Crankshaft Machining Process Planning: Principles and Datum Selection

Crankshaft Process Planning Principles and Datum Selection

Process-Route Design Principles

1. Establish datums first: In process planning, the first or second operation should, where possible, machine the crankshaft’s precision datums. Subsequent operations can then use those precision datums to locate the part, making it easier to ensure the accuracy of other surfaces.
2. Machine surfaces before holes: When holes are required in crankshaft journals or flange faces, machine the journals and flange faces first, then machine the holes in those surfaces. This makes it easier to ensure hole accuracy.
3. Machine major surfaces before secondary surfaces: Process planning should focus on the important surfaces of main journals and crankpin journals. Secondary surfaces can be inserted between these operations.
4. Rough before finish, with separate roughing and finishing: The purposes of separating rough and finish machining are as follows.
   1. Ensure machining quality: During rough machining, large stock allowance produces high cutting forces and cutting heat. The process system therefore experiences elastic deformation, thermal deformation, and high workpiece residual stress. Rough machining cannot achieve high accuracy or low surface roughness. Subsequent operations must gradually reduce cutting parameters and machining errors to meet the part-quality requirements. For blanks with residual internal stress, rough machining can redistribute that stress and cause deformation over time. Separating roughing and finishing avoids the effect of stress-related deformation on finish machining; remaining deformation can also be eliminated in later finish-machining steps.
   2. Detect blank defects early: Rough machining removes a large allowance and can reveal blank defects early, allowing timely scrapping or repair and avoiding waste from continued machining.
   3. Use machine tools appropriately: Rough machining can use machines with high power and rigidity but moderate accuracy, while finish machining uses high-accuracy machines. This makes full use of each machine’s capability. Separating roughing and finishing applies both to a single-surface process and to the whole part process. As casting and forging accuracy for modern automotive blanks continues to improve and total stock allowance decreases, the distinction between roughing and finishing stages is becoming less pronounced.
5. Arrange heat-treatment operations and surface-strengthening processes rationally.
6. Crankshafts are slender parts, so machining deformation should be minimized. Crankshaft process planning generally follows these principles: rough before finish; datum before other features; surfaces before holes; major surfaces before secondary surfaces; and main journals before crankpin journals.

When developing a process plan, further principles should be considered: compatibility with the production program, the best economic result, and active adoption of new processes and technologies suited to the production program.

A process plan is mainly evaluated on three criteria: quality, cost, and delivery time.

Overall Process-Route Development

For an integral crankshaft, the main blank materials are forged steel and nodular cast iron. Common process routes are as follows:

• Forged-steel crankshaft: blank preparation, forging, normalizing, rough machining, surface quenching and tempering, and finish machining.
• Nodular-cast-iron crankshaft: casting, normalizing, tempering, rough machining, surface quenching and tempering, and finish machining.

Rough-Machining Process Development

Common crankshaft rough-machining methods include turning, internal milling, turn-broaching, and turn-turn-broaching. All four methods are currently used. The selected method depends on the product, production volume, takt time, process requirements, and investment.

Turning: offers the strongest versatility. Internal milling: is suitable for machining crankpin journals and crankweb sides, particularly when the blank has a large stock allowance. Turn-broaching: places high demands on the blank and adapts poorly to product changes, so it is suitable for high-volume production of a single product. Turn-turn-broaching: combines turning and turn-broaching; its accuracy reaches the level of turn-broaching, its efficiency is lower than internal milling, and its flexibility is higher than turn-broaching and internal milling.

The 4H crankshaft uses a combination of turning and internal milling for rough machining.

Grinding Process Development

Grinding sequence and machining quality: Whether to grind main journals or crankpin journals first has no absolute answer.

1. Main-journal grinding followed by crankpin-journal grinding: Stress release caused by crankpin grinding can make main-journal runout exceed tolerance and is difficult to control.
2. Crankpin-journal grinding followed by main-journal grinding: Converting the datum can cause crank-radius error to exceed tolerance, is difficult to control, and requires additional equipment.

As crankshaft-journal rough machining and quenching technologies develop, grinding allowance has been greatly reduced. Grinding deformation and grinding stress are lower, and precision grinding of journals can be completed in one pass, eliminating semi-finish grinding and simplifying the process.

Grinding cracks and burns, especially on wide thrust faces, main-journal runout, and the positional accuracy of crankpin journals relative to main journals, including radial runout, crank radius, and parallelism, are key considerations in grinding-process design.

From a manufacturing-process perspective, if the effect of crankpin-journal grinding force on crankshaft deformation can be controlled within the allowable main-journal runout, grinding main journals before crankpin journals is an optimized process with a simple route and low investment. Operations used solely to establish process location are non-value-added activities and should be reduced or eliminated.

Crankshaft Locating-Datum Selection

Using a 4H engine crankshaft as an example, the crankshaft has four crankpin journals and five main journals. For radial datum selection on an in-line engine crankshaft, the center holes at both ends or main journals, such as the line joining the centers of the first and fifth main journals or the line joining the two center holes, can be used as radial datums.

Several axial datums are available. To limit axial movement and prevent axial drift caused by axial forces, the shoulder, or thrust face, of the fourth main journal and the end face of the flange journal are used as axial datums.

Because a crankshaft is a complex part, it also has several angular locating datums, such as the angular process boss on the balance weight of the first main journal, the first crankpin journal, and process holes in the flange.

1. Selection of the 4H crankshaft rough datum: Crankshaft blanks are generally curved. To ensure that the center holes at both ends are drilled at the geometric centers of the end faces, the rough datum in the radial direction is selected from the outside diameters of journals near both ends. The axial locating datum is the shoulders on both sides of the third main journal. Because the crankwebs on both sides of the third main journal lie in the middle of the crankshaft, using them as a rough datum reduces positional error in other crankwebs. The angular process boss on the balance weight of the first main journal is selected as the angular rough datum.
2. Selection of the 4H crankshaft precision datum: Because the crankshaft surface is complex, it has multiple precision datums. Two are used throughout the machining process: the most important is the center holes at both ends, and the other is the first and fifth main journals. Several axial precision datums are also available, including the center holes, the flange-journal end face, and the shoulder, or thrust face, of the fourth main journal. Most angular precision locating uses process holes in the flange to restrict rotational freedom while grinding the main and crankpin journals. The first crankpin journal is also occasionally used as an angular locating datum.