Garolite Machining: G10 vs FR4, CNC Tooling, Tolerances and Inspection

Garolite is a trade name. In machining, the parts are usually G-10 or FR-4 under ASTM D709-25 Type IV. The standard governs supplied laminate, not the finished CNC feature. Three things decide the outcome of a shop order: grade identity on the purchase order, tool and fixture choices against glass abrasion, and separate drawing control over holes, slots and flatness.

1. What Is Garolite, and How Do G-10 and FR-4 Differ?

Garolite is a commercial descriptor used for thermosetting laminates. In CNC work it almost always means G-10 or FR-4: woven glass fabric bonded with epoxy. The cut face exposes glass fiber ends and resin interfaces, which decide cutter wear and the appearance of hole and slot edges.

For broader glass-reinforced composite work (hand layup, pultruded profiles, filament winding), see our fiberglass GFRP manufacturing guide. This article stays with sheet-form glass epoxy for CNC-milled components.

1.1. ASTM D709-25 Grades and Glass Epoxy Construction

ASTM D709-25 (Standard Specification for Laminated Thermosetting Materials) classifies laminates by reinforcement, resin, product form and performance. G-10 and FR-4 sit under Type IV, glass fabric with epoxy resin.

Table 1 maps each grade to a MIL-I-24768 slash-sheet designation. FR-4 differs from G-10 in the resin only, with modifications that limit continued burning after ignition is removed (Appendix X1.25). G-11 and FR-5 are heat-resistant counterparts (Appendix X1.24 and X1.26); do not treat them as drop-in substitutes for G-10 or FR-4.

Grade	ASTM D709 construction	MIL-I-24768	Primary selection driver
G-10	Woven glass fabric with epoxy resin	/2, Type GEE	Mechanical strength and electrical insulation; no called-out flame requirement
FR-4	Woven glass fabric with flame-resistant epoxy	/27, Type GEE-F	Same mechanical/electrical envelope as G-10 plus flame resistance
G-11	Woven glass fabric with heat-resistant epoxy	/3, Type GEB	Elevated-temperature flexural performance
FR-5	Heat-resistant, flame-resistant glass epoxy	/28, Type GEB-F	Elevated temperature combined with flame resistance

Standard requirements are organized by form. Section 7 covers sheet; Section 11 and Section 12 cover tubes and rods. Sheet acceptance values live in Table 7 (flexural strength minima by grade, nominal thickness and LW/CW direction), Table 8 (impact and bonding), Table 9 (permittivity and dissipation factor by frequency and thickness band), Table 10 (dielectric breakdown parallel to laminations, in kV minimum), Table 11 (water absorption maximum), Table 12 (arc and tracking) and Table 13 (flame resistance). Every number carries a conditioning code from Section 14.1: A (as received), C (humidity, e.g. C-24/23/50 means 24 h at 23 °C and 50% RH), D (immersion, e.g. D-48/50 means 48 h in distilled water at 50 °C), E (temperature). A single Table 7 LW flexural value does not transfer to a thread, a slot wall or an inspected exit edge.

An FR-4 designation under ASTM D709-25 does not establish a UL rating, a copper-clad PCB qualification or a specific IPC-4101 slash sheet. IPC-4101 governs copper-clad base materials for printed boards; an FR-4 mechanical plate ordered to ASTM D709-25 /27 is not interchangeable with an IPC-4101/21 or /24 PCB laminate for electronic qualification.

1.2. G-10 vs FR-4: Which Grade Belongs on the Drawing?

Specify G-10 when the part needs mechanical and electrical performance with no called-out flame requirement. Specify FR-4 when flame resistance is required and documented against Table 13. Sheet color is not a grade identifier; natural and green sheets have shipped in both grades depending on supplier and lot.

Before releasing the drawing, confirm three callouts:

• Grade on the purchase order: ASTM D709-25, Type IV, grade (G-10 or FR-4), MIL-I-24768 slash sheet.
• Sheet orientation when holes, loads or insulating distances depend on ply direction (LW parallel to the stronger flexural direction).
• Separate electrical, flame, environmental and finished-part acceptance callouts. Mechanical plate ordering does not pull PCB or UL qualification along with it.

A 10 mm nominal FR-4 blank meeting Table 25 thickness tolerance can still fail flatness, slot or hole-edge requirements after machining. Those are finished-part checks, inspected after release from the fixture. State the grade on the PO; state finished acceptance on the drawing.

2. How to Machine Garolite Without Excessive Tool Wear or Delamination

G-10 and FR-4 machining cuts hard glass fibers in cured epoxy. Edge wear on the cutter rises faster than it does in aluminium, and breakthrough thrust can split the last plies at a hole exit. The engineering levers are tool substrate and geometry, feed per flute, chip evacuation and local support at the exit face.

For the general machining pipeline (CAM, fixturing, inspection flow), see our CNC machining overview. The sections below cover the laminate-specific items.

2.1. CNC Tooling for G-10 and FR-4

For prototype and short-run work in 10 mm or thinner G-10 and FR-4, uncoated carbide with a sharp, polished flute will cut several plates before the edge rounds. Diamond-coated carbide extends edge life in abrasive composites and suits mid-volume runs. For production, PCD (polycrystalline diamond) is the usual choice; per-tool cost is higher, and the edge holds against glass through longer runs between changes.

Common geometry notes for G-10 and FR-4:

• Low-helix end mills pull less on the laminate and reduce lift at the top ply.
• Compression (up-down) routers cut the top and bottom plies in opposing directions, which helps when both faces are visible.
• Twist drills with a short web and a split or four-facet point centre consistently in glass. Brad-point wood drills do not suit layered glass epoxy; the single center spur deflects at the first fibre bundle.

Replace a cutter when burrs grow between pieces, slot or hole dimensions drift toward the top of the tolerance band, or spindle load climbs for the same program. These three signals usually appear together before an obvious edge failure. Section 6.5 of ASTM D709-25 confirms the material can be drilled, tapped, sawed and machined; it does not quantify a tool-life window.

2.2. Cutting Conditions and Transferable Numbers

Spindle rpm by itself does not transfer between jobs. Cutting speed Vc = π × D × n, with D in mm and n in rpm, gives the comparable number. Feed per tooth Fz = Vf / (n × z) gives the comparable chip load. The case plate in Section 5 ran at Vc 226 m/min contour (6 mm, 2-flute PCD at 12,000 rpm) with Fz 0.025 mm/tooth, and Vc 113 m/min drill (6 mm, 2-flute carbide at 6,000 rpm) with feed 0.030 mm/rev at 2.0 mm peck.

For a new combination, bracket Vc in the range typical of glass epoxy with the chosen substrate (carbide, diamond-coated or PCD) and the machine’s rigid envelope, then verify with a first-article on the actual sheet. Record cutter diameter, flute count, axial depth of cut ap, radial engagement ae, support arrangement and the resulting slot and hole condition. Do not reuse a PCD feed on a carbide tool at the same rpm; usable Fz on carbide in G-10 is lower, because edge wear accelerates with chip load.

Keep the workpiece supported across the entire machined area. Use a sacrificial layer where through-features break out. When the final contour releases the part, confirm clamps and vacuum zones still hold without a shift.

2.3. Exit Support, Delamination and Edge Quality

The last one or two plies of a through-hole or slot sit without support below. At breakthrough, drilling thrust loads them in tension rather than shear, and the laminate can split or lift along the fibers. The fix is a tight, replaceable backing board in direct contact with the exit face, a sharp drill, controlled thrust (feed matched to peck for small-diameter drills), and a chamfer on the exit side when the drawing allows.

Inspect both faces of every through-hole. Record chip depth, loose fibers at the edge and any crack trace continuing into the ply stack. A light chamfer removes a superficial lift. It does not repair a crack that continues beneath the chamfer face.

Set edge acceptance against function: maximum chip depth in mm, no continuous crack, no visible interlaminar separation, no loose fibers at datum surfaces. Numeric limits are what inspection can act on.

2.4. Dust Extraction and Dry or Wet Machining

Glass and cured epoxy particles come off dry cutting as fine, abrasive dust. Collect at the cutter with an enclosed or local extraction nozzle. Compressed air broadcasts the dust across the shop and should not be used for cleanup. Filter selection, airflow and operator PPE come out of the equipment and workplace risk assessment, with the material SDS as a reference. ASTM D709-25 does not set an occupational exposure limit, an extraction airflow or a dust explosibility rating.

Wet machining is possible but changes cleanup and conditioning. Trapped moisture in a laminate affects dielectric response and dimensional recovery (Section 14 conditioning codes). Match any coolant to the downstream electrical use before switching from dry cutting.

3. Garolite CNC Design Rules for Holes, Slots and Threads

After the general DFM review (feature access, cutter radii, tolerance envelope), add three laminate-specific checks: exit integrity on through-features, remaining material around slots and ribs, and thread or insert loading relative to the ply direction. The DFM design guidelines set the general reviews; the items below are the laminate add-ons.

3.1. Drilled Holes, Counterbores and Hole-Wall Integrity

Specify the hole diameter and the functional location against the datum system, not center distances alone. A hole can pass the gauge for diameter and still miss the fixture because the pattern is rotated, or the exit is torn beyond the edge allowance. Use positional tolerance against defined datums for alignment holes.

A counterbore or countersink thins the laminate under the fastener. Check that the remaining thickness supports the clamp load and that the back side still has ply count to resist breakout. Specify the counterbore diameter, depth and acceptable back-face condition on the drawing.

3.2. Slots, Thin Walls and Laminate Edges

For narrow ribs and long slots, confirm cutter access, remaining material and loading after machining. A slot too close to the plate edge can leave less than one screw-bearing width on the retained side. Specify cutter-compatible internal radii and measurable edge criteria: maximum chip depth in mm, no visible interlaminar separation, no loose fibers. The words “clean” and “smooth” are not acceptance limits.

Insulating fixtures in consumer electronics manufacturing often combine closely spaced metal mounts with electrical separation. Verify creepage, clearance and fastener contact on the assembly drawing, separately from the mechanical callouts.

3.3. Tapped Holes, Inserts and Fastener Loads

G-10 and FR-4 can be drilled and tapped (ASTM D709-25 Section 6.5), but a tapped composite does not hold the same load as a metal thread of equal nominal size. Pullout depends on engagement length, thread direction relative to the ply stack, edge distance, torque, humidity and assembly cycles.

Threads cut with the axis parallel to the thickness direction load individual plies in shear at every thread flank. This is the usual case in a flat plate. For repeated assembly or sustained clamp load, specify a brass or steel threaded insert (heat-set or press-fit, sized to the engagement) or a through-bolt with a load-spreading washer. Validate pullout, strip torque and crush on the actual stack-up when the joint affects function or safety. A Table 7 flexural number does not translate to thread pullout.

Feature	Primary concern	Acceptance check
Through hole	Exit breakout, torn fibers	Chip depth and crack trace at both faces
Closed slot	Wall damage, remaining rib	Slot width, visible delamination, side-face finish
Counterbore	Thickness under fastener	Clamp load vs remaining ply count
Tapped hole	Thread pullout, ply split	Pullout or torque test on sample stack
Flat plate	Clamp release, local warp	Flatness measured on relaxed part

4. What Tolerances Can Garolite Machining Actually Hold?

There is no universal CNC tolerance for G-10 or FR-4. Sheet thickness, ply orientation, cutter wear, fixturing, local humidity and the inspection setup each move the achievable number. Set tolerance against the function; validate on the first piece released from the fixture.

4.1. ASTM D709 Stock Thickness vs Finished CNC Tolerances

Section 10 and Tables 22 through 27 of ASTM D709-25 cover supplied sheet dimensions and allowable variation. Table 25 sets thickness tolerance by nominal sheet thickness band and grade group; Table 26 covers certain special electrical grades; Table 27 lists available thicknesses by grade. Table 5 bounds stock warp and twist. These control incoming stock, not the finished dimension of a milled face.

Appendix X3.6, X3.7 and X3.8 give typical machining tolerance guidance for drilled, punched and finished features. These are nonmandatory and labeled “typical.” They are a reasonableness check on a drawing callout, not a vendor commitment and not an acceptance limit.

A 10 mm nominal sheet is not 10.00 ± 0.00 mm. Specify final thickness and which faces are machined on the drawing.

4.2. Flatness, Clamping and Laminate Orientation

Vacuum and clamps can flatten a bowed sheet during cutting and leave it to spring back once released. Inspect flatness on the functional face in a defined support condition (free rest on three points, or on the mounting datums named in the drawing).

ASTM D709 lengthwise (LW) is the stronger flexural direction; crosswise (CW) is 90° to LW (Note 17 under Section 15.1). Carry the sheet direction onto the drawing when flexural load, thermal recovery or insulation distance depends on it. A sheet that meets Table 5 warp can still show warp after machining and relaxation.

4.3. Hole Position and Inspection Strategy

Use positional tolerance against named datums for alignment features. Hole-to-hole distance alone will not catch pattern rotation against the mounting datum. Reference the CNC drawing tolerances guide for GD&T callouts, and the tolerances and standards page for the comparison envelope. Add chip-depth and visible-delamination limits in the inspection notes for every hole and slot that crosses a laminate ply.

5. EPOC CRAFTER Shop Floor Case: FR-4 Insulating Fixture Plate

EPOC CRAFTER machined 20 electrical-insulating locating fixture plates from Norplex-Micarta NP510A, an FR-4 glass epoxy to ASTM D709-25 Type IV, grade FR-4 (MIL-I-24768/27, Type GEE-F). Nominal plate 120.00 × 80.00 × 10.00 mm. The plates locate hardware on a low-voltage tooling surface and provide separation. They are not safety insulation.

Setup

Three-axis vertical machining centre, high-speed spindle. Full-area flat sacrificial base plate under the sheet. Vacuum hold-down with perimeter stops. Replaceable backing board under each drill breakthrough. Dry cutting with local negative-pressure dust extraction at the cutter.

Tooling and Parameters

• Contour and slot: 6 mm, 2-flute PCD end mill. 12,000 rpm, 600 mm/min feed. Vc 226 m/min, Fz 0.025 mm/tooth. Contour ap 2.0 mm, ae 1.5 mm. Slot ap 1.0 mm, ae 6.0 mm.
• Through-hole: 6 mm, 2-flute solid-carbide drill. 6,000 rpm, 180 mm/min feed. Vc 113 m/min, feed 0.030 mm/rev. Peck 2.0 mm (0.33 D).
• Features: outer contour, one closed slot 60.00 mm long, 8.00 ± 0.10 mm wide, 3.00 ± 0.10 mm deep; four Ø 6.00 ± 0.10 mm through-holes at 90.00 ± 0.15 mm transverse and 50.00 ± 0.15 mm longitudinal spacing. Both hole-edge faces received a light chamfer.

Operation Checks

Tool edges were examined for wear when burrs grew between pieces, dimensions drifted, or spindle load climbed on repeat runs. The job stopped for a cutter check when continuous glass stringing appeared on a slot wall. These settings apply to this NP510A plate on this machine with this fixture. They are a starting point, not a universal envelope for every G-10 or FR-4 job.

Inspection Results (20 pieces)

Characteristic	Measured range	Drawing requirement
Overall length	119.94 to 120.08 mm	120.00 ± 0.20 mm
Overall width	79.95 to 80.07 mm	80.00 ± 0.20 mm
Thickness	9.96 to 10.07 mm	10.00 ± 0.15 mm
Slot width	7.96 to 8.05 mm	8.00 ± 0.10 mm
Slot depth	2.97 to 3.06 mm	3.00 ± 0.10 mm
Hole diameter	5.97 to 6.05 mm	6.00 ± 0.10 mm
Transverse hole spacing	89.95 to 90.07 mm	90.00 ± 0.15 mm
Longitudinal hole spacing	49.96 to 50.08 mm	50.00 ± 0.15 mm
Working-face flatness	0.06 to 0.12 mm	0.20 mm maximum

Measurement conditions: calibrated vernier (resolution 0.01 mm), micrometer (0.001 mm), depth gauge (0.01 mm), height gauge on a granite surface plate, tool microscope at approximately 23 °C. Edge inspection at 10× with directed side lighting; 50× on suspect damage. Edge acceptance: 0.20 mm maximum chip depth, no continuous crack, no visible interlaminar separation, no loose glass fibers at the edge.

All 20 pieces met the dimensional and visual limits. Hole diameter alone did not qualify the hole; both-face inspection and the chip-depth check closed the acceptance. This batch also informs low volume production of insulating fixtures. These settings do not establish general PCD tool life, electrical certification, or tolerances for other stock thicknesses.

6. Inspection and RFQ Checklist for G-10 and FR-4 Parts

Use the materials and properties reference to compare alternatives, then pin the exact G-10 or FR-4 grade and form on the RFQ. ASTM D709-25 Section 18.2 lists the minimum shipment identification: type, grade, form, quantity, manufacturer, and contract or order number. It does not require a particular CoC format or a full-test dossier per shipment. Add those requirements explicitly on the PO.

RFQ or inspection field	What to state or verify
Laminate specification	ASTM D709-25 edition, Type IV, grade (G-10 or FR-4), MIL-I-24768 slash sheet (/2 for G-10, /27 for FR-4)
Separate electrical or flame qualification	IPC-4101 slash sheet or UL file where PCB qualification or UL rating is contractually required; not pulled by the ASTM grade alone
Stock form	Sheet, nominal thickness, width, length, and machining oversize per Table 25 or 26
Orientation	LW or CW direction where flexure, insulation distance or thermal recovery depends on ply direction
Surface and finish	Functional faces, edge treatment, visible-defect limit, cleaning
Finished geometry	Feature sizes, hole and slot locations, flatness, functional datums (positional, flatness, perpendicularity callouts)
Features under load	Fastener bearing surfaces, tapped holes or inserts, and the application loads they resist
Inspection method	Gauge or optical method, magnification, acceptance limits in mm
Identification and traceability	Grade, quantity, manufacturer, production lot per Section 18.2; add CoC format and test report scope if contractually required

Verify incoming laminate grade and condition first, then inspect finished geometry and edges. Specify separate end-use electrical tests when post-machining performance drives acceptance. A rapid prototyping run confirms tool access, fixture support and the inspection method before repeat orders; it does not replace production acceptance.

7. Garolite Machining FAQ

7.1. Can G-10 and FR-4 Be CNC Machined With the Same Tools?

Carbide, diamond-coated carbide and PCD cut both grades. Resin formulation, sheet thickness and tool geometry still change edge life and the surface produced. Share the substrate; verify Vc and Fz on each job.

7.2. Can Garolite Be Drilled and Tapped?

Yes (Section 6.5). Support the exit, chamfer where allowed, and inspect for fiber pullout. Verify the actual threaded joint against its load and reassembly count. ASTM D709 does not list threaded-hole retention; use test data on the specific stack-up or specify a brass or steel insert.

7.3. Does ASTM D709 Specify Finished CNC Tolerances?

No. Tables 22 through 27 cover supplied sheet. Appendix X3.6 to X3.8 give typical machining tolerance ranges, nonmandatory. Finished tolerance belongs on the drawing and is accepted on the inspected part.

7.4. How Do You Inspect Delamination Around a Machined Hole?

Both faces, consistent lighting, 10× magnification; 50× on a suspect feature. Record chip depth, fiber lift and crack trace. A passing diameter does not close the inspection.

For an RFQ, state the grade, the hole and slot functions, the finished dimensional limits and the inspection condition used to accept the delivered plate.