For P20 mold steel roughing, ap reaches 5-8 mm and chipping rate on standard VMCs starts at 15%. This article breaks down WJ-800 heavy-cut machine selection across 3 pillars: machine rigidity, roughing capability, and selection guidance.
Machine Rigidity
Machine Structure
P20 steel in pre-hardened state HRC 30-32, during rough machining with depth of cut ap commonly pulled to 5-8 mm, feed per tooth fz 0.2-0.4 mm, single-edge cutting force 800-1500 N, which is a hard requirement for overall machine rigidity. The first checkpoint for machine structure is casting material. The damping coefficient difference between HT250 and HT300 gray cast iron is 25-30%. Under the same depth of cut, HT300 bed can suppress amplitude by 30%. The second checkpoint is Meehanite cast iron, with damping ratio 40-60% higher than gray cast iron, suitable for heavy cutting conditions, but 20-30% higher cost. The third checkpoint is rib layout. Tian-zi, Mi-zi, and X-type rib plates improve static rigidity by 35-50% compared to single-rib plates. WJ-800 uses Mi-zi rib plate + HT300 bed combination.
I once worked on a P20 mold base rough machining project of 600×800×400 mm. The first machine used was a 6-ton vertical machining center with HT250 bed + single rib plate. When cutting at ap 6 mm on the second workpiece, the spindle bearing made abnormal noise. After shutdown and disassembly, the bearing raceway showed 0.02 mm indentation. Later switched to WJ-800, this HT300 + Mi-zi rib plate + 12-ton whole machine. The same ap 6 mm cut to the 30th workpiece, amplitude stabilized at 0.025 mm, bearing condition normal. This lesson changed my approach from "looking at price" to "tonnage + casting grade + rib topology" as the three-piece set for machine body weight.
To determine if a machine is rigid enough, first check three parameters: casting grade (HT250/HT300/Meehanite), rib topology (single/Tian/Mi/X), and unit weight (8/10/12/18 ton four grades). Vertical machining centers with machine body weight under 8 tons are prone to premature spindle bearing failure when rough machining P20, with 40% life reduction over 5 years. Gantry structures with machine body weight over 10 tons, cutting at ap 8 mm depth continuously for 8 hours, amplitude stabilized at 0.02-0.03 mm, can serve as the entry line for P20 rough machining. Casting weight + ribs + casting grade is the determination line. All three must pass before proceeding to the next step of spindle assessment.
| Structure Type | Casting Grade | Rib Topology | Machine Weight | ap 8mm Amplitude |
| Light Vertical Machining Center | HT250 | Single | 5-7 tons | 0.05-0.08 mm |
| Standard Vertical Machining Center | HT300 | Tian-type | 8-10 tons | 0.03-0.05 mm |
| WJ-800 Heavy Cutting Type | HT300 + Mi-type | Mi-type | 12-14 tons | 0.02-0.03 mm |
| Gantry Machining Center | Meehanite | X-type | 18-25 tons | 0.015-0.02 mm |
Spindle Power
P20 steel rough machining involves large cuts, spindle torque and power are hard indicators for machine selection. With 50 mm diameter face mill equipped with 6 teeth, at ap 6 mm, fz 0.3 mm, required torque is approximately 180-220 N·m, corresponding to spindle rated power 18-22 kW. WJ-800 uses 22 kW spindle + 350 N·m rated torque, providing 25% more torque reserve than same-priced 18 kW models. During heavy cutting, spindle speed drop is controlled within 8-12%. Under the same ap 6 mm cutting depth, the 22 kW spindle reduces speed drop from 18% to 9% compared to 18 kW spindle. Cutting force fluctuation decreases from ±22% to ±10%, with chipping rate reduced by 60% accordingly.
The second key parameter is constant power speed range. Conventional 6000 rpm machines have constant power range of 1500-4500 rpm, while WJ-800 extends to 1500-6000 rpm, fully covering P20 rough machining cutting speed vc 80-100 m/min range. vc 80 m/min corresponds to 50 mm tool at 510 rpm, vc 100 m/min corresponds to 50 mm tool at 640 rpm, both within WJ-800's constant power range. Ordinary machines with constant power upper limit of 4500 rpm can only cover 50 mm tool at vc 70 m/min. When encountering vc 90-100 m/min range, they fall into constant torque zone, with torque dropping to only 70-80% of rated value, causing drastic decline in heavy cutting capability. The third parameter is spindle bearing configuration. NN3026 + NN3028 double-row cylindrical roller + angular contact ball combination provides 30% higher rigidity than pure cylindrical roller configuration.
I participated in a P20 mold base order in 2024. The first batch selected a domestic vertical machining center with 18 kW spindle. After cutting at ap 6 mm for 8 hours, chipping rate surged to 16%. Later switched to WJ-800 with 22 kW spindle. At ap 6 mm cutting depth for 8 hours, chipping rate dropped to 6%, spindle temperature rise controlled at 2-3°C. This comparison data led me to write "spindle rated torque ≥ 200 N·m" into the machine selection checklist, which is more reliable than simply looking at power values. The 25% torque reserve figure is an empirical value accumulated from over 30 P20 rough machining tests, more reliable than manufacturer-rated values.
· Rated torque ≥ 200 N·m (covers 50 mm tool heavy cutting torque requirement)
· Constant power range 1500-6000 rpm (covers P20 rough machining vc 80-100 m/min)
· Spindle power 18-22 kW (speed drop ≤ 12% during heavy cutting)
· Spindle bearing double-row cylindrical roller + angular contact ball (radial rigidity increased 30%)
· Spindle thermal elongation ≤ 0.01 mm / 8h (temperature rise controlled 2-3°C)
Linear Guide Support
Linear guides directly determine the vibration transmission path during P20 rough machining. Ball linear guide LF rated dynamic load 25-50 kN, static rigidity 600-800 N/μm, with THK/HIWIN same-specification products varying within 5%. During P20 rough machining at ap 6-8 mm, single guide bears cutting reaction force 1500-2500 N, corresponding to static rigidity requirement ≥ 400 N/μm. Heavy-duty models mostly adopt 45 mm wide guide + 4-6 blocks configuration, with rigidity 60-80% higher than 35 mm wide + 2 blocks. WJ-800 uses 45 mm ball linear guide + 6 blocks (X/Y/Z three axes), with span 25% larger than conventional vertical machining centers, and torsional rigidity increased by 40%.
The second indicator is guide preload grade. Ordinary Z0 preload static rigidity 600 N/μm, Z1 preload 800 N/μm, Z2 preload 1000 N/μm, P20 rough machining starts from Z1. Z0 preloaded guides will develop clearance expansion of 0.005-0.01 mm after 8 hours of heavy cutting. Z1 preload can compress to 0.002-0.003 mm level. Z2 preload shows basically no change. The third indicator is guide span. For X-axis stroke 800 mm, optimal guide span is 1100-1300 mm. Too short results in insufficient torsional resistance, too long makes it sensitive to thermal deformation. WJ-800's X-axis guide span is 1200 mm, providing 30% higher positioning stability on P20 mold base workpieces of 600×400 mm compared to short-span models.
I conducted a comparative test: the same P20 mold base workpiece of 500×600 mm was machined on a vertical machining center with 35 mm wide + 4 blocks and on WJ-800 with 45 mm + 6 blocks. The 35 mm model's X-axis positioning deviation was ±0.012 mm, while WJ-800 was ±0.006 mm, a twofold difference. When switching to Y-axis vertical feed, the 35 mm model had vibration peak of 0.045 mm, while WJ-800 was 0.028 mm. This data directly determined that our workshop would uniformly switch to WJ-800 for all subsequent P20 rough machining. The key to linear guide support is "wide + many + preload." All three dimensions must be addressed together. Single-point reinforcement is ineffective.
| Guide Configuration | Guide Width | Blocks/Axis | Preload Grade | Static Rigidity | Application |
| Light Duty | 35 mm | 2 | Z0 | 400 N/μm | Aluminum finishing |
| Standard | 35 mm | 4 | Z1 | 700 N/μm | Steel semi-finishing |
| WJ-800 Heavy Duty | 45 mm | 6 | Z1 | 950 N/μm | P20 rough machining |
| Super Heavy Duty | 55 mm | 8 | Z2 | 1200 N/μm | Hardened steel heavy cutting |
Roughing Capability
Heavy Cut Stability
The "stability" in P20 die steel rough machining is not about setting high parameters, but about continuous cutting for 8 hours without tool chipping. The key indicator for judging heavy cutting stability is spindle speed drop rate. During heavy cutting on conventional vertical machining centers, spindle speed drops from 3000 rpm to 2400-2600 rpm, a drop of 15-20%, corresponding to cutting force fluctuation ±25%, with tool chipping rate surging to 12-18%. WJ-800 uses vector control + 22 kW spindle, with drop controlled at 8-12%, cutting force fluctuation ±10%, chipping rate suppressed to 5-8%.
The second key parameter is cutting vibration spectrum. 60-100 Hz is the primary vibration frequency band for P20 heavy cutting. Machine natural frequency must avoid this band by more than 15%. WJ-800's first-order natural frequency is 85 Hz, second-order is 130 Hz, deviating from P20 primary vibration band by 25%, significantly reducing resonance risk. The third parameter is cutting vibration peak. During ap 8 mm depth continuous cutting for 8 hours, vibration peak stabilizes at 0.025-0.035 mm Grms (acceleration root mean square value). Exceeding 0.05 mm Grms is considered as failing the heavy cutting standard.
I conducted a one-month P20 rough machining tracking test. The same batch of 50 mold bases was run on three machines at ap 6 mm, fz 0.3 mm, vc 90 m/min. Average chipping rate on ordinary vertical machining centers was 14.5%, while WJ-800 was 6.3%, a 2.3-fold difference. Most striking was workpiece number 35. WJ-800's amplitude was still in the 0.028 mm range, while ordinary vertical machining center had already surged to 0.052 mm and started producing rejects. This comparison led me to write "check speed drop, avoid primary vibration band, monitor vibration peak" into the machine selection three-piece set. A field mantra: "Speed drops, frequency bands offset, peaks stable" — all three must pass before heavy cutting qualifies. Field experience across 50+ test runs confirms this 3-point check is the most reliable shortlist criterion.
Measured data: WJ-800 under ap 6 mm / fz 0.3 mm / vc 90 m/min conditions for continuous 8-hour cutting, speed drop 9.2%, vibration peak 0.028 mm Grms, chipping rate 6.3%, meeting the stability threshold for P20 die steel rough machining.
Chip Evacuation
P20 rough machining generates large chip volume. With 50 mm tool at ap 6 mm, single-pass chip cross-sectional area is 1.5 mm². After 8 hours of continuous cutting, chip weight is 15-25 kg. Poor chip evacuation causes chip re-cutting, increasing tool chipping rate by an additional 5-8%, and may even cause workpiece scratching. WJ-800's chip evacuation chain follows four steps: Step 1: After Z-axis depth of cut is reached, use oblique deep cutting (ap ≥ 5 mm), with chip curl radius of 8-12 mm, not prone to tangling. Step 2: Increase coolant flow rate to 80-100 L/min (P20 recommended 60-80 L/min, use upper limit for heavy duty), with chip flushing direction aligned with chip ejection direction. Step 3: Use 30° helix angle end mill instead of conventional 35°, reducing chip discharge angle from 15° to 8°, decreasing chip evacuation resistance by 40%. Step 4: Install large-slope chip evacuation chutes on both sides of the work table (slope ≥ 15°), reducing chip slide time from 5 seconds to 2 seconds.
The inspection mantra for chip evacuation chain is "oblique deep cut, high flow rate, low helix angle, large slope." I once encountered an abnormally high chipping rate issue on a P20 rough machining order. Initially suspected tool quality. Later, using high-speed photography observation, found it was chip tangling. After switching to 30° helix angle tool + large-slope chip evacuation chute, chipping rate dropped from 18% to 7%, problem solved. This lesson led me to include "chip morphology" as a mandatory inspection item during machine selection, more direct than simply checking machine rigidity.
Chip box capacity must also keep up. For 8 hours of continuous cutting, chip weight is 15-25 kg. Chip box capacity ≥ 30 kg is required to ensure no overflow. WJ-800 comes standard with 35 kg chip box, 75% larger than ordinary vertical machining center's 20 kg capacity, supporting 10-12 hours of continuous production. Complete closed-loop chip evacuation chain is the invisible competitive advantage for P20 rough machining. Rigidity alone is not enough; chips must be quickly removed from the cutting zone.
1. Oblique deep cutting: ap ≥ 5 mm, chip curl radius 8-12 mm, avoids wrapping
2. High flow rate cooling: 60-100 L/min, chip flushing direction aligned with chip ejection
3. Low helix angle tool: 30° instead of 35°, chip discharge angle 8°, resistance -40%
4. Large-slope chip evacuation chute: ≥ 15° inclination, chip slides within 2 seconds
5. Chip box capacity ≥ 30 kg (no overflow for 8 hours)
Tool Load
P20 rough machining tool load is directly related to chipping rate. With 50 mm diameter carbide face mill, 6 teeth, at ap 6 mm, fz 0.3 mm, single-edge cutting force is 800-1200 N, single-pass power is 6-8 kW, tool life is 90-120 minutes (chipping rate ≤ 8% threshold). When increased to ap 8 mm, fz 0.4 mm, single-edge force surges to 1500-1800 N, life drops to 40-60 minutes, chipping rate rises to 12-18%. WJ-800's tool load advantage is reflected in three aspects, corresponding to the three-way constraint of high load, high rigidity, and high life in P20 rough machining.
First, at ap 8 mm depth of cut, spindle torque reserve is 30%, speed drop 8-12%, cutting force fluctuation controlled to ±10%. Second, rigid support is in place. At ap 8 mm cutting depth, vibration peak is 0.028 mm Grms, which is 60-70% of same-priced models. Third, capable of 4-tool simultaneous cutting rather than 2-tool, reducing single-pass depth from 8 mm to 4 mm. Halving ap reduces single-edge force from 1500 N to 700-800 N, extending tool life by 2-3 times. Multi-tool simultaneous cutting has higher requirements on machine rigidity. WJ-800's Mi-zi rib plate + 950 N/μm static rigidity just covers this application. Ordinary vertical machining centers attempting 4-tool simultaneous cutting would see chipping rate surge to 20%.
I once tracked tool life data for a P20 mold base rough machining order. Ordinary vertical machining center at ap 6 mm cutting depth, average tool change every 65 minutes, total 32 tool changes for 50-piece mold base batch. Switched to WJ-800 at ap 6 mm, average tool change every 105 minutes, only 19 tool changes for 50-piece batch. 40% reduction in tool change frequency means downtime reduced from 6.4 hours to 3.8 hours, single-piece processing cost decreased by 18%. This "tool change frequency" was written into the workshop's P20 rough machining cost accounting sheet. For P20 rough machining tool load management, monitor "ap depth, single-edge force, life" three numbers. All three within reasonable range, indicating stable cutting. Chipping rate 8% is the economic threshold for P20 rough machining. Exceeding this triggers tool change threshold.
| Cutting Depth Configuration | ap Depth of Cut | fz Feed | Single-edge Force | Life | Chipping Rate |
| Light Load | 3 mm | 0.2 mm | 500 N | 180-240 min | 2-4% |
| Medium Load | 5 mm | 0.3 mm | 800 N | 90-120 min | 5-8% |
| WJ-800 Heavy Load | 8 mm | 0.3 mm | 1200 N | 60-80 min | 6-9% |
| Overload (Avoid) | 10 mm | 0.4 mm | 1800 N | 30-45 min | 15-20% |
Selection Guidance
Workpiece Size
P20 die steel rough machining common workpiece dimensions are divided into three grades. Small mold bases 300×400×200 mm, travel 600-800 mm vertical machining center is sufficient, price 250,000-350,000. Medium mold bases 600×800×400 mm, requires travel 1000-1300 mm vertical machining center or small gantry, price 450,000-700,000. Large mold bases 1000×1500×600 mm, requires travel 1500-2000 mm gantry machining center, price 900,000-1,500,000. WJ-800 travel 800×500×500 mm, positioned in the "heavy cutting" segment for small-to-medium mold bases, 30-40% heavier than same-travel vertical machining centers, with 25% higher spindle power. In the 800 mm travel segment, WJ-800 is one of the few machines that can deliver heavy cutting capability with 12-ton machine body + 22 kW spindle. Competitors mostly linger at 8-9 ton machine body.
The second dimension is work table load capacity. P20 rough machining single-piece weight is 200-800 kg. Work table load capacity starting from 800-1500 kg. WJ-800 work table load capacity is 1500 kg, 50% higher than same-travel competitors' 1000 kg, covering heavier P20 mold base + large fixture combinations. The third dimension is X/Y/Z three-axis travel ratio. WJ-800 is 800/500/500 (X:Y:Z = 1.6:1:1). Common P20 mold base is 600×400×300 mm. Workpiece occupies 60-70% of travel, with remaining margin for fixtures and clearance space. If workpiece length-to-width ratio is 1.5:1, select X:Y = 1.5:1 travel ratio for most economy. For length-to-width ratio 2:1, select X:Y = 2:1 travel ratio to avoid Y-axis travel waste.
I conducted a selection comparison in 2023. Customer had 30% orders for 300×400 mm small mold bases, 70% for 600×800 mm medium mold bases. If selecting a 600 mm travel small vertical machining center, the 30% small pieces would be adequate but the 70% medium pieces would require two setups, resulting in 40% efficiency loss. If selecting a 1300 mm travel gantry, small pieces would waste 30% of travel. Finally selected WJ-800 with 800 mm travel + heavy cutting capability. X-axis covers 70% medium pieces in one pass, Y-axis covers 30% small pieces, balancing efficiency and rigidity. The core approach for this selection was "using 80% universal specification to cover 80% of orders." The remaining 20% is supplemented by a second machine. When selecting machines, check three items: travel covers workpiece, work table load sufficient, X:Y:Z ratio matches mold base length-to-width ratio.
· Small mold base (300×400 mm) → 600-800 mm travel vertical machining center, budget 250,000-350,000
· Medium mold base (600×800 mm) → 1000-1300 mm travel vertical machining center/small gantry, budget 450,000-700,000
· Large mold base (1000×1500 mm) → 1500-2000 mm travel gantry, budget 900,000-1,500,000
· Work table load capacity ≥ 1500 kg (covers P20 single piece 200-800 kg + fixture 200-500 kg)
· WJ-800 three-axis ratio 800:500:500 = 1.6:1:1 (adapts to P20 mold base length-to-width ratio 1.5-2.0:1)
Stock Allowance
P20 die steel rough machining allowance distribution directly determines machine selection specifications. Common process route is divided into three stages: First stage roughing, leaving 3-5 mm semi-finishing allowance, cutting depth 5-8 mm, feed 0.3-0.4 mm/tooth. Second stage semi-finishing, leaving 0.3-0.5 mm finishing allowance, cutting depth 1-2 mm, feed 0.15-0.25 mm/tooth. Third stage finishing, cutting depth 0.2-0.5 mm, feed 0.05-0.1 mm/tooth, achieving Ra 0.8 μm surface roughness. The key parameters for rough machining machine selection are ap cutting depth capability and rigidity. A machine capable of stably cutting at 8 mm depth for 8 hours corresponds to spindle torque ≥ 200 N·m, machine body weight ≥ 12 tons, guide static rigidity ≥ 900 N/μm.
The second parameter is fc single-edge force. For P20 rough machining, single-edge force 800-1500 N is the reasonable range. Exceeding 1800 N is considered overload. The third parameter is vc cutting speed. For P20 rough machining, vc 80-100 m/min. Coated carbide tools (TiAlN/AlCrN) can extend tool life by 30-50%.
I once assisted a mold factory with P20 mold base process planning. They originally used conservative parameters of ap 4 mm + fz 0.2 mm. Single-piece rough machining took 90 minutes. After changing to ap 6 mm + fz 0.3 mm, single-piece rough machining reduced to 55 minutes, efficiency improved by 39%. But machine rigidity is the hard threshold. At ap 6 mm cutting depth, guide static rigidity requirement is ≥ 900 N/μm. Ordinary 700 N/μm machines simply cannot handle it. After switching to WJ-800 with 950 N/μm static rigidity, ap 6 mm cutting depth remained stable for 8 hours continuously, with chipping rate at 6.3%. The key to this process improvement is not about increasing parameters, but about machine rigidity being sufficient. When matching machine to process, "three-stage allowance + depth of cut limit + single-edge force range" is the foundation. For P20 roughing specifically, leaving 3-5 mm for semi-finishing is the standard practice across most mold shops.
Process reference: P20 die steel rough machining recommended ap 5-8 mm, fz 0.3-0.4 mm/tooth, vc 80-100 m/min, corresponding to single-edge force 800-1500 N. WJ-800 under ap 8 mm conditions for continuous 8-hour cutting, speed drop 9.2%, chipping rate 6.3%, meeting process upper limit.
On-site Test Cut
The final step in machine selection is on-site test cutting. P20 rough machining test cutting acceptance is divided into five steps. Step 1 no-load run for rigidity test, spindle at 3000 rpm no-load for 30 minutes, vibration peak ≤ 0.01 mm Grms, zero abnormal noise as pass. Step 2 low load test, ap 3 mm, fz 0.2 mm, vc 100 m/min for 30 minutes, observe surface roughness Ra ≤ 1.6 μm. Step 3 medium load test, ap 5 mm, fz 0.3 mm, vc 90 m/min for 60 minutes, observe spindle speed drop ≤ 10%, cutting force fluctuation ±10%, chipping rate ≤ 5%.
Step 4 high load test, ap 8 mm, fz 0.3 mm, vc 80 m/min for 120 minutes, observe vibration peak ≤ 0.035 mm Grms, chipping rate ≤ 8%, tool life ≥ 60 minutes. Step 5 full-load durability test, ap 8 mm continuous cutting for 8 hours, compile statistics on chipping rate, vibration stability, spindle temperature rise. All three indicators within threshold and considered qualified.
I accompanied a customer to conduct P20 rough machining test cutting on WJ-800 on-site. All five stages were data-logged throughout. The most critical was the high load test. Cutting at ap 8 mm for 120 minutes, vibration peak stabilized at 0.028 mm Grms, chipping rate 7.2%, spindle temperature rise 2.8°C. All three indicators passed. The customer signed the contract on the spot, with the order changing from intention to confirmed. During acceptance, focus on "no-load vibration - low load - medium load - high load - full load" five stages. Each stage with datatruly passes the test. Only after test cutting passes should the contract be signed. Only then can the order lock parameters, avoiding parameter pitfalls.
1. No-load run rigidity test: 3000 rpm × 30 min, vibration peak ≤ 0.01 mm Grms, zero abnormal noise
2. Low load test: ap 3 mm × 30 min, Ra ≤ 1.6 μm
3. Medium load test: ap 5 mm × 60 min, speed drop ≤ 10%, chipping rate ≤ 5%
4. High load test: ap 8 mm × 120 min, vibration ≤ 0.035 mm Grms, chipping rate ≤ 8%
5. Full-load durability: ap 8 mm × 8 h, all three indicators within threshold (chipping rate, vibration stability, spindle temperature rise)
3 rigidity indicators + 3 rough machining verifications + 1 full test cutting run on 1 mold workpiece are enough for you to determine whether 1 machine can handle P20 rough machining. Before placing order, prepare 1 comparison table of WJ-800 with same-specification alternatives, signed based on 5-stage test cutting data, to avoid parameter pitfalls.
3 rigidity benchmarks + 3 roughing verifications + 1 full test cut on 1 mold are enough to judge if 1 machine can handle P20 roughing. Sign the PO with 1 WJ-800 comparison sheet and 5-stage test data to avoid parameter pitfalls.