Key Takeaways
- Always preload the jaws outward (or inward, depending on grip direction) before boring — this ensures the bore matches the clamping state you'll use in production.
- For outside-diameter gripping, bore the jaw ID to 0.05–0.10 mm less than the part OD — the slight interference creates a proper grip. Do not bore to exact part size.
- Boring jaws on the machine they'll run on (or at least on an identical chuck) is critical — jaw runout measured on one chuck will not be the same on another.
- Soft jaws distribute clamping force over a larger contact arc, making them essential for thin-wall parts, delicate finishes, and accurate second-operation gripping.
Why Soft Jaws Instead of Hard Jaws
Standard hard chuck jaws grip the workpiece on three small contact points. For rough stock, this is fine. But for second operations — gripping a finished OD, holding a thin-wall turned part, or locating a casting accurately — the hard jaws can:
- Deform the part (thin wall, soft material)
- Leave jaw marks on a finished surface
- Provide poor concentricity because only three point contacts locate the part
Soft jaws (also called pie jaws or machinable jaws) are bored to match the exact part diameter. The contact arc becomes 120° per jaw — the part seats like a ring fitting into a bore, giving 10–50× better repeatability than hard jaws on a finish-turned OD.
Jaw Materials
| Material | Best For | Notes |
|---|---|---|
| Aluminium (6061) | Most applications; light to medium parts | Easy to machine; low grip mark risk; limited to ~100 reboringcycles before worn thin |
| Mild steel (1018) | Heavy parts; high clamping force | More durable; slightly higher risk of marking soft ODs |
| Nylon / Delrin | Polished ODs, glass, ceramics | Excellent for delicate surfaces; limited torque capacity |
| Copper / Brass shim | Protecting finished surfaces | Applied as a liner over hard or soft jaws; not a standalone jaw |
Step-by-Step: Boring Lathe Soft Jaws
Step 1 — Mount the Blank Jaws
Install the blank soft jaw set into the chuck in the same jaw slots and positions you will use in production. Match the jaw number to the chuck slot (most chucks have numbered slots — always respect this). Snug up the jaws but do not fully tighten yet.
Step 2 — Preload the Jaws
This is the most important step that beginners skip. Before boring:
- Outside-diameter gripping: Insert a ring spacer (steel ring or a piece of bar stock slightly smaller than your intended bore) into the jaws and clamp the chuck to a moderate clamping force — similar to what you will use in production. This opens the jaw faces outward against the scroll under load.
- Inside-diameter gripping (expanding into a bore): Use a ring of the appropriate ID around the outside of the jaws and clamp outward. This preloads the jaws inward.
Preloading ensures that when you bore the jaws, you are cutting in the same deflected state as when gripping the part. Without preloading, the bore is accurate only at zero clamping force — under actual grip, the jaws spring inward and the bore becomes a non-circular shape.
Step 3 — Face the Jaws
Before boring the ID, face the front faces of the jaws flat. Use a facing tool and take a light skim cut across all three jaw faces simultaneously — this guarantees the jaw faces are all in the same plane perpendicular to the spindle axis. This is the reference datum for part shoulder location.
Step 4 — Calculate the Bore Diameter
For outside-diameter gripping:
The 0.05–0.10 mm undersize creates a slight interference fit. When the part is seated, the three jaw arcs flex slightly and grip the part without allowing radial movement. If you bore to exact part OD, the part will slip under cutting forces. If you bore more than 0.15 mm undersize, the jaws will mark or deform the part.
For aluminium parts (softer surface), use the smaller end of the range (0.05 mm under). For hardened or ground ODs, 0.05 mm. For rough turned or commercial tolerance stock, 0.08–0.10 mm.
Step 5 — Bore the ID
Using a boring bar (not a drill — a boring bar gives the correct ID geometry and surface finish), rough bore to within 0.2 mm of target, then finish bore to calculated diameter. Take the finish pass in a single continuous pass around all three jaws without stopping. Check the diameter with an internal micrometer or telescoping gauge — do not rely solely on the machine readout.
Step 6 — Remove the Preload Ring and Test
Remove the preload ring. Load the first production part and check runout with a test indicator — 0.01–0.03 mm TIR is achievable on a well-maintained chuck with correctly bored jaws. If TIR is high, check jaw faces are clean, seating surfaces are free of chips, and the jaw-to-scroll engagement is correct.
Gripping Thin-Wall Parts
For thin-wall parts (wall thickness less than 10% of diameter), clamping force must be reduced to prevent distortion. Recommendations:
- Use the minimum clamping force that prevents slipping under cutting loads — not maximum spindle key torque
- Bore the jaws to exact part OD (no undersize) and rely on friction rather than interference
- Use a larger contact arc: consider full-bore pie jaws that contact 120° or more of the part circumference
- For very thin wall (under 2 mm wall), use a mandrel or expanding arbor instead of an OD chuck grip
Reboring Used Jaws
Soft jaws can be rebored many times. After machining, mark the jaws with a permanent marker or scribe the new bore diameter and date. Track the remaining wall thickness — when the jaw wall falls below 3–4 mm, replace the jaw set. Keep used jaws in labelled bags or on jaw boards marked by diameter range — a set bored to 63 mm can be quickly retrieved for another job near that diameter rather than starting from blanks.