(Draft, in progress) Collets and tool sizes for routers, drilling machines are a bit confusing at first but in reality there are not tat many to consider for the small workshop or DIY, unless you have specialized needs.
Lets start with the small and work upwards.
Cylindrical collets
Dentists size; 2mm
Dentist tools and dental tech tools usually usually 2mm diameter shanks. While there may be imperial sizes, to my knowledge they are not much used.
1/8″ of an inch (3.175mm)
For “Dremel” size tools, the 1/8″ dominate totally. While there are metric equivalents in 3mm and 4mm these are not used much. Hand held machines this size are usually supplied with collets in several sizes but 1/8″ is used more than any other. This size is also used in engraving machines but above all, in the electronics industry for PCB manufacturing. The PCB industry use drill bits this size by the thousands.
6mm, 1/4″ and 8mm
This size is used much for hand held routers of the intermediate size. The 1/4″ (6.35mm) totally dominate in the US and UK. In Europe, the larger 8mm is quite common. Many machine spindles that that take 1/4″ usually take 8mm too, and the 8mm have significantly stiffer shanks. Try to get collets for both 1/4″ and 8mm. The 6mm is a size you can omit as it, to my knowledge, is hardly used at all. It is too close to 1/4″. Unfortunately some European manufacturers make 6mm machines that do not take 1/4″ collets and you will be very limited in the selection of bits for these machines.
12mm, and 1/2″(12.7mm)
This size is used for compact hand held machines, where the larger conical fixes would make the machine too big. The size is used for large hand held routers. Again, the imperial size 1/2″ dominates, but 12mm is quite common in Europe.
Other sizes
There are machines that use other collet sizes, especially lathes, but this is typically to hold a workpiece directly, not really made to hold a tool shank.
One note about the collets is that the collets themselves, the exterior, the nut etc are available in a large number of types. For hand tools these are essentially different for each manufacturer, even if the inner diameter is the same.
Conical
Cylindrical sizes above 1/2″ are seldom seen and adjustable chucks are typically limited to 13mm. Above this size, the conical mounts dominate.
Machine cones, Morse taper, MT
The next size up found in workshops is the “Morse” cones in a few sizes. The geometries are specified in ISO 296, also known as MT-sizes. While there are 8 sizes, zero to seven, you will mostly find size 2 and 3 in smaller workshop.
Other, larger conical mounts
In larger machines, there is a large number of standards and alternatives; Brown & Sharpe, R8, Jacobs, Jarno, NMTB, BT, ISO Tapers. I will not go into these here as it is a fairly large subject.
Summary
| Metric | US/UK | Use |
| 2mm | ? | Dentists, clockmakers |
| 3mm, 4mm | 1/8″ | Fine mechanics, PCB drilling, inlay work |
| 6mm, 8mm | 1/4″ | Fine mechanics, deburring, small/medium hand routers |
| 12mm | 1/2″ | Large hand routers |
| Morse 2 & 3 | Morse 2 & 3 | Bench and pillar drill presses, routers, lathes |
| SK, HSK, BT | ANSI 30, ANSI 40 | Metal mills, routers, industrial wood routers |
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