LASER INTERFEROMETER GRAVITATIONAL WAVE OBSERVATORY LIGO Laboratory/LIGO Scientific Collaboration LIGO-T040111-00-D Advanced LIGO 29 December 2004 Galling Tendencies and Particles Produced by Ultra Clean Screw Threads Mike Gerfen, Larry Jones and Calum Torrie Distribution of this document: LIGO Laboratory This is an internal working note of the LIGO Project. California Institute of Technology LIGO Project – MS 18-34 1200 E. California Blvd. Pasadena CA 91125 Phone (626) 395-2129 Fax (626) 304-9834 E-mail: [email protected] Massachusetts Institute of Technology LIGO Project – NW17-161 Cambridge, MA 02139 Phone (617) 253-4824 Fax (617) 253-7014 E-mail: [email protected] LIGO Hanford Observatory P.O. Box 1970 Mail Stop S9-02 Richland WA 99352 Phone (509) 372-8106 Fax (509) 372-8137 LIGO Livingston Observatory P.O. Box 940 Livingston, LA 70754 Phone (225) 686-3100 Fax (225) 686-7189 WWW: http://www.ligo.caltech.eduLASER INTERFEROMETER GRAVITATIONAL WAVE OBSERVATORY 1. ABSTRACT A test was conducted to evaluate the tendency to gall and to collect the particles generated by screw threads that were thoroughly cleaned for ultra high vacuum service. Bars of stainless steel and aluminum were tapped to size ¼-20 using a wide variety of tap configurations, with 20 holes per bar. Screws of various materials and treatments were inserted, torqued and removed 10 times per hole, on 10 of those holes, to check for the effects of material and tap choices on galling tendencies. Particles from these were collected by means of a strip of transparent tape placed before insertions under the row of holes in each bar. After testing, each strip of tape was transferred to a report sheet, which was then scanned at 1600 dpi. The data were evaluated and recommendations are made to minimize particle generation and galling in future designs. 2. INTRODUCTION The LIGO Project has experienced difficulties from galling of screw threads that have been thoroughly cleaned for ultra high vacuum application. Most of these problems have been overcome by means of material selection and having the holes tapped at 0.005” oversize. High Precision Devices, a contractor on a value engineering task, presented a report which showed that a significant amount of particles are generated by some ultra cleaned screw threads. Since some of the LIGO optics are located below threaded joints utilized for optics installation, we wanted to better understand this effect and how to minimize it. This task was performed to better understand the potential for galling and particle generation within the resources available to the LIGO Project. Sensitivity of LIGO’s optics to condensed hydrocarbons demands that no lubricants are to be used in our vacuum chambers, and all parts must be thoroughly cleaned and baked to minimize outgassing risks. 3. HARDWARE PREPARATION Test blocks were cut from 0.5” x 1.0” rectangular bar stock: 12 from Type 304 Stainless Steel, and 12 from 6061-T6511 Aluminum. Taps were purchased of various materials and designs, to evaluate the effects that the tap would have on the tendency for galling and generating particles during use. Specific ¼-20 taps purchased were as follows: 1. TiN coated , spiral point tap, H3 pitch dia., 3-flute, 2 ½” overall length. 2. TiCN coated, spiral point tap, H3 pitch dia., 3-flute, 2 ½” overall length. 3. Oxide-over-Nitride treated, spiral point tap, H3 pitch dia., 3-flute, 2 ½” overall length.LASER INTERFEROMETER GRAVITATIONAL WAVE OBSERVATORY 4. Nitride treated, spiral point tap for aluminum, H3 pitch dia., 3-flute, 3.15” overall length. 5. Bright finish, High Strength Steel, spiral point tap, H3 pitch dia., 2-flute, 2 ½” overall length. 6. TiN coated, High Strength Steel, spiral point tap, H3 pitch dia., 2-flute, 2 ½” overall length. 7. TiCN coated, High Strength Steel, spiral point tap, H3 pitch dia., 2-flute, 2 ½” overall length. 8. Cobalt steel, spiral point tap, H3 pitch dia., 2-flute, 2 ½” overall length. 9. High performance, TiN coated, spiral flute tap, H3 pitch dia., 3-flute 10. High performance, TiCN coated, spiral flute tap, H3 pitch dia., 3-flute 11. Bright finish, High Strength Steel, spiral flute tap, plug, H3 pitch dia., 3-flute 12. TiN coated, High Strength Steel, spiral flute tap, plug, H3 pitch dia., 3-flute 13. Bright finish, High Strength Steel, thread-forming tap, plug, H4 pitch dia., 2 ½” overall length. 14. Chrome plated, High Strength Steel, hand tap, plug, H3 pitch dia., 4-flute 15. 0.005” oversize, spiral point tap, H11 pitch dia., 2-flute, 2 ½” overall length. 16. 0.005” oversize, High Strength Steel, hand tap, plug, H11 pitch dia., 4-flute Not all of the taps were used, due to similarities between them. The “H” number designates the oversize dimension in pitch diameter; H3, considered as standard, is +0.0010” to +0.0015”; H11 is +0.0050” to +0.0055”. Some of the taps are better for tapping aluminum and some better for stainless steel; others are more universal. The blocks were drilled and tapped with 20 holes per block in a uniform sequence, as shown in Figure 1. Tapping on each block was started with a new tap, which then proceeded through the 20 tapped holes (except as noted below). The bars were sectioned through the odd-numbered holes to permit close examination of the helical surfaces and thread edges. These steps were taken in order to: a. check the effects of tap wear for tapping 20 holes, on the helical surfaces and edges of the threads, and b. check the effects of that wear on the tendency to gall and generate particles during use. We had expected that wear effects might be noticeable, especially with tapping the stainless steel blocks. LIGO requirements forbid the use of cutting fluids that contain sulfur, chlorine or silicone, and require that fluids be water soluble. Tapping was started using Hangsterfers S-500 CF (water soluble coolant concentrate, chlorine free) with machine tapping, and no problems were experienced in aluminum. However, tap seizing caused great difficulties in stainless, and some broken taps. The procedure was changed to hand tapping, with little improvement. Finally, the cutting fluid requirement was relieved in order to make progress. Rapid Tap cutting oil was used, with good results in machine tapping. The LIGO Project should reconsider their current requirement on cutting fluids. Although