ChipLoad

Troubleshooting

Every fix links to the tool or machine manufacturer document it comes from. Starting points only: test in scrap.

Woodworking (router) troubleshooting

Burn marks on the cut edge

Cause: The chip is too thin. Freud: a very small chip, or sawdust, does not carry enough heat away from the edge, so the heat stays in the bit and the wood. Onsrud: a larger chip removes more heat. Dwelling makes it worse: at 18,000 RPM a two-edge tool contacts the part about 600 times a second (Onsrud).

Fix: Raise chip load into the manufacturer's range by feeding faster (or lowering RPM). Freud's solid-carbide range for a 1/4" bit in hardwood is .008"-.011" per tooth. Avoid stops and dwells in the cut.

When this fix doesn't apply: If raising feed makes the part move on the hold-down or the finish drops, back feed off 10% (Onsrud method, as published by ShopBot). Burning from a dull bit won't fix with feeds.

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Sources: Freud: Get the most out of your Freud bits (CNC feed & speed guide) · LMT Onsrud: Fixturing and Routing of Plastics with CNC · ShopBot Tools: Feeds and Speeds Charts (machine manufacturer; quotes Onsrud's optimizing method)

Tear-out or chipped veneer on plywood/laminate

Cause: Spiral direction lifts or pushes the face fibers: an upcut spiral pulls the top face up, a downcut pushes the bottom face out.

Fix: For through-cuts in double-sided laminated or veneered panels, use a compression spiral (Onsrud lists its compression series for 'double-sided laminated and veneered wood composites'). Cut deeper than 1xD only with reduced chip load: -25% at 2xD, -50% at 3xD (Amana, Freud).

When this fix doesn't apply: A compression bit run in shallow multi-pass steps only uses its upcut tip, so the top face can still tear.

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Sources: LMT Onsrud 2017 Production Cutting Tools Catalog (compression spiral usage, p. 40-42) · Amana Tool: Plastic O-Flute speed chart v5 (depth-of-cut rule) · Freud: Get the most out of your Freud bits (CNC feed & speed guide)

Fuzzy or rough edge finish

Cause: Chip load outside the manufacturer's range. Freud: too small a chip overheats and dulls the edge; too large a chip leaves a rough surface.

Fix: Bring chip load into the published range, e.g. Freud solid carbide 1/4" in plywood .006"-.009", MDF .013"-.017".

When this fix doesn't apply: Deeper than 1xD per pass: reduce chip load 25% at 2xD and 50% at 3xD (Freud).

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Sources: Freud: Get the most out of your Freud bits (CNC feed & speed guide)

Parts shifting on the vacuum table

Cause: Holding force drops as cutting removes the sealed area, and small parts have too little area for vacuum to hold against cutting force (Onsrud).

Fix: Rout a grid connecting the vacuum ports so vacuum reaches the part edges; seat gasket tape in an oversized channel; leave tabs or a thin skin and cut them last. Onsrud's starting points for the pump: flow-through ~400 CFM at 15 inHg; dedicated fixture ~80 CFM at 25 inHg.

When this fix doesn't apply: A plain MDF sheet with weather stripping and a few holes isn't enough for demanding work (Onsrud). Onsrud publishes no skin thickness, so choose one by test.

Sources: LMT Onsrud: The Importance of Spoilboards in the Machining Process · LMT Onsrud: Fixturing and Routing of Plastics with CNC

Plastic melting or chips welding back into the cut

Cause: Heat builds when chips are small or the tool dwells; chips re-weld (Onsrud).

Fix: Increase the feed rate or switch to a single-edge (1-flute) tool. With a downcut spiral, cut a slot in the spoilboard so chips have room to expand (Onsrud Soft Plastic chart). Use the published chip load, e.g. Onsrud 63-750 1-flute upcut O-flute 1/4": .008"-.012".

When this fix doesn't apply: Onsrud warns that incorrect chip loads can cause cratering, so stay inside the published range.

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Sources: LMT Onsrud: Soft Plastic cutting data · LMT Onsrud: Fixturing and Routing of Plastics with CNC · LMT Onsrud: Hard Plastic cutting data

Knife marks on plastic edges

Cause: Chip load too low: chips don't curl properly, and the edge shows knife marks (Onsrud).

Fix: Use an O-flute tool and keep chip load in Onsrud's 0.004"-0.012" range for mechanical plastics (ABS, acetal, Delrin, polycarbonate).

When this fix doesn't apply: Hard plastics that make a splintered chip instead of a curl: Onsrud recommends V-flute tools for those, not O-flutes.

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Sources: LMT Onsrud: O-Flutes for Mechanical Plastics · LMT Onsrud: Soft Plastic cutting data · LMT Onsrud: The Router Way (O-flute vs V-flute for plastics)

Small parts come loose or get thrown on the last pass

Cause: On a flow-through (universal) vacuum table, holding area shrinks as material is cut away, so small parts lose hold-down (Onsrud spoilboard article). Small scrap pieces can come loose, chatter on the table, hit the bit or be thrown (Onsrud fixturing article).

Fix: Leave tabs or an onion skin (a thin bottom layer) and remove them afterwards (Onsrud). Use a smaller-diameter cutter to reduce cutting pressure and open area, and cover unused spoilboard with scrap or plastic sheet. For repeat parts, a dedicated spoilboard with a raised pedestal lets offcuts fall away from the bit.

When this fix doesn't apply: If the part is large and still moves, the issue is vacuum supply or leakage, not part size (see 'Vacuum hold-down is weak'). Screws or riveted tabs through scrap are Onsrud's last resort because they slow setup.

Sources: LMT Onsrud: The Importance of Spoilboards in the Machining Process · LMT Onsrud: Fixturing and Routing of Plastics with CNC

Vacuum hold-down is weak across the table

Cause: Leakage and an undersized vacuum system. Onsrud: unsealed spoilboard edges, uncovered areas, grooved or glazed spoilboard surfaces and small supply lines all cut holding force. Open-cell weather stripping used as gasket doesn't spring back.

Fix: Seal spoilboard edges (rubberized paint), cover unused areas, and resurface periodically with a large spoilboard cutter (surface both sides when new). Onsrud's starting points: flow-through about 400 CFM at 15 inHg on 3" minimum lines (4-5" preferred); dedicated fixtures about 80 CFM at 25 inHg on 3/8" minimum lines (1/2" preferred). On dedicated fixtures, use closed-cell gasket in a routed channel.

When this fix doesn't apply: If one small part slips while big parts hold, see 'Small parts come loose'. These CFM and inHg figures are Onsrud's starting points, not specs for any particular pump.

Sources: LMT Onsrud: The Importance of Spoilboards in the Machining Process · LMT Onsrud: Fixturing and Routing of Plastics with CNC

Bit makes dust instead of chips

Cause: Chip load is too low, or chips are being re-cut. Onsrud: 'the goal is to produce chips, not dust'; dust means re-cutting or too low a chipload. Freud likewise warns that a very small chip, or sawdust, leaves heat in the bit.

Fix: Raise chip load by feeding faster, or lower RPM if feed is already maxed (Onsrud). Make sure dust collection is clearing the cut so chips aren't re-cut. Use the highest chip load that still gives an acceptable finish.

When this fix doesn't apply: If faster feed makes parts move or finish degrade, back off 10% (Onsrud method, published by ShopBot). Very fine dust is normal on MDF and doesn't by itself mean a low chip load.

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Sources: LMT Onsrud: Fixturing and Routing of Plastics with CNC · Freud: Get the most out of your Freud bits (CNC feed & speed guide) · ShopBot Tools: Feeds and Speeds Charts (machine manufacturer; quotes Onsrud's optimizing method)

Blow-out or melting where the bit plunges

Cause: Straight plunges make the bit dwell. While it bores in, it keeps re-contacting the cut surface, and unless it's a spiral or shear tool, chips don't evacuate well (Onsrud). In plastics that heat re-welds chips. When a hole is bored straight through, the plug can blow out the underside.

Fix: Ramp in: plunge gradually on Z while traveling in X/Y, then travel back to clean up the ramp if needed. For holes, use a smaller bit to ramp in a circle and rout the hole to size rather than drilling it. This holds tolerance and reduces underside blow-out when the plug comes free (Onsrud).

When this fix doesn't apply: A true drilling cycle with a drill bit is a different operation. This advice is for router bits making holes and pocket entries.

Sources: LMT Onsrud: Fixturing and Routing of Plastics with CNC

Edge finish differs from one side of a part to the other

Cause: On nested or mirrored parts cut in a single pass, some edges are climb-cut and others conventional-cut, and finish changes with cut direction (Onsrud).

Fix: Onsrud: conventional cutting generally gives the better edge in a single pass. If you add a finish pass, it can be a climb cut. Otherwise cut in two passes with a smaller tool, reversing direction so each exposed edge gets its best pass. On thick stock, rough first and leave about .080" for the finish tool.

When this fix doesn't apply: If every edge is rough no matter the direction, check chip load and bit sharpness instead (see 'Poor finish').

Sources: LMT Onsrud: Fixturing and Routing of Plastics with CNC