Cutting Ab rasive 

A gre at deal of re search, design and testing  was c ondu cted as part of the gr inding  wheel  developm ent  process. We bega n our investigatio n of grindi ng wh eel materi als with an emphasis on diam ond p roducts.  These are the hardest mat erials availa ble, will operate at rel atively hi gh surface speeds and are subjected  to low force s where tou gher m aterials are n ot neede d. We experim ented  primarily wi th syntheti c  diamon ds, li miting ou r investigation  of natural di amonds. Th e relatively min or addition al hardne ss of  natural di amonds doe s not outweig h the more pre dictable and repeatable p ropertie s of th eir syntheti c  counterparts. We te sted la rge single cry stal diam onds faceted to f orm a rake and reli ef angl e nee ded fo r  cutting a nd m ultiple large a nd sm all dia mond crystals bonded in a sin gle laye r to a steel substrate. In all,  we te sted at least 5 0 diffe rent mat erial and g eometric combi natio ns. Upon  investigatio n we  learned th at  the tips of th e diam ond grains dev eloped flat spot s that preve nted furthe r grin ding within o ur bounded  force limits . This discovery came as  a surprise to us, as the force per grain is qu ite low and diamonds are  much h arder than the basa lt that we were using a s our test stand ard. These small flats create d too much   surfa ce area for the lo w loa ding of the to ol on the rock to have any cutting effect.  A diamond a brasive with   the ability to self sha rpen was needed. In this ap proach, when th e diamon d grains wear, they  fall out an d  expose fresh, sh arp g rains. Resin diamond wheels are designed especially fo r this purpose. Early testin g  of resi n diam ond wheel s led to very po or results be cause wh ole wheel s were used th at pre sented a la rge  surfa ce area to the ro ck. A se cond approach, atte mpted late r on, was designed to u se resin diamo nd  wheels also with very little down force. In this app roach, the whe els were cut along two cord s formin g two  small pa ds of  resin alo ng the perim eter of the “w heel” to engag e the ro ck. Re sultantly, the contact area  with the ro ck was reduced, causing a higher fo rce pe r grai n. Ultim ately, we settl ed on a g rain size of 100120 me sh (0. 1-mm diam eter) an d a co ncentration o f 112 (whi ch has a volum e of 28 perce nt diamond ).  The dia monds were coated with 56 weight perce nt Ni ckel that e nhan ces the bond characteristics of the   diamond to the resin. T he resin i s a phenolic with sili con carbide and cryolite fillers. Thi s seem ed to be the  best formula for gri nding within the giv en pa ramete rs. The sacrificial consumption of the grinding p ad  thickne ss was about 0.5 -0.64 mm (20-25 mil s) per grind  event i n hard rock i n Earth atmo sphere. T his  gave u s a tool life of ab out five grindi ngs in stron g basalt. We th en investigate d the geomet ry of the a ctual  resin diam ond pad s. 

The size an d shape of th e grindi ng pads affected th e level of succe ss of ea ch grinding experi ment in h ow  deep th e bit was abl e to grind  as well  as how mu ch time a nd energy was expend ed. Addi tionally, ho w  much wear each grinding pad en dured was impo rtant in the development of th e pad de sign. If the pads   were large in  terms of surface a rea contact with th e rock, it wo uld se e less wear than o ne of smalle r  conta ct area, yet the p enetration rate would suffer. Furthermore, the shape of t he pads was importa nt in  minimizi ng in duced vibrati on and p ad bond integ rity with the su bstrate. The arc l ength of ea ch pa d mu st  be inversely proportional to the wi dth of the pad i n orde r to m aintain a give n surface area. Throug h  experim entati on, we kne w that the surface  area of each pa d must b e on the orde r of 13 square  millimeters or less. F or vibration co ncerns, a large ar c length would be better but thi s would make the  width of the pads un acce ptably thin in  terms of bond integ rity and man ufacturability. Through out th e  process of filli ng in an elaborate te sting matrix, we settled on an arc lengt h of 6.35 mm  and a width of 1.5  mm. The u sable thickne ss of the re sin material i s 2.6 mm. T his thickness is governed by parameters set  earlie r in the design that i nvolve amon g other thi ngs, the top ograph y of the rock t hat can b e dealt with a nd  what cl earan ce shoul d be afforded in o rder to comme nce the g rinding algo rithm.  

Force an d Torque Mar gin Analy sis 

After com pleting the d esign of the RA T, we b uilt four units. Th e first two are lab eled Engin eering Mod el 1  (EM1) and E ngine ering Model 2 (EM2 ). These unit s are ex actly the sam e as the flight units but are used   for te sting here on Earth, EM1 at the  Jet Prop ulsion Laborato ry (JPL) an d EM 2 at Honeybee Robotics.  We also built  two flight units, Flight Model 1 (FM1 ) and Flight M odel 2 (FM2 ). FM1 was in stalled on the  Oppo rtunity rover an d FM 2 wa s installed on th e Sp irit rove r. FM1 and FM2 unde rwent Qualification  Testing  under a range of temperature s and lo ading condition s at Mars atmo sphere. Th e Ele ctrical Ground  Suppo rt Equi pment (E GSE) used fo r issuin g the te st comm ands and colle cting the data p roducts had an  anomal ous behavior that wa s not di agnosed until both flight units we re deliv ered. The E GSE can be   defined  as the set of el ectronics and software  that di rectly interfa ces with th e RAT actuators and se nsors  282