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SDSS Engineering

Results of the 1/94 Drilling Tests

Sloan Digital Sky Survey Telescope Technical Note 19940412-02

Russell Owen

Introduction

This is an extension of the drilling tests performed 3/93 and described in engineering report 19930430. In these new tests we increased the maximum drill entry angle from one to four degrees and tried center drilling before drilling to see if it improved the holes.

Plates and Drilling

We drilled 22 plates. The plates were 3.5" diameter 1/8" thick disks of aluminum. 50 holes were drilled in each test plate, 10 each in five concentric circles. The top surface of each disk was formed of annuli of varying slopes, so that the entry angle (the angle at which the bits contacts the plate) for the inner through outer circles was 0.0, 1.0, 2.0, 3.0 and 4.0 degrees. Several different drilling techniques were tried, as follows:

Twist Drill

Five plates (#1-5) were drilled using a carbide twist drill. Drill flute length was 3/16". The drill extended 0.5" below the collet. Holes were drilled with a 0.1" peck cycle (3 pecks per hole, one more peck than the 3/93 tests ). Each plate required 5.7 minutes to drill. This was longer than the 3/93 tests (4.8 minutes), presumably due to the extra peck.

Center Drill Followed By Twist Drill

Six plates (#6-11) were drilled first with a center drill and then a twist drill. The center drill went down far enough that the shoulder carved a full diameter hole on the higher end of the plate (since the holes were drilled at an angle). The first three plates (#6-8) were drilled with a 0.1" peck cycle. These tool 9.3 minutes each to drill. The other three (#9-11) were drilled with no pecks, and took 7.9 minutes each to drill.

Spade Drill

Five plates (#12-16) were drilled using a carbide spade drill. Spade drills did very well in the 3/93 tests , even though those were a little longer than necessary. This time we bought special short bits, which could be held so they extended 0.5" below the collet. Peck cycle and time to drill were identical to the twist drill.

Center Drill Followed by Twist Spade Drill

Six plates (#17-22) were drilled using first a center drill and then a spade drill. Details were the same as the center drill followed by the twist drill, including the first three plates being drilled with a peck cycle and the second three being drilled without.

Details

The diameter of each bit was 0.0867 +0/-0.000,05" (2.202 +0/-0.001 mm). This diameter was the optimal value for our plug plates determined from the 1/92 drilling tests. All bits were made of carbide steel by Johnson Carbide Products, Inc., Saginaw, Mich. A fresh bit was used for each test plate unless otherwise noted. For both spade and twist drill bits the unsupported length (distance between tip and collet) was 0.5".

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.

Measurement and Analysis

The test plate holes were measured with a coordinate measuring machine. Each hole was measured at 24 points, 8 each at 3 depths (approximately 0.1" away from top surface, near the middle and approximately 0.1" away from the bottom surface). At each depth the 8 data points were processed to give four numbers: x position, y position, diameter and "non-circularity".

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.

Results

The results are shown in tables 1-4. Table 1 shows the errors averaged over all plates drilled using a given method. Table 2 shows the errors as a function of entry angle of the bit into the plate, again averaged over all plates drilled using a given method. Table 3 shows the errors measured at different depths along the holes. Table 4 shows the errors plate by plate. Note that the first plate was measured twice to test reproducibility.

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.

Table 1: Results by Method 

           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

Table 2: Results by Entry Angle 

           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     2.5
We 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 ).

Table 3: Results by Depth 

           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    18.3
We 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.

Table 4: Results for each Plate 

           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     1.8
Plate 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.

Conclusions

The current results have some disturbing inconsistencies when compared to the 3/93 results . The spade drills were shorter for the current tests, which we thought might improve position accuracy and reduce diameter variation; but the twist drills had the same geometry as in the 3/93 tests, so should have yielded similar results. Instead, we observe that position accuracy improved by approximately a factor of two for both spade and twist drills. Worse, average diameter increased by 2 µm for the spade bits, and an incredible 9 µm for the twist bits (out to 1 degree). The diameter tolerance for both kinds of bits was +0/-1 µm. We measured the bits with a micrometer, and the diameters appear to be within specification, or close enough that simple bit diameter variation cannot account for the errors. We suspect that centering of the tip or some other factor is causing the variation, and are discussing the issue with Johnson Carbide (manufacturers of the bits).

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.

Acknowledgments

Frank Ward ordered the bits and made the blank plates. Ron Musgrave made the collet, drilled the holes, took careful notes while doing so, and gave useful advice for improving the tests. Robert Riley and others in the Fermilab Quality Control Lab measured the plates. Siriluk Limmongkol measured the diameters of the drill bits.