Slow-Speed Grinders

Slow Speed Has a Wide Range

Slow is slow, right? Not exactly. The wheel’s surface travels much faster on some machines than on others, and that makes quite a difference in heat buildup. Higher surface speeds create more friction and more heat. A wheel’s surface speed, measured in surface feet per minute (sfpm), depends on two factors:  the motor’s revolutions per minute (rpm) and the wheel’s circumference.

You might think all slow-speed grinders spin at the same speed, but they don’t. At ther slowest settings, they range from 1,120 to 2,000 rpm (see Profiles, pages 68 and 69). Most are in the range of 1,725 to 2,000 rpm.

Slow-speed grinders have 6- or 8-in.-dia. wheels. The distance around a 6-in. wheel is about 1.5 ft.; the distance around an 8-in. wheel is about 2 ft. A 6-in. wheel’s surface travels 33 percent slower than an 8-in. wheel’s surface, if both wheels rotate at the same speed.

Combine both factors, speed and circumference, and you get the sfpm number, the one number that tells you the most about heat buildup on any particular grinder. It’s easy to figure out: Speed (rpm) multiplied by a wheel’s circumference equals surface speed (sfpm). We’ve done the math for each machine.

It’s no surprise that 6-in. machines grind at cooler temperatures than 8-in. machines do. The minimum sfpm rate for 6-in. grinders varies from 1,760 to 3,140 sfpm, for 8-in. grinders, from 3,610 to 3,770 sfpm. Both work well, but you must exercise a lighter touch and quench more often when using an 8-in. grinder.

Have a Good Rest

Tool rests on most slow-speed grinders don’t offer the precision, adjustability and convenience we want for woodworking tools. Only one machine, the Palmgren 82064, has tool rests that we like.
We recommend adding at least one aftermarket tool rest on any other grinder (see photos, left). This adds about $40 to a grinder’s cost and requires mounting the grinder on a plywood base or benchtop. Two excellent tool rests are available (see Sources, below). We prefer the Veritas for its larger locking levers and smoother operation. A single aftermarket rest will do, because you’ll primarily work on a 60-grit wheel when grinding chisels and plane irons. Two aftermarket rests aren’t absolutely necessary but are a nice luxury.
An aftermarket tool rest offers multiple benefits:

Tool-free adjustability. No wrenches are necessary to adjust the rest’s angle. In contrast, manufacturer-supplied tool rests require one or two wrenches to adjust and typically are balky.

Continuous positioning. You can raise or tilt the rest to grind any bevel between 15 and 90 degrees.  With two pivoting points, fine-tuning an angle is quite easy. Manufacturer-supplied tool rests often have detents, limiting your choice of angles.

Comfortable grinding position. When grinding at 25 degrees, which is one of the most commonly used bevel angles, we prefer to adjust an aftermarket rest to sit high and at a shallow angle to the grinder’s base. This puts your wrists at a comforable angle. To grind the same bevel, most rests that ship with the grinders must be positioned at a steep angle to the grinder’s base, where it’s awkward to grind a straight edge because your wrists are uncomfortably bent. These rests simply slide in and out; they can't be raised to a more comfortable angle.

Minimum gap. You can easily set the rest 1/8 in. or less from the wheel at any angle. That’s important for safety. When grinding 25- to 30-degree bevels on many manufacturer-supplied rests, the smallest gap you can set is much greater than 1/8 in.

Broad support. The platform is deep and wide. Extra depth helps support long-handled tools. Extra width helps your fingers guide a plane blade, so you get a straight edge. Most manufacturer-supplied tool rests are too small.   



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White Wheels Produce Less Heat

Many grinders come with one or two grey hard-bond wheels. We prefer white soft-bond wheels because they provide more insurance against overheating your tools.

We recommend installing a medium 60-grit white wheel on every grinder. Some machines come with this wheel, but many don’t. It’s easy to replace a grey wheel with a white one, but it does add $20 to $31 to the machine’s cost.

Hard- and soft-bond wheels have very different properties. Bond refers to the strength with which abrasive particles are bound together. Grey hard-bond wheels wear very slowly. They retain their shape well but, when grinding tool steel, can quickly load and become glazed with metal particles, increasing the friction on a tool. They work better with softer steel. White soft-bond wheels are preferable for chisels and plane irons. Soft-bond wheels come in other colors, too, but white is the most common. Soft-bond wheels are more friable, meaning they wear faster, but that helps prevent them from loading.

The grinders we tested come equipped with coarse 36-grit and medium 60-grit wheels or medium 60-grit and fine
120-grit wheels. We prefer the medium and fine combination, as long as both are soft-bond wheels. The finer the grit, the slower and hotter a wheel cuts. We recommend a medium-grit white wheel for grinding chisels and plane irons. It removes material fast without too much heat buildup. Coarse wheels are good for major reshaping. Fine wheels are good for shaping delicate tools, such as carving gouges, and for putting a final edge on some tools, such as a scrub plane iron.

A Wheel Dresser Is a Must

Dressing your wheels keeps them clean and round. It also creates a flat face. Clean wheels create less heat, round wheels minimize vibration and a flat face creates a straight edge. Dressing is routine maintenance. For the best results, you should dress the wheels each session if you grind more than a few minutes.  

We prefer a T-style diamond dresser for white wheels. It costs about $15. This dresser works fast and doesn’t require finesse. Some grinders come with one. If a grinder doesn’t include this dresser, we recommend getting one when you buy your grinder. New wheels must often be trued round after they’re mounted, so your machine will run with the least vibration.

Wide Wheels Make Grinding Easier

Slow-speed grinding wheels come in two widths: 3/4 and 1 in. Among the machines we tested, all  the 6-in. grinders come with 3/4-in. wheels and all the 8-in. machines have 1-in. wheels. Most woodworkers find it easier to grind a straight edge when using a 1-in. wheel. A 3/4-in. wheel requires more finesse.

We recommend getting an 8-in. machine if you will do lots of grinding, such as sharpening turning tools. Wheels that are 1 in. wide with an 8 in. diameter have much more material than narrower and smaller wheels, so they’ll last longer.

High-Speed Option Is Handy

Many slow-speed grinders have dials that allow you to increase the motor’s speed. At the top end, these variable-speed motors spin about as fast as a high-speed grinder (3,450 rpm). One model, the Woodcraft 144291, has two separate speeds, low and high.

High speed is a very handy feature, particularly if you also use your grinder for general metalworking. You can remove a lot of metal much faster at high speed, and some grinding jobs do require hogging off lots of material.

Lightweights Are Easy To Store

Many woodworkers store their grinders on a shelf or under a bench when not in use. A lightweight machine is easier to lift and maneuver into cramped quarters than a heavier one.

Lightweight 6-in. slow-speed grinders generally weigh about 27 lbs. Moderately heavy 6-in. models and most 8-in. models weigh about 45 lbs. One very heavy 8-in. model, the Delta 23-725, tips the scales at 86 lbs. Moderately heavy machines can be difficult to lift above or below your waistline. The Delta 23-725 requires a permanent home.

Tip

To eliminate annoying vibration from your grinder, draw pencil lines all the way around the surface of both wheels. Dress the wheels until the lines are completely gone. Sometimes you must remove quite a lot of material to make the wheels perfectly round and balanced. New wheels often require this, too.

A Light Is a Bonus

We really like grinders that come with a built-in gooseneck light. It’s a lot easier to accurately set a tool rest’s angle and grind with precision when you can clearly see what you’re doing. A flexible gooseneck lamp is preferable to fixed task lighting. You can bend the lamp to shine directly on your tool’s edge on either wheel. All the lamps on the grinders we tested take a maximum 40-watt bulb.

Power Isn’t a Big Issue

Chisels and plane irons don’t require lots of power to grind. Even the smallest grinder does a good job. Extra power usually doesn’t get this job done faster, but it does help if you’re shaping large turning gouges, altering the bevel on a large chisel or plane iron or using the grinder for metalworking.

In general, 8-in. grinders are more powerful than 6-in. grinders. For a rough guide, look at an individual model’s amperage. Variable-speed machines have more torque at high speed than at low speed. In our experience, low-amp 6-in. variable-speed grinders don’t have enough power at low speed for reshaping large turning tools at a reasonable pace.

Tool-Free Arbor Nuts

Changing wheels on most grinders can be a hassle, although you probably won’t do it often. To change a wheel, you must dig out a wrench and  hold the wheel with a thick leather glove or remove the other wheel cover and hold the opposite nut with another wrench. Two grinders from Delta, the GR275 and GR450, have special arbor nuts that don’t require any tools. Folding two wings toward each other loosens the nut so that it can be unscrewed by hand. Clever! Unfortunately, this nut won’t fit on other grinders, because their wheel covers don’t have sufficient clearance.