Prototyping and testing

Example 1 Hole compensation from a test piece

Mean hole sizes measured on three pieces per size (hypothetical values).

import statistics as st

design = [3.4, 5.0, 8.0, 12.0]            # mm
measured = [3.12, 4.74, 7.76, 11.78]      # mm

mx, my = st.mean(design), st.mean(measured)
a = sum((x - mx) * (y - my) for x, y in zip(design, measured)) / sum((x - mx) ** 2 for x in design)
b = my - a * mx
print(f"measured = {a:.4f} x design {b:+.3f} mm")
for d, m in zip(design, measured):
    print(f"design {d:>4} mm -> measured {m:5.2f} mm (error {m - d:+.2f} mm, fit residual {m - (a * d + b):+.3f})")
for target in (3.4, 8.0):
    print(f"to get {target} mm, draw the hole at {(target - b) / a:.2f} mm")

measured = 1.0067 x design -0.298 mm
design  3.4 mm -> measured  3.12 mm (error -0.28 mm, fit residual -0.005)
design  5.0 mm -> measured  4.74 mm (error -0.26 mm, fit residual +0.004)
design  8.0 mm -> measured  7.76 mm (error -0.24 mm, fit residual +0.004)
design 12.0 mm -> measured 11.78 mm (error -0.22 mm, fit residual -0.003)
to get 3.4 mm, draw the hole at 3.67 mm
to get 8.0 mm, draw the hole at 8.24 mm

A slope near 1 means holes shrink by almost the same amount at every size, about 0.2–0.3 mm, with small holes shrinking slightly more. Enlarging holes in CAD using this equation is more accurate than a single compensation value. It applies only to that printer, material and set of settings.

Example 2 Is the camera mount strong enough?

The camera weighs 0.3 kg. The team requires the mount to survive an impact of 20 times its weight in a hard landing (about 60 N) with a safety factor of at least 2.0. Results for five pieces (hypothetical values):

import statistics as st

failure_n = [182, 175, 190, 168, 186]     # N load at which the mount failed
M_KG, G, SHOCK, SF_REQUIRED = 0.3, 9.80665, 20, 2.0

required = M_KG * G * SHOCK
mean, sd = st.mean(failure_n), st.stdev(failure_n)
conservative = min(min(failure_n), mean - 2 * sd)
sf = conservative / required
print(f"required load {required:.1f} N")
print(f"failure loads: mean {mean:.1f} N, sd {sd:.1f} N, minimum {min(failure_n)} N, mean - 2sd {mean - 2 * sd:.1f} N")
print(f"safety factor on the conservative value {sf:.2f} -> {'PASS' if sf >= SF_REQUIRED else 'FAIL'}")

required load 58.8 N
failure loads: mean 180.2 N, sd 8.8 N, minimum 168 N, mean - 2sd 162.6 N
safety factor on the conservative value 2.76 -> PASS

It passes, but five pieces is still a small sample and the test was a single static load. Vibration and temperature tests are needed too, because PLA softens at modest temperatures (HDT about 55 °C in the datasheet). In the Thai sun, dark parts on the airframe may approach that level, so measure the real temperature.