Grade-45 steel (GB/T 699-2015 high-quality carbon structural steel, carbon content 0.42%–0.50%, tensile strength 600MPa, hardness HB201) is the most widely used medium-carbon steel material for mold and die structural components. Before CNC machining, raw block blanks must undergo systematic preprocessing—including surface oxide scale inspection, blank dimensional verification, hard spot detection, dual-face milling datum establishment, and properly calculated stock allowance. This guide covers the complete pre-processing workflow for 45 steel blocks (thickness 20–200mm) prior to CNC machining, helping process engineers establish standardized work procedures to reduce positioning errors and out-of-tolerance machining caused by blank defects.
Check Raw Blocks
Surface Scale
Hot-rolled or forged 45 steel blanks are typically covered with a layer of oxide scale (a mixture of FeO, Fe₂O₃, and Fe₃O₄) ranging from 0.1 to 0.3mm in thickness. This oxide scale accelerates tool wear during subsequent machining—measured data shows that untreated oxide scale surfaces reduce cemented carbide tool life by approximately 23%, because the scale hardness reaches HV500–800, far exceeding the 45 steel substrate hardness (HB201), and the uneven surface creates impact loads during entry. Even more critically, the gap between oxide scale and substrate causes uneven carburizing/nitriding during heat treatment, compromising final hardness uniformity across the part.
Oxide scale inspection follows a two-step protocol: Step one is visual inspection—use a wire brush to locally remove oxide scale and examine the substrate surface for cracks, folds, or inclusions. Common 45 steel defects include forging folds (depth exceeding 0.5mm requires evaluation for usability) and raw material cracks (linear defects in any direction mandate rejection). Step two uses an ultrasonic thickness gauge to measure oxide scale thickness. Typical hot-rolled oxide scale ranges from 0.05 to 0.25mm; if single-face scale thickness exceeds 0.3mm, communicate with the supplier to improve rolling or request annealing treatment. Crucially, blanks with heavy oxide scale require increased depth of cut on first milling (recommend removing at least 0.5mm per side), which directly affects the blank's initial stock calculation.
A practical case from an automotive transmission housing manufacturer illustrates the cost of neglecting scale inspection: their 45 steel blanks (200 pieces per batch) showed heavy mill scale averaging 0.22mm per face. Operators set first-face depth of cut to 0.5mm without accounting for the thick scale layer, leaving residual oxide islands embedded in the surface after milling. When the CNC finish-milled these workpieces, carbide inserts encountered hard oxide inclusions and shattered prematurely—one insert set lasted only 12 workpieces instead of the normal 80. The root cause investigation revealed that the 0.5mm cut had only grazed the tops of 0.22mm scale peaks, leaving scale valleys intact. The corrective action was to increase first-face depth of cut to 0.8mm for all future batches with similar scale thickness, which successfully eliminated insert fracture incidents. This case demonstrates why a two-step inspection protocol—visual plus ultrasonic thickness measurement—is essential before setting machining parameters.
A secondary but equally important consideration is oxide scale removal method. Abrasive blasting (shot peening with fine steel grit, 0.3–0.8mm particle size) is the most effective industrial method for removing heavy mill scale from large 45 steel forgings, achieving 95–98% scale removal efficiency in a single pass and exposing the substrate for visual inspection. Mechanical grinding with a angle grinder and wire brush wheel is suitable for localized scale removal on smaller blanks but introduces surface scratch marks of 0.05–0.15mm depth that must be accounted for in RSA calculations. Chemical pickling (immersion in 10–15% sulfuric acid solution at 60–70°C for 10–30 minutes) is effective for complex shapes but requires neutralization and anti-corrosion treatment afterward, adding process time and environmental compliance cost. The choice of removal method directly affects the surface condition entering duplex milling and therefore must be specified in the preprocessing specification document.
Block Size
Blank dimensional deviation directly affects CNC machining process planning and final yield rate. For 45 steel hot-rolled flat steel, thickness tolerance per GB/T 709-2019 is ±0.4mm for thickness 20–60mm (standard precision) and ±0.8mm for thickness 60–120mm. Taking a nominally sized 45 steel blank of 200mm × 150mm × 80mm as an example, actual thickness may fall between 79.2–80.8mm (higher precision grade)—this ±0.8mm deviation causes inconsistent depth of cut parameters during CNC finish machining, affecting dimensional accuracy and surface quality consistency.
Dimensional verification upon receipt uses vernier calipers (accuracy 0.02mm) or digital calipers to measure length, width, and height at three points each, recording maximum and minimum values. For batch production, establishing a blank dimensional distribution log to track supplier consistency is recommended. When blank thickness deviation exceeds the reserved allowance specified in the process documentation (typically nominal size +0.5mm/-0.3mm per side), communicate with the programming engineer in advance to adjust depth of cut settings in the machining program, or classify that batch separately. Additionally, blank flatness deviation must be monitored—hot-rolled blanks typically exhibit flatness of 0.3–1.0mm/1000mm; severely warped blanks create positioning gaps when clamped on vacuum or magnetic worktables, causing datum tilt and machining out-of-tolerance.
A die base plate manufacturer processing 45 steel blanks (dimensions 600mm×400mm×100mm, batch size 50 pieces) discovered that 30% of incoming blanks from a new supplier had thickness deviation exceeding ±1.0mm—far beyond their ±0.5mm stock allowance tolerance. Without proper incoming inspection, these blanks would have required two additional rough milling passes to correct thickness before entering the CNC program, adding 45 minutes of auxiliary milling time per workpiece and exceeding the daily production schedule by 37%. After implementing a mandatory incoming inspection protocol with 100% dimensional logging, the quality team identified the root cause: the supplier's rolling mill rollers were overdue for replacement, causing progressive thickness drift. The corrective action—a supplier corrective action request (SCAR)—resulted in the supplier replacing their rollers within two weeks, restoring blanks to ±0.4mm tolerance. This case demonstrates that dimensional inspection of raw blanks is not merely a receiving QC step but a critical yield and schedule protection mechanism.
Beyond thickness, length and width tolerances also affect machining economics. When blank length deviation exceeds nominal by more than +1.0mm, the CNC program must account for the extra material by increasing rapid traverse paths, extending cycle time by 3–8% depending on the part geometry. Width deviations on the other hand affect the clamping margin—if the blank is 1.5mm narrower than nominal, it may not fit the fixture pocket designed with only 0.5mm clearance per side, requiring shimming or fixture modification before setup can proceed. A practical tolerance allocation approach is to specify the blank tolerance as one-third of the finished part tolerance for critical dimensions, and one-half for non-critical dimensions. For a part with finished thickness tolerance of ±0.06mm, the blank tolerance should be specified as ±0.02mm per side, which typically requires requesting the higher precision grade from the steel mill at a 12–18% premium—but this premium is far offset by the reduction in CNC setup adjustments and rework.
Hard Spots
Hard spots (also called hard inclusions or carbide segregation) are among the most dangerous hidden defects in 45 steel blanks. There are two formation mechanisms: first, MnS inclusion aggregation during steelmaking, and second, carbide alignment along the rolling direction during rolling (banded structure). When banded structure is severe (rating exceeding GB/T 13299 Grade 3), local carbon content can reach above 0.8%, causing that region to spike to HRC55–60 hardness while adjacent areas remain at normal HRC15–20—a hardness differential exceeding 40HRC. This severe non-uniformity causes abnormal tool wear or even insert fracturing during CNC machining, because cutting force in hard spot regions reaches 3–4× that of normal regions, sufficient to generate crater wear on ordinary coated cemented carbide inserts within seconds.
Three standard methods detect hard spots: First, Macro Hardness Survey—place hardness indents on the blank surface on a 10mm grid, record abnormally high readings, and flag regions exceeding HB250 as suspected hard spots. Second, ultrasonic testing uses the acoustic impedance difference between hard spots and substrate (carbide-rich zone acoustic velocity approximately 6.5km/s versus 45 steel substrate 6.0km/s) to detect internal defects, suitable for blanks over 20mm thickness. Third, post-annealing metallographic analysis—heating a sample to 850°C, holding 1 hour, air cooling, then examining microstructure banded structure grade. The most practical workshop-level detection uses a portable Leeb hardness tester (accuracy ±5HL) scanning 5 points on each of the six blank faces; when any point exceeds 30% above the average value, retest to confirm. If two consecutive points exceed the threshold, downgrade that blank or return it to the supplier.
One mold frame manufacturer (45 steel, dimensions 1,000mm×600mm×150mm, batch 12 pieces) experienced three consecutive insert fracture incidents during rough milling of the same workpiece batch, causing USD 2,400 in tooling damage plus 6 hours of downtime. The root cause investigation involved sectioning one damaged blank and performing metallographic analysis, which revealed banded structure rating of GB/T 13299 Grade 4—far exceeding the acceptable Grade 2 threshold. The affected blanks originated from a single heat batch from a new raw material lot. The corrective action was to establish a mandatory Leeb hardness survey protocol for all incoming blanks exceeding 80mm thickness: any blank with hardness variation exceeding 25% across the measurement grid is rejected and returned. This protocol has now been in place for 18 months with zero insert fracture incidents attributable to hard spots, validating the investment in incoming hardness inspection.
The economic threshold for hard spot inspection deserves explicit analysis. The cost of a Leeb hardness survey on one blank (labor 5 minutes at CNY 0.83 plus instrument depreciation CNY 0.17) totals approximately CNY 5 per blank. The cost of one spindle damage incident from hitting a hard spot ranges CNY 8,000–80,000 in repair plus lost production. For any shop running more than 100 blanks per year, the expected value analysis clearly favors the inspection investment. Conversely, for job shops machining fewer than 20 blanks per year, selective inspection (e.g., only the first and last blank of each heat batch) may be a practical risk-based compromise, provided blanks come from a qualified supplier with metallurgical certification and a documented hard spot rejection rate below 0.5%.
Mill Two Faces
First Flat Face
The first step in duplex milling is machining the first flat face—the datum plane that establishes all subsequent machining references. Which face should be selected as the first datum face? The principle: prioritize machining the top surface of the blank (typically the least flat, thickest oxide scale, and likely bearing scratches and indentation marks from the rolling process). The machining quality of this face directly determines the workpiece's clamping stability on the CNC machine table—if the first face has center convexity (center higher than edges), contact with the table occurs only at the edges, with actual contact area potentially falling below 30% of the theoretical contact area. Under cutting forces (side milling thrust reaching 800–1,500N), this causes elastic tool deflection.
First-face milling parameters: using a φ63mm face mill on 45 steel, cutting speed Vc=180–200m/min (spindle speed 910–960rpm), feed F=1,000–1,200mm/min, feed per tooth 0.15–0.2mm, depth of cut ap=0.5–1.0mm (removing oxide scale and surface defect layer). For workpieces exceeding 100mm thickness, two-stage rough and finish milling is recommended: rough milling at ap=2.0mm to establish the large flat plane, followed by finish milling at ap=0.3mm to improve surface accuracy. After finish milling, flatness must reach 0.02mm/300mm or better, with surface roughness Ra≤1.6μm. After the first face is milled, measure flatness with a dial indicator—if out of tolerance, continue finish milling to remove stock. Never proceed to the next operation with center convexity error present.
The economic consequence of skipping first-face flatness verification is illustrated by a mold base plate manufacturer who machined 8 pieces of 45 steel (400mm×300mm×100mm) before checking flatness. All 8 workpieces showed center convexity ranging 0.06–0.12mm/300mm due to a worn machine table insert on the milling center. When these workpieces were loaded onto the CNC vacuum table, the center gap caused insufficient vacuum seal pressure at 600mm Hg, and two workpieces shifted during rough milling of the cavity pocket—resulting in a 0.35mm positional error that scrapped both parts. The total cost of this incident (two scrapped parts plus 16 hours of re-machining) was CNY 18,400, compared to the CNY 200 cost of a simple dial indicator flatness check that would have caught the problem before CNC loading. This case is now used in the shop's training program as a mandatory example of why first-face flatness inspection is non-negotiable.
Opposite Face
Milling the opposite face relative to the first is the critical step in establishing two parallel datum planes. These two parallel faces constitute the most important datum system for mold structural components—they simultaneously serve as the thickness direction (Z-axis) process datum and measurement datum. Parallelism error between them transfers at a 1:1 ratio to all subsequent cavity depth dimensions. Therefore, machining the second face is not merely material removal—it is the core precision control operation. Second-face stock calculation before milling is critical: based on the measured thickness after first-face milling, combine with nominal finished thickness and finish milling allowance (0.3–0.5mm) to determine the rough milling depth of cut.
After rough milling the second face, measure the distance between both faces with a micrometer (three measurements at the same point, average) and calculate parallelism error. If parallelism exceeds 0.03mm/300mm, immediately adjust cutting parameters (reduce feed, increase number of cuts) and re-rough mill to correct. During finish milling, a symmetric milling strategy (alternating entry from both ends of the workpiece) is recommended to eliminate machine screw backlash effects on parallelism. After finish milling, the flatness of both parallel faces must reach 0.01mm/300mm or better with surface roughness Ra≤0.8μm.
A precision die-set manufacturer requiring parallelism of 0.005mm/300mm for 45 steel die sets used in stamping operations discovered that their two-station duplex milling setup was producing parts with parallelism ranging 0.008–0.015mm/300mm—well outside specification. Root cause analysis revealed that the machine tool's spindle thermal drift during the second-side milling (after 45 minutes of continuous first-side milling) was causing the spindle to lift by 0.006–0.009mm, producing systematic second-side thinness. The corrective action was to implement a mandatory 10-minute thermal soak period between first-side completion and second-side start, allowing the spindle and workpiece to reach thermal equilibrium. After implementing the soak period, parallelism settled consistently within 0.003–0.006mm/300mm, meeting the IT5 requirement. This case illustrates that second-face parallelism is not solely a geometric accuracy issue—thermal management during the milling sequence is equally critical for achieving tight parallelism tolerances.
Advanced toolpath strategies can further improve second-face parallelism beyond what symmetric milling alone achieves. Climb milling (down milling) is preferred for the second face because the cutting force direction pushes the workpiece against the workholding surface throughout the cut, minimizing lifting tendency at the entry edge. Conventional milling creates a lifting force at entry that can cause the workpiece to lift slightly before the insert fully engages, producing an entry-edge thickness error that compounds across the full workpiece length. For workpieces exceeding 500mm in length, a two-pass strategy with 50% step-over overlap eliminates stepmarks at the pass junction, and the second pass should run in climb milling mode to consolidate all entry effects within the overlap zone rather than distributing them across the full workpiece. Additionally, climb milling reduces cutting temperatures by 15–20% compared to conventional milling because chip evacuation is more efficient, which further reduces thermal distortion of both the workpiece and the machine spindle during the critical second-face pass.
Keep Parallel
Maintaining dual-face parallelism involves not only milling parameters but also workpiece clamping strategy and precision monitoring methods. In batch production, blank thickness deviation varies piece by piece—after milling the first face of each blank, measured thickness may distribute between 79.5–81.2mm. If the second face is milled using a uniform depth of cut, the finished product thickness dispersion will completely replicate the original blank thickness variation. Therefore, high-precision duplex milling must employ a "measure-adjust-mill" closed-loop mode: after rough milling each first face, measure actual thickness, dynamically adjust second-face depth of cut based on measurements rather than using a fixed machining program.
The closed-loop adjustment procedure: Step 1, immediately after rough milling use a digital micrometer (accuracy 0.001mm) to measure thickness at five points (four corners plus center), recording maximum, minimum, and parallelism deviation; Step 2, judge whether deviation falls within process requirements—if out of tolerance, modify the CNC program's Z-axis stock allowance and re-rough mill; Step 3, re-measure after finish milling for verification. One hydraulic valve block batch production line (200 pieces/day, material 45 steel, thickness tolerance requirement ±0.02mm) using this method compressed thickness dispersion from ±0.18mm to ±0.012mm, raising yield rate from 89% to 99.2%.
For higher precision requirements (±0.005mm), use a coordinate measuring machine (CMM) in a constant-temperature workshop (20±1°C), placing the workpiece for 30+ minutes to eliminate temporary deformation from milling heat before final measurement. An aviation components manufacturer requiring 45 steel structural members (tolerance ±0.005mm on thickness, IT4 grade) implemented a full closed-loop duplex milling system combining in-process laser thickness measurement (resolution 0.001mm, update rate 100Hz) with real-time CNC program adjustment. This system reduced thickness variation from ±0.025mm to ±0.004mm, achieving a first-pass yield of 99.6% on a 500-piece batch—eliminating the need for costly post-machining grinding operations that previously added CNY 180 per workpiece in processing cost.
Beyond closed-loop thickness control, clamping force monitoring is an underutilized but highly effective method for maintaining parallelism in production runs. Using strain gauge sensors embedded in the fixture base (sampling rate 1kHz), the clamping force waveform during initial clamping can reveal workpiece flatness defects before the cut begins—if the force curve shows a sudden drop at 40% of full clamping pressure (indicating the workpiece has bottomed out at a high point while edges are still floating), the operator can stop and re-shim before any cutting occurs. One heavy-duty plate machining facility (45 steel, 800mm×600mm×120mm, 30 pieces per day) integrated clamping force monitoring into their duplex milling cell and reduced parallelism-related rework by 73%, because the monitoring system catches flatness defects before they become costly cutting errors. The ROI calculation for a CNY 8,000 clamping force monitoring system (installed on one milling machine) showed full payback within 4 months through reduced rework alone.
Leave Safe Stock
Rough Stock
Rough Stock Allowance (RSA) is the metal layer reserved on the blank surface before CNC rough machining. Its purpose is to eliminate errors from heat treatment deformation, fixture deflection, and tool wear that occur during rough machining, while providing a uniform and consistent machining datum for finish machining. For 45 steel forgings, RSA standards per JB/T 9161.3-2014: for thickness 20–100mm forgings, single-face RSA is 1.5–3.0mm; for thickness 100–200mm, RSA is 2.0–4.0mm. Note: this is the allowance from blank to rough-machined state, not including finish machining allowance—total reserve = RSA + FSA.
Hidden dangers of excessive RSA are often overlooked: every 1mm of additional stock means extra material removal cost (milling time, tool wear, electricity), plus increased chip handling burden. More importantly, oversized RSA extends rough machining stroke length, increasing machine spindle thermal deformation and cutting fluid temperature rise—these factors combined can actually reduce dimensional accuracy after rough machining. Conversely, undersized RSA (below minimum) poses more direct harm: if local hard spots or inclusions are exposed during rough machining, the insert absorbs impact loads—light cases cause insert fracture, severe cases damage the spindle.
A gear box housing manufacturer mistakenly set RSA from 2.5mm to 1.2mm in a cost-reduction initiative; when machining a batch of 45 steel flanges they encountered banded structure hard spots, insert fracture occurred, and an 80,000 CNY spindle was destroyed. The lesson: RSA is not arbitrary but is derived from statistical analysis of material property variation. A data-driven approach to RSA setting uses a minimum of 30 measurements of incoming blank hardness variation to calculate the minimum RSA that covers 99.7% of all cases (three standard deviations). For the 45 steel from their previous supplier with hardness variation within ±12HB, RSA of 1.8mm was sufficient; for the new supplier with ±22HB variation, RSA needed to be increased to 2.5mm—verified by zero hard spot incidents in 18 months of production following this adjustment.
Dynamic RSA adjustment based on real-time cutting load monitoring represents the next level of RSA optimization. Modern CNC systems with adaptive control (such as SIEMENS Sinumerik Adaptive Control or Mazak Smart Function) can monitor spindle load in real time and automatically adjust feed rate when cutting resistance increases, which is a reliable proxy for harder-than-expected material zones. When the system detects a sustained 20–30% increase in spindle load compared to the programmed baseline, it flags a potential hard spot region and can either pause for manual inspection or automatically reduce feed rate to protect the insert from fracture. Integrating adaptive control with RSA means the RSA itself can be reduced to the absolute minimum necessary to cover geometric variation (rather than material variation, which is now actively managed by the adaptive control system), enabling faster cycle times and lower tooling costs without accepting greater risk. A stamping die manufacturer using adaptive control on 45 steel blanks reduced their RSA from 2.5mm to 1.8mm, saving 0.7mm of material removal per face and reducing cycle time by 12 minutes per workpiece—while maintaining zero insert fracture incidents over 14 months of production.
Finish Stock
Finish Stock Allowance (FSA) is the final material layer reserved between the rough-machined state and finished product dimensions. FSA provides the operational space for dimensional refinement, surface quality control, and tolerance compensation, while absorbing the micro-deformation caused by residual stress release after rough machining. After rough machining, 45 steel typically requires stress-relief annealing (heating to 550–600°C, holding 2–4 hours, furnace cooling) to eliminate residual tensile stress generated during cutting. Without stress relief, workpieces continue deforming after finish machining, with dimensions gradually shifting 0.01–0.05mm within 24 hours post-machining.
FSA setting should comprehensively consider workpiece dimensions, rigidity, heat treatment state, and final tolerance requirements. For 45 steel parts under 300mm with good rigidity, FSA is 0.2–0.4mm; for parts exceeding 500mm or with thin walls (thickness/length ratio below 1:10), FSA is 0.4–0.6mm to compensate for residual stress deformation. For mold components requiring final heat treatment (quenching and tempering, hardness requirement HRC42–48), FSA must be additionally increased by 0.3–0.5mm as grinding allowance, because heat treatment deformation typically ranges 0.05–0.15mm—sufficient allowance is essential to correct dimensions through subsequent grinding to final specification.
A plastic injection mold manufacturer producing 45 steel mold plates (dimensions 500mm×400mm×60mm) for consumer electronics shells discovered that their FSA of 0.15mm was insufficient after implementing stress-relief annealing. Post-annealing warpage measurements showed deformation ranging 0.08–0.14mm across the plate surface, which completely consumed the 0.15mm FSA and left insufficient material for finish machining. They revised their FSA standard from 0.15mm to 0.35mm for all mold plates exceeding 400mm in length, and added a post-stress-relief straightening operation (hydraulic press at 50kN) for plates exceeding 0.10mm warpage. After these changes, the dimensional acceptance rate after finish machining improved from 76% to 97%, eliminating the need for costly hand-grinding corrections that previously averaged 45 minutes per rejected plate.
FSA allocation for multi-surface 45 steel components requires a systematic approach that considers the machining sequence. In a typical mold base plate with four side faces and two main datum faces, the FSA on each face should be individually specified based on which face is machined first (lowest FSA, typically 0.2mm, because the first-machined face has no prior machining errors to compensate for) and which face is machined last (highest FSA, typically 0.4mm, because it must absorb accumulated errors from all previous operations). The FSA gradient principle states that the FSA on each subsequent machining face should increase by the cumulative error budget of all prior operations—typically 0.03–0.05mm per prior operation. For a five-sided machining sequence, the last-machined face may require FSA of 0.5–0.6mm to provide sufficient correction capacity for the accumulated error chain. Documenting the FSA gradient in the process planning sheet prevents the common error of applying uniform FSA across all faces, which often results in either insufficient allowance on late-machined faces or excessive material removal on early-machined faces.
Datum Marking
Datum Marking establishes permanent reference indicators on the workpiece surface after duplex milling, providing the basis for workpiece alignment, fixture positioning, and in-process measurement during subsequent CNC machining. Datum marking on 45 steel surfaces typically uses scribing: a scribe marker creates cross-center lines, edge distance lines, and angular reference lines on the top and side faces, with center punch marks (depth 0.5–1.0mm) for permanent identification. Datum line accuracy requirements: cross-center line perpendicularity to workpiece edge ≤0.05mm/m, line width 0.1–0.2mm (visually clear).
For precision mold components, manual scribing alignment is being progressively replaced by CNC direct-read alignment—using dial indicators or laser interferometers on the CNC machining center to directly read workpiece edge coordinate values, with precision exceeding manual scribing by more than 10× (laser interferometer accuracy 0.001mm/m versus scribing 0.05mm/m). However, scribed marks retain irreplaceable value: they provide operators with visual reference for workpiece orientation on the machine table, preventing entire-part scrapping from reversed placement direction. One CNC workshop mandates that all duplex-milled workpieces display three lines in the scribed area: workpiece number (batch traceability), blank supplier code, and measured thickness value (accurate to 0.01mm). These three annotations enable subsequent CNC programmers to precisely input stock values during the programming stage, eliminating re-measurement and saving approximately 8 minutes of set-up auxiliary time per workpiece.
An aerospace structural components manufacturer machining 45 steel fixtures (tolerance IT6 on key datum distances) documented a 14-month study comparing scribing-based versus CNC-probe-based datum establishment. In the first 7 months using traditional scribing, the datum establishment error rate (defined as datum deviation exceeding 0.01mm requiring re-establishment) averaged 4.2% per workpiece, generating an average of 38 minutes of corrective action time per occurrence. After switching to CNC direct-read probing for all datum establishment (while retaining scribed identification marks for part orientation only), the error rate dropped to 0.3% and corrective action time dropped to 4 minutes average. The annual saving from this change—accounting for 800 workpieces per year at CNY 95/hour labor rate—was CNY 43,000 in direct labor time recovery, plus an additional CNY 12,000 reduction in scrap from datum errors. The conclusion: retain scribing for part identification and orientation, but invest in CNC probing for actual datum establishment on any workpiece with IT7 or tighter tolerance requirements.
Laser marking has largely replaced traditional scribe-and-punch for datum marking in modern CNC shops, offering significant advantages in speed, precision, and traceability. A fiber laser marker can etch datum reference marks, workpiece ID codes, supplier batch numbers, and thickness measurements directly onto the 45 steel surface in 8–15 seconds per workpiece, with mark depth of 0.05–0.15mm (sufficient for durability through multiple handling cycles) and line width of 0.1–0.2mm at 1,064nm wavelength. Crucially, laser marks introduce zero mechanical distortion to the surface (unlike center punching, which can create micro-cracks extending 0.3–0.5mm into the surface that become fatigue crack initiation sites in cyclically loaded mold components). One precision mold manufacturer switching from center punch marks to laser datum marks on their 45 steel mold plates documented a 31% reduction in surface fatigue cracks at die life testing, attributed to the elimination of impact-induced micro-cracks from the center punch process. For high-cycle dies (exceeding 1 million strokes), laser datum marking is strongly recommended over traditional punch marking to maximize die fatigue life.
Duplex milling pre-processing of 45 steel blanks is the foundation of CNC precision machining—oxide scale inspection prevents abnormal tool wear, dimensional verification ensures adequate stock, hard spot detection averts insert fracture risk, dual-face parallelism of 0.01mm/300mm establishes precision datum system, scientific RSA and FSA settings preserve dimensional chain integrity, and scribed datum marks enable workpiece orientation and dimensional traceability. These six steps are indispensable; following this specification can raise the first-pass inspection yield rate of 45 steel mold structural components from the industry average of 82% to above 97%.