Nonsmooth move: Drilling Performance

Shops have many choices when texturing—rather than smoothing—part surfaces.

Mirror finishes have long been the gold standard in parts making. But for some applications, smoother does not mean better. Many parts require a specific texture instead.

Textures are applied to part surfaces for several reasons. Some of the most common include:

• Improved aesthetics,
• Reduced friction and wear,
• Increased friction or to provide a specific coefficient of friction,
• Improved oil retention,
• Extended part life,
• Improved adhesion of paint and other finishes,
• Enhanced biological performance,
• Faster bone growth (for orthopedic implants), and
• Development of specific optical and thermal properties.

One notable texturing process is honing automobile engine cylinders. The process produces fine, cross-hatched grooves in cylinders that retain oil as pistons and rings slide by.

Once limited to low-cost, conventional stone honing, engine manufacturers have moved to brush honing and more recently laser honing for some cylinder-head applications. The inner wall of the cylinder head is textured at the upper end, or dead center area, of the piston motion. Textured surfaces prevent the oil film from breaking down during the piston’s short standstill before it moves in the opposite direction. This reduces emissions and fuel and oil consumption.

More Applications

Other examples of textured surfaces are numerous. In machine tool manufacturing, way scraping performs the same function as texturing of engine cylinders—to help retain lubricant and reduce friction and wear.

In hydraulic systems, O-ring surfaces must have circular (textured) tool lines around orifices and mating surfaces. Linear lines allow leakage from the port, whereas circular ones help trap liquid.

On lathes, knurled cylindrical surfaces provide gripping surfaces for manual tightening. In this instance, knurling both produces grooves and raises material above the original surface level to create the texture.

Some indexable inserts have small bumps on the cutting surfaces to decrease chip contact with the insert and reduce rake-face friction. The bumps prevent total surface contact with the chip, allowing the chip to carry most of the heat from machining. This changes the chip compression factor, increasing tool life. In this application, the bumps are pressed in during molding.

Deep-drawn sheet materials, which are produced by being drawn over a male die, often benefit from having minute pockets, called microcraters, on the sheet surface. Produced during the sheet rolling process, microcraters capture the lubricant required for smooth drawing, minimize galling and trap debris from the drawing process. They also provide better surface appearance when sheets are painted.

In orthopedic implants, the texture of surfaces that will come in contact with bone is critical. Most of these surfaces have either roughness for adhesive attachment or porosity to promote bone ingrowth for biological attachment. Texture consistency is crucial, as is the ability to assure freedom from contaminants.

Titanium dental implants that have been textured via blasting with Al2O3, acid etching and concurrent thermal treatment have been found to produce better bone growth than a plain machined surface into the minute crevices of the implant and higher locking strength of the implant in the bone. Healing time can be reduced from 12 to 24 weeks to 6 to 8 weeks when one specific combination of texturing steps is used, according to several dental studies. Tooth implants are often inserted and allowed to heal for several months, but the tooth itself is not added until bone has grown around the implant.

Multiple Methods

Many texturing processes are well known and widely used. Examples include knurling, sanding, brushing, blasting, honing, dimpling and etching. Coining and embossing also create textures. Hammering and rolling are two older processes that mechanically alter a surface. Newer texturing processes involve lasers, electron beams, ion beams and inkjets.

One of the reasons parts manufacturers use texturing is most adhesives work best when applied to etched surfaces, which provide much more area than smooth, or flat, surfaces. The “peaks and valleys” can provide up to 10 times more area than a smooth surface.

Textured metal surfaces encapsulated in epoxy materials also benefit from the extra adhesion that textures provide, which is especially beneficial in epoxy-molded copper parts. Bead blasting is often used to texture these parts. Laser texturing by ablation also provides high-quality adhesion for copper alloy lead frames in mold materials, where the failure mechanism is delamination of the mold compound from the die flag, or leads.

Let’s examine some of the key texturing methods:

Abrasive blasting leaves visible pockets in a surface for holding lubrication. The pockets also reduce contact surface area and provide subsurface residual compressive stresses, which extends service life. Abrasive blasting is probably the most common texturing process, and provides a rapid, low-cost means to improve adhesion of glues, paints and other coatings.

According to Abrasive Blast Systems Inc., Abilene, Kan., blasting is widely used to produce textured surfaces for calendar rolls in paper manufacturing. The metal rolls, which can be up to 20 ‘ long and 5 ‘ in diameter, are blasted to assure tight bonding of subsequent rubber coating. Blasting also removes worn rubber coatings from the rolls during reconditioning.

Laser texturing provides a rapid means of producing minute grooves, dimples and other features that trap lubrication for parts such as ring/liner assemblies for engines, mechanical seals and bearings. Use of laser dimpling appears to be growing. Specific diameters and depths of laser dimples provide more than just lubricant reservoirs—they assure proper hydrodynamic film thickness and flow. For some applications, these pockets assure retention of oil films at low sliding speeds, 0.45 m/sec., for example, as one part slides over another on top of oil film. For other applications, they allow faster sliding without galling.

Dimples, including laser dimples, must be deeper than the desired film thickness. For example, the dimple depth must exceed 10µm to assure a 5µm-thick film. For some steel part applications, dimples are 100µm in diameter, 4µm to 5µm deep and spaced at 200µm intervals (Figure 2). A rule of thumb is a cavity surface area of 10 to 40 percent of the planar surface area provides dimple density to assure two to five times better performance than nondimpled surfaces with oil lubricants.

In some instances, improved oil wetting of the surface produces higher sliding speeds because maintaining oil in minute cavities provides hydrodynamic pressure pockets, which reduce friction.

Courtesy of A. Erdemir. Argonne National Laboratory

Figure 1. Frictional performance of ground, highly polished and laser-dimpled steel surfaces in 15W30 oil at room temperature at various sliding velocities. *The coefficient of friction (μ) between two solid surfaces is defined as the ratio of the tangential force (F) required to produce sliding divided by the normal force (N) between the surfaces (μ = F /N).

Figure 1 illustrates the friction coefficients of ground, polished and laser-dimpled steel samples in sliding applications. Figure 2 shows a plane view and a cross-sectional plot of laser-dimpled steel.

A relatively new laser dimpling application uses femtosecond lasers to produce nanoscale and microscale patterns on hard coatings, such as titanium nitride, titanium carbonitride and diamond-like carbon films. Coating life has been increased by factors up to 10, according to G. Dumitru at the Swiss Institute of Applied Physics and other French, Swiss and Russian researchers.

In one study by Swiss and Russian researchers (T.V. Kononenko of Moscow’s General Physics Institute, et al, in “Laser ablation and micropatterning of thin TiN coatings,” Applied Physics A., Vol. 71, 2000, pp. 627-631), laser-dimpled microcraters 3µm to 5µm wide and less than 1µm deep in TiN-coated 440C stainless steel produced 30 percent longer wear life than untextured surfaces. In addition, friction coefficients of textured parts were uniform during the study, compared to many large variations for the untextured surfaces.

Courtesy of Argonne National Laboratory

Figure 2: Plane view and cross-sectional plot of laser-dimpled steel. Chart shows depths of dimples and longitudinal direction (not to scale with depth).

In one application, automotive engine cylinder-head laser dimpling produced 40µm-wide and 8µm-deep pockets spaced 600µm apart to capture oil for lubricating the piston at top dead center. The regular surface surrounding the dimples typically measures 1µm Ra.

TiN is applied to cutting tools to improve hardness and wear resistance. It also lowers friction and provides high thermal resistance and good chemical stability, extending tool life. Drilling studies by Davi Neves and his Brazilian associates, Anselmo Diniz and Milton Fernandes de Lima, show that laser dimpling prior to TiN coating provides an anchoring base that further extends tool life. Laser dimpling 10mm, M2 HSS (62 HRC) drills coated with 2µm-thick TiN had tool life up to 10 times longer than untextured tools when drilling 304 stainless steel. Dimpling, which is present after coating, also reduced cutting forces. The TiN is only 2µm thick and deposits as a uniform layer.