Try special drills for special savings Essay

For many years, engineers sought improved twist-drill performance.In the quest, they developed solid-carbide drills, carbide tips,superabrasive inserts, powder-metal steel, new HSS alloys, and, mostrecently, titanium nitride (TiN) coatings. However, these new materialsdo not work alone.

To yield longer tool life and better performance,they must be coupled with proper geometry and good tool design. Hereare some cases where more expensive special tools proved to be cheaperin the long run because they cut overall production cost or did a betterjob. Often, we didn’t even need the exotic new tool materials.

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For example, Figure 1 shows an application where a standard drillwandered off course. To drill a 0.1122″-dia through-hole into aninjector nozzle, our customer first tried a jobber-length drill workingthrough a bushing.

He found he couldn’t get the bushing closeenough to the workpiece surface to handle the oblique entry and exit.Because of the angled entry, holes were often off-center, and heavylateral forces acted on the drill, causing excessive land wear andpremature drill failure. The solution was simple. Our trouble-shooting engineers wanted tokeep a good length of the plain portion of the drill shank engaged inthe bushing before the tool point made contact with the work. Therefore,they reduced the flute length by 50 percent, leaving the overall drilllength the same as before. This design greatly increased drillrigidity, and, coupled with a special 140-degree drill point, solved theproblem.

Yes, the special drill cost twice as much as a standard, buttool performance quadrupled, and overall productivity increased eighttimes. Another customer had a simple job, but wanted longer tool life. Hewas drilling sheet packs, each consisting of 10 sheet-metal plates31.5″ X 59″ X 0.118″. The 0.394″ (10 mm) jobber drills originally used could not cope. Frequent breakages occurred onthe corners of the cutting edges.

To solve this problem, our engineers modified standard jobbers bygrinding a double-angle point, using 118 degrees and 90 degrees. Thisdesign, similar to that used for cast iron, doubled tool life. The pointwas also specially web thinned to reduce thrust load and pressure on thesheets that formerly had tended to bend. A further benefit, thedouble-angle design aided breakthrough, avoiding grabbing and leaving aclean, burr-free finish to the underside of the hole. Graphite sandwiches The difficulty in machining GRP is well known to many toolengineers.

In the application shown in Figure 2, a simple 1/4″-diahole was needed, but conditions were severe. The graphite material wasembedded in a sandwich. The top layer consisted of stainless steel, thebottom, aluminum. The first choice would normally be a heavy-dutydrill, but this won’t deliver the exact concentricity and burrlessperformance demanded by the user. Beginning with a tungsten carbide base, we designed a special stepdrill with no margins at the pilot diameter. The step served forpredrilling and as a guide pilot. The guide allowed the larger diameterto finish the hole through the sandwich with minimum torsionalvibration. To reduce burr formation, we reduced the standard 32-degree helixangle to 22 degrees.

Tool life proved to be 200 holes between regrinds.The tools served in pneumatic drilling guns, suggesting that carbide isnot as fragile as some might think. It will handle severe jobs as longas it can be kept from vibrating and facing intermittent cutting. In another case, the task was to drill 0.2953″-diathrough-holes 0.6693″ long in 100 Cr 6 having a hardness of 99.

5RB, Figure 3. The drill ran horizontally at 835 rpm with a feed of0.0039 ipr. Coolant was soluble oil.

Unfortunately, even a drillbushing could not keep the hole concentric. And there were burrs at theexit hole. The user tried solving the problem by dividing the operation intotwo steps. First he predrilled a pilot hole, and then finished with acore drill. But this was too expensive.

Our solution used a HSSsubland drill with special diameters of 0.2953″ and 0.2362″,and a step length of 0.2362″. The smaller diameter handledpredrilling and centering, and also served as a pilot guide. To aid centering, we gave the tool a split point to form C withcoinciding center lines.

Thus, the pilot stabilized the countersinkdiameter (0.2953″), which worked as a core drill in this case.Good chatter-free cutting was achieved with improved hole finish. Flexible challenge Here’s a problem we could blame on the workpiece, but ofcourse we had to solve it just the same. After a bending operation, thepiece shown in Figure 4 required realignment of prepunched holes.

Themanufacturer tried to use a core drill to correct the average0.0197″ error. It would have worked except for flexing of therelatively flimsy workpiece. To meet the challenge, we modified our standard No 533 core drillto a point angle of 195 degrees to improve centering ability and greatlyreduce burr formation. The required accuracy ruled out use of aconventional pilot diameter. Instead, we built a drilling jig designedto compensate for thrust load and eliminate flexing of the joint plate.After some effort to finalize drill-bushing guidance, the setupincreased production rates.

A deep-hole assignment is shown in Figure 5. The 0.1024″-diahole had to be drilled 3.

622″ deep with maximum axial-alignmenterror of only 0.03937″. Depth-to-diameter ratio was 35, and thematerial was C 45 round bar.

We first thought of our standard deep-hole drills, which tackledepths up to 15 times drill diameter, usually without pecking. Butwould the design work with a ratio of 35? To find out, we built ahigh-speed cobalt drill with a specially optimized profile similar tothe GT 100 type. Its length was 5.512″, with flute length of3.937″, point angle of 130 degrees, and standard web thinning. Trial runs were better than expected. Average alignment accuracywas 0.

0177″, with 0.0374″ maximum. We told the customer hecould use a setup with 17 withdrawals, 2820 rpm, peripheral cutting of75.5 sfm, and 0.

000 59 ipr feed. On the job, the tools gave 42 holesper regrind, some 20 percent more than originally specified. Vanadium body We dealt with a really intractable material for the workpiece shownin Figure 6. It was a rotor body made of V4A vanadium steel, requiringa 0.3937″-dia center hole. The user’s plan required firstdrilling with a 3/16″ carbide-tipped tool, then countersinking witha solid-carbide tool having the required 60-degree form.

He used acutting speed of 213 sfm for both operations, feeding manually andflooding with a soluble-oil coolant. If lucky, he got six pieces pergrind. Often, one of the tools broke before he even got six! The solution was to use a cheaper tool. We decided that cobaltsuper-high-speed steel would work better than carbide. We designed aspecial centerdrill made of cobalt high-speed steel and told the user torun it at a much slower surface speed than used for carbide.

This losswas amply made up by the increased feed rate made possible. Moreefficient cutting angles eliminated the breakages that plagued theformer setup, and drill life between regrinds increased to 18 or moreparts. Thus, a lower-cost tool allowed a single setup, longer too life,and less downtime. Carbide isn’t always the answer. No drill would work in the hard laminated silicon sheets shown inFigure 7.

The user wanted to drill a 0.150″-dia blind hole 1″deep in the hard sheets–in the direction of the laminations! Allcombinations of drills, feeds, speeds, and coolants failed. After aslight penetration, all cutting surfaces became completely blunt.

We found that the sheets formed hard grainy swarf when drilledtogether in the plane of the laminations. The hard swarf immediatelydulled cutting edges. Furthermore, drills would not withstand the greatheat generated in the operation. Our solution took the form of a drill made of cobalt alloyedhigh-speed steel.

It works better in sophisticated material and cantake much more heat. A high-pressure air-hose pipe was installed toblow chip fragments off the hole before they could affect the drill. Wetook 20 cuts at 30 sfm and 0.0024 ipr. After 20 holes, only slight wearmarks appeared on the cutting edges, and the lands showed no visiblewear.

Benefits of hard coating As suggested earlier, many given tools, whether specials orstandards, can be improved by coatings that make tool-edge surfaces veryhard without affecting the condition of the base material. Forinstance, a TiN coating (what we call an S coating) can dramaticallyincrease tool life, sometimes by more than 2000 percent. We have tested coated twist drills on a wide range of workpiecesand find they almost always offer longer life than identical uncoatedtools.

This is particularly true when the tool must cut highly abrasivematerials, workpieces prone to cold welding, hard or abrasive nonferrousalloys, and difficult materials including synthetics. The coating treatment also improves finish in the hole–so much sothat secondary reaming can often be eliminated. Through holes are generally much more demanding on a drill thanblind holes.

Surprisingly, tests show an even greater increase inperformance in through holes than in blind holes. These results areattributable to the wellbonded, wear-resistant coating. The coating also reduces cutting forces. We tested thrust andtorque in a number of applications. For example, in a through-holeoperation on 35 Rc plate steel 1.

1811″ thick, using0.2953″-dia drills, thrust loading of an S-drill was 17 percentless than that required for an untreated tool. Also, the range ofthrustload variation was reduced by 75 percent with the coated drill.In the same test, torque load was lessened and was more uniform with thecoated drills. These benefits stem from reduced resistance to chip flowwith treated drills.

Also, the coating has a smooth, oily surface thataids cutting action. TiN coatings at work Coating benefits range from significant to spectacular. Forexample, in 1045 medium carbon steel with a hardness of 16 to 23 Rc,Figure 8, performance increased 500 percent. The rocker bearingrequired a 0.315″-dia through-hole, 1.338″ deep, bordering ona tangential 1.575″-dia hole.

Cutting speed with acompetitor’s standard cobalt HSS drill was 66 sfm at 0.005 iprfeed. The setup used a horizontal spindle, hydraulic feed, andsoluble-oil coolant. Tool life was 105 holes, with occasional drillbreakage brought about by unstable performance and drill flexing.

Switching to a GT 100 S-treated drill of the same type increasedperformance 500 percent, producing 530 holes before regrind. Even afterregrinding, the drills machined 320 holes, allowing safe programming for300 holes per tool change. In cutting a 0.394″-dia hole 0.591″ deep in armor platehardened to 38 to 43 Rc, production increased from 12 holes per tool to50 holes, an increase of 415 percent. Cutting speed was 33 sfm at0.

0051 ipr in a vertical spindle with automatic feed drive. Standardcutting oil served as coolant. The yoke plate in Figure 9 is made of free-cutting 12L13resulphurized carbon steel. Two 0.512″-dia through holes aredrilled to a depth of 0.224″. The original taper-shank toolmachined 2100 parts before regrinding, compared to 20,500 parts for anS-coated drill of the same type. Even after repointing, the TiN-coatedtool processed 11,300 parts.

The improvement amounts to 980 percent forfirst use, and 550 percent after regrinding. Spot drilling was notrequired. Instead, the vertical spindle fed the drill through abushing. Very rich soluble oil served as coolant for the 85-rpm,0.008-ipr operation.

Finally, consider this last case. The workpiece was 316 stainlesssteel, requiring a 0.236″-dia through-hole to a 1-3/16″drilling depth. Three tools were tested at 26 sfm, 0.0024 ipr, in avertical spindle with mechanical drive. The operation used soluble-oilcoolant and required two drill withdrawals. A premium cobalt HSS toollasted for only 30 cuts; an HSS drill with TiN coating survived 110holes, but a cobalt HSS tool with TiN coating drilled 675 holes.

That’s a performance increase of 2250 percent! For small holes or large, nonferrous materials, soft workpieces,cold-extruded materials, superhard tool steels–and almost anythingelse–TiN coatings generally provide significant tool-life improvement.Even if tool life is only doubled, the coating pays for itself in toolcost alone, not to mention overall productivity gains and reductions inmaintenance costs. Application hints Nothing is as easy as it first appears, and there are some thingsto watch for in applying TiN coatings. For example, the coatingincreases surface hardness of the tool from 80 Rc to greater than 2000Vickers. It’s harder than tungsten carbide. Because hardmaterials are sensitive to vibration and chatter, you must make setupsas rigid as possible, with no axial or radial play in thedrilling-machine spindle or backlash in the feed mechanisms.

The drills are resharpened in exactly the same way as standard orcobalt HSS tooling. Although coated-drill performance is loweredsomewhat by regrinding, it remains superior to that of untreated drills. Maximum performance of coated drills is achieved by increasing thecutting speed while maintaining the same feed rate. This results inbetter tool life and superior hole surface finish. However, you do haveto watch details carefully.

Drill geometry can drastically affect thelife of coated drills, as illustrated in Figure 10. For more information about special drills, circle E21.


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