The coil is your tooling for heat. When it matches the part and the job, you get fast, even, repeatable results. When it doesn’t, you fight hot spots, slow cycles, and drift. Use this as a practical guide to pick and tune a coil that works on the floor, not just on paper.
Start With the Part and the Heat Pattern
Define the target zone first. Mark what you must heat and what you must protect. A shaft journal needs a narrow band with clean edges. A ring gear needs uniform tooth roots without burning the tips. A busbar joint needs a small molten puddle while the nearby plastic stays cool. Write the temperature window and the depth you need. If you harden, note case depth and quench. If you braze, note filler alloy and atmosphere. That map drives everything else.
Now tie the pattern to geometry. Thick sections and high-mass parts need more penetration and longer dwell. Thin features overheat fast if you flood them with surface power.
This is where frequency and turns interact. Higher frequency concentrates heat near the surface. Lower frequency pushes heat deeper. You don’t have to lock frequency yet, but knowing your target depth keeps you from choosing a coil that fights physics from day one.
Single-Turn vs. Multi-Turn
Pick a single-turn when you want intensity and a small heat footprint. You get strong coupling, short ramps, and tight control. It shines on spot hardening, small shrink fits, and brazing lugs or terminals. The trade-off is coverage. A single turn can leave temperature variation over length, so you often rotate the part or traverse the coil to “paint” the zone.
Pick a multi-turn when you need even heat over distance. Extra turns spread the field and flatten peaks, which helps on journals, longer shaft sections, ring gears, and stacked laminations. You trade some ramp speed for uniformity, but you gain consistency and fewer reworks. If you run high mix, plan for both. Keep a fast single-turn for small, localized work and a multi-turn for long or wrapped features. Build quick-change plumbing so you can swap without stressing the copper.
Mind the layout. A helical multi-turn wraps a cylinder and gives solid 360° coverage. A pancake style focuses on a face or short axial band. A split coil opens like a clamp for closed rings and hubs. Match the form to how the operator will load and unload. If the fixture forces awkward gaps at one side, uniformity will suffer no matter how many turns you add.
When to Add Ferrite Blocks
Ferrite is a flux concentrator. Think of it as a lens for magnetic fields. You use it to push energy into a zone, sharpen a boundary, or steer heat away from something sensitive. It helps you tighten a gear tooth pattern, reduce stray heating on nearby hardware, and cut generator power for the same part temperature.
Place ferrite where the field leaks or where you want more push into the part. Add it behind the copper on the “cold side” of a joint to drive heat across the gap. Add it along the outside of a ring gear coil to hold the footprint at the roots. Watch for two failure modes. Ferrite saturates if you overdrive it and then stops helping. It also overheats if you don’t cool it. If blocks discolor or crack, back off current, improve water flow, add a thicker ferrite cross-section, or break it into more pieces so heat can escape. Use ferrite to fine-tune. Do not use it to cover a bad coil shape. Fix geometry first, then polish with ferrite.
Gap and Alignment Decide Your Cycle Time
Coil-to-part gap sets coupling. Coupling sets ramp time and uniformity. Keep the gap tight and even. As a simple target, hold ~1.5–2.0 mm on small/medium parts; open slightly (~1.9 mm or more) only for scanning or larger-tolerance setups. Every extra millimeter adds seconds and creates hot and cold bands. You will try to “buy safety” by opening the gap. Don’t. You’ll chase heat forever.
Build fixtures that lock the part where the field is strongest. Use hard stops so operators can’t float the part inside the coil. Use feeler gauges during setup and record the number in the recipe. If the feature is offset from the part OD—like a narrow journal on a stepped shaft—center the coil on the feature, not the outer diameter. If runout is a fact of life, consider rotating the part under a stationary coil or moving the coil along the feature to average out small geometry errors. A vision sensor that nudges the coil ±0.5 mm beats turning up power to cover a mis-aim.
Cooling and Durability Matter
Coils die from heat and handling. Give the copper an easy path for water, enough flow, and clean chemistry. Size hoses to keep flow turbulent so you strip heat off the tube wall. Keep supply water in the low 20s °C and limit the return rise to a few degrees. Strain the circuit so oxide flakes don’t clog small passages, especially in tight corner bends.
Protect the insulation. Nicks and carbon tracks turn into shorts. Use guards where operators grab coils during changeover. Add strain relief so hoses don’t lever on the copper. Mark inlet and outlet so people reconnect the same way every time. If you change coils often, use quick-connects that seal cleanly and don’t force twisting loads. Schedule inspections by hours, not by calendar. Look for darkened insulation, pitted copper, and any change in flow. Replace before a failure takes your cell down in the middle of a shift.
Practical Next Steps
Choose one target part and draw the heat zone with the temperature and depth you need. Ask for two coil options that suit that map: a single-turn for speed and a multi-turn for uniformity. Build a simple fixture that sets gap with hard stops and makes it easy to repeat. Start with no ferrite, tune turns and gap until the pattern is close, then add ferrite to sharpen edges or block stray heat. During first runs, log temperature versus time and note how one millimeter of gap changes the cycle. Lock those numbers in your recipe and photograph the setup. Do this once and you stop guessing. Your coil becomes a tool you can trust, not a variable you chase.