Advancing the Art of Professional Airless Spraying
Geometry Measurement There are a number of challenging aspects of airless spray tip design.The foremost is geometry measurement.Without the ability to measure the geometry of finished components it is not possible to effectively advance the art of airless spray orifice design. A significant portion of our work has been focused on developing the ability to measure critical geometry of finished spray orifices. The following discussion will focus on the orifice geometry that are the most critical and difficult to measure.They will relate to the internal dome and external slit.The passageways leading to the internal dome can affect spray performance.However, they are easily measured and will not be discussed. The critical geometry that we will focus on are listed below.Descriptions are given for each. Some of the geometry in the figures below are exaggerated so that the values represented can be indentified.Some of these values are typically so small, that without exaggeration, they would not be discernable.
Internal Geometry Dome Diameter- This term is used loosely since the dome usually is not a true sphere.However, the surface of the internal dome is assumed to be defined by a radius. Eccentricity- This represents the location of the center of curvature for the Dome Diameter relative to the axis of the internal geometry.Consider how the punches used to form the internal geometry of the orifice blank are fabricated.Punches are turned on a CNC grinder which forms all the diameters along its length and also sweeps an arc on the tip of the punch, which forms the dome.If the center of curvature lies on the axis of the punch, the geometry formed will be a perfect sphere.However, either due to design intent or manufacturing variation, very rarely does this center of curvature lie on the punch axis.If the center of curvature is not on or past center, the internal dome will be flattened at the end.If the center of curvature is past center, the internal dome will have a cathedral shape.
Z0 Height- Is the distance along the center axis of the blank from the bottom of the blank, Datum A, to the center of curvature.It is a point in space that cannot be physically measured.It is a derived value.
Throat Diameter- This is the diameter of the internal passageway leading to and joining with the internal dome.This value is easily measured; however, it is included here since it plays a key role in defining the next measurement, Delta Throat Height.
Delta Throat Height- This represents the location of the end of the throat relative to Z0 Height.An assumption could be made that the surface of the Dome is tangent to the Throat.This is not always the case.Therefore, it is possible that the end of the Throat is higher than or equal to the Z0 Height.It can never be lower than the Z0 Height.
Internal Spherical Geometry
Internal Cathedral Geometry
Internal Flat Top Geometry
External Orifice Geometry
External Geometry V Angle- Again, this value is relatively easy to measure; however, it is included here since it plays a very critical role in forming the orifice geometry.
Slit Radius- The value determined for this is really an “equivalent” value since the actual form is not quite a true radius.However, for practical terms in how it affects spray performance, we can assume it is a true radius.This value changes as the grinding wheel used to form the “V” slit wears.It gets larger over time.As it gets larger, the net effect is the Minor Axis length increases with a given Major Axis.This value needs to be watched during the production process and once it reaches a set limit, the wheel needs to be redressed.
Distance to Center- This value represents the distance from the bottom of the “V” Slit to the Z0 Height.It should never be negative, i.e. past center of curvature.Also, the “V” Slit should not cut into the Throat.As mentioned earlier the Throat may extend past the Dome center of curvature if the surface of the Dome is not tangent with the Throat.
Delta Y- This is the measurement of how far off center the “V” Slit is with the center axis of the internal geometry.This value has to be controlled below a given limit.This limit is relative to the size of the internal dome.Larger domes can allow for more variation without detriment to spray performance.If Delta Y is too large, the pattern will have a crescent moon shape, which tends to be undesirable.
Example of Measured Orifice Geometry EnduraSpray Technologies, LLC has developed a proprietary measurement system which can determine all the above mentioned parameters with considerable accuracy. We have used this system to analyze not only our finished product, but also finished product of other leading manufactures. This has given us the ability to thoroughly evaluate and map the current art of airless spray orifice design.
The following are measurements made on an airless spray orifice currently available on the market.
Sample: GX-1239.1 Internal Geometry Dome Diameter: 0.0821" Eccentricity: -0.0018" Z0 Height: 0.0820" Delta Throat Height: 0.0000" Throat Diameter: (Throat is tangent to dome.)
Internal Geometry: V Angle: 26.76° Slit Radius: .0029" Distance to Center: .0100" Delta Y: .0009"
Other Metrics: Major Axis: .07581" Minor Axis: .01874"
