A belt-driven head in two model sizes
This is a cast iron milling head for a manual mill or a drill-mill: a box section with four mounting holes on top, a spindle through the middle on a pair of tapered roller bearings, and a V-belt pulley at the back. It is a component, not a machine. The motor, the belt and the guard are supplied by the buyer.
Two model sizes are published, MT3 and MT4, differing in power, mass, centre height and spindle length. Both drawings carry the same note: the two series share their dimensions and differ only in the nose. The figures below come from those two tables and the dimensioned views.
Three things are not in them and matter as much as what is: no bearing tolerance class, no spindle runout, and no statement of whether the taper socket is hardened. Those are the questions at the end.
The MT3 row as published
| Item | MT3 |
|---|---|
| Lubrication method | High speed bearing grease |
| Motor power (motor self-provided) | 1.5 kW |
| Fixing bolt | M12 x 45 |
| Bearing | 32207 / 32207 |
| Maximum speed | 2,000 r/min |
| Weight | 24 kg |
| Spindle length | 360 mm |
| Centre height | 60 mm |
| Mounting hole size | 107 x 160 mm |
| Overall dimension | 386 x 135 x 120 mm |
| Section size | 135 x 120 mm |
| Belt disc model | BX200-2A |
The MT4 row as published
| Item | MT4 |
|---|---|
| Lubrication method | High speed bearing grease |
| Motor power (motor self-provided) | 2.2 kW |
| Fixing bolt | M12 x 60 |
| Bearing | 32209 / 32210 |
| Maximum speed | 2,000 r/min |
| Weight | 35 kg |
| Spindle length | 420 mm |
| Centre height | 70 mm |
| Mounting hole size | 120 x 150 mm |
| Overall dimension | 430 x 155 x 140 mm |
| Section size | 155 x 140 mm |
| Belt disc model | 35 x 120-3A |
More power and more iron, same ceiling
Going from MT3 to MT4 adds 46.7% more motor power and 45.8% more mass. The spindle is 60 mm longer, the section grows by 20 mm in each direction, and the centre height rises 10 mm.
And the maximum speed does not move. Both are 2,000 r/min.
That is the single most important thing on the page, because it means the larger head is not faster. It removes more metal per revolution and the same number of revolutions. Torque at the ceiling is 7.16 N·m for the 1.5 kW unit and 10.51 N·m for the 2.2 kW unit, which is exactly the 46.7% difference in power at the same speed.
For a buyer choosing between the two: you are buying rigidity and depth of cut, not speed. If your problem is that the finish is slow because the cutter is small, neither head solves it.
What actually caps it at 2,000 r/min
The bearing is 32207, a metric tapered roller bearing, 35 mm bore by 72 mm outside diameter by 24.25 mm wide. Published limiting speeds for that bearing on grease are 5,300 to 5,500 rpm.
| Source | Figure | Share of the bearing limit |
|---|---|---|
| 32207 published limiting speed, grease | 5,300 to 5,500 rpm | 100% |
| This milling head, as stated | 2,000 r/min | 36% to 38% |
| 200 mm pulley at 2,000 rpm | 20.9 m/s belt speed | about 70% of V-belt practice |
| 120 mm pulley at 2,000 rpm | 12.6 m/s belt speed | about 42% of V-belt practice |
So the bearing is not the constraint. At 2,000 rpm it is running at a little over a third of what it will take.
The belt is the constraint. The MT3 row gives the pulley as BX200-2A; if 200 is the pitch diameter in millimetres, then at 2,000 rpm the belt surface is travelling at 20.9 m/s. V-belt practice runs to roughly 25 to 30 m/s, so 2,000 rpm sits at about 70% of the belt's own limit.
That is why the ceiling does not move when the motor gets bigger. The motor is not what is holding it back either. To go faster you would change the transmission to a poly-V or a direct drive, and then the bearing becomes relevant. As sold, this is a belt-driven head and the belt sets the redline.
The motor is self-provided
Both rows carry the same parenthetical: Motor self-provided. What arrives is the iron, the spindle, the bearings and the pulley. What does not arrive is the motor, the belts, the guard and whatever carries the motor.
The 24 kg and 35 kg figures are the head alone. A 1.5 kW four-pole motor adds roughly 15 to 18 kg and a 2.2 kW roughly 20 to 24 kg, before the pulley and the mount. A buyer comparing this against a complete power head is comparing a component with a machine.
The Motor power row is therefore a rating, not a supply. Read it as: this head is built to take up to 1.5 kW, and you decide what drives it.
Morse for drilling, BT for milling, and why that is not marketing
One drawing compares the two families side by side. Morse MT3, MT4 and MT5 are marked suitable for drilling. BT30, BT40 and BT50 are marked suitable for milling. The note under the comparison says the two series share the same dimensions and differ only in the milling head part.
That is a correct engineering statement, not a sales split.
A Morse taper is a self-locking friction taper at roughly 1:20. It holds because the surfaces grip, and it transmits torque through that grip. That works for drilling, where the load is axial and the rotation is one way. Under milling it fails: side load and vibration break the friction lock, and the toolholder creeps or spins.
A BT taper is 7:24, about eight degrees half angle, and it does not lock at all. It is retained by a pull stud and a drawbar, and it drives through two keys. It is made for side load and for reverse. It also needs a drawbar, which a Morse spindle does not.
The comparison drawing does contain two spelling errors in the taper name, which is worth noting only because a buyer searching for the correct term will not find it in that image.
The taper comparison in millimetres
The second reason the split is real is simply size. At the same nominal position in the range, the BT nose is a third to over half again larger.
| Morse | Gauge diameter | BT equivalent | Gauge diameter | BT is larger by |
|---|---|---|---|---|
| MT3 | 23.83 mm | BT30 | 31.75 mm | 33% |
| MT4 | 31.27 mm | BT40 | 44.45 mm | 42% |
| MT5 | 44.40 mm | BT50 | 69.85 mm | 57% |
Bigger gauge diameter means a stiffer connection and a larger bearing bore behind it. A BT30 spindle carrying a 31.75 mm taper is a different structure from an MT3 spindle at 23.83 mm, even though the cast box around them measures the same.
This matters at the ordering step. The title of this listing says BT30, BT40 and BT50 in one line, and the drawings show MT3 and MT4 tables. Those are not two names for the same product.
What 2,000 rpm can cut, by diameter
Surface speed is the product of diameter and revolutions, so a fixed ceiling of 2,000 rpm fixes the diameter range that any given cutting speed allows.
| Cutter diameter | Surface speed at 2,000 rpm | What that suits |
|---|---|---|
| 10 mm | 63 m/min | Between high-speed steel and carbide, neither well |
| 16 mm | 101 m/min | Between the two |
| 20 mm | 126 m/min | Between the two |
| 25 mm | 157 m/min | Carbide in steel |
| 32 mm | 201 m/min | Carbide in steel |
| 40 mm | 251 m/min | Top of carbide in steel, start of aluminium |
| 50 mm | 314 m/min | Carbide in aluminium |
| 63 mm | 396 m/min | Carbide in aluminium |
| 80 mm | 503 m/min | Aluminium only, at its limit |
| 100 mm | 628 m/min | Too fast for steel |
| 125 mm | 785 m/min | Too fast for steel |
The useful window with carbide in steel, at 150 to 250 m/min, falls between roughly 24 and 40 mm diameter. That is a face mill or a large slot drill.
The other end is the problem. A 10 mm carbide end mill wants 150 to 250 m/min, which is about 5,000 rpm. This head cannot give it. Running that cutter at 2,000 rpm puts it at 63 m/min, which is too slow for carbide and too fast for high-speed steel, so the tool rubs and the finish suffers.
If your work is small pockets and profiles, this is the wrong class of head, whatever the power rating says. If your work is facing and squaring large parts, the 2,000 rpm ceiling is not a limitation at all, because the cutter is large.
What 1.5 kW removes
At about 20 W per cm3/min in steel, the 1.5 kW head removes on the order of 75 cm3/min and the 2.2 kW about 110.
| Head | Stock removal in steel | ap 2 mm, ae 50 mm | ap 3 mm, ae 63 mm |
|---|---|---|---|
| MT3, 1.5 kW | about 75 cm3/min | 750 mm/min, fz 0.094 mm | 397 mm/min, fz 0.050 mm |
| MT4, 2.2 kW | about 110 cm3/min | 1,100 mm/min, fz 0.138 mm | 582 mm/min, fz 0.073 mm |
These are plausible face-milling numbers at 2,000 rpm with a four-tooth cutter, and they describe the machine this head was designed for: a manual mill or a drill-mill doing flat work, not a high-speed machining centre. The unit power figure is an order-of-magnitude constant; treat the feeds as a starting point to adjust at the spindle load meter.
40Cr and a process chain that stops at tempering
One detail drawing points at the spindle nose with a red note: the mandrel is 40Cr, processed by precision forging, rough machining, tempering, finishing, rough grinding, semi-finishing and fine grinding.
40Cr is a Chinese standard alloy structural steel, roughly equivalent to AISI 5140, with about 0.4% carbon and 1% chromium. It is a quench-and-temper steel.
Quenched and tempered 40Cr lands around 28 to 32 HRC. That is a sound core for a spindle: tough, dimensionally stable, and machinable. It is not a wear surface. A BT or Morse taper socket sees sliding contact every time a toolholder goes in, and at 30 HRC it will polish and then bell out, which shows up later as runout that no longer repeats.
The chain as written contains no case hardening and no induction hardening of the taper. Either the process is incomplete on the drawing, or the socket really is left at core hardness. The grinding steps give geometry; they do not give wear resistance. This is the first thing to ask, and the answer decides whether the head lasts five years or twenty.
The same drawing loop does say the front and rear bearings are precision bearings and that they eliminate shake and play. It does not say which grade, which preload, or who made them.
Cast iron and "aging treatment"
Another note says the casting is aged after casting so that it does not deform later and the machining accuracy holds.
That is the right thing to do and it is the reason a cast iron head keeps its geometry. But aging covers two very different processes. Natural aging leaves the casting outdoors for months, which costs floor space and time. Artificial aging heats it to a few hundred degrees and holds it, which costs energy and takes days. Both are sold as aging treatment.
For a buyer, the practical difference is whether the head still holds alignment after two years of thermal cycling in an unheated shop. It is a fair question and it has a specific answer the supplier knows.
Two bearings front and rear, and no grade
MT3 uses 32207 in both positions. MT4 uses 32209 in front and 32210 at the back, so the shaft is stepped: 45 mm bore at the nose and 50 mm behind it. That is a sensible arrangement for a spindle that must be stiff at the tool and merely supported at the drive end.
What is missing is the tolerance class. The same 32207 exists as P0, P6, P5 and better, and the class is most of the price difference and all of the runout difference. A designation without a class is half a part number.
Tapered roller bearings also need a defined preload to be stiff, and preload is set at assembly. No preload value is published, which means the buyer cannot check it and cannot reset it after a rebuild without a specification.
The mounting pattern that shrinks as the box grows
| MT3 | MT4 | Change | |
|---|---|---|---|
| Box length | 255 mm | 300 mm | +45 mm |
| Hole spacing, width | 107 mm | 120 mm | +12% |
| Hole spacing, length | 160 mm | 150 mm | -6.2% |
Across the two models the box grows 45 mm in length, the width spacing grows 12%, and the length spacing falls 6.2%. A longer casting with closer bolts along its length is not a scale-up, and it is the kind of thing that only matters when someone tries to fit the larger head onto a pattern drilled for the smaller one.
Get the mounting drawing for the exact model before drilling anything.
Centre height decides the envelope
Centre height is 60 mm on MT3 and 70 mm on MT4: the distance from the mounting base to the spindle axis. It is not a size badge. It sets how tall a workpiece can pass under the nose and how short a one still gets reached by the quill.
Ten millimetres is a different machine envelope. On a drill-mill, 60 mm centre height with a 360 mm spindle suits small and medium plate; 70 mm with 420 mm suits a taller setup. Neither is better, and choosing the bigger one can put the nose out of reach of the work you actually do.
Eleven questions for the quotation
The tables are good enough to buy against, which is exactly why the remaining unknowns are worth pinning down in writing before the order rather than after.
| Question | Why it decides the purchase |
|---|---|
| Is the taper Morse or BT, and which number | The title lists BT30/BT40/BT50, the tables are headed MT3/MT4, and the drawings say Taper 7:24 BT30 while the row says MT3 |
| Is the bearing grade P0, P6 or P5 | 32207 is given but its tolerance class is not; the class sets the runout you will measure |
| Is the spindle nose hardened after tempering | The process chain names 40Cr, rough machining, tempering, grinding; no surface hardening is mentioned |
| Natural or artificial aging of the casting | "Aging treatment" covers both, and they differ in months and in cost |
| Bearing preload and how it is set | A paired tapered roller spindle needs a defined preload; none is published |
| What motor frame and pulley the head expects | The motor is self-provided, and the pulley model alone does not fix the motor shaft |
| Pulley groove count and belt section | BX200-2A and 35 x 120-3A do not follow one numbering rule |
| Spindle runout at the taper gauge line, measured | The only accuracy number that matters, and there is none on the page |
| Drawbar or pull stud for the BT version | A 7:24 taper does not self-lock; without a drawbar the toolholder is not retained |
| Whether the nameplate is third-party | The plate reads as a machine-tool works mark, not the brand on the listing |
| Mounting pattern drawing for the actual model | The published long-direction spacing shrinks as the box grows |
If you already run BT30 tooling
The BT30 spindle clamp article covers the retention side of the same interface. The taper and the clamp have to match, and this listing does not document the drawbar at all, so the clamp question is genuinely open on the BT version.
Frequently asked questions
Is this a BT30 head or an MT3 head?
The listing is ambiguous and you should confirm it in writing. The title names BT30, BT40 and BT50; the two published tables are headed MT3 and MT4; and the dimension drawings carry the note Taper 7:24 BT30 next to an MT3 heading. They are different products with different noses and different uses.
Why is the maximum speed only 2000 rpm?
Not because of the bearing. A 32207 tapered roller bearing is rated 5300 to 5500 rpm on grease, so 2000 is about a third of that. The limit is the belt: a 200 mm pulley at 2000 rpm puts the belt at 20.9 m/s, close to normal V-belt practice. Raising the speed would mean changing the transmission.
Does the motor come with it?
No. Both tables say Motor self-provided. The 24 kg and 35 kg are the head alone; a 1.5 kW motor adds roughly 15 to 18 kg and a 2.2 kW about 20 to 24 kg, plus pulley, belts and guard.
Can I run a 10 mm carbide end mill on it?
Not usefully. At 2000 rpm a 10 mm cutter runs at 63 m/min, below the 150 to 250 m/min carbide wants and above what high-speed steel likes. This head suits 24 to 40 mm cutters in steel, or 50 to 63 mm in aluminium, which is face milling work.
What does the 40Cr note tell me?
That the spindle is a quench-and-tempered alloy steel, about 28 to 32 HRC. That is a good core but not a wear surface, and the process chain as printed has no case or induction hardening of the taper. Ask whether the socket is hardened, because that decides how long the taper holds its accuracy.
Why does MT4 have different bearings front and back?
32209 at the nose and 32210 behind it means a stepped shaft, 45 mm bore in front and 50 mm behind. It is normal: stiff at the tool, merely supported at the drive end. What is not given is the tolerance class, which is most of the price and nearly all of the runout.
Will the MT4 bolt onto a pattern drilled for MT3?
No. The width spacing grows from 107 to 120 mm while the length spacing shrinks from 160 to 150 mm, so neither direction matches. Get the mounting drawing for the exact model before drilling.
Key specifications
Frequently asked questions
- Is this a BT30 head or an MT3 head?
- The listing is ambiguous and you should confirm it in writing. The title names BT30, BT40 and BT50; the two published tables are headed MT3 and MT4; and the dimension drawings carry the note Taper 7:24 BT30 next to an MT3 heading. They are different products with different noses and different uses.
- Why is the maximum speed only 2000 rpm?
- Not because of the bearing. A 32207 tapered roller bearing is rated 5300 to 5500 rpm on grease, so 2000 is about a third of that. The limit is the belt: a 200 mm pulley at 2000 rpm puts the belt at 20.9 m/s, close to normal V-belt practice. Raising the speed would mean changing the transmission.
- Does the motor come with it?
- No. Both tables say Motor self-provided. The 24 kg and 35 kg are the head alone; a 1.5 kW motor adds roughly 15 to 18 kg and a 2.2 kW about 20 to 24 kg, plus pulley, belts and guard.
- Can I run a 10 mm carbide end mill on it?
- Not usefully. At 2000 rpm a 10 mm cutter runs at 63 m/min, below the 150 to 250 m/min carbide wants and above what high-speed steel likes. This head suits 24 to 40 mm cutters in steel, or 50 to 63 mm in aluminium, which is face milling work.
- What does the 40Cr note tell me?
- That the spindle is a quench-and-tempered alloy steel, about 28 to 32 HRC. That is a good core but not a wear surface, and the process chain as printed has no case or induction hardening of the taper. Ask whether the socket is hardened, because that decides how long the taper holds its accuracy.
- Why does MT4 have different bearings front and back?
- 32209 at the nose and 32210 behind it means a stepped shaft, 45 mm bore in front and 50 mm behind. It is normal: stiff at the tool, merely supported at the drive end. What is not given is the tolerance class, which is most of the price and nearly all of the runout.
- Will the MT4 bolt onto a pattern drilled for MT3?
- No. The width spacing grows from 107 to 120 mm while the length spacing shrinks from 160 to 150 mm, so neither direction matches. Get the mounting drawing for the exact model before drilling.