Problem Overview
The Hardinge Cobra 42 is a slant-bed turning center equipped with a servo-indexed turret. Like other Hardinge lathes in the Cobra product family, the machine uses a Fanuc CNC that controls turret rotation through an A-axis commanded from a belt-driven rotary encoder. When the turret fails to clamp reliably — operators describe the issue as "clunking into place," "going tooth-to-tooth" on the curvic coupling, or simply not clamping at all — production stops and tool offsets drift.
The two most common failure modes for this complaint are:
- Mechanical play introduced at the encoder belt or curvic coupling, causing the reported A-axis position to shift after clamping.
- Grid shift drift between commanded and actual station positions, which can be corrected in software using Fanuc parameter 1850.
This article walks through both the mechanical and the parametric root causes, the diagnostic procedure for each, and the steps to bring turret clamping back into specification without removing the spindle or rebuilding the turret disc.
Symptoms and Operator Observations
Before opening any covers, document the failure mode precisely. The same alarm text can hide three different problems.
| Symptom | Most Likely Cause |
|---|---|
| Turret rotates to commanded station, then "clunks" and stops short of clamping | Curvic coupling worn; encoder belt slipping |
| Turret clamps, but first cut is off-position by a few thousandths | Encoder belt stretched; grid shift drifted |
| Turret rotates back-and-forth ("hunting") before clamping | Excessive belt play; coupling not fully engaged |
| Turret occasionally lands "tooth-to-tooth" and refuses to clamp | Indexer indexer pin / clamping piston timing off; or severe coupling wear |
| Hole drills oversized or bored feature appears shifted on one part | Encoder belt knocked out of position (bump, large drill impact) |
Root Cause Analysis
The Hardinge Cobra 42 turret is positioned by a servo-driven worm-and-gear reducer and held by a curvic coupling — a Hirth-style face coupling with curved teeth machined into the mating faces of the turret disc and the spindle flange. When the clamping piston applies axial force, the two halves are pulled together and the curved teeth self-center. If there is any rotational play between the encoder and the spindle, the displayed "A position" after clamp will not equal the displayed position before clamp.
On this machine family the rotary encoder is mounted under the turret sheet metal and driven by a timing belt from the turret input shaft. Any of the following will introduce the symptom:
- Belt elongation (typical after several years of service)
- Belt contamination from coolant or oil
- Belt tooth shear from impact (large drill exit, crashed tool)
- Encoder mounting fastener loosening
- Curvic coupling tooth wear
- Clamping piston seal leakage (low clamp force)
Software compensation — using Fanuc parameter 1850 (grid shift) for the A-axis — can mask small amounts of repeatable error. It cannot compensate for sloppy mechanics, and it will drift again as soon as the belt continues to stretch.
Safety and Prerequisites
- Machine in MDI or reference-return complete; spindle stopped; turret at a safe station (typically station 1).
- Main air supply at rated pressure (verify against machine nameplate; the Cobra 42 uses shop air to actuate clamping).
- Cover keys / Torx drivers for the turret sheet metal.
- Fanuc parameter backup procedure known (see Parameter Manual B-64310EN for the control series fitted to your Cobra 42 — typically a Series 0i-T or 18i-T).
- 0–25 mm micrometer or test indicator for mechanical verification of station-to-station repeatability.
Mechanical Diagnostic Procedure
- Display the A-axis position. On the Fanuc position screen, toggle to All axis display (or enable parameter to show the A-axis). Watch the A position while you index each station manually from MDI.
- Record "pre-clamp" vs. "post-clamp" A position. Index to station 2, note the displayed value (target 30.000° for an 8-station turret, 45.000° for 12 stations, etc.). Watch the value change as the curvic coupling engages. A shift of a few thousandths is normal; a shift of 0.01°–0.05° is the signature of belt or coupling play.
- Remove turret sheet metal. Locate the encoder belt on the input side of the turret housing. Inspect belt teeth for rounding, contamination, or missing teeth.
- Check belt tension. With a finger at mid-span, the belt should deflect roughly 6–10 mm under moderate thumb pressure on a properly tensioned installation. If it bottoms out, the belt is stretched and must be replaced.
- Check encoder mounting. Verify the encoder body fasteners are torqued and the encoder shaft coupling to the belt pulley is not slipping.
- Check turret alignment. With a test bar in the spindle and the turret clamped, sweep each station for runout. Stations that show >0.01 mm TIR indicate a worn curvic tooth or a bent locating pin.
- Push test. With the turret clamped at station 1, attempt to rotate the turret disc by hand using a strap wrench. Any visible rotation indicates the clamping piston is not holding — investigate the air/clamp solenoid and seals before touching parameter 1850.
Software Compensation — Fanuc Parameter 1850 (Grid Shift)
If the mechanical diagnosis above shows the belt, encoder, and coupling are all in good condition, the residual repeatable position error at each station can be eliminated in software. On the Fanuc control fitted to the Cobra 42 the relevant parameter is parameter 1850, the grid shift value for the A-axis (rotary axis).
Tuning Procedure
- Place the control in MDI mode and put the parameter write-enable bit to 1 (Parameter setting screen, PWE = 1).
- Index the turret to station 1. Record the A-axis displayed position after clamp. The commanded position is 0.000°. Calculate
error = actual − commanded. - Repeat for every station. Build a table similar to the one below.
| Station | Commanded (°) | Actual (°) | Error (°) |
|---|---|---|---|
| 1 | 0.000 | +0.012 | +0.012 |
| 2 | 30.000 | 29.985 | −0.015 |
| 3 | 60.000 | 60.020 | +0.020 |
| 4 | 90.000 | 89.994 | −0.006 |
| 5 | 120.000 | 120.011 | +0.011 |
| 6 | 150.000 | 149.998 | −0.002 |
| 7 | 180.000 | 180.014 | +0.014 |
| 8 | 210.000 | 210.009 | +0.009 |
- Average the errors. Compute the mean. In the example above the mean error is roughly +0.0056°.
- Set parameter 1850 to the negative of the mean error (so the average residual becomes ~0). Resolution is one detector unit; the sign convention is "subtract to compensate" if the displayed position overshoots after clamp.
- Re-index every station and re-record. Repeat until no station is outside the repeatability envelope of the indexer.
Encoder Belt Service
If the belt shows any of the failure modes below, replacement is the only correct fix. Do not rely on parameter 1850 to compensate for a stretched belt — the value will drift again within weeks.
- Belt tooth wear: rounded tooth profiles, fuzz on the belt sides.
- Belt contamination: coolant-soaked, swollen, or glazed from oil.
- Belt tooth shear: any missing tooth visible on inspection.
- Tension loss: mid-span deflection > 12 mm under the same thumb force used at install.
Replace with the OEM timing belt specification. After installation, run a full index of every station and re-verify parameter 1850 — the grid shift will almost always need to be reset to a smaller value once the new belt eliminates the dominant mechanical slop.
Curvic Coupling Inspection
The curvic coupling is the face coupling between the turret disc and the spindle flange. Inspect by:
- Removing the turret from the machine (rarely required if the indexer is healthy).
- Looking for spalling, fretting, or chip indentations on the curved tooth faces.
- Measuring tooth profile with a contour gauge if visible wear is present.
Couplings with chipped teeth, heavy spalling, or visible plastic deformation of any tooth must be replaced as a matched set — Hardinge supplies the disc and the mating flange as a pre-matched pair. After replacement, the turret must be re-aligned to spindle centerline and the encoder zero reference re-established; parameter 1850 must then be re-tuned from scratch.
Verification
After mechanical service and parameter adjustment, perform the following checks before returning the machine to production:
- Station-to-station repeatability test. Index each station ten times. The A-axis displayed position at each station must repeat within the encoder resolution (typically 0.001°).
- Test bar runout. Mount a precision test bar in the spindle and sweep each turret station with a 0.0005" DTI. Runout at the OD of the test bar should be < 0.0005" at every station.
- Tool-offset verification cut. Cut a simple OD turning feature at each station and measure. Tool offsets should not need to shift by more than 0.0001" between stations.
- Spindle bore / Z-axis alignment. Confirm turret face-to-spindle-face squareness is unchanged — a turret crash can bend the locating pin and shift station offsets in X or Z as well as A.
- Alarm history clear. Clear the alarm history and run a full automatic part program with multiple tool changes to confirm no clamp alarms recur under production duty cycle.
Service and Support Options
Hardinge continues to support the Cobra 42 platform, including spare parts for the encoder belt, curvic coupling assemblies, clamping pistons, and replacement turret discs. When contacting Hardinge, request the Cobra 42 service group specifically — older Cobra and T-series product support is handled by a dedicated group within the company and they will route the call directly.
Third-party rebuild shops familiar with Hardinge lathes and Fanuc-controlled turning centers can also supply parts (curvic couplings, belts, encoders) for owner-operator rebuilds. When evaluating a service provider, ask specifically for:
- Direct experience on the Cobra 42, not generic CNC lathe experience.
- Fanuc control certification on the 0i/18i series.
- Travel time billing — confirm whether portal-to-portal or only on-site hours, as travel is a significant cost on regional service calls.
- Willingness to ship parts to the customer for owner-performed mechanical work if budget is constrained.
Preventive Maintenance Recommendations
- Quarterly: Visual inspection of encoder belt condition through any access window; confirm belt tension.
- Semi-annually: Index every station and log the A-axis displayed position. A drift trend across stations is the earliest warning of belt stretch.
- Annually: Verify parameter 1850 against the station map; re-tune if any station exceeds ±0.005° from commanded.
- After any impact event (crash, broken insert in a large drill, stuck workpiece): inspect encoder belt tension immediately and re-verify station offsets before the next part.
What does Fanuc parameter 1850 actually do on the Hardinge Cobra 42?
Parameter 1850 is the grid shift value for the A-axis (rotary indexer axis). It adds a fixed offset to the A-axis feedback so that the displayed and commanded positions match after the curvic coupling clamps. It can correct small repeatable error — typically up to a few hundredths of a degree — but cannot compensate for sloppy mechanics or a stretched encoder belt.
Why does the turret "hunt" before clamping on a Cobra 42?
Hunting is the signature of play in the encoder belt or curvic coupling. The indexer overshoots, the encoder reads overshoot, the servo reverses, the encoder reads reverse — and the cycle repeats until the curvic teeth engage. Pull the turret sheet metal and inspect the belt first; this is the cheapest and most common fix.
How tight should the encoder belt be on a Hardinge Cobra 42 turret?
With the belt at mid-span between pulleys, moderate thumb pressure should produce roughly 6–10 mm of deflection. More than 12 mm means the belt is stretched and must be replaced. Less than 4 mm means over-tensioned, which will preload the encoder bearings and shorten their life.
Can a curvic coupling be repaired or must it be replaced?
Light fretting can be lapped out with a precision lapping plate. Spalling, chip indentations, plastic deformation, or any broken tooth requires replacement of both halves of the coupling as a matched set. After any coupling service the turret must be re-aligned and parameter 1850 re-tuned from zero.
Will adjusting parameter 1850 hide a real mechanical problem?
Yes — and it is the most common reason a Cobra 42 comes back with the same complaint weeks later. If the post-clamp error is larger than roughly ±0.02°, or if it changes from day to day, the cause is mechanical (belt or coupling) and parameter tuning alone will not hold. Always complete the mechanical diagnostic procedure before changing parameter 1850.