Factors that affect tool wear
September 26, 2002
Most metal forming operations use lubricants to protect the tooling and part from excessive wear caused by scuffing, scratching, scoring, welding, and galling. Four lubricant families are commonly used in pressworking, and thousands of formulations are available within each chemical family. The physical characteristics of the lubricant and metal forming operation involved determine the application method to be used.
Many variables affect tool wear in stamping operations. Just a few of them are:
Most metal forming operations use lubricants to protect the tooling and part from excessive wear caused by scuffing, scratching, scoring, welding, and galling.
Lubricants work by forming lubricating films between two sliding surfaces in contact with each other. When these metal surfaces are viewed under magnification, peaks and valleys become apparent, even on finely ground surfaces.
The lubricating film needs to prevent the asperities (peaks) on the two surfaces in sliding contact with each other from damaging the mating surface. In metal stamping dies this is typically accomplished with mixed film lubrication. Some areas of the die are in hydrodynamic or full-film lubrication; other areas operate under boundary film conditions; and the most difficult high-pressure stations sometimes function with chemical extreme-pressure (EP) films.
Under hydrodynamic or full-film lubrication, two surfaces are completely separated by a fluid film, with no contact between the asperities. This condition could change as speeds vary during start and stop modes or if the pressure and temperature increase beyond the lubricant's film strength.
At this point the asperities may come into contact, so a boundary film lubricant is required. Certain additives are blended into metal forming lubricants to provide boundary film lubrication. The additives adsorb onto the metal surface and prevent the asperities from damaging the mating surfaces.
Hundreds of different additives in a variety of chemical strengths, forms, and quality grades are used to provide boundary film lubrication. These additives include esters, soaps, fatty acids, polymers, animal fats, vegetable oils, and fish oils.
Boundary lubricants work up to a certain temperature and pressure, and then the boundary additive breaks down and metal contacts metal. The working temperature varies with the type and amount of additive used and its interaction with other additives.
At this point EP lubrication is required. The three EP additives used in metal forming lubricants are chlorine, sulfur, and phosphorus compounds. These compounds form a chemical film that reduces the coefficient of friction and prevents damage. This film is effective within a particular temperature range, depending on which of the three EP compounds is used.
Many additives are available using these three elements reacted onto a wide choice of substrates. A lubricant formulator can choose from many boundary and EP additives to develop lubricants for protecting sliding surfaces over a range of temperatures and pressures.
Four lubricant chemical families commonly are used in stamping, and thousands of formulations are available within each chemical family. Generally, the lubricant families can be rated from the heaviest duty (as far as protecting tooling) to the lightest duty as follows:
The order of this list becomes inverted when these chemical families are rated for ease of use in manufacturing. Vanishing oils then become the best choice because they evaporate and leave no residue, followed by chemical solutions, because they are easy to clean and use in secondary operations. Next come macroemulsions and compounded oils.
Tool wear problems in standard metal forming operations of common metals can be addressed by using a heavier-duty product within the same chemical family or switching to a heavier-duty family, such as from a macroemulsion to a compounded oil.
Reducing tool wear and extending die life in more difficult forming and drawing applications, at higher speeds, at high production volumes, and with tough materials usually require a combined engineering effort between the lubricant manufacturer and the user.
Typically, the formulating chemist and technicians must evaluate more than one combination of additives and treat levels to develop data points. This information then is used to recommend the best lubricant from performance and cost standpoints.
Even the finest lubricant will not prevent tool wear if it does not get to the tool when needed. The physical characteristics of the lubricant and metal forming operation determine the application method to be used.
Five commonly used application systems are:
Following are examples of these application methods.
Tool wear can be reduced in tough transfer press operations that use water-soluble flood systems by applying with a roller coater or by drip method a high-viscosity, EP-type compounded oil to the stock going into the die. The compounded oil reduces tool wear on the draw rings in the cupping stations.
An easy and effective way to increase tool life on punching and perforating equipment is to lubricate the bottom of the strip. Punch wear occurs on the return stroke, after the punch breaks through and the metal springs back onto the punch. Using a roller coater or spray system to apply lubricant on the bottom of the strip allows the punch to relubricate itself for the return trip.
Airless spray systems are suitable for applying lubricants onto progressive dies or transfer tools. They can be actuated to apply lubricant to the top or bottom of the stock or onto only the station or tool needing it. This application method helps to increase the life of extruding punches, sizing tooling, shaving punches, and in-die taps.
The part shown in Figure 1(along with the worn-out extruding punch) was run with roller-applied heavy compounded oil. The oil was being squeezed off the punch, and the punch galled up and eventually welded and broke off in the extruded hole.
A low-viscosity compounded oil with boundary film additives that provided good penetration and anti-wipe and an EP additive were applied by airless spray to this station. The new lubricant penetrated with the extrusion punch and provided adequate lubrication for the return stroke.
Modern micrometering mist systems are suitable for applying lubricants to high-speed stamping dies running narrow material. A controlled, fine coating of lubricant can be applied to the top and bottom of the strip. Loss of lubricant even for a few minutes on a high-speed stamping die can cause thousands of dollars in tooling damage. Sensors can be built into the lube system to shut down the press automatically in case of pressure loss or low fluid level.
The cleanness of lubricants used in recirculating flood systems is important. Metal stamping dies generate metal particles and metal oxides. If left to build up in the fluid, these abrasive particles and oxides recirculate back into the tooling, and abrasive tool wear occurs. Most recirculated lubricants should be filtered to 15 to 25 microns.
Controls need to be established for water-extendible lubricants to obtain consistent tool life and performance. In recirculating systems, the concentration of water-based lubricants changes, and the emulsion stability can be weakened by a number of factors, including microbes and hard-water salts. If left unchecked, these changes can affect tool wear and the overall performance of the fluid.
Many products can be controlled with two easy tests: checking pH with pH paper or a hand-held pH meter and checking the concentration with a refractometer. Controls for recirculating fluids, premix tanks, and reclaimed lubricants can be established with a control sheet.
The material entering the tooling must be checked for dirt, scale, rust, grit from polishing operations, smut from annealing, and other types of contamination that can contribute to tool wear.
When stock comes in with heavy mill oil, the lubricant must be able to work on that oil surface. This can be a problem for oil-free chemical solutions, because these products need to reach the metal surface to provide lubrication but tend to float off an oily surface. A roller coater helps work the chemical solution through the oil coating.
Some materials generate particles or oxides while being worked in dies. Lubricants with good flushing and penetration qualities can help keep these abrasive particles and oxides from building up on tools. Spray application systems also are helpful in keeping the tool clean.
A rule of thumb is to run metal stamping dies as coolly as possible. This is true for all steels, aluminum, and yellow metals.
Exceptions to this rule are certain grades of stainless steel that work-harden quickly, such as 302 and 304.
When these alloys are run with water-extendible lubricants, the part cools after each tool progression, which results in work hardening of the part and accelerated tool wear. These alloys should be run with EP-type compounded oils that can operate at very high temperatures. Getting the oil and die up to temperature and keeping the part hot help reduce work hardening.
Misalignment or deflection in the press or tooling can cause tool wear that sometimes can't be prevented with lubricants. The press ram needs to be parallel from front to back and side to side, and the die needs to be set in the press so that the force for making the part is centered under the ram.
The die must be rigid from the bolster plate through the die set and die components. This is especially important for working high-strength material.
For a die run on rails, the rails should be located properly to prevent deflection and the die marked so the rails are located the same each time. It is not uncommon for very large dies to have worn leader pins, which can lead to misalignment of the top and bottom dies.
Galling or tool wear on only one side of the die sometimes can be corrected by floating the tool in. After loosening the top die, the operator uses the material being formed to center the die and then locks the die in place.
The tooling material always should be dissimilar from the metal being worked to aid in lubrication and extend tool life. For example, chrome-plated tooling never should be used with stainless steel.
Tool coatings are very helpful in extending tool life. In many cases, a lighter-duty lubricant can be used with coated tooling but not with uncoated tools.
Tool clearance must be adjusted for the material being worked. Proper tool clearance can enhance tool life without any changes to lubricants, tool steels, or other variables.