Manual Milling vs. CNC Milling: A Comprehensive Comparison
In the world of precision machining, choosing between manual milling and CNC milling can feel like navigating a complex maze.…
I. Common causes and solutions for milling machine faults
1. Large vibrations during milling
Large vibrations during milling can easily damage tools, increase the surface roughness of the workpiece, and prevent the use of climb milling and normal milling feed rates. The causes and solutions for large vibrations during milling are as follows.
(1) Spindle looseness
The main causes of spindle looseness are excessive spindle bearing clearance and pitting on the spindle bearing raceway. The former can be solved by adjusting the spindle bearing clearance, while the latter requires bearing replacement.
(2) Worktable looseness
The main cause of worktable looseness is that the worktable guide strip is too loose. It needs to be readjusted, and the tightness can be controlled using a 0.03mm feeler gauge. If the guide strip is not straight, it should be scraped or replaced.
(3) Large clearance between worktable leadscrew and nut
The solution is to readjust the clearance by adjusting the position of the adjustable nut through the worm gear, ensuring appropriate clearance between the leadscrew and nut, and tightening the fixing screws.
(4) Other factors
- If the machine foundation is poor, it should be rebuilt according to requirements.
- If the main motor vibrates excessively, dynamic balancing of the motor rotor should be performed.
- If the main drive gears move unevenly or produce loud noise, inspect and replace the faulty gears.
- If the milling cutter taper doesn’t match the taper hole or the milling cutter isn’t tightened properly, check and grind the taper, and tighten the milling cutter.
2. Uneven movement when manually cranking the worktable longitudinally
The main causes are leadscrew bending or partial wear, or misalignment between the leadscrew axis and longitudinal guide. The former requires straightening, repairing, or replacing the leadscrew; the latter requires reinstallation of the leadscrew and reaming of the positioning pin holes.
3. Crawling of the elevating table during low-speed elevation
The causes are that the column guide pressure bar is not loosened and poor lubrication. The former requires loosening and adjusting the pressure bar, while the latter requires maintaining proper lubrication.
4. Worktable rapid feed fails to start or disengage
The main reason for the worktable rapid feed failing to start is excessive clearance in the friction clutch. The clutch needs to be overhauled, and the electromagnet should also be checked for failure.
The main reasons for the worktable rapid feed failing to disengage are excessive residual magnetism in the electromagnet or insufficient spring force in the slow return spring. Electricians and mechanics should repair and adjust it. This fault is rare but very dangerous and should be given special attention.
5. Feed system safety clutch failure
There are two manifestations of feed system safety clutch failure: one is that the worktable stops feeding with slight resistance; the other is that the feed doesn’t automatically stop when overloaded.
The main reason for safety clutch failure is improper torque adjustment. The safety clutch should be readjusted to rotate at a torque of 157-196 N·m.
6. Poor spindle rotation or failure to start
If the spindle doesn’t stop within 0.5s after pressing the “Stop” button or even reverses, the cause is improper adjustment of the spindle braking device or relay failure in the electrical circuit. If the spindle fails to start after pressing the button and the motor makes a “humming” sound, it’s an electrical fault that needs to be repaired by mechanics and electricians.
7. Difficulty in engaging speed change gears
When changing spindle speed, if the speed change lever can’t be pushed to its original position, the cause is that the speed change micro switch hasn’t conducted. An electrician should check and repair it.
8. Failure of worktable horizontal and vertical feed control handles or manual and automatic interlock device
When the worktable horizontal and vertical feed control handles fail, there’s no horizontal or vertical feed, or even both directions feed simultaneously. The main causes are drum position changes or electrical switch rod position changes. Electricians and mechanics should adjust and repair it.
As for the failure of the horizontal and vertical manual and automatic interlock device, the main cause is the detachment of the driving lever or stop pin in the interlock device. Mechanics should be called for inspection and repair.
II. Safe operation of milling machines
Safe operation of milling machines includes the quality safety of workpiece processing as well as the safety of operators and equipment. To achieve both quality workpiece processing and safe production, milling machine operators must follow the “General Process Rules for Cutting Operations” (see Table 1) and “General Process Rules for Milling Operations” (see Table 2).
Table 1 General Process Rules for Cutting Operations
| Item | Main Rules | |
| Preparation before processing | 1) After receiving the processing task, the operator should first check if all required product drawings, process specifications, and related technical documents are complete 2) Understand and clearly read the process specifications, product drawings, and their technical requirements. If there are any questions, consult relevant personnel before proceeding with processing 3) Verify if the workpiece blank or semi-finished product meets the requirements according to the product drawing and/or process specification. If any issues are found, report them to relevant personnel immediately and only proceed with processing after the issues are resolved 4) Prepare all required process equipment according to the process specification and address any issues promptly. For new fixtures or molds, familiarize yourself with their usage requirements and operation methods first 5) Place the process equipment in designated positions, do not misplace them, and especially do not place them on machine tool guideways 6) Do not disassemble or modify process equipment arbitrarily 7) Check the machine tool equipment to be used for processing and prepare all necessary accessories. Before processing, lubricate the machine tool and perform a dry run as specified | |
| Clamping of tools and workpieces | Tool clamping | 1) Before clamping any tools, always clean the tool shank, tool holder, guide sleeve, etc. 2) After clamping the tool, check its correctness using a tool setting device or test cut |
| Workpiece clamping | 1) When installing fixtures on the machine tool worktable, first clean the locating base surface and align its position relative to the tool 2) Before clamping the workpiece, clean the locating surface, clamping surface, shims, and the locating and clamping surfaces of the fixture, ensuring there are no burrs 3) Clamp according to the locating datum specified in the process specification. If not specified, the operator can choose the locating datum and clamping method. Select the locating datum according to the following principles: ① Try to make the locating datum coincide with the design datum ② Try to use the same locating datum for all machining surfaces ③ For rough machining, choose a flat, unprocessed surface or a surface with minimal machining allowance as the locating datum, and use it only once ④ For finish machining operations, the locating datum should be an already machined surface ⑤ The chosen locating datum must allow for convenient positioning and clamping of the workpiece, ensuring stability and reliability during machining 4) For workpieces without specialized fixtures, follow these principles when aligning: ① For scribed workpieces, align according to the scribe lines ② For unscribed workpieces, if the surface requires further processing in subsequent operations, ensure sufficient machining allowance for the next operation when aligning ③ For surfaces machined to final dimensions in this operation, the alignment accuracy should be less than 1/3 of the dimensional tolerance and position tolerance 5) When clamping assemblies, pay attention to check the positioning of mating surfaces 6) When clamping workpieces, the clamping force should act through support points or surfaces. For workpieces with low rigidity (or with overhanging parts during machining), add auxiliary supports at appropriate positions to enhance rigidity 7) When clamping precision-machined surfaces or soft material workpieces, use soft pads such as pure copper foil 8) When using pressure plates to clamp workpieces, the support point of the pressure plate should be slightly higher than the workpiece surface, and the clamping bolts should be as close to the workpiece as possible to ensure clamping force | |
| Processing requirements | 1) To ensure processing quality and improve productivity, choose appropriate cutting parameters based on workpiece material, precision requirements, and machine tool, cutting tool, and fixture conditions. When machining castings, to avoid damaging tools due to surface sand inclusions or hardened layers, the depth of cut should be greater than the depth of sand inclusions or hardened layers when conditions allow 2) For dimensions with tolerance requirements, try to machine to the middle of the tolerance range 3) For rough machining operations where surface roughness is not specified in the process specification, the machined surface roughness Ra should not exceed 25μm 4) The surface roughness Ra before reaming should not exceed 12.5μm 5) The surface roughness Ra before precision grinding should not exceed 6.3μm 6) During rough machining, chamfers, rounds, and groove depths should be machined or deepened according to finish machining allowances to ensure design requirements are met after finish machining 7) For workpiece surfaces that require surface hardening, ultrasonic testing, or roller burnishing in subsequent operations, the surface roughness Ra machined in this operation should not exceed 6.3μm 8) If deburring cannot be performed in subsequent operations, burrs produced in this operation should be removed in this operation 9) During the machining of large workpieces, regularly check if the workpiece has loosened to prevent quality issues or accidents due to loosening 10) When rough and finish machining are performed on the same machine tool, generally loosen the workpiece after rough machining, allow it to cool, and then reclamp it 11) If abnormal sounds occur or surface roughness suddenly worsens during the cutting process, immediately retract the tool, stop the machine, and inspect 12) In batch production, the first piece must be inspected, and only after it passes inspection can production continue 13) During the machining process, operators must perform self-inspection of workpieces 14) When inspecting, use measuring instruments correctly. When using gauges, micrometers, etc., gently push or turn them in, avoiding excessive force. When using calipers, micrometers, dial indicators, etc., adjust the zero position beforehand | |
| Post-processing handling | 1) After each operation, workpieces should be free of chips, water, and dirt, and neatly arranged in designated fixtures to avoid dents, bumps, or scratches 2) Surfaces that will not undergo further processing immediately or have been finish machined should be treated for rust prevention 3) Workpieces held by magnetic fixtures during machining should be demagnetized after processing 4) For related parts processed in groups, markings (or numbering) should be made after processing 5) Workpieces completed in each process can only be transferred to the next process after being inspected and approved by dedicated inspectors 6) After using process equipment, it should be wiped clean (apply rust-preventive oil), and returned to the designated location or tool storage 7) Product drawings, process specifications, and other technical documents used should be kept clean and neat, with modifications strictly prohibited | |
Table 2 General Process Rules for Milling
| Item | Main Rules |
| Selection and Clamping of Milling Cutters | 1) Selection of milling cutter diameter and number of teeth ① The diameter of the milling cutter should be selected based on the milling width and depth; generally, the larger and deeper the milling width and depth, the larger the cutter diameter should be ② The number of teeth on the milling cutter should be selected based on the workpiece material and processing requirements; generally, for milling plastic materials or rough machining, choose coarse tooth milling cutters; for milling brittle materials or semi-finishing and finishing, choose medium or fine tooth milling cutters 2) Clamping of milling cutters ① When clamping a milling cutter on a horizontal milling machine, keep the cutter as close to the spindle as possible without affecting processing, and the support bracket close to the cutter. If the cutter needs to be far from the spindle, an auxiliary support bracket should be installed between the spindle and the cutter ② When clamping a milling cutter on a vertical milling machine, choose a short tool holder whenever possible without affecting milling ③ After clamping the milling cutter, if necessary, use an indicator to check the radial and end face runout of the cutter ④ When milling wide flat surfaces with two cylindrical milling cutters simultaneously, choose two cutters with opposite spiral directions |
| Workpiece Clamping | 1) Clamping on a machine vise ① Ensure the correct position of the machine vise on the worktable; if necessary, use an indicator to align and fix the vise jaw face to make it parallel or perpendicular to the machine table movement direction ② Place parallel spacers of appropriate thickness under the workpiece, and when clamping, ensure the workpiece is tightly pressed against the parallel spacers ③ The workpiece should not protrude too much above the vise jaws or extend too far from the ends of the jaws to prevent vibration during milling 2) Requirements for using a dividing head (clamping) ① When clamping a workpiece on the dividing head, first lock the dividing head spindle. When tightening the workpiece, it is forbidden to use a pipe on the handle to apply force ② After adjusting the tilt angle of the dividing head spindle, tighten the four screws on the base to prevent zero position movement ③ When clamping shaft-like workpieces between the two centers of the dividing head, ensure that the centerlines of the front and rear centers align ④ When indexing with the dividing head, the indexing handle should be rotated in one direction. If the position is overshot, rotate back more than the exceeded distance and then rotate to the correct position to eliminate backlash ⑤ When indexing, slowly insert the positioning pin on the handle into the hole of the indexing plate. Do not release suddenly to avoid damaging the indexing plate |
| Milling Process | 1) After adjusting the machine before milling, lock the unused movement directions 2) When approaching the workpiece with rapid feed, switch to normal feed rate to prevent tool and workpiece collision 3) When milling helical grooves, first conduct a trial run with the calculated gears (change gears) to check if the lead and spiral direction are correct, then proceed with processing 4) When milling with form cutters, to improve tool life, the cutting parameters should generally be about 25% less than those for cylindrical cutters 5) When profile milling using the copying method, maintain good contact between the roller and the template, but avoid excessive pressure to allow the roller to rotate freely 6) When cutting off, the milling cutter should be as close to the fixture as possible to increase stability during cutting 7) Selection of climb milling and conventional milling ① Conventional milling is recommended in the following situations: When the backlash between the milling machine table leadscrew and nut is large and cannot be easily adjusted; when the workpiece surface has a hard layer, scale, or uneven hardness; when the workpiece surface is significantly uneven; when the workpiece material is too hard; when step milling; when the cutting depth is large ② Climb milling is recommended in the following situations: When milling workpieces that are difficult to clamp securely or are thin and long; for precision milling; when cutting materials like bakelite, plastics, or acrylic glass In addition, the following points must be noted: |
1) Before starting the milling machine, first check if all handles are in the correct position, lubricate all sliding parts, then run at low speed for 3-5 minutes. Only start working after confirming that all parts are operating normally.
2) When loading/unloading workpieces or measuring dimensions, the workpiece must be moved away from the milling cutter position and the machine must be stopped; however, do not stop the machine before the workpiece has moved away from the tool.
3) When loading/unloading heavy workpieces, choose reliable lifting equipment and methods, and ensure the workpiece is securely clamped.
4) Correctly install cutting tools and regularly check for tool wear and tightness.
5) All sliding surfaces of the milling machine should be clean and free of objects. Installation reference surfaces such as the spindle taper hole and worktable surface should be clean and free of scratches, and regularly lubricated.
6) It is forbidden to heavily strike workpieces on the machine or to step on the machine table surface or place objects that could damage the table surface.
7) When changing spindle speeds on the milling machine, generally avoid changing more than three times continuously. Otherwise, each speed change should be done with a 5-minute interval to prevent damage to the motor circuit due to excessive starting current.
8) When manually feeding the worktable in longitudinal, transverse, and vertical directions, if the required distance in a certain direction is exceeded, do not directly return to the required scale. Instead, turn the handle back one revolution and then rotate it again to the required value.
9) Concentrate while working, keep your head and hands away from rotating cutting tools, do not touch the milling cutter with your hands, and never use your hands to brake.
10) Do not wear gloves while operating; those with long hair must wear safety helmets. Do not remove chips directly with your hands or blow them away with your mouth; use a soft brush to gently sweep them away.
When milling at high speeds, wear safety goggles to prevent high-speed flying chips from damaging your eyes. When milling cast iron workpieces, it’s best to wear a mask to prevent inhaling chip powder through your mouth and nose.
11) Pay attention to the operation of all parts at all times. If any abnormality is detected, stop the machine immediately, inspect, and eliminate the fault.
12) When leaving the machine, always disconnect the power supply; before ending work, ensure the worktable is positioned in the middle of each feed direction.
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