ENGLISH EDITION

CNC machining of watch cases

Process planning and precision machining of internal pockets

Machining the interior of a watch case means removing substantial material while preserving thin walls, tight corners and seating features at different heights. Planning must therefore address both the cutter path and the material that remains to support subsequent operations.

Use a rigid cutter for bulk pocket removal and a smaller cutter for residual corners. Equalize the remaining allowance before finishing the functional walls and floors. Decide which regions may become thin early and which material must remain as support; these choices influence corner vibration, wall deflection and final geometry.

01. Divide the interior into distinct machining regions

Separating the interior into machining regions makes tool selection more precise. Broad cavities favor efficient removal; narrow corners require access and stiffness from smaller cutters. Retained bosses, islands and floors at different elevations require separate consideration of how the remaining material supports the cut.

Assign functions before selecting paths. A seating face is governed by its height and form; a locating wall by its position and clearance relative to the specified datums. A clearance cavity may not require the finish or accuracy of a mating surface. Allocate finishing operations according to function and tolerance instead of applying the same treatment everywhere.

Distinguish the internal corner radius seen in plan view from the fillet where a wall meets a floor. The former relates to cutter diameter and the planar path; the latter relates to the cutter’s end geometry. A square, corner-radius or ball-end tool does not produce the same floor and transition geometry.

Where design changes are possible, identify the exact contact or interference that requires a tight corner. An unnecessarily small radius or deep narrow slot may force a smaller, less rigid tool. Review mating clearance and remaining wall thickness before agreeing a more machinable feature.

02. Plan the intermediate workpiece, not only the final shape

Pocketing changes the workpiece’s stiffness. A thick floor or connecting region can support the next cut; removing it early can increase deformation or vibration even when cutting parameters remain unchanged. Optimizing roughing, rest machining and finishing independently can therefore undermine the complete route.

The following sequence is a planning example. Initial operations differ for bar stock and preformed blanks, including forged or cast material. Combine or reorder stages according to access, fixturing and the loads imposed by later operations.

1. Establish datums and holding regions.

   Define repeatable height, center and angular orientation, including references needed after reversal or side machining.

2. Rough the main pockets.

   Remove bulk material while retaining support around thin walls and local features for as long as needed.

3. Rest-machine corners and equalize finishing stock.

   Remove residual regions with smaller tools and use semi-finishing where necessary to make the final cut more consistent.

4. Coordinate side features with internal finishing.

   Account for exit burrs, reclamping loads and deburring access. Check whether later operations could damage completed functional surfaces.

5. Finish walls and floors, then verify the completed condition.

   Select the order against the available support. Inspect after deburring, cleaning and any required finishing, under the drawing-defined condition.

Datums and fixture support

A circular locating surface establishes a center but does not establish angular orientation. Asymmetric pockets and side holes need a repeatable rotational reference. A possible arrangement uses a specified plane for height, a cylindrical feature for centering, and a separate hole or feature for orientation.

Changing to whichever surface is easiest to locate in each operation can preserve individual dimensions while losing their relationship. Where datum transfer is necessary, measure the relationship between the old and new references and assess its contribution to the assembly requirement.

Separate the roles of locators, supports and clamps. Locators establish position, supports react loads, and clamps maintain seating. Forcing a thin ring into location with excessive pressure can produce acceptable measurements while clamped and a different shape after release.

For soft jaws or a contoured nest, evaluate the actual load path as well as contact area. Trapped chips can change height or tilt, so include relief and cleaning access. During initial qualification, compare the same part clamped and released to distinguish workholding distortion from cutting error.

03. Rough the pocket with the next cutter in mind

Roughing removes bulk material and creates the stock condition inherited by the following operation. A thin allowance on a straight wall and a large residual mass in a corner expose a finishing cutter to very different loads along an otherwise continuous path.

For closed pockets, compare helical entry, linear ramping and entry through a prepared hole. Helical entry can distribute the entry cut, but the pocket must accommodate the required orbit. Respect the cutter’s permitted ramp angle, pitch and end-cutting capability.

A center-cutting tool is not automatically suitable for unrestricted vertical plunging. Consider end engagement and a viable chip exit path. If the pocket cannot accommodate a suitable entry, evaluate a prepared hole or a different tool arrangement, including the effect of the hole’s location and depth on subsequent geometry.

Engagement and finishing allowance

Radial engagement, a_e, describes the width engaged across the cutter; axial depth, a_p, describes the cutting height along its axis. Equal programmed stepovers do not guarantee equal engagement. Contact can increase substantially around an internal corner or island.

Adaptive and trochoidal paths can reduce abrupt engagement changes. They do not guarantee constant cutting force: material, wear, runout, chip recutting and workpiece stiffness also contribute. In a small cavity, compare the space required by the path and the resulting noncutting travel as well as material-removal capability.

Set radial wall stock and axial floor stock separately where their purposes differ. Wall stock provides material for correcting form and finish. Floor stock relates to final height and may contribute support during intermediate operations.

Insufficient allowance may leave no material to remove roughing error or surface damage. Excessive allowance can concentrate heavy cutting in the final operation, when the structure is already thin. Allowance is not the same as drawing tolerance: establish it using the actual remaining stock and distortion after roughing. Address thick corner residues through rest machining or semi-finishing before expecting a uniform finishing pass.

04. Concentrate small tools on residual corners and narrow features

A larger cutter can offer useful stiffness and removal capacity where it fits. A smaller cutter reaches tighter details but may be more sensitive to deflection and breakage at the same depth. Assigning distinct roles avoids repeatedly machining the entire cavity with its smallest required tool.

Rest machining must use an accurate representation of what the previous operation left behind. Previous cutter diameter, end radius, depth and machined region all matter. A path based only on the finished model may include excessive air cutting or unexpected heavy engagement.

If an earlier cutter or machining depth changes, update the stock used for subsequent rest machining. With multiple setups, verify the stock model’s coordinate relationship to the new orientation. Examine entry engagement and remaining material below the cutter, not just the highlighted residual region.

Corner radius and cutter access

For an ideal planar internal corner of radius R and a cutter of diameter D, the geometric requirement is D ≤ 2R. As D approaches 2R, however, the cutter-center turning radius approaches zero. Geometric access alone does not establish stable cutting.

A smaller tool alone may not resolve corner vibration. Divide heavy residual corner stock into controlled cuts and adjust the entry path and feed before engagement rises. Check that the small cutter is not unexpectedly entering a nearly full-width cut.

Reach and cutting length

Distinguish reach from flute length. A deep pocket requires sufficient reach, but does not always require a cutting edge as long as the complete depth if the wall is machined in levels. A short-flute, relieved-neck tool may provide access for that strategy.

Conversely, a long neck does not make a tool suitable for full-height wall finishing. The engaged height must fit within the effective cutting length, and the neck, shank and holder must clear the wall. Validate the assembled tool geometry rather than relying only on diameter and overall length.

05. Select wall and floor finishing order from the support condition

Thin walls are sensitive to lateral loading; thin floors can respond strongly to axial loading depending on support. Finishing both simultaneously couples different error mechanisms. Separate passes can help, but separation alone does not determine which surface should be finished first.

Evaluate distortion from later removal, not just the surface produced by the current cut. If temporary connecting material or a supporting web is retained, include its eventual removal and the subsequent inspection in the route.

Uniform stock before the final pass

Semi-finishing can equalize stock before the final pass. Roughing may leave uneven allowances and distortion, making finishing load unpredictable. If the workpiece is reversed or reclamped after semi-finishing, recheck the references and available stock in the new holding condition.

Repeating a nominally zero-stock path is not a universal correction. A repeat pass may remove material left by elastic deflection, but it may also add rubbing and cycle time. Measure the first-pass error and the change after repetition before making that pass part of the production route.

Full-height and staged finishing

Full-height wall finishing can reduce witness lines between depth levels, but requires adequate cutting length, tool and workpiece stiffness, and dynamic stability. Low radial engagement does not automatically make a large axial depth safe or accurate.

Dividing the wall into levels reduces the engaged height, but different deflection at each level can leave steps. Review transitions, overlap, compensation and measurements at multiple heights. Choose the method that repeatedly satisfies the required form and finish, rather than the one with the fewest passes.

For floors, verify both height and local surface condition. Pass boundaries, entry marks and burrs can prevent seating even when nominal depth is correct. Investigate end-edge condition, axial runout, spindle alignment relative to the reference plane and chip recutting. Average roughness alone does not describe every local defect affecting contact.

06. Select speed and feed together with material, tooling and engagement

Confirm material grade and hardness, cutter geometry and coating, overhang, cutting width and depth before selecting parameters. Tools with the same diameter can have different edge geometry and effective tooth counts. Begin with toolmaker data for the relevant material, then validate the conditions at the actual reach and workholding arrangement.

Chip thickness, runout and corner feed

Feed per tooth is the advance associated with each tooth passing the cut. Actual chip thickness changes with position and engagement. In micro-milling, when chip thickness becomes very small relative to the edge radius, deformation and rubbing can become significant instead of effective chip formation. Further feed reduction may therefore worsen burr formation or surface condition.

Runout makes the teeth remove unequal amounts of material. Its relative effect can be substantial at small feeds per tooth. If finish is poor or small tools repeatedly fail, inspect interface cleanliness, holder condition, runout at the actual working extension and edge damage before adjusting feed alone.

During an internal circular move, the cutter center travels at a different radius from the machined wall. The relationship between programmed center feed and feed at contact changes from a straight cut, while increased corner engagement adds another load effect. Check CAM and controller compensation to avoid applying the same feed correction twice.

Coolant delivery and chip removal

Chip evacuation belongs in parameter planning. Chips retained in a deep narrow pocket can be recut, scratch the surface and produce transient overload. Confirm that coolant or air reaches the cutting region and floor, and that the fixture does not obstruct the exit path. Choose cooling and lubrication to suit the material and cutter; where delivery is poor, adjust nozzle placement and the machining sequence together.

07. Separate CAM verification from dimensional acceptance

A smooth-looking animation does not verify the complete machining process. First, use realistic initial and intermediate stock to check gouging, remaining material and entry engagement. Second, verify the neck, shank and holder against the workpiece and fixture. Third, use applicable machine simulation and verification procedures to assess rotary motion, travel limits and postprocessed movements.

Conventional material-removal simulation evaluates geometry swept by the cutter. It does not automatically establish tool deflection, thin-wall elastic response or distortion from residual stress redistribution. First-off machining and measurement are needed to reveal these physical effects.

Geometric verification and part tolerance

Toolpath tolerance relates to geometric approximation between the model and calculated path. Finished-part error also includes tool diameter error, runout, machine motion, temperature, workholding and cutting deformation. If smoothing is applied, include its permitted geometric deviation in the total error budget. Do not treat a CAM tolerance setting as a guaranteed part tolerance.

Inspection after machining and release

On-machine probing helps detect setup error and process drift, but clamped measurements may differ from released geometry. Measuring force and support position can themselves deform a thin part. Reproduce the support condition specified by the drawing and inspection procedure, and select the method against access, optical reflections, tolerance and required uncertainty.

Plan access to burrs where side holes break into pockets or where steps meet. Define allowable edge geometry so that deburring does not remove excessive material from functional faces. For polished regions, evaluate dimensional removal and protect seating or mating surfaces as required.

After cleaning, check small pockets and holes for chips and polishing residue. Assembly verification should establish full seating, absence of local interference, suitable fastening, and the required operation of side features. This connects pocket machining to the function of the completed component.

08. Use the location and timing of an error to narrow the cause

Reducing every feed or changing all tool offsets makes diagnosis difficult. Compare straight sections with corners, the top and bottom of walls, and clamped with released geometry to identify where the error begins.

Compare changes under consistent measurement conditions. If only corners are unstable, inspect their stock and engagement before slowing the complete program. If geometry changes mainly on release, separate fixture and sequencing effects before relying on cutter-diameter compensation.

The complete route retains support during roughing, controls local stock through rest machining, establishes functional surfaces during finishing, and verifies their relationships in final inspection. Keeping these stages connected allows improvements in tooling, CAM and fixturing to translate into repeatable assembly quality.