Applications , page 7
Exercise 6. Determine blade length cutting specifications. Because the blade length increases during the hardening process the sawn length has to be shorter than the hardened length. To determine how much shorter to cut the blades, conduct an experiment as follows: harden 50 blades using the cutoff saw stock. Keep track of the before and after lengths to determine the mean growth. Subtract the mean growth from the center of the blade length specifications to set the saw target. Develop a work instruction for saw length specifications.

Figure 1.
The saw is initially set at 9.990 inches to produce 10 inch blades, allowing 0.010 inches for growth. N is set at 50.
Figure 2. Saw production data.

Start production with the green key on the control panel.
After the blades are cut, go back to the saw production data to copy the data to the clipboard and save them in Excel, for example (Figure 2).
Next, heat-treat the same 50 blades. To do this, go to Hardening Furnace menu. Do a test run using the default settings. In the Production Run menu, set the run size to 50 and sample size to 50. (Figure 3). The simulation will measure the blades consecutively if the sample size and the run size are the same, using either random or systematic sampling.
Choose Begin Heat Treatment to open the heat treatment production window (Figure 4). Click use cutoff saw stock in order to harden the same blades you cut to length .
After hardening the blades, copy the length data to the clipboard, then to Excel or other analysis software.
Figure 3. The run and sample size are both 50.
Figure 4. Heat treatment using the same 50 blades in cutoff saw stock, taken in the same order.
Estimating the average length change
The length change for each heat-treated blade was calculated by taking the treated length minus the sawn length for each blade. The histogram in figure 5 shows the length changes for all 50 blades are fairly balanced around a mean of .0112
Figure 6 shows a confidence interval for the length change, calculated in Excel as 0.0112 +/– 0.0095482. The 95% C.I. is (0.010, 0.012).

The length specification for10 inch knives is –0.008 to +0.000, according to the current work instruction. Therefore, the saw target for 10 incn knives should be 0.011 + 0.004 = 0.015 inches less than the finished length. Set the saw to cut at 9.85 inches. Monitor the finished lengths using a run chart or an x bar s chart.

The change in length was about 0.1% of the total length. Does this figure hold for other blade lengths?
Figure 5. Length changes. (Difference = HT length minus saw length)
Figure 6. Descriptive statistics
Statistics and SPC: 1 2 3 4 5 6 7
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