End mill holders: side-lock, power milling chuck, ER collet chuck

These three solve different problems, and picking wrong is expensive in a specific way — a cutter pulling out of a collet under heavy radial load does not just ruin the part, it can ruin the holder. This page covers the Weldon flat standard, the size threshold where set-screw retention stops being optional, and where a power milling chuck fits.

The Weldon flat

A Weldon (side-lock) holder uses one or two set screws bearing on a flat ground into the tool shank. The flat is what makes it work: it locks the cutter both axially and torsionally, so unlike a collet, there is no friction to overcome and nothing to pull out.

Weldon flat positional dimensions, in inches. From one tooling manufacturer's published specification sheet — see the note below on tolerances and hand-ground flats.
Shank Ø	A	B	C	D
0.1250	0.155	0.500	0.020	—
0.1875	0.155	0.500	0.025	—
0.2500	0.155	0.500	0.030	—
0.3125	0.295	0.750	0.040	—
0.3750	0.295	0.750	0.050	—
0.4375	0.345	0.850	0.060	—
0.5000	0.345	0.850	0.060	—
0.6250	0.415	0.900	0.065	—
0.7500	0.470	0.900	0.075	—
1.0000	0.530	1.000	0.075	0.900
1.2500	0.530	1.000	0.095	0.900

Published tolerances: A ±0.004, B ±0.015, C +0.015 / −0.000, D ±0.008. Sizes up to 0.500 take a single flat; 1.000 and above are commonly specified double (two flats, 180° apart).

Do not grind the flat by hand. The specification sheet that publishes this table states it directly: hand-ground flats induce runout. If you need a flat, have it ground — or buy cutters that come with one, which most general-purpose end mills do.

Where the flat has to sit

Position matters more than most people expect. The cutting flutes must not be buried inside the holder bore, and the tool must not hang out further than it needs to. The published reason for the table above replacing the older high-speed-steel Weldon specs is exactly this: the old figures were based on overall tool lengths, and modern cutters are sized by flute wash position or reduced-neck transition instead.

The 3/4-inch threshold

Above roughly 3/4", set-screw retention is not optional. The recurring failure mode is a 3/4" or larger end mill in a collet-only holder: broken tools and scrapped parts. A side-lock holder removes the pull-out failure entirely at those sizes. A practical starting inventory is 3/8", 1/2", 3/4" and 1" side-lock holders — that covers most face, profile and slot milling in a general shop.

Power milling chucks

A power milling chuck (sometimes called a heavy-duty or needle-bearing milling chuck) clamps mechanically through a roller or needle-bearing arrangement rather than a plain split collet. The practical consequences:

Highest torque transmission of any of the four systems on this page — this is the reason to own one.
More clamping force than a standard collet chuck.
Higher runout than hydraulic or shrink fit, and less damping — so it is a roughing tool, not a finishing tool.

Choosing between the four

	ER collet chuck	Side-lock (Weldon)	Power milling chuck	Hydraulic
Holds	Any round shank in range	Only shanks with a flat	Round shank, discrete sizes	Round shank, h6
Pull-out resistance	Depends on nut torque	Mechanical — cannot pull out	Very high	High
Torque capacity	Good	High	Highest	Moderate
Runout	5 – 20 µm typical	5 – 15 µm typical	Higher	≤ 3 µm
Size range	Broadest	Discrete sizes only	Discrete sizes	Discrete sizes
Best for	General work, drills, taps	Heavy roughing ≥ 3/4"	Heavy roughing at high torque	Finishing, reaming, chatter

Side-lock holders and milling chucks

Nose diameters scale with shank size — a published CAT40 side-lock range runs 19.81 mm at 1/4" through 31.75 mm at 1/2", 44.45 mm at 3/4" and 63.5 mm at 1-1/4". For the machine-side interface see 7:24 tapers; for collet-nut torque see nuts and torque.