Guide

Mini grinder or 5-inch grinder: which one, and which disc

Two sizes, two kinds of work. Here is how to pick between a 3-inch mini grinder and a 4-1/2 to 5-inch grinder, and how to match the disc to the job.

A man in safety glasses grinding steel with a red TEENO angle grinder, sparks flying

Two sizes

What each grinder is for

A mini grinder takes a 3 in disc. The 3″ Mini Angle Grinder spins it at 19,500 RPM and fits in one hand, so it reaches a bolt behind a bracket or a pipe under a sink. It cuts well and grinds a little.

A full-size grinder takes a 4-1/2 or 5 in disc. The 5″ Brushless Angle Grinder has three speeds up to 8,500 RPM and a side handle for two hands. It grinds welds and rust over a wide area and cuts thicker stock.

If most of your jobs are small cuts, start with the mini. If you clean up welds or strip paint off a trailer, the full-size grinder will save you hours. Plenty of people end up owning both.

Discs

The disc decides the job

The grinder spins it. The disc does the work. Use each one the way it was made to be used.

  1. 1

    Cut-off disc

    Thin, for cutting with the edge: bolts, rod, angle iron, thin pipe. Do not grind with its flat face.

  2. 2

    Grinding wheel

    Thick and stiff, for taking down welds and rough edges on steel.

  3. 3

    Flap disc

    Overlapping sanding flaps, for rust, paint and blending a weld smooth.

  4. 4

    Diamond or tile disc

    For tile, stone and masonry edges. For long straight cuts, a saw with a base keeps the line.

Speed and guard

Match the disc to the grinder’s speed

Every disc has a maximum speed printed on it. OSHA’s rule for workplaces puts it plainly: The spindle speed of the machine shall be checked before mounting of the wheel to be certain that it does not exceed the maximum operating speed marked on the wheel. OSHA, 29 CFR 1910.243 For the 3″ Mini Angle Grinder, that means a disc rated for at least 19,500 RPM.

The same rule says Abrasive wheels shall be used only on machine provided with safety guards OSHA, 29 CFR 1910.243, and for hand-held grinders like ours, the top half of the wheel shall be enclosed at all times. OSHA, 29 CFR 1910.243 Keep the guard on, and turn it so it sits between you and the disc.

OSHA also asks for eye protection that provides side protection when there is a hazard from flying objects. OSHA, 29 CFR 1910.133 Sparks and disc fragments fly sideways as often as straight ahead.

Before you start

A short checklist for every cut

  1. 1

    Check the disc

    No cracks or chips, and a speed rating at or above the grinder’s top speed.

  2. 2

    Set the guard

    Between you and the disc, covering the top half.

  3. 3

    Clamp the work

    A clamp or a vise holds it still, so both hands stay on the tool.

  4. 4

    Let it stop

    Wait for the disc to stop before you set the grinder down. The 5″ Brushless Angle Grinder brakes on its own when you let go.

Side by side

3-inch mini grinder or 5-inch grinder

Compared3″ mini grinder5″ grinder
Disc size3 in4-1/2 in and 5 in
Top speed19,500 RPM8,500 RPM, three speeds
Batteries in the kitTwo 2.0 AhTwo 4.0 Ah
How you hold itOne handTwo hands with the side handle
Better choice whenCuts are small or in tight spotsYou grind welds, rust and paint

Questions

Questions about grinders

Can I put a 4-1/2 inch disc on a 3-inch mini grinder?

No. The 3″ Mini Angle Grinder takes 3 in discs only. For 4-1/2 and 5 in discs, use the 5″ Brushless Angle Grinder.

Which grinder is better for rust removal?

The 5″ Brushless Angle Grinder: a 5 in flap disc covers more surface, and the low 3,000 RPM speed keeps it under control on thin sheet.

Sources