R8 collets: sizes, drawbar thread and how to identify yours

R8 is the taper fitted to most Bridgeport-style knee mills and the many copies of them. It is not part of the ER system, and it is not a 7:24 taper — that mistake is common enough to be worth clearing up first.

R8 at a glance

Commonly published R8 collet and spindle figures. Where sources disagree, see the note below.
Parameter	Value	Notes
Drawbar thread	`7/16"-20 UNF`	Agreed by collet makers, industrial suppliers and Bridgeport references
Collet body diameter	0.949 – 0.950 in (24.1 mm)	Not a round fractional size — measure rather than assume
Collet overall length	≈ 4.0 in (101.6 mm)	Taper portion is much shorter than the total
Usual collet range	1/8" to 3/4" (3 – 19 mm)	Above 1/2" the grip becomes marginal — see below
Torque transmission	Keyed drive tang + taper	A slot in the collet engages a pin in the spindle bore
Retention	Drawbar from the top of the spindle	R8 is not self-holding; it needs the drawbar

Two published values we could not reconcile. The cone geometry of R8 appears in the wild as both an included angle of about 16°51′ and as 3.5° per side (7° included); these do not describe the same angle and at least one is in error. Separately, one reference gives the drawbar thread as 3/8"-16 UNC while three others give 7/16"-20 UNF — we have followed the majority. If a replacement drawbar is on your list, thread-check the one you have before ordering.

Standard R8 size sets

Unlike ER, an R8 collet holds essentially one size — so sets are bought by increment. These are the sets actually stocked by industrial suppliers:

Pieces	Increment	Sizes covered	Use
8	1/8"	1/8" – 1"	Basic coverage, fewest collets
13	1/16"	1/8" – 7/8"	General imperial work
20	1/32"	1/8" – 3/4"	Fine coverage for mixed tooling
27	1/32"	1/16" – 7/8"	Widest imperial range offered
18	1 mm	3 – 20 mm	Metric tooling in an R8 spindle

The 1/2 inch ceiling. Standard R8 collets are rated for shanks up to about 1/2". Larger shanks can be clamped, but the gripping length left in the collet becomes too short to hold properly under side load. If you routinely run 3/4" tooling, an R8 collet is the wrong tool — use an R8-shank end mill holder or a proper milling chuck instead.

Drawbar torque

The drawbar is what holds the collet in the taper, and over-tightening is the most common way R8 collets are ruined. Tighten to roughly 15–20 ft-lbs and do not exceed 25. Beyond that the collet distorts, gripping accuracy falls and service life drops noticeably.

If you've been leaning on a long wrench to stop tools slipping, the problem is more likely a dirty taper or an undersize shank than insufficient torque.

R8 versus ER — which do you actually have?

	R8	ER
Held by	Drawbar through the spindle	Collet nut threaded onto a holder
One collet covers	One size	About 1 mm of range
Tool changing	From the top of the head	At the spindle nose with a hook spanner
Typical machine	Bridgeport-type manual knee mill	CNC routers, mills, tool holders
Interchangeable?	No — different tapers, different retention, no adapters in the collet itself

An R8 spindle can take an ER collet chuck — a holder with an R8 shank and an ER nose. That's the usual route if you want ER convenience on a manual mill.

Identifying an R8 spindle quickly

Measure the bore at the spindle mouth. Close to 0.950" points strongly to R8.
Get the drawbar out and read the thread. 7/16"-20 UNF confirms it. This is the single most reliable test.
Look for the drive pin inside the bore — R8 is keyed, and a plain taper bore means it isn't R8.