The spacer that looks too easy
I have seen purchasing send spacer prints to three different shops and get back three completely different parts, all within drawing tolerance. A spacer is a tube, but the difference between a spacer that works and one that binds a shaft or wobbles on a pin is concentricity, wall thickness consistency, and the surface finish on the ID.
The key callouts on any spacer drawing are: ID tolerance (H7 or similar for slip-fit over a shaft or pin), OD tolerance (often h6 or h7 if the OD is a bearing journal), concentricity between ID and OD (sometimes called runout in DIN drawings), and end-face perpendicularity to the bore. Miss any one of these and the spacer does not perform.
Why concentricity fails on thin-wall spacers
On a lathe with a three-jaw chuck, the clamping pressure squeezes the thin-wall spacer out of round while machining the OD. When the chuck opens, the part springs back and the OD ends up lobed, typically 0.03 to 0.08 mm circularity error depending on wall thickness and chuck force. The ID was bored while the part was squeezed, so it springs back into a non-circular shape too.
- Use a collet chuck or an expanding mandrel instead of three-jaw for thin-wall spacers under 3 mm wall
- Bore the ID and turn the OD in the same setup whenever possible; this is the single biggest concentricity improvement
- Leave 0.3 mm finish stock on the OD after boring the ID, then take a light finish pass to correct any spring-back
- For production volumes, a Swiss-type lathe with a guide bushing is ideal for long, thin sleeves under 20 mm OD
Wall thickness: thin is possible, thin and round requires process
| Wall thickness | Length/diameter ratio | Concentricity (TIR) | Recommended process |
|---|---|---|---|
| 3-5 mm | Up to 3:1 | 0.01-0.02 mm | Standard lathe, collet or soft jaws |
| 1.5-3 mm | Up to 4:1 | 0.02-0.03 mm | Mandrel work or sub-spindle transfer |
| 1-1.5 mm | Up to 5:1 | 0.03-0.05 mm | Expanding mandrel, light cuts, stress relief |
| Under 1 mm | Over 5:1 | 0.05-0.1 mm | Swiss-type or specialized thin-wall process |
ID finishing: reaming vs. boring vs. honing
For spacers with an ID under 12 mm, a reamer is the fastest way to get a consistent H7 bore. Reamers follow the existing hole, though. If the drilled hole is off-center, the reamer follows it and the ID ends up offset from the OD. Boring the ID on a lathe (single point) gives concentricity to the OD because the part is rotating around the spindle axis. For H6 tolerances or a very fine surface finish (Ra 0.4 or better), honing is the final step. It corrects minor size errors and produces a crosshatch finish that holds lubricant.
One trap with reamers: always drill the reamer pilot hole to the correct size for the reamer. A drill that is too big means the reamer removes too little material and follows the existing hole; a drill that is too small causes reamer chatter and oversize bores.
Volume production: choosing the right machine
For runs under 100 pieces, a standard CNC lathe with a sub-spindle is more than capable. For 500-5,000 pieces, a twin-spindle lathe with a bar feeder can run spacers unattended: part off from the main spindle, transfer to the sub-spindle, face and chamfer the back end. Swiss-type machines with a guide bushing excel at long, small-diameter spacers where a standard lathe would chatter or deflect the part.
Brass C360 and 303 stainless are the easiest materials for high-volume spacers: free-cutting, good chip control, and they do not work-harden as aggressively as 304. Aluminum 6061 cuts fast but produces longer chips that can wrap around the part and mark the OD if chip control is not set up properly.
End faces: perpendicularity and chamfer
A spacer that is not face-perpendicular to the bore tilts when clamped between two components. The practical tolerance is 0.01 mm per 10 mm of OD, measured as runout of the face relative to the ID. I face both ends from a setup that references the bore to achieve this, not from a chuck jaw clamping the OD.
Frequently Asked Questions
What concentricity (runout) is realistic on a precision spacer?
For a spacer with 3+ mm wall thickness machined in a single lathe setup, 0.01-0.02 mm TIR between ID and OD is standard. Thin-wall spacers (1-3 mm) typically hold 0.02-0.05 mm with proper fixturing.
Do I need honing for a precision ID?
For H7 fits, a sharp reamer or single-point bore is usually sufficient. For H6 tolerance or Ra 0.4 surface finish (bearing journals or hydraulic spool sleeves), honing or roller burnishing is needed.
How thin can a spacer wall be?
On a Swiss-type lathe, walls down to 0.5 mm are possible on small-diameter parts. On a standard lathe, 1-1.5 mm walls are practical with proper tooling. Below 1 mm, expect concentricity to degrade without specialized fixturing.
Can spacers be made from aluminum for shaft applications?
Yes, but aluminum spacers will wear against steel shafts under load. For light-load positioning, 6061-T6 or 7075-T6 is fine. For rotating or high-load applications, steel or bronze is a better bushing material.
What determines if a spacer should be Swiss-turned?
Length-to-diameter ratios above 3:1, diameters under 20 mm, and volume runs over 500 pieces favor a Swiss-type lathe. Short, large-diameter spacers are more economical on a standard lathe.