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What a Turntable Platter Needs From the Shop That Machines It

September 10, 2026

A platter looks like an easy part. It is a disc. The trouble is that almost every requirement on it is a relationship between features, not a size, and relationships are what get lost between your CAD file and the shop floor.

The four things that actually matter

1. Flatness of the record surface

The record sits on this face. If it is dished or crowned, the record sits on its edge or on its label area and the stylus sees a changing vertical tracking angle across the side.

Specify flatness as a form tolerance, not as a thickness tolerance. A platter machined to a thickness of 20 mm plus or minus 0.05 mm can still be visibly dished. A flatness callout of 0.05 mm over the whole top face says what you actually mean.

Note that a platter finished on a lathe will usually have a fine spiral tool path on the top face. That is normal and a mat covers it. If the top face is exposed, say so.

2. Bearing bore fit and runout

The spindle bore is the datum for everything. Two numbers matter:

Bore diameter and fit. A slip fit, an interference fit for a pressed bearing sleeve or a clearance for an inverted bearing are three different parts. Say which.
Runout of the outside diameter and the top face relative to the bore. If the top face runs out relative to the bore, the platter wobbles vertically once per revolution. Specify total indicated runout, for example 0.02 mm TIR on the top face relative to datum A, the bore.

This is the single callout that separates a platter drawing from a disc drawing.

3. Mass distribution

Rotational inertia is what resists speed change. Mass at the rim counts far more than mass at the centre, because inertia scales with the square of the radius. A rim weighted design gets more flywheel effect per kilogram than a solid slab.

That has manufacturing consequences. A rim weighted platter is usually a pocketed underside, which means more machining time and a part that can distort as material is removed. If your design relies on a thin web with a heavy rim, expect the shop to rough it, let it rest and finish it in a second operation. Say on the drawing that the top face is finished last.

4. Balance

Pocketing, a set screw hole, a strobe window or an off centre logo all shift the centre of mass. Balancing removes the remainder, usually by drilling small blind holes in the underside.

Specify it as a residual unbalance in gram millimetres rather than writing “balanced”. Also state where the shop may remove material, because a balance hole in a visible face is a defect.

Material

Material	Density	What you get	Watch out for
Aluminum 6061	2.70 g/cm3	Stiff, fast to machine, anodizes well	Rings unless damped by a mat or a sub-platter
Aluminum 7075	2.81 g/cm3	Stiffer, better surface off the tool	More expensive, anodizes less predictably
Brass C360	8.50 g/cm3	Mass and internal damping	Material cost, weight on the bearing, tarnish
Stainless 304	8.00 g/cm3	Mass, wear resistance	Slow to machine, hard to finish evenly
Acrylic (PMMA)	1.18 g/cm3	Cheap, damped, no ringing	Moves with temperature, scratches, stress crazing
POM (Delrin)	1.41 g/cm3	Damped, stable, easy to cut	Not decorative, creeps under sustained load

Many designs use two materials: an aluminum platter with a damping layer, or a metal platter with a POM or cork mat. That is a cheaper way to kill ringing than choosing brass for the whole part.

The sub-platter and the belt groove

If you use a belt drive, the sub-platter groove is a precision feature. The groove diameter sets the speed ratio, so its tolerance is a speed tolerance. Concentricity of the groove to the bearing bore is what stops speed varying once per revolution.

Put both on the drawing:

Groove diameter with a tolerance you have derived from your target speed error
Runout of the groove relative to the bore

Surface finish

A platter has up to four surfaces with different requirements:

Top face: flat, and cosmetic only if no mat covers it
Outside diameter: always cosmetic, usually brushed or bead blasted then anodized
Underside: functional, not seen, the right place for balance holes
Bore: functional, needs the fit and the surface the bearing expects

Say which is which. A shop that treats all four as cosmetic quotes high. A shop that treats none of them as cosmetic delivers a platter with a scuffed rim.

A drawing checklist

Datum A: the bearing bore, with its fit and tolerance
Flatness on the record surface
TIR on the top face and the outside diameter relative to datum A
Thickness and overall diameter
Belt groove diameter and runout, if belt driven
Mass target and residual unbalance limit
Which faces are cosmetic and which finish each one gets
Where the shop may remove material to balance
Spindle hole diameter for the record, usually 7.24 mm nominal
Any strobe markings, screw holes or mat recess

Frequently asked questions

Does a heavier platter always sound better?

Heavier is not the point. Where the mass sits is. Mass concentrated at the rim raises rotational inertia far more than the same mass near the centre, which is what smooths speed variation. A heavy platter with badly distributed mass and poor balance is worse than a lighter one that is balanced.

Do I need the platter dynamically balanced?

For a platter turning at 33 rpm the forces are small, but an out of balance platter still shows up as rumble and as a bearing that wears unevenly. Single plane balancing to a stated residual is cheap insurance and most shops can arrange it. Specify a grade rather than saying balanced.

Aluminum, brass or acrylic?

Aluminum is the practical default: light enough to machine fast, stiff, and it takes a finish. Brass adds mass and damping at a much higher material cost. Acrylic is cheap to cut but moves with temperature and scratches, so the finish has to be protected all the way to the customer.

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This guide is part of Knobs, Platters, Cups and Feet, one of four sections in the guide library.