Jig grinder head over travel problem.

One of my Moore jig grinders wont hold the stroke length when chopping.
Here's my problem.
I have the stroke set short approx. .600".
I hit go and the head starts chopping as it should.
It will chop to the set length for anywhere from 10 to say 15 chops. Then coming down it will go past the set distance by about .5" hit the lock then go rite back to running correctly for another 10 to 15 chops and on the way down run past again.
It will do this over and over. I took the reversing valve off and cleaned it out but it made no difference. I ordered a new valve to try but I'm wondering if it could be one of the cups or seals in the head ? Has anyone had this same problem?

Moore jig grinder hydraulic pnematic circuit

The hydraulic-pneumatic circuit for the jig grinder is shown on page 117 of the Holes, Contours and Surfaces book. There are three possible explanations for the problem you described. One, there is an intermittent fault in the lower electrical limit switch This is unlikely if it is working reliably for 15 cycles before a fault. Two, there is a intermittent fault in the electric four way valve that directs the compressed air to the double acting cylinders. This is also unlikely if it is working consistently for 15 cycles before a fault. Three, you are gradually losing air pressure during the spindle reciprocation.

A air pressure of between 70 psi to 90 psi is required to both balance the weight of the quill assembly and operate the double acting reciprocation cylinders. The pressure required is determined by the weight of the quill and the weight of the grinding spindle. There is a regulator and accumulator installed to maintain the correct PSI. If the pressure is set too low the quill/spindle assembly will run past the lower limit switch. Your grinding spindle shares the 100 psi machine air inlet supply with the balance and reciprocation cylinders.

As a guess, it takes 15 grinding cycles to run the air supply pressure down low enough to prevent the reciprocation circuit from doing its job. After a fault the air supply has time to catch up and bring the pressure up the minimum necessary to balance the quill/spindle assembly.

What does the pressure gauge installed on the air balance circuit show as the machine is cycled? You need a gauge that is not filled with glycerin.

There may be a fault in your air supply, there may be a leak in the air accumulator tank, there may be a jammed check valve leading to the accumulator, the accumulator bleed valve may be leaking, the air filter may be plugged, or the pressure regulator may be damaged.

A bad air seal on the balance cylinder would not fail intermittently. The quill would stay at the lower hard stop.

Robert

Thank you sir.
I will check these areas out.
I appreciate the info.

There is one easy test that may identify the problem. The air accumulator for the balance piston should be able to hold a air pressure of 70 psi for 12 hours with the jig grinder air line disconnected. If the grinder quill drops down to the lower stop overnight then you will know that there is a air leak.

Robert

Robert is likely right. These machines use a LOT of air. If your compressor is adequate in size then look at your plumbing. Could be a restriction somewhere or a dryer too small.