Quick answer

To set up the low-speed timing belt on your HiTorque large bench mill, replace the poly-v belt with the 3M-420 timing belt when you need more torque at spindle speeds between 100 and 1800 RPM. Follow the steps below to complete the swap safely.

Why use the low-speed timing belt?

The HiTorque large bench mill (and SIEG SX3.5) comes with two belt options: a poly-v belt for speeds between 280 and 5000 RPM, and a timing belt for speeds between 100 and 1800 RPM. The timing belt provides more torque at lower speeds, which is useful for heavy cuts or when using larger end mills.

Switching to the timing belt is a straightforward maintenance task that can be done in about 15 minutes with basic hand tools. Knowing which belt to use and how to install it correctly will keep your mill running smoothly and prevent premature wear.

Step-by-step timing belt setup

  1. Disconnect power and remove the belt cover: Unplug the mill and remove the belt cover on the head. This is usually held by a few screws. Set the cover aside. Check that the power is off before touching any moving parts.
  2. Loosen the motor mounting bolts: Loosen the bolts that secure the motor to the head. This allows the motor to move and releases tension on the current belt. Do not remove the bolts completely; just loosen them enough to slide the motor.
  3. Remove the poly-v belt: Slide the motor toward the spindle to create slack, then remove the poly-v belt from the pulleys. Inspect the old belt for cracks or wear before discarding it.
  4. Install the timing belt: Place the timing belt (3M-420) over the pulleys, ensuring the teeth engage properly. The belt is 15 mm wide, so it should sit flush on the pulleys. Make sure the belt is not twisted before you tension it.
  5. Tension the belt and tighten the motor bolts: Push the motor away from the spindle to tension the belt, then tighten the motor mounting bolts. The belt should deflect about 1/4 inch when pressed firmly. Over-tightening can damage bearings; under-tightening causes slipping.
  6. Reinstall the belt cover and test: Replace the belt cover, plug in the mill, and run the spindle at a low speed (e.g., 500 RPM) to verify smooth operation. Listen for unusual noise and check that the belt stays on the pulleys.

Product facts

  • Belt, Timing, HiTorque Large Bench Mill, Low Speed: This timing belt is marked 3M-420 and is 15 mm wide. It is the low-speed belt for spindle speeds between 100 and 1800 RPM. The 15 mm width and 3M-420 pitch ensure a proper fit on the HiTorque large bench mill and SIEG SX3.5, so you get reliable power transfer at low speeds.

    Belt, Timing, HiTorque Large Bench Mill, Low Speed

    Belt, Timing, HiTorque Large Bench Mill, Low Speed