shim local thinning
Depth control is what separates stainless steel shim chemical milling from ordinary through-etching: instead of cutting a profile, the etchant is stopped partway and leaves material behind — a pocket, a step, a relieved seat, or a whole field thinned by a fraction of its gauge. On a shim that is usually the feature that does the work, because a shim is specified by the gap it creates, and a milled depth is how one part creates two gaps at once. The consequence is that depth, not outline, becomes the dimension that decides whether the part functions — and depth is the one dimension the process cannot stop at by itself.
1. Depth Is a Different Problem from Outline
An outline is finished when the etchant breaks through, which hands the process a natural end point: the part is done when the last ligament opens. A milled depth has no such signal. The sheet stays solid, the etchant keeps working, and the only thing that ends the operation is taking the part out of the bath. Every milled shim is therefore a timed part, and its depth tolerance is a tolerance on time multiplied by a rate that drifts with chemistry, temperature and bath load. There is no stop layer and no electrochemical brake on stainless, so nothing in the metal resists the etchant once the floor is exposed.
That is why chemical milling on a shim is specified differently from a through-etched contour. The drawing states a depth, a tolerance and a reference face; the process plan states a rate, a dwell and a check. A shop that quotes a milled depth from the same table it uses for outline tolerance has not separated the two problems, and the first article is where that shows.
2. Rate, Dwell and the Stop Point
Depth is rate times dwell, and both terms move. Rate rises with etchant temperature and concentration, falls as dissolved metal accumulates, and changes with alloy: a 316 shim mills more slowly than a 304 shim in the same bath, and a tempered grade can differ again. Dwell is set by the machine, but the effective dwell at a feature depends on how quickly fresh etchant reaches the surface, which is why a spray etcher and a static bath do not deliver the same depth from the same nominal time.
The practical result is a depth window: a nominal depth plus a band the shop can hold across the panel. That band is not a fixed number. It widens as the nominal depth grows, because a percentage error in rate becomes a larger absolute error the longer the etch runs. It widens again on a part with mixed feature sizes, because a narrow pocket exchanges etchant more slowly than an open field, and the two fall away from nominal at different rates.
3. Steps, Pockets and Milled Geometry
Three shapes carry most milled depth work on shims. A pocket is a closed recess with a floor and a wall on every side. A step is a recess that runs to an edge or to a land, so the part carries two levels. A relieved seat is a pocket sized to clear a fastener head, a fillet or an insert. All three are made by leaving resist where metal must stay and opening it where metal must go, and all three share one geometry problem: the etchant removes metal sideways as well as downwards.
That sideways cut is why a milled pocket is not a straight-sided box. The opening is the widest part, the floor is narrower, and the wall is a taper rather than a vertical face. The taper follows depth, so a deeper pocket has a more strongly inclined wall and a smaller floor than its plan outline suggests. A pocket also cannot be arbitrarily deep for its width: as depth approaches width, fresh etchant reaches the floor more slowly, the floor flattens, and the practical limit becomes a ratio rather than a single depth.
4. Local Thinning: The Lowered Field
Some shims are not pocketed at a point; a whole region is thinned. A diaphragm, a flexure, a spring tongue or a pressure-sensing disc works because its thickness is lower than the surrounding land, and chemical milling is how that thickness is set without a die and without a mechanical load.
Thinning differs from pocketing in two ways. The milled area is large, so etchant exchange is fast and the depth is more even across the field than in a narrow pocket — but panel-scale variation now matters, because the field spans much of the sheet. And removing metal from one face of a thin strip unbalances the residual stress the mill rolled into it, so a part that was flat before the mill can bow afterwards. One-sided thinning is therefore a flatness decision as much as a depth decision; two-sided thinning keeps the balance but doubles the depth stack. Stiffness falls with the cube of thickness, so on a spring tongue a small depth error becomes a large force error.
5. The Depth Datum: Which Face Is Zero
A milled depth is meaningless without a face to measure it from, and drawings often leave that face implicit. The convention should be stated: depth is measured from the original strip surface on the milled side, before any metal was removed. That surface is the datum. It survives on the land around the pocket, which is why a land wide enough to seat a gauge is part of the design rather than a leftover.
The alternative is to specify remaining thickness instead of depth, and on a shim that is often the better call. Strip arrives with its own thickness tolerance, so a given depth and a given remaining thickness are not the same requirement: the first fixes how much is removed, the second fixes what is left. When both faces are milled, or when a part carries two levels, the datum question compounds, and the safest drawing gives the datum face, the nominal depth, the tolerance and a minimum remaining thickness.
6. Measuring Depth and What Each Method Proves
Four methods cover most milled-depth inspection, and they do not prove the same thing. A depth micrometer, or a dial gauge on a stand zeroed on the surrounding land, measures depth directly and is the cheapest check on the floor; a ball tip beats a flat anvil because a flat anvil can bridge a rough or dished bottom and read shallow.
A stylus profilometer traces a step and returns depth plus bottom roughness, but its tip radius limits it to pockets wider than the stylus. Optical three-dimensional instruments — focus variation, confocal, white light — map the floor areally and show whether a pocket is flat or dished, which no single point can. A cross-section, mounted, polished and viewed under a microscope, is the only method that shows wall taper, floor radius and true depth together, and it is destructive, so it belongs to the first article rather than to the lot. An ultrasonic gauge reads remaining thickness from the flat face and suits a large thinned field, provided it is calibrated on the same alloy and finish.
7. What the Depth Tolerance Contains
A quoted depth tolerance is a stack, and it is worth asking what is inside it. The first term is the process band: the variation a controlled line holds across a panel at that nominal depth. The second is the feature effect: a narrow pocket and an open field do not reach the same depth in the same dwell, so one tolerance across mixed features is optimistic unless the shop has measured both. Ask for the band per feature class, not one band for the part.
The third term is the strip. Depth is measured from the surface, but where the floor sits relative to the opposite face depends on incoming thickness, so a depth tolerance and a remaining-thickness tolerance are not interchangeable. The fourth is measurement uncertainty: a gauge that resolves a micron on a rough etched floor is not delivering a micron of knowledge. A supplier who answers a depth question with one number, without naming the feature size and the datum, has answered a different question.
8. Milled Depth on the Drawing and in the PO
A drawing for a milled shim should carry, in order: the datum face, the nominal depth, the tolerance, the minimum remaining thickness, the pocket’s plan size at the opening, the wall-taper allowance and the floor condition. Where two levels exist, each level needs its own depth callout, and the drawing should say which level the tolerance applies to, because two levels do not share a tolerance unless the drawing says they do. A note that pocket edges are not to be used as a datum saves an argument at first article.
The purchase order should add the measurement method agreed for acceptance, whether the first article includes a cross-section, and how a depth outside the band is dispositioned. A milled depth is the one characteristic that cannot be recovered by reworking the outline, so the check is worth agreeing before the parts run. Ask for the coupon as well: a test strip etched alongside the parts is the cheapest evidence that the depth on the drawing was actually reached.
