What is a boring mill? How it works and when to use one
Contents
Two bearing seats, one common axis
A gearbox housing has a bearing seat in each of two opposing walls. Both bores must meet their diameter requirements and align with one another. A mating face also has a specified position and orientation relative to the bearing axis. Producing one accurately sized bore does not resolve the whole task.
A horizontal boring and milling machine approaches the component from the side. A rotary table can expose further faces without releasing the clamps, reducing the need to re-establish alignment. Whether all features are accessible in that setup is checked before machining.
The name comes from boring: an adjustable cutting tool brings an existing hole to its final size. Modern boring mills also perform milling, drilling and threading operations. In this article, boring mill refers to the horizontal type.
How the spindle, column and table work together
- Bed: supports guideways and assemblies.
- Column: guides the headstock vertically.
- Headstock: sets the tool height.
- Boring spindle: extends towards the workpiece.
- Rotary table: presents further workpiece faces.
- Workpiece: plan access and clamping together.
On a table-type boring mill, several movements position the tool and workpiece relative to each other:
- The bed and column form the supporting structure. Guideways, rigidity and installation influence how the machine behaves under cutting forces.
- The headstock travels up and down the column. The rotating work spindle drives the tool; an extendable boring spindle increases its reach. A large extension does not mean that tools of any length can be used.
- The table positions the workpiece. A rotary table turns it towards the spindle. Clamps and obstructing features also determine which faces remain accessible.
- For long boring bars, an outboard support can support the bar on the opposite side. This depends on the equipment and setup; it is not part of every machining operation.
Y often denotes vertical movement, W boring spindle travel and B table rotation. X and Z describe further relative movements. When comparing manufacturers, check the actual directions and moving assemblies as well as the axis letters.
Five operations and their purpose on the component
The operations serve different purposes on the housing. Availability depends on tooling and machine equipment:
- Drilling creates an initial hole, such as a through hole or a tapping hole.
- Boring brings an existing hole to its specified final size. The bearing seat diameter and the geometric requirements on the drawing are considered separately.
- Milling creates mating faces, slots and pockets. On the housing, it can produce the mounting face.
- Facing machines an end face using a radially adjustable tool, for example with a suitable facing head. The tool and face are aligned with the bore axis.
- Thread cutting or thread milling produces screw connections. The choice depends on the thread, material and machine equipment.
A common setup can reduce re-clamping errors. It guarantees neither final size nor geometric tolerance: machining strategy, tooling, workpiece condition and the agreed inspection also matter.
Types: table-type boring mill and floor-type boring mill
These names mainly describe the arrangement of the workpiece and machine. A rotary table alone does not distinguish the two types.
On a table-type boring mill, a moving table carries the workpiece. The table and column move along different axes depending on the design; table rotation is common. Table load, travel and space for clamps limit the application.
On a floor-type boring mill, the workpiece is typically clamped to floor plates while the column travels alongside. This suits large and heavy parts. Additional rotary tables are possible: floor plates do not rule out workpiece rotation.
A travelling-column milling centre also has a moving column. A swivelling milling head provides further tool directions. Its applications overlap with those of a boring mill; selection depends on the particular machine, head, spindle and working space.
CNC travelling-column milling centre for long and tall components
Which machine suits the machining task?
The table describes typical designs. Additional axes, milling heads and driven tools can extend their capabilities.
| Machine | Typical strength | Check before assignment |
|---|---|---|
| Table-type boring mill | Side bearing seats, mating faces and hole patterns on housings and machine columns | Table load, swept space, tool reach and access to reference surfaces |
| Travelling-column milling centre | Long and tall components such as machine beds, frames and mast assemblies | Column travel space, head positions, tool access and clamping |
| Gantry milling machine | Large faces and hole patterns on plates, crossmembers and frames | Clearance and head equipment; side machining, for example with an angle or swivelling head |
| Radial drilling machine | Holes and threads whose requirements can be met using marking out, stops or a drilling jig | Required positional accuracy, tool guidance and suitability of the fixture |
| Vertical lathe | Diameters, flat faces and runout on rings, flanges and hubs | Turning envelope and clamping; additional drilling and milling require suitable equipment |
Machine names do not replace a feasibility assessment. The drawing, access and available equipment determine whether another machine or setup is required.
Three limits that travel figures do not explain
Even when the nominal machine dimensions appear to suit the workpiece, these questions remain:
- Tool access: can the cutting edge reach the face or bearing seat without colliding with housing walls and clamps? A deep bore may require long tooling; tool overhang affects machining.
- Clamping and movement: is the component safely supported and secured? Is there enough space to rotate it, including protrusions and clamps? Table dimensions and travel alone do not describe this space.
- Workpiece condition and sequence: further welding or material removal may distort a welded housing. Roughing, any specified stress-relief operations and finishing therefore need to suit the required final geometry.
This assessment also determines whether roughing and finishing use the same machine or whether several machines should be combined.
The housing example: what H7 specifies and what it does not
Back to the gearbox housing from the start: two bearing seats Ø 160 H7 in opposing walls, with base face C below, on which the housing sits on the machine frame. The drawing specifies three requirements separately. The values are an example:
| Drawing callout | What it specifies | What it leaves open |
|---|---|---|
| Ø 160 H7 on each bearing seat | The diameter of each bore between 160,000 and 160,040 mm (ISO 286-1) | Whether the two bores lie on one axis |
| Coaxiality Ø 0.02 to common axis A-B | The axis of each bearing seat lies within a cylinder of 0.02 mm diameter around the common axis of both bores (ISO 1101) | How this axis relates to the base face |
| Parallelism 0.03 to base face C | The common axis lies between two planes 0.03 mm apart, parallel to C | The height of the axis above C; that needs its own dimension or a position tolerance |
Each tolerance applies on its own (independency principle, ISO 8015): two bearing seats can both be within H7 and still not align.
How aligned bearing seats are produced on the boring mill
A boring mill has two basic routes for two bearing seats in opposing walls. In the first, a long boring bar passes from one side through both walls; an outboard support can steady it at the far wall. Both bores come from the same spindle axis without rotating the workpiece.
In the second, each wall is machined from the outside and the rotary table turns the part 180° in between. The tools stay short and rigid. In return, the indexing accuracy of the table and the exact position of its rotation axis enter the alignment.
Which route fits depends on wall spacing, bore diameter, tolerance and obstructing features in the housing. The diameter of each bearing seat is measured, the common axis in a separate measuring step. The order defines which inspection record belongs to the part.
For the enquiry, the dimensioned drawing with references and tolerances, material, blank condition, quantity and deadline are enough. We decide the machining route.
Choosing between the machines at Bock
At Bock in Lorch, three identical TOS-Varnsdorf WHN 13 CNC horizontal boring mills have a boring spindle diameter of 130 mm. Travel is 3,500 mm crosswise, 2,500 mm vertically and 1,200 mm longitudinally. A rotary table and Heidenhain iTNC 530 support multi-side machining. Other geometries can be assigned to:
- the travelling-column milling and boring centre Soraluce FR 20,000 with 20,000 mm × 4,000 mm × 1,600 mm travel
- two gantry milling machines Zayer Thera W with up to 26 m travel
- four machines for radial drilling and tapping: drilling up to Ø 90 mm, threads M2–M60, reach up to 3,000 mm
- three vertical lathes up to Ø 4,800 mm turning diameter
For a housing or frame, a gantry mill might first machine a reference face before a boring mill produces side bearing seats. This is a possible route, not a fixed assignment. Our manufacturing team assesses the drawing to select the machines. We do not operate a floor-type boring mill; larger components are assessed for travelling-column and gantry milling.
Frequently asked questions about the boring mill
What is a boring mill?
A boring mill is a machine tool for drilling, boring and milling. This article covers the horizontal type, whose work spindle lies horizontally. It is often used for large housings, frames and other components whose bores and surfaces must be positioned precisely relative to one another.
What is the difference between a boring mill and a radial drilling machine?
A boring mill positions tool and workpiece through its axes and can machine faces and pockets as well as bores. A radial drilling machine moves its drill head to the hole position on a swivelling arm. Required positional accuracy, machining operations and the available fixture guide the choice.
What is the difference between a table-type boring mill and a floor-type boring mill?
On a table-type boring mill, a moving table carries the workpiece. On a floor-type mill, the component is typically clamped to floor plates while the column travels alongside. A floor-type mill can also have additional rotary tables. Bock operates table-type mills but no floor-type boring mill.
What does boring (Ausspindeln) mean?
Boring uses an adjustable cutting tool to remove material from an existing hole until it reaches the specified size. Alignment of several bearing seats also depends on clamping, machining strategy and the specified geometric requirements.
Boring mill or gantry mill – which machine suits my component?
Side bores and related faces may favour a boring mill. For large faces and long components, a gantry mill may offer favourable tool access. Angle or swivelling heads extend its applications. The actual geometry decides; we assess it from your drawing.
Do I have to specify which machine my component is to be produced on?
Normally you specify the requirements for the finished component. The manufacturing team selects the machine and setups. Any binding requirement for the manufacturing route should be stated explicitly in the enquiry.
Which boring mills does Bock use?
Three identical CNC horizontal boring mills TOS-Varnsdorf WHN 13 with boring spindle Ø 130 mm, travels of 3,500 mm × 2,500 mm × 1,200 mm, 360° rotary table and Heidenhain iTNC 530 control. For larger parts there are also the travelling-column milling centre Soraluce FR 20,000 and two gantry milling machines Zayer Thera W.
Standards and technical sources
The standards distinguish size tolerances from geometric requirements. Manufacturer examples document designs and equipment variants; their performance data do not describe Bock machines.
- Positional tolerances of the bores relative to each other (coaxiality, parallelism, perpendicularity) and how they are specified on the drawing DIN EN ISO 1101 – Geometrical product specifications (GPS) – Geometrical tolerancing – Tolerances of form, orientation, location and run-out
- Fits of the bearing seats, such as H7 DIN EN ISO 286-1 – Geometrical product specifications (GPS) – ISO code system for tolerances on linear sizes – Part 1: Basis of tolerances, deviations and fits
- Independency principle – size and geometrical tolerances each apply on their own DIN EN ISO 8015 – Geometrical product specifications (GPS) – Fundamentals – Concepts, principles and rules
- Floor-type mill with floor plates and optional rotary tables; W spindle travel and Z ram travel WRF HEAVY – Horizontal Floor-Type Boring Mill
- Machines combining turning, drilling and milling as an equipment variant Technische Daten Maschinen – VERTIMASTER
