Week 7: Computer-Controlled Machining
Our assignment:
Group Assignment: 1. do your lab's safety training 2. test runout, alignment, fixturing, speeds, feeds, materials, and toolpaths for your machine
Individual Assignment: 1. make (design + mill + assemble) something big (~meter-scale)
Group Assignment
Find our group assignment documented in full here and my documented learnings below.
Our lab's machine for large scale cutting is the Avid CNC Router Machine. This work area includes a dust collector and a fan for circulating air.
Workflow
We learned the basic steps for operating the machine:
- Turn on the machine (two switches on side)
- Open Mach4 Software for Avid CNC control
- Press Enable button, Reset button, and Keyboard Jog switch on. For our machine, "jog" the machine (moving it along X, Y, and/or Z axes using the keyboard) to ensure it's working properly.
- Press Home XYZ Axes button. The machine will then slowly move to its "home" which is the furthest corner on the left/front of the machine.
- Send the machine back and away for safety.
- Ensure the collet (the device that holds the cutting tools) is tight and not loose. Press Spindle Warmup button and let the operation run until complete. This warms up the grease in the motor which allows the bearing sto run at an optimal state.
- Fixture piece for cutting. Our fixturing methods so far have been 1) double-sided tape 2) clamping
- Change out bit to desired bit using wrenches. TIP: To check if a bit is down-cut or up-cut, hold it downwards, twirl the bit clockwise, and observe -- you can tell visually if the bit appears to be twirling town or up.
- Set X, Y, and Z axes heights by using the Auto Z and Corner Finding Touch Plate. Make sure in the window that the tool diameter is correct for the bit! This is easy to miss
- Load G-code file(s)
- OPTIONAL: Run thru an air cut test run by adjusting zero Z to ensure there is enough clearance between the drill bit and the material. For us, which zeroed Z at 2 inches above the top of the material. Note: you can bump up the feed rate lots here since we're not actually cutting material.
- Turn on dust collector and fan
- Finally, run your cut! Hit the Cycle Start G-Code button. A good practice after starting is to hover the mouse over the Feed hold button so that you can pause and restart without a big hassle in non-emergency situations (for emergencies hit the E-Stop button!).
- After moving the machine a safe distance back (being mindful of objects in the path), remove the cut pieces. Depending on the fixturing method used, you may need to use a wedge and mallet to loosen the pieces.
- You did it! Now you can go on to any finishing steps (like sanding the wood, painting, etc.)
Safety Training
This is something small I designed in Fusion 360 (see design files below) for us to use as an example to walk through the process of using the CNC machine and putting our safety training into practice.
From our lab safety training, we learned:
- Wear appropriate safety equipment when operating the CNC machine: eye goggles, ear protection, closed toed shoes
- No running. Be conscious of how you and others are moving
- Take your time
- Don't stand within the path of the cut while operating the machine
- Have the E-stop (aka emergency stop) button at the ready for when you need to stop the machine in an emergency situation (if its not emergency you can pause the machine instead which makes it much easier to continue your work without having to reset everything)
- Give a heads up to everyone in the lab about machine operation
- When walking away from the machine, make sure to hit the Disable button AND hit the Keyboard Jog OFF switch so that accidental touches to the keyboard or screen will not cause the machine to run.
- Do not start cutting without inspecting the cutting area and secure fixturing of the cutting material
Mirror design file's G-Code uploaded into Mach 4 software
Machine Testing
For our machine testing, we collectively cut the small mirror design, a couple of joint tests, and some tests for each of individual assignments.
We each made joint tests in the V-Carve software we have available at the lab. Castor made a joint test to test for variation in width of a joint and how that fits within a given thickness of our 18mm plywood (which actually measured to about 0.74 inches). Meanwhile, I made a joint test to test for variation in the length of a dogbone joint to observe how well a block of wood cut nominally at 3'' long would fit within that length. We each also created two sets of these test joints to run at varying amounts of passes.
Dogbone joint test starting nominally at 3'' then increasing by 0.05'' with each successive cut. The top and bottom rows are copies of each other but the top row's toolpath is set for 2 passes whereas the bottom row's toolpath is set for 3 passes.
See group page for more images. I observed that the cuts at nominal length of 3.0'', 3.05'', and 3.10'' were best. The 3.0'' made for an extremely tight fit that was hard to push all the way thorough, especially with 2 passes. The 3.05'' was nice and snug but gave more wiggle room to more easily press through. The longer width joints had much more noticeably visible space (I aligned the 3 inch long wood block to bottom corner of the joint to observe this)even though the fit was snug around the thickness of the material.
For general testing and learning about the CNC machine, see our group page. Here are some lessons learned from group instruction that I'll document here too:
Runout
This refers to the spindle runout. While the spindle is spinning, in reality it is not truly 100% concentric. Poor runout can lead to inaccuracies and rough cuts and wear on the tools. When I made the mistake of not reseting the height using Auto Z touch plate, I caused damage to the drill bit because it ended up knicking a nail unexpectedly. Then I used the same drill bit for the following cut and the cut was noticeably much more rough along the edges even though it has essentially the same settings. I had to sand it and we threw out the drill bit. Lesson learned!
Alignment
Despite the error mentioned above, we had lots of practice with properly aligning the CNC machine. I learned how to use the Auto Z touch plate to help position not only the height in relation to the cutting tool but also the origin of the X and Y axes as well and how to precisely align the drill bit with each access as the CNC machine runs this function.
Fixturing
We learned 1) double sided tape and 2) clamping with wooden blocks and nails as methods for fixturing.
Speeds and Feeds
Speed is the cutting speed -- how fast the tool moves around. Feed is the feed rate -- how fast the tool moves through the material itself. Increasing these these can cause risks due to the heat generated at high speeds. Also, it can reduce quality of a cut. At the same time, we aim for efficiency with time and finding the balance.
Materials
We used 12mm plywood and 18mm plywood so far with the CNC machine.
Toolpaths
These are routes charted for the cutting tool. So far, I've done pockets and contours, practicing on Fusion 360 and V-Carve. I've learned that you must be aware of drill bit size and appropriately set maximum step down to the bit diameter (to be conservative). Rather than having the drill bit plunge down too deeply into a pocket, for instance, you set the max distance it can go at a time. This can help with cleaner and less wearing cuts. I also learned that order matters! Do essential features first. And be mindful of the fixturing as material is being cut away -- ensure throughout all parts of the cut each piece is still firmly held down. Another common thing to add is a finishing pass, which gives us a better finish.
Make Something Big!
Resources
For my make something big assignment, I decided to make a bookshelf that resembles a cello. First, I looked into lots of resources for design inspiration + technical how-to + general learning. I'm still working my way through, here's what I found / some that Will reccomended:
- A commercial bookshelf installation guide - I see lots of ads for this and similar designs. The waves made me think to have the silhoutte resemble a classical instrument
- Encyclopedia of Furniture Making by Ernest Joyce (1970) - an old book we had in the lab with a helpful section on jointing techniques and methods
- A few Youtube tutorials: a couple on The Fusion Essentials channel on common woodworking joints, modelling a bookshelf; one by Product Design Online on dovetail joints
- This blog on Digital Fabrication for Designers
- This episode on Joinery with a CNC by Chris Fitch
- This book Make: Design for CNC - Furniture Projects and Fabrication Techniques by Anne Filson, Gary Rohrbacher & Anna Kaziunas France
- Make: Getting Started with CNC by Edward Ford
Design
This week, I'm back to being humbled by Fusion 360 😅
My basic idea was a corner bookshelf with the silhouette of a cello, a few shelves, and some design elements that referenced the cello. I wanted to have one of the shelves look like the bridge piece on a cello. So I found this cello image online and this bridge image, imported to Inkscape, and got to work to create an image ready for import into Fusion so it could be extruded as a sketch.
Edited sketch on Inkscape
My sketch for the bridge shelf piece. I added some rectangular edges so it would slide into place
I started out eager to incorporate all kinds of joint types for assembly. Learned that more complicated doesn't necesarily translate to a better design. In my first attempt, designed mortise and tension joints by extruding slots from the shelving and cutting into the panel. I also laid out an array of dovetail joints to join the two main panels.
Original design for shelving joints
Remnants of my dovetail joints along one of the panels
With guidance from Will, I simplified my design so that my main panels came together using a rabbet joint and so that my shelves slide into place using dado joints based on the thickness of the material. This made way more sense!
Timelapse of design on Fusion 360
Milling - Prep & Execution
Next, in Fusion 360 I went to the MANUFACTURE workspace to set-up the toolpaths for each cut to be exported into G-Code and uploaded for the CNC machine to cut. To do this for each piece cut, I completed setup, and generated toolpaths for 2D pocket(s) and 2D contour. This went like...
In Setup:
- Select Machine - Avid CNC EX 3 Axis
- Set operation type as milling
- Select Work Coordinate system (I chose Z axis/plane & X axis for these cuts) + set up appropriate axes and origin points (typically origin is set at stock box point). Note: Z needs to be pointing up and away from the material. Set origin so that its on the machine left corner.
- Select body of the model being referenced for this toolpath
- remaining settings as is
- Under "stock" tab choose relative size box and set stock side offset at 0.25in
Then add 2D Pocket(s) as needed:
- Select tool (for my main panels this was 1/4'' flat drill bit; for my shelf because of some of the more intricate details this was the 1/8'' flat drill). When selecting tool in the user library, we input values within the cutting data that work well with NODA's CNC machine
- Set feed & speed (see screenshot below for example numbers)
- under "Geometry" tab select the feature to be pocketted
- under "Passes" check "finishing passes" and input 1 for number of finishing passes (or more if desired)
Settings for Feed & Speed on one of the 2D pocket toolpaths created in Fusion 360
Now add 2D Contour(s) as needed:
- Go through same steps as above
- Chose the selection for what needs to be contoured (esentially whats being cut out of the stock)
- Under "passes" make sure to add a finishing pass too. And also check "Multiple depths. Ensure maximum roughing stepdown is set at the bit diameter (so 0.25inch or 0.125 inch in my case)
Now, let's simulate to check if everything is working as expected.
- Right click the "Setup" folder for the toolpath to simulate
- Click simulate and change display settings to desired outputs
- Press play to simulate. Adjusting speed by adjusting the pin on the bar below
Next, I had to right click each set-up/tool path and click "Generate" (or simply Ctrl-G). I need to do this every time I make any change in Fusion. This regenerates out of date options.
Finally, for each cut, I right-clicked and selected "Post Processing." Input machine configuration, file name, output folder, unit (inches) and click "post." Now my files were ready to upload in the Mach4 software.
Pocket cut for one of the main shelving panels uploaded on Mach 4 software
For each cut, I walked through the basic steps as outlined above under group assignments. For this project, I did need to change out my drill bit which I did using a couple wrenches. I also set up using a combination of double-sided tape and clamping down with wood block scraps and nails.
Fixturing with tape
Fixturing one of my small cuts with clamping rig. This involved nailing in a block of wood, then pressing a wedge piece of wood with a mallet to hold the material to be cut into place
Using the auto z touch plate to set height
Cut bookshelf panel
Once finished, it does take some effort sometimes to remove the pieces. The double sided tape is surprisingly strong! To ease this process, we learned a method to use a thin wedge of wood and a mallet to get the pieces off the machine bed.
Accidentally forgot to Auto Z for one of the cuts! Luckily it did not cause too much damage but one of my main panel pieces got cut all the way through because the Z-plane reference was slightly off. So, instead of keeping my original design for the shelves I wanted to put these pieces to good use and recycle them as material for the shelves. For these shelves, I ended up using the lab's table saw to cut them down.
Assembly
After making the cuts, I worked on sanding the pieces using the air random orbital palm sander we have in the lab and also a circle of sandpaper for the more detailed edges. I made sure to sand it in the direction of the frayed edges rather than against that grain.
Sanding away! Had to get some of the dirt off too.
To assemble, I slotted the two main panels together, slid in each of the three panels, and rested the bottom panel on the two bottom pieces of wood.
My assembled cello corner shelf!
I haven't had time yet to add finishing touches like paint, gluing, etc. but will continue to work on it. One way to improve the design would be to add more CNC friendly cuts within the dado joints so that the shelves can lock into place more sturdily and still be removed for easier transport. I'd also like to re-cut the top and bottom shelves to look more uniform.
Design Files
Link to design files