Week 7 Group Assignments -¶
- Group assignment:
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Complete your lab’s safety training
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Test runout, alignment, fixturing, speeds, feeds, materials and toolpaths for your machine
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Document your work to the group work page and reflect on your individual page what you learned
Safety Training¶
The safety training for the lab and specifically the wood shop room was completed. We discussed the overall safety rules of the shop and rules related to the Shopbot and wood cutting tools.
- Some highlights are:
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Always wear eye and ear protection while using the Shopbot
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Never work alone (buddy system)
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Know where the closest fire extinguisher is (In our shop it is right outside the Shopbot room)
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Never leave the machine while it is running
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Always run the dust extractor while cutting
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Be within arms length of the emergency stop while the machine is powered up
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Clean areas after working to prevent trips, falls, and cuts
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Use gloves to handle wood to prevent splinters
Testing¶
At the Super Fab Lab at UNCC we have a Shopbot PRS5 Alpha with a 8’ x 5’ table. All of the testing and cutting was performed on this machine.
Runout¶
The runout test was to figure out how tru the spindle is. We setup a micrometer on a magnetic arm onto the bet and moved the arm such that that the plunger on the micrometer was touching the shank of the 1/4” router bit that was chucked up into the machine. Making sure that the plunger was engaged, we then spun the spindle by hand and observed the motion of the micrometer dial. No noticable movement of the dial was observed, thus proving that the spindle is true.
Alignment¶
The alignment was evaluated from the 4” square and circular test pieces that were made below. We evaluated these parts with calipers to check the dimensional cut accuracy and they measured within .005” from the CAD geometry.
We also checked the square test part for squareness. We have a digital angle finder in our lab so we used this to measure the squareness of the part. We measured all 4 angles and they measured between 89.8 and 89.9 degrees proving that the alignment of the machine is accurate.
Fixturing¶
There are number of different ways that material can be fixtured to the table bed. Below are examples.
Screws¶
One way to fixture material is with metal screws. A drill can be used to easily screw the workpiece down to the table. This provides a secure anchor for the material, but the drawback is that you risk hitting it and damaging the bit if the toolpath happens to contact it. With care this is a great way to anchor.
Plastic Nails¶
Another great way to fixture material on the Shopbot is to use plastic nails. These are installed with a pneumatic nail gun and provide secure anchoring of the work piece. The upside is that if they are runover by the router bit that they will machine down without damaging the bit. The downside is that these are only suitable for materials like wood, as they will not penetrate plastics. One must also be careful to cut them off the sacrificial material after the job as they can stick into the substrate and are hard to see and can cause fixtureing problems for the next job.
Clamps¶
It is possible to use bar clamps to secure material. However, they are not a preferred way on this machine. It can be hard to find a good place to secure the clamps to the machine and the piece at the same time. It also poses a significant risk for a collision with the machine. It is recommended to not use this technique unless a unique situation requires it.
Double Sided Tape¶
Double sided tape is another way to hold down work pieces. This is effective only when one can use large enough pieces of tape to get good holding force and when the surface of the table is clean and dust-free to get good adhesion. This is a good option to use in concert with other fixturing techniques like nails or screws to hold down pieces during a finishing pass. This technique was used to cut acrylic parts shown below.
Tabs¶
While not technically a hold-down technique, tabbing a part can help hold it in place during machining. Tabs are created in the V-Carve software to create do-non-machine areas on a profile path that cuts through the full thickness of the part. It leaves a little bit of material attached to the stock material so that the machined piece will stay in place. At the conclusion of the machining pass, the part can then be easy cut or chiselled out from the stock material and the remnant tab cut and sanded to create a smooth surface.
Speeds and Feeds¶
We started out by researching suggested feeds and speeds from the Shopbot manual. A selection of the suggested feeds and speeds for common materials is listed below:
| Material | Bit | Speed (rpm) | Feed (ips) |
|---|---|---|---|
Wood |
1/8" End Mill |
18,000 | 1.8-3 |
Wood |
1/4" End Mill |
18,000 | 3-4.8 |
Wood |
1/2" End Mill |
18,000 | 3-4.8 |
Acrylic |
1/4" End Mill |
16,000 | .8-1.2 |
So the acrylic requires a much slower feed rate, likely due to heat buildup at the interface of the tool.
Then we did a test to see the effect of altering the suggested feeds and speeds on wood. We made a file with four 6” long lines. Then we created a profile path for each of them, with a 1/4” 2-flute end mill with a depth of 1/4” and set the speed to 18,000 RPM. Then for each line we set a different speed. The bottom line had 3, the next one up 6, the next 9, and the top one 12.
We chose these based on the maximum chip load from the chipload calculator inside of the Shopbot software.
Then we ran the job to see how it would perform. The slow speed definitely seemed slower than the machine could handle. None of the speeds seemed to cause any trouble for the machine. Looking at the cut paths, the highest speed was a little bit rougher on the edges, but did not have any burning or signs of over-stress.
Materials¶
There are a number of materials that are suitable to cut on the Shopbot including all types of wood, mdf, plastics like acrylic and ABS and even thin aluminum. For this assignment we tested both wood and acrylic as we felt like those would be the most popular materials used in the lab and for our projects.
Wood¶
The details of cutting wood are outlined elsewhere on this page.
Acrylic¶
Acrylic is an interesting and easy material to cut with the Shop Bot. We had some 9mm thick acrylic at the SFL that we purchased to make awards for the College of Engineering so we used that to test cut settings. Based on information in forums on cutting acrylic, the feed should be about 1 ips so about 3X slower than wood. So I setup the same cirle and square with a pocket as I ran on the wood for the acrylic. I used the same 1/4” end mill and recompiled the tool path using 1ips speed, 18,000 RPM, and .1in cut depths. I did a .020” thick final pass to keep the ch
For workholding I put double sidede tape on the bottom of the sheet and stuck it to the bed of the machine. Then we drilled holes in the corners of the sheet and installed screws as additional clamping. I did a .020” thick final pass to keep the chip load low for the last pass to make sure the parts would hold on the double sided tape.
Then we ran the tool path and in about 10 minutes we had an acrylic square and circle.
We then took this a step further and did a test cut for the trophies we are making. We used the UNCC C logo and created a cut path in VCarve and used the same settings as with the square and circle. We cut it with the front of the C on the table so that we would keep the front of the C in good shape.
Once the C was cut out we took the backing paper off the back of the C and put it in the laser cutter and etched the back of the C with reversed lettering to make the trophy.
Toolpaths¶
We explored both pocket and profile toolpaths as those are 2 key operations that will be used to process many parts. To do this, we made a circle and square with corresponding pockets in them
Job Setup¶
We setup the stock to 48” x 48” to give ample room for the cut shapes and set the thickness to .75”. Then we set the orgin to the table zero.
Pocket Setup¶
We opened a new part in V-Carve and drew a 4” square with a 3” square inside of it for the pocket. Similarly, we drew a 4” circle with a 3” inner circle. Then we selected the inner small square and small cirle and selected the pocket operation. We set the cut dept th .25” and chose the 1/4” end mill tool. We edited the tool and changed the feed and speed to 18,000 RPM and 3 inches/sec to match the chart provided in the Shopbot manual. We left the default 40% stepover of .1” Then we selected climb milling and set the number of passes to 2 to be conservative. Then we calcultated the toolpath.
Profile Setup¶
We selected the outer contours of both shapes and selected the profile tool path. We selected the same 1/4” end mill from the pocket operation. We set the cut depth to .75 to match the material thickness, and selected climb cutting. We selected 4 passes to keep a decent chip load. Then we added tabs to the tool path. We set a length of .2 and a thickness of .1 and then we clicked ‘Edit Tabs’ and selected 3 places on each tool path to add the tabs. Then we calculated the toolpath and checked it on the viewining window.
Toolpath and Export¶
Once we created the toolpaths we were able to view the full job in the main window. We could see the toolpaths and the breaks in the profile for the tabs.
Next we selected both tool paths, selected the Shopbot from machine list and chose the ‘Shopbot inches’ post processor and save the file. The total cut time was estimated to be a litte under 5 minutes.
Zero Axes¶
A 1/4” upcut mill was installed in the Shopbot. Then the Z axis was zeroed. We put the zeroing plate on the sacrificial layer and under the mill. Then we clamped the alligator clip to the collar of the collet holder to allow for an electrical connection between the end mill and the clip.
Then we opened the Shopbot control software and ran the Z-zeroing routine. Once we verified with the software that the zeroing plate was installed correctly it dropped the z-axis until it touched the plate. Then it raised up about an inch and lowered down to the plate more slowly to find an accurate Z- zero. Then it prompted us to put the plate away.
Then we zeroed the X and Y axes. We used the arrove keys on the control computer to move the mill to place on the stock that would be suitable starting location for the lower left side of our art to have enough room to machine the shapes. Then we clicked the ‘Zero Axes’ button and selected the X and Y axes to complete the zeroing.
Cutting¶
Now it was time to cut. We raised the Z-axis and put the brush shoe on the spindle. Then we selected ‘Cut Part’ from the Shopbot software. Then we went through the software prompts telling it that the axes were zeroed. We put the key in the spindle lock-out on the control box of the machine and when prompted, pushed the green start button on the E-stop pendant to start the spindle. Then we turned on the dust extractor that is located under the machine.
We confirmed with the software to start the cut and it dutifully started running the pockets and eventually the outer profiles.
Result¶
The result parts looked great. The tabs worked perfectly, and the parts stayed attached to the working piece. The surface finish was a little bit choppy, but it could be the nature of the material as we used upcut and climb settings which should yeild the best finish.
Then we chiseled them out with a small wood chisel to check the dimensions. The outside of the square measured consistently at 4.004” with the calipers, so just a little bit over. The circle measured out at 4.001-4.003” depending on the measurement location. So the cut parts are fairly accurate to the CAD.
