Benchtop Disk Sander Using a Washing Machine Motor

This Instructable describes how to make a powerful and professional benchtop disk sander with the following features:

• Variable speed and high torque
• Dust extraction port
• Disk guard for safety

The following are not essential to the machine and are included as options:

• Oak edging strips on the plinth
• T-track for use with a mitre gauge or other tools

You might be considering omitting the dust extraction port and/or the disk guard – I would recommend against doing so as they are essential safety features.

Key dimensions:

• Overall: 450mm wide x 545mm deep x 265mm high
• Disk diameter: 225mm
• Plinth dimensions: 250 x 450mm, or slightly larger if you add edging strips

Construction:

Sanding disk:

• 12mm MDF: 230mm x 230mm

Plinth:

• Base: 450mm x 250mm
• Top: 2 of 450mm x 110mm 

Cabinet:

• Cabinet base: 12mm plywood 420mm x 255m
• Cabinet front / control panel: 12mm plywood 420mm x 160mm
• Cabinet back: 12mm plywood 420mm x 160mm
• 2 Cabinet sides, 12mm plywood each 216mm (approx.) x 160mm high
• Cut these to length when doing the final assembly
• Cabinet top: 12mm plywood 240mm (approx.) x 420mm
• Check and cut the top to size when doing the final assembly

Guard:

• 2 pieces of 12mm plywood, each 150mm x 150mm

Dust extraction:

• Front baffle: 9mm plywood or similar: 81mm x 302mm
• Back baffle: 9mm plywood or similar: 39mm x 338mm
• Extraction base: 5mm plywood or similar: 65mm x 302mm

Other:

• 2 battens, each 44mm x 44mm x 350mm
• 2 plinth supports, each 86mm x 300mm x 18mm. The length is probably more than required. We’ll cut them to length during final assembly.
• Oak edging (optional): 15mm x 24mm x 1000mm (approx.)

Electrical

• Universal 3-phase washing machine motor
• 3 phase variable frequency drive, rated at least 1kW, e.g. https://www.amazon.co.uk/gp/product/B0CKX4DSCH/
• Mains cable with plug, rated for your power socket voltage (240v in this case) 
• Rocker on/off switch, e.g. https://www.amazon.co.uk/gp/product/B083NLSRTT Note that the on/off switch is in the control circuit, not in the AC power circuit and is rated 5v DC.
• Miscellaneous short lengths of cable and cable connectors

Hardware and finishing products

• U bolts, 32mm (1.26") Inner Width 46mm Length, thread size M6, e.g. https://www.amazon.co.uk/gp/product/B0BGXCZQGD/ 
• T-track at least 450mm long (optional), e.g. https://www.amazon.co.uk/gp/product/B0CQK5ZJ9V/
• Sticky backed Velcro large enough to cover the 225mm diameter disk
• Flange, to attach the disk to the motor shaft, e.g. https://www.amazon.co.uk/gp/product/B0BTHQNPQC/
• 225mm diameter sandpaper disks, e.g. https://www.amazon.co.uk/Sanding-Discs-Holes-Drywall-Sander/dp/B09VL3M22L/
• Water-based polyurethane varnish
• Miscellaneous wood screws

Tools and equipment:

• Hacksaw
• Locking pipe grip pliers
• Table saw or hand saw
• Router with a cutter bit: a 4mm diameter spiral cutter bit is ideal
• Circle cutting jig
• Router table (optional)
• Coping saw
• Drill press or hand drill and a selection of wood drill bits
• Screwdriver
• Safety equipment:
• Wear eye protection when using any power tool
• Wear a dust mask when using power tools with wood or MDF
• Use ear defenders when using the table saw or router

The pulley is attached to the shaft using a powerful industrial press, so removing it can be difficult. Online, you’ll find various methods for removing it, most of which require specialist equipment. However, for this project, we don’t need the pulley, so it doesn’t matter if we break it and that greatly simplifies the challenge. Here’s how to remove it:

Turn the motor on its end so that the shaft is facing upwards. Support it on blocks of wood if needed: you need it to be stable.

Using locking pipe grip pliers, clamp the shaft below the pulley so that it can’t rotate. You’ll need to hold the pipe grip so that the shaft doesn’t move during the next step: an assistant may be helpful here.

Using a hacksaw or other metal saw, cut the pulley in a straight line vertically downwards, about 1mm from the shaft. Cut all the way through it, taking care that the cut is maintained 1mm from the shaft.

The remaining 1mm of metal between the cut and the shaft should crack due to the strain of the press fit and the pulley should now be loose enough to pull off with a pair of pliers. If it won’t come off, make another cut at 90 degrees to the first, closer to the shaft if possible.

If you are not embedding a T-track, simply cut two pieces of 12mm plywood to size (450mm by 250mm) and glue or screw them together one on top of the other.

To embed the T-track, we use three pieces of plywood: the base (Part C in the diagram) and two top pieces between which the T-track will go (Parts A and B in the diagram).

The plywood is 12mm deep and the T-track is 12.8mm deep, so we need to deepen the channel by 0.8mm. Glue parts A and B to part C and then using a router with a profiler bit (this one for example: https://www.amazon.co.uk/gp/product/B011861BS4) to extend the channel 0.8mm into the base. Check that the T-track will now fit flush with the top of the plinth.

If you are adding the edging strips, cut them to size, make the channel for the T-track and glue the strips to the edges of the plinth.  

Apply three coats of polyurethane varnish to the top and sides of the plinth.

Cut the T-track to length, lightly file the ends to remove any burrs and then secure it into the channel with screws.

Cut the base board for the cabinet to size, then mark the holes for the U-bolts which will secure the motor to the base. Drill the holes for the U-bolts, and screw holes to attach the base to the battens. Screw the base onto the battens. At this point, I also attached the back baffle for the dust extraction, which you can see in the photo.

Attach the motor to the board with the U-bolts. Don’t tighten the bolts at this stage, as you’ll need to adjust the position of the motor later.

Cut out the cabinet front from 12mm plywood.

Mark out the holes for the motor spindle, on/off switch and the four corners of the variable frequency drive unit, then drill the holes.

Use a coping saw or router with a cutter bit to cut out the square hole for the variable frequency drive.

Cut the MDF to size: 230mm x 230mm

Mark the centre of the square and drill a hole to match the circle jig

Mark the holes to attach the flange – it’s easier to do this when the material is still square rather than after you have cut it into a circle.

Cut out the circle of MDF using the router – either using a jig on a router table or a circle jig attached to a freehand router.

Drill the holes to attach the flange, then attach it. It is important that the flange is positioned exactly in the centre of the disk. Depending on what bolts you use, you’ll need to counter-sink or recess the bolt heads on the front of the disk, so they don’t protrude. 

Peel off the backing paper of the Velcro and carefully attach it to the face of the disk, taking care to avoid creasing it. Attach a sand paper disk using the Velcro.

Attach the plinth supports to the inside of the battens, with the back of the plinth supports butting up against the front of the cabinet base.

Drill a hole in the centre of the front baffle to fit the hose of your vacuum cleaner or dust extraction unit. Mine is 35mm diameter.

The dust extraction parts are shown in the diagram. The front baffle is attached to the inside of the plinth supports, in front of the sanding disk. The back baffle is attached to the inside of the battens, behind the sanding disk. I used small wooden brackets to secure the baffles. Alternatively, you could use glue or metal brackets. The base is glued to the bottom of the two baffles. 

The guard is made from two pieces of 12mm plywood glued together and cut in the same way as the disk, using the circle jig. The radius of the inner circle is 5mm larger than the sanding disk: 117.5mm. The radius of the outer circle is 140mm.

Cut the outer circle, then cut the inner circle. Remove the newly made ring from the jig. Measure the length of the arc that you’ll use as guard and cut it to size. I also had to make a straight cut to avoid obscuring the VFD control panel.

Adjust the guard so that there is a gap of 5mm all around between the guard and the disk, then screw the guard to the front of the cabinet. Depending on the position of the cabinet front relative to the sanding disk, you might need to put shims behind the guard to bring it forward a little. Ideally the sanding disk should protrude no more than 5mm in front of the guard.

There are two circuits: the power circuit which is mains voltage AC and the control circuit which is 5v DC.

Power circuit:

Connect the three wires of the motor to the VFD (output) terminals marked U, V and W. You can connect any of the three wires to any of the terminals. This is shown as A in the photo below.

Drill a hole in the back of the cabinet and push the AC power cable through it. Connect the Live and Neutral wires of the AC power cable to the two (input) AC terminals of the VFD. This is shown as B in the photo below.

At this point, I recommend you connect to the mains and test that that motor works and can be controlled by the VFD. If the motor ‘forward’ direction is opposite from what you expect, simply swap any two of the three cables in terminals U, V and W: this will reverse the direction.

Control circuit

Connect the on/off switch between the COM and X1 terminals of the VFD control circuit. This is shown as C in the photo below.

Finalise the position of the motor, by checking the amount of shaft overhanging the cabinet base and the orientation with respect to the hole. You want the back of the disk to be about 3 to 5 mm from the front edge of the cabinet base.

Once positioned correctly, tighten all the nuts on the U-bolts underneath the machine.

Now you can position the front of the cabinet, just back from the forward edge of the base. The exact position will depend on the dimensions of the flange and motor. Once you have finalised the position of the cabinet front, cut the cabinet sides to length.

I used wooden brackets to secure the cabinet front, back and sides to the base. Alternatively, you could use screws from beneath the cabinet or pocket holes.

The VFD is extensively programmable. Detailed instructions on how to programme it are provided in the manual supplied, so we don’t cover that here. The essential configurations are:

• P022 set to 1: activates the on/off switch to use the control circuit through terminals X1 and COM.
• P003 set to 85: This sets the base frequency – you might need a different value if you are using a different brand of motor
• P004 set to 310: This sets the highest frequency – you might need a different value if you are using a different brand of motor

The flange is secured to the shaft using two grub screws, adjusted by a hex (Allen) key. I found that there wasn’t sufficient space between the disk and the cabinet to insert the hex key to tighten the grub screws. To solve this, I modified the hex key – it’s not pretty but it works. You’ll need a wooden dowel or plastic rod about 120mm long:

• Using a hacksaw, cut off the short part of the hex key. We only need the long part.
• Drill a hole in the end of the wooden dowel or plastic rod just big enough to push in the long part of the hex key and about 25mm deep.
• Apply some epoxy (or similar) glue around the end of the dowel from which the hex key protrudes and wait for it to set.
• Using the new tool, you can tighten the grub screws and secure the flange to the shaft.

Alternatively, you could cut a slot in the front panel of the cabinet from the base to the hole for the shaft, so that you can remove it with the flange and disk attached and access the flange from the back.

Cut the cabinet top to size, drill screw holes and attach it to the cabinet.

Check the position of the plinth – it should be 3 – 5 mm in front of the sandpaper disk. Mark and cut the plinth supports to length (on my unit the length is 289mm) and screw them to the inside of the battens.

Attach the plinth to the plinth supports. I used pocket holes and screwed it on from underneath. Alternatively, you could use brackets.

I added some brackets to the bottom of the plinth supports so that I could clamp the machine to the workbench. However, it’s stable without them, so they aren’t essential. 

A standard mitre gauge will fit the T-track. I made my own from some oak offcuts, fixed at 90 degrees.