When Surface Lay Matters
For some machined parts satisfactory performance depends on correct orientation of the “lay.” Get the lay wrong and consequences can include premature failure and warranty costs. Here’s an introduction to surface lay and why it matters.
Machining Direction and Lay
“Lay” is the term given to surface finish directionality. It describes the direction of the dominant pattern or texture.
For an example, consider a shaft turned on a lathe. The cutting tool leaves a shallow groove in the surface as it removes metal. This groove spirals along the shaft like a screw thread only shallower and finer-pitched.
If the face of the shaft is machined the same thing happens: the tool cuts a groove, this time almost like that on an old LP record.
So what’s the lay on these surfaces? On the shaft diameter it runs around the circumference perpendicular to the axis. Run a fingernail down the shaft in the axial direction and you’ll feel the grooves. Run the same fingernail around the shaft and it will feel smooth.
This directionality is apparent on the shaft end face too. Here the surface is smooth in the circumferential direction yet rough in the radial orientation.
Lay and Surface Roughness
Surface roughness is defined as the shorter frequency of real surfaces relative to the troughs. On a mirror this deviation is almost nothing; on sandpaper it’s visible to the naked eye.
This isn’t the place to discuss surface roughness measures: let’s just say there are many. The relevance to lay is that clear or visible lay means the surface roughness is different when measured in the longitudinal or transverse directions.
This can be quantified by the texture aspect ratio. Essentially, this is the ratio of the roughness across and along the lay.
Surface Roughness Measurement
Roughness is usually measured with a stylus. Drawing this across a short length of surface produces a chart of vertical displacement against horizontal distance.
On our machined shaft example above, drawing the stylus down the shaft in the axial direction will reveal the grooves left by the cutting tool. Turn the stylus through 90° and pull it along the groove and the vertical displacement is much less.
Impact on Machined Part Function
On a single part surface roughness and lay determine appearance and not much else. Light shining on the surface in the direction of the lay will reflect straight off, making it look shiny. Light from the transverse direction will be scattered by the machining grooves, giving the part a dull appearance.
Where lay matters is when two surfaces come into contact. When this happens the direction of lay on the two surfaces determines the friction between them.
Using the shaft example again, if two screws are brought together so the threads mesh it takes considerable effort to slide one axially over the other. This is because the interlocking peaks and valleys resist movement. But in the direction of the threads they’ll slide easily.
Lay also influences friction through its effect on lubrication. This is down to how the surface retains an oil film. Any rough or textured surface can hold oil in the valleys, but when combined with a lay it enables almost frictionless movement in the lay direction.
Engineered Lay Applications
Lay is relevant whenever a design requires that one machined surface slide over another, but two examples stand out in particular: sliding element bearings and cylinder bores.
In bearings lay determines friction through both surface roughness directionality and lubricant retention. Likewise, in a cylinder the lay holds oil and helps ensure smooth movement of the piston or piston rings. A particular feature of cylinder bores though is that they are often honed.
Honing is a specialized machining process that puts a specific texture on the surface of the bore. In cross-section this usually looks like shallow valleys separated by large plateaus. Here the goal is to minimize oil consumption, blow-by and of course friction.
Consider Lay in Precision Machining Projects
At Impro we’re leaders in precision machining. We make components that go into hydraulic products like pumps and valves. Lay is always a key consideration when machining parts like these. Contact Us to learn more about our precision machining expertise.