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ASME B94.33-1996 (2015) — Jig Bushings
📐 ASME B94.33-1996 is the American National Standard covering the sizes, types, tolerances, and identification of jig bushings and the locking devices used to secure them in a jig or bushing plate. Reaffirmed by ASME in 2015, it is prepared by Technical Committee 8 (Jig Bushings) of the ASME B94 Standards Committee on Cutting Tools, Holders, Drivers, and Bushings. Jig bushings are the core guidance elements of drill jigs: they position the cutting tool (drill, reamer, tap, spot facer, counterbore, etc.) precisely with respect to the workpiece. Standardizing these components guarantees interchangeability across manufacturers, shortens fixture design cycles, and reduces tooling cost in machining operations.
💡 Jig bushings are essentially a “standard interface” inside a fixture: as long as bushings, liners, and lock screws are made to B94.33, components of the same designation from different suppliers are interchangeable and work reliably in the same bushing plate.
1. Scope and Bushing Families 📌
This Standard establishes American National Standard practice for the sizes, types, tolerances, and identification of jig bushings and of the locking devices used to secure bushings in the jig or bushing plate. Three families are defined:
- Press Fit Bushings: installed directly into the jig without a liner; used mainly for short production runs where the bushing will not require replacement, and where close hole center distances prevent the use of liners and renewable bushings. Two types: with heads and without.
- Renewable Bushings: used in liners, which in turn are installed in the jig; specified where bushings wear out or become obsolete before the jig, or where several bushings must be interchangeable in one hole. Usually knurled-headed for removal. One type is standardized: the slip-fixed renewable bushing, whose head combines a “fixed” side (left in place until worn) and a “slip” side (changed frequently, e.g., when drilling is followed by reaming, tapping, spot facing, or counterboring in a single jig).
- Liner Bushings: provided with and without heads, permanently installed in the jig to receive renewable bushings; sometimes called “master bushings.”
⚠️ Standard lengths of the press-fit portion are based on a standardized uniform jig plate thickness. If the actual plate thickness deviates from the standard range, verify that the press-fit length still engages correctly before specifying the bushing.
2. Designation System 🔧
Section 4 defines a complete identification system. A bushing is specified by its decimal inside diameter plus a type letter and a two-part size code:
| Type Code | Description | Key Features |
|---|---|---|
| P | Press Fit, Plain | Pressed directly into the plate; no head |
| H | Press Fit, Headed | Pressed directly into the plate; head provides location and anti-pullout |
| L | Liner, Plain | Permanent installation; receives renewable bushings |
| HL | Liner, Headed | Permanent installation; headed version |
| SF | Renewable, Slip-Fixed | Head carries fixed and slip notches for locking |
Carbide bushings add the letter “C” after the type designation (e.g., SFC = slip-fixed renewable carbide bushing). A handy rule: all body diameters are multiples of 1/64 in. and all body lengths are multiples of 1/16 in. Example: a bushing designated 0.5000 and SF-48-16 means inside diameter 0.5000 in., type SF (renewable, slip-fixed), body diameter 48/64 in., body length 16/16 = 1.000 in.
3. Sizes, Tolerances, and Specifications 📏
Tables give the dimensions and tolerances of each family: Table 1 covers head-type press fit bushings (Type H) and Table 2 plain-type press fit bushings (Type P); Table 3 covers slip-fixed renewable bushings (Type SF); Tables 4 and 5 cover plain-type and head-type liner bushings (Types L and HL). Tolerances govern interchangeability and fit, ensuring that hole positions established by the jig remain accurate over the life of the tooling.
| Bushing Family | Table | Key Controlled Dimensions | Typical Application |
|---|---|---|---|
| Head-type press fit | Table 1 | Body OD, overall length, head thickness, drill bearing length | Short runs, limited plate thickness |
| Plain-type press fit | Table 2 | Body OD, press-fit length, bearing length | Close center distances, no liner space |
| Slip-fixed renewable | Table 3 | Body OD, length, notch and fixed-side dimensions | Multi-operation holes, rapid replacement |
| Plain / head-type liner | Tables 4 / 5 | Body OD, length, receiving bore | Permanent installation for renewable bushings |
💡 Understanding the slip-fixed head: the fixed side takes the cutting torque so the bushing does not rotate; the slip side allows quick interchange of bushings inside the same liner size (for example, replacing a drill bushing with a reamer bushing in the same fixture).
4. Lock Screws and Clamps 🔩
Section 5 covers the designation of lock screws and round clamps used with slip-fixed renewable bushings. Lock screws thread radially through the liner wall and seat in the bushing notch, holding the renewable bushing in the liner; the round clamp is an optional device used only with the fixed side of slip-fixed renewable bushings for extra axial retention. Tables 6, 7, and 8 give dimensions for the lock screw, the round clamp, and the locking mechanism respectively. Table 9 provides decimal equivalents of fractional, wire, and letter drill sizes so designers can derive bushing bores directly from tool sizes.
🚨 Lock screw torque must be controlled: too loose and the bushing rotates under cutting torque and scores the liner bore; too tight and the bushing body is squeezed out of round, distorting the tool-guiding bore. Verify tool passage and free rotation after locking.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: size the guided tool (drill/reamer/tap) → read decimal bore from Table 9 → choose family (press fit, or liner plus renewable) → select body diameter and length from plate thickness and center distances → add “C” for carbide.
- 🔧 Fit control: press-fit bushings and liners must be driven square to avoid damaging the plate bore; renewable bushings need the correct clearance in the liner so they can be changed easily yet remain concentric.
- 📊 Wear management: replace only the renewable bushing when it wears—the liner and jig stay in place, minimizing downtime; for long-run, high-cycle lines prefer liner-plus-renewable construction.
- 📋 Procurement: write the full designation on POs (e.g., 0.2500 P-32-8) including carbide suffix, to prevent mis-supply and mismatch.
6. Frequently Asked Questions ❓
❓ Q: When should I choose a press-fit bushing versus a renewable bushing?
A: Use press-fit bushings for short runs, close center distances, or where replacement is unlikely. Use liner-plus-renewable construction when the bushing wears faster than the jig or when several operations (drill, ream, tap, countersink) share one hole.
A: Use press-fit bushings for short runs, close center distances, or where replacement is unlikely. Use liner-plus-renewable construction when the bushing wears faster than the jig or when several operations (drill, ream, tap, countersink) share one hole.
❓ Q: What does the designation SF-48-16 mean?
A: SF = renewable, slip-fixed type; 48 = body diameter 48/64 in.; 16 = body (effective) length 16/16 = 1.000 in.; a decimal bore such as 0.5000 is stated separately.
A: SF = renewable, slip-fixed type; 48 = body diameter 48/64 in.; 16 = body (effective) length 16/16 = 1.000 in.; a decimal bore such as 0.5000 is stated separately.
❓ Q: How are carbide bushings identified?
A: Add the letter C after the type letter, e.g., SFC for a carbide slip-fixed renewable bushing. All other designation rules are the same as for steel bushings.
A: Add the letter C after the type letter, e.g., SFC for a carbide slip-fixed renewable bushing. All other designation rules are the same as for steel bushings.
❓ Q: Why are diameters in multiples of 1/64 in. and lengths in multiples of 1/16 in.?
A: It is the Standard’s incremental series that covers common tool sizes while limiting the number of catalog items—simplifying manufacture, stock control, and interchangeability.
A: It is the Standard’s incremental series that covers common tool sizes while limiting the number of catalog items—simplifying manufacture, stock control, and interchangeability.
