How to Accurately Calculate Material Removal Rate in Roll Grinding

 

Dan Goad, Application Engineer III, Norton Abrasives

 

W

hen calculating the material removal rate (MRR) for a roll grinding process, many may read and understand some of what they are looking at, but it quickly turns into a science class with formulas typically not used in roll grinding. Ultimately, the important information to determine is how does it affect the operating budget?

 

In a previous article, we explained how an operator can measure to see if the grinding wheel is being used efficiently by following some easy test parameters. This helps us to understand how the wheel is really being used without wasting valuable wheels to get the required stock removal and determine the money saved during the process.

 

Now we want to go a step further and look at the MRR of the wheel and compare MRR rates between two roll grinding wheels. There are a few items to take into consideration.

 

1. The Roll Grinder: A roll grinder has limitations such as the horsepower to the spindle and the stiffness of the grinder. Most operators know these limits, and in determining the best grinding wheel to purchase, these machine limits will need to be considered.

 

2. The Setup: The focus of this article is directed at hot and cold mill roll grinding. When grinding on centers or journals, the test would be conducted differently with respect to power used during the operation.

 

3. The Roll Type: The same type of roll must be used for MRR testing; material removal rates will vary due to material. For example, iron rolls will have higher removal rates than a material such as HSS, which will have lower values during testing. Testing should be performed with a roll that has been ground after mill use to ensure the “mill hard” surface has been removed prior to the grinding process.

 

4. Consideration should be taken to perform the grind test that best fits your situation, time availability, test wheels, and test roll.

 

  • Plunge grinding is a fast way to measure the MRR difference between two wheels. Most CNC machines have this feature, and this may be the best time saver.

 

  • Traverse grinding will yield the results you are looking for; however, there are a couple of points to look out for. In some cases, the wheel's contact with the workpiece can affect the results if you cannot have almost full contact between the workpiece and the wheel face. This will vary depending on the shape of the workpiece, but as long as the exact same cycle is used with the two test wheels, the results will produce data that shows the best MRR.

 
  • In most cases, testing the wheel at the beginning of its life while in new condition will provide data that will show the best MRR for that wheel. This should be similar to the results that you would see at the end of the wheel’s life; however, it may also be good for MRR testing to be done when the wheel when it is at the two thirds worn state.

 

  • You can also figure out your MRR over time with a check test, which tracks the data over several wheels. Although less accurate, it's an option for busy roll shops. Keep in mind that grinding time has an effect on the MRR outcome.

 

 

Calculate Q’w (Q prime)  Definition of MRR Material Removal Rate

Depth of cut in millimeters times feed rate in mm/s.

The calculated Q prime value will be in mm2/sec.

Ratio of volume of material removed to the grinding time (Minutes/Seconds).

Better known as Q’w (Q prime).

This is typically measured in cubic centimeters per minute (cm3/min).

 

When you're ready to perform the grind test, follow these steps:

  1. If your grinder is capable of measuring the dimensions of the wheel and the roll, then you can measure the roll and wheel before and after each grind test.
  2. Keep an accurate time count for your grind cycle; depending on the type of test being run to determine the MRR, time will be a key value at the end of the test.
  3. If your grinder is totally manual, you will need to use another item such as a Py tape to measure the wheel and roll.
  4. First, set up the test roll in your machine. Measure the ends and middle of the roll or the profile of the roll, using the end or middle for diameter reference. 
  5. Test the first wheel; measure the wheel diameter before the test grind.
  6. After the grind cycle has been set, run several short cycles and record the data. Enter the data into a system documentation form, which can be supplied by your Norton Application Engineer.
  7.  After each grind cycle, measure the finished diameter of the wheel and the diameter of the roll, and enter all data into the system documentation form.
  8. The next step of the test is to mount the second test wheel and repeat the above steps, making a new data sheet for this test.

 

Example:

    CYCLE 1 CYCLE 2 CYCLE 3
G Ratio (Test)   3.49 3.60 1.83
MRR' (Q'w) Average in^3/min 4.7138 3.8536 3.8048

 

At the end of the day, you can determine what your grinder's MRR is for any given test wheel. To have a Norton Application Engineer support your testing, feel free to contact us or reach out directly to your Norton salesperson to request help with a MRR test.

 

For more roll grinding resources, check out our articles on determining if your roll grinding wheel is being used efficiently and how to best optimize coolant parameters.