Pneumatic Bending-cum-Punching Fixture with Toggle Booster for a Rajkot Grill Fabrication Shop, with Cycle-Time Study

Walk through the fabrication lanes of Rajkot or Ahmedabad and you will see the same scene in dozens of small shops: a worker bending mild-steel flats for window grills and gates by hammering them over a vice, and another drilling bolt holes one at a time. The work is slow, noisy, tiring and inconsistent; two "identical" grill panels rarely match.

This diploma project designs, builds and tests a pneumatic bending-cum-punching fixture for a fictional shop, Shree Khodiyar Grill Works, Rajkot, which makes about 40 window grills and 6 gates a week from 25 × 3 mm and 20 × 3 mm MS flats. The fixture runs on the shop's existing compressor at about 6 bar. A double-acting cylinder drives an interchangeable V-bending die directly, and drives a punch through a toggle-lever booster for holes, because a direct pneumatic stroke alone cannot produce the shear force needed to punch steel. A two-hand control arrangement keeps both of the operator's hands away from the punch.

The team calculates bending and punching forces, sizes the cylinder and toggle, draws the pneumatic circuit to ISO 1219 symbols, fabricates the frame and tooling in the polytechnic workshop and installs the fixture at the shop for trials. A stopwatch cycle-time study compares the old method with the fixture for a standard job (two 90° bends and two holes per flat), followed by a cost and payback calculation. The report follows GTU Diploma Project-II (4361904) with its logbook, work-allocation matrix and test reports.

Abstract

A pneumatic bending-cum-punching fixture with a toggle-lever force booster was designed, fabricated and tested for a small grill and gate fabrication shop. Bending and punching forces for 3 mm MS flats were calculated, a cylinder and toggle mechanism were sized to deliver them at 6 bar, and a two-hand control circuit was designed for operator safety. Shop-floor trials compared cycle time, bend-angle consistency and hole position accuracy with the existing hammer-and-drill method. The fixture reduced manual effort and was evaluated for cost per piece and payback.

Introduction

Gujarat has a large number of micro fabrication units serving housing construction. Most rely on manual bending and drilling because hydraulic press brakes and punching machines are expensive for their volume. Compressed air is already available in many such shops for spray painting and grinding tools, which makes low-cost pneumatic fixtures a practical step up.

Existing method versus proposed fixture

• Existing: flats marked by hand, bent with a hammer over a vice jaw, holes drilled on a pillar drill. Angles vary, holes shift, and the worker tires.
• Proposed: a stop-located flat is bent in a V-die by one cylinder stroke; for holes the operator swaps to the punching station, where the toggle multiplies the cylinder force and the punch shears the hole in one stroke under two-hand control.

Literature gap

Pneumatic bending and punching machines are common diploma topics, but many reports size the cylinder only for bending and assume punching will "also work", which the force calculation does not support. Few designs include a safety circuit, and fewer measure productivity in a real shop.

Feasibility

• Technical: cylinders, valves and an FRL unit are standard catalogue items; the polytechnic workshop can weld the frame and turn the punch and die.
• Economic: estimated build cost ₹22,000–28,000, shared with the sponsoring shop.
• Operational: the shop owner allows two days of trials and provides material.

The sponsoring shop bends and drills hundreds of mild-steel flats every week by hand. The manual method is slow, inconsistent in bend angle and hole position, and physically demanding, and hole drilling is a bottleneck before welding. Hydraulic press brakes and punching machines are too expensive for the shop's volume. The problem is to design and fabricate a low-cost pneumatic fixture, using the shop's existing compressed air, that can bend 25 × 3 mm MS flats to 90° and punch 6 mm holes safely, and to measure its effect on cycle time, accuracy and cost per piece compared with the manual method.

The fixture handles MS flats up to 25 × 3 mm for 90° V-bends and 6 mm round holes. Thicker sections, rolling of curves (scrolls) and automatic feeding are excluded. Trials are conducted at one shop over two days, so long-term durability of the tooling is not established. The design uses manual valve operation; PLC control is listed as future work.

Approach

Problem study → design calculations → drawings → fabrication → shop trials → time and cost analysis, recorded in a daily logbook as required for Project-II.

Step 1 — Shop study

Observe and time the existing process for 30 pieces; record products, flat sizes, hole sizes and weekly volume; note air pressure available (gauge at compressor outlet) and compressor capacity.

Step 2 — Force calculations

• Punching (shear) force: F = π · d · t · τ, where τ for MS is taken as about 0.8 × UTS (UTS ≈ 410 MPa). For d = 6 mm, t = 3 mm: F ≈ π × 6 × 3 × 330 ≈ 18.7 kN.
• V-bending force: F = k · L · UTS · t² / W, with k ≈ 1.33, bend length L = 25 mm, die opening W = 8t = 24 mm: F ≈ 1.33 × 25 × 410 × 9 / 24 ≈ 5.1 kN.
• Cylinder force: F = p · A · η. A 125 mm bore at 0.6 MPa with η = 0.9 gives about 6.6 kN, enough to bend directly but well short of punching.
• Toggle booster: mechanical advantage MA ≈ 1 / (2 tan α) for a symmetric toggle, where α is the link angle from the line of action. Designing the punch to contact the strip when α ≈ 6–7° gives MA ≈ 4, so punching force ≈ 26 kN, a margin of about 1.4 over the required 18.7 kN.
• Punch-die clearance: about 5–10% of sheet thickness per side; punch in tool steel, hardened.
• Frame check: bending stress in the C-frame throat and weld size for the peak punching reaction.

Step 3 — Pneumatic circuit (ISO 1219)

FRL unit → two-hand control (two 3/2 push-button valves in series feeding a pilot to the main 5/2 valve) → double-acting cylinder with meter-out flow control on both ports → quick exhaust for fast return. Bending station uses a single foot-operated 5/2 valve with guard.

Step 4 — Fabrication

Frame from ISMC channel and MS plate, welded; cylinder mounting plate; interchangeable V-die and punch-die blocks located on dowels; stop gauge for repeatable flat positioning.

Step 5 — Trials and time study

Standard job: two 90° bends and two holes per flat. 30 pieces by the manual method and 30 with the fixture by the same worker after a short practice; stopwatch elemental times (load, locate, operate, unload); bend angle measured with a protractor; hole position with a vernier.

Step 6 — Analysis

Mean cycle time and standard deviation; percentage reduction; angle and position variation; air consumption = cylinder swept volume × (p_abs / p_atm) × strokes per hour; cost per piece including labour, air and depreciation; payback.

Timeline (14 weeks)