Run Morse taper-shank drills above roughly 20 mm on a CNC machining center in an MTB-style holder — its retention screw threads into the drill's tapped shank and secures it against the pull-out loads of peck retracts and through-hole breakthrough. Reserve MTA-style (tang-slot) holders for unmodified tang-end drills in blind-hole work with conservative retracts, where taper friction plus positive tang drive is sufficient. Keep the runout stack to one interface: a ground holder bore typically contributes 0.005-0.015 mm TIR, and each added reduction sleeve contributes 0.015-0.020 mm more per DIN 228.
Taper-shank twist drills were designed for drill presses and radial drills, but a BT, CAT, or HSK holder with a female Morse taper bore lets them run in a machining-center spindle. The decision layer this guide covers is that holder: MTA (tang slot, friction plus tang drive) versus MTB (tapped shank, clamping-screw retention), how each transmits torque and resists withdrawal, what the taper stack does to runout, and which MT size matches which drill diameter. For the broader spindle-interface picture across BT, CAT, and HSK, see the BT CAT HSK comparison; for the full tool-holding landscape, see the tool holding complete guide.
When Taper-Shank Drills Still Earn a Place on a Machining Center
Taper-shank HSS drills remain the lowest-capital route to holes above roughly 20 mm on a machining center when volumes are low and the drills are already in the crib. Solid carbide dominates hole-making under ~20 mm and indexable insert drills win on cost-per-hole in production runs above ~20 mm, but both assume you are buying new tooling. A shop that already owns a set of large taper-shank drills — common wherever radial drills or manual lathes ran before the machining center arrived — only needs one Morse taper holder, typically $25-$70 for a BT40 version at distributor pricing, to put that inventory back to work.
Three conditions justify the taper-shank route:
- Low volume — one-offs, repairs, and short jobbing runs where insert-drill economics never amortize. The cost-per-hole advantage of indexable drills builds over hundreds of holes, not five.
- Large diameters at low spindle speed — HSS taper-shank drills run at typically 20-30 m/min in mild steel, so a 40 mm drill turns roughly 200 RPM at 25 m/min. No machining-center spindle is stressed by that speed; the constraint is torque and thrust, not RPM.
- Capital already owned — DIN 345 taper-shank drill sets purchased for manual machines carry over directly; the holder is the only new purchase.
HSS taper-shank drills tolerate the rigid, interrupted entry conditions of large-hole work because the tough high-speed-steel body resists the chipping that can destroy a brittle carbide tool of the same size. The trade-off is speed: carbide typically runs 3-4x faster in steel, so once volume rises, the cycle-time penalty of HSS outweighs the capital savings. When that crossover arrives, the drill-type economics — indexable versus solid carbide versus HSS cost-per-hole — deserve their own analysis, which is outside this guide's scope.
Run the Crossover Math Before Buying New Tooling
Multiply holes-per-year by the cycle-time difference between HSS (typically 20-30 m/min in mild steel) and an indexable drill (typically 3-4x faster). Below roughly 200-500 holes per year per diameter, the $25-$70 holder that reuses owned HSS drills usually beats new insert tooling on total cost; above that, cycle time dominates and the taper-shank drill becomes the expensive option.
MTA vs MTB Holder Anatomy: Tang Slot vs Retention Screw
MTA holders drive the drill through taper friction plus a tang slot, while MTB holders add a retention screw that threads into the drill's tapped shank for positive axial security. Both have a BT, CAT, or HSK spindle end and a female Morse taper bore per DIN 228; the difference is entirely in how the small end of the drill shank is handled.
| Factor | MTA (Tang Slot) | MTB (Retention Screw) |
|---|---|---|
| Torque drive | Friction + positive tang engagement | Friction only |
| Axial pull-out security | Friction only | Positive (screw thread) |
| Drill shank end | Standard tang (DIN 345 as-delivered) | Tapped hole (modification often needed) |
| Removal method | Drift key through slot, seconds | Unscrew retention bolt first |
| Peck/through-hole cycles | Conservative retracts only | Preferred — no walk-out risk |
| Typical BT40 price | $25-$60 | $30-$70 |
Most DIN 345 taper-shank twist drills arrive with a tang, not a tapped shank, so MTB use typically means grinding the tang off and drilling-and-tapping the shank end — a common, accepted shop modification. Taper-shank tooling that is designed for drawbar retention (some core drills, taper-shank end mills, and machine reamers) often comes with the tapped end already in place. Verify which end your drill has before ordering the holder; the two styles are not interchangeable in service.
Torque Transmission and Axial Security Under Peck Retract
The Morse tang is a positive anti-rotation drive and removal feature, not an axial retention device — in an MTA holder, friction alone holds the drill against pull-out. The self-holding Morse geometry (approximately 1:20 taper per DIN 228) generates enough cone friction to transmit steady drilling torque in most conditions, with the tang as a mechanical backstop when torque spikes — at breakthrough, on chip jams, or at engagement — exceed what friction carries. A drill that spins in the bore without a tang gals and scores both tapers, typically scrapping the holder bore.
Axial loading is where the two styles separate. During cutting, thrust pushes the taper deeper into its seat, so security is not the issue while the drill is feeding. The risk appears on withdrawal:
- Peck retracts — chips packed in the flutes can grip the hole wall, so each rapid retract applies a withdrawal load to the taper joint.
- Through-hole breakthrough — as the point exits, the web stops cutting and the lips can grab; the drill's helix then tries to screw the tool forward out of the holder, the classic "corkscrew" failure on manual machines.
- Tapping-style reversal — any reversed-spindle operation in the same holder loads the joint in the release direction.
Breakthrough grab and packed-flute peck retracts generate withdrawal loads that taper friction alone cannot reliably resist, which is why MTB-style screw retention is preferred for unattended CNC drilling cycles. On a drill press an operator feels the grab and eases off; a machining center executes the programmed retract at full rapid regardless.
A Walked-Out Drill Is a Crash, Not an Inconvenience
If a friction-only taper releases during a retract, the drill stays in the hole while the spindle moves up — the next rapid or tool change then drives the spindle assembly into a drill standing proud of the work. When running tang-end drills in MTA holders on through-holes, reduce breakthrough feed by typically 30-50%, keep pecks shallow, and confirm full chip evacuation. Better: tap the shank and run MTB.
This withdrawal-security logic mirrors the tang-versus-drawbar split on manual machines, where drawbar retention is the standard answer for vertical spindles — the manual-machine side of that decision belongs to the Morse taper adapter and sleeve system guide.
Runout Stack-Up: What the Hole Sees
Every interface in the taper stack adds runout — a ground holder bore typically contributes 0.005-0.015 mm TIR, and each Morse reduction sleeve adds 0.015-0.020 mm more per DIN 228 tolerance. The full chain on a machining center is: spindle taper → holder shank seat → holder MT bore → (optional sleeve) → drill shank. With a clean spindle and a ground holder, total indicated runout at the drill shank of roughly 0.01-0.03 mm is a realistic expectation; add a sleeve and 0.03-0.05 mm becomes typical.
For large twist drills this is usually acceptable. Two-flute twist drills tend to cut oversize by roughly the runout at the point, so a 0.03 mm TIR stack adds roughly 0.03 mm to hole diameter — well inside the IT12-IT13 accuracy class that large HSS twist drills produce as-drilled in typical conditions. Holes destined for an H8-H9 fit get a boring or reaming pass afterward regardless of holder choice; for matching the finishing tool to the tolerance target, see the drill bit and reamer selection guide.
Two practices keep the stack honest:
- Buy the holder in the drill's native MT size instead of sleeving down. A single MT2-to-MT3 sleeve adds 0.015-0.020 mm; stacking two sleeves roughly doubles the addition and softens the self-locking grip. One interface, one tolerance.
- Indicate the assembly at the drill shank after mounting. Place a dial indicator on the ground shank just below the holder face and rotate the spindle by hand. A reading more than roughly 2x the holder's specified grade points to a contaminated bore, a bruised taper, or a worn sleeve — not a bad holder.
Where Taper-Shank Tooling Fits in the Runout Hierarchy
Hydraulic and shrink-fit holders reach ≤0.003 mm TIR at 3xD, integrated drill-chuck holders 0.05-0.06 mm, and a Morse taper stack typically 0.01-0.05 mm depending on sleeves. That middle position is exactly right for large twist drilling, where the drill's own point geometry — not the holder — usually limits hole accuracy. Setup fundamentals are covered in the CNC tooling setup beginner guide.
Matching Morse Taper Size to Drill Diameter
Match the Morse taper size to the drill diameter the way manufacturer catalogs allocate them — roughly 14-23 mm drills carry MT2 shanks, 23-32 mm carry MT3, and 32-50 mm carry MT4. DIN 345 defines the taper-shank twist drill series whose shank size steps up with diameter, which is why a 25 mm DIN 345 drill arrives with an MT3 shank (23.825 mm large-end diameter) and a 20 mm drill with an MT2 shank (17.780 mm large-end diameter). The allocation exists because torque capacity of the friction joint scales with cone area: a bigger drill needs a bigger cone.
| Morse Taper | Typical Drill Diameter Range | Typical Holder Availability | Practical Note |
|---|---|---|---|
| MT1 | up to ~14 mm | BT30, BT40, CAT40 | Rarely used on machining centers — straight-shank tooling covers this range better |
| MT2 | ~14-23 mm | BT40, CAT40, HSK63A | The most common entry point for taper-shank work |
| MT3 | ~23-32 mm | BT40, BT50, CAT40, CAT50 | Workhorse size for large-hole jobbing |
| MT4 | ~32-50 mm | BT50, CAT50 (BT40 versions exist but are torque-marginal) | Prefer a 50-taper spindle for the thrust involved |
| MT5 | ~51-76 mm | BT50, CAT50 only | Radial-drill-class holes; verify machine thrust capacity first |
Diameter ranges are typical manufacturer catalog allocations for taper-shank twist drill series; individual product lines vary at the boundaries.
BT shanks per JIS B 6339 dominate Asian-built machining centers, CAT V-flange shanks per ANSI/ASME B5.50 dominate North American shops, and HSK hollow tapers per DIN 69893 serve high-speed spindles — Morse taper holders are cataloged for all three, but the deepest MT4-MT5 selection sits in BT50 and CAT50. ISO 12164 is the international equivalent of the DIN 69893 HSK specification, so it is the designation to check when a non-German machine builder lists HSK spindle compatibility. Spindle speed is rarely the constraint: standard BT/CAT V-flange holders carry a practical limit of 12,000 RPM from manufacturer testing, while a 40 mm HSS drill at 25 m/min needs roughly 200 RPM. Thrust is the real limit — large HSS drills generate multi-kilonewton axial loads at catalog feeds, so check the machine's Z-axis thrust rating before programming a 50 mm drill at full feed, and consider pilot drilling (typically at 25-30% of final diameter) to cut thrust substantially on 40-taper machines.
Removal Procedure and Taper Bore Maintenance
A drift key removes a tang-end drill from an MTA holder in seconds, while an MTB drill releases only after the retention screw is backed out completely. For MTA holders, insert the drift through the lateral slot so it bears on the back of the tang, support the drill, and strike the drift once or twice with a soft-face hammer — the same procedure used on drill-press quills. For MTB holders, never hammer with the screw engaged; back the screw fully out, then tap the drill loose through the access hole or with a soft drift at the shank.
The female taper bore is the holder's precision surface, and it degrades three ways:
- Chip contamination — a single chip between cones can shift runout by typically 0.02-0.05 mm and concentrates the friction load on two contact lines instead of the full cone. Wipe the bore and the drill shank with a clean, dry, lint-free cloth before every insertion.
- Galling from spin-through — a drill that slips in the bore under torque scores both surfaces. Inspect after any suspected slip; a scored bore typically cannot be restored economically and keeps damaging every shank inserted afterward.
- Fretting and rust — keep the bore dry and plugged in storage. Do not oil the cone: the joint relies on friction, and lubricant measurably reduces both torque capacity and self-holding grip.
Blue-Check Before Blaming the Holder
If runout or slipping persists after cleaning, blue-check the taper fit: a thin film of marking blue on the drill shank should transfer over more than 80% of the cone surface after seating. Lower contact means a bruised shank, a worn sleeve, or a mismatched taper family — DIN, JIS, and ANSI Morse dimensions are close but not identical, and a mixed pair can seat on the ends of the cone only.
Quick Selection Table
Use this matrix as a first-pass shortlist matching the drilling scenario to a holder style, MT size, and spindle interface.
| Scenario | Holder Type | MT Size | Spindle Interface | Why |
|---|---|---|---|---|
| Blind holes 14-23 mm, low volume, conservative pecks | MTA (tang slot) | MT2 | BT40 / CAT40 | No breakthrough grab in blind holes; friction + tang drive carries the torque with zero drill modification |
| Through-holes or deep peck cycles, 23-32 mm | MTB (retention screw) | MT3 | BT40 / BT50 | Screw retention resists the withdrawal loads of breakthrough grab and packed-flute retracts that friction alone cannot guarantee |
| Large holes 32-50 mm in steel | MTB (retention screw) | MT4 | BT50 / CAT50 | Multi-kilonewton thrust and high torque favor the stiffer 50-taper interface and positive axial retention |
| Tang-end drill that cannot be modified | MTA (tang slot) | Match shank | BT40 / BT50 | Tang slot accepts the as-delivered DIN 345 shank and allows drift-key removal in seconds |
| Hole needs H8-H9 fit after drilling | MTA or MTB + boring/reaming pass | Match shank | Any | As-drilled accuracy is typically IT12-IT13; the finishing tool, not the holder, sets final size |
| Mixed MT2/MT3 drill crib, one-holder budget | MT3 holder + single MT2-to-MT3 sleeve | MT3 | BT40 | One ground sleeve adds only 0.015-0.020 mm runout per DIN 228; avoid stacking two sleeves |
Choose MTB screw retention for CNC drilling cycles; use MTA tang holders only for unmodified drills in blind-hole work.
On a machining center, the deciding factor is axial security under programmed retracts: MTB holders thread a retention screw into the drill's tapped shank and eliminate walk-out during pecks and breakthrough, while MTA holders rely on taper friction that a drill press operator can feel slipping but a CNC cannot. Match MT size to drill diameter per the catalog allocation (roughly MT2 for 14-23 mm, MT3 for 23-32 mm, MT4 for 32-50 mm), buy the holder in the drill's native taper instead of sleeving down, and keep the bore clean and dry — a 0.01-0.03 mm TIR stack is normal and adequate for the IT12-IT13 holes large twist drills produce.
Can I run a standard tang-end Morse taper drill in a CNC machining center?
Yes — an MTA-style BT or CAT holder with a tang slot accepts the as-delivered drill, typically for $25-$60 in BT40. Restrict it to blind holes or conservative through-hole cycles, because the taper joint resists pull-out by friction alone, and reduce breakthrough feed by typically 30-50% on through-holes.
What is the difference between MTA and MTB Morse taper holders?
MTA holders have a tang slot: taper friction plus positive tang drive, but no axial retention beyond friction. MTB holders have no tang slot and instead use a retention screw — commonly M12 for MT3 — threading into the drill's tapped shank, giving positive pull-out security for peck retracts and through-hole breakthrough.
What Morse taper size do I need for a 25 mm taper-shank drill?
A 25 mm taper-shank twist drill in the DIN 345 series typically carries an MT3 shank (23.825 mm large-end diameter), so order a BT40-MT3 or CAT40-MT3 holder. Catalog allocations run roughly MT2 for 14-23 mm drills, MT3 for 23-32 mm, and MT4 for 32-50 mm — verify the shank before ordering.
Why does my taper-shank drill pull out of the holder during peck drilling?
Friction-only retention is releasing under withdrawal load — chips packed in the flutes grip the hole wall during each rapid retract, and at breakthrough the lips can grab and corkscrew the drill out. Switch to an MTB holder with screw retention, keep pecks shallow, and clean both cones; a contaminated taper can lose a substantial share of its grip.
How much runout does a Morse taper holder add on a machining center?
A ground MT holder typically contributes 0.005-0.015 mm TIR bore-to-shank, giving a realistic total stack of 0.01-0.03 mm at the drill shank with a clean spindle. Each reduction sleeve adds 0.015-0.020 mm more per DIN 228 tolerance, which is why buying the holder in the drill's native MT size beats sleeving down.
