How to choose discs for a random orbital sander: diameter and holes

The first thing to choose for a random orbital sander is the abrasive disc that matches the backing pad: the same diameter (125 and 150 mm are the most common in the collection) and the same hole pattern (6, 8 or 14 holes) as your machine. Then the grit: 40-80 for removing paint or levelling, 120-180 for preparation, 240-320 for finishing. Aluminium oxide is enough for work on wood and paint; for metal and heavier work, look at zirconia.

The types

• Hook-and-loop discs: round discs with a hook-and-loop back and dust extraction holes. This is the most common type, in diameters between 50 and 225 mm. One example is the mesh abrasive disc DTM3103-QZ.
• Mesh discs: the abrasive is bonded to an open mesh and the dust passes through. They clog less, for example the mesh disc DTM3123-QZ.
• Abrasive sheets: rectangular, for sheet orbital sanders. One example is the mesh abrasive sheet DTM3022-QZ.
• Sandpaper: the traditional paper version, for dry work on wood and filler.
• Sponges and pads: abrasive sponge discs or nylon discs with silicon carbide, for touching up and finishing without marking the surface.
• Backing pads: the pads on which the disc is fitted, to be replaced when the hook-and-loop face no longer grips.
• Polishing accessories: the collection also includes lambswool pads for polishing.

How to choose by job

In practice it pays to standardise: choose one diameter and one hole pattern for all the machines in the department and keep 80, 120 and 240 grits in stock. You reduce ordering errors and your staff always know which disc to fit.

The features to look at

• Diameter (mm): in the collection 50, 75, 100, 115, 125, 150, 178 and 225. What matters is the diameter of your backing pad.
• Number and arrangement of holes: 6, 8 or 14. If the holes do not line up, dust extraction loses efficiency and the disc clogs.
• Grit: from 40 to 320. Low for stock removal, high for finishing.
• Material: aluminium oxide (also stearated, which resists clogging better), zirconia, silicon carbide, nylon with silicon carbide. Silicon carbide is typical of fine finishing.
• Pack size: 5, 10, 25, 50, up to 100 pieces for heavy users.

Common mistakes

• Buying a disc with a different diameter from the backing pad: if it is larger, the edges get damaged; if it is smaller, the backing pad scratches the workpiece.
• Ignoring the hole pattern: the holes must line up with those of the backing pad, otherwise dust extraction does not work.
• Not stepping through the grits: after 40 grit, do not go straight to 240. Go through 80 and 120.
• Pressing hard: with a random orbital sander the weight of the machine is enough. Pressing heats the disc and clogs it.
• Holding the disc at an angle: the machine must be held flat, otherwise the edge of the disc digs in.

Frequently asked questions

Which grit should I start with?

It depends on the workpiece: 40-80 for old paint or rough surfaces, 120 for wood that is already fairly smooth. If you are not sure, start at 80 and do a test.

Perforated or non-perforated disc?

If your machine has dust extraction, use a perforated disc with the holes aligned to the backing pad. On a non-perforated backing pad, choose a non-perforated disc.

Can I use the same disc on metal and wood?

It is better not to: once used on metal the disc is already worn, and metal particles can mark the wood. Keep the abrasives separate.

When should I change the disc?

When it is clogged, smooth, or the hook-and-loop face no longer grips.

All the sizes and grits are in the Abrasive Sheets and Pads for Random Orbital Sanders collection; to choose with the AMES technicians or request a quote for quantities, get in touch.