Design of an Arc Surface Drilling Device

• 1
  Why a drill walks on a curved surface
• 2
  How an arc surface drilling device holds the axis
• 3
  Spotting, flat-bottom contact, and tool choice
• 4
  Feed, speed, and the first 2 mm
• 5
  When a fixture stops being the right answer
• 6
  What the method can and cannot deliver
• 7
  Which setup suits which part

A standard twist drill is a two-edge cutter. On a flat surface both lips touch the material at the same moment and the thrust load stays on the drill axis. On an arc surface only one lip lands first. That single contact pushes the point sideways, and the drill bends along its length until the second lip catches up. By then the hole has moved.

The effect gets worse as the wall gets more curved. On a Ø200 mm boss the surface drop over a 10 mm drill diameter is around 0.06 mm, which a rigid setup can absorb. On a Ø30 mm shaft it is closer to 0.4 mm. That is larger than most position tolerances on the drawing, and it happens before the drill has cut its full diameter.

Walking is not random. The drill moves toward the side that touched first, so the error repeats if the part is loaded the same way. Operators often see a hole that is consistently off in one direction and read it as a fixture problem. It is usually a contact problem.

Three things decide how far the point slides: how sharp the drill is, how much thrust the operator applies, and how far the drill overhangs from the spindle. A blunt drill needs more thrust, which increases the side load. Long overhang turns that load into visible bend.

The design idea is simple: give the drill a guide that is independent of the curved surface. A bushing plate, sometimes called a drill jig plate, carries a hardened steel bushing at the hole location. The bushing sets the axis. The curved workpiece only has to sit still underneath it.

The plate must be positioned from the part, not from the machine table. Locate on a machined face, a bore, or the arc itself with a V-block or a matching radius nest. If the plate floats on the surface, the hole axis floats with it. On brake shoe type parts the web or the bore is usually the better datum.

Bushing clearance matters. A 10 mm drill in a 10.05 mm bushing is fine for rough work. For position within ±0.1 mm, use a bushing bore of drill diameter plus 0.01 to 0.02 mm, and keep the bushing length at least one drill diameter. Short bushings let the drill tilt.

Support the far side of the part. Drilling thrust on a thin arc wall pushes the wall away from the bushing, which changes the effective depth and can crack the part at breakout. A simple back-up block, shaped to the inner radius, removes most of that deflection.

A spot drill or a center drill creates a cone that the following drill can follow. On an arc surface, spot to a diameter about 1.5 times the final drill diameter and deep enough that the cone sits fully inside the material. If the cone only touches the peak of the arc, the drill still walks.

A flat-bottom spot, made with an end mill or a flat spot drill, gives an even better start. It removes the arc crown and presents a flat face to the drill. This costs one extra tool change but removes the walk almost completely on parts with a small radius.

For holes that need a seating face, cut the spot face before the through hole. A spot face cutter with a pilot works on curved walls, but the pilot must enter the drilled hole, so the sequence is spot, drill, then spot face. Reversing it produces a crescent-shaped face.

Carbide drills run faster but tolerate less side load. On a guided setup they are a good choice, because the bushing takes the bending. On a hand-fed radial arm drill without a bushing, high-speed steel with a 118° or 135° point is more forgiving.

Most of the error is created in the first 2 mm of cut. Run that section at half the normal feed and let the drill establish both lips before you lean on it. On a 10 mm drill in mild steel, a normal feed of 0.15 mm/rev becomes 0.07 to 0.08 mm/rev for the entry.

Speed follows the material, not the surface shape. Aluminium 6061 runs at 80 to 120 m/min with carbide, 30 to 50 m/min with HSS. Mild steel 1018 runs at 25 to 35 m/min with HSS. Stainless 304 drops to 12 to 20 m/min and needs constant feed, because rubbing work-hardens the surface.

Coolant reaches the point better if you use through-spindle supply or an external nozzle aimed at the entry. On a curved wall the chips tend to ride up the arc and fall back into the hole. Pecking every 1× diameter clears them.

Measure the first part, not the last. Check hole position with a pin gauge and a height gauge while the setup is still warm. If the first hole is off by 0.15 mm in a repeatable direction, shift the bushing plate and rerun. Adjusting after a full batch is expensive.

A guided drill fixture makes sense for low volume, for parts that are awkward to clamp on a table, and for holes that sit on a radius where a flat table cannot reach. It is cheap to build and fast to put into production. It also needs an operator who understands the entry condition.

It stops making sense when hole position must hold tighter than about ±0.05 mm, when holes are not parallel to each other, or when the part has several faces to machine. At that point a 3-axis or 4-axis machine indexes the part and drills from the correct normal direction.

A curved surface with holes at different angles is a natural fit for 4-axis or 5-axis work. The tool always approaches along the surface normal, so there is no walk to fight. Setup cost is higher, but the geometry is handled by the machine instead of by the operator.

For one or two holes on a large welded frame, the fixture usually wins. For a family of parts with 20 holes each, the CNC wins. Count the holes, count the setups, and compare the two numbers before you commit.

A well-built arc surface drilling device with a hardened bushing can hold hole position within ±0.1 mm on a rigid part. With a tight bushing and careful spotting, ±0.05 mm is possible on short holes. That is a fixture result, not a machine result, and it depends on the operator.

Hole diameter follows the drill, not the fixture. A 10 mm drill cuts 10.02 to 10.10 mm in most steels. If the drawing calls for a reamed fit, drill undersize and ream through the same bushing. The bushing keeps the reamer on axis.

Surface finish inside the hole is Ra 3.2 to 6.3 μm after drilling, better after reaming. On curved walls the finish can be uneven where the drill exits at an angle, because the exit surface is not perpendicular to the axis.

The method does not correct a bad datum. If the locating face is rough or the part rocks in the nest, no bushing will save the position. Check the datum before you check the drill.