CPM Pellet Mill Rebuild Guide: Dies, Clamps & Mainshaft

Changing a die or setting a mainshaft on a CPM pellet mill is straightforward once you know the sequence, and unforgiving if you skip a step. This guide walks through both jobs the way we do them on the floor, with the details that save you a ruined snap ring or a scarred set of threads. The clips throughout are from Chandler's own rebuilds.

Changing the die: bear claw (Acme) clamps

Newer CPM mills hold the die with bear claw clamps, also called Acme clamps, that ride on a threaded post. Each clamp has a 50 millimeter center nut and a pair of jack-off (push-off) bolts. The whole job comes down to knowing which fastener does which job.

Removing the die

1. Back out the two jack-off bolts. With the die still clamped tight, remove both regular die-clamp (push-off) bolts from the clamp.
2. Barely loosen the 50 mm nut. Just a bump is plenty. Loosen it too far and you roll over the snap ring.
3. Break the clamp loose with the jack-off bolts. Run the push-off bolts all the way into the quill flange, with an impact gun if you like, until the clamp breaks free of the die. This is the only correct way to free it.
4. Run the clamp up the post. Now use the 50 mm nut and the impact gun to run the clamp up far enough to clear the die. Do not run it all the way off the post.
5. Work around the die. Do the same to all six clamps. The die will come free, so support it well and take your time. Leave all six clamps on the flange.

Installing the new die

1. Mount the die. Set the new die in place and get everything lined up.
2. Draw it in with the 50 mm nuts. Run the center nut on each clamp down to pinch the die eye and suck the die in tight. You can bump it little by little and let the nut pull the die into place.
3. Torque to spec. Bring each clamp to 350 foot-pounds. The Acme design makes it easy, with no busted knuckles.

Why the design works

The post uses Acme, or square, threads. Square threads push and pull far more load than a tapered thread, which is why the clamp can draw a die in and hold it. The post itself stays bolted to the quill flange at 500 foot-pounds and does not move. You only run the clamp up and down the Acme thread.

Setting the mainshaft

The mainshaft job is all about patience and one thing above all: the shaft has to go in dead straight.

1. Start it straight. Line the shaft up and tap it gently with a block, like a 4x4, just to start the wear sleeve into the bearing. You are not driving it, only engaging it.
2. Walk it in by hand. Once it engages, move it in a half inch to an inch at a time. The newer brown seals are looser than the old green ones, so with brown seals expect short moves.
3. Check the seal on every move. Reach around and confirm the mainshaft seal stays seated and in position as you walk the shaft in. You may reseat it a dozen times, and that is normal.
4. Take it home. With everything straight and lightly oiled, finish seating the shaft. Oil on the bearing race and the wear sleeve keeps it slick.

Mainshaft seals

The seals are where a clean, careful hand pays off.

• Loctite the retainers. Use Loctite on the stainless bolts that hold the seal-holder retainers.
• Thin film of oil, metal only. These are two-piece seals with metal-to-metal contact. Put a light film of oil on the metal between the two seals, on the inner race of the mainshaft bearing, and on the mainshaft wear sleeve. Keep the seal cup surface clean and dry.
• Viton is what matters. The important part is a Viton mainshaft seal. Green versus brown does not matter; the green ones are just being phased out.
• On a used machine, a little oil on the splines at the back of the hub helps. A new machine does not need it.

These notes come from Chandler's own rebuilds. Always follow CPM's manual and torque specs for your exact machine, and work safely: support the die and follow your lockout and confined-space procedures.