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In general, adjust feeds and speeds for a lower chipload at the same rpms for a better finish. Take a finish pass that’s relatively light as well. Make sure as you’re doing this to avoid rubbing, which is very hard on tool life.

Engineering prints call out a great many things in their attempt to make sure the part that gets made matches the designer’s intent. Aside from dimensions and tolerances, another important callout is Surface Finish.

Optical lenses

Like all plano-convex lenses, these lenses have a positive focal length and near-best-form shape for infinite and finite conjugate applications. They can be employed to converge collimated beams or collimate light from a point source. To minimize the introduction of spherical aberration, a collimated light source should be incident on the curved surface of the lens when being focused and a point light source should be incident on the planar surface when being collimated.

Surface Roughness speaks to fine detail imperfections, but there may also be much coarser irregularities. For example, a surface may be warped or deflected from the ideal.

Another important consideration is friction. Reducing surface roughness typically reduces friction, which can be critical to reducing wear and increasing the efficiency of sliding parts.

There are a number of different characteristics we might measure with respect to Surface Finish. Two of the most important are the height of the roughness peaks and their separation, often referred to as “Ra” and “D”:

Cylindricallens

The plot shown below indicates the performance of the standard coatings in this family as a function of wavelength. Broadband coatings have a typical absorption of 0.25%, which is not shown in the reflectance plots.

There are a variety of considerations, and they may even vary for different stages in the manufacture of a particular part. For example, we may have a surface finish requirement on a casting that ensures the finish (think of it as the deviations of the surface of the casting from an ideal) is good enough that the allowance made for extra material that will be machined off in a future step is sufficient. If the casting is too imperfect, the trough of imperfection may be below the expected machined surface of that future step.

This is a very ROUGH estimate and not an exact conversion!Avg Max height of the profileRz ISO Roughness HeightRPM Average max profile peak heightRMSRMS = SQRT( (M1^2 + M2^2 + M3^2 + M4^2) / 4 )Root Mean Square

Although uncoated lenses are highlighted on this page, these N-BK7 plano-convex lenses are also available with one of five antireflective coatings (-A, -AB, -B, -C, or -D), which reduce the amount of light reflected from each surface of the lens over the specified wavelength range. Links to each of these pages can be found in the selection guide table below. Please see the Graphs tab for coating information.

Rz is often preferred to Ra in Europe, particularly Germany. Instead of measuring from the centerline like Ra, Rz measures the average of the 5 largest peak-to-valley differences within five sampling lengths. While Ra is relatively insensitive to a few extremes, Rz is quite sensitive since it is the extremes it is designed to measure.

Ra is by far the most commonly used Surface Finish parameter. One reason it is so common is that it is fairly easy to take the absolute value of a signal and integrate the signal using analogue electronics, so Ra could be measured by instruments that contain no digital circuits.

when used to calculate the principal plane locations of plano-convex lenses. fb is the back focal length of the unmounted lens, which is often referred to as the working distance. For mounted lenses, the working distance is measured from the back of the threaded housing.

These high-performance multilayer AR coatings have an average reflectance of less than 0.5% (per surface) across the specified wavelength ranges (except for the -AB and -D coatings, which provide <1.0% average reflectance) and provide good performance for angles of incidence (AOI) between 0° and 30° (0.5 NA). For optics intended to be used at large incident angles, consider using a custom coating optimized at a 45° angle of incidence; this coating is effective from 25° to 52°.

Our G-Wizard Calculator software has a handy “Tortoise-Hare” slider that makes it easy to dial in proper feeds and speeds for a finishing pass:

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The lenses sold on this page are also available with broadband antireflective coatings, the reflectance traces of which are shown in the graph below.

The Lay is the direction of the predominant surface pattern and is usually determined by the production method used to process the surface. Here are some typical Lay patterns:

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We are group of engineers together working to help the engineer and give more information about the Engineering industries.

How do product designers decide what surface finishes to require, and how should CNC machinists think about surface finishes?

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Ra, while common, is not sufficient to completely characterize the roughness of a surface. Depending on the application, surfaces with the same Ra can perform quite differently. Here are 4 surfaces with the same Ra and quite different shapes:

The goal of the designer is to specify surface finishes that are as coarse as possible but will still function within the part’s desired operating parameters. The goal of the machinist is to achieve surface finishes on parts that are as good as those required by the designer but not better as that results in the cheapest-to-manufacture parts. It’s important for the designer and manufacturer to agree on exactly which parameters (Ra, Rz, etc.) are to be used for inspecting and accepting parts. While there are commonly accepted conversions between these, actually using the same parameters for the inspection that the designer used with the design always yields the best results.

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Surface Finish is a measure of the overall texture of a surface that is characterized by the lay, surface roughness, and waviness of the surface. Surface Finish, when it is intended to include all three characteristics, is often called Surface Texture to avoid confusion, since machinists often refer to Surface Roughness as Surface Finish. Another term, analogous to Surface Texture, is Surface Topology.

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Convex lens

One approach to Surface Finish is to use abrasives or sand the surface. This chart converts from abrasive grit to Surface Finish Ra values:

Infraredlens

When using the thick lens equation to calculate the focal length of a plano-convex lens, R1=∞ and R2=-R. Note that the minus sign in front of R is due to the sign convention used when deriving the thick lens equations and values of R are reported in the specification tables below. Therefore, via substitution, the thick lens equation becomes:

– Profiling Techniques: Here, a high-resolution probe is used to measure the surface. Think of a sensitivity more in line with a phonograph needle than a typical CNC Probe.

If you’re 3D Profiling, each pass of the ballnosed cutter leaves a “scallop” in the surface of the material. Our G-Wizard Calculator software has a special Mini-Calc that will help you adjust for a scallop height that delivers the required surface finish:

These uncoated Plano-Convex Lenses are fabricated from RoHS-compliant N-BK7. N-BK7 is typically chosen whenever the additional benefits of UV fused silica (i.e., good transmission further into the UV and a lower coefficient of thermal expansion) are not necessary. These Ø1/2", Ø1", Ø1.5", and Ø2" lenses are mounted in SM05 (0.535"-40), SM1 (1.035"-40), SM1.5 (1.535"-40), and SM2 (2.035"-40) compatible mounts, respectively, for compatibility with our line of SM-threaded lens tubes. An SM-threaded retaining ring is used to secure each lens within the mount. Each mount is engraved with the item #, lens type, focal length, and an arrow pointing toward the curved surface of the lens.

There are many factors that affect surface finish, with the biggest being the manufacturing process (see table above). For machining processes, such as milling, turning, and grinding, factors such as cutting tool selection, machine tool condition, toolpath parameters, feeds, speeds, tool deflection, cut width (stepover), cut depth, coolant, and vibration are just a few of the many.

The focal length of a thick spherical lens can be calculated using the thick lens equation below. In this expression, nl is the index of refraction of the lens, R1 and R2 are the radii of curvature for surfaces 1 and 2, respectively, and d is the center thickness of the lens.

Rmax is particularly sensitive to anomalies such as scratches and burrs that may not be obvious from measures such as Ra that rely on averages.

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Enter the maximum scallop height and G-Wizard will tell you the resulting RA and RMS surface finish as well as calculate the stepover. BTW, if you want to know how to pick optimal stepovers and tool diameters for 3D Profiling, we’ve got an article on that too!

Here n is the index of refraction and R is the radius of curvature of the lens surface. For more information, please see the Tutorial tab.

– Area Techniques: With an area technique, a finite area is measured and this provides a statistical average of the peaks and valleys. Examples of area techniques include optical scattering, ultrasonic scattering, and capacitance probes. Area techniques are easier to automate and faster to execute, but profiling techniques are often more accurate.

CaF2lens

Surface roughness is a measure of finely spaced surface irregularities. Surface roughness is usually what machinists refer to when talking about “surface finish.” When talking about all three characteristics, they may use the term “Surface Texture” more properly.

Custom coatings are also available. Please contact Tech Sales for a quote. N-BK7 lens kits are also available. Please click here for information.

The transmission graph below is for uncoated N-BK7. It represents total transmission through a 10 mm thick, uncoated sample, and the losses within the 350 nm to 2.0 µm wavelength range are primarily due to surface reflections.

– Microscopy Techniques: These techniques are usually more qualitative because they rely on measurement of contrast to provide information about peaks and valleys.

The Surface Finish Units we would use for parameters like Ra would be either micro-inches (English or Imperial) or micrometres (Metric). Our chart of surface finishes by manufacturing process (see above) gives both.

In the United States, surface finish is usually specified using the ASME Y14.36M standard. The rest of the world commonly uses International Organization for Standardization (ISO) 1302.

The required surface finishes are very much determined by the function and use of the component. Ship’s propellors start out with fairly high surface finish standards, but in actual use, they erode pretty quickly. Optical and especially components used with X-rays have some of the finest surface finish requirements achievable.

Ra is also known as Arithmetic Average (AA) or Center Line Average (CLA). It is the average roughness in the area between the roughness profile and its mean line. Graphically, Ra is the area between the roughness profile and its centerline divided by the evaluation length. The evaluation length is normally five sample lengths where each sample length is equal to one cutoff length.

Surface Finishes vary tremendously by the manufacturing process used to achieve them. A flame-cut plate edge has a radically different surface finish than a ground surface, for example. Choosing a process that’s capable of achieving the desired surface finish is the first step in figuring out how to manufacture a part that requires a particular finish. Sometimes, more than one process must be overlaid to achieve the desired result cost-effectively.

Turning is very similar to 3D profiling in that the stepover, in this case determined by the feedrate while turning, results in scallops that match the shape of the turning insert. We use this to good effect when turning threads, but if we want a smooth shaft, we’ll have a surface finish requirement that must be met. That will determine feedrate and insert nose radius we can use for the job. G-Wizard has a nice calculator for that too:

The focal length of the lens calculated using the simplified thick lens equation directly above is the distance between the second (back) principal plane (H") and the position at which a collimated beam incident on the curved surface of the plano-convex is focused. The principal plane positions of a thick lens can be calculated with the following equations:

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M1, M2, … Mn are measure valuesAverage Roughness in micro-meters or micro-inches. Ra is the arithmetic mean deviation of the profileRc Mean height of profile irregularitiesRku Kurtosis of the profileRmax Maximum roughness depthRmr Material Ration of the profileRpRp = MAX( M1, M2, M3, … )Max profile peak heightRqRq = RMSRoot mean square deviation of the profileRsk Skewness of the profileRt Max height of the profileRvRv = MIN( M1, M2, M3, … )Max Profile Valley DepthRy Max height of the profileRzRz = Ra x 7.2

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