The type and specification of the coolant / oil used in Honing determine a range of process outcomes such as tool life, surface quality, dimensional consistency and so forth.
Work-piece material, abrasive stone composition, desired amount of lubricity, heat dissipation capacity, affinity to clean & recycle, etc., are few of the characteristics that drive the selection of Honing Coolant.
Appropriate selection of Honing Coolant, is a crucial aspect for efficient & cost-effective production.
▸ Evacuate chips to enable clean cutting, reduce tool glazing & maintain consistent surface finish
▸ Lubricate the cutting zone to minimize undesired friction - to enhance tool life
▸ Dissipates process heat quickly and effectively - to ensure consistent bore-size & geometry
▸ Protect machine tool, fixtures, work-pieces from unwarranted rust
Two classes of coolants are used for honing, or in fact any metal cutting operation:
1) Oil-based coolants
2) Water-based coolants
Oil-based Honing Coolants
▸ Superior lubricity compared to water-based coolants, hence enables much cleaner cutting, less heat generation & better tool life.
▸ Less effective in heat-dissipation, but can be offset by using coolant-chiller & sufficient flow rate. Better lubricity too offsets heat dissipation to certain extent.
▸ Disposal of oil-based coolants is relatively complex, needs statutory compliances and calls for comprehensive recycling &/or disposal SOPs.
Water-based Honing Coolants
▸ Superior heat dissipation compared to oil-based coolants, hence providing a cooler process setting - which enhances thermal stability of work-piece and tool.
▸ Very effective for tooling with diamond abrasives, but do not possess good affinity with vitrified abrasives.
▸ Three basic types are: (i) Emulsions: mix of water & oil, (ii) Synthetics: Water-soluble chemicals, (iii) Semi-synthetics: Toned down synthetics with water base
An overview of Honing Coolants & their make-up
Honing coolants / oils are formulated with a select blend of base fluids and additives for specific types of materials. Certain types of oils work well with different classes of materials, but the best results are obtained with right blend is used for specific materials. A compromise or more aptly - a balance can be struck if a shop-floor is engaged in honing of different types & classes of work-piece materials - by choosing a blend of oil that works decently well across all varieties of materials.
Generally, Honing oils are blended with chlorine and/or sulfur additives, antioxidants, extreme-pressure additives, and synthetic lubricity additives.
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Coolants used for general metal cutting operations - despite being usable for Honing in some cases, generally do not work quite well with Honing - when cutting efficiency is taken in account.
A few generic approaches to the coolant make-up would be:
1) Materials with stringy / continuous chips such as Steel & similar alloys:
Coolant is preferred to have good sulphur content and possess high lubricity to minimize surface-tearing & rubbing.
2) Materials with discontinuous or powder-form chips such as Cast Iron & similar brittle materials:
Light honing oil with low viscosity and just enough lubricity to enable clean cutting.
3) Non-ferrous materials such as Aluminium, Brass and materials with similar metallurgical make-up:
Light honing oil that possesses high fat content to enable clean cutting of softer materials.
As a generic principle - high lubricity of honing oil helps heralds longer tool life, faster cutting speeds, and better surface quality.
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▸ Lowest possible viscosity, while retaining good lubricity - a balance
▸ Choice of right additives keeping in mind the compatibility with tool-abrasive & bonding agent, and work-piece material
▸ Filterability i.e., affinity to separate out the fine chips generated during cutting
▸ Allowability to settle sludge and fine particles - a parameter that drives choice of viscosity
▸ Resistance to oxidation and chemical stability for extended periods of time - to allow for recycle and reuse
Considerations of make up of Honing Coolants:
Composition of Honing Coolant determines its aptness to application. A few key aspects that need comprehension and consideration are hereunder.
1) Polar based additives:
Induce good sludge separation & settling properties.
Provide good rinsing properties - which reduces sludge-deposition and thereby better honing performance.
2) Extreme Pressure (EP) additives:
Enhance tool life by greatly decreasing tool-wear and thereby enable accomplishment of consistently good surface finish.
3) Refined base-stocks:
Provide good oxidation-stability - which enables longer sump-Iife of the coolant, hence decreasing operational costs.
3) Lower Viscosity:
- Enhances heat dissipation / cooling & enables better swarf removal.
- Minimizes oil-misting - which reduces carry-over loss and hence lowers consumption of coolant.
- Better oil spreadability also enhances heat generation during cutting.
- Easy filterability due to better affinity to separation of chips & swarf.
4) Lower Copper-strip corrosion:
- Minimizes / eliminates stain marks on work-pieces during machining.
- Typically used for honing of Aluminium and other nonferrous materials.
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