2026-04-04
A 25.4 mm (1.000″) round bar in a flat-jaw vise contacts at two lines. The total contact area is approximately zero. Every newton of cutting force acts against friction alone.
Round parts have no flat parallel faces. A standard flat-jaw vise holds by friction at two tangent lines — one on each jaw. The contact geometry is a line, not a surface. Clamping force concentrates at those two lines, and the part can roll under any lateral cutting load.
This shows up in automated fixture selection. The surface analyzer looks for Z-aligned flat faces to determine seating suitability. A cylinder returns none. The flat_ratio — the fraction of total surface area contributed by flat faces — approaches zero. Fixture plates need a seating face; there is not one. Zero-point plates need a flat bottom; there is not one. The vise strategy scores best by default (compact_bonus), but flat jaws still hold by line contact only.
The fix is not to clamp harder. The fix is to change the contact geometry.
A 5C collet block clamps radially with uniform pressure around the circumference. The collet is a split sleeve — typically three or four segments — that compresses when the draw bar pulls it into the taper. Contact is distributed around the full OD.
Typical total indicator runout (TIR): 0.013 mm (0.0005″). This is the concentricity between the collet bore axis and the spindle/block reference. For secondary operations on turned parts where OD concentricity matters — drilling a cross-hole on center, milling a flat parallel to the axis — collets are the standard answer.
Each collet fits a narrow diameter range, approximately 0.4 mm (0.015″). A 25.4 mm collet holds 25.0–25.4 mm stock. Outside that range, the segments do not close uniformly and TIR degrades. A shop holding round stock in many sizes needs a full collet set.
Limitation: the collet contacts only the OD. There is no axial location unless a stop is added behind the part. Under axial cutting forces (face milling, drilling along the axis), the part can push back into the collet. A positive stop behind the part solves this but adds setup time.
A V-block self-centers the round part via two angled contact lines, typically at 90 degrees. The part settles into the V under gravity or light clamping. The centerline of the part aligns with the bisector of the V angle, regardless of diameter (within the block’s range).
Contact at two lines means less total friction than a collet’s circumferential grip. Concentricity depends on the V-block’s machining precision and the part’s OD tolerance. A part that is out-of-round seats differently in the V depending on orientation — the center shifts. Practical concentricity: 0.025 mm (0.001″) for precision-ground V-blocks with round stock.
The weak axis is along the V-block length. The part can slide axially under cutting loads. A heel pin or end stop is required for any operation that generates axial force. Without it, the part walks out of the block during the cut.
Best for: inspection setups, light machining, layout work, and operations where concentricity tolerance is relaxed (>0.025 mm). V-blocks accept a wide range of diameters in a single fixture, which makes them efficient for mixed-diameter work.
A conformal jaw has a pocket machined to match the part’s cylindrical profile. Each jaw contacts an arc of the OD — typically 90–120 degrees per side. Total contact spans 180–240 degrees of the circumference. This is surface contact, not line contact.
Maximum contact area means maximum friction and maximum rigidity under cutting forces. The part cannot roll because the jaw cradles it. Concentricity depends on the jaw machining accuracy relative to the vise centerline. Typical TIR: 0.010 mm (0.0004″) — better than V-blocks, comparable to collets.
The tradeoff is setup cost. Each conformal jaw set is custom to one diameter (or a very narrow range, within the clearance fit). Machining the jaw profile takes time on the first setup. For repeat production of the same part, the amortized cost is low. For one-off work across many diameters, the jaw machining time dominates.
Conformal jaws also require the part to be accurately round. An out-of-round part does not seat in the machined arc — it contacts at high spots and rocks. If the stock has significant ovality (common in drawn bar), the conformal jaw may hold worse than a V-block, which tolerates ovality by design.
Method TIR (mm) Setup Time Diameter Range Rigidity
5C Collet 0.013 Low Per-collet Medium
V-Block 0.025 Low Wide Low
Conformal Jaw 0.010 High (1st) Per-diameter HighChoose collets when concentricity matters and you have the right collet size. Secondary ops on turned parts — cross-holes, flats, keyways — are the classic use case.
Choose V-blocks when you need quick setup across varying diameters and the concentricity requirement is relaxed. Inspection, layout, and light machining.
Choose conformal jaws when rigidity under heavy cutting loads is the priority and the part is accurately round. Production runs where the jaw machining cost amortizes across many parts. This is also the method that automated fixture generation targets — the jaw profile is derived directly from the part geometry, so the machining-time penalty of a custom diameter applies only once.
The clamping force required depends on the cutting loads and friction coefficient at the jaw-part interface. Arc contact (conformal) provides the highest friction force per unit of clamping pressure. See Clamping Force and Part Deflection for the force calculations.
Hex stock, square stock, and parts with machined flats are prismatic — they have parallel faces and seat in standard flat-jaw vises without any of the above considerations. If the part has even one machined flat, it may be possible to fixture on that flat and avoid the round-part problem entirely. See Fixture Selection for Irregular Parts for how the strategy selector handles mixed geometry.
Parts held by an internal diameter — expanding mandrels, arbors, or ID collets — are a different problem. The holding surface is the bore, not the OD. The concentricity relationship reverses: bore-to-OD runout depends on the mandrel, not the external clamping method.
Conformal jaw profiles can be generated directly from the part’s STEP geometry. Upload a STEP file to see the result.
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