Grinding & EDM

Milling and turning can handle 90% of machined parts. But when you need tolerances tighter than ±0.01mm, surface finish better than Ra 0.8μm, or you're cutting material harder than 50 HRC, conventional machining hits a wall. That's where grinding and electrical discharge machining (EDM) come in. Neither is cheap, and neither is fast — but both can achieve things that no milling cutter ever will. This page helps you decide which one you actually need, and avoid paying for capability you don't.

CNC Machining vs Grinding vs EDM — When to Use Each

Start here. The table below maps your part's requirements to the right process. Most parts that end up on a grinder or EDM machine got there because someone specified a tolerance or material that milling can't handle. Knowing which process to request up front saves time, money, and the back-and-forth that kills RFQ turnaround.

What Your Part Needs	Use This	Why	Cost Factor
Standard tolerances (±0.025–0.05mm), Ra 1.6–3.2μm, material <40 HRC	CNC milling / turning	Fastest, cheapest, most versatile. Handles 80%+ of all machined parts. No reason to look further.	1.0x (baseline)
Tight tolerance (±0.005–0.01mm), Ra 0.4–0.8μm, flat or cylindrical geometry	Grinding	Abrasive wheel removes material in tiny increments. Achieves dimensional accuracy and surface finish that cutters can't. Best for flat surfaces, cylindrical bores/ODs, and precision slots.	2.0–4.0x
Very hard material (>50 HRC), heat-treated tool steel, carbide	Grinding or Wire EDM	Grinding cuts hard materials with abrasives. Wire EDM erodes them with sparks. Both work on fully hardened material — no need to machine before heat treatment (for EDM).	2.0–5.0x
Sharp internal corners (zero or near-zero radius)	Wire EDM	No cutting tool has a sharp corner. A wire EDM electrode is 0.1–0.33mm diameter — it can cut internal corners that no end mill can reach. The wire follows any 2D path.	3.0–6.0x
Blind cavities, complex 3D internal features, mold cores	Sinker EDM	A custom electrode burns the negative shape into the workpiece. Can produce cavities, ribs, text, and organic 3D shapes that no rotating tool can access.	4.0–8.0x
Thin walls, delicate features in hard material, no cutting force allowed	Wire EDM	EDM is a non-contact process — there is zero cutting force. The wire never touches the part. This means no deflection, no chatter, no distortion of thin walls.	3.0–6.0x
Ultra-precise holes (start holes for wire EDM, cooling holes in turbine blades)	Fast hole drilling EDM	Hollow electrode tube blasts coolant and sparks through the material. Drills holes as small as 0.3mm diameter at rates of 30–60mm/min in hardened steel.	5.0–10.0x
Large flat surfaces requiring Ra 0.1–0.4μm and flatness <0.01mm	Surface grinding	Magnetic chuck holds the part flat. The grinding wheel skims the surface in precise passes. Achieves flatness and parallelism that milling cannot.	1.5–3.0x

The most common specification mistake Calling out Ra 0.4μm and ±0.005mm on a part that could function fine at Ra 1.6μm and ±0.025mm. Every tolerance step tighter than necessary adds cost — often exponentially. If the part is a bracket that bolts to a frame, Ra 1.6 and general tolerance are probably fine. Reserve grinding and EDM for features that genuinely need them: sealing surfaces, bearing fits, mold cavities, gauge blocks, and precision tooling.

Grinding Types at a Glance

Grinding is not one process. The type of grinder you need depends on the geometry you're trying to achieve. Here's a quick comparison of the four main grinding types used in precision machining.

Grinding Type	What It Does	Achievable Tolerance	Achievable Ra	Typical Cost Factor
Surface grinding	Flat surfaces — the workhorse. Part sits on a magnetic chuck, wheel traverses back and forth. Used for die plates, fixture bases, precision flats, seal faces.	±0.005 mm	0.1–0.4 μm	1.5–2.5x
Cylindrical grinding	Round parts — OD and ID. Workpiece rotates between centers (OD) or on a chuck (ID). Used for shaft journals, bearing seats, precision bores, punch pins.	±0.003 mm	0.1–0.4 μm	2.0–3.5x
Jig grinding	Precision holes and contours. Like a vertical milling machine, but with a small grinding wheel on a high-speed spindle. Used for die sets, precision hole locations, taper bores.	±0.002 mm	0.05–0.2 μm	3.5–6.0x
Creep-feed grinding	Deep material removal in a single pass. The wheel takes a deep cut (up to 10–20mm) at slow feed. Used for turbine blade root forms, slot grinding in hard materials, profiles.	±0.01 mm	0.4–1.6 μm	2.0–4.0x

Grinding is always a finishing operation Grinding is not an alternative to milling for roughing out a part. It removes material too slowly for that. The standard workflow is: mill (or turn) to within 0.1–0.5mm of final dimension, heat treat if needed, then grind to final size. The pre-grind stock allowance matters — too much and you burn up expensive grinding wheel time; too little and you don't have room to clean up heat treatment distortion. Typical grinding allowance: 0.1–0.3mm per side for surface grinding, 0.2–0.5mm diameter for cylindrical grinding.

EDM Types at a Glance

EDM removes material using electrical sparks — no cutting tool touches the part. That single fact makes it indispensable for hard materials, sharp corners, thin walls, and any geometry where cutting force would cause problems. There are three main types.

EDM Type	What It Does	Achievable Tolerance	Achievable Ra	Speed	Cost Factor
Wire EDM	Cuts through the part like a bandsaw, but with a wire electrode. Follows any 2D path. Used for punch/die profiles, sharp internal corners, thin walls, gear teeth, extrusion dies.	±0.005 mm	0.2–0.8 μm	20–300 mm²/min (depends on thickness and material)	3.0–6.0x
Sinker EDM	A custom electrode plunges into the workpiece to burn a cavity. The electrode is the negative of the desired shape. Used for injection mold cavities, forging dies, stamping die impressions, textured surfaces.	±0.005–0.01 mm	0.4–1.6 μm	5–50 mm³/min (depends on electrode size and surface area)	4.0–8.0x
Fast hole drilling EDM	Hollow tubular electrode blasts through material. Used for start holes for wire EDM, cooling holes in turbine blades, oil passages in hardened shafts.	±0.05 mm (position), ±0.02 mm (diameter)	1.6–3.2 μm	30–60 mm/min depth rate	5.0–10.0x

EDM is material-blind EDM doesn't care how hard the material is. It erodes anything electrically conductive: hardened tool steel at 60+ HRC, tungsten carbide, titanium, Inconel — all cut at roughly the same speed. That's the opposite of milling, where harder material means slower feeds, faster tool wear, and higher cost. If your part is made from heat-treated H13, S7, or D2 tool steel, EDM is often cheaper than milling despite the higher hourly rate — because you can EDM the finished part directly, instead of rough-machining before heat treat and then finishing after.

Surface Grinding Deep-Dive

Surface grinding is the most common finishing operation in a tool-and-die shop. It produces flat surfaces with tight dimensional control and excellent surface finish. If your part has two parallel faces that need to be flat within 0.01mm and smooth enough for a seal, surface grinding is the answer.

When Surface Grinding Is Needed

Sealing surfaces — O-ring gland faces, hydraulic manifold mating surfaces, flange faces
Die plates and fixture bases — where flatness and parallelism are critical
Gauge blocks and master plates — reference surfaces for inspection
Mating surfaces on precision assemblies — where Ra must be ≤0.4μm
Parts after heat treatment — to clean up distortion and achieve final dimensions
Stamping die surfaces — where flatness directly affects part quality

Achievable Results

Parameter	Rough Grinding	Standard Precision	Ultra-Precision (Lapping Grade)
Surface finish (Ra)	0.4–0.8 μm	0.2–0.4 μm	0.05–0.1 μm
Dimensional tolerance	±0.01 mm	±0.005 mm	±0.002 mm
Flatness	0.01 mm / 100mm	0.005 mm / 100mm	0.002 mm / 100mm
Parallelism	0.01 mm / 100mm	0.005 mm / 100mm	0.002 mm / 100mm
Stock removal rate	5–20 mm³/min/mm wheel width	1–5 mm³/min/mm	0.1–0.5 mm³/min/mm
Cost per hour	$40–60	$60–90	$90–150

Material Limitations

Surface grinding works on virtually any metal — steel, stainless, cast iron, aluminum, titanium, carbide. But there are practical considerations:

Aluminum and copper: Soft materials load the grinding wheel (abrasive particles get embedded in the material). Requires open-structure wheels and frequent dressing. Adds cost.
Non-magnetic materials: Aluminum, copper, titanium, and austenitic stainless won't hold on a magnetic chuck. Need special fixturing (vacuum chuck, mechanical clamps, or adhesive mounting). More setup time.
Thin parts: Parts under 3mm thick may warp from grinding heat. Use light cuts, coolant, and stress-relieve before grinding.
Very large parts: Surface grinders are limited by chuck size. Most shops have 300–600mm x 1000–2000mm capacity. Larger parts need a Blanchard grinder or way grinder.

The grinding allowance rule Always leave 0.1–0.3mm per side for surface grinding after milling. Less than 0.1mm and you may not have enough stock to clean up the entire surface (especially after heat treatment, which can cause 0.05–0.2mm distortion). More than 0.3mm and you're paying for grinding time that milling could have done faster. Tell your shop what the pre-grind stock allowance is so they can plan the grinding passes accordingly.

Wire EDM Deep-Dive

Wire EDM is the go-to process when you need to cut profiles in hard material with zero cutting force, sharp internal corners, or extremely thin walls. The wire (typically brass or coated brass, 0.1–0.33mm diameter) is continuously fed through the part while electrical sparks erode the material. The result: a profile cut with accuracy that no mechanical cutter can match, in material that no cutter would survive.

When Wire EDM Is Needed

Hard materials: Heat-treated tool steel (>50 HRC), carbide, hardened bearing steel — material that would destroy end mills
Sharp internal corners: True internal corner radii down to the wire radius + spark gap (typically R0.08–0.18mm). No end mill can do this.
Thin walls and delicate features: Zero cutting force means no deflection. Walls as thin as 0.3mm in hardened steel are achievable.
Tall, thin slots: Wire EDM can cut slots with aspect ratios (depth-to-width) of 50:1 or more. Milling maxes out around 4:1.
Punch and die profiles: The traditional application. The punch is cut, then the die is cut with an offset to create the required clearance.
Gear teeth and splines: External and internal gears in hardened material, where the tooth profile must be precise.
Extrusion and stamping dies: Complex 2D profiles with tight tolerances and smooth finishes.

Accuracy and Capability

Parameter	Standard Cut	Skim Cut (Precision)	Multiple Skim Passes
Positional accuracy	±0.01–0.015 mm	±0.005 mm	±0.003 mm
Dimensional accuracy	±0.01–0.02 mm	±0.005–0.008 mm	±0.003–0.005 mm
Surface finish (Ra)	0.8–1.6 μm	0.4–0.8 μm	0.2–0.4 μm
Corner radius (min)	Wire radius + spark gap (~R0.15–0.20mm)	Same	Same
Taper capability	Up to 15–30°	Up to 15–30°	Up to 15–30°
Max workpiece height	300–500mm (most machines)	300–500mm	300–500mm

Speed vs. Thickness

Wire EDM speed is measured in area per minute (mm²/min). The thicker the workpiece, the slower the cut because the wire has more material to erode and the flushing (dielectric coolant) is less efficient at greater depths.

Workpiece Thickness	Typical Cutting Speed	Notes
≤ 20mm	150–300 mm²/min	Fast. Good flushing. Standard applications.
20–50mm	80–150 mm²/min	Common range for stamping dies and punch profiles.
50–100mm	40–80 mm²/min	Slower. Flushing becomes critical. May need special nozzles.
100–200mm	20–40 mm²/min	Slow. Requires experienced operator to prevent wire breakage.
200–400mm	10–20 mm²/min	Very slow. Specialized equipment. Often cost-prohibitive vs. alternative methods.

Wire Types

Wire Type	Diameter	Best For	Cost
Brass (standard)	0.25mm most common	General purpose. Good balance of speed, accuracy, and cost. The default choice.	1.0x
Zinc-coated brass	0.25mm	Faster cutting. Zinc coating improves spark generation. 20–30% faster than plain brass.	1.2–1.5x
Diffusion-annealed wire	0.25mm	Best accuracy for skim passes. Multi-layer coating for precise, consistent spark gap. Used for finishing passes on precision parts.	2.0–3.0x
Fine wire	0.10–0.15mm	Very small internal radii. When R0.10mm or smaller is required. Slower cutting, more fragile.	3.0–5.0x
Molybdenum	0.10–0.18mm	High-temperature cutting and very thick workpieces. Molybdenum wire doesn't stretch like brass.	2.0–3.0x

The start hole requirement Wire EDM cannot start from the edge of a solid block — the wire must thread through a hole in the workpiece. If you need an internal profile (like a die opening or gear teeth), someone has to drill a start hole first. This is usually done by fast hole EDM or a small drill. For external profiles (like a punch), the wire can start from the outside edge. Factor the start hole into your cost and lead time.

Sinker EDM Deep-Dive

Sinker EDM (also called cavity EDM or ram EDM) uses a custom-made electrode that is the mirror image of the desired cavity. The electrode plunges into the workpiece while electrical sparks erode the material between them. Where wire EDM cuts 2D profiles, sinker EDM creates 3D cavities — the only practical way to produce complex internal shapes in hard materials.

When Sinker EDM Is Needed

Injection mold cavities: Complex 3D shapes with undercuts, ribs, and textured surfaces — the classic sinker EDM application
Forging die impressions: Deep cavities in hardened die steel that cannot be milled
Blind features: Pockets, slots, and cavities that don't go through the part — wire EDM can't reach these
Text and logos: Engraving part numbers, logos, or textures into mold surfaces
Sharp internal corners on 3D features: Where wire EDM can't access (no line of sight through the part)
Complex turbine blade root forms: Fir-tree and dovetail slots in nickel superalloys

Electrode Materials

Electrode Material	Wear Ratio (electrode:workpiece)	Best For	Cost
Copper tungsten	1:1 to 1:3 (low wear)	Precision cavities, fine detail, long production runs where electrode wear must be minimal. The premium choice.	3.0–5.0x
Graphite	1:3 to 1:8 (higher wear)	Large cavities, roughing operations, mold bases. Fast material removal. Easy to machine the electrode. Most common choice.	1.0x
Copper	1:1 to 1:2 (low wear)	Fine detail, small features, finishing electrodes. Good surface finish. Harder to machine than graphite.	1.5–2.5x
Brass	1:1 (very low wear)	Small holes, fine detail tubes. Limited to simple shapes because brass is hard to machine into complex 3D forms.	1.2–1.8x

Accuracy and Capability

Parameter	Roughing	Semi-Finish	Finish
Dimensional tolerance	±0.02–0.05 mm	±0.01–0.02 mm	±0.005–0.01 mm
Surface finish (Ra)	3.2–6.3 μm	1.6–3.2 μm	0.4–1.6 μm
Material removal rate	10–50 mm³/min	2–10 mm³/min	0.5–2 mm³/min
Electrode wear	High	Medium	Low
Spark gap	0.05–0.15 mm	0.02–0.05 mm	0.01–0.02 mm

Electrode cost is a significant factor The electrode itself must be machined — usually by CNC milling or high-speed milling. A complex injection mold cavity electrode can take 4–16 hours to machine, and you typically need 2–5 electrodes (roughing + finishing, plus spares). Graphite electrodes are cheapest to make (graphite mills fast) but wear faster during EDM, requiring more electrodes. Copper tungsten electrodes last longer but cost more to make. The total EDM cost is usually 30–50% electrode cost and 50–70% EDM machine time.

Cost Comparison

All costs are approximate and vary by region, shop, and part complexity. Use these as relative benchmarks for process selection, not as quotations.

Process	Hourly Rate (Approx.)	Achievable Tolerance	Achievable Ra	Material Removal Rate	Best Batch Size
CNC Milling (3-axis)	$40–80	±0.025 mm	1.6–3.2 μm	50–500 cm³/min	1–10,000+
CNC Turning	$40–70	±0.025 mm	0.8–3.2 μm	30–300 cm³/min	1–10,000+
Surface Grinding	$50–100	±0.005 mm	0.1–0.4 μm	5–20 cm³/min	1–1,000
Cylindrical Grinding	$60–120	±0.003 mm	0.1–0.4 μm	2–10 cm³/min	1–500
Jig Grinding	$80–150	±0.002 mm	0.05–0.2 μm	0.5–3 cm³/min	1–100
Wire EDM	$60–120	±0.005 mm	0.2–0.8 μm	20–300 mm²/min	1–500
Sinker EDM	$60–130 (+ electrode cost)	±0.005–0.01 mm	0.4–1.6 μm	5–50 mm³/min	1–100
Fast Hole EDM	$80–150	±0.05 mm	1.6–3.2 μm	30–60 mm/min depth	1–10,000+

The total cost equation Hourly rate alone doesn't tell the story. A wire EDM at $100/hr that cuts the part in 4 hours ($400) might be cheaper than milling at $60/hr that takes 10 hours plus a heat treatment cycle ($600 + $200 + setup). And the EDM part might have better accuracy and no distortion from cutting forces. When comparing costs, look at total cost per finished part — including material, all setups, heat treatment, and any scrapped parts from failed milling attempts on hard material.

Common Mistakes

Mistake	Consequence	Fix
Specifying Ra 0.4μm on all surfaces	Every surface gets ground. Cycle time explodes. Cost doubles or triples for no functional benefit.	Ra 1.6 for non-critical surfaces. Ra 0.8 for mating surfaces. Ra 0.4 only for seals, bearings, and cosmetic visible areas. Call out specific surfaces that need fine finish.
Calling out ±0.005mm on features that milling can hold at ±0.025mm	The entire part gets priced at grinding rates. Features that could be milled for $5 now cost $20 because the shop assumes everything needs grinding.	Apply tight tolerances only to specific features using GD&T. Let everything else float at general tolerance.
Requesting wire EDM for a simple external profile in soft aluminum	Wire EDM takes 5–10x longer than milling for the same cut. The part costs 3–5x more than it should.	Wire EDM is for hard material, sharp corners, and thin walls. If the material is aluminum and the profile has standard corner radii, use milling.
Not providing a start hole for internal wire EDM profiles	The shop has to add a fast hole EDM operation ($50–150) or drill operation before wire EDM can begin. Adds time and cost that wasn't in the original RFQ.	Specify "provide start hole" or "drill start hole" on the drawing. Or design the part so the wire can thread from the outside edge.
Specifying sharp internal corners (R0) on a milled part	Impossible with standard tooling. The shop must add wire EDM ($100–500+) or tell you it can't be done. Either way, delays and cost overruns.	Internal corner radius = end mill radius. Minimum R0.5mm, prefer R1.5–R3mm. Only specify R0 if you're prepared to pay for EDM.
Leaving insufficient grinding stock (under 0.05mm per side)	After heat treatment, there's not enough material to clean up the surface. The part is undersize or has areas that weren't ground. Scrapped or rework.	Leave 0.1–0.3mm per side for surface grinding. 0.2–0.5mm on diameter for cylindrical grinding. Account for heat treatment distortion.
Leaving too much grinding stock (over 0.5mm per side)	The grinder spends hours removing stock that the mill should have taken off. Grinding wheel wear increases. Cost goes up dramatically.	Mill to within 0.1–0.3mm of final dimension. Grinding is a finishing operation, not a roughing operation.
Forgetting about the EDM recast layer	EDM produces a thin (0.01–0.05mm) recast layer on the cut surface. This layer is hard and brittle. If the surface is a bearing seat or fatigue-critical, it may crack in service.	For critical surfaces, specify "remove recast layer" — usually by grinding or polishing after EDM. Adds a secondary operation but prevents field failures.
Not accounting for electrode wear in sinker EDM	The cavity dimensions drift as the electrode wears. The first cavity is dimensionally correct; subsequent cavities are progressively smaller. On multi-cavity molds, this is a serious problem.	Specify electrode material and expected number of cavities. For multi-cavity molds, use copper tungsten (low wear) and plan for electrode replacement.
Specifying sinker EDM for a simple open pocket that milling can handle	Sinker EDM costs 4–8x more than milling for the same feature. The electrode alone can cost hundreds of dollars and take days to make.	Use sinker EDM only for blind cavities, undercuts, or features in material too hard to mill. Open pockets should always be milled.
Not specifying GD&T datums for ground surfaces	The grinder doesn't know which surfaces are critical. It grinds everything to the same precision, increasing cost on non-critical surfaces. Or worse, it grinds the wrong surfaces first and loses the datum.	Mark datum surfaces (A, B, C) on the drawing. Specify which surfaces need grinding and which are "as-machined." The grinder will reference from the datums.
Requesting grinding on a part with non-magnetic material and no mention of fixturing	The shop discovers it can't hold the part on a magnetic chuck. Needs to build a custom fixture or use vacuum clamping. Adds $100–500 and delays the job.	If the part is aluminum, copper, titanium, or austenitic stainless, note "non-magnetic — require vacuum chuck or mechanical clamping" on the drawing.

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