Tool holder interface: the spindle-to-holder fit (steep taper, dual-contact or HSK), the retention knob and the drawbar pull that clamps it
Machining-centre spindle interface · 7:24 steep taper vs. taper-and-face contact vs. HSK hollow taper · retention knob thread, angle and torque · drawbar pull per taper size and how it is checked
The tool holder interface is the joint between a machining-centre spindle and whatever holder the tool changer puts into it: the taper and, on some systems, the flange face that locate the holder, the retention knob or HSK clamping bore that the drawbar grips, and the pull-in force that keeps the two together under cutting load. Getting it wrong shows up as runout, fretting and tools that shift in Z, so the holder, the knob and the spindle have to be ordered against the same standard and the same clamping force.
The spindle-side interface of machining-centre tool holders: steep 7:24 tapers (CAT, BT, SK), dual-contact taper-and-face variants, HSK hollow-shank tapers, retention knobs (pull studs), and the drawbar or HSK clamping unit force that holds them. The holders themselves, with their collet, hydraulic and shrink-fit clamping, runout and balance, are covered on the Tool holder page; motor spindles on the High-speed spindle page; turret quick-change clamps on the Turret tool clamping unit page; tool change arms, magazines and robot tool changers on their own pages.
CNFX editorial research · Sources checked October 19, 2026 · 5 sources
Where the holder touches the spindle: gauge line, flange face or hollow taper
BIG DAISHOWA describes a conventional steep-taper holder as supported on a reference diameter at the gauge line, whereas its BIG-PLUS holders, built on ASME B5.50, JIS B6339 and DIN 69871, are pulled into contact with the spindle taper and the spindle face at the same time. [1]
The company explains how that is possible with ordinary tolerances: every machine sees an axial movement of roughly 0.0005 to 0.001 in when the drawbar clamps, and BIG-PLUS uses that elastic movement to make taper contact first and face contact second. A BIG-PLUS holder still fits a standard spindle, where it seats on the taper only. [2] Editorial calculation: 0.0005 to 0.001 in is 0.0127 to 0.0254 mm (× 25.4).
The contact diameters BIG DAISHOWA lists show why the face matters: 1.750 in at the gauge line against 2.500 in at the BIG-PLUS face for CV40, and 2.750 against 3.875 in for CV50. It names three outcomes on the spindle: proper fit with taper and flange contact after clamping; flange contact only, with poor runout, chatter and possible spindle damage; and taper contact only, which behaves like ordinary taper-only tooling. The company also claims ATC positioning within 1 micron for BIG-PLUS. [1]
HSK reaches taper and face contact differently. OTT-JAKOB describes the hollow-shaft taper as shorter and lighter than the steep taper, allowing quicker tool changes and higher speeds, with flange and taper contact surfaces clamped together. [3] BIG DAISHOWA notes that HSK relies on the thin-walled short taper expanding elastically under very high clamping forces; in its own testing, longer holders were stiffer on BIG-PLUS, while it recommends HSK for high-speed work. [2]
BIG DAISHOWA warns that dual-contact holders from makers without its licence lack the gauges for the taper-to-flange relationship; if a dual-contact spindle is specified, ask which gauge the holder supplier works to. [1]
02
Drawbar pull by taper size, and why the gripper limit is a different number
TAC Rockford compiles drawbar forces from standards and gripper makers and states that the machine builder sets the actual value for a given machine. The columns below come from different bases, so compare within a column, not across. [4]
The ISO 12164-1 values are recommendations chosen so that the share acting on the flange face never falls below 75 % of the total; lower forces can do for light finishing loads and heavy cutting can need more. TAC Rockford describes the BERG gripper figures as maximum allowable for the gripper and high for the machine taper, and the HSH gripper as built for very high clamping-force machines. [4]
Editorial calculation: for HSK-A/C 63 at the recommended 18,000 N, at least 0.75 × 18,000 = 13,500 N must act on the flange face. The HSH gripper limit of 40,000 N is 40,000 / 18,000 = 2.2 times the recommendation, so a gripper rating is not a target for the spindle. [4]
OTT-JAKOB attributes the pull-in force of its HSK clamping units to the geometry of the gripper segments and clamping cone, with segments that move in parallel so the holder is clamped between two surfaces; its Type C unit has a holding function intended to shorten tool-change times. [3]
Retention knob: thread, angle and tightening torque that must match the drawbar
On steep-taper holders the spindle gripper pulls the holder in by the retention knob, so the knob is the part that ties holder and drawbar together. Hoffmann Group notes that pull studs come in different versions and that ISO 7388-3 replaces the older DIN 69872. [5]
As one data point, Hoffmann's Form A pull stud for SK 40 in 18CrNiMo7 has an M16 thread, a 15° angle, 54 mm overall length, 26 mm gauge length, a 19 mm head, a 7 mm coolant bore and a maximum tightening torque of 50 Nm; the same listing tells users to check the tightening torque when fitting it. [5] A knob with the wrong angle or length for the spindle gripper will not seat the holder correctly, so the knob drawing belongs in the order alongside the taper size.
Ask the machine builder for the knob standard, form, angle and length its drawbar is made for, and for the clamping force it sets; quote those on the holder order.
04
Checking clamping force once the machine is in service
TAC Rockford writes that tool clamping force "is often never measured after a machine is put into service", and that regular checks let a weakening drawbar be found before it causes a breakdown. It lists wired drawbar force gauges, measuring bars, HSK drawbar position gauges for setting the clamping stroke, and machine-integrated wireless gauges stored in the tool changer that report force to the control. [4]
The same reference asks for drawbar force to be given in newtons, kilonewtons or pounds-force, never in kilograms, pounds or foot-pounds, and gives the conversion 1 N = 0.2248 lbf. [4] Editorial calculation: the CAT40 suggestion of 10,230 N × 0.2248 = 2,300 lbf, matching the figure TAC Rockford prints beside it.
05
Interface data to send with a spindle or holder order
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Spindle: [CAT, BT, SK or HSK; size; taper only or taper and face]
Drawbar: [clamping force set by the builder; gripper type]
Retention knob: [standard; form; angle; thread; length; coolant bore]
Holders: [dual-contact licence or gauge used]
Checks: [force gauge; HSK stroke gauge]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 19 October 2026
Buyer questions
What is the difference between taper-only and dual-contact holders?
A conventional steep-taper holder seats at the gauge line; a dual-contact holder such as BIG-PLUS also touches the spindle face after clamping. [1]
How much drawbar force does an HSK-A63 spindle need?
ISO 12164-1, as compiled by TAC Rockford, recommends 18,000 N for HSK-A/C 63; the machine builder sets the actual value. [4]
Is HSK always better than a steep taper?
Not by every measure. BIG DAISHOWA reports longer holders were stiffer on its dual-contact steep taper and recommends HSK for high-speed work; OTT-JAKOB cites HSK's lighter, shorter design and faster changes. [2][3]
Sources and what still needs a direct check
Sources checked 2026-10-19. BIG DAISHOWA on taper-and-face contact, OTT-JAKOB on HSK clamping units, TAC Rockford's drawbar force reference compiled from ISO 12164-1, ASME B5.50 and BERG gripper data, and a Hoffmann Group pull stud listing.
