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CNC turning

Ten CNC lathes, mostly Mazak, many with live tooling and a few of those also with a Y-axis. Bar-fed or chucked components up to Ø500 mm.

Diameter Ø500 mm

Spindle speed alone says nothing

A lathe data sheet gives the maximum spindle speed. But the tool does not feel spindle speed — it feels how fast the material passes beneath it, and that depends on the part diameter. Pick a diameter and see the cutting speed each lathe in the park reaches.

Part diameter [mm]

Cutting speed wanted [m/min]

Lathes that reach it

The spindle speeds are those in each machine’s catalogue documentation. The cutting speed you want is your own choice: it depends on the material, the tool and the coolant, not on the machine.

The formula, once

Cutting speed is vc = π × D × n / 1000, with diameter D in millimetres and spindle speed n in revolutions per minute; the result comes out in metres per minute. From this follows something that looks counter-intuitive: at a large diameter, a low spindle speed already gives a high cutting speed. A lathe for large parts does not need high revolutions — they would be unusable.

Why a park holds different lathes

At Ø20 mm, a 2,000 rpm lathe barely reaches 126 m/min — too little for a modern insert. At Ø400 mm, the same lathe gives 2,513 m/min, far beyond what most materials take. One lathe cannot cover both ends well, and that is why the park has different spindle speeds, not merely different sizes.

Allowance, passes, feed

Roughing removes the bulk and leaves an allowance — a little material above final size. The allowance is not waste: it keeps the finishing tool away from the rolled skin and from the stresses in the stock. Finishing then passes at a low feed, because surface roughness follows the feed: halving it smooths the surface and doubles the time. This is where the trade between price and surface quality is decided.

Why the clamping decides

A long part held only in the chuck bends under the cutting force, and the diameter comes out tapered — not because the machine is wrong, but because the part moves away from the tool. From a length-to-diameter ratio of roughly three onwards, the far end is supported. That is not a quality option; it is the condition under which the dimension on the drawing means anything.

Fits: what H7 and h6 ask of machining

A letter and a digit beside a dimension are not a formality: they say where the material sits relative to the nominal size and how narrow the field is. Upper case is for a hole, lower case for a shaft; the digit is the tolerance grade, the same throughout the system. The values below are for Ø50, from ISO 286.

Fit	Deviations at Ø50	Where it is used
H7	+25 / 0 µm. Hole: the nominal size is the lower limit; material may only be missing upwards.	Bearing bore, bush seat, dowel-pin hole.
h6	0 / −16 µm. Shaft: the nominal size is the upper limit; material is only taken off.	Shaft end entering an H7 bore, guide journal.
g6	−9 / −25 µm. Shaft a few microns thinner than nominal.	Guaranteed clearance: the part must slide or turn in the bore.
k6	+18 / +2 µm. Shaft thicker than nominal, with the same 16 µm field.	Transition fit: the bearing ring sits firm yet can be drawn off.
f7	−25 / −50 µm. More clearance, wider field.	Plain bearing, where the clearance carries the oil film.

Deviations from ISO 286 for the 30–50 mm range. They change with diameter: the same h6 is 0 / −9 µm at Ø10 and 0 / −29 µm at Ø200. The width of the field is exactly the grade in the name — IT6 or IT7 — while the letter only says where that field sits relative to nominal.

The system: hole basis or shaft basis

A fit comes from two parts, but both are not held tight. Common practice is hole basis: the hole stays H, and clearance or interference is chosen through the shaft letter — g for clearance, k for transition, p or r for press. The reason is practical: a hole is made with a tool of fixed size, whereas a shaft diameter can be adjusted on the lathe by a few microns without a new tool.

Clearance, transition, interference

A g6 in an H7 always leaves clearance, however unfavourably the two parts fall within their fields. At k6 the pair may come out either with slight clearance or with slight interference — hence transition. Beyond that, at p6 or r6, interference is certain and assembly is no longer done by hand: it needs a press, or the outer part heated.

What a fit asks of the shop

A 16 µm field does not come out of roughing. It needs a finishing pass with small stock, a sharp edge and a steady chip, sometimes grinding after heat treatment. It also needs measurement to match: at 16 µm a caliper is no longer enough, and the temperature of the part enters the reckoning — on 50 mm of steel, five degrees above reference shift the size by nearly 3 µm, a fifth of the field.

What helps on the drawing

The fit written on the dimension rather than left to the general note, and — if you know it — the role of the joint: what goes into what, whether it is ever taken apart, whether it is assembled warm. Two drawings with the same H7 can need different things, and the difference follows from the function, not from the dimension.

The machines that run this operation

Swing over bed: ~845 mm·
Bar capacity: ~184 mm·
Chuck size: 18 ″·
Spindle power: 37 kW·
Max spindle speed: 2,000 rpm·
Turret stations: 12·

One catalogue figure exceeds the public machining ceiling and is not shown. Parts above the ceiling are discussed on the drawing.

The largest CNC lathe in the park, for large-diameter parts and heavy bar stock.

High-capacity three-axis CNC turning centre for heavy parts and large diameters. A rigid structure and a main spindle of high power and torque allow intensive roughing as well as accurate finishing. It suits shafts, flanges, rollers, bushes and components used in energy and plant construction, including long or bulky parts.