Fine Grinding aka Lap Grinding, is an abrasive machining process that combines the best of two fundamental finishing processes - Grinding & Lapping.
With a combined effect of - High MRR of Grinding and Accuracy of Lapping, Fine Grinding produces flat & parallel work-pieces.
Fine Grinding Machines evolved from traditional vertical lapping machine with two-wheels (double-wheel) working with a planetary drive which is typically known as the 'lapping kinematics'.
In Fine Grinding Machines, Grinding wheels are used in place of Lapping wheels. And the grinding wheels are cooled through labyrinth-arrangement in the wheels, wherein coolant is fed through the holes in the upper wheel.
Relative of lapping, Fine Grinding warrants higher cutting forces and operates at much higher cutting speeds; Hence, drive motors, gears-boxes and the overall structural rigidity of the machines are much stronger in Fine Grinding Machines.
High Accuracy + Fast Cutting, place Fine Grinding Machines in a class of their own, offering a much cleaner, consistent and faster solution to producing accurate flat parts with high surface finish.
Take a quick look at the following video for a visual understanding of Double Side Fine Grinding. If you already savvy with it, skip to the 'Contents' section.
Video shows iGrind LG 720, installed at one of the leading Appliance Manufacturers in India.
Part being machined is a Valve Plate used in Refrigerator Compressors.
Practically any flat part with stringent size, form & surface roughness requirements - can be finished in iGrind LG Series
Before we get to Fine Grinding, let's take a look at Double-disk Grinding.
Although traditional double-disk grinding and double side fine grinding are quite different classes of processes, fundamental elements of both the process are very similar. So then, let's quickly get through an overall view of double side grinding.
Why Grind with two wheels simultaneously?
As the name suggests - double disk grinding employs two opposing abrasive grinding-wheels to grind two sides of a work-piece at the same time.
This simple yet elegant kinematic principle makes double-disk grinding - a very efficient grinding method to achieve tight size-tolerance, flatness, parallelism and high surface finish.
Compared to a typical Blanchard grinder - where material removal is on one side of the work-piece, quite simply because the material is being removed at once from both side of the work-piece in a double disk grinder - the MRR is nearly doubled - if not exactly.
MRR of course - like any abrasive machining process - is a function of type of abrasive, the bond and other variables. All said and done - the tolerances achieved in double disk grinding are far superior to what can be achieved with a Blanchard grinder.
A host of varieties of work-piece materials can be ground using double-disk grinders - ranging from various alloy steels, stainless steel, tool & die steels, sintered metals, to non-ferrous metals like copper, aluminium, bronze, and a many more.
Compared to single side grinding, generally speaking - double disk grinding process achieves much higher degrees of dimensional accuracies in terms of tolerances, parallelism, and flatness. It is also quite cost-effective in applications such as thickness control of small metal parts.
Stock removal mechanics in double disk grinding
With two opposing abrasive wheels or so to say - the disks, double disk grinding removes equal stock of material from two sides of the workpiece simultaneously. This cutting kinematics - equalizes & normalizes the degree of stress on the workpiece, which in effects enables this process to:
Significantly reduce internal stresses and improve structural stability of the work-piece during machining.
Achieve quite a high degree of dimensional accuracy and parallelism.
Attain flatness values that would otherwise be very difficult to achieve a single-side grinding.
Double disk grinding process also provides much better surface finish than general grinding methods - as it greatly minimizes / eliminates grind-patterns progressively through the processing cycle.
Applications of double disk grinding are quite wide - from die-castings, stampings, blankings, valves, rings, rotary compressor pistons, chambers, sprockets, and so many others. And when it comes to automotive components - double disk grinding is widely used in finishing of brake drums, brake pads, bearings, and a host of engine parts.
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[Aptly called as Lap-Grinding or simply Fine Grinding]
Fine Grinding is a batch-mode ‘super-finishing process’ for flat surfaces, that imparts:
▸ High-precision form | Flatness & Parallelism
▸ Tight size control | Part Thickness
▸ High surface finish | Consistently & Precisely
The process is a fusion of lapping & grinding (aptly named ‘Lap-Grinding’), combining the best of:
▸ Speed & aggressive cutting of grinding +
▸ Accuracy of lapping kinematics
In ‘Lap-Grinding’ kinematics, work-pieces are driven around in an epicyclic trajectory whilst being sandwiched between upper & lower grinding wheels.
Upper & lower wheels, as a pair, remove the material from both sides of the work-pieces simultaneously. Both wheels are made with an abrasive layer of diamonds or CBN.
Epicyclic trajectory of work-pieces, is accomplished by planetary motion of carriers i.e., work-piece holders.
Coolant through holes & labyrinths in the upper wheel ensure reach of coolant to the cutting zone in a systemic fashion; While the evacuation holes in the bottom wheel act as pathways for chip removal.
iGrind LG series is sort of similar to a typical horizontal double disc grinder, albeit with such differences as-
(a) batch-mode grinding (b) epicyclic carrier drive (c) a self-aligning upper wheel via a universal torque-joint
Relative to double-disc grinding, the construction & process design of Lap-Grinding Machines, enables far-superior precision & size accuracy.
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At a fundamental level - Fine Grinding differs from regular grinding since it is considered to be a cooler process with minimal or no sparks.
And looking deeper into this subject - for efficient Fine Grinding a flood of cooling fluid is a necessity to control the temperature and to remove the chips / swarf from the machined parts. If insufficient coolant is provided, wheel loading increases as a side effect and accuracy starts degrading, in addition to the possibility of thermal damage to the parts and/or the wheel. Hence, coolant system and distribution schema, are critical to the outcome of Fine Grinding.
Furthermore, when compared to traditional Double Disc Grinding, Fine Grinding is deceptively similar to an untrained eye, but fundamentally different.
Elementary yet a key differentiator being - Fine Grinding is a batch-mode process unlike conventional Double Disc Grinders where parts are loaded through continuous feeding system. Batch-mode processing means - a fixed quantity of parts are processed simultaneously (in a batch!).
Epicyclic trajectory of each work-piece - ensures that all the areas of Grinding Wheels come in contact with the work-piece. And hence, owing to normalization of all parts during the Grinding cycle, and extended contact of the parts with the abrasive wheel, Fine Grinding produces far superior results in comparison to traditional single face or dual face grinding techniques, in terms of much better normalization of size & parallelism.
However, when looked upon closely - both Fine Grinding & Double Disc Grinding, have their specific merits. Where there's high stock removal - double-disk grinding is preferred; Let's say in large-scale production, single parts like say gears - can be finish ground with a cycle time of less than 0.5 seconds with continuous-feed loading on a Double Disc Grinder.
Whereas, Fine Grinding is preferred where there's a need of high precision and accuracy. Components such a pump parts or compressor parts - can be finished with an accuracy of ± 0.001mm, and form tolerances of less than the size tolerance band.
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▸ Performed with super abrasive diamond or CBN wheels.
▸ Material/stock removal is effected by micro grooving, fine cutting & abrasion.
▸ Fine-ground surface has cross-hatched marks, which is desired for applications where work-pieces need to retain lubricant.
▸ Coolant is filtered with simple filtration systems, re-circulated and reused.
▸ Work-pieces are coated with only a thin layer of coolant and hence work-piece cleaning is minimal to nil.
▸ Cutting speeds are of order of 3 to 18 m/s
▸ Typically, MRR is 4 to 25 times faster than lapping
▸ In lapping - work-piece and tool slide over each-other over a loosely applied finishing medium i.e., the lapping compound aka slurry, and the work-pieces are subjected to continuously changing direction of rotation - to normalize the surface topography.
▸ Material/stock removal is effected by rolling and sliding action of abrasive grains of the lapping medium.
▸ Topography of lapped surface is dull matte-kind and a sort of micro-crater type of surface structure.
▸ Lapping medium/compound are not possible to recycle due to contamination caused by removed stock.
▸ Due to the fine grains of lapping compound and fine sludge generated - lapped parts are considered to be contaminated and hence require cleaning.
▸ When done right - with right selection of abrasive, grit size & compound make-up, lapping process possesses the potential of producing better surface roughness compared to any other flat finishing process, typically of order of 20 to 30% better Ra values. Of course - at the expense of operational economics and MRR.
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Overall aspects of Fine Grinding process
▸ Achieves accuracies that have traditional been achieved only by lapping.
▸ Superior process quality & consistent repeatability of size, form & surface finish.
▸ Large size work-pieces can be effectively machined - with appropriate size of the wheels - in turn the appropriate size of the machine. Insight's iGring LG 1200 can machine parts of size envelop of up to dia.400mm.
▸ Batch processing of work-pieces provides sufficient finishing time - spread over multiple work-pieces, hence accomplishing high accuracies as well as faster process times.
▸ Work pieces are loosely held in carriers and hence - are machined (ground) stress-free; This aspect greatly helps in grinding of thin section parts - as it minimizes distortion significantly.
▸ Work-pieces of various different shapes can be machined, be it round, rectangular or irregular. Ground surface of work-pieces can be either full surface or can be with cut-outs i.e., interrupted cuts.
▸ The pellet structure of the Grinding Wheels allows different matrices of layouts - corresponding to desired kinematics conditions.
▸ Strategically structured voids (space) between the pellets are conducive to a free & high flow rate of the coolant, thereby promoting very good chip flow and significantly minimizing temperature rise through systemic heat dissipation.
▸ Heat dissipation effect is further enhanced/maximized by comprehensive design of labyrinths & flow-distribution channels in Insight's iGrind LG line up.
▸ With right tweaking of material removal per cycle, quite long and constant cycle times can be attained without frequent dressing process.
▸ Process can be quite easily automated in terms of L/UL. Insight's Automation division offers different levels of Automation to end-users by helping in choosing the right way and method of Automation - drawing from actual end-use requirements and future-proofing.
Faster cycle times and in-turn lower takt
▸ Usage of CBN or diamond super abrasives enables high MRR - typically 4 to 25 times faster than lapping.
▸ In most cases significant pre-machining steps can be avoided; For example sintered parts with decent input-size tolerances can be directly ground without milling or pre-grinding. It is however a trade-off between desired tool life as a function of amount of material removal in fine grinding.
▸ Super-abrasive pellets help in extending the time interval between resharpening i.e., dressing, hence maximizing run-time.
▸ Within the working envelop of the machine, pretty much any part can be ground. A wide variety of work-piece materials can be fine ground in iGrind LG line up, offering a high level of flexibility.
▸ L/UL automation solutions enable unmanned operation and provide the ecosystem of consistent productivity throughout the shift.
Fine Grinding is a cleaner process
▸ Compared to lapping, in Fine Grinding - the coolant can be filtered & re-circulated for much longer time intervals.
▸ As a result of extended re-circulation life, wastage is tremendously less than lapping.
▸ In most cases, owing to continuously fresh coolant infusion at higher flow-rates - post process work-piece cleaning is minimal to nil.
▸ In cases where work-piece cleaning is necessary, rinsing mostly suffices - as compared to lapping where ultrasonic or chemical cleaning may generally be employed.
▸ Extended tool life, much less frequent tool resharpening / dressing, easier coolant recycling - herald reduced downtime.
Operational Economics of Fine Grinding
▸ High MRR heralds reduced capital costs and manpower expenditure.
▸ Low wheel wear, or more aptly the long life time of the grinding wheels - greatly reduces the tool costs, and hence the CPC.
▸ Owing to extending coolant life and less effort warranted for recycling and reuse, waste disposal cost goes down, thereby reducing running costs.
▸ Lower or no work piece cleaning - translates into considerably decreased running costs.
▸ Possibility of saving / reducing pre-machining steps - poses the merit of reduced pre-processing costs.
▸ Using standard pellet-structure wheels - makes maintenance of tool quite easy, translating into lower upfront cost and hence the life-time cost of tool.
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Single or Double Side Lapping - is the abrasive surface finishing process that removes material at low-speed - using loose abrasive grains in a liquid mixture aka abrasive slurry.
These loose abrasive grains is slide/glide over the work-piece surface under pressure. Cutting action is effected by abrading, kneading, chipping or scraping away the peak-points on the work-piece surface. Peak-points are broken away by applying sufficient pressure to exceed the bending strength & break the peaks away.
Single or Double Side Fine Grinding - is the abrasive machining process that removes material at low-speed (higher than lapping though) - using a bonded Super-abrasive Grinding wheel with coolant to dissipate heat and flush away the chips.
Positionally affixed abrasive grains - act like cutting tips with undefined geometry. Cutting action is effected by a sort of localized plowing - which effects material removal by micro-grooving, cutting & abrading. Formation of chips happens by way of exceeding the shear strength.
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▸ Fine Grinding is very similar in kinematic principle as lapping.
▸ Workpieces are held in carriers that are driven around by an inner pin-ring w.r.to a fixed outer pin-ring - like planets - providing an epicyclic trajectory to work-pieces.
▸ Such all-round motion kinematics enable exposure & contact of work-pieces to full surface of the Fine Grinding wheel.
▸ Outer pin-ring is kept fixed and the inner pin-ring rotates in either the same direction of the lower Fine Grinding wheel or in the opposite direction, and some times changing directions.
▸ Such motion profile creates an ecosystem of a series of epicyclic cutting paths - hence normalizing the surface topography of work-pieces.
Insight’s Machine building division, Tooling division & Automation division, accord a single source of build of entire system, end-to-end, effectively enhancing productivity, quality consistency, and thereby reduce overall ownership cost of the system.
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