large pieces that exceed the length of the table, clamps may be applied to secure the material. Be sure to note the location of the clamps and check that they will not interfere with the movement of the cutter and router casing.

Double-sided tape is useful for dampening vibration and for ensuring that cutouts remain in place once they are free of the main block of material. Keep in mind that tape will not hold an object completely rigid, and using tape alone to secure materials will result in less precise cuts. For the best results, tape should be used in conjunction with clamps and screws. Also be aware that double sided tape will elevate the material from the table slightly, and that the tape may vary slightly in thickness.

What types of cutters can the ShopBot use?

The router may be equipped with a variety of cutters suitable for milling. A device called a collet holds the cutter securely in place by clamping tightly around the non bladed shank of the cutter. Collets may be designed to receive a half-inch cutter shank or a quarter-inch cutter shank.

The blades of a cutter are called the flutes. Straight flute cutters have vertical or slightly angled blades. Spiral flute cutters have helical blades to direct removed material upward or downward. The flute length is the length of the cutter shank that is occupied by cutting blades. Cutters with longer flutes can make deeper cuts in a single pass if the material is suitable. At no time during milling should the router plunge beyond the flute length of the cutter.

Shouldered cutters (also called step-down cutters) have a larger diameter shank on the non-fluted portion. This is necessary for cutters with odd diameters that would not otherwise fit in a router collet. Extra care must be used when milling with shouldered cutters to avoid interference between the material being cut and the larger diameter shank.

The Model Shop possesses a small assortment of cutters that may be used with the ShopBot CNC router. End mill cutters have a flat end and will cut a square channel. These cutters are typically used for three-dimensional roughing and two-dimensional finishing work. Ball nose cutters have a rounded end and will cut a scalloped channel. These cutters are typically used for three-dimensional finishing work. V-bit cutters have a pointed end and will cut a V-shaped channel. These cutters are typically used for cutting two-dimensional designs and text. The following is a list of cutters that are currently available.

  • 1/16" diameter end mill with 3/16” straight flute, shouldered cutter, quarter-inch shank
  • 1/8" diameter end mill with 7/16” straight flute, shouldered cutter, quarter-inch shank
  • 1/4" diameter end mill with 15/16” straight flute, quarter-inch shank
  • 1/4" diameter end mill with 3/4” spiral flute, shouldered cutter, half-inch shank
  • 5/16" diameter end mill with 11/16” straight flute, quarter-inch shank
  • 5/16" diameter end mill with 1-11/16” flute, half-inch shank
  • 1/2" diameter end mill with 2-3/8” spiral flute, half-inch shank
  • 1/4" diameter ball nose with 1/2” straight flute, quarter-inch shank
  • 1/2" diameter ball nose with 1-3/16” flute, half-inch shank
  • 1/2" diameter V-bit with 45 degree angled flute
  • 3/4" diameter V-bit with 45 degree angled flute

Material density limits the volume of material that the cutter may remove in a single pass. Depth of cut should be adjusted to balance the size of the cutter. Although a larger diameter cutter will cover more area in a pass, it must compensate by making shallower cuts. Deeper cuts may be made with a smaller diameter cutter only if the cutter is strong enough to avoid deflection and breakage.

For three dimensional models and two dimensional area clearing, it is generally desirable to choose a cutter that maximizes horizontal coverage. Consider the resolution required by the model or design and select the largest diameter cutter that will perform the task. Smaller diameter cutters are capable of finer resolution, but generally take longer to mill. To understand the resolution of a cutter, think of a paint brush of equal diameter, and consider the amount of detail that is afforded.

It is important to consider how the cutter and router need to maneuver around the model or design. Although the router can be positioned anywhere within the three-axis system, sudden height variations within the model or design can lead to interference between the object being milled and the non-fluted cutter shank or the router casing. Use a cutter with a longer shank to elevate the router casing above tall obstacles. Use a non-shouldered cutter to avoid conflicts with steep slopes or deep pockets.

What does the ShopBot operating software do?

The operator interfaces with the ShopBot CNC router through the ShopBot’s proprietary operating software. The operating software receives and processes user commands and then relays the instructions to the ShopBot control box. The control box then directs the step motors to maneuver the router as instructed. The operating software also stores various values and settings that are used to calculate the instructions needed to execute commands.

Values stored by the operating software include categorical information about the cutter. Always check that the operating software settings reflect the current cutter’s type and size, as well as the desired speed and step-over values. After switching cutters, remember to modify the operating software’s settings to reflect the new cutter’s values.

The operating software also stores information about the current location of the cutter within the axis system. After switching cutters or mounting new material, remember to change the Z-axis coordinate value to reflect the new vertical distance between the cutter tip and the indexing surface. The Z-axis may be indexed from the bed of the machine or from the top surface of the material being milled, depending on the application. For cuts in which the top surface of the material will be removed during the milling process, it is best to index from the bed of the machine.

Be careful not to physically bump the gantry axis system when switching cutters or mounting material, as this will alter the location of the cutter. If the X-axis or Y-axis gantries are bumped, they must be re-indexed manually by the operator.

Commands may be entered into the operating software at the command line to be executed in real time, or they may be entered into a ShopBot part file to be executed in succession. Commands may include instructions to change the value stored in a setting, or instructions to change the location of the cutter. There are also commands for calculating and cutting simple shapes. For a complete list of commands, see the ShopBot operation manual.

The operating software can run in cut mode or in preview mode. In cut mode, all commands are sent to the ShopBot control box to be executed by the router. In preview mode, all commands are sent to a simulation which records settings changes and traces graphical representations of tool paths into a virtual model of the router. Commands entered in preview mode have no affect on cut mode settings or router location.

Preview mode is an essential tool for testing cuts before milling. The majority of operator error can be avoided by simulating part files in preview mode before running them in cut mode. While in preview mode, the operator can carefully review the tool paths displayed in the virtual model to check for cutter clearance.

Simple part files may be written by hand in a text editor. More complex part files may be written by software applications such as PartWizard for two-dimensional vector designs, or MillWizard for three-dimensional vector models. The operating software also has algorithms for converting various sources such as G-code (standard CNC router code) into ShopBot part files.

Never attempt to direct cutting manually in real time using keyboard commands. Part files are required for all cutting maneuvers.

Once a part file has been tested in preview mode, it is ready to be run in cut mode. The operator must be present and attentive at all times during milling. Ear and eye protection is mandatory. Once the router is powered on at the appropriate rotational speed, the part file may be loaded. If the operator needs to leave the room before milling is complete, the part file must be paused and the router must be powered down.

The remote stop button should remain with the operator when observing. If emergency requires an immediate interruption of milling, activating the remote stop button will freeze the axis system’s motion. Never switch off power to the router while the machine is in motion.

The following checklist can help the operator prepare to run a part file. No student may operate the ShopBot without the assistance of a Model Shop approved technician.

  1. Secure the material to the machine table.
  2. Mount the cutter in the router collet.
  3. Change the setting values in the operating software.
  4. Set the cutter’s Z-axis height
  5. Check the coordinate location of the cutter.
  6. Test the part file in preview mode.
  7. Locate the remote stop button.
  8. Acquire ear and eye protection.
  9. Set the router’s rotational speed.
  10. Turn the router on.
  11. Run the part file.

How does PartWizard write part files for two-dimensional designs?

The following vector file formats are recommended for two-dimentional cutting. Bitmap images must be converted into vector images before a part file can be written. Preferred formats are listed in bold.

  • Adobe Illustrator (*.ai)
  • AutoCAD 2D (*.dxf; *.dwg)
  • CorelDRAW (*.cdr)
  • Duct Picture (*.pic)
  • EPS (*.eps)
  • Windows Metafile (*.wmf)

We presently use a software application called PartWizard to write part files for milling two-dimensional designs on the ShopBot. Part Wizard has a complete vector tool set capable of creating, importing, editing, and saving two-dimensional vector paths. One may choose to create an original vector graphic using Part Wizard, or import graphics created in other software applications such as Adobe Illustrator or CorelDRAW. These vector paths may then be used by Part Wizard to write various tool paths. Once written, any or all tool paths may be assembled by Part Wizard in any order and saved into a part file.

PartWizard can write tool paths which cut along a vector path, cut out the exterior or interior profile of a closed vector path, clear the area within a closed vector path, and drill holes at specific coordinate locations. PartWizard can also write part files which carve text using a V-bit cutter for a small selection of true type fonts.

Each tool path created in Part Wizard is calculated for the specified cutter. Information specific to each cutter is stored by Part Wizard, including recommended speed and step-over values. Be sure to review these values before selecting a tool. Minimize tool changes by choosing the most versatile cutters suitable for the design and grouping together tool paths executed by the same cutter. Tool paths for each cutter must be saved in separate part files.

When milling cutout designs or pockets and holes which penetrate through the design, remember to secure a substrate material beneath the material being milled to prevent cutting into the machine table. Also be sure that off-cuts will remain secured during milling as each cut is made.

How does MillWizard write part files for three-dimensional models?

The following file formats are recommended for three-dimentional cutting. Preferred formats are listed in bold.

  • 3DStudio (*.3ds)
  • AutoCAD (*.dxf; *.dwg)
  • formZ Version 3.8 (*.fmz)

We presently use a software application called MillWizard to write part files for milling three-dimensional models on the ShopBot. As MillWizard has no editing and only coarse scaling capabilities, finished vector models created using three-dimensional vector modeling software such as 3-D Studio or formZ must be imported into MillWizard. MillWizard can then calculate roughing and finishing tool paths to a specified depth based upon the three-dimensional model. A large cutter making aggressive cuts removes the bulk of the material during the roughing pass. A finer cutter making slower, shallower cuts is used for the finishing pass, increasing the model’s resolution.

A border no smaller than the radius of the largest cutter to be used is added by MillWizard to the overall size of the model. This is to allow the cutter room to maneuver around the exterior of the model as it is milled. The border must remain consistent for all roughing and finishing passes. Discrepancies in border size can lead to major problems during milling.

In some cases, extreme local variations in model height may make it impossible for the cutter to safely maneuver around the complete model. This may be resolved by dividing the model into separate horizontal or vertical sections to be joined after milling.

MillWizard uses a raster approach to creating a three-dimensional product. For the roughing pass, the cutter makes multiple passes over the model area achieving greater depth with each pass. Each pass consists of partially overlapping horizontal paths across the model length. The horizontal distance between each path is called the stepover. The vertical distance between each layer is called the step-down.

For the finishing pass, the cutter makes a single full-depth pass over the model area.

The greatest distinguishing characteristic between a roughing and a finishing pass is in the cutter’s tool path. In a roughing tool path, the ShopBot starts cutting from the top of the model and steps downward incrementally, as determined by programming in optimal lateral speed and plunge movement for the combination of material and cutter to be used. For example, when milling hardwood, the ShopBot will make its first pass over the model at a depth of 1/4”, the second pass along the programmed tool path will occur at 1/2”, the third pass at 3/4”depth, etc. until the full depth of the model is reached.

In a finishing pass the ShopBot makes a single cut at full depth, removing the tolerance amount programmed into the roughing pass. This means that the cutter goes to the lowest part of the model and begins to move laterally. If there is not enough working cutter length to go all the way down in depth, or if the geometry of the given tool places non cutting tool shank surfaces beyond the area where the cutter blades may be effective, as in the 1/8” cutter illustrated above, the ShopBot will attempt to move the non cutting portion of the cutter shank directly through the solid wall of tolerance material allowed by the roughing pass. This will cause the ShopBot to rack its gantries, which requires that the machine be re-built and squared all the way down to its legs.

To correct for potential discrepancies between roughing and finishing passes, we have devised an algorithm and make what is called a rectangle pass between roughing and finishing passes. The rectangle pass alleviates the possibility of crashing the ShopBot in the outer perimeter of the model, wherein typically there is a sheer drop off. For our purposes we have determined that a standard tolerance for the rectangle pass of .05 is acceptable.

Here is an example of correcting for cutter geometry, as you would use with the eighth-inch end mill cutter pictured above.  The shank diameter of this cutter is 1/4” and the working portion of the cutter is 1/8” diameter. This means that there is 1/16 of an inch (.062) wherein the shank has the potential of crashing before the working portion of the cutter makes contact with obstacles. To correct for this tools geometry we add 1/16” to our standard rectangle tolerance: .062 + .05 = .112, which is inserted as our desired tolerance in the rectangle pass algorithm.

Click here to download the spreadsheet calculator for determining the dimensions and positioning of the rectangle clearance pass. Double click on the table to modify data.

Below are some software simulations of various computer generated topographs, using various cutters and tool paths.

This is a simulation of a roughing tool path shown in MillWizard.

This is a model simulation of the tool path shown above using a half-inch ball nose cutter.

This is a model simulation of the same model after a finishing pass using a quarter-inch end mill cutter.

How can I make the ShopBot work for my project?

  

The ShopBot CNC Router is an excellent tool for many applications which would be prohibitively complex or time consuming using traditional methods. The ShopBot is designed to be versatile, and while its strengths lie in its ability to repetitively cut part files, prototypes are often more laborious and time consuming on the ShopBot For many applications, traditional methods are often more efficient or economical and should be seriously considered.

As with any project you bring to the ENV Model Shop, it is important to choose the best and safest tool for the job. Consult with the Shop Supervisor for help in choosing the most appropriate method. Here are some things to consider when determining whether the ShopBot is the right tool for your project.

Every cut to be made using the ShopBot must first be modeled in the computer as a vector graphic. Vector graphics are a versatile and powerful tool. If one is proficient with the graphics software, designing and modeling with two-dimensional and three-dimensional vector graphics can be fun and easy. Keep in mind that learning to work with vector graphics will take some time and patience if you are not already comfortable with them.

Not every model that fits within the ShopBot’s physical limits is well suited for milling. Models with tall obstacles, steep slopes, and deep pockets require careful planning. Some problems with maneuverability cannot be solved by changing cutters, and will require a piecewise approach if milling is the chosen option.

Not all materials are well suited for milling, and many materials that are well suited can be expensive. Milling is often more wasteful or costly in terms of materials than traditional construction methods.

Material removal takes time. Three dimensional models and two dimensional designs with large amounts of area clearing can be very time consuming. Remember that machine time is purchased by the minute.

Some projects demand milling because of a need for accuracy or detail, however some projects can be completed more quickly and cleanly using traditional methods. It is important to consider all options and weigh the end results against the time, effort, and resources invested before you commit.

Pricing is $10.00 setup fee plus $0.20 per minute of part file runtime.

 

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