Characterize the CNC Machine¶
This week’s group assignment is to complete the safety training for this extremely powerful machine and to characterize the relevant concepts of runout, alignment, speeds, feeds, materials, and toolpaths. These are all very important to take into a account since all of these will affect the quality of the product. We found these values on our lab’s ShopBot Desktop MAX.
- Alignment - Having the machine zero-ed to the workspace of the material.
- Speeds - The rate of rotation of a cutting tool in revolutions per minute. Source
- Feeds - The rate at which a material is milled/ how quickly the tool follows its path, which impacts safety, efficiency, tool life, and surface finish; the distance a tool travels during one spindle rotation.
- Toolpaths - The path/ directions a tool follows as it cuts through material to achieve a desired geometry of a shape.
- Radial Runout = When the rotating tool is not centered with the main axis and is slightly offset to a side, also resulting in the but roating off center.
- Axial Runout = The term refers to when the rotating tool is misaligned at an angle to the axis, thus causing it to rotate off center.
Speeds and Feeds¶
Throughout the process of characterizing the speeds and feeds for our cnc, we used ShopBot’s provided Speeds and Feeds Documentation. This documentation outlines the meanings of some terms and where they are derived from. These terms and their brief definitions are provided below.
- Chip Load = thickness of chip that is removed per cutting edge, or for each revolution of the tool.
- IPS = Inches Per Second.
- IPM = Inches Per Minute.
- RPM = Revolutions Per Minute.
- Number of cutting edges = Number of flutes.
- Chip Load = Feed Rate (IPM) / (RPM x # of cutting edges)
- Feed Rate (IPM) = RPM x # of cutting edges x Chip Load
- Speed (RPM) = Feed Rate (IPM) / (# of cutting edges x Chip Load)
- IPM = IPS x 60 (and inversely, IPM = IPM / 60)
The documentation also includeds Speeds and Feeds charts for different materials on the ShopBot, as well as steps to optimize these speeds and feeds which are listed below.
- Start off using an RPM derived for the chip load for the material being cut (see charts).
- Increase the cutting speed (feed rate) until the quality of the part’s finish starts to decrease or the part is starting to move from hold downs. Then decrease speed by 10%.
- Decrease RPM until finish deteriorates, then bring RPM back up until finish is acceptable.
- This optimizes RPM and speed to remove the largest possible chips.
Before continuning with our machine testing, we had to align the machines axes. Our shop bot comes equppped with proximity switch endstops, as well as a z-probe plate, that made this aligment relatively straight-forward. We first ran the command
In the ShopBot terminal, we homed the x-axis and y-axis by using the machine’s proximity endstops. Once completed, the gantry was located on the machine’s orgin, and we jogged the machine towards the center of the workplane where we positioned the z-probe plate under the tool. This plate remains connected to the machine throughout the milling process, so this task simply required moving the plate from its mount onto the workplane underneath the tool. With the z-probe plate positioned underneath the tool, we ran the command
in the ShopBot terminal, and watched as the machine slowly lowered the tool towards the z-probe plate until it made an electrical connection between the plate and the tool. Lastly, we removed the z-probe plate from the material, placing it back into its mount for safe-keeping until its usage would be necessitated again after swapping the material again. After placing the z-probe plate back into its mount, we re-homed the machine, completing the alignment process of the CNC machine. While the alignment process is tedious and is required every time the material on the machine is switched, learning how to complete this process is massively important as it increases the lifespan of the bit and the quality of components that are milled on the machine.
Runout occurs when the tool on a CNC machine runs off of the correct axis, increasing the potential for errors to occur throughout the milling process and drastically reducing the expected lifetime of the tool. There are two types of runout: axial runout and radial runout:
Runout is important to avoid since it can decrease tool life and can increase the chance of machining defective parts. Runount may result from excess debris, dirt, coolant that enters the spindle and interferes with the clamped tool, which often results in defective machinery parts or shortened tool life. There are several ways to mitigate tool runout, including using higher quality tool holders, performing frequent maintenance and cleaning on the bit, and decreasing the stickout of the bit.
Axial runout is when the bit is not perfectly perpendicular to the surface you are attempting to cut. This can cause rotation in the bit that is not good for a cut. If you are using a flat end mill bit, normally you would like a perfectly vertical profile for your parts, but if there is any axial runout, that edge will not be perfectly straight. Below is an image that is a visual representation of axial runout.
Radial runout occurs when the drill bit is not perfectly centered in the spindle. This can affect every sort of outline, whether it be interior or exterior. If it affects an interior outline, any sort of slot or interior cutout will be slightly larger than intended. This means that if something is meant to be snap fit, the slot will be too large and the joint will be too loose.
To test for runout, we used the test indicator to determine how the angle of the bit changes as the machine runs. As the bit spins, if it is even barely misalligned, it will have a different position along the bit at different angles. Ideally, we would have a change of 0, but it is difficult to get it perfect. We found that our bit was slightly misalligned, but it was such a miniscule amount that it could be ignored.
Toolpaths: The path/ directions a tool follows as it cuts through material to achieve a desired geometry of a shape.
To test our lab’s shopbot, we used our lab’s desktop max shopbot to run a few simple toolpaths. We designed the toolpaths in aspire, and cut out a square, circle, star, and a crescent shape out of wood. The shapes came out fairly well. Our toolpath ran 4 passes, which means that each pass it cut out 1/4 of the material. This is important becuase with materials as thick as 3/4” wood, cutting out too much material at once can damage the mill bits. With 4 passes, the machine cut out 1/4*3/4=~.19”, which is safe for our bit.
Another important thing to add to toolpaths is a tab. This holds the material in place on the last path so that the material is not moving while a part is being cut out. We added 2 tabs for each part on opposite ends of the cut, and the material was held in place.
Since the ShopBot can only read files in the .spb format, we had to export all of the designs from Aspire in that format.