Grinder: Can't Lock Spindle to Change the Grinding Wheels

For a malfunctioning button, refer to springs, caps, shoulder pin and o-ring repair sections. All of these parts make up the button and will all need to be examined.

If the gear box housing has broken in, and around, the lock button assembly, then you will have to replace the entire gear box. Some angle grinders have a button lock built into the gear case that cannot be repaired and therefore requires you to replace the entire gear box. To do this, remove the gear box from the grinder and then the spindle assembly from the gear box. On models with a removable button, gently pry the cap of the button off with a small screwdriver then take out the spring and shoulder pin. Locate the small rubber pin inside the gear box housing and remove it. This pin helps secure the upper spindle bearing. Install the rubber pin into the new gear box housing. Install the shoulder pin into the new gear case and then grease the new gear box housing. Install the spindle assembly to the gear box housing and tighten the screws in an alternating pattern, a little at a time, to seat the o-ring on the shoulder pin. Install the gear box assembly onto the grinder and reinstall the handle and wheel guard.

The shoulder pin is located under the button on the grinder head. When you push the button, the shoulder pin locks on the spindle gear to prevent movement allowing you to remove the grinding disc. The most common problem with this pin is the end gets sheared off, if it stays locked down, and it turns the grinder on, or if the returning spring inside the button assembly fails. When this occurs, you will no longer be able to lock the spindle to change the grinding disc.

To replace this pin, remove the side handle and then the wheel guard. Take out the gear box housing from the grinder and then remove the spindle assembly on the gear box. Pry the cap off the button assembly with a small, flat-blade screwdriver. Remove the compression spring and the shoulder pin, followed by the o-ring from the shoulder pin and transfer it into the new shoulder pin. Now, Install the shoulder pin in the gear housing. Reinstall the spindle assembly back in the gear box housing, tightening each screw in an alternating pattern, a little at a time, to seat the o-ring on the shoulder pin. Reinstall the compression spring and the pin cap. Put the gear box assembly back on the grinder, followed by the wheel guard and the handle.

If the spindle has sheared off, you will not be able to lock it in place to remove the grinding disc. To check and replace the spindle, remove the wheel cover and the handle. Remove the spindle box assembly from the gear box housing. This procedure will vary between makes and models. Remove the spindle bearing using a bearing separator or a bearing puller. Take out the retaining ring, wave washer (if your unit has one) and the spindle gear with the woodruff key. Remove the spindle housing from the spindle, with the labyrinth ring. Install the labyrinth ring and spindle housing into the new spindle.

Next, install the spindle gear, with the woodruff key, making sure the woodruff key stays in place during installation. Reinstall the wave washer and retaining ring. Install the bearing, using a socket that makes contact with the bearings inner race to avoid any damage to the bearing. Tap the bearing in place with a hammer. Pack the spindle box assembly with new grease and then reattach the spindle assembly box to the gear box housing. Install the wheel cover and handle. Reinstall the inner flange, grinding wheel and outer flange on the spindle and tighten with a lock nut.

Some angle grinders have a button on top of the gear housing that requires you to press down in order to lock the spindle in place, allowing you to remove the grinding disc with only the removal wrench. If this spring fails to return to the upright position, you will not be able to lock the pin down to allow you to change the grinding disc. It can also keep the shoulder pin stuck in the down position, allowing the shoulder pin to shear off during use. To replace the spring, pry the cap off the button assembly with a small, flat-blade screwdriver. Remove the compression spring and install the new compression spring. Reinstall the cap back on top of the spring. You may need to use an adjustable wrench to apply pressure downwards on the cap, to lock it in place.

Before you begin, make sure you are using the correct wrench for your tool, the removal wrench is in good working order and the pins are not bent or broken. Replace the removal wrench if there are signs of damage. Some grinders require you to hold the nut behind the grinding disc as you remove the front nut.

The o-ring prevents grease from the gear box from traveling up into the button. If the o-ring fails, it can prevent movement of the shoulder pin and allow grease to leak out of the button. To replace the o-ring, begin by removing the grinding wheel guard and the handle. Take the gear box out of the grinder, followed by the spindle assembly from the gear box. Pry the cap off the button assembly using a small, flat-blade screwdriver and remove the compression spring and the shoulder pin. Now, inspect the o-ring – if it is deteriorating, or broken, and is preventing free movement of the pin, then the o-ring needs to be replaced. Reinstall the shoulder pin with the new o-ring back in the gear box housing, followed by the spindle assembly. Tighten each screw in an alternating pattern, a little at a time, to seat the o-ring on the shoulder pin. Reinstall the compression spring, the pin cap and then put the gear box assembly back on the grinder. Reinstall the wheel guard and the handle.

The cap is located on top of the button. The cap gives you a surface to push down on in order to lock the shoulder pin in place. It also houses the compression spring, which returns the shoulder pin and cap to the upright position. If the cap breaks, it will not be able to house the compression spring, nor will you be able to push on it. If there is a piece of cap left on the button, pry it off with a screwdriver and install the new cap by pushing down on it, to lock it in place. You may need an adjustable pair of pliers for this.