How to use a 125mm angle grinder hook and loop backing pad on a Bosch 5 inch random orbit sander

If you receive my newsletter, you’ll know I wrote a rave review of the Bosch GEX 125-1 AE (the 120V version is coded ROS20VSC).

It’s a fantastic random orbit sander. I use it for sanding skimboards and cupboards.

But the hooked backing plate wore out too quickly.

I sanded half a cupboard at full speed with a 40 grit sanding disc, and the hooks on the backing plate disappeared.

I contacted Bosch about this, asking them whether this was standard for their backing plates.

Bosch replied, saying that it happens sometimes, especially with coarse grit sanding discs, and that they’d send me a free backing plate replacement.

I was elated at their response, since these backing plates are expensive.

In the meantime I needed to sand down a cupboard and skimboards.

Cheap backing plate

I bought a cheap hook and loop sanding plate for a 125mm angle grinder from a local hardware store. Compared to the Bosch backing plate it costs mere pennies.

But it can’t mount to the Bosch ROS. It’s as different from the Bosch backing plate as seawater is from cola.

The Bosch’s backing plate locates onto the sander via four pins and is secured to the sander with four screws.

The cheap backing plate secures to a 125mm angle grinder via 14mm thread.

Adaptor

I’d have to make an adaptor.

So I drew up a concept.

Here’s a screenshot of the adaptor drawn up in Sketchup.

Sketchup screenshot of adaptor to use angle grinder hook and loop backing plate on Bosch ROS20VSC random orbit sander.

I’m not sure if this adaptor will work for other random orbit sanders. Some ROS machines use three screws for the backing plate, while others, like the Bosch, use four.

Also, take note that the angle grinder hook and loop backing plate does NOT take a 125mm (5 inches) sanding disc, but a 115mm (4.5 inches).

How to attach it

I follow three steps to use my backing plate adaptor.

1. I secure the 14mm socket cap screw to the adaptor with a washer and lock nut.
   • I use a 12mm Allen key and 22mm spanner (wrench, if you’re in North America).
2. I screw the adaptor to the Bosch random orbit sander with four button head cap screws.
   • The Bosch GEX 125-1 AE uses 4mm thread.
   • I use a 2.5mm Allen key.
3. I screw the angle grinder backing plate to the 14mm bolt.

Wood, plastic or metal?

I contemplated making the adaptor from wood, but realised it wouldn’t hold up against the random orbit sander’s vibration. There’s not much meat on the adaptor, so it would fall apart.

I thought about having it made from mild steel, but realised that it might be too heavy for the machine.

I would have been happy with plastic, but the man who made the adaptor gave me a good price on aluminium.

I got a happy medium between the worries of wood and malady of metal.

Why not DIY?

Designing the adaptor took me back to my toolmaker days, when I worked on lathes, milling machines and other machinery. (I’m tempted to say, “those were the days!”)

The right machinery destroys limits. Not having the right machinery hamstrings. Limitations call for innovation or delegation.

I don’t have a milling machine and a lathe, both of which were required for making the adaptor. Delegation was my only option.

Does it work?

Sanding plywood with the Bosch GEX 125-1 AE with an adaptor for an angle grinder sanding plate.

Sadly, it doesn’t work well.

Using the adaptor places the sanding surface much farther from the machine. This makes the machine unstable.

In fact, I need to use two hands to keep the machine stable. I also have to drop the ROS’ speed, otherwise the machine vibrates too much.

I don’t advocate using this sort of adaptor as a permanent replacement. I had it made to help me out of a sticky situation.

When I need to sand down cupboards and don’t have a Bosch backing plate, or can’t find one at a local store, this tool will help me out of a tight spot.

But it’s best to use a Bosch ROS20VSC original backing plate.

Check this short video to see how the Bosch GEX 125-1 AE (ROS20VSC) works with the custom adaptor.

Excellent machine

Despite the hiccup with the Bosch sander’s backing plate, I give the tool two thumbs up.

It doesn’t remove material as fast as a belt sander, but it’s safer. It’s also quicker than an orbital sander, yet gives the same finish.

But the outstanding feature is the Bosch’s apathy towards sanding direction. Whereas a belt sander, orbital sander and hand sanding require sanding with the grain, the random orbit sander puts no restriction on your sanding direction. It doesn’t scratch your project, no matter how you sand it.

I suspect the problem with the backing plate is not restricted to Bosch sanders; it’s one of the drawbacks of using a random orbit sander with a hook and loop system.

In future I’ll start with a 60 or 80 grit sanding disc, instead of 40. That should increase the backing plate’s life.

Read my review of the Bosch random orbit sander and get yourself one if you’re in the market for less stressful sanding.