Short answer: CNC machined shims and spacer plates are useful when thickness, flatness, parallelism, burr control, hole alignment, and material stability are controlled together. They may be simple parts, but they often decide whether a machine, fixture, motor, bracket, or precision assembly can be aligned repeatably.
For buyers, the main risk is treating a shim as just a thin rectangle. If the part has thickness variation, raised burrs, warped faces, or shifted holes, the final assembly can tilt, preload unevenly, or lose the adjustment that the shim was supposed to provide.
What Are CNC Machined Shims and Spacer Plates?
Shims and spacer plates are thin or medium-thickness components used to adjust height, compensate tolerance stack-up, set bearing or rail alignment, space brackets, level fixtures, and tune assembly preload. They may be made from aluminum, stainless steel, brass, steel, copper, bronze, or engineering plastics.
Compared with stamped or laser-cut shims, CNC machined shims are useful when holes, pockets, slots, special outlines, controlled faces, or matched-set thicknesses are required. Their role is closely related to tolerance stack-up control in CNC machining tolerances.
Key Features to Control
| Feature | Common Risk | Factory Control |
|---|---|---|
| Thickness | Incorrect alignment or preload | Measure at multiple points and label matched sets when needed |
| Flatness | Rocking, false contact, or uneven load | Use stable stock, light finishing passes, and surface plate checks |
| Parallelism | Assembly tilt even when average thickness is correct | Finish both faces from controlled datums |
| Hole alignment | Screws pull the shim out of position | Inspect hole pattern from functional datums |
| Burr condition | Raised edges change effective thickness | Deburr without rounding functional contact faces |
Thickness Is Not Enough
A shim can have the correct nominal thickness but still fail if it is tapered, cupped, or burred. For precision assemblies, the real question is how much contact area is flat and parallel under load. A raised burr around a hole can change the effective stack height more than the specified tolerance.
When shims are used as matched sets, each piece should be identified clearly. A batch may include 0.10 mm, 0.20 mm, 0.50 mm, and custom values, and mixing them during assembly can create errors that are hard to diagnose later.
Machining Strategy for Thin Plates
Thin shims and spacer plates are sensitive to clamping pressure, residual stress, and heat. A part can look flat while clamped and then move after release. For thin or wide plates, the factory may use vacuum fixtures, sacrificial plates, adhesive workholding, soft jaws, or light finishing passes to reduce distortion.
If the spacer plate includes slots, countersinks, or relief cutouts, the machining order should preserve flatness and datum accuracy. Similar flatness concerns appear in CNC machined cover plates.
Material Selection
Stainless steel shims resist corrosion and hold up well in many assemblies. Brass and copper are useful when conductivity, softness, or galling resistance matters. Aluminum is lightweight but can dent under high bolt load. Engineering plastics can isolate vibration or electricity, but they may creep under compression.
Surface treatment should be reviewed carefully. Plating, anodizing, or coating can change thickness, burr feel, and contact behavior. If the shim thickness is critical, the final dimension should be inspected after finishing or the treatment should be excluded from the contact faces.
Inspection Methods
Useful inspection methods include micrometers, height gauges, CMM checks, surface plate and indicator checks, flatness measurement, pin gauges for holes, and visual burr inspection. For very thin shims, measuring force matters because squeezing the part can change the reading.
For first article inspection, report thickness at multiple locations, flatness, parallelism, hole position, burr condition, and final fit with the mating screws or locating pins. If shims are shipped as a kit, labels and packaging should protect the measured order.
RFQ Tips for Buyers
- State whether the shim is for alignment, preload, spacing, vibration isolation, or electrical separation.
- Define thickness tolerance and whether multiple measurement points are required.
- Clarify flatness and parallelism only where the shim contacts functional surfaces.
- Specify material and whether surface treatment changes final thickness.
- Ask for matched-set labeling when multiple shim thicknesses are supplied together.
- Control burrs around holes because they can change the effective stack height.
FAQ
Why use CNC machined shims instead of stamped shims?
CNC machining is useful for custom outlines, precision holes, small batches, thicker spacer plates, and controlled flatness or parallelism requirements.
Can burrs affect shim thickness?
Yes. Burrs around edges or holes can raise the effective contact height and create alignment errors.
How should shim flatness be inspected?
Flatness can be checked on a surface plate with an indicator, by CMM, or by suitable optical and height-gauge methods depending on thickness and tolerance.
Should shim dimensions apply before or after coating?
If final stack height matters, dimensions should apply after coating or the contact faces should be masked.
Final Thoughts
CNC machined shims and spacer plates are small adjustment parts with large assembly influence. By defining thickness, flatness, parallelism, material, burr limits, hole alignment, and inspection method early, buyers can make alignment and preload adjustments repeatable instead of uncertain.