Guide · Conversion
Converting a straight-knife planer or jointer to a spiral cutterhead means replacing the straight-knife shaft with a helical shaft that holds indexable carbide inserts. On most machines the body, bearing housings, pulley and drive are reused, and the machine itself is not modified. RTING ships new bearings with every head — fit them together with the new shaft (factory pre-installation on request). The job takes about 1.5–2.5 hours with a torque driver, and needs nine caliper measurements before you order: cutting-circle diameter with inserts installed, body diameter, overall length, body length, bearing model or journal diameter at both ends, rotation direction seen from the pulley end, keyway size and thread spec, pulley-end shaft diameter, and the spiral rows and insert count you want. Leave at least one knife on the shaft when sending a sample, otherwise the cutting-circle diameter cannot be measured accurately.
A straight-knife head cuts with two or three long edges that run straight across the full width. A spiral head uses dozens of small carbide inserts set in helical rows, so only a few inserts contact the wood at any moment. The result is a shearing cut instead of a chopping cut: quieter, less tear-out on figured grain, and no knife setting.
| Item | Spec | Note |
|---|---|---|
| Torque driver | 5.4–5.6 N·m (48–50 lb·in) | Pre-set if possible |
| Torx bit | T20 | Match the insert screw |
| Insert screws | 10#-32 or M6 thread | Use the screws supplied with the head |
| Carbide inserts | 15×15×2.5 / 14×14×2 / 30×12 mm (1.5 / 2.5 mm thick — state when ordering) | Four usable edges per 15 mm and 14 mm insert |
| Caliper | 0.02 mm resolution | For body diameter |
| Dial indicator | — | For runout check |
Unplug the machine and lock out the switch. Every later step assumes the machine cannot start.
Take off the belt guard and drive belt, then unbolt the bearing housings and lift the straight-knife shaft out. Keep the housings, bearings and pulley — they are reused.
Measure the old shaft on the bench before you order anything.
| Measurement | Where | Why it matters |
|---|---|---|
| Shaft length | End to end, excluding the shaft ends | Sets the head overall length |
| Body diameter | Across the knife body | Sets insert row geometry |
| Shaft end diameter | At each journal | Must match your existing bearings |
| Knife count and size | On the old shaft | Confirms the machine's cutting width |
Most machines built for the global market use 15×15×2.5 mm inserts with T20 screws. Many European machines use 14×14×2 mm — also with T20 screws. Heavy-duty moulder heads use 30×12 mm (1.5 / 2.5 mm thick — state when ordering) wedge-clamped inserts. Order the system your machine geometry allows, not the one you prefer.
Slide the spiral head into the bearing housings, refit the pulley and belt, and check that the head rotates freely by hand before any power is applied.
Seat every insert flat in its pocket with the screw finger-tight, then torque each T20 screw to 48–50 lb·in (5.4–5.6 N·m). Do not over-torque: carbide cracks before the screw yields.
Mount a dial indicator against the head body and rotate it by hand. Runout should stay within 0.03 mm. A head that is dynamically balanced before shipping normally passes this check without adjustment.
Rotate one insert 90 degrees to confirm the screw loosens cleanly. Then make a test cut on a scrap board and listen: a spiral head produces a continuous low hum, not the thumping of a straight-knife head.
| Spec | Value |
|---|---|
| Insert sizes | 15×15×2.5 mm · 14×14×2 mm · 30×12 mm (1.5 / 2.5 mm thick — state when ordering) |
| Screw thread | 10#-32 or M6 |
| Torx size | T20 (15 mm and 14 mm inserts) |
| Insert torque | 48–50 lb·in (5.4–5.6 N·m) |
| Edges per insert | 4 (15 mm and 14 mm) · 2 (30 mm) |
| Conversion time | 1.5–2.5 h |
| Mistake | What actually happens | Do this instead |
|---|---|---|
| Tightening insert screws "as hard as possible" | The carbide insert cracks — and in the worst case shatters — before the screw yields. Fragments can fly, and a cracked edge often only shows up later as chipping | Use a pre-set torque driver at 5.4–5.6 N·m (48–50 lb·in) and stop there. Wear eye protection |
| Ordering by machine model alone | The same model name covers different production years with different shaft dimensions | Send the nine measurements along with the model |
| Reusing the old screws | Old screws have stretched threads and will not hold torque | Use the screws supplied with the new head |
| Skipping the power-disconnect step | Indexing an insert on a live machine is the most common cause of injury | Unplug and lock out before any insert work |
Most thickness planers and jointers with a removable straight-knife shaft can be converted without modifying the machine. RTING holds fitment data for 932 machines across 141 brands, of which 798 are verified against actual measurements. Look up your model in the machine fitment database — if it is there, the nine measurements are already on file and a drawing can be issued within 24 hours.
No. The conversion reuses the machine body, bearing housings, pulley and drive; only the shaft is swapped. New bearings ship with every head and should be fitted at the same time.
1.5–2.5 hours for a first-time conversion, including the runout check.
48–50 lb·in (5.4–5.6 N·m) for T20 screws. Re-check periodically.
15 mm and 14 mm inserts have four usable edges; 30×12 mm (1.5 / 2.5 mm thick — state when ordering) inserts have two.
Yes, but heavy-duty heads use 30×12 mm (1.5 / 2.5 mm thick — state when ordering) wedge-clamped inserts rather than 15 mm screw-on inserts.
The carbide insert cracks — and in the worst case shatters — long before the screw itself yields, and fragments can fly. Stop at 48–50 lb·in (≈5.4–5.6 N·m) with a pre-set torque driver instead of brute force, and wear eye protection when handling or indexing inserts.
Prices are not published on this site — quoted once the dimensions are confirmed. Insert torque, sizes and conversion time are given as general guidance; always follow the specifications supplied with your cutterhead.