3R is abbreviation for the three principles of reducing, reusing and recycling. It was proposed by British scholars in 1959, then gradually recognized and adopted by scientific and technological workers around the world. 3R fixture is an auxiliary fixture that is manufactured based on 3R principle. It has good manufacturability, is easy to process, adjust, assemble and inspect, reduces production cost of workpiece, workpiece is convenient and fast to load and unload, positioning is accurate, installation time is shortened, and production efficiency is improved.
After 3R fixture reference system is calibrated, it can quickly position workpiece, save time for processing workpiece, can firmly clamp electrode or workpiece in working area of machine tool. Through unified 3R reference sheet interface, electrode or workpiece can be fixed. A 1-minute changeover is realized on machine tool in each process.

Conformal positioning tooling is shown in Figure 1. Material is polypropylene (PP), which is a colorless, odorless, non-toxic, translucent thermoplastic synthetic resin with chemical resistance, heat resistance, electrical insulation, high strength mechanical properties and high wear resistance. This material does not have conductivity and can only be processed by CNC milling machines. Strength of polypropylene is lower than that of steel. CNC milling machine adopts vise to clamp easily to deform tooling to be formed and produce shear stress. After tooling is processed, it will be deformed due to stress release. Considering stress, tolerance of polypropylene tooling is ±0.05 mm.

 

Figure 1 Conformal positioning tooling

Key dimensions of tooling are (see Figure 1): ①Tolerance of cavity surface is ±0.02 mm; ②Tolerance of positioning waist hole is ±0.005 mm; ③Dimension of groove from bottom surface of workpiece is (20±0.01) mm. It is necessary to ensure relative position accuracy of contact surface of tooling fixture and workpiece, as well as tolerance of key size, as shown in Figure 2.

 

Figure 2 Positioning tooling

After analysis, main difficulties in manufacturing conformal positioning tooling are as follows.
(1) Material processing is difficult. Polypropylene material is soft and easy to deform when clamping, and workpiece is also easy to deform after finishing milling and stress is released.
(2) Positioning waist hole is too deep, working length of milling cutter is too long during finishing milling, and milling cutter vibrates too much during processing. If you turn over and finish milling positioning waist-shaped hole, it is easy to cause datum to be inconsistent, relative position of working surface and positioning hole will deviate, and workpiece material is polypropylene, which is non-conductive, can not be processed by slow-moving wire.
(3) Size and position accuracy requirements are high. For example, general tolerance of cavity surface is ±0.02 mm, and positioning waist-shaped hole tolerance is ±0.005 mm, etc.
In summary, use of traditional CNC milling machines to manufacture conformal positioning tooling has many drawbacks. For example, multiple clamping is easy to cause accumulation of machining errors, machining is difficult, clamping is not easy, relative position accuracy cannot be guaranteed, and operator's proficiency is high.

Principle of 3R fixture is shown in Figure 3. Positioning piece and positioning prism are only in line contact. Prismatic positioning teeth of chuck are inserted into positioning piece to pre-position. After clamping, positioning piece is elastically deformed, flexible positioning piece cleans up chips, contact surface is minimized to minimize impact of chips and particles.

 

Figure 3 Principle of 3R fixture

Through structural analysis of conformal positioning tooling, determine machining reference surface and position of the process step, so that connecting surface of tooling to be formed and process step is set on auxiliary surface that does not affect use as much as possible, and model is established using UG NX 12.0 software, as shown in Figure 4.

 

Figure 4 Model structure

After confirming tooling shape and process steps, use UG NX to create a block command to contain tooling and process steps to form a containment block. When making blank, set single-sided machining allowance of maximum size of tooling and process steps to 0.5 mm (see Figure 5). Due to large size deviation during cutting of sawing machine, blank margin can be appropriately enlarged when cutting material. After completion, use a bench drill to process threaded bottom hole connected with 3R reference plate, installation and connection state of 3R reference plate is shown in Figure 6.

 

Figure 5 Blank size

 

Figure 6 Blank is connected to 3R reference plate

After calibrating 3R fixture base datum, blank is directly connected with 3R datum. Because processing datum is center of 3R datum (see Figure 7), blank is transferred by 3R datum to achieve purpose of fast clamping and conversion using same datum. When one working surface is processed, only need to disassemble reference piece and rotate it to another working surface to be processed, then lock reference piece to continue processing directly without re-aligning reference. Figure 8 shows effect of conformal positioning tooling after processing.

 

Figure 7 Manufacturing benchmark

 

Figure 8 Processed objects

Use of 3R fixtures to process and manufacture workpieces mainly has following four characteristics.
(1) 3R fixture guarantees repetitive positioning accuracy of tooling to 2 μm. Control accumulated error from source and keep accuracy of machine tool and workpiece (electrode) equal.
(2) Uniform coordinate zero reference. No need to re-calibrate, greatly reducing machine downtime and maximizing equipment utilization.
(3) Standardized interface. Realize high-precision and high-efficiency conversion between different machining types such as milling, drilling, turning, grinding, and EDM.
(4) Urgent processing. Fixed reference point and repeatability of system can interrupt processing of workpiece, remove pallet, complete urgent processing, then continue to process unprocessed workpiece without correction or readjustment.

In actual processing and application of 3R fixture, commonly used numerical control and digital display machine tools of enterprise, including vertical/horizontal machining centers, CNC lathes, coordinate boring machines, coordinate grinders, electric discharge machine tools, wire cutting machine tools, slow wire machine tools, three coordinate measuring machines, etc. are connected into a unified reference platform and integrated into a flexible manufacturing system, which shortens production preparation cycle, improves production efficiency, reduces production costs, and improves corporate competitiveness. And 3R system has Macro, Refix and other systems suitable for small parts processing in mold industry, and Delphin Big large-scale workpiece positioning system suitable for equipment industry, with a maximum workpiece bearing capacity of up to 10 tons. Regardless of size of mold and part, 3R fixture can meet needs of normal cutting and powerful cutting of workpiece.