A complete reference for clamping systems, locating principles, tolerance stack-up, GD&T, and gauge R&R — from theory to workshop floor.
A fixture is a work-holding device that locates, supports, and clamps a workpiece so that machining or assembly operations can be performed accurately and repeatedly. Unlike a jig, a fixture does not guide the cutting tool — it only holds the part.
The golden rule of fixture design: locate first, then clamp. Clamping before proper location introduces positional error that no amount of precision can recover.
A fixture is a production tool that locates, holds and supports a workpiece during a manufacturing operation. It does not guide the cutting tool but ensures repeatability of part position.
| Type | Application | Key Feature |
|---|---|---|
| Milling Fixture | Milling operations | Rigid base with T-slots |
| Turning Fixture | Lathe / chuck work | Balanced for rotation |
| Drilling Jig-Fixture | Multi-hole drilling | Drill bush guides |
| Welding Fixture | Assembly & welding | Heat-resistant clamps |
| Inspection Fixture | CMM / gauge checks | Datum reference surfaces |
| Assembly Fixture | Part sub-assembly | Poka-yoke features |
Every fixture must achieve: (1) Accurate location, (2) Rigid clamping without distortion, (3) Easy loading and unloading, (4) Chip/swarf clearance, (5) Economic manufacture.
Q: What is the primary difference between a jig and a fixture?
Every rigid body in space has six degrees of freedom (DOF) — three translational (X, Y, Z) and three rotational (Rx, Ry, Rz). A locating scheme must constrain all six DOF to ensure a unique, repeatable part position.
3 on primary, 2 on secondary, 1 on tertiary plane to fully constrain 6 DOF.
More than 6 locators creates over-constraint. Part may rock or distort.
Diamond (relief) pin — used with round pin to allow thermal expansion. The diamond pin prevents rotation only. Round pin — locates in both X and Y. Always use one round + one diamond for two-hole location.
Q: How many degrees of freedom does the primary locating plane constrain?
Clamping holds the workpiece against the locators during machining. Correct clamping force is essential: too low allows part movement; too high distorts thin-walled parts or damages surfaces.
μ = friction coefficient (0.15–0.4), SF = safety factor (2–3 typical).
MA = mechanical advantage of the toggle linkage (typically 20–200).
D = bore diameter (m), P = supply pressure (Pa). Retract: use rod area.
P_hyd = system pressure (typically 70–210 bar for hydraulic fixtures).
| Clamp Type | Force Range | Best For | Limitations |
|---|---|---|---|
| Strap clamp | 0.5–20 kN | Simple machining fixtures | Slow — manual tightening |
| Toggle clamp | 0.2–5 kN | Welding, assembly | Fixed stroke |
| Pneumatic clamp | 0.5–30 kN | High-volume production | Needs air supply |
| Hydraulic clamp | 10–500 kN | Heavy-duty machining | Needs hydraulic unit |
| Cam clamp | 0.1–2 kN | Lightweight, fast release | Not for high vibration |
Always apply clamp force directly over a locator whenever possible. Clamping between locators can flex the part and introduce dimensional error.
Q: A milling operation generates 1,200 N cutting force. Friction coefficient is 0.3 and safety factor is 2.5. What minimum clamping force is required?
Tolerance stack-up (accumulation) occurs when individual part tolerances combine to affect an assembly's critical dimension. Two main methods are used: Worst Case (WC) and Root Sum Squares (RSS).
Sum of all absolute tolerances. Guarantees fit but gives widest tolerance range. Used for safety-critical assemblies.
Statistical method — assumes tolerances are independent, normally distributed. More realistic for high-volume production.
aᵢ = +1 or −1 (direction), dᵢ = nominal dimension. Algebraic sum of all loop dimensions.
Cp ≥ 1.33 is generally required for production. Ensures process spread fits within tolerance band.
| Aspect | Worst Case | RSS |
|---|---|---|
| Risk of failure | Zero (100% guarantee) | ~0.27% (3σ level) |
| Part cost | Higher (tighter individual tols) | Lower (relaxed individual tols) |
| When to use | Safety-critical, low volume | High volume, statistical process |
| Tolerance result | Larger (conservative) | Smaller (realistic) |
1. Identify the critical gap or dimension. 2. Draw a closed loop through all contributing dimensions. 3. Assign +/− direction to each link. 4. Calculate nominal gap. 5. Apply WC or RSS to find tolerance on gap.
Q: Three parts have tolerances ±0.05, ±0.08, ±0.06 mm. What is the RSS tolerance on the assembly gap?
Geometric Dimensioning and Tolerancing (GD&T) is the language of engineering drawings. As a fixture engineer, you must translate GD&T callouts into physical datum surfaces and checking methods on your fixture.
| Symbol | Characteristic | Fixture Implication |
|---|---|---|
| ⊕ | True Position | Locating pins must place part within position tolerance zone |
| ◻ | Flatness | Fixture base must be flatter than part flatness requirement |
| ○ | Circularity | V-block or three-point support for round parts |
| ↗ | Angularity | Angle block or sine plate locating required |
| ⌒ | Profile | Custom profiled locating nest or CMM fixture |
| ⊘ | Concentricity / Runout | Concentric chuck or precision mandrel locating |
ΔX, ΔY = deviation from nominal. TP must be ≤ drawing tolerance zone diameter.
When part is not at MMC, position tolerance increases by the difference. Allows looser fixtures for economic production.
Additional positional freedom when datum feature departs from Maximum Material Boundary.
Functional gauge pin diameter = MMC hole size minus the positional tolerance. Simplest go/no-go check.
The fixture datum reference frame (DRF) must exactly match the drawing datum order (A, B, C). Building a fixture with datums in the wrong order is one of the most common — and costly — mistakes in fixture design.
Q: A hole has a true position callout of ⌀0.2 at MMC. The hole is 12.05 mm actual vs. 12.0 mm MMC. What is the total allowed position tolerance?
Gauge Repeatability and Reproducibility (R&R) is a statistical study that evaluates how much variation in measurements comes from the measurement system itself — rather than actual part variation.
Total study variation = part-to-part variation plus gauge variation (R&R).
Repeatability (equipment) + Reproducibility (operator) combine to give total gauge variation.
Under 10% — acceptable. 10–30% — marginal (review). Over 30% — unacceptable (fix gauge).
Must be ≥ 5 for an adequate measurement system. Fewer categories means the gauge cannot distinguish parts.
| Step | Action | Minimum Requirement |
|---|---|---|
| 1 | Select parts | 10 parts spanning full process range |
| 2 | Select operators | 2–3 operators (normal production team) |
| 3 | Measure each part | Each operator measures each part 2–3 times |
| 4 | Randomize order | Blind measurements — operators don't see prior results |
| 5 | Analyse data | Use ANOVA or Average & Range method |
| 6 | Accept or act | %GRR < 10% pass; > 30% requires gauge improvement |
Worn gauge anvils or contact points · Inconsistent clamping force on gauge · Operator technique variation · Part surface roughness or burrs at measurement point · Thermal expansion of gauge during study · Insufficient calibration frequency.
Q: A gauge study gives σ_GRR = 0.018 mm and σ_Total = 0.090 mm. What is %GRR and is the gauge acceptable?
Locators are the fixture elements that accurately position the workpiece and prevent it from moving during the operation. They are classified into internal locators (using holes) and external locators (using outer surfaces).
Internal locators engage holes in the workpiece and are the most precise location method. External locators reference the outer profile or flat surfaces and are common for castings, forgings, and BIW sheet metal panels.
| Type | Description | Best Use |
|---|---|---|
| Dowel screws | Lag-threaded studs (headless) used for workpieces with threaded holes | Precise alignment in threaded bores |
| Press fit | Part inserted tightly into a hole — can be ejected after operation | Locking workpiece, batch production |
| Threaded | Locator with external thread engages threaded hole in workpiece; also assists clamping | Combined locate-and-clamp applications |
| Pin Style | Profile | Application Note |
|---|---|---|
| Round | Hemispherical nose | Standard hole location — constrains X & Y |
| Bullet | Conical tapered nose | Self-guiding entry into close-tolerance holes |
| Plain | Flat top cylinder | Used where nose lead-in is not needed |
| Conical | Sharp cone tip | Countersunk or tapered hole location |
| Dowel | Straight cylindrical rod | High-precision fixture-to-fixture alignment |
| Relieved (Diamond) | Cylindrical with two flats ground off | Second pin in a two-pin scheme — prevents rotation, allows thermal expansion |
Always use one round pin (constrains X and Y) paired with one diamond/relieved pin (constrains rotation only). This combination fully locates the part without over-constraint and accommodates differential thermal expansion between part and fixture.
| Type | How It Works | Best For |
|---|---|---|
| Nesting locator | Encloses workpiece on all sides — nest shape mirrors part profile | Complex cast or forged shapes |
| Vee (V-block) locator | Two inclined faces create self-centering contact on round stock | Circular and round-edged rectangular parts |
| Fixed-stop locator | Stop buttons constrain movement of parts that cannot use vee or nesting | Flat sheet metal, brackets, frames |
| Dowel pins | Precisely ground steel pins — Normal, Split, or Grooved variants | Sub-micron fixture plate alignment |
| Adjustable stop | Lock nut, locking screw, or set screw allow fine position tuning | Irregular or varying surfaces, prototype fixtures |
Dowel pins come in three common variants: Normal (solid cylinder for highest precision), Split (spring-like, for press-fit without interference buildup), and Grooved (longitudinal grooves allow air escape during press-fit insertion — prevents hydraulic locking).
Q: In a two-hole location scheme, why must the second pin always be a diamond (relieved) pin rather than a second round pin?
Supports act as the base for the workpiece on its external surface. They carry the weight of the part, provide stability, and must be placed at optimal positions to avoid deflection during cutting or welding.
| Type | Description | Load Capacity | Typical Use |
|---|---|---|---|
| Solid support | Fixed-height pad — transmits load directly to base plate | Heavy (high rigidity) | Castings, heavy machining fixtures |
| Adjustable — threaded | Height set by rotating threaded body into base plate hole | Medium | Parts with height variation between batches |
| Adjustable — spring | Spring-loaded plunger retracts when part loads; knob locks it at that height | Medium (damped) | Delicate surfaces, vibration-sensitive work |
| Equalizing support | Two linked contact points — if one depresses, the other rises to maintain contact | Distributed | Uneven or warped surfaces, castings with draft |
Place supports directly below clamp points wherever possible. This creates a compression path through the part into the support, minimising bending. Never cantilever a clamp over an unsupported section of a thin-walled part.
| Accessory | Function | Notes |
|---|---|---|
| Spring-stop buttons | Flange-mounted spring plungers — apply side force to hold part against locators before clamping | Not a primary locator — used for pre-positioning only |
| Spring locating pins | Press-fit inserts applying compact side force for positioning or light clamping | Extremely compact; ideal for blind holes in BIW panels |
| Ejectors | Remove workpiece from close-fitting locators after operation — reduces cycle time | Essential when locating pins have tight clearance fits |
| Lifting devices | Aid the worker in loading and unloading heavy or awkward workpieces | Critical for BIW panels and large sub-assemblies to ensure precise, safe mounting |
| Drill bushings (jig bushings) | Guide drill bits, reamers, countersinks — position and support the cutting tool | Also used in built-up tool bodies for repeatable multi-hole drilling |
| Body Type | Material | Advantages | Limitations |
|---|---|---|---|
| Cast | Cast iron / aluminium | Complex shapes, low secondary machining, good vibration damping | Long lead time, heavy |
| Welded | Steel / aluminium plate | Low cost, easy to modify | Distortion from welding heat — secondary machining often needed; not for high precision |
| Built-up | Individual steel elements | Fastest from design to use, high strength-to-weight, easy to modify for design changes | More assembly joints than cast body |
Q: An equalizing support is preferred over two solid supports when the workpiece surface is:
Clamps firmly hold the workpiece against its locators during the operation. Selecting the wrong clamp type results in part movement, surface damage, or poor repeatability. Note that clamping force (applied by the drive mechanism) and holding force (maximum the gripper arm can sustain without failure) are not the same quantity.
| Clamp Type | Mechanism | Force Range | Ideal For |
|---|---|---|---|
| Lever clamp | Lever + trigger + ratchet — fast release in tight spaces | Light to medium | Assembly, welding, confined access areas |
| Hinge clamp | Swinging eye bolt swings into slot; nut tightens to clamp | Medium | Applications needing speed and positional accuracy |
| Sliding clamp | Two modes: clamp slides on fixed plate, or plate slides over fixed clamp | Medium | Adjustable fixturing, thin sheet metal |
| Latch clamp | Pull bar (hook) seats over opposing latch and locks — heavy duty | High | BIW and automotive heavy sub-assemblies |
| Screw clamp | Threaded element with knurled collar / Allen key — self-locking once set | Light | Light secondary clamping, prototype fixtures |
| Swing clamp | Pivots about shoulder screw — rotates clear, then descends to clamp | Medium | Where clear part loading path is needed |
| Hook clamp | Swings into position then clamps straight down by screw action — ideal in tight spaces | Medium–high | High force in confined zones; mounted in reamed hole or holder block |
| Cam-action clamp | Cam shifts mating surface — very rapid action; reaches interior recesses | Light to medium | Interior clamping where other clamps cannot access |
| Wedge clamp | Wedge action — compact; can hold two workpieces simultaneously | Medium | Round and rectangular parts, space-saving designs |
| Toggle clamp | Lever + pivot pins lock over-centre — single fast movement; remains locked until released | 0.2–5 kN | Welding fixtures, press tools, inspection fixtures |
Place clamp force directly over a support whenever possible. Clamping between support points bends the part under clamping load — a common cause of dimensional error on thin-walled BIW stampings.
L1 = input lever arm, L2 = output arm to pivot. Toggle linkages can reach MA of 20 to 200 depending on geometry.
T = torque applied, r = cam radius, α = cam lift angle, φ = friction angle. Self-locking when α < φ.
SF = safety factor (2.0–3.0), μ = contact friction coefficient (0.15–0.4 for steel-on-steel).
The clamp arm must not yield under the reaction to its own applied force. Select clamp rated above calculated F_clamp.
Clamping before locating (part lifts off datum) · Clamping over unsupported spans (part deflects) · Using a clamp rated below the cutting force (clamp yields) · Placing all clamps on one side (part tilts) · Forgetting to re-clamp after tool change on a multi-setup part.
Q: You need a clamp that can reach an interior recess of a BIW sub-frame where no other clamp has physical access. Which type is most appropriate?
Body in White (BIW) is the automotive manufacturing stage where the car's sheet-metal shell is assembled before doors, engine, chassis, or any moving parts are attached. Fixtures are central to every BIW operation — they hold stacked sheet metal panels at exact pre-decided locations before welding.
BIW stands for Body in White — the welded sheet metal structure of a vehicle (roof, floor, pillars, sills, firewall) before painting and trim. It is entirely made of spot-welded sheet metal. Any dimensional error at BIW stage propagates through the entire vehicle build.
| Step | Action | Purpose |
|---|---|---|
| 1 | Load panel onto locating pins | Initial XY alignment via hole references |
| 2 | Engage supports | Panel sits at correct Z-height; weight distributed |
| 3 | Close clamps (manual or auto) | Constrain all 6 DOF — eliminate spring-back and panel gaps |
| 4 | Verify panel gap (go/no-go) | Confirm joint is within weld specification before gun fires |
| 5 | Weld (robotic spot gun) | Join panels at pre-programmed weld spots |
| 6 | Release clamps and eject | Remove welded sub-assembly; fixture resets for next cycle |
If degrees of freedom are not fully constrained, the welding gun pressure alone can shift the panel. This misplaces weld spots, causes panel gaps, and creates rework. Unconstrained welding is the leading cause of BIW dimensional non-conformance.
| Aspect | Manual Welding | Robotic Welding |
|---|---|---|
| Operator skill | High — experienced welder required | Programmed — repeatable after teach-in |
| Access | Good for tight interior regions robots cannot reach | Excellent for open, repetitive weld patterns |
| Repeatability | Variable — human fatigue, technique drift | High — same path, force, current every cycle |
| Error risk | Over-welding, distortion, bead build-up on panel | Misfire if fixture or robot offsets drift; no fatigue errors |
| Secondary operations | Often needed to remove excess bead | Rarely needed — weld parameters tightly controlled |
| Industry preference | Limited to sections robots cannot access | Preferred for all accessible weld points |
| Gun Type | Configuration | Characteristics | Best For |
|---|---|---|---|
| C-type gun | Operating cylinder connected directly to moving electrode — in-line with force axis | Cheapest, most common, simple mechanics; many frame/arm shape variants | Open, accessible weld areas; high-volume production lines |
| X-type gun (Scissors / Pinch) | Operating cylinder remote from electrode; force applied via lever arm | Greater reach into tight areas; balanced force; heavier than C-type | Deep-reach weld spots on pillars, sills, tunnel sections |
For pneumatic spot guns: pressure × piston area. Typical BIW force: 2–5 kN per weld spot.
HI = kJ/mm. V = volts, I = amps, v = travel speed (mm/min). Critical for distortion control in thin BIW sheets.
t = thinnest sheet thickness (mm). Rule of thumb for nugget diameter acceptance — verified by destructive peel test.
BIW fixtures typically require 3-sigma repeatability within ±0.5 mm on body-critical datums. Verified by CMM audit.
In modern BIW lines, robotic arms both load the panels (using gripper end-effectors) and weld them (using welding gun end-effectors). The same robot controller handles both tasks sequentially, reducing cycle time and eliminating the human-robot handoff point that historically caused the most misloading errors.
1. Match datum scheme to vehicle master datum (N-S-E-W body coordinates). 2. All locating pins must enter from the same direction as the robot loader path. 3. Clamps must not contact A-class (visible) surfaces. 4. Build in ejector pins at all close-fit locator holes. 5. Every fixture must have a CMM-auditable reference nest for periodic calibration. 6. Design for scrap-panel flush — closed cavities trap welding spatter.
Q: In BIW spot welding, what does an X-type (scissors/pinch) welding gun offer compared to a C-type gun?
This reference chapter consolidates the detailed classification of every fixture component used in BIW and general manufacturing — from the 3-2-1 locating principle through all locator types, support variants, clamp families, tool body constructions, and fixture accessories. Use it as a quick-reference companion alongside the analytical chapters.
To perform any work on a workpiece its movement must be constrained. The 3-2-1 principle achieves this systematically across three mutually perpendicular planes, eliminating all 6 degrees of freedom.
3 — Planar contact on Plane 1 (primary). 3 location points constrain 3 DOF (Z-translation + 2 rotations).
2 — Line contact perpendicular to Plane 1 (secondary). 2 points constrain 2 more DOF (Y-translation + 1 rotation).
1 — Point contact on the third plane (tertiary). 1 point constrains the final DOF (X-translation).
| Plane | Contact Type | No. of Points | DOF Constrained |
|---|---|---|---|
| Primary (Plane 1) | Planar | 3 | Z-translation, Rx, Ry |
| Secondary (Plane 2) | Line | 2 | Y-translation, Rz |
| Tertiary (Plane 3) | Point | 1 | X-translation |
a) Supporting — base support from below. b) Positioning (Locating) — accurate placement against datums. c) Clamping (Holding) — firmly securing after locating.
Locators are fixture elements that accurately position and constrain the workpiece. They can be fixed or adjustable, and are classified as Internal or External type.
Locate the workpiece using internal holes. Divided into Large Hole and Pin Hole types.
| Type | Description |
|---|---|
| Screwed & Doweled (Dowel Screws) | Lag-threaded headless dowel studs used to locate workpieces that have both threads and holes. |
| Press Fit | One part inserted tightly into a hole in another. Locks the workpiece in place; can be ejected later. |
| Threaded | Used for threaded holes in the workpiece. Assists in clamping through thread engagement. |
| Type | Characteristics |
|---|---|
| Round | Hemispherical head; standard general-purpose pin hole locator. |
| Bullet | Tapered/conical nose guides the pin into the hole — ideal for quick loading. |
| Plain | Flat-faced cylinder; used where minimal engagement guidance is needed. |
| Conical | Full conical profile; self-centring for holes with moderate positional variation. |
| Dowel | Precisely ground cylindrical rod; high accuracy, high toughness, usually hardened steel. |
| Relieved (Diamond) | Relief ground on two sides; allows for slight inter-hole distance variation when used with a round pin (the round-diamond pair is standard BIW practice). |
Used to locate workpieces with reference to their external surfaces, particularly flat surfaces.
| Type | Description | Best For |
|---|---|---|
| Nesting Locator | Completely encloses the workpiece from all sides; locator shape can mirror the workpiece profile. | Irregular or contoured external shapes. |
| Vee Locator | V-groove seats a circular diameter; also works for rectangular parts with rounded edges. | Round bar stock, cylindrical features. |
| Fixed-Stop Locator | Uses stop buttons to constrain workpiece movement; for parts that cannot use a vee or nest. | Prismatic parts, sheet edges, irregular profiles. |
| Dowel Pins (External) | Precisely ground steel cylinders pressed into the base plate. Three variants: Normal, Split, Grooved. | High-precision external surface location. |
| Adjustable Stop Locators | For varying/irregular surfaces. Variants: Lock nut, Locking screw, Set screw — all allow height/position adjustment. | Castings, forgings, and variable-surface parts. |
Q: A round pin and a diamond (relieved) pin are used together to locate a BIW panel. What is the purpose of the diamond pin?
Fixture supports act as the base for the workpiece from its external surface. In addition to locators, they absorb a significant proportion of the load, give stability, and their correct placement is critical for repeatable results.
| Support Type | Description | Application |
|---|---|---|
| Solid Supports | Fixed-height; absorb high loads and transmit directly to base plate. | Heavy-load machining and welding operations. |
| Adjustable — Threaded | Threaded into base plate; height adjusted by rotation. Fixed once set. | Where support height varies workpiece-to-workpiece. |
| Adjustable — Spring | Mounted on base plate with internal spring; plunger retracts under workpiece load, then locks via knob at that height. | Delicate or variable surfaces requiring gentle seating. |
| Equalizing Support | Two connected contact points — if one depresses the other rises, maintaining dual contact on uneven surfaces. | Castings, forgings, and other non-flat surfaces. |
Always place clamp force directly over a support point. Clamping between supports causes the workpiece to bow under clamping load — a leading source of thin-panel dimensional error in BIW.
Clamping Force — force applied by the drive mechanism to the workpiece via the clamp arm during gripping. Holding Force — the maximum force the workpiece can exert on the gripper arm without destroying it. These are not the same; always select a clamp whose holding force exceeds the calculated clamping requirement.
| # | Clamp Type | Operating Principle | Best Application |
|---|---|---|---|
| 1 | Lever Clamp | Quick-release via lever + trigger + ratchet mechanism; single-hand operation. | Tight spaces; fast cycle assembly and welding. |
| 2 | Hinge Clamp | Swinging eye bolt seats into open slot; nut tightens to clamp. Works like a door hinge. | Where speed and positional accuracy are both required. |
| 3 | Sliding Clamp | Two modes: (a) clamp body slides on fixed slotted plate; (b) slotted plate slides over fixed clamp body. | Adjustable fixturing, thin sheet metal panels. |
| 4 | Latch Clamp | Pull-bar (hook) placed around opposing latch then pressed down to lock. Heavy-duty rated. | BIW heavy sub-assemblies; high-load applications. |
| 5 | Screw Clamp | Threaded element (knurled collar, Allen key, tommy bar, spanner flat). Self-locking once torqued. | Light clamping; prototype fixtures; secondary clamping. |
| 6 | Swing Clamp | Rotates in plate plane about shoulder screw; knurled head screw descends to clamp. Swings clear for loading. | Where a clear loading path is required above the part. |
| 7 | Hook Clamp | Manually swings into position, then clamps straight down by screw action. Mounts in reamed hole or holder block. | High force in confined spaces; tight BIW interior zones. |
| 8 | Cam-Action Clamp | Cam shifts mating surface — very fast action; reaches interior recesses inaccessible to other clamps. | Interior BIW clamping; high-speed automated lines. |
| 9 | Wedge Clamp | Compact wedge action; can hold two workpieces simultaneously between its clamping faces. | Round and rectangular parts; space-saving fixture layouts. |
| 10 | Toggle Clamp | Single plate; lever + pivot pins lock over-centre in one rapid movement; remains locked until lever released. | Welding fixtures, press tools, inspection fixtures. |
| Type | Construction | Advantages | Limitations |
|---|---|---|---|
| Cast Tool Bodies | Cast iron or alloy cast to near-net shape. | Excellent dimensional stability; complex shapes with less machining; good vibration damping. | Long lead time; high tooling cost; suited to permanent work holders only. |
| Welded Tool Bodies | Steel or aluminium plate/section, MIG/TIG welded. | Inexpensive; easy to modify; suitable for roughing operations. | Heat distortion during welding may require secondary machining; not for extreme precision. |
| Built-Up Tool Bodies | Modular individual elements bolted together (most common). | Easy to build; shortest design-to-fixture time; excellent strength-to-weight ratio; easily modified. | More fasteners than cast; requires careful assembly for precision work. |
| Drill Bushings (Jig Bushings) | Hardened sleeves pressed or screwed into the jig body to guide drill bits, counterbores, countersinks, and reamers. | Guides, positions, and supports the cutting tool precisely; replaceable when worn. | Specific to a drill size; requires accurate hole in jig body. |
Welding heat causes distortion in the tool body. Always stress-relieve welded fixtures before finish-machining critical datum surfaces. Skipping stress relief is the most common cause of welded fixture drift after initial setup.
| Accessory | Description | Function |
|---|---|---|
| Spring-Stop Buttons | Flange-mounted spring plungers with a large contact face. | Apply side force to hold the workpiece against locators until clamps engage; used for light clamping jobs. |
| Spring Locating Pins | Press-fit inserts with an internal spring-loaded pin. | Compact way to apply side force for positioning or light clamping — no external hardware required. |
| Ejectors | Mechanical pins or pushers built into close-fitting locators. | Remove workpiece from tight locators quickly, reducing cycle time and increasing productivity. |
| Lifting Devices | Mechanical or pneumatic assist arms integrated into fixture. | Enable safe and precise loading/unloading of heavy workpieces irrespective of their weight or size. |
Always design ejectors into any fixture with close-fit locating pins. Without them, operators use screwdrivers or mallets to remove panels — damaging both the workpiece and the pin — adding seconds of manual force that negates the precision of the location.
All 6 rigid-body DOF (3 translational + 3 rotational) must be exactly constrained — no over- or under-location.
Locator positional accuracy should be at most 1/5 of the tightest workpiece tolerance to avoid consuming the full tolerance budget.
Diamond pin minor width set to accommodate pitch variation δ_pitch between two locating holes.
For a simply supported panel of span L under central clamp force F. Minimise L by placing supports close to clamp points.
Q: Which built-up vs cast vs welded tool body type is most commonly used and offers the shortest design-to-fixture lead time?