Problem Details
A Moore jig grinder (#2 Jig Grinder) fails to hold the programmed chop length during reciprocation. With the stroke set to approximately 0.600", the head completes 10-15 correct strokes, then on the down-stroke travels roughly 0.5" past the set point, contacts the lower hard stop (lock), and resumes normal chopping for another 10-15 cycles. The fault is intermittent, repetitive, and reappears at a near-constant cycle interval.
Initial troubleshooting focused on the reversing valve (cleaned with no effect; replacement ordered). The actual root cause is almost certainly upstream of the valve, in the air supply / balance / accumulator circuit that controls the double-acting reciprocation cylinders.
Moore Jig Grinder Hydropneumatic Circuit Overview
The jig grinder uses a combined hydraulic-pneumatic circuit. Compressed air at 100 psi feeds the machine inlet, where it is split between:
- The balance cylinder that supports the weight of the quill and grinding spindle assembly
- The double-acting reciprocation cylinders that drive the up/down chop stroke
- The spindle air supply for the grinding spindle itself
A pressure regulator and an air accumulator tank are installed in the balance / reciprocation branch to maintain the working pressure. A four-way directional valve (the reversing valve) routes the regulated air to either side of the double-acting cylinder to produce the chop motion. Lower electrical limit switches signal the valve to reverse at each end of the stroke.
Root Cause Analysis
Three failure modes can produce an intermittent over-travel of this exact signature:
| # | Suspect | Symptom Match | Probability |
|---|---|---|---|
| 1 | Intermittent lower electrical limit switch | Fault appears after consistent correct cycles | Low |
| 2 | Intermittent four-way reversing valve coil / spool | Fault appears after consistent correct cycles | Low |
| 3 | Gradual loss of regulated air pressure during reciprocation | Fault appears every 10-15 cycles, recovers after dwell | High |
Mechanism #3 is the most likely cause because the pattern is cyclic. Air consumed by the reciprocation cylinders during each chop is normally replenished instantly from the regulator and the stored volume in the accumulator. As the supply-side pressure decays (leak, undersized compressor, restricted dryer, or faulty regulator), the balance cylinder can no longer support the full quill/spindle weight at the bottom of the stroke. The head coasts past the limit switch, hits the mechanical hard stop, and the dwell at the bottom of the stroke (while the valve is energized) allows the regulator to recover before the next cycle.
Required Operating Pressure
The balance and reciprocation circuit requires a regulated pressure of 70 psi to 90 psi at the gauge. The exact setpoint is a function of:
- Quill assembly weight
- Grinding spindle weight
- Net downward force at the lower limit
If the pressure decays below the lower bound while the spindle is reciprocating, the quill/spindle assembly will overrun the lower limit switch and slam into the hard stop. The hard stop is what you are hearing and seeing as the "lock." Once the cycle pauses at the bottom, line pressure recovers and the next 10-15 strokes are clean until the supply is drawn down again.
Step-by-Step Diagnostic Procedure
- Install a dry-case pressure gauge on the air balance circuit. The discussion specifically warns against glycerin-filled gauges because the glycerin dampens the needle and hides fast pressure transients. A 0-160 psi dry-case industrial gauge with 1 psi resolution is adequate.
- Watch the gauge while the machine chops continuously. Set the stroke to 0.600" and let the head run. A healthy circuit will hold the gauge rock-steady at the regulator setpoint. A failing circuit will show the needle decay by 5-15 psi between cycles, with the lowest reading exactly when the head over-travels.
- Perform the 12-hour accumulator leak-down test. Disconnect the jig grinder air line at the machine inlet. Pressurize the balance-circuit accumulator to 70 psi and shut off the supply. Wait 12 hours. If the pressure has dropped below 70 psi, the accumulator bladder, check valve, or bleed valve is leaking. If the quill drops to the lower hard stop overnight with the line disconnected, the leak is on the balance cylinder side.
- Inspect the air supply upstream. Verify compressor output matches nameplate CFM at 100 psi. Check the air dryer for adequate capacity (an undersized dryer is a common cause on older Moore installations). Inspect the FRL (filter-regulator-lubricator) for a plugged filter element, which starves the regulator of flow.
- Check the check valve feeding the accumulator. A jammed or fouled check valve prevents the accumulator from recharging between cycles. Crack the line at the accumulator inlet and confirm one-way flow into the tank.
- Inspect the accumulator bleed valve. A leaking bleed valve bleeds pressure back to atmosphere and prevents the tank from holding its charge.
- Inspect the pressure regulator. A damaged regulator seat or diaphragm causes the downstream pressure to sag under high flow. Adjust the regulator through its full range; if it will not hold setpoint, rebuild or replace it.
- Rule out the balance cylinder seal. A failed balance piston seal would not be intermittent. The quill would stay at the lower hard stop continuously. If the quill does drop fully and stay down with the air applied, the seal has failed.
Field Test Sequence Summary
| Test | Tool Required | Pass Criterion | Failure Indicates |
|---|---|---|---|
| Live pressure reading during chopping | Dry-case 0-160 psi gauge | Steady at setpoint, ±1 psi | Regulator, supply, or accumulator fault |
| 12-hour isolated leak-down | Air line disconnect, gauge | Holds 70 psi for 12 hr | Accumulator bladder, check valve, or bleed valve leak |
| Overnight quill drop test | None (line disconnected) | Quill stays in position | Balance cylinder seal leak |
| Compressor CFM check | Tank pump-up timer, spec sheet | Matches nameplate CFM | Compressor or dryer undersized |
Solution Path
Based on the 10-15 cycle interval, the most probable sequence of fixes is:
- Verify the live pressure trace first. If the gauge sags, repair the supply side before touching the valve.
- Replace or rebuild the FRL filter element and the pressure regulator if the gauge is steady but the leak-down test fails.
- Replace the accumulator check valve and inspect the bladder if leak-down fails with the FRL in good order.
- Re-evaluate the four-way reversing valve only after the air circuit holds pressure. A replacement valve ordered before this diagnosis may have been unnecessary; bench-test it before installing.
Verification
After the corrective work, run the chop cycle for a minimum of 30 minutes continuously and observe the head behavior. The fault is considered resolved when:
- The head stops at the programmed 0.600" position on every down-stroke, with no contact with the mechanical hard stop.
- The balance-circuit pressure gauge holds within ±1 psi of the setpoint throughout the run.
- The 12-hour isolated leak-down test passes (no pressure loss).
Additional Resources
- Putting our Jig grinder into action - YouTube
- Adjusting for taper on Moore Jig grinder - YouTube
- How To Operating A Jig Grinder - YouTube
Why does my Moore jig grinder head run past the set stroke every 10-15 chops?
The most likely cause is decaying regulated air pressure in the balance and reciprocation circuit. Each chop cycle consumes air faster than the supply can replenish it, so after 10-15 strokes the pressure falls below the 70-90 psi operating window and the quill overruns the lower limit switch.
What air pressure does a Moore jig grinder balance circuit require?
70 psi to 90 psi regulated at the balance circuit gauge. The exact setpoint depends on the combined weight of the quill and grinding spindle and is set so the head stops cleanly at the limit switch without sagging.
How do I test the air accumulator on a Moore jig grinder?
Disconnect the machine air line, pressurize the balance-circuit accumulator to 70 psi, and isolate it for 12 hours. If pressure drops, the bladder, check valve, or bleed valve is leaking. A dry-case gauge (not glycerin-filled) must be used for accurate readings.
Can a bad reversing four-way valve cause intermittent over-travel?
Yes, but the pattern usually does not repeat on a clean 10-15 cycle interval. Diagnose the air supply and accumulator first; the reversing valve is statistically the least likely cause of a periodic fault with full recovery between cycles.
What happens if the balance cylinder seal fails on a Moore jig grinder?
The quill drops to the lower hard stop and stays there. A balance seal failure is not intermittent. If the head cycles normally between faults, the seal is not the problem.
