The Fundamentals of Honing Filtration

Clean fluids lead to longer tool and equipment life. Learning about filter media and methods will help to bolster your bottom line.


In today’s manufacturing environment it is essential that every process be as efficient and cost effective as possible. Operating costs must be minimized in order to compete. Parts must be produced at a faster rate, with more-consistent quality, and productivity must be maximized. The honing process is no exception to the rule.

The goal, therefore, is to minimize the operating costs associated with the honing process. Certainly there are many ways to do this. We can select a better honing stone and tooling, and we can select a better metalworking fluid. All of these are viable options, but many times these variables are changed to treat the symptom of the problem and not the problem itself.

For example, if a manufacturing engineer is faced with the problem of short honing stone life the usual result is to select a harder stone material (at a higher cost per tool). The tool life goes up, but the real problem may not have been addressed, because dirty fluid may have been the reason for the reduced stone life. A better, more cost effective solution in the long run may have been to optimize the filter media selection and/or replace the filter system with a more efficient one. With clean fluid the less-expensive tooling could be used, thereby reducing operating costs. Coupled with the other benefits of a better filter, such as reduced disposal costs, reduced makeup fluid costs, and reduced labor to clean sumps, the overall operating costs may have been substantially reduced by addressing the problem–a poor filter system–instead of treating the symptom. Figure 1

The Honing System
The honing system consists of the following items, at minimum:
• Hone
• Honing stone/tooling
• Metalworking Fluid
• Product
• Filter System

It is our job to minimize the operating costs associated with this system, which include–but are not limited to–the following:


• Maintenance
• Utilities

Honing Stone

• Acquisition Cost
• Life
• Effectiveness

Metalworking Fluid

• Acquisition Cost to fill the system
• Makeup Costs to maintain the fluid level in the system
• Disposal Costs


• Reject
• Rework

Filter Sy

• Maintenance
• Utilities
• Filter Media Costs
• Dump and Recharge Costs
• Fluid Disposal Costs
• Solids Disposal Costs
• Media Costs

How do we minimize the operating costs through better filtration? While there is no perfect filter or filter media, there is an optimal combination for every application. With a proper understanding of the filtering process, available filters, and available filter media, it is possible to select the best system for any application. The following information is designed to assist in making the proper choices.

Principles of Filtration
All filters utilize a force to drive fluid through a barrier (filter media) where the solids are collected. Higher differential pressure across the media translates to more and finer particles being removed; a better, more-efficient filter. In some cases the barrier merely prevents the solids from entering the clean side of the system (screening). In this case the solids are not removed from the system, but are prevented from entering the clean side. In other cases the solids are trapped and removed from the system (filtering). The filtration process can take three forms: 1) surface filtration; 2) depth filtration, and; 3) cake filtration. All three forms of the filtering process are utilized to remove honing solids from metalworking fluid. Note: For this discussion the filter barrier is usually non-woven disposable media, or filter paper.

The grinding process produces a good mixture of large and small solids. The honing process generally does not. In this case we have two choices: select a larger filter, or add the larger particles in the form of a filter aid. The filter aid establishes the cake and traps the smaller particulate. It also extends the filtering cycle. Many different types and grades of filter aids are available, and some of these are described later in this article.

In cake filtration, the filtration media need only be tight enough to catch the solids to establish the cake. The filter media for this application can be a cleanable belt.

Common Liquid/Solid Separators
There are many devices that can be utilized to remove solids from liquids. Some of these are as follows:

• Centrifuge
• Separators: Hydrocyclone, Magnetic
• Dragout
• Gravity
• Vacuum: Hydro-Vac, Air-Vac
• Pressure: Manual Clean, Bag, Cartridge, Automatic Clean (Tubular Backwashing, Flat Bed)
• Combinations: Tubular Backwashing with Flat Bed Pressure Filter, Tubular Backwashing with Dragout

All of these are used in metalworking applications, but many are not efficient enough for honing. This article will detail only the most commonly used devices for honing.

Gravity Filter
(Surface Filtration Mode)

The gravity filter is a simple filter where the force driving the fluid through the filter media is gravity. As the media plugs and the fluid level rises, a float switch is tripped and the media is indexed. The gravity filter usually must be combined with a magnetic separator, and in some instances a bag or cartridge filter, to increase efficiency when used for honing. The magnetic separator will remove ferrous particles. The bag or cartridge filter is usually placed in the clean supply line to protect the hone from any particulate that has passed through the separator and filter media. Figure 2

Hydro-Vac Vacuum Filter
(Surface, Depth, or Cake Filtration Modes)

The hydro-vac is essentially a drag tank that utilizes filter media. The media lay below the fluid level in the dirty tank. It is held in place by the weight of the chains and flights. The filter pump draws a suction under the media to pull the fluid through it. The solids are contained on the top. When the media becomes plugged with solids the pump suction is redirected to a clean reservoir and the vacuum on the filter media is released. The conveyor motor is energized and the media is indexed 12″-24″. The spent media may be re-rolled for easier disposal. After the index cycle the pump suction is returned to the vacuum box and fluid is once again drawn through the filter media. The clean reservoir is re-filled for the next index cycle.
Figure 3

Some vacuum filters can utilize cleanable belts, as well as disposable non-woven filter media. In the honing application, the cleanable belt must usually be utilized with filter aid. The filter aid protects the belt from plugging.

Tubular Backwashing Pressure Filter
(Cake Filtration Mode)

Tubular filters require the use of filter aid (diatomaceous earth, Perlite, cellulose, etc.) to function properly. Tubes are pre-coated with the filter aid material by pumping a slurry of the filter aid and process liquid into the vessel. This provides a filtration barrier down to sub-micron levels. Dirty liquid, combined with filter-aid slurry (body feed), is pumped into the filter during the filtration cycle to further enhance the filter’s efficiency and filtration cycle length. When the filter reaches its terminating pressure (usually 20-30 psi) the filtration cycle is ended and the tubes are automatically backwashed with clean liquid. The dirty backwash slurry is drained into a collection tank for dragout or de-watering/de-oiling with a flatbed pressure filter. Figure 4

Virtually any number of tubes can be provided within single or multiple housings. Common sizes include 248 tube (186ft2 of filtering area) and 376 tube (282 ft2 of filtering area) housings.

Flat Bed Pressure Filter
(Surface, Depth, or Cake Filtration Modes)

The filter can be described as a single, horizontal chamber pressure filter that is fully automatic and operates at up to 50 psi differential pressure. The filter chambers are closed with the filter media (filter paper or cleanable belt) between them. Dirty liquid is pumped into the filter and solids begin to accumulate on the filter media, thereby increasing the filter pressure. When the pressure reaches the terminating set point the dirty liquid flow is stopped and compressed air is brought into the upper chamber to dry out the collected solids. When the solids are dry the chambers are separated and the filter media is advanced until a fresh section of media is in position between the chambers. The process is repeated. The dry solids are separated from the filter media for disposal. Figure 5

Types of Filter Media
There are many product families available to choose from. Each family has a number of different grades or basis weights. Each product therefore has a basic set of measured properties that indicate overall performance. These include material, weight (oz/yd2 in US), grab strength (in machine and cross directions), burst strength (psi in U.S.), and air permeability (CFM/ft2 in U.S.). Heavier basis weights within the same family will be stronger and tighter. However, this is not true when comparing different families of products. A 1 oz/yd2 rayon is not equivalent to a 1 oz/yd2 spunbond polypropylene.

In addition, “lofted” depth media are proprietary in material and construction. These are usually heavy products starting at 2.0 OSY, although some are as heavy as 9.0 OSY. Some examples are PowerLoft, CrystaLoft, HolliFlo, and MasterFlo. Figure 6

Proper media selection is dependent upon many factors, including but not limited to: type of filter, filter condition, fluid, material being honed, mode of filtration, and maximum particle size allowable. I recommend consulting with a filter media supplier to obtain the optimal selection. In general, select the media that delivers the best performance/operating cost ratio. A tighter, more-efficient media may be more costly per roll but may deliver lower operating costs, i.e. longer life, better part quality, less scrap, and less fluid usage.

Filter Aids
Filter aids are solid, intricately shaped, porous particles that are utilized as the large particles to remove smaller particles in the cake filtration process. There are numerous filter aids available, although most will fall into three categories; cellulose, perlite, or diatomaceous earth (DE). Each has properties that make them attractive. Figure 7

Filter aids extend filtering cycles and increase filter efficiency. The smaller solids are trapped by the larger, porous filter aid solids in the depth of the filter cake. This provides a flow path for the fluid and prevents the filter media from plugging prematurely. Types of filter aids include:

Cellulose: There are a number of different grades of cellulose fiber filter aids. They are generally classified by the average fiber length. The longer the fiber, the more easily it will bridge the filter medium. The shorter the fiber, the more efficient it will be at removing smaller particles. Notes: not as efficient as perlite or DE; ashless; can be incinerated; inert; organic.

Perlite: Consists of naturally occurring siliceous rock (volcanic glass). When perlite rock is heated quickly the water trapped in it is released and expands from four to 20 times its original size. Several grades are available, and they are generally classified by relative porosity. Notes: more efficient than cellulose; inert.

Diatomaceous Earth (DE): An industrial mineral composed of the skeletal remains of microscopic aquatic plants. These are called diatoms, hence the name “diatomaceous earth.” DE is available in numerous grades and are usually classified by median pore size. Notes: most efficient filter aid; composed mainly of silica; 10-200 micron in diameter; human carcinogen, though not dangerous with proper handling.

Proper filter aid selection is dependent upon many factors, including but not limited to: type of filter, filter condition, fluid, material being honed, mode of filtration, and maximum particle size allowable. Again, I recommend consulting with a filter aid supplier to obtain the optimal selection. In general, select a filter aid that has about the same size particles and an equal amount as those to be removed. As always, select the filter aid that provides the best performance/operating cost ratio. Figure 8

Benefits of Good Filtration

We’ve analyzed the process, selected the best filter media or filter aid or have selected a different type of filter, but what do we stand to gain? What are the benefits of good filtration? At the very minimum they are:

• Proper Honing Tool Action
• Expansion
• Contraction
• Extended Stone Life
• Extended Fluid Life
• Reduced Disposal Costs
• Fluids
• Solids
• Increased Productivity
• Better Quality Parts