Table of Contents
- 1. Introduction
- 2. What Is a Facing Operation?
- 3. How Facing Is Performed on BOS DS50 and DS60
- 4. Applications — When Facing Is Needed
- 5. The Swing Bearing Mounting Face — Most Common Facing Job
- 6. Facing vs Grinding vs Milling
- 7. The 5-in-1 Capability — Where Facing Fits
- 8. Facing Operation Parameters
- 9. Tolerances Achievable in Field Conditions
- 10. Frequently Asked Questions
- 11. References
1. Introduction
The BOS DS50 and DS60 are marketed as 5-in-1 machines. Four of the five functions — line boring, inner circle welding, sector welding, and jump welding — are discussed frequently. The fifth function, facing (also called flange facing), is less well understood but equally valuable in the field. In some heavy equipment maintenance scenarios, it is the facing capability that determines whether a complete repair can be done on-site or requires a workshop.
This guide explains what facing is, how it works on a line boring machine in India, when you need it, and the specific applications in Indian heavy equipment maintenance where on-site flange facing saves significant time and cost.
I am Prakhar Bhardwaj, Founder of BOS — Bore On Site. What follows is a complete technical explanation of the facing operation and how the BOS DS50 and DS60 perform it on-site — drawn from six years of field operations across mining, construction, and marine sectors in India.
2. What Is a Facing Operation?
Facing is a machining operation that restores the flatness and surface finish of a flat face — a surface that is perpendicular to the axis of a bore or shaft. The cutting tool travels radially (outward from the centre) across the face while the boring bar rotates, removing material to produce a flat, smooth surface.
The most familiar facing operation in automotive workshops is engine head facing — resurfacing the cylinder head's gasket mating surface to restore flatness after warping or corrosion. In heavy equipment maintenance, facing is used on a wider range of surfaces: slewing ring mounting faces, swing bearing housing flanges, hydraulic manifold faces, cylinder end cap faces, and any other flat surface that must seal against a mating component or provide a precise reference plane.
The key characteristic of a facing operation is that it produces a flat surface perpendicular to the bore axis. Because the boring bar establishes the axis, and the facing head travels radially from that axis, the faced surface is automatically perpendicular to the bore — provided the boring bar was correctly aligned when the operation began. This is why facing on a line boring machine produces better geometric relationship between the bore and the face than separate boring and facing operations on different machines.
3. How Facing Is Performed on the BOS DS50 and DS60
Both the DS50 and DS60 can perform facing operations using a facing head attachment fitted to the boring bar. The process differs from boring in one key way: instead of the cutting tool advancing along the bore axis (axial feed), the facing head advances the tool radially outward from the bore centre (radial feed) while the bar rotates.
Bar Alignment
The boring bar is aligned along the bore axis using self-centering setup cones — exactly as for a boring operation. Alignment is critical for facing: any tilt in the bar axis will produce a faced surface that is not perfectly perpendicular to the bore, affecting sealing and mating accuracy. Verify alignment with a dial indicator before beginning the facing pass.
Facing Head Attachment
The boring head is replaced with the facing head — a radial slide attachment that holds the cutting tool and advances it outward from the bar centreline. The facing head is set with the cutting tool at the inner diameter of the face to be machined. The depth of cut is set on the facing head: typically 0.1–0.5mm per pass for finish facing, 0.5–2mm per pass for material removal facing.
Facing Pass
The boring bar rotates while the facing head's automatic feed advances the cutting tool radially outward. The tool removes material from the face in a spiral pattern, moving from the inner edge toward the outer edge. Feed rate (how fast the tool advances per revolution) determines the surface finish: slower feed produces a finer finish with closer-spaced tool marks. For sealing faces requiring Ra 3.2 or better, a slow finish pass is used after any roughing passes.
Measurement and Verification
After each facing pass, flatness is checked with a straight edge or dial indicator traversed across the faced surface. Surface finish is assessed visually and with a surface roughness gauge where required. For gasket sealing faces, the target is typically Ra 1.6–3.2 micrometres. For precision mating faces (slewing ring mounting, bearing housing flanges), Ra 0.8–1.6 may be required — achievable with a fine finishing pass and the correct cutting insert grade.
Dimension Verification
After facing, the total material removed is measured to confirm the face is still within required dimensional limits — the face must remain within the minimum thickness specification of the component. For components where face position is critical (e.g., the relationship between a bearing housing bore depth and the mounting face), verify the final dimension against the manufacturer's specification before releasing the setup.
4. Applications — When Facing Is Needed in Heavy Equipment Maintenance
Facing is required whenever a flat mating surface on a heavy equipment component has been damaged, corroded, or distorted and must be restored to flatness and surface finish before reassembly. The table below covers the primary applications encountered across Indian mining, construction, and marine operations.
| Application | Component Faced | Why Facing Is Needed | BOS Machine |
|---|---|---|---|
| Swing bearing / slewing ring replacement | Slewing ring mounting face on excavator upper structure | Mounting face must be flat within 0.1mm to prevent ring distortion and premature bearing failure | DS50 or DS60 |
| Hydraulic cylinder end cap replacement | Cylinder tube end face | End face must be flat and square to the bore for the end cap to seal correctly | DS50 or DS60 |
| Engine / pump mounting face repair | Equipment frame mounting face | Mounting pads on excavator frames develop corrosion and impact damage, preventing accurate engine or pump seating | DS50 or DS60 |
| Hydraulic manifold face repair | Manifold body mating face | Manifold faces develop scoring and corrosion from seal failure and contamination | DS50 |
| Crane slewing ring mounting | Upper and lower slewing ring mounting faces | Large crane slewing rings require precise flatness on both mounting faces to distribute load evenly and prevent ring distortion | DS60 |
| Marine flange facing | Propeller shaft flange, stern tube flange | Marine shaft flanges develop fretting corrosion at the mating surface. Facing restores the sealing interface before seal replacement | DS60 |
| Industrial plant flange facing | Pump, compressor, and turbine mounting flanges | Industrial plant flanges develop corrosion, gasket erosion, and weld distortion. On-site facing avoids unit removal | DS50 or DS60 |
Table 1 — Facing applications in heavy equipment and industrial maintenance. Machine selection depends on bore and face diameter. Contact BOS to confirm the correct machine for your specific application.
5. The Swing Bearing Mounting Face — The Most Common Facing Job in Indian Heavy Equipment
The most frequently performed facing operation on Indian construction and mining excavators is the swing bearing mounting face. Here is why it matters and how it is done.
An excavator's swing bearing (slewing ring) connects the upper structure — cab, boom, arm, counterweight — to the undercarriage. It is a large ring bearing that allows the upper structure to rotate 360 degrees. The mounting face on the upper structure and the mounting face on the undercarriage frame must be flat and parallel within tight tolerances (typically 0.1mm flatness over the ring diameter) for the slewing ring to seat correctly and distribute load evenly.
When a slewing ring is replaced — a common maintenance operation on excavators with more than 8,000–10,000 hours of operation — the mounting face frequently requires resurfacing. Weld splatter from field repairs, corrosion in the bolt hole counterbores, and the distortion caused by uneven bolt torquing during the previous installation all contribute to a mounting face that is no longer flat enough for the new ring to seat correctly.
Resurfacing this face in a workshop requires removing the entire upper structure from the excavator — a major disassembly job. BOS DS60 facing capability allows the mounting face to be resurfaced with the excavator in its operating position. The boring bar is aligned through the swing bearing bore (the central bore in the slewing ring mounting), and the facing head machines the mounting face flat and perpendicular to the bore axis — exactly the geometric relationship required for correct slewing ring installation.
6. Facing vs Grinding vs Milling — Why Boring Bar Facing Is Preferred On-Site
When a flat face needs restoration on heavy equipment, three methods are sometimes used: grinding, milling, and boring bar facing. Here is how they compare for on-site heavy equipment work in Indian field conditions.
| Method | How It Works | Geometric Accuracy | On-Site Suitability | Surface Finish |
|---|---|---|---|---|
| Angle grinder (manual) | Operator removes material by hand with abrasive disc | Poor — highly operator-dependent, no geometric reference | Possible but not recommended for precision faces | Rough — Ra 6.3–12.5 typical |
| Portable milling / facing machine (dedicated) | Machine clamps to component, mills face with rotating cutter | Good — machine provides geometric reference | Possible for large faces | Good — Ra 1.6–3.2 |
| Boring bar facing (BOS DS50 / DS60) | Facing head on boring bar — automatic radial feed from bore axis | Excellent — bore axis provides perpendicularity reference simultaneously | Fully on-site compatible — same setup as bore repair | Good — Ra 1.6–3.2 achievable |
| Workshop surface grinder | Precision grinding of removed component on workshop machine | Excellent | Workshop only — component must be removed | Excellent — Ra 0.4–0.8 |
Table 2 — Facing methods compared for on-site heavy equipment work.
The key advantage of boring bar facing over dedicated portable facing machines is integration. When the facing operation follows a bore repair — which is the most common scenario — the boring bar is already set up and aligned through the bore. Adding a facing pass requires only changing from the boring head to the facing head attachment. There is no second setup, no second alignment procedure, no opportunity for geometric error between the bore centreline and the face perpendicularity.
For a swing bearing mounting face repair that follows a swing bearing bore repair on the same setup, this integration means the entire repair — bore weld, bore to specification, face mounting surface flat and perpendicular — is completed in a single bar setup. This is the fundamental advantage of a machine with boring and facing capability combined. For a detailed comparison of the DS50 and DS60, see our DS50 vs DS60 comparison guide.
7. The 5-in-1 Capability — Where Facing Fits in the BOS Machine
BOS describes the DS50 and DS60 as 5-in-1 machines. The five functions together mean the BOS machines can address virtually every machining requirement that arises at a heavy equipment bore repair job — without bringing multiple different machines to the site.
Restores worn pin-and-bore joints to original diameter and alignment. The primary and most used function. The boring bar rotates while advancing axially through the bore, removing material to the target diameter with H7 tolerances.
Automatic bore welding with full 360-degree circumferential weld deposit. Used for uniform bore wear. Prerequisite for line boring when material build-up is needed before machining back to the correct diameter.
Programmable arc welding — deposits material only on the worn face of asymmetric bore wear. Reduces heat input and material waste versus inner circle welding for one-sided wear patterns common in crane and mining applications.
Programmable segmented welding — skips defined intervals around the bore circumference. Used for partial bore wear and to protect grease holes and ports from weld deposit.
Radial facing operation using the facing head attachment. Restores flat mating surfaces perpendicular to the bore axis. Covers swing bearing mounting faces, hydraulic cylinder end faces, crane slewing ring faces, marine flanges, and industrial plant flanges.
A single portable line boring machine in India handles the complete job: weld the bore, machine the bore, face the mounting surface. No second machine. No second mobilisation. No workshop.
8. Facing Operation Parameters — What Controls the Result
Understanding these parameters allows operators to plan each facing job correctly before starting — selecting the right depth of cut, feed rate, and insert grade for the material and required surface finish.
| Parameter | What It Controls | Typical Setting |
|---|---|---|
| Rotation speed (RPM) | Controls cutting speed and chip formation. Higher RPM for smaller diameters and softer materials | 50–200 RPM for facing (lower than boring — larger tool path diameter at the face outer edge) |
| Radial feed rate | How fast the cutting tool advances outward per revolution. Slower feed = finer finish, more tool marks per mm | 0.05–0.2 mm/rev for finish facing; 0.2–0.5 mm/rev for roughing |
| Depth of cut | Material removed per pass. Roughing passes remove 0.5–2mm; finish passes remove 0.1–0.3mm | 0.1–0.3 mm for finish pass; up to 2 mm for roughing passes |
| Cutting insert grade | Determines surface finish and tool life. ISO P (steel), K (cast iron), N (non-ferrous) grades | ISO P10–P30 for structural steel; K10–K20 for cast iron |
| Coolant / cutting fluid | Reduces heat and improves surface finish. Field facing often done dry with compressed air for chip clearing | Dry with compressed air is typical for field work |
| Maximum facing diameter | Determined by facing head arm length. Both DS50 and DS60 cover standard heavy equipment face diameters | Up to 600mm facing diameter with appropriate head attachment |
Table 3 — Facing operation parameters and settings for BOS DS50 and DS60 in field conditions.
9. Tolerances Achievable in Facing Operations on BOS Machines
The facing capability of BOS DS50 and DS60 achieves the following tolerances in field conditions. These values are based on correctly set-up boring bar alignment and appropriate parameter selection.
Over 300mm face diameter — achievable in field conditions with correctly aligned bar
Over face diameter — inherent to facing from bar setup geometry
On finish pass — suitable for sealing and bearing mating applications
Face depth/height in field conditions
These tolerances meet the requirements for slewing ring mounting, hydraulic manifold sealing, cylinder end cap seating, and most other heavy equipment facing applications. For applications requiring Ra 0.8 or better, a precision finishing pass with the correct insert grade achieves this in field conditions on BOS machines.
For the full technical specifications of both machines, see the BOS DS50 product page and the BOS DS60 product page. For an overview of all line boring machines available in India, including pricing, see our dedicated hub page.
10. Frequently Asked Questions
11. References
- Mactech On-Site — Portable Line Boring and Flange Facing Equipment (mactechonsite.com, 2026)
- CLIMAX Portable Machining — FF Series Flange Facing Machines (climaxportable.com)
- Specialty Gear Drives — Detailed Guide to Choosing a Line Boring Machine (specialtygeardrives.com, July 2026)
- PR Crane and Earthmovers / BOS — DS50 and DS60 product specifications: 5-in-1 function documentation (wearebos.in)
- ISO 1101 — Geometrical Product Specifications: Tolerances of Form, Orientation, Location and Run-out
Get a same-day quote for on-site bore repair and facing, or request a product demo before purchasing a BOS machine. Tell us your equipment type, bore diameter, and location.
Prakhar Bhardwaj founded BOS with a modern approach to on-site machining and bore repair solutions. With 6+ years of hands-on field experience, he leads BOS's product development and customer operations across India and international markets.