G-10 and FR-4 look like plastic and machine like nothing else in the shop. They are glass cloth held in epoxy, and the glass is the story: it is why the material is stiff, strong and stable enough to replace a metal bracket that also has to insulate, and it is why a part that looks simple on the drawing can wear out tooling, fill a room with dust, or come off the machine with frayed, delaminated edges.
Here is what glass epoxy does at the cutter, and how to design and specify a part that comes out clean.
What G-10 and FR-4 are
G-10 and FR-4 are NEMA grades of the same family: layers of woven glass fabric bonded with epoxy resin and pressed into sheet or made into rod. They behave alike on the machine. The difference is in the resin: FR-4 is the flame-retardant grade, G-10 is not. If the part has a flammability requirement, it is FR-4, and the grade belongs on the drawing.
They are laminates, not plastics in the usual sense. The values below are the G-10 rows on our G-10/FR-4 material page, from the Norplex-Micarta NP500A data bulletin:
| Property | G-10 (NP500A, typical) | What it means at the machine |
|---|---|---|
| Tensile strength, ultimate (ASTM D638) | 43,000 psi | Far stronger than any unreinforced plastic we machine |
| Flexural modulus (ASTM D790) | 3,400,000 psi | Very stiff; thin sections hold their shape under the cutter |
| Hardness (ASTM D785) | Rockwell M99 | Hard, and the glass inside it is harder still |
| Thermal expansion, in-plane (IPC-TM-650 2.4.24) | 0.67 × 10⁻⁵ in/in/°F | Close to steel |
| Water absorption, 24 h (ASTM D570) | 0.22 % | Low, about the same as acetal over 24 hours |
Treat these as typical published values for one maker's G-10, not a specification. Crosswise values are lower, and FR-4 publishes its own numbers.
Why glass epoxy wears out tooling
Every cut goes through glass fibre. Glass fibre is highly abrasive: it wears high-speed steel out almost at once and wears even carbide quickly, so a cutter that would run all day in acetal can be dull after a short run in G-10. A dull cutter does not just leave a worse finish here: it starts to tear fibres out of the surface and push the layers apart.
- Carbide is the minimum. High-speed steel is not a practical tool material for glass epoxy.
- Diamond-coated tooling earns its cost on production runs. It holds an edge many times longer in abrasive laminates, which keeps size and edge quality consistent from the first part to the last.
- Plan tool changes, do not wait for failure. Edge quality and size drift together as the tool wears. Inspect early parts and late parts, not just the first article.
The dust is the hazard, not the chips
G-10 does not make chips. It makes a fine dust of glass fibre and cured epoxy, and because it is glass, it is harder and more abrasive than the dust from any other material we machine, phenolic included.
- Capture it at the cutter. Extraction at the tool, not general room ventilation, is what keeps the dust out of the air and out of the machine.
- It is usually cut dry, with extraction. Some shops cut it wet to hold the dust down instead; either way the dust has to be captured, not left to settle.
- Keep it off the machine's slides and spindle. Glass dust is abrasive to the machine as well as the tooling, which is part of what a glass epoxy job costs.
None of this is the buyer's problem to solve, but it is why glass epoxy parts are quoted differently from a thermoplastic part of the same shape.
Delamination and edge breakout
A laminate fails along its layers. The places it does that are predictable:
- Where the cutter exits. The last layer has nothing behind it, so the tool pushes it away instead of cutting it. Backing the material with a sacrificial plate supports that layer.
- At the top and bottom faces of a routed edge. An up-cut tool lifts the top layer, a down-cut tool pushes the bottom one. Compression cutters, which cut toward the middle of the sheet from both faces, are the usual answer for clean edges on sheet parts.
- Where a drill breaks through. Support the work directly under the hole, keep the drill sharp, and ease the feed as the point breaks through.
- At sharp outside corners. A small radius or chamfer on exposed corners keeps a corner from chipping off in handling.
Design rules for glass epoxy parts
- Load it in-plane. The glass carries load along the sheet. Through the thickness, the part is held together only by the resin between the layers, so it is weaker and expands more with heat. Put the strength where the fabric is.
- Do not tap into the edge. A thread cut into the edge of the sheet, parallel to the layers, can split them apart. Threads through the face of the sheet are the safer layout.
- Use inserts for repeated assembly. Tapped threads in G-10 are fine for occasional assembly; a part that will be taken apart again and again is a candidate for threaded inserts.
- Keep holes off the edge. A hole close to an edge leaves a thin web of laminate that can split when a fastener is tightened.
Tolerances a G-10 part can hold
This is where glass epoxy is easy. In-plane, it expands at about the same rate as steel and a fraction of the rate of any unreinforced plastic on our plastic thermal expansion chart. A 24" G-10 panel that warms 50 °F grows about 0.008" along the sheet (24 in × 6.7 µin/in·°F × 50 °F); the same panel in cast acrylic grows about 0.047". It also takes up little water, so it does not swell between the machine and the assembly.
What still needs saying on the print:
- Thickness. Laminate sheet is supplied to the thickness tolerance of its grade. If thickness is a critical dimension, call it out so it can be machined to size. Surfacing a face cuts through the resin-rich skin and exposes glass, which changes how the surface looks and feels, so say whether that is acceptable.
- Through-thickness features. Dimensions across the layers are where the laminate is weakest and least stable. Hold the critical features in the plane of the sheet where the design allows.
- Which dimensions are critical. As with every material, mark the dimensions that matter and put a sensible general tolerance on the rest.
When G-10 or FR-4 is the right call
- Insulators, standoffs and terminal boards that carry a mechanical load.
- Electronics hardware, including test fixtures and boards.
- Aerospace structural and insulating components.
- Tooling and fixtures where stiffness and electrical insulation both matter.
When it is not
- Bearing and sliding parts. The glass is abrasive to whatever it rubs against. Delrin, UHMW or PTFE are the usual answers.
- Parts that only need to insulate. If the part carries no real load, an unreinforced plastic is cheaper to buy and to machine.
- Lower-cost insulating laminate. Where the strength of glass is not needed, phenolic is the cheaper fabric-reinforced laminate.
How we quote G-10 and FR-4 parts
Send the drawing and tell us what the part does: whether it needs the flame-retardant grade, what voltage and temperature it sees, which dimensions are critical, and how the edges and faces must look. We machine G-10 and FR-4 sheet and rod to print in Linden, NJ, alongside the rest of the engineering plastics we have been CNC machining since 1968. Properties and stock forms are on our G-10/FR-4 material page.