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: http://www.apo.nmsu.edu/Telescopes/SDSS/eng.papers/19940412_drill_1_94.html
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Russell Owen
All holes were drilled with a Dahlil Machining Center, running at a spindle speed of 3500 RPM and a feed rate of 5.0"/min. The peck cycle, when used, was 0.1" (3 pecks per hole). A coolant (Blasocut 2000 universal water soluble solution) was used during all drilling.
The bits were held by a custom-made aluminum collet which was held in a standard Nikken Mini-Mini chuck. To minimize runout of the bit the hole in the collet was machined while the collet was mounted in the NC mill. Scratches were made on the collet and machine so that the collet could be re-inserted in the same orientation. The collet was split using EDM to minimize burring from the splitting operation. Runout of the collet was measured at 0.000,1" (0.003 mm) or better for all bits, and half that for 17 of the 22 bits.
The center drill had a diameter of 0.0788" (2.00 mm) for the smaller cutter at the tip. It was a conventional bit with conventional accuracy. It was held in a standard Nikken BT40-NPU13-80 drill chuck, which held it with a measured TIR of 0.0002" (0.005 mm). The same center drill was used for all holes.
During drilling, the plate was held in a custom jig flat against a backing plate. The backing plate had an over-sized pit under each hole in the test plate, so the bits never contacted the backing plate. After drilling the plates were run through an automatic dishwasher; this removed most or all metal shavings from the holes.
Non-circularity is defined as the difference in radius between the point closest to the hole's center and the point farthest from the hole's center. Note that this definition is based on the extreme measurements of the eight measurements at the given depth; hence it will tend to be dominated by noise in the measuring machine and residual dirt in the hole. The measuring machine introduces an average of approximately 5 µm of noise into non-circularity according to Robert Riley (one of the people made the measurements). Non-circularity may be useful for comparing different drilling methods, but is not easily used to predict hole morphology.
The x-y position data for each plate was corrected for overall errors in offset and rotation--both meaningless artifacts of the way the plate was mounted in the measuring machine. Only the middle depth x-y data was used for this correction, to avoid problems due to damage of the ends of the holes. Scale errors were not removed.
Hole tilt was computed by dividing the offset between the hole position measured at the top and bottom of the hole by the distance between the top and bottom measurements.
Position error is the radial distance between the measured hole and the desired hole. Diameter error is the measured diameter minus the nominal diameter. Non-circularity and tilt are described in section 3. The mean and standard deviation of the diameter error are given in addition to the RMS because if the diameter is sufficiently reproducible, the RMS error can be reduced to the standard deviation (at minimum) by using a different size bit.
Method Pos Err Diameter Error Non-Circ Tilt RMS mean std dev RMS RMS RMS (µm) (µm) (µm) (µm) (µm) (mrad) Twist 3.6 10.5 5.5 11.9 9.5 1.9 Center & Twist 3.6 5.2 3.3 6.1 9.1 2.0 Ctr & Twist, No Pecks 3.9 5.6 5.0 7.5 11.5 2.0 Spade 4.8 8.9 4.3 9.8 9.1 1.9 Center & Spade 3.3 7.4 4.9 8.9 5.8 1.5 Ctr & Spade, No Pecks 4.4 7.7 4.9 9.1 11.1 1.8
Method Entry Pos Err Diameter Error Non-Circ Tilt Angle RMS mean std dev RMS RMS RMS (deg) (µm) (µm) (µm) (µm) (µm) (mrad) Twist 0.0 4.3 10.4 6.2 12.1 10.1 2.6 1.0 3.8 10.7 5.7 12.1 12.5 2.3 2.0 3.3 10.6 5.1 11.7 8.5 2.0 3.0 3.4 10.4 5.5 11.7 8.6 1.6 4.0 3.2 10.4 5.0 11.5 7.0 1.6 Center & Twist 0.0 4.8 4.8 4.3 6.4 9.6 1.5 1.0 3.9 5.0 3.1 5.8 12.5 1.3 2.0 2.7 5.3 1.9 5.6 6.0 2.4 3.0 3.2 5.4 2.7 6.1 7.9 2.9 4.0 3.0 5.4 4.1 6.8 8.4 2.2 Center & Twist 0.0 3.2 6.1 3.3 6.9 7.4 1.7 No Pecks 1.0 4.3 5.7 4.6 7.3 12.4 1.4 2.0 3.5 5.9 3.8 7.0 8.0 1.4 3.0 4.3 5.2 5.5 7.5 14.0 1.3 4.0 4.2 5.2 6.8 8.6 13.7 1.4 Spade 0.0 4.9 7.0 3.7 8.0 10.6 2.0 1.0 3.1 7.7 1.9 7.9 4.3 2.4 2.0 2.7 8.4 2.4 8.7 4.9 2.7 3.0 4.5 9.6 3.5 10.2 6.8 2.1 4.0 7.2 11.5 6.6 13.3 14.4 1.9 Center & Spade 0.0 3.0 5.7 1.9 6.0 4.2 1.7 1.0 3.8 5.2 2.2 5.6 7.5 1.5 2.0 2.5 6.0 1.6 6.2 3.9 1.8 3.0 3.0 8.1 3.7 8.9 4.5 2.3 4.0 3.8 12.2 8.1 14.6 7.7 1.6 Center & Spade 0.0 4.4 5.7 4.7 7.3 12.3 1.3 No Pecks 1.0 4.1 6.4 3.4 7.2 11.2 1.2 2.0 3.6 6.6 1.9 6.8 3.9 1.3 3.0 3.5 8.3 4.4 9.4 4.1 1.8 4.0 6.0 11.4 6.5 13.1 17.5 2.5We see from table 1 that spade drills are slightly better than twist drills except for position error. This is no surprise, as we have obtained the same results in the 3/93 and 1/92 drill tests. Center drilling helps, especially with a normal peck cycle for the final bit.
The Sloan Digital Sky Survey will require holes drilled at up to 2 degrees; at these angles spade drills are superior to twist drills in all respects (see table 2 ).
Method Meas Pos Err Diameter Error Non-Circ Depth RMS mean std dev RMS RMS (µm) (µm) (µm) (µm) (µm) Twist Enter 3.1 12.3 4.3 13.0 5.7 Mid 2.5 13.2 4.8 14.0 5.7 Exit 4.9 6.0 4.4 7.5 14.4 Center & Twist Enter 3.0 4.4 1.2 4.6 4.7 Mid 2.3 6.5 1.9 6.8 5.0 Exit 4.9 4.6 5.0 6.8 14.2 Center & Twist Enter 3.1 3.4 2.5 4.2 7.2 No Pecks Mid 2.8 9.1 4.2 10.0 7.9 Exit 5.4 4.4 5.7 7.1 16.8 Spade Enter 4.1 10.9 3.2 11.3 5.1 Mid 3.9 9.0 2.6 9.4 4.2 Exit 6.1 6.7 5.3 8.6 14.2 Center & Spade Enter 2.7 7.1 4.0 8.1 4.4 Mid 2.5 7.6 4.1 8.6 4.0 Exit 4.3 7.6 6.4 10.0 8.1 Center & Spade Enter 3.4 7.9 3.7 8.7 4.1 No Pecks Mid 3.6 8.1 3.9 9.0 4.0 Exit 5.8 7.0 6.5 9.5 18.3We plan to drill the plates from the sky side, so "Enter" refers to the sky side and "Exit" to the fiber side. Hence the expected position error with a plain spade drill is 4.1 µm RMS, and probably even less for angles out to 2 degrees.
Method Plate Pos Err Diameter Error Non-Circ Tilt RMS mean std dev RMS RMS RMS (µm) (µm) (µm) (µm) (µm) (mrad) Twist 1 2.8 12.8 4.2 13.5 5.5 1.5 1 3.1 12.3 4.5 13.1 7.9 1.7 2 3.8 10.7 4.9 11.7 10.5 2.1 3 3.2 14.9 5.2 15.8 7.3 1.6 4 4.6 4.5 3.0 5.4 11.4 2.2 5 3.4 10.2 3.8 10.8 9.8 1.7 Center & Twist 6 3.4 5.2 1.3 5.4 5.4 1.9 7 3.2 6.0 3.0 6.7 8.4 1.6 8 4.1 4.3 4.6 6.3 12.3 2.4 Center & Twist 9 3.8 7.2 4.9 8.7 10.3 2.2 No Pecks 10 3.2 6.3 4.0 7.5 8.7 1.9 11 4.7 3.3 5.1 6.0 14.7 2.1 Spade 12 4.3 9.1 3.1 9.7 4.6 1.2 13 3.6 9.3 3.7 10.0 7.0 1.5 14 4.2 9.4 4.0 10.2 8.9 1.7 15 4.2 8.8 3.3 9.4 6.7 1.5 16 6.9 7.7 6.2 9.9 14.7 2.9 Center & Spade 17 3.7 7.6 6.1 9.8 6.5 1.8 18 2.8 6.8 4.6 8.2 4.4 1.1 19 3.2 7.9 3.8 8.8 6.2 1.5 Center & Spade 20 4.7 8.6 5.3 10.1 14.6 2.2 No Pecks 21 2.8 7.0 4.7 8.4 8.4 1.0 22 5.3 7.4 4.4 8.6 9.3 1.8Plate 1 was measured twice, and the two measurements are quite consistent. For the most part, the results for the various plates drilled by a given technique agree well. However, non-circularity varies quite a bit from plate to plate. This is not surprising; the measure for non-circularity relies on outliers and hence is intrinsically noisy.
Once again, I recommend drilling with simply a spade drill. A twist dill produces holes with significantly higher standard deviation in diameter. Center drilling helps both techniques, but plain spade drilling is so good that the additional accuracy is probably not worth the extra cost.