1. Control of process system deformation under stress: increasing stiffness and optimizing load
The force deformation of the process system is what directly causes pieces to be the wrong size. For example, when using a gantry milling machine to manufacture huge flat surfaces, the crossbeam can bend 0.1–0.3 mm under the weight of the tool holder, which directly causes the workpiece to be overly flat. To fix these kinds of problems, optimization needs to be done from the following three points of view:
Strengthening contact stiffness
The contact stiffness can be greatly improved by making sure that important parts like machine tool guides and spindle bearings fit better. For instance, preloading the nut with pre-tightened rolling bearing technology removes the space between the main shaft and the bearing. This increases the actual contact area by 30% to 50% and the contact stiffness by more than double. After using this technology, a company that makes car parts cut the roundness error of the crankshaft connecting rod neck from 0.015mm to 0.005mm.
Making the workpiece more rigid
Auxiliary supports are needed to make thin-walled items like cylinder blocks and transmission housings stiffer. For processing thin-walled casing, the "guard shaft+tip" composite clamping method is used. First, the inner hole is processed to the right size. Then, the guard shaft that fits the gap between the inner hole is inserted and fixed with the front and rear tips. This makes the workpiece 4-6 times stiffer during outer circle processing and keeps the cylindricity error within 0.02mm. Also, things like tool holders and center frames can help keep slender shafts from vibrating too much. A case study on machining a transmission shaft found that using a dual wheel floating support system cut the amplitude of machining vibration by 80% and the fracture rate from 15% to less than 2%.
Optimizing the allocation of loads
If you design the direction and application point of the clamping force correctly, you can avoid local stress concentration. For instance, when working with aluminum alloy wheels, radial clamping is employed instead of axial clamping. This spreads the clamping force equally throughout the wheel, which cuts down on workpiece deformation by 60%. Finite element analysis (FEA) can be used to model the distribution of clamping force and improve the structure of the fixture for pieces that are not regular.
2. Controlling thermal deformation: balancing the temperature field and improving heat dissipation
The main reason pieces distort when they are cut is because of heat. For example, when turning at high speed, the cutting edge temperature of the tool can exceed 850 °C, and the tool holder can stretch by 0.05–0.1 mm/m, which directly causes the size of the machining to change. To solve the problem of thermal deformation, we need to look at two things: source control and process compensation:
Separating the heat source and making it easier for heat to escape
Put heating parts like electric motors and hydraulic pumps on the outside of the machine tool, and use insulation materials like ceramic fiber boards to keep heat from moving. A forced air cooling system can keep the temperature rise within 5 °C for important parts like spindle bearings and guide rails. A particular maker of CNC machine tools improved the coolant circulation system such that the thermal deformation of aluminum alloy cylinder bodies during milling went from 0.08mm to 0.02mm.
Design for temperature field balance
When machining the gearbox body, the gears and bearings are arranged in a symmetrical way to keep the temperature of the gearbox wall rising evenly and cut down on deformation by half. To reduce axial tension buildup and make room for thermal expansion, the "one end fixed, one end elastic tip" clamping method is employed on thin shaft parts.
Improving the settings for cutting
It is possible to cut faster (Vc), which can shorten the cutting time, but it is important to keep the cutting heat and tool wear in check. Tests have showed that increasing Vc from 100m/min to 200m/min lowers the unit cutting heat by 30% but shortens the tool's life by 40%. So, the best combination of parameters should be chosen based on the type of material (such aluminum alloy or cast iron). For instance, when working with cast iron cylinder bodies, you may keep the thermal deformation within 0.03mm by setting the feed rate to f=0.2mm/r and the cutting speed to Vc=150m/min.
3. Managing stress inside: designing the procedure route and blank pretreatment
Internal tension is a major reason why parts change shape after they have been processed. Residual stress can be as high as 30% to 50% of the yield strength of quenched steel parts. This can cause the parts to bend or shatter while they are being stored or used. When it comes to internal tension, it needs to be controlled in the following two ways:
Optimizing the route of a process
More than 80% of residual stress can be removed by following the steps of "rough machining → stress relief annealing → precision machining." For instance, when working with crankshafts, stress relief annealing at 600–650 °C after rough machining, holding for 4–6 hours, and then precision machining to dimensional accuracy might stop them from changing shape when they are used again. For items with thin walls, the "symmetrical machining" method is used. This implies cutting one side to a given depth first and then flipping the other side to relieve the stress.
Preparing raw materials
Use forging, rolling, and other methods to make the blank structure more even and get rid of interior flaws. One company, for instance, employs "isothermal forging" technique to make connecting rod blanks. This technology refines the grain size to ASTM 6-8 level and cuts down on distortion by 70% after processing. Also, the blank grouping approach (grouping by size and hardness) can make the cutting allowance the same every time and cut down on mistakes.
