Drilling superalloy parts for the space shuttle.

Drilling

Superalloy parts for the space shuttle 

With the exception of some refractory metals, Ni-base superalloys are probably the least machinable of the high-strength thermal-resistant materials. Strain hardening, the generation of high heat during cutting, welding to the cutting tools, and high shear strength combine to offer serious machining problems. These materials are also abrasive and chemically reactive.

This was the situation faced by engineers at United Space Boosters Inc (USBI), a subsidiary of United Technologies, Huntsville, AL, when they undertook the machining of a group of Inconel 718 parts for the NASA space-shuttle program.

One part, a large hex nut that is used to hold the shuttle and booster rockets in postion on the launch pad, proved exceptionally challenging. It is 5 1/2 long and requires drilling a 3-dia hole that is later threaded.

The drilling operation is performed on a 20-year-old Warner & Swasey Model 3A, 15-hp heavy-duty turret lathe. According to Bill Perry, machine-shop foreman at USBI, most of the difficulty encountered in machining the Inconel 718 material is a result of tremendous tool pressures required because the material simply does not want to separate.

Initially, Perry tried a variety of conventional tools in an attempt to drill the holes. The problem was slow machining rates and poor tool life.

"We tried premium high-speed steel drills,' says Perry, "but the material was too tough for these tools. Even with a 10-percent cobalt drill, we were lucky to get 1/4 into the part before the tool needed resharpening. And if we waited too long before stopping to resharpen the drill, the material would work harden and force us to scrap the part.'

Results of tests with conventional carbide drills proved equally disappointing. The cutting edges of the carbide tool wore quickly, causing work hardening of the part that led to premature tool failure.

At this point, Perry decided to try a 3-dia CarboDrill(TM) indexable insert drilling bar from Carboloy Systems Dept, General Electric Co. The cutting end of the drill has two flutes, each of which has one or more recessed pockets to locate the indexable inserts.

The inserts are held in place with locking pins. The cutting edge across the diameter of the drill is divided into segments using three inserts in the larger sizes above 2 3/8 dia. Two inserts are used in the smaller sizes.

One insert is placed adjacent to the centerline and carves out an annular ring with a very small ID. The other inserts are positioned father out radially to cut the remaining annular ring.

Using the drilling bar, the operation is performed at 93 rpm and 0.0075-ipr feed, with a speed ranging from zero at the center cutting edge up to 74 sfm at the outside of the hole. Now, two of the 3-dia 5 1/2-long parts can be completed before the inserts must be indexed.

Perry attributes the CarboDrill's success in handling the tenacious Inconel 718 material to the use of Grade 390 inserts for the triangular (inboard) center cutting edges and titanium-carbide-coated ProMax(TM) 518 inserts for the square (outboard) cutting edges.

"In drilling from solid,' says Perry, "the cutting pressures are concentrated at the center of the drill. So, it's vital that the center cutting edges remain sharp. Grade 390 inserts have the tough-ness to stand up to the requirements of center machining.

"Since coated carbide is much less prone to built-up edge, the Grade 518 inserts in the outboard position are more resistant to welding with the workpiece material. Together, these inserts provide improved drilling performance that no single insert grade can.'

Specially designed, advance chip-groove geometry (-55 degrees) used in the drill is also an integral element in the performance of the total drilling system. The -55-degree chip groove enables force reduction and efficient chip breaking, helping ease the chip pressure at the cutting edge and aiding chip removal.

Consequently, the superior performance of the total drilling system is due to the combination of the drill design and associated use of the coated grade-geometry system. Based on the results of this successful application, USBI now uses the system in all Inconel 718 drilling applications related to the space-shuttle program.

For information on the CarboDrill system, circle E37.

Photo: CarboDrill indexable-insert drilling bar. Dimension "D' shows drill diameter; dimension "A' indicates maximum drill depth (up to 7 1/2 in 3-dia drill size) Note coolant ports inside shank of drill.

Photo: Wear-resistant coating and specially designed chip-groove geometry of Carbo-Drill indexable drilling bar provide significant increase in productivity and quality improvement in machining Inconel 718 parts at United Space Boosters Inc, Huntsville, AL. Picutred are, left to right: Bill Perry, supervisor; Bob Allender, machine operator; and Jake Cole, sales engineer for Carboloy Systems Dept.