— Engineering Practice & Technical Insight from FDON GROUP
A Small Detail That Controls the Whole System
In the manufacturing of slewing bearings, the quality of the mounting holes directly determines:
- Installation concentricity
- Uniformity of bolt preload
- Rotational accuracy
- Long-term service life of the entire bearing system
In real production and on-site machining, engineers and customers often ask one simple question:
Should mounting holes be machined with water-based coolant or cutting oil?
However, experienced engineers know that coolant selection alone is not enough.
Behind every stable slewing bearing installation lies a complete control system involving:
- Coolant and lubrication
- Machining sequence
- Hole accuracy
- Chamfering and edge treatment
- Internal chip removal and cleaning
- Corrosion protection
Based on long-term engineering practice in heavy equipment and new energy projects, FDON GROUP shares key technical guidelines on how to guarantee reliable mounting hole quality.
1. Coolant Selection: Water vs. Cutting Oil — What Is the Real Difference?
During drilling, reaming and boring of mounting holes, the coolant must perform three core functions:
Remove heat from the cutting zone
Flush chips out of the hole
Improve surface finish and dimensional stability
🔹 Water-Based Coolant (Emulsion)
Advantages:
- Strong cooling capability
- Low cost and easy cleaning
Risks:
- Weak corrosion protection
- Residual moisture may cause micro-corrosion inside holes and threads
- Not suitable for high-precision mounting holes and long-term reliability
Typical applications:
- Rough machining stages
- Standard structural holes
- Low precision requirements
🔹 Oil-Based Cutting Fluid (Strongly Recommended for Finishing)
Advantages:
- Excellent lubrication, reducing vibration and chatter
- More stable hole diameter and roundness
- Better surface finish
- Strong rust-prevention performance
Engineering conclusion:
- For the finishing process of slewing bearing mounting holes, high-quality cutting oil is strongly recommended instead of water-based coolant.
This is essential to ensure:
- Uniform bolt preload
- Accurate flange contact
- Smooth and stable slewing motion
2. Five Key Controls to Guarantee Mounting Hole Accuracy and Installation Safety
From FDON GROUP’s engineering practice, the real determinants of long service life are not only the coolant type, but the following five critical controls.
1. Reference Surfaces Must Be Finished Before Drilling Holes
Principle:
- Upper and lower mounting faces must be machined first
- Mounting holes must be processed afterwards
Otherwise:
- Hole perpendicularity deviates from mounting face
- Hidden eccentric loading appears after installation
- Early raceway fatigue and abnormal noise may occur
2. One-Time CNC Clamping Is Mandatory
Wrong practices:
- Multiple re-clamping operations
- Manual marking and positioning
Risks:
- Accumulated pitch circle errors
- Uneven hole distribution
- Highly uneven bolt load
Correct method:
CNC machining with one-time clamping for the full bolt circle to guarantee:
- Pitch circle accuracy
- Uniform hole spacing
- Stable concentricity
3. Drill → Enlarge → Ream (or Fine Boring) — Never One-Step Machining
Standard process route:
- Rough drilling for positioning
- Enlarging for size correction
- Reaming or fine boring to final tolerance
Purpose:
- Control hole diameter tolerance
- Improve roundness
- Reduce residual machining stress
4. Chamfering (Beveling) the Hole Entrance — A Small Step with Big Safety Impact
This step is often ignored but extremely important.
Every mounting hole should be properly chamfered (beveled) after machining.
Functions of chamfering:
- Remove sharp edges and burrs
- Prevent injury to operators’ hands during installation
- Avoid scratching bolt surfaces and damaging protective coatings
- Improve bolt insertion and seating accuracy
- Reduce stress concentration at the hole edge
Without proper chamfering:
- Operators may be injured during manual positioning
- Bolts may be scratched or jammed
- Local stress concentration may accelerate fatigue failure
Engineering recommendation:
- Use a standard 45° chamfer or controlled edge radius according to bolt size and drawing requirements.
5. Complete Internal Chip Removal and Final Cleaning — The Most Neglected but Critical Step
Many early installation failures originate from remaining metal chips inside mounting holes.
Mandatory actions:
- High-pressure air or flushing to remove internal chips
- Mechanical brushing if necessary
- Thorough cleaning to remove coolant and oil residues
- Final application of anti-rust oil or corrosion inhibitor
Otherwise, remaining chips may cause:
- Bolt seating deviation
- False tightening torque readings
- Uneven preload distribution
- Local corrosion at thread roots
- Risk of bolt loosening after several months of operatio
