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A killer combination for ideal grinding conditions

Nov. 1, 1998
Recent developments in grinding wheel and metalworking fluid technology can boost production while cutting costs for grinding operations.

Recent developments in grinding wheel and metalworking fluid technology can boost production while cutting costs for grinding operations.

Combining Milacron's MSB grinding wheels and Quantalube 270 metalworking fluid on its grinding machines has helped Acme Grinding Inc., Rockford, Ill., dramatically increase production, while cutting down on wheel changes, machine downtime, and production costs.

As the metal removal rate increases, specific energy drops. The MSB/Quantalube combination decreases Uvalues the most.

The MSB/Quantalube combination gives the best grinding efficiency ( E), and, in fact, produces an ideal grinding condition.

Grinding is a fairly straightforward process, combining four key elements: a grinder, a grinding wheel, a metalworking fluid, and a part. Two of these components are fixed — the grinding machine, which is rather expensive to replace or modify, and the part, which must be manufactured to a defined specification. The grinding wheel and the metalworking fluid, however, are variables that can be modified. And given the right combination, they can significantly improve the grinding process by improving part quality, increasing production, and decreasing overall cost.

For years, many believed that ideal grinding conditions could only be obtained combining a superabrasive grinding wheel and an undiluted straight oil metalworking fluid. But researchers at Milacron Inc., Cincinnati, Ohio, have shown that pairing Quantalube 270, a water-soluble metalworking fluid with lubricity properties comparable to that of straight oils, with MSB grinding wheels, which use sol-gel alumina abrasives, can optimize grinding of steel parts.

Another benefit is that, compared to conventional abrasive products such as fused alumina, MSB wheels have demonstrated superior grinding performance. They are also less expensive than superabrasive products, for example diamond and cubic boron nitride (CBN).

Defining parameters
To see how Milacron combined its new grinding wheels and metalworking fluid to optimize grinding conditions, one must first understand several key parameters that explain the grinding process and the advantages of this new technology. These key parameters are grinding efficiency, grinding ratio, specific energy, and specific metal removal rate.

Grinding efficiency (E) is the grinding ratio (G) divided by the specific energy (U) and is expressed by in. 3 /hp/min. Increasing the G ratio or decreasing the specific energy increases efficiency.

The following formula is used to determine grinding efficiency:

E = G/U

In this equation, G is the grinding ratio, which is defined as the volume of material removed per unit volume of wheel wear. G increases with less wheel wear and/or higher metal removal. The higher it is, the better the grinding conditions, especially in terms of longer wheel life.

To determine G, one takes the equation:

G = Vw/Vs

Where Vw = Volume of metal removed (in. 3 ) and Vs = Volume of wheel wear (in. 3 ).

The U in this equation is the specific energy, which is defined as the grinding power required to remove one unit volume of material per one unit of grinding time (hp 3 min/in. 3). Lower U values are better than high ones, and U can be lowered by either decreasing power consumption or increasing the volume of metal removed.

To determine U, the following equation is used:

U = N/(Q´ 3 W)

Where N= Grinding power (hp) and W= Effective wheel thickness (in.).

The in this equation is the specific metal removal rate, which is defined as the volume of metal removed, per unit of grinding time, per unit of effective wheel thickness (in. 3 /min/in.).

To determine this value:

Q´ = Vm/W

Where Vm = Volume of metal removed per grinding time (in. 3 /min).

An ideal grinding condition (IGC) is achieved when the efficiency (E) remains constant as the metal removal rate () is increased.

Milacron research proved that IGC can be achieved without superabrasives and without straight oil by combining MSB abrasives with Quantalube fluids.

The skinny on metalworking fluids
Typically, metalworking fluids are defined by the amount of mineral oil contained in the concentrate. The chart below breaks fluids into four types.

Type of metalworking fluid % mineral oil in concentrate Typical use dilution
Straight oil approximately 100% undiluted
Soluble oil 60-90% 5-10%
Semi-synthetic 3-30% 5-10%
Synthetic 0 3-10%

Many of these products may also include extreme pressure (EP) lubricants to enhance their performance. Typical EP lubricants in metalworking fluids contain sulfur and/or chlorine. The semi-synthetic used in the first phase of Milacron's testing contained a chlorinated EP lube package, while the one used in the second phase did not.

The Quantalube 270 metalworking fluid contains a sulfurized EP package, but does not fit any of these standard definitions that could classify it as a particular type of fluid. In appearance, it is similar to a soluble oil.

A bit about grinding wheels

The grinding wheel is composed of three major components, abrasives, bond, and porosity. The solgel alumina chemical composition of MSB wheels are similar to fused alumina used by conventional wheels.

However, the MSB's crystal sizes are significantly smaller. In fact, the average crystal size of these abrasives are submicron, less than 1 µm, while that of fused alumina is about 200 µm.

MSB has a microfracturing capability that results in self-sharpening characteristics, as well as high fracture toughness. Grinding wheels containing MSB abrasives can therefore demonstrate excellent grinding performance.

In testing Milacron's new products, researchers used wheels containing different combinations of fused alumina and sol-gel alumina. The Milacron 3MSB wheel, for example, contains 30% sol-gel alumina and 70% conventional fused alumina, while the 5MSB wheel contains 50% sol-gel and 50% fused alumina. These combinations make for a more cost-effective grinding wheel.

Performance testing
In the first evaluation, researchers used a 220-8 RK centerless grinder to evaluate different combinations of grinding wheels and metalworking fluids. The workpiece was 52100 steel with a hardness of 60-62 Rc. The wheel grade for the aluminum oxide was 1XA801-Y-BWK and for the MSB was 3MSB801-Y-BWK. Both wheels were 20 3 6 3 12 in.

Wheel wear and metal removal were measured in each test to calculate a G ratio. A value of 0.167 was used and the horsepower draw was measured in order to calculate the U value.

During each test, researchers removed 0.500 in. from the diameter of the workpiece. They compared a chlorinated semi-synthetic and Quantalube 270, running both at their recommended concentration of 5%.

The grinding ratio significantly improved when the abrasive was changed from conventional aluminum oxide to MSB, and then doubled again when the fluid was changed to Quantalube 270.

Normally, when an MSB wheel replaces a conventional grinding one, wheel performance significantly improves while energy requirements increase.

Researchers found an unexpected drop in energy when they replaced conventional soluble oil with Quantalube 270. This drop indicated a synergy between the MSB wheel and the Quantalube 270. Grinding efficiency increased significantly as did grinding production.

In the second phase of testing, conventional and MSB wheels were once again evaluated, but this time three different specific metal removal rates were used.

A universal center-type grinder plunge ground 0.500 in. from the diameter of a 4150 cylindrical steel workpiece with a hardness of 48-52 Rc.

The part rotated at 200 sfpm, and wheel speed was 1,718 rpm. The wheel measured 14 3 0.5 3 5 in. values were 0.4, 0.6, and 0.8 in. 3 /min./in., and the infeed rates for these values were 0.0417, 0.0625, and 0.0833 in./min respectively.

The fluids used in this test were a standard semi-synthetic and Quantalube 270, both run at the same concentration.

In this evaluation, the G ratio once again increased significantly, first by changing from a conventional abrasive to MSB, and then by changing the fluid to Quantalube 270.

However, the ratio did decrease, as would be expected, at the higher metal removal rate.

The specific energy, U, was also determined under these same conditions.

U decreased as the metal removal rate was increased, as the wheel was changed to MSB, and as Quantalube 270 was used, further supporting the combination.

Finally, the grinding efficiency was calculated.

E was increased, and the most significant results were seen with the combination of MSB and Quantalube 270. In fact, an ideal grinding condition (IGC) was observed with this combination. Ideal conditions are when the grinding efficiency remains unchanged (or in this case actually improves) with increasing metal removal rates (). It was previously considered impossible or contrary to scientific principles for this to occur in grinding operations.

This performance advantage has also been demonstrated in actual production-type grinding operations on steel parts. At one plant, a conventional abrasive and standard soluble-type fluid were being used. Immediately after introducing MSB grinding wheels, wheel life increased, meaning less machine downtime for changes, which yielded increased production. The next step was to replace the soluble oil with Quantalube 270. The change boosted productivity by 30% and further reduced wheel usage by 40%.

Jobshop reaps the benefits of Milacron

Gene Hodges, manager at Acme Grinding Inc., runs a job using a 20 X 4 X 12 grinding wheel and Quantalube fluid on a Cincinnati grinder.

Acme President Judy Pike and Milacron Inc. Representative Byron Thompson hold a component that was finish ground to customer specifications. The part was machined on one of Acme's grinders using new Milacron wheel and fluid technologies.

Acme Grinding Inc. in Rockford, Ill., which finish grinds specifications, was not only having problems meeting but costs were mounting up. The company opted of Milacron's MSB grinding wheels and its Quantalube 270 metalworking fluid. In Acme's estimation, the results have been outstanding.

According to Acme President Judy Pike, "We were losing money, so we decided to evaluate our grinding process. That's when we started looking at the combination of the MSB wheels and the Quantalube fluid." Previously, the jobshop used a 60LF semi-synthetic coolant and VRW grinding wheels, but found that the combination was not providing optimal results for their particular operations.

Acme Managers Gene Hodges and Kevin Thompson report that, besides cost, Acme was also having problems involving wheel wear and machine downtime.

Pike says, "With the process we were using, down-time was astronomical. So we worked with our Milacron representative, Byron Thompson, to come up with some new ideas. Among the four of us, we decided to try the combination of the MSB ceramic wheel and the Quantalube."

The company now uses Quantalube 270 fluid with a 20 X 4 X 12 in. 3MSB801-J6-VSA grinding wheel. Since the changeover, reports Hodges, production has increased dramatically, wheel changes are less frequent, machine downtime has decreased, and costs have taken a plunge. "Originally," Hodges says, "we were getting about 60 pieces to a dress on the wheel. That was dressing about 0.015 in. stock off the wheel. Now we're getting 875 pieces to a dress, and, each time, we're only dressing off 0.007 in."

In addition, he remarks, "Before, we were able to run about 8,000 pieces on a wheel. With the new wheel, we're able to run over 250,000 pieces."

Obviously, Acme is happy with these results. The company is also pleased with the money it is saving. "Our production cost savings is about 26 1 /2 cents a part — that's down from 37 1 /2 cents to 11 cents a part," says Hodges.

Surface finishes have been good too, reports Thompson, "We're taking 0.065 to 0.075 in. stock removal in one pass and holding a 32 µin. finish." Pike adds, "We tried several operations and it never helped — we tried roughing and finishing the parts, but it never changed the outcome. With the change of the coolant and the wheel, we were able to hold it to a one-pass operation."

In summing up the Acme experience, Pike says the cost of the grinding wheels and coolant shouldn't scare anyone off from trying the combination. "A lot of shops may complain that the wheels are expensive, and the coolant is definitely not cheap. But, if you review some case studies and look at the statistics, you can really see the cost savings. It's overwhelming — really, the cost of the wheel and the coolant becomes totally insignificant."

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