3D Scanner Turntable Base Die Casting

A turntable base is a circular or cross-ribbed platform with an integrated bearing housing. The scanner rotates the part, not the sensor, so any error in the base becomes a positional error in every point cloud. Ra 0.4 μm on a housing bore does nothing for you when the outer edge of the platter runs out by 40 μm. The datum stack is what matters: bearing bore, thrust face, and motor mount register. Those three surfaces set the axis of rotation, and they must stay concentric and perpendicular after the casting has been through heat treat and thermal cycling.

Concentricity and perpendicularity on those datums should hold within ±0.02 mm on the finished part. That is not a machining tolerance alone, it is a process tolerance. A casting that moves 30 μm in the weeks after machining will drag the bore out of position no matter how well it was cut. Residual stress locked into the grain structure relaxes slowly, and the part drifts. Machining a casting to ±0.005 mm is straightforward for us. Keeping the part at that tolerance six months later is a heat treat and stress-relief problem, not a spindle problem.

Wall thickness drives this more than most designers expect. Thick bosses next to thin ribs create a thermal gradient during solidification, and the thick section stays hot while the thin section freezes. The result is a locked-in stress field that shows up as a slight bow or a twist once the part reaches room temperature. Uniform 3–4 mm walls with ribs sized to the load usually beat a thicker base plate with heavy bosses.

Vibration damping is the second job the base does. Mass distribution matters more than total mass here. A base that puts most of its weight at the bearing housing and little at the outer rim will ring. Adding ribs under the platter perimeter, or thickening the outer ring, raises the first natural frequency and cuts settling time. For a scanner running a step-and-settle cycle, that is the difference between a 2 second scan and a 4 second scan.

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  Datum stackBearing bore, thrust face, motor register — machined in one setup.
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  Wall ruleKeep 3–4 mm nominal; avoid thick-to-thin transitions.
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  Mass biasWeight at the rim damps better than weight at the hub.

Gravity die casting and high-pressure die casting both work for a turntable base, and both use aluminium-silicon alloys such as A380 or ADC12. The rapid solidification of HPDC produces a fine dendritic microstructure. After a proper T5 or T6 heat treatment, that structure gives the dimensional stability needed for under 5 μm runout at the outer edge of a 300 mm platter. Near-net shape is the other benefit — deep ribs, hollow bosses, and varying wall thickness come out of the tool without the long machining times you would pay on billet stock.

Porosity is where the process bites. Turbulent fill during injection folds oxide films into the melt and can create cold shuts right at the thin-to-thick transitions where ribs meet the base plate. Those defects are invisible from the outside. A casting can pass a visual check and still crack under cyclic torque after a few thousand rotations. The fix is upstream: gate position, vent grooves, and overflow wells sized so the last metal to freeze sits in a well, not in a structural rib. Mould flow simulation before the tool is cut is cheaper than a tool rework after the first batch.

The other limit is section thickness. HPDC likes walls between 2.5 mm and 5 mm. Go below 2 mm and the metal freezes before it fills the rib. Go above 6 mm and you get shrinkage porosity in the center of the section. If your design needs a 12 mm thick mounting pad, cut it as a machined feature on a 5 mm cast wall rather than casting the full thickness. That is a design change, and it is usually the right one.

For low-volume or large-format bases, gravity casting or vacuum casting can make sense. Tooling cost is lower, wall sections are more forgiving, and you can cast a 600 mm platter without the tonnage a cold-chamber HPDC machine would need. The trade-off is a coarser surface and slower cycle time. We run both processes and will say which one fits your volume and geometry.

The casting is only half the part. Everything that touches the scanner's accuracy is machined after the casting cools. The bearing bore, thrust face, and motor mount register should be cut in a single setup so the datums stay related to each other. Move the part between two fixtures and you introduce a stack-up you cannot measure your way out of.

On a 5-axis machine with a Ø400 mm rotary table, we can bore the housing, face the thrust surface, and drill the motor pattern in one operation. That holds the bore-to-face perpendicularity and the bore-to-motor-pattern concentricity without relying on a fixture that may itself be off. For a base with a large platter, the rotary table also lets us interpolate the outer rim in the same cycle, so runout at the edge stays tied to the same axis as the bore.

A common mistake is machining the casting before stress relief. Cut a green casting and it will move as the internal stress redistributes. The bore you measured at 20 °C in the inspection room may be 25 μm oval after a month on the bench. Sequence matters: cast, heat treat, rough machine, stress relieve, finish machine. That is five steps, and skipping the rough-and-relieve pair is the usual shortcut that fails.

Surface finish on the datums is secondary but not irrelevant. Ra 0.8–1.6 μm on the thrust face gives a stable bearing seat. Going finer offers little, because the bearing, not the base, defines the running surface. Where finish does matter is any optical reference feature — a machined flat used for scanner alignment should be flat and clean, and laser marking for part traceability needs a minimum character height of 1.5 mm to stay readable after anodizing.

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  One setupBore, thrust face, and motor register in the same operation.
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  Right sequenceCast, heat treat, rough, stress relieve, finish.
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  MarkingKeep traceability text at 1.5 mm minimum height.

You cannot inspect a turntable base into accuracy. You can, however, catch the defects that will cause drift before they reach assembly. Start with the raw casting. Check wall thickness at the rib roots with ultrasonic gauging, and X-ray the transition zones where thin sections meet thick ones. A cold shut at a rib root is a fatigue crack waiting for a load cycle. Porosity in a non-structural web is not worth scrapping a part over, but porosity in the bearing housing is.

After machining, measure the datums, not the part outline. Bore diameter and roundness, thrust face flatness, and perpendicularity of the bore to the face tell you whether the part will rotate true. Runout at the outer edge of the platter is the number your scanner actually sees, and it should be measured with the part mounted the way it will run. A base that measures 8 μm runout on a surface plate can show 20 μm once bolted to a frame that is not flat.

We inspect 100% of parts before shipment, with raw material check, in-process monitoring, and final inspection. Reports are available on request. Tolerance capability on machined features is ±0.005 mm (±0.0002 in), and fine surfaces can reach Ra 0.2–0.8 μm where an optical reference calls for it. Those numbers are the machining capability, not a promise about the casting. The casting has to be right first.

One more check that pays for itself: measure the part again after a thermal cycle. Run a sample through the temperature range it will see in service, then re-measure the datums. A base that moves more than 10 μm between the two measurements will cause calibration drift you will spend weeks chasing. Catching that at the supplier is far cheaper than catching it in a shipped scanner.