Week 7: Computer Controlled Machining

group assignment: • do your lab’s safety training • test runout, alignment, fixturing, speeds, feeds, materials, and toolpaths for your machine

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:

1. Turn on the machine (two switches on side)
2. Open Mach4 Software for Avid CNC control
3. 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.
4. 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.
5. Send the machine back and away for safety.
6. 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.
7. Fixture piece for cutting. Our fixturing methods so far have been 1) double-sided tape 2) clamping
8. 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.
9. 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
10. Load G-code file(s)
11. 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.
12. Turn on dust collector and fan
13. 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!).
14. 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.
15. You did it! Now you can go on to any finishing steps (like sanding the wood, painting, etc.)

Safety Training

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

Machine Testing - Practice & Joint Tests

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). Angela 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.

Joint, Speed, & Passes Test

This is the test for both fit tolerance, speed, and Passes.

On the boards are 10 rectangles. From left to right I programmed the machine to cut them to be .72, .725, .73, .735, 1.0. THi is all in inches. We chose these numbers because the plywood me measured was .72in in thickness.

The top rows on each were cut in 3 passes, and the bottom rectangle in 6 passes. Overall I did not notice a signifigant difference. The 6 passes were smoother in feel and were alightly looser than their 3 pass counterparts.

Overall we found that slight differences do matter. some joints were tighter but still worked, which would be good if you wanted to fit two things together without glue. The looser joints are more ideal for if you want to add glue, since they are easier to slide together.

Lastly there is speed. THe board on the left is 100% speed, and the right is 150% speed. The faster cuts were roughter and shaved away slightly less wood.

Dogbone Joint Test

Creating dogbone joint test design + setting toolpath settings for 1/4’’ drill bit in V-Carve

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.

Very tight fit at 3.00’’ and 2 passes

Another very tight fit at 3.00’’ and 3 passes (slightly less tight). Side angle view

Very snug fit at 3.05’’ and 2 passes. You can just barely see the gap

Another very snug fit at 3.05’’ and at 3 passes

Showing an example from the other end of the scale. Very large gap at 3.25’’ and 2 passes.

The cuts at nominal length of 3.0’‘, 3.05’‘, and 3.10’’ were best (depending on desired fit). 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’‘and 3.10’’ were 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.

Machine Testing – Definitions, Practices, Observations

Runout: When the bit in the machine is is spun rapidly to cut something, it is not perfectly centered. There are minor deviations. This is called runout

Alignment: The process where we calibrate the machine so it “knows” where to cut. Please note our machine uses relative location. It does not “know” where the cutting tool is and must be calibrated everytime you start it up.

Fixturing: You must secure your material onto the cutting bed so that it does not shift during cutting. Our studio uses double sided tape. If needed, sometime we use pieces of scrap wood and screws to secure a piece of wood onto the bed.

Speeds: How fast the bit is cutting. Our program uses RPM, rotations per minute.

Feeds: How fast the bit is cutting the material. This is measured in IPM, or inches per minute.

Materials: For far our studio has been using plywood for our projects.

Toolpaths: Different tools need to be programmed to cut differently. We used the “cut through” toolpath, whih follows a line where there is a vector. The other type of toolpath we used is “pocket”. This is good for hollowing out inside sections.

Passes: The amount of times the machine repeats a path, each time cutting away more matrial.

Downcuts, upcut, and compression cut: In machineing there are different types of bits. 3 of these types are downcut bits, upcut bits, and compression bits. Downcut bits give the cleanest look for the top of the aterial, they push the woodchips downword. If you look at them point facing down,the flutes rotate clockwise. Upcuts bits push the wood chips up, this is good because theres less sawdust getting in the way of your cuts. Their flutes rotate counterclockwise. The last type is compression bits. They are expensive. They push the woodchips to the center, giving it both the benefits of the upcut and the downcut bits.

eflection: The aomunt the bit tilts in response to it being pushed against the wood. Bigger force, Bigger deflection = less clean cuts and tighter joints.