Die grinder

A Die Grinder is a small high speed rotary power tool. They take an array of tools with 1/8" (3.17mm) shank, and typically upto 1.5" diameter.

Die grinders are often referred to as:

Dremel ( a popular brand)
Rotary tool

Tools

Die grinders use a 1/8" collet rather than a Jacobs chuck, so all tools need a 1/8" diameter shaft (3.17mm).

Collets

Available in a few other sizes
Enables use of other drill bit sizes
The set on the right covers 1/16" to 1/8"

Drill bit

Only 1/8" (3.17mm) bits fit
Other size collets are available, enabling a few other drill bit sizes
Die grinders drill far faster than mains or cordless drills
Thin drill bits are very weak, sideways force or movement should be carefully avoided
Don't be tempted to try solid carbide bits in die grinders, in small sizes they are unsuitable and unsafe for handheld tools.
Its possible, upto a point, to grind 1/8" bits down to get smaller hole sizes
Larger holes can be drilled in plastic using a different method

Grind stones

Various shapes, & a few compounds & grades.
Aluminium oxide is used for most grinding stones, and isn't very good at grinding steel.
Alox stones come in various light colours.
Carbide stones are good for steel. These are dark grey, and several times the price of alox.
Use depends on the material and shape of the bit

Drilling with grind stone

Plastics can be drilled with a pointed cylindrical alox grind stone.

Drilling occurs by both abrasion & melting
Speed of cut is good once the stone warms up.
No risk of cracking the plastic
1/4" holes drill easily & quickly
Withdraw the tool without delay once it penetrates
Makes a mess of the stone

Felt polishing pads

Polishing metal, plastics,
Use with polishing compound
Used to create the circular swirl pattern finish sometimes used on metalwork

Screw mandrel

These hold felt polishing pads
The screw thread tip just screws into the felt pad

Cutoff discs

Very weak and break very readily. Expect to use a few even for a small job
Take care to hold the grinder very steady, or disc will break impracticably quickly
Reinforced cutting discs are tougher, but still not strong
Slow cutting compared to an angle grinder
The very narrow kerf of a fraction of a mm is occasionally useful
Half worn discs can be used for access into very tight places

Sanding drum

Usually alox grit belts
Loosen the end screw to release the belt

Rasps

High speed coarse cutters
Fast shaping of soft metals, plastics, wood etc
Don't try to use on harder materials such as steel or masonry

Flap wheel

Sanding flaps
For sanding non-flat surfaces
Normally alox abrasive

Spiral saw

1/8" spiral saw bits can be used in die grinders
Sideways cutting with a thin weak bit isn't the best combination
Limited uses. Best with thin and soft materials eg plastics

Sanding discs

Sandpaper discs are used with a rubber backing pad
Typically weak and fine grit, making progress slow
Not really suitable for stock removal sanding

Cotton polishing wheel

Polishing
Used with abrasive paste, rouge etc

Diamond coated engraving bits

For engraving glass
Security marks hard surfaces
- Postcode and often house number are the usual info engraved
Assorted shapes

Diamond cutoff disc

Not fragile

Metal wire wheels

For removing rust, corrosion, scale & paint
very effective
Full size angle grinder wire wheels are much quicker for most work
Can cause rust staining if used on concrete
Raises grain in wood
Brass wire wheels remove rust, paint etc without damaging steel workpieces

Nylon brush wheel

Cleaning
Removal of soft materials
Removal of loose paint
Plastic bristles mean it won't damage a much wider range of workpiece materials compared to wire wheels
- detailed info

Abrasive types

Tools use the following abrasives

Aluminium oxide

The most common abrasive
Medium performance
Slow going on steel
Not effective on hard steel
Come in a range of colours, mostly pink, also white and assorted light colours
Occasionally grey tools are alox rather than carbide

Carbide

Silicon carbide is a much more effective abrasive
Several times the price
Grey
Cuts hardened steel nicely

Sand

Not very common
Low performance
Can be described as silicon oxide or dioxide

Diamond

The planet's best abrasive, can cut anything bar another diamond
...if it stays attached to the tool, and is coarser than fine dust
Too often neither is the case

Steel

Used as the cutting edge in rasps
Good for plastics, blunt rapidly on anything harder

Brass

Rust & paint removal

Nylon

Cleaning mainly

Tool Maintenance

Grinding stones can clog with metal. This can be removed either by a wirebrush, a coarse abrasive or a spiked wheel.

When grinding soft metals such as aluminium and copper, clogging can be a real problem. Rubbing the disc or stone firmly with chalk first can reduce clogging.

Performance

Linear speed of a 1" 30,000 rpm tool is around 2/3 of that of a 4.5" 11,000 rpm angle grinder tool. Also angle grinder discs & brushes are much more robust. So more speed and force are available from an angle grinder than a die grinder.

The advantage of die grinders lies in their tiny size, which enables access in restricted spaces, and makes fine work with miniature tools practical.

They are also generally much quieter than angle grinders, occaionally an advantage.

Finally more types of tool are available for die grinders than angle grinders.

Usefulness

Personal experiences:

I rarely use mine, but when I do, nothing else will do.
I last used one for cutting out some bearings. I don't know any other tool that could have done the job.
They can be used for cutting pipes in awkward places.
They de-rust old tools well
I use mine with a small grinding bit to trim my toenails. The tiny little grinders the cosmetic firms sell for this purpose are utterly useless, but the Dremel is first rate!

Speed

These tools operate at extremely high rotational speed, and one should check that the tools used are rated for the speed of the machine. There are tools available rated at below typical machine speeds, and these should not be used above their rated speed on safety grounds.

Reducing speed

Some machines have variable speed for lower rated speed tools.

For machines with no speed control there are workarounds for speed reduction, though they're far from ideal. They all rely on the fact that speed is proportional to voltage. This is true for both mains and low voltage machines.

Running the tool off a 110v transformer gives a max speed of 110/240 x the usual run speed
Low voltage tools can be run off a supply of lower voltage to reduce speed.
If no better option is to hand, running a lightbulb in series with the die grinder reduces voltage and speed.
- A multimeter is used to measure the offload voltage and thus calculate max speed. The downside of this arrangement is that as the tool is loaded its speed will reduce at a greater than usual rate as load increases.

See Droppers for more information

Safety

Explosive breakage of the cutting/grinding tool at very high speed can be an issue. Running tools over rated speed should be avoided.

Cutting discs break extremely easily.

Wire brushes are prone to shedding bristles. These shed bristles come off at high speed, often sharp point first. Some wire brushes have a reduced speed rating.

Eye protection shold be used, and one should stay out of the plane of spin of the tool.

Catching a spinning tool on something can yank the machine. The machine should be held firmly, hands kept out of the way of the tool, and force on the workpiece should be kept light.

Tools that cause vibration should be retired. These are much more liable to fail dangerously in use. They also put high stresses on machine bearings causing premature failure.