Week 7: Computer Controlled Machining - Richard Shan & Alana Duffy¶
Assignment¶
For this week, we were assigned to:
- Complete your lab’s safety training
- Test runout, alignment, fixturing, speeds, feeds, materials and toolpaths for your machine
- Document your work to the group work page and reflect on your individual page what you learned
Work Distribution¶
| People | Description |
| Alana/Richard | Runout Test |
| Alana | Fixturing |
| Alana | Speeds |
| Alana | Feeds |
| Richard | Toolpaths Test |
| Richard | Materials Test |
| Richard/Alana | Alignment Test |
| Richard/Alana | Documentation |
Runout¶
For the runout test we tested the variation of the spindle’s diameter from its true axis as it rotates by using a dial indicator (measures how much pressure is put on the gauge of the indicator as it rotates). We set up the indicator next to the spindle and manually pushed the spindle so that it would rotate. We then recorded the values on the indicator as the runout value
Fixturing¶
As defined by Nevatio Engineering, “CNC Fixtures are devices designed for holding CNC machined workpieces (or parts) during cutting operations on CNC machines.”
Although there are multiple ways to fixture materials, our lab mainly uses tabs. In our CAM designs we include tabs to prevent cut out pieces from shifting/moving around while the rest of the job occurs.
Another fixturing method is to use screws, however, due to the risk of the bit hitting the screws and shattering and sending metal shards flying, our lab does not allow us to use them.
Shop Bot has additional documentation on their reccomendations for fixturing materials. They talk about screws, T-track tables and rails, clamps, press-fit jigs, adhesives, and vacuum hold down systems. They reccomend: screws for spoilboard; T-track tables and rails for movable fixturing; clamps for strong holds and when there is no excess material in which to drive screws into; press-fit jigs for good holding power and holding small pieces, thick stock, or materials where there is no space for screws to be used; adhesives for thin materials where a press-fit jig doesn’t work and small pieces of material; and vacuum hold down systems for conveniency and repetitive plywood/other large sheet material cuttings.
Speeds¶
As Autodesk puts it, “speed refers to the surface speed that the cutting edge of the tool is moving and is needed to calculate the spindle RPM.” This is measured in revolutions per minute and is inversely proportionate to feed rate. An example of this is that, if feed rate is held constant and the bit is spinned faster, the bit will travels less per revolution. Thus, if eed rate is held constant and the bit is spinned slower, it will travel more distance per revolution.
Feeds¶
Referencing Autodesk again, “feeds specifically refers to the feed rate the tool advances through the material.” This is measured in inches per revolution. It’s important to moniter this as too fast or slow of a feed rate will affect the quality of your cut.
Toolpath Test¶
We first created a simple Aspire file with three 2x2 inch squares. When creating their toolpaths, we selected either Inside, Outside, or On for each of the square toolpaths.
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We then ran the cut.
Here is the wood after cutting.
We then removed the tabs.
Here are the results of our toolpath test.
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We see that running the cut on the Inside setting has the machine cut on the inside of the line so that the furthest outer edge cut is the line. Running on the Outside setting cuts on the outside of the line so that the further inner edge is the line. Cutting on the On setting has the machine cut on the line, which means that this setting isn’t really useful unless the file is designed specifically to be cut using the On setting.
Materials Test¶
Although the Shopbot Alpha User Manual does not specifically mention its materials, researching online has revealed that our CNC machine is capable of cutting wood, plastics, wax, soft metals, and other CNC-able materials.
Here is the table in markdown format:
| Material | Turning | Drilling | Reaming | End Milling (Roughing) |
|---|---|---|---|---|
| Aluminum | 400-1000 | 250-600 | 100-300 | 600 |
| Brass | 225-300 | 150-300 | 130-180 | |
| Bronze | 150-225 | 100-250 | 75-180 | Medium: 250, Hard: 125 |
| Cast Iron | ||||
| - Soft | 100-150 | 75-150 | 60-100 | 60 |
| - Medium | 75-120 | 70-110 | 35-65 | |
| - Hard | 50-90 | 60-100 | 20-55 | 50 |
| Copper | 100-200 | 60-100 | 40-60 | |
| Magnesium | 600-1200 | 300-650 | 150-350 | |
| Stainless Steel | ||||
| - Free machining | 100-150 | 65-100 | 35-85 | 304: 55 |
| - Other grades | 40-85 | 15-50 | 15-30 | 17-4PH: 35 |
| Carbon and Alloy Steel Free Machining | ||||
| - Less than 0.3% C | 125-200 | 100-145 | 60-100 | Low C: 75 |
| - 0.3% to 0.6% C | 75-175 | 70-120 | 50-90 | 4140: 50 |
| - More than 0.6% C | 65-120 | 55-90 | 45-70 | 4340: 50 |
| Titanium | 25-55 | 30-60 | 10-20 | Ti-6Al-4V: 25 |
Alignment Test¶
To test the machine’s alignment, we used the Shopbot Jog function to move the spindle. For example, using Jog -> X, 60 would move the spindle to a position where the x-axis value is 60, keeping Y and Z axis values constant. A similar procedure with Jog -> Y and Jog -> Z can be used to jog those positions, and you can also use the Jog command to set specific points on all 3 axes to jog to.
We used the jog function in running air-cuts. We would first jog the Z axis up 2 inches then zero the Z axis at that point, meaning that when we run a cut, it would cut the entire job 2 inches above the actual material and allow us to test the job.