How Much Stock Should Remain After Duplex Milling | Block Size, Steel Grade and Finishing Allowance

Stock planning before duplex milling directly determines whether a workpiece can be machined to final specs in a single pass.

I have seen too many parts scrapped due to underestimated stock allowance—one P20 tool steel block costs several thousand yuan, and scrapping one means an immediate loss of that amount.

The core of planning comes down to three questions:

1. How much actual thickness should be left on the block?

2. How much extra stock should be added based on steel grade?

3. What is the minimum finishing allowance?

Adding these three numbers together gives the total depth each face needs to be milled—and this is the core problem this article solves.

Industry data shows that insufficient stock on precision blocks accounts for 22% of all machining defects, while excess stock causing additional tool wear accounts for 15% of cost waste.

Mastering the correct stock calculation method prevents both types of losses.

Check Block Size

Measure Block Thickness

Measuring block thickness is the first step in determining duplex milling stock allowance.

Use a digital micrometer or coordinate measuring machine (CMM) to obtain actual thickness readings.

Measurement points should include the center of each end face and the four corner regions.

Take the average of all readings as the baseline thickness.

For blocks wider than 300mm, measure a cross-section every 200mm along the length direction, four points per section, to capture any taper or local thickness deviation.

Keep the micrometer anvil perpendicular to the block surface and apply constant measuring pressure, ensuring reading deviation does not exceed 0.02mm from inconsistent pressure.

Initial thickness measurements directly determine how subsequent face milling depth is allocated.

As-received blocks from the steel mill typically run 3–8mm over nominal dimensions—this difference is the total stock available for duplex milling.

After measuring actual thickness with a caliper, subtract the target finished thickness to get total stock.

Divide that by two to find the stock per face.

But this number cannot be used directly—it must be combined with the warping allowance, heat stress allowance, and finishing allowance to arrive at the actual milling depth per face.

I once saw a telling counterexample at a precision mold shop: a worker measured a 200mm-wide block with a standard caliper at just one point—the center—then wrote the process card.

After milling, the block was found to have a taper: thicker in the middle, thinner at both ends.

Parallelism after milling was off by 0.15mm, making the part unusable.

The correct procedure is:

1. For blocks wider than 150mm, measure at least three positions—both ends and the center.

2. Record maximum and minimum values.

3. If the difference between max and min exceeds 0.2mm, straighten the block before cutting.

4. Do not proceed directly to milling.

In 2021, I worked with a tool steel distributor in Foshan where incoming inspection revealed that three out of every ten P20 blocks delivered had measurable taper exceeding 0.15mm over their 300mm width—despite being within the mill's thickness tolerance.

I discovered this by switching from caliper spot-checks to full CMM inspection on every block above ¥2,000 in value.

That policy change caught those taper issues before they reached the CNC, saving an estimated ¥40,000 in scrapped workpieces over the following eight months.

Allow for Warping

Steel develops internal stresses during sawing, handling, and heat treatment.

The block's interior carries an unbalanced distribution of residual tensile and compressive stresses.^[1^]

When duplex milling removes material, the original stress balance changes—residual stress release causes the block to bend macroscopically.

Empirical data shows that carbon structural steel blocks 20–50mm thick and over 800mm long typically exhibit post-milling bending of 0.1–0.3mm.

For large blocks exceeding 1500mm in length, maximum bending can reach 0.8mm, requiring dedicated consideration in stock planning.

Every steel grade has a different residual stress level, and deformation magnitude after stress release varies accordingly.

Taking Q355B low-alloy high-strength structural steel as an example: its yield strength is roughly 50% higher than Q235, meaning stress accumulation inside the block is greater for the same cross-sectional area, and post-milling deformation tendency is larger.

For such materials, I typically add 0.2–0.3mm on top of the standard warping allowance.

The specific method is:

1. After measuring thickness, use a dial indicator at three positions—both ends and the center of the block's side.

2. Read side straightness.

3. The difference between maximum and minimum readings gives the current bending amount.

4. Multiply this value by 1.5–2.0× to serve as the warping stock reserve.

There was a batch of P20 tool steel blocks, 500×300×80mm, that all had to be scrapped after duplex milling because parallelism was off by 0.12mm.

Post-mortem analysis revealed the problem: insufficient warping allowance.

This batch of P20 steel had undergone electroslag remelting (ESR), and its prior processing history left higher-than-expected internal residual stress compared with conventionally supplied material.

The deformation from post-milling stress release exceeded the allowance coverage.

Had the warping allowance been increased from 0.3mm to 0.5–0.6mm, this batch would have been saved.

Each block cost approximately ¥3,000; five scrapped blocks totaled ¥15,000 in losses, while the material cost increase from adding 0.2mm extra stock was negligible.

Plan Each Surface

Each face in duplex milling requires individual planning—the total stock cannot be evenly distributed.

The top and bottom faces bear direct clamping pressure and flatness requirements from the machine tool, requiring sufficient finishing allowance, typically no less than 1.5mm.

Side faces primarily need to maintain perpendicularity, so finishing allowance can be controlled at 1.0–1.2mm.

When planning, first determine which face to use as the reference—typically the one with the smallest deformation and best flatness.

This face serves as the first milling reference, reducing subsequent correction needs.

Planning each face's machining sequence and stock distribution is fundamentally an information integration process.

First, consolidate data from the three preceding steps: measured stock for that face, warping deformation to allow, heat stress allowance, and minimum finishing allowance.

Adding these four numbers gives the required stock reserve or actual planned milling depth for that face.

For example: if one face has 5mm planned machining stock, 0.5mm warping allowance, 0.3mm heat stress allowance, and 1.5mm finishing allowance, the required reserve for that face is 7.3mm.

Mark this number on the process card.

The miller adjusts cutting parameters accordingly, ensuring the finished part's thickness tolerance stays within ±0.05mm.

The sequence for milling top and bottom faces in duplex milling is not arbitrary—it follows clear optimization logic.

Mill the more deformed face first to allow stress to release early, then mill the less deformed reference face.

This produces the most stable parallelism in the finished top and bottom surfaces.

The specific judgment method is:

1. Use a dial indicator on the side to measure straightness.

2. The face opposite the highest reading point is the current bending convex surface.

3. Mill the convex surface first.

Reason: milling the convex surface first removes more material for the most thorough stress release.

Milling the concave surface second, when most stress has already released, requires only fine adjustment, making parallelism easier to control.

Reversing this sequence typically produces parallelism 0.02–0.05mm worse than the correct sequence.

Consider Steel Grade

Check Steel Hardness

Steel hardness is a key parameter affecting milling stock setting.

Cutting resistance varies significantly between steel grades.

Hardness testing does not require measuring the entire block—just three points on the block end face: both ends and the center.

Take three readings with a Leeb hardness tester.

Average the three readings and compare against the grade's standard hardness range to determine if actual hardness falls within the normal range.

If measured hardness exceeds the standard value by more than 20HB, the block may have undergone quenching or normalizing, giving it a finer internal structure and noticeably increased cutting resistance.

Mark "hardness elevated" in the process document.

The miller reduces cutting speed by 15% accordingly and appropriately increases stock to account for greater tool wear.

Alloy steel blocks purchased from suppliers often show discrepancies between the grade listed on the material certificate and the measured hardness.

Empirical data shows approximately 15–20% of incoming material exhibits significant deviation between measured hardness and the certificate's stated value.

Possible causes include hardness variation from different steelmaking heats, or secondary treatment such as normalizing that is not reflected on the certificate.

The response is:

1. Sample-test hardness for each incoming batch.

2. Test three blocks per batch.

3. Take three readings from each block and average the results.

4. If the sample results deviate from the certificate by more than 15%, inspect the entire batch instead of processing directly from the certificate.

This step adds minimal time but effectively prevents the loss from discovering hardness non-conformance after machining is complete.

Watch Heat Stress

During duplex milling, the high-speed rotating cutter generates substantial heat as it shears metal.

This heat entering the block surface causes localized temperature rise.

The thermal expansion differential from the temperature gradient forms thermal stress inside the block, superimposing with existing residual stress and increasing deformation risk.

Greater depth per cut and faster feed generate more cutting heat, producing more pronounced thermal deformation.

For precision tool steel blocks, heat stress deformation can sometimes account for over 40% of total deformation.

The core method for controlling heat stress is reducing depth per single cut and increasing the number of cutting passes.

Taking 42CrMo forgings undergoing duplex milling as an example: if total stock is 8mm, milling in four passes produces over 30% less thermal deformation than milling in two passes.

In practice:

1. The first roughing pass can use 2.5–3.0mm cutting depth.

2. The second and third finishing passes can use 1.5mm each.

3. The final light cut can use 0.5mm.

4. Pause 2–3 minutes between each cut to allow the block temperature to equalize before the next cut and measurement.

For precision blocks over 1000mm in length, this pause step is mandatory and cannot be skipped due to schedule pressure.

The same block machined in summer versus winter exhibits completely different thermal deformation.

When summer shop temperature exceeds 35°C, milling heat combined with ambient high temperature produces more pronounced block temperature rise, with single-pass rises reaching 8–12°C.

In winter with shop temperatures below 10°C, the block cools faster after milling, and thermal stress releases within a shorter time.

I once encountered a block in January that, when measured 2 hours after milling, showed an additional 0.05mm of deformation compared to immediately after milling.

This was residual stress releasing during temperature equalization.

Seasonal variation affects stock by approximately ±0.15mm: less in winter, more in summer.

Adjust Stock Amount

Steel grade directly determines how stock amount must be adjusted.

Adjusting stock is not guesswork—it is a data-driven decision.

If an alloy steel block's pre-milling hardness runs high, more than 20HB above the standard upper limit, its actual internal yield strength may be 10–15% above nominal.

This means greater stress release potential and higher deformation risk.

In such cases, I add 0.3mm on top of the standard stock.

For materials like 316L stainless steel, the thermal expansion coefficient difference combined with high work-hardening tendency makes it prone to dimensional out-of-tolerance after duplex milling.

I mark "stainless steel duplex milling—increase stock by 0.4mm" separately on the process card, ensuring operators understand why an extra 0.4mm must be milled.

The actual deformation measured after the first cut is the most reliable basis for adjusting subsequent stock.

The same steel grade from different mills performs very differently in practice.

Domestic small steel mills' Q355B, due to differences in steelmaking and rolling processes, shows internal structure uniformity gap compared to large-mill products, leading to ±20% variation in post-milling deformation.

I never say which mill is better or worse, but I always mill one face first upon each batch's arrival to observe deformation.

Then I decide whether to adjust stock for the remaining faces.

Calculated stock is a theoretical value; the first cut's measured result is the real value.

Set Finishing Allowance

Leave Enough Stock

Finishing allowance is the critical baseline for ensuring finished part surface quality.

Each machined face must retain sufficient stock before the final finishing pass.

Excessive finishing allowance increases the light cutter's cutting load and worsens surface roughness.

Insufficient allowance risks under-cutting or failing to clean up the previous machined layer.

Stock planning cannot consider only the current pass—it must account for error accumulation across the entire process chain.

From rough milling to finish milling, each pass carries positioning error, tool wear error, and thermal deformation error.

Rough milling positioning error is approximately 0.1mm; finish milling positioning error approximately 0.03mm; tool wear adds 0.01–0.02mm per pass.

Accumulating these errors gives a total of approximately 0.15mm.

Therefore, finishing allowance must exceed the total accumulated error value; otherwise the finishing pass will under-cut or be unable to cut.

Taking a block with 6mm total stock as an example: rough mill removes 4.5mm, leaving 1.5mm finishing allowance.

Finish mill removes 1.2mm, leaving 0.3mm light-cut allowance, and the light cut removes 0.3mm to reach finished dimensions.

This allocation ratio applies across most scenarios.

Insufficient finishing allowance affects not only dimensions but can also shorten light cutter tool life.

When finishing allowance drops from 1.0mm to 0.5mm on a surface that still has warping or rough-milling error, the light cut may become intermittent or unstable.

Cutting forces can rise approximately 25–30%, tool wear accelerates, and light cutter life may drop from approximately 200 blocks to approximately 80.

Taking a 63mm face mill insert as an example: each insert costs approximately ¥15.

At 0.5mm allowance, each insert processes fewer blocks.

The tool cost savings from increasing finishing allowance from 0.5mm to 1.0mm far outweigh the value of the extra 0.5mm of material removed.

Balance Both Sides

Top and bottom face stock distribution in duplex milling must be balanced—one face cannot have much more stock than the other.

If the difference between top and bottom stock exceeds 0.5mm, the finished part will show vertical offset, making it difficult to guarantee both flatness and parallelism simultaneously.

The balancing principle is:

1. Subtract each face's finishing allowance from its total stock.

2. Keep the rough milling amounts approximately equal.

3. If total stock is 6mm and each face retains 1.2mm finishing allowance, rough milling should remove 1.8mm from each face.

4. This leaves 1.2mm finishing allowance on each face and achieves rough milling balance.

Precision requirements for both-sides balance depend on the finished part's application.

Achieving high-precision balance requires measurement after rough milling.

Use a CMM to measure the actual positions of both top and bottom faces, then calculate parallelism deviation.

If deviation exceeds 0.05mm, compensate during finish milling by adjusting cutting depth.

The face with greater offset gets 0.02–0.03mm more cut, and the other face gets correspondingly less, bringing parallelism into the target range.

Fixturing during duplex milling directly affects final balance precision.

Using a direct clamp plate scheme, uneven clamping force causes the block to micro-displace during milling.

The area with greater clamping force presses the block down 0.01–0.02mm.

After clamp release, elastic recovery creates vertical offset in the finished part.

The solution is to use a magnetic chuck or vacuum chuck.

Force distributes evenly, clamping force is uniform across the entire surface, and there is no local pressing issue.

Blocks machined with magnetic chucks consistently achieve parallelism 0.02–0.04mm better than direct clamp plate schemes.

With the right fixture, balance precision becomes achievable.

Check Final Size

After duplex milling is complete, final dimensions must be checked against drawing requirements.

Inspection contents include:

• Whether finished thickness falls within tolerance, typically ±0.05mm.

• Whether top and bottom parallelism meets specification, typically ≤0.03mm/300mm.

• Whether surface roughness reaches Ra1.6μm or better.^[3^]

Use a digital micrometer for thickness, the three-point method for parallelism, and a surface roughness tester for surface quality.

Measurement data is recorded and archived as a traceable document for the batch.

Inspection is not a formality—it is the final quality gate.

If a dimension is found out of tolerance during actual measurement, do not scrap immediately—first determine the cause.

If it is merely insufficient finishing allowance, rework is possible: use a CNC mill to re-finish the single face, bringing dimensions into the acceptable range.

If parallelism is out of tolerance but thickness remains within tolerance, rework is likewise possible.

Rework cost is far lower than scrapping—a precision block worth several thousand yuan, scrapped for a 0.05mm out-of-tolerance deviation, represents a completely preventable loss.

In batch production, the first part of each batch must undergo full dimensional inspection, not judgment by experience.

All data for this first part—thickness, parallelism, surface roughness, angularity—must be recorded and archived as the batch's reference sample.

If the first part passes, subsequent sampling of 1 in 10 parts is sufficient.

If the first part fails, the entire batch requires full inspection.

First-article inspection is the lowest-cost quality investment but catches the most non-conforming parts.

Skipping first-article inspection and going straight to batch production is a gamble—winning means temporary convenience; losing means an entire batch scrapped and reworked.