CNC Spring End Facing: How the MJK90-2 Grinder and Control System Work

• 1
  What CNC spring end facing actually removes
• 2
  How the MJK90-2 control system closes the loop
• 3
  From loading tray to finished spring: the cycle in order
• 4
  Where end facing helps and where it does not
• 5
  What to measure on a faced spring
• 6
  End facing versus other spring end options

A compression spring leaves the coiling machine with two open, tapered ends. The wire has a helix angle, so the last turn does not sit flat. CNC spring end facing grinds that last turn back until the end face is square to the spring axis. The result is a spring that stands upright, seats flat in a pocket, and carries load over the full first coil instead of a single contact point.

The cut is small. On a 1.5 mm wire spring, the facing pass may remove 0.4–0.8 mm of axial length per end. That sounds trivial, but it sets free length, squareness and the point where the load curve starts. Grind too little and the spring leans. Grind too much and free length drops below the drawing, so the spring is scrapped even though the wire itself was fine.

The MJK90-2 does this on a water-cooled grinding head that travels in the X direction while the spring sits in a rotating work area. Two ends are faced in one cycle, one after the other, with the spring flipped or indexed by the loading tray. The operator sets wire diameter, coil count and target free length; the control system works out the in-feed depth and the number of passes.

The key point for engineers: this is a material removal process, not a forming process. It does not close the end coil, it does not change pitch, and it does not fix a spring that was coiled out of tolerance. If the coiler produced a spring with the wrong number of coils or the wrong wire diameter, end facing will simply square off a bad spring.

The control system is a Delta-based architecture. An EH series PLC runs the sequence and talks to a bank of M series inverters that drive the grinding head and the work-area rotation. A 10.4 inch TFT color screen is the operator interface. Grinding head position is not read from an encoder alone; the PLC reads back high-speed pulses so the axis position is known at the moment the head touches the spring.

That pulse read-back matters because the facing depth is tiny. At 0.4 mm total stock removal, a lost pulse or a delayed stop turns a good spring into scrap. The PLC compares commanded position against returned pulses every cycle, and it holds the axis at the set depth for a dwell time rather than reversing immediately. The dwell lets the wheel spark out, which is what keeps the end face flat instead of crowned.

The inverter side controls spindle speed and the feed rate of the head. Coarse grinding runs at a higher in-feed; fine grinding drops the in-feed and raises the spindle speed slightly. On a typical spring, coarse grinding removes about 70–80% of the stock and fine grinding removes the rest. The transition point is set in the PLC recipe, not by hand.

Automatic compensation is the part that keeps a batch consistent. After every cycle, the PLC adds a small increment to the commanded depth to offset wheel wear. The increment is a parameter, usually in the 0.001–0.005 mm per cycle range. Without it, the first springs in a batch run long and the last ones run short. With it, free length drifts by only a few hundredths of a millimeter across a run.

The spring machine delivers coiled wire into the left or right loading tray. The operator starts the cycle. The left tray presses down and the spring wire enters the work area. The upper grinding head is brought to the reference position, and the PLC checks that the spring is present before it allows the in-feed to start. A missing spring is the most common cause of an empty cycle.

Coarse grinding follows. The head advances to the coarse depth and the work area indexes so both end faces are presented. The PLC holds the coarse dwell, then retracts and repositions for fine grinding. Fine grinding uses a lower in-feed and a longer dwell. The head retracts to a safe height, the tray indexes, and the spring drops or is picked out.

Cycle time depends on wire diameter and required finish. Thin wire springs under 1 mm face quickly because there is little material to remove. Heavy wire springs above 4 mm take longer because the coarse pass has more stock and the wheel has to be dressed more often. A typical mid-range spring of 2 mm wire runs a cycle in the range of a few seconds per end, not minutes.

The operator does not re-measure every spring by hand. The control system counts cycles and applies compensation. A first-article check sets the recipe; after that the batch runs on the stored parameters. If a batch is interrupted, the PLC remembers the wheel wear state, so restarting does not begin with an uncompensated wheel.

End facing is the right process when the spring needs a flat seat, a defined free length, and a squareness spec that a coiler cannot hold on its own. It suits compression springs from roughly 0.3 mm to 6 mm wire, with a spring index that lets the wire sit stably in the work area. Closed and ground ends are the classic case. Open ends that only need a light deburr are usually not worth the cycle.

It is the wrong process when the spring is too short to grip, when the wire is so thin that the grinding force bends it, or when the end face will never touch anything. Torsion springs and extension springs with hooks are not end-faced at all, because the wire end is formed into a hook rather than squared off. Sending those to a facing grinder wastes setup time.

There is also a material boundary. Hardened spring steels grind cleanly with a water-cooled head. Very soft wire can smear instead of cut, which loads the wheel and raises the risk of a burr on the end face. If the end face must be burr-free, the fine pass and a light brush are part of the process, not an afterthought.

Springs that will be plated or coated need the facing done before plating. Grinding after plating removes the coating on the end face and leaves a bare edge that corrodes first. Sequence the operations correctly and the coating covers the ground face.

The three numbers that matter are free length, squareness and end-face flatness. Free length is measured on a length gauge or a load tester at a defined load. Squareness is checked by standing the spring on a surface plate and reading the gap under the end face, or by measuring perpendicularity to the axis. Flatness is often checked with a light gap or a dial indicator while the spring rotates.

Load at a given height is the functional check. A spring can be dimensionally inside tolerance and still miss its load point if too much material was removed from one end. That is why the facing depth is tied to the load curve, not just to overall length. If the drawing calls out load at 50% deflection, the recipe should be set from that number.

For production runs, record the compensation increment and the wheel dress interval. Those two parameters explain most drift. When free length starts walking in one direction, the cause is usually wheel wear outpacing compensation or a dress that removed more wheel than the recipe assumes.

GreatLight runs CNC machining and finishing for spring-related hardware, control housings and fixtures that hold these parts during grinding and inspection. Tolerances on those parts hold to ±0.005 mm, with surface finish from Ra 0.2–0.8 μm when a fine finish is required. Inspection is 100% before shipment, with reports on request.