What is facing machining? How can lathes/milling machines achieve high-precision facing machining?

In the field of mechanical manufacturing,facing machining is one of the key processes that determine the assembly accuracy and performance of parts. Whether it is the bearing mounting end face of shaft parts, the flange connection end face of disc parts, or the sealing end face of box parts, the machining quality directly affects the stability, sealing and fatigue life of the equipment. However, different processing equipment and processes have significant differences in principles, precision control and application scenarios. This article will systematically analyze the core concepts of facing machining, and deeply compare the process characteristics, operation points and applicable scenarios of lathes and milling machines in facing machining, so as to provide industry-related personnel with a clear basis for process selection.

1. What is facing machining?

Facing machining is the process of machining the plane (called "end face") on the part that is perpendicular to the axis (or at a specific angle) through cutting, grinding and other processes to make it meet the technical requirements of dimensional accuracy, flatness, surface roughness, verticality, etc.

(1) Core purpose:

● Precision control: ensure the distance from the end face to the reference plane (dimensional accuracy), the flatness/straightness of the end face itself (shape accuracy), as well as the perpendicularity between the end face and the axis, and the parallelism with the adjacent plane (position accuracy).

● Surface quality: reduce surface roughness (such as Ra≤1.6μm), eliminate defects such as burrs and knife marks, and improve the assembly performance and service life of parts.

(2) Application scenarios:

● Shaft parts: both end faces of motor shafts and transmission shafts (for bearing installation, verticality must be guaranteed).

● Disk parts: end planes of gears and flanges (for bolt connection or sealing, flatness must be guaranteed).

● Box parts: top surface of engine cylinder block, end face of gearbox housing (sealing surface, high-precision flatness required).

2. How to perform facing machining on a lathe?

(1) Processing principle

● Main motion: the workpiece rotates with the lathe spindle (clockwise or counterclockwise).

● Feed motion: The end turning tool feeds along the horizontal (X axis) straight line, and forms a plane through the cutting trajectory of the blade.

● Processing range: Mainly used for the outer end faces of rotationally symmetrical parts (such as shaft ends and outer cylindrical end faces of disk-type parts). In a few cases, the inner end faces can be processed (such as deep hole end faces, which require special boring tools).

(2) Equipment type:

● Ordinary lathe (such as CA6140): suitable for single-piece small batches, accuracy IT10-IT8, surface roughness Ra 6.3-1.6μm.

● CNC lathe (such as Fanuc Oi-T system): suitable for precision machining, accuracy IT8-IT6, Ra 1.6-0.8μm, supports automatic tool setting and parameter programming.

(3) Clamping method:

● Three-jaw chuck: automatic centering, suitable for cylindrical workpieces (positioning accuracy ±0.02mm).

● Four-jaw chuck: manual adjustment, suitable for eccentric or irregular workpieces.

● Center + chuck: Long shaft parts (such as length > 5 times the diameter), reduce bending deformation during rotation.

(4) End face turning tool type:

● Right-side tool: The main rake angle is 90°, used to process the right end face (close to the chuck side) to avoid direct force on the tool tip.

● Left-side tool: The main rake angle is 95°, used to process the left end face (free end of the shaft) to reduce the end face boss.

● End face boring tool: Processing large diameter inner end faces (such as the inner hole end face of flange with a diameter > 300mm).

(5) Critical angles:

● Rake angle (γ₀): affects cutting force and tool life (γ₀=10°-15° for high-speed steel tools, γ₀=5°-10° for cemented carbide tools).

● Main rake angle (κᵣ): 90° The main rake angle can make the radial cutting force zero and avoid leaving a boss in the center of the end face.

● Blade inclination angle (λₛ): Control the chip flow direction (finishing λₛ=0°, chips are discharged to the surface to be processed; roughing λₛ=-5°, enhance the strength of the tool tip).

(6) Process parameters

(7) Processing steps

● Tool setting: Determine the Z axis zero point (the starting point of facing machining), and usually use the trial cutting method (measure the size after turning the end face and enter the machine tool coordinate system).

● Roughing: Quickly remove the allowance, leave 0.5-1mm finishing allowance, and pay attention to avoid excessive tool wear.

● Finishing: Reduce the feed rate and back cutting amount to ensure surface accuracy. The last cut uses "finishing processing" (no feed light cutting to eliminate tool marks).

● Inspection: Use a micrometer to measure the size, a knife edge ruler to check the flatness, and a dial indicator to check the end face runout (verticality).

(8) Common problems and solutions

● End face boss: tool tip wear or main deflection angle < 90°, replace with a new tool and adjust the tool angle to 90°.

● Chattering: spindle bearing is loose or workpiece is not clamped tightly, tighten the bearing or increase the chuck clamping force (≥500N).

● Roughness tolerance: cutting speed is too low (<50m/min), increase the speed or replace with a sharp tool.

3. How to perform facing machining on a milling machine?

(1) Processing principle

● Main motion: milling cutter rotates at high speed (multi-blade cutting).

● Feed motion: the workpiece is fed linearly along the X/Y axis with the worktable, and a plane is formed by the envelope trajectory of the milling cutter blade.

● Processing range: suitable for external end faces, internal end faces, step faces, irregular end faces (such as complex end faces of box-type parts), especially suitable for large plane processing.

(2) Equipment type:

● Vertical milling machine (such as X5032): The spindle is vertical, suitable for processing vertical end faces and step surfaces.

● Horizontal milling machine (such as X6132): The spindle is horizontal, suitable for processing horizontal end faces of long shaft parts.

● Machining center (such as Mazak VCN-530C): The CNC system supports automatic tool change, with an accuracy of IT8-IT6, suitable for high-precision complex end faces.

(3) Clamping method:

● Flat-nose pliers: For small parts, the verticality of the pliers and the spindle needs to be corrected (error ≤0.02mm/m).

● Pressure plate bolts: For large parts, the clamping force is evenly distributed to avoid deformation of the workpiece.

● Square ruler positioning: Ensure that the end face is perpendicular to the reference plane (such as using a 90° square to fit the reference edge of the workpiece).

(4) Types of face milling cutters:

● Face milling cutter (disc milling cutter): 50-300mm in diameter, with carbide inserts, used for large-area facing machining (such as the top surface of the engine cylinder block).

● End milling cutter: 10-50mm in diameter, with both end and peripheral cutting edges, suitable for small face or step surfaces (such as the end surface of the gearbox).

● Indexable milling cutter: with replaceable inserts (such as Sandvik R390 series), suitable for high-speed processing of aluminum alloy, stainless steel and other materials.

(5) Key parameters:

● Milling cutter diameter (D): select D>1.5 times the processing width to reduce the cutter mark (such as processing a 100mm wide face, select D=160mm).

● Number of teeth (z): coarse teeth (z=4-6, large chip space, suitable for rough milling); fine teeth (z=8-12, smooth processing surface, suitable for fine milling).

● Main deflection angle (κᵣ): 90° main deflection angle milling cutter processes right-angle end faces, 45° main deflection angle milling cutter can reduce axial cutting force (suitable for machine tools with insufficient rigidity).

(6) Processing parameters

(7) Processing steps

● Tool setting: Use edge finder to determine the X/Y axis zero point (workpiece reference edge), and the Z axis zero point is usually set at the highest point of the end face (to avoid tool collision).

● Rough milling: Divide the tool into 2-3 passes, remove 80% of the excess, and the milling width each time is ≤ 2/3 of the milling cutter diameter (reduce tool load).

● Fine milling: Use down milling (the tool rotation direction is consistent with the workpiece feed direction, and the surface is smoother). A single pass covers the entire end face, and the overlap amount is ≥ 20% to eliminate tool marks.

● Inspection: Use a feeler gauge to detect flatness (insertion gap ≤ 0.01mm), use a roughness meter to measure the Ra value, and use a dial indicator to detect the verticality of the end face and the reference surface.

(8) Common problems and solutions

● Obvious cutter marks: The milling cutter diameter is too small or the overlap between two passes is insufficient. Increase the milling cutter diameter or set the overlap to ≥30%.

● Poor surface roughness: The milling cutter swing is too large (>0.02mm). Use a dial indicator to correct the tool runout during installation (≤0.01mm).

● Dimension out of tolerance: The worktable screw clearance is not compensated. Enable the reverse clearance compensation function of the CNC system (compensation amount ≤0.01mm).

4. Summary: Lathe vs. milling machine facing machining

Choose the appropriate processing method based on the part shape, precision requirements and production batch. Lathes are suitable for precision small end faces, and milling machines are suitable for large planes or complex end faces. The two together constitute the core technology of facing machining in mechanical processing.