Couplers are extremely useful devices that are used to connect mechanisms together. There are dozens of variations on the coupler theme, from flexible couplers to rigid couplers. One of the most common needs in a classroom is a coupler that can connect a motor shaft to a wheel, or an axle to a wheel. These are typically used in robotics and various physical science projects in a STEM classroom.
Purchasing couplers is expensive and limiting. You must have the correct dimensioned motor shaft and wheel spacing for the couplers you buy, and the cost is quite high. Fortunately this poses a STEM opportunity for teachers who have a CNC equipped classroom.
Students can design and manufacture couplers based on the parts they may have harvested from a dead electronic device, saving time, money and encouraging innovation. If the parts will not be subjected to a heavy load (such as a mouse trap car project), plastic can be an appropriate material for couplers. In this case students can design and then manufacture couplers using a 3D printer. However, for heavier loads metal is best, and if you have a CNC mill you can use SprutCAM software to take student designs to making metal chips in short order.
This tutorial is meant to guide you through the steps to make a coupler that attaches a shaft to a wheel. It is to be used as both a teaching example in the classroom and as a general guide to getting started with SprutCAM software. This is a simple part with few machining operations required, and will be a good start for a useful part that is also instructive. As an extension, when you see students have mastered this lesson have them download a more complex piece from Thingiverse.com or GrabCAD.com and see if they can simulate machining operations in SprutCAM.
SprutCAM is easy to learn, as long as students are actively engaged with it. The endgame for beginners is a successful simulation with no error messages. This is a very powerful teaching method that students have fun with, and despite its complexity you will be amazed at how many students can run parts that require lots of analysis and manipulation of variables to get right. By the same token, students will most likely not learn how to use SprutCAM by looking at a Powerpoint or video of someone else doing it, so be sure you have the computer resources available for maximum effectiveness in the classroom environment.
If you have your students design a similar coupler using their favorite CAD software, your students can follow this tutorial and get a good simulation as an end product no matter what the dimensions, if you make adjustments to tool type and size, as well as certain parameters such as top and bottom z level.
One caveat about this lesson for teachers. We have left out the ‘feeds and speeds’ portion of the machining parameters as that is a subject in and of itself. For the moment, you can turn to any number of popular feeds and speeds calculators for your particular stock material and and available tools if you want your students to manufacture this part or one like it. Student will learn the basics of SprutCAM just fine with this lesson and using the default values for feeds and speeds.
Be sure and go through the short introduction to SprutCAM lesson posted here to learn how to complete the initial setup of SC for your machine and tools.
Begin by opening SC and setting the machine parameters to your particular mill. Next, go to your preferences and set the units to metric or imperial. For this in class lesson we are using the built in tool tables in SC. When you machine actual parts, be sure and substitute your tool table measurements into a new library. Verify that the tool table library is set to the same units as that of the machine. In other words, if you are using Imperial units be sure you are using the inch based tool library. Metric units go with metric tools.
Import the part. Be sure that you have saved it as an .IGS file in your CAD program. You need to orient the axes next by using the transform tab to set Z to the top of your work piece, from here
In order to get the Z axis oriented, you will need to transform the model. Be sure you are in the model tab, then drag select and then sew all the faces together. Tip: mouse over any tabs to get an idea of their function/name. Next, select the transform button and start by moving the Z axis to the top. You should see a preview of what changes will take place with this modification. If the piece is not oriented correctly, you may also need to select the rotate tab. Rotate around the various axes, generally in 90 degree increments until you piece looks like the one above.
Now that the axes are where they should be, students need to start thinking a bit like the machine. You have a block of metal that needs to be carved into a particular shape. How can this shape be created in as few steps as possible? What will the part look like after each machining operation, and does it logically lead to the next one?
In order to help students determine what operations might work for their part, use SprutCAM’s create button while on the machining tab. This is a very powerful tool that helps beginners so much that you should make it a mandatory part of this lesson. The drop down menu provides an animated preview of each machining operation, and this give students a visual preview of what exactly is going to happen when this operation is selected. They should click through these animations, and imagine which ones would be best to carve out the part they are going to machine,-and in which order they should take place. There will be lots of ‘Aha!’ moments after this exercise.
Next writing a quick summary of what might operations might work best is good to put things together.
These are not locked in stone, and are just a starting point. Operation order and/or choice may need to be altered along the way, so don’t let students get bogged down trying to make something work that may need revision.
For the coupler, here is a summary that you can start with. We are going to try and do it with 2 machining operations.
- Roughing Waterline
- Drill or pocket holes. Generally a good idea to drill hole first if possible to avoid the part shifting out of position. That didn’t work on this piece for reasons explained a bit later. Instead consider leaving a bit of material on the bottom of part as shown below to accomplish the same thing.
The roughing waterline tool is a very versatile yet can be overeager, so watch it carefully during the simulation. It will pick up the holes for the screw heads and mill out the flat.
In order to prevent the part from shifting, you can use tabs(how to create them is shown in this video tutorial )or you can leave a small amount of material, say .01 in that can be punched out and trimmed/deburred. So in parameters the bottom level is set to .99 rather than the full 1 inch thickness of the stock.
Use the recognize hole function in hole machining, but beware it will pick up everything.
What you don’t want drilled, just delete. Quick and easy.
Be sure and do the operations in an order that makes sense given the tools you have. For instance, if you tried to drill first, SC would give you an error on both operations. Errors require a bit of sleuthing, which is a learning opportunity for students. Digging into the gcode reveals that the drill bit in our tool table was too short and would bottom out. So in this case it was imperative to machine the piece flat before the drilling operation in order to have enough drill length for the holes. After every adjustment, click ‘run’.
If you get green checks, go to the simulation tab and run the simulation.
Roughing operation continued. Notice the very thin layer of stock that remains in order to keep the piece in place.
Next the holes are drilled. Select the edges of the holes in the machining tab, and make sure they appear in job assignments. The parameters are set to completely go through the stock. It is important to have the drill retract to a safe height so as not to crash into the piece as it travels from hole to hole. Generally SC will catch that type of error but it is better to be safe than sorry. 1 inch is probably overkill but it is a very safe height.
CNC machining is now done for this piece. Remember that once the completed part is pulled from the machine the thin layer remaining stock must be removed with a deburring tool. Two things remain for the coupler to be functional in the real world. It will be manually drilled and tapped for a set screw to keep it in place and will be broached for a keyed axle. Broaching and tapping are subjects in and of themselves, and we will present tutorials on those machining techniques in the near future.