Ejector drilling helps smooth landings of Beech Aircraft

Modern Machine Shop, Jan, 1991

For the better part of two decades, Beech Aircraft Corporation, a general aviation manufacturer in Wichita, Kansas has been gundrilling aircraft landing gear struts to provide housings for shock absorber assemblies.

Recently, the company began looking for an economical method of drilling deeper holes and achieving larger hole diameters in the 7075 forged aluminum struts required by recently updated part specifications. The struts were redesigned to larger overall dimensions that now require three-inch diameter bores running 42 inches deep.

To meet the need for larger, deeper drilling, Beech installed a DeHoff Model EDR 94, available from Dehoff, Inc., 15 Worthington Road, Cranston, RI 02920. This machine is designed with a maximum stroke capacity of 84 inches and can drill hole diameters up to 4.5 inches.

The gear struts, some of which weigh up to 75 pounds, are loaded and unloaded into the machine manually. They are positioned with help from an overhead hoist and manually clamped. There are 25 different strut variations which are currently gundrilled to meet the requirements of 17 aircraft models produced by Beech. Cycle times range from two minutes to eight minutes depending on workpiece size.

The initial 24 inches of the strut housing corresponds to the length of the absorber piston. Considered a critical area, this hole diameter is held to a tolerance of 0.002 inch. The ejector drilling operation produces a 63 rms or better finish on the inner diameter.

The strut is drilled to a 42-inch depth in an effort to remove metal and lighten the overall weight of the landing gear. This weight factor can be converted to either passenger or fuel capacity.

The company feels the machine's programmable control feature offers them an advantage over previous gundrilling equipment. Substantial improvements in controlling the spindle feed rate (ten inches per minute), the spindle speed (700 rpm), and the depth control (42 inches) have resulted in an overall productivity gain of 10 to 15 percent. With the programmable control, the operators enter the work parameters in the computer and the machine takes over from there.

Other built-in machine operation controls include a traveling whip guide located between the spindle and the guide bushing support--both of which reinforce the elongated tool and help eliminate thrust forces which can create a whip effect.

By utilizing the EDR 84, Beech has found that they can remove a large amount of stock in minimum time. The ejector drilling process and the drill configuration produce chips that divide into separate pieces and break up further into short coils. Ample clearance is provided by the drill ending to assure chip removal. High pressure oil, which travels through a center-hole the length of the gun drill to point of workpiece contact, is also a major factor in the effective chip clearance system associated with gundrilling. The company has found they get better concentricity and can hold closer tolerances.

Most recently, Beech has added another gearbox to the DeHoff EDR 84 ejector-type machine that reduces rpm and increases torque up to a point required for drilling steel tubes. The move from aluminum to steel struts is part of a new process associated with a more current Beech aircraft model.

Since its installation, the EDR 84 has been running a full eight hour shift daily, five days a week.