How to Drill Straight Holes on Curved Surfaces
By the Wescon Engineering Team · Last updated October 18, 2026To drill a straight hole on a curved surface without a drill press, hold the drill bit in a hardened bushing set in an egg cup bushing holder. The holder’s legs rest on the curve, and the bushing holds the bit perpendicular (90 degrees) to the surface so it cannot wander off the mark. Wescon makes the WES5703-2-T tripod egg cup drill guide for strongly curved surfaces and the WES5723 egg cup drill guide for flat and curved work, both with slip fit drill bushings.
On an expensive part you often get one shot at each hole. A hole drilled off square comes out elongated instead of round, which weakens the fastener’s grip. The usual fixes are drilling oversize to clean it up or scrapping the part.
Why holes go wrong on a curved surface
On a curved fuselage skin, a leading edge, tubing or any slippery or contoured part, the drill point tends to walk or skate off the mark before it starts cutting, and judging square by eye is hit or miss. The drill has to stay at right angles to the surface the whole time, however the part curves. Tilting it as the bit enters or leaves the material turns a round hole into an egg shape.
The error grows with thickness. One degree off perpendicular moves the exit point about 0.017 inch for every inch of material.
How an egg cup bushing holder keeps the bit square
An egg cup is a hand held drill guide for drilling done away from a drill press, with a hand drill on the part itself. A hardened steel drill bushing sits in the cup, and the cup stands on legs that rest on the part. The bushing bore is sized for one drill, so the bit is guided from the moment it touches the surface and enters at 90 degrees to it. Even the FAA mechanic’s handbook lists the egg cup as the bushing holder that allows drilling on both flat and curved material.
Wescon egg cups take WESSF slip fit drill bushings, the slip/fixed renewable design that ANSI B94.33 calls Type SF. Each one cross-references to its ANSI part number. A 1/2 inch OD by 3/4 inch long WESSF-32-12 bushing is an ANSI SF-32-12, with bores from 0.0625 to 0.3438 inch. SF bushings use a slide and lock method. The bushing slides into the cup and locks into the locking notches built into every Wescon egg cup. Swap the bushing to change drill size, so the same cup guides the pilot hole and the final size. Every Wescon cup body is anodized aluminum rather than plastic.
Tripod egg cup or WES5723
Wescon makes two egg cup drill guides for curved surfaces.
The WES5703-2-T tripod egg cup is for extremely curved or contoured surfaces, where a standard three leg foot cannot sit stable. Its tripod base adds stability on curves and gives a steady footing on rounded parts such as tubing. It takes 1/2 inch OD by 3/4 inch long SF bushings, stands 1.85 inches tall and is 1.20 inches across.
The WES5723 egg cup drill guide covers flat and gently curved work. Its three leg foot suits contoured or slightly cambered surfaces. It comes as a mini egg cup and in four bushing sizes, each matched to an ANSI SF bushing:
ANSI SF-32-12
ANSI SF-20-12
ANSI SF-32-12
ANSI SF-32-16
ANSI SF-48-16
The WES5723 mini egg cup and WES5723-1 have the smallest footprint, 1.00 inch across, for tight or hard to reach spaces. Shown with the heavy duty plastic foot, photos not to scale.
Choose the heavy duty plastic foot to protect a finished surface from scratches, or the anodized aluminum foot for wear resistance. For bushing compatibility tables and the threaded bushing versions, see Choosing Your Drill Guide.
How to drill perpendicular holes on curved surfaces
- Mark the hole location.
- Make a light punch mark, about the width of the chisel edge of the drill. Too small and the drill can walk off it, too heavy and it can deform the part.
- For larger holes, start with a pilot hole and fit a slip fit bushing sized for the pilot drill. Pilot drilling is recommended for holes 3/16 inch and larger in sheet metal (FAA Airframe Handbook (FAA-H-8083-31A), chapter 4, “Hole Drilling Techniques”, p. 4-20).
- Set the egg cup over the mark with every leg resting on the part, and hold it firmly.
- Run the drill through the bushing, starting slowly with light pressure.
- Swap to the bushing for the final size and drill again.
Frequently asked questions
Can I use an egg cup drill guide on composite parts?
Yes. Drill guides are recommended for composite work, especially for thicker laminates.
Do I need a different bushing for each drill size?
Yes. Each slip fit bushing is sized for one drill. The WESKIT008 kit has 16 bushings from 3/32 to 5/16 inch with an egg cup guide and a mini egg cup, and the WESKIT009 kit has 9 bushings from #40 to 1/4 inch with egg cup guides.
Is there an egg cup for threaded drill bushings?
Yes. The WES5721 and WES5722 threaded egg cup drill guides take 3/8-16 and 1/2-13 threaded drill bushings. The tripod and WES5723 take slip fit bushings.
