How Plumbing a CNC Grinding Machine is Trickier than Running One (by Eric Schwarzenbach):

I have never been an advocate for overhead coolant delivery and return to a centralized filtration system, but I have to admit that sometimes there are no other alternatives that make sense. In this feature, I examine the various considerations when connecting a CNC grinding machine to a coolant filtration system.

You have three choices to deliver and return the coolant oil from the grinding machines to a centralized filtration system. Traditionally, the pipes are run just above the floor and the coolant is returned by way of gravity feed. This will restrict you somewhat since the discharge heights on grinding machines are generally low and only allow a certain length of reach to the filter. The slope recommended for water drainage in PVC piping is 2%, however with the grinding sludge present in the discharged oil, the recommendation is more around 3 - 4%, in other words up to 3/8” and 1/2” per linear foot.

This approach is typically used when grinding machines are grouped into smaller cells which are then served by a filtration system for each cell. Depending on the size of your factory, you need to position the filters close to your machines inside your workshop.

A more effective and less expensive way is to position the filters in a separate, adjacent room so that the noise is kept away from the grinding area. A free-standing room also allows you to keep the heat output of the coolant chiller away from the machines. The heat generated by the chillers could then ideally be ducted through the ceiling or wall into the open air. Alternatively, the refrigerant could be plumbed to an outdoor heat condenser.

It goes without saying that the temperature of the coolant oil must be controlled. The only means to do that is to run the oil through a chiller or a heat exchanger after filtration. Unrestraint coolant temperatures invite issues with machine stability, part integrity (geometrical variations during grinding), reduced grinding wheel performance, premature spindle failures, surface integrity and more.

It has never made sense to me to position chillers inside the grinding area and allowing the heat to discharge into the factory. Even during winter months, these chillers produce a substantial amount of ambient heat that can only be removed a second time with air conditioning. A large part of keeping the grinding machines thermally stable is to constrain the ambient room temperature which in most geographical regions means air conditioning. The cost of removing heat from the grinding process and from the workshop is thereby easily doubled.

The golden rule is that the oil temperature in grinding machines should be kept at the same level as the ambient temperature in the building.

Water-based chiller systems have gained popularity in the grinding industry. This makes sense for a cellular system (with several oil filters inside a factory). Water mixed with Glycol has the ability to absorb and release large amounts of heat without changing its temperature. This makes it ideal for use in transporting the heat from a heat exchanger on the oil filtration system to an outside chiller which then only chills the water (not the oil). Cost-effectiveness is another consideration here. You don’t need a chiller for each filtration system, you just need a simple and inexpensive heat exchanger.

Another solution is to dig trenches and run your coolant pipes underneath the floor. If the factory design is such that the filters can be positioned at a lower level, then gravity feed can potentially return the coolant to the filter. Else, pumps have to be installed to return the discharged grinding oil.

This approach will esthetically look tidy and appealing, however it locks you into a pre-determined machine layout and it does not present any flexibility which is commonly needed for the growth of the business, adaptation to new market trends and the capacity expansion with new equipment.

Another method is to plumb the piping for both the clean oil delivery and the discharge of dirty oil overhead. This allows flexibility in terms of repositioning the machines as needed, or easily expanding the grinding capacity. A separate coolant pump that allows dirty oil to be ejected needs to be installed at the back of every grinding machine.

The drawback with the overhead solution is the fact that the return pump for the dirty coolant has to be sized sufficiently to overcome the hydraulic height difference and this induces heat into the oil. Additional heat is against the principle of lowering cost for cooling.

Another distinctive disadvantage with overhead pumping manifests itself when the coolant finally returns from the ceiling and plunges into a tank. Now, the oil is plummeting 15’ or 25’ downwards into a tank like the Niagara waterfall and inevitably air is introduced into the oil. These air pockets and bubbles have to be purged and removed quickly before the oil is cleaned and returned to the machine. The only way to reduce air pockets is to have a tank with a large surface area where the air can naturally escape.

Entrained air in the oil is the enemy of grinding. Mixture of air and oil above 30% are generally referred to as foam. Air barriers created by air turbulences or by other parameters can deflect coolant oil from the workpiece causing numerous issues. There is a whole science of how to design coolant nozzles which reduce turbulences and air intrusion into the grinding process.

Company owners and plant managers have to find their own specific machine layout and filtration design that best suit their situation and their future potential. Nonetheless, for someone who may be very familiar with designing and grinding cutting tools, coolant oil and chillers can be a tricky subject.

My advice to managers is not to be shortsighted, but to plan the layout of their factories with plenty of potential expansion in mind.

Lean CNC Tool Grinding – Standardized Process (by Eric Schwarzenbach)

Workflow and workplace organization, standardization and waste reduction are not traditionally addressed in CNC tool grinding. CNC tool grinding is sometimes looked at an "art" rather than a "standard process". In some cases, the standard principles of manufacturing cannot be applied to CNC tool grinding, however it is worthwhile discussing some areas of grinding where a harmonized procedure is of benefit.

Lean manufacturing provides a systematic method for minimizing waste within a manufacturing system, while staying within certain margins of control such as productivity and quality. It was originally invented by the founding engineers of Toyota Motor Corporation in Japan, and it was known as the TPS (Toyota Production System).

These days, it doesn’t matter how big or how small your company is – now is the right time to incorporate some of these “lean” tactics into your operating processes to get the most out of your team and equipment. It’s vital to the integrity of your product!

New systems and tools are available for users of tool and cutter grinding machines where some of the philosophies of lean manufacturing can be utilized. Following the logical progression of preparing grinding wheel packs, managing/storing the wheels and integrating them into the manufacturing process using the latest equipment and software available, you can see small ways to make adjustments that translate into big differences for your business.

RFQs – Customer Quotations:

In the initial quotation stage, the tool manufacturer needs to have a clear idea of the tooling needed for the job and the cycle time. Desktop tool design software provide a realistic three-dimensional model of the tool together with an accurate total grinding time. An animation of the grinding process inside the machine including all axes and the tooling helps the manufacturer identify the accessories and wheels needed for immediate production of the tools, in case the purchase order comes through. Shorter runs force a manufacturer to do setups effectively and productively.

If such a program is used to establish the feasibility and cycle time at the time of quoting, then the production process is already established once the customer submits the purchase order.

Wheel Dressing:

After determining the wheel shape and finding the appropriate wheels, the wheels are assembled on the actual grinding arbor and dressed both on the periphery and on the side. This is essential for both new wheels as well as for worn wheels. Here are some advantages of off-line dressing (on a separate machine rather than the actual grinding machine):

Machine time is not taken up for wheel preparation.

Wheel packs can be prepared during the working hours of qualified setup personnel.

Wheel pack management is centralized in the factory and wheel packs can be shared on different equipment.

Silicon-carbide or aluminum-oxide wheels can be used for dressing which work best for all common bonds on superabrasive wheels (diamond or CBN). Trueing directly on a CNC tool grinding machine necessitates the use of a diamond roll which is only suitable for regenerating the profile of a form wheel.

These are usually specially-designed dressing machines which utilize a silicon-carbide or aluminum-oxide wheel as a trueing wheel. Some machines have capabilities for radius dressing, others only for linear dressing. Some machines incorporate a camera and monitor in order for the operator to be able to dress more accurately and produce shapes. Other available models are CNC controlled whereby an entire wheel pack can be programmed and automatically dressed without operator intervention.

The smaller the tool diameter is, the more care has to be taken in getting the wheels dressed without any run-out.

For larger tools, it is advisable to add a small radius to any sharp corner. Tool design software are capable of incorporating such radii into their calculations and provide accurate setup information. A sharp corner has little edge retention which requires the operator to “baby-sit” and make adjustments on the first few tools of the batch until the edge radii are broken in. It also helps to have a better surface finish from the beginning.

Wheel Balancing:

Another area to improve efficiency and cut down on setup time is during the wheel balancing process. Without proper wheel balancing the following errors can occur:

Reduced finish.

Pre-mature wheel wear.

Loss of corner holding on wheels.

Unnecessarily longer cycle times.

More frequent need for wheel sticking (manual or automatic application of the dressing stick into the wheel bond to regenerate the grinding wheel.

Reduced length of unattended operation.

It is also important to figure out where the origin of the imbalance is. Some possibilities include:

Undressed or improperly dressed wheels.

Excessive diameter on wheel holes.

Exceptionally heavy wheel bodies.

Stacking up of wheels on long arbors with poorly made spacers.

Using adaptors while trueing the wheels on a dressing machine.

To keep in line with lean principles, the same team that dresses wheels should balance the wheels and make them ready for the next step of measuring.

Wheel Presetting:

Presetting your wheels is a useful practice whether a single machine or many machines are in operation. The presetting can be performed on a stand-alone presetter and the same team that dresses the wheels can do the presetting right after wheel balancing. For the sake of accuracy, it is advisable to avoid touching off of wheels on the CNC tool grinding machine.

A presetter typically comes with a camera and 2 linear glass scales. The camera provides a live feed into the programming software where the entire profile of the wheel shape including corned radii can be accurately acquired. Once the tool design is completed, it will be sent to the machine including the wheel pack information. The purpose is to decrease any errors in wheel measurement which would otherwise cause issues down the line. Setup time will be reduced, and the first good tool will be attained much quicker with an accurately measured wheel pack.

Wheel file management:

Another way to manage your grinding processes is by keeping accurate track of what wheels are used to grind which parts. This sounds basic, but you’d be surprised by how many companies ignore this crucial step. It’s a good idea to work with either your machine tool builder or someone within your company to develop a system for filing wheel types and shapes and cross-reference them with your product listing.

The best way may be to develop a fillable form that shows the most commonly used wheel shapes and is specifically designed for use with your machines, whether they are multi-axis grinding machines or cylindrical grinders. Each form can be saved separately and can be identified with the corresponding wheel pack.

A library of these files can be made available to all operators to share in order to use standardized wheel packs. This way the files can be viewed or accessed by the tool designer who issues the customers’ quotations. The operator on the machine will also have direct access from the CNC machine.

Tool Measurement and statistical process control:

The most important and most critical parameters of your final tool are geometrical integrity in terms of size and concentricity. In particular, high-performance endmills with uneven flutes and helixes need this type of inspection. Obtaining a uniform standard of dimensional measurements on rotary cutting tools, regardless of the number of flutes, can be done via laser directly on the machine or on stand-alone vision measurement systems. Collecting and organizing these data logs allow traceability and recording of measurements in a streamlined method. It provides your production team with the visibility to accurately assess efficiency and quality.

Machine calibration:

The last piece of the puzzle in reducing setup time is an accurately calibrated machine. Once the steps outlined above on wheel trueing, balancing and measuring are followed, and setup stills takes excessively long, the culprit is in the machine calibration. Even the best quality grinding machine needs periodic and careful calibration. These tool grinders generally run on 5- or 6-axes and often two or more axes are built on top of each other. One axis out of calibration can cause geometrical errors that affects other axes. Sometimes, operators believe that they can cheat their way through a setup knowing where and how to make adjustments. Lean manufacturing teaches repeatable processes and standardized operations. The aim is to take the “black art” out of tool grinding.

If you start to incorporate these methods into your processes, you may be surprised at how quickly they make a difference.

Cylindrical Grinding Operations - Back to Basics (by Eric Schwarzenbach)

The following piece covers the fundamental principles of cylindrical grinding. The oldest cylindrical grinding process is centerless grinding that was invented in the United States in the 1920’s to support the grinding of roller bearings for the bicycle industry.

Centerless Grinding:

Centerless grinding is an operation in which the workpiece rests on a knife-edge support, rotates through contact with a regulating or feed wheel and is ground by a grinding wheel. This method allows grinding long, thin parts without steady rests, and also lessens taper problems created by deflection.

Centerless grinding is similar to centered grinding except that there is no work holding spindle. This allows high through-put since the parts can be quickly inserted and removed from the process. There are three main types of centerless grinding: through-feed, in-feed and end-feed grinding.

For through-feed grinding, the grinding wheel is canted with respect to the other two axes so that a horizontal feed of the workpiece is achieved. This auto feeding characteristic is useful for rapidly processing many parts in quick sequence. Through-feed grinding is performed by traversing a part from one side of the machine to the other. Only straight parts can be ground. The wheel cannot be dressed to grind more complex shapes.

In-feed grinding differs from through-feed grinding in that the part is not fed axially so that the ground surface does not need to be a right circular cylinder. The grinding wheel can be dressed to accommodate the part. Once the workpiece part is in place, the grinding wheel is fed in radially.

In end-feed grinding, the part moves in axially between the grinding wheels, stops for grinding, and then backs out again. The wheel can be dressed to form more complex shapes, but the part can only get progressively smaller in diameter.

Peel and Pinch Grinding (Form Grinding):

Peel grinding is a process that removes the entire stock in one single grinding pass. The precondition is to arrive a grinding method that generates the smallest possible amount of heat on the workpiece surface. This process can be compared to longitudinal form grinding with a narrow wheel following a path, and is similar to the turning process.

Grinding wheels with single-point contact usually need to be low-wearing superabrasive wheels running at higher-than-normal RPM’s. Single-point contact or pin-contact reduces the contact zone between the abrasive wheel and the workpiece resulting in lower heat generation. This grinding method offers a high degree of flexibility, together with a high stock removal capacity. Different part geometries only require a program change.

A further development in the area of peel grinding is the method of “pinch” grinding. Pinch grinding involves forcing a rotating blank past two opposing grinding wheels, whereby the workpiece is “pinched” (all on the same center line) between a roughing and a finishing wheel with vastly different grit sizes. The finishing wheel trails behind the roughing wheel both axial and radially by a very small distance, just enough for the finishing wheel to remove the grooves created by the rough grinding.

In addition to enhance the advantages of pinch grinding, concentric workholding is of the utmost importance. To achieve highly concentric parts, a collet holding system is no longer practical. A V-block is the only system that can guarantee a run-out after grinding below .0001”. The blank spins against the two angular surfaces of a V-block and a guide roller then rests on top of the blank applying pressure to the rotating blank and holding it firmly against the V-block surfaces.

Non-round grinding utilizing the pinch/peel approach is available and used for precision form punches of all shapes, pill (tablet) punches and other unround components (patented by Rollomatic SA).

Cylindrical, Plunge and Internal Grinding:

Cylindrical and plunge grinding involves fixturing the part on a spindle axis as it is being ground. This can be done with or without a tailstock.

External cylindrical grinding is a process where the grinding wheel traverses along the axis of the workpiece in axial and radial direction. The contact of the wheel with the workpiece is usually broad and material removal occurs by rolling or sliding.

Internal cylindrical grinding is done by inserting an abrasive wheel which is smaller than the internal hole to be ground into the center of the component. In most instances, the RPM of the grinding spindle needs to be at high frequency in order to achieve sufficient surface footage on a relatively small wheel.

Plunge grinding is a method where the grinding wheel is pre-formed to the required component form and plunged in a radial motion into the workpiece.