Surface Finishing Technologies, Process Types and Methods
There are many surface finishing technologies and methods that you can use to finish your parts, each method produces a different surface finish and flatness.
Lapping is a precision operation and is and based on the cutting power of either a free abrasive grain in a carrier or a fixed abrasive particle within a composite lapping plate matrix. There are 2 types of Lapping processes, Diamond or Conventional. Either type of lapping process can produce flatness results down to 0.0003mm as long as the flatness of the lapping plate is controlled and monitored. The lapping process is a gentle stock removing process that transfers the flatness of a lapping plate to the component being lapped without introducing any stress to the components because the whole surface is being processed at the same time. This differs from typical CNC, turning, milling, and grinding processes where the cutting is focused on a particular area at any one time. Although any lapping process has the ability to generate flatness, diamond and composite process combinations have a far wider range of surface finishes achievable.
Polishing is often carried out after a lapping operation, to produce the ultimate in surface finish. Some of the common reasons for polishing are; To produce mirror surfaces, to improve appearances, to obtain the optimum sealing surfaces, to optically measure flatness, to improve electrical contact, to improve the optical qualities of materials.
Chemical mechanical polishing or planarization is a process of smoothing surfaces with the combination of chemical and mechanical forces. It can be thought of as a hybrid of chemical etching and free abrasive polishing (Lapping). A CMP process is common when either a very low Ra is required, or where scratch free microscopic images are required such metallurgical samples for micro hardness testing.
Centrifugal Polishing generate a very high gravitational force, the drive mechanism is designed to produce high ‘G’ forces of 5-25 times than normal gravity with 3 or 4 hexagonal/circular barrels mounted on a turret. The turret and the barrels rotate at high speed in opposing directions and the resultant centrifugal force increases the weight of the abrasive media in the barrels which slides against the components, also in the barrels, to produce a rapid cutting action.
Vibratory finishing is a type of mass finishing process used to deburr, radius, descale, burnish, clean, and brighten a large number of relatively small workpieces using specially shaped pellets of media.
Grinding is an abrasive machining process that uses coarser abrasive material.
Drag finishing is a specialised version of vibratory finishing. It is different in that the parts to be deburred and finished are mechanically dragged through the media while attached to fixtures. This prevents the parts from contacting each other.
Surface Finishing Standards
There are a multitude of surface finishing standards but by far the most common are Ra and Rz. Ra is the roughness average over a given sample length and although very common, since it is an average, it does have the potential to miss significant scratches that may not be within the spirit of an Ra target. The Rz standard gives a better overall roughness figure for a process, first splitting a sample length into smaller sectors and then taking the worst-case peak to valley measurement for each sector and then displaying the average of the smaller sectors combined value. Surface Finishing standards is a detailed subject within itself.
Arithmetical mean roughness valve RA
Total height of the roughness profile Rt, mean roughness depth Rz and maximum roughness depth Rz1max
The mean groove spacing RSm is the mean value of the spacing Xsi of the profile elements
The material component curve of the profile depicts the material component Rmr(c) of the profile as a function of the section height c(Abbott-Firestone Curve)
Why Surface Finishing is Important
The surface finish of a component needs to be defined for many different reasons. The most basic is for aesthetics, but the surface finish can also control the wear characteristics of a part, the ability of a surface to retain lubrication, the ability for 2 hard faces to produce a good seal and many other important objectives all reliant upon the surface finish that is generated. The surface finish on an engineering drawing is shown with a tick. This symbol represents the flatness required
Below is an extract from a technical drawing showing examples of the surface finish required
Typical statements of surface finish on a technical drawing:
Symbol A How to specify maximum roughness value in Ra microns.
Symbol B How to specify maximum and minimum roughness values.
Symbol C How to specify maximum roughness and finishing process.
How Surface Finish is Measured
There are 2 methods of measuring a surface finish. Contact and non-contact measuring systems. Contact systems use either a ruby ball or a diamond stylus that runs across the surface of a part in a single short trace considering the peaks and valleys on the surface. Using complex algorithms and formulas this is then converted to a surface finish figure. This is the lower cost method and simple handheld tools can be reasonably priced. With non-contact systems like an interferometer a laser is bounced off a surface giving a 3D render of the surface. This tends to be a far more expensive solution but does take into account the whole surface being measured rather than just a single trace. This type of measuring system is more essential for optical surface finish measurement. For most engineering applications a contact system is acceptable.
Surface roughness is the property of a material that defines its surface texture or flatness. Deviation from the normal vector of a real surface from the ideal form is used to quantify the roughness of the material. Several changes over a length for the given surface are termed roughness. It plays a very important role in determining the interaction of objects with the environment. It has multiple applications that define whether its value shall be high or low. For microscopy, we need the surface roughness to be low so we can exactly determine the microscopic image of the sample. It can be measured by using atomic force microscopy or optical profilometry. It can be controlled to fit the material in the required application. Surface polishing or grinding are the techniques that can be used to control to reduce the surface roughness of the material.
Surface Finish versus Flatness
These two terms are extensively used while defining the quality of the surface. Surface finish is a micro-level measurement that defines the texture of a surface and it is mostly characterized by the surface roughness of the material. The higher the surface finish, the lower will be the surface roughness. While flatness is the property that is based on macro-level features and we can feel it by looking at a flat surface from an angle. If we rotate the object, its flatness will be impacted by its surface finish will not be changed. Thus, flatness is observed at the macro level. But it is still impacted by the surface roughness as the lower the surface roughness, the more flat appearance will be there.
Surface Finish and Tolerance
Tolerance is the existence of a margin or allowance in a device or part of the machine that can occur in the surface finish without impacting the quality of the product or service. Surface finish tolerance is the specification in terms of micrometres to microinches from 0.001 to 2000. A model surface is selected to compare the surface finish of the material. Dimensional tolerances are based on several dimensions involved in a particular setup. A high-grade surface finish is said to be of low surface roughness and tolerance grades are assigned according to it. A low tolerance grade will have higher surface roughness and vice versa.
Related Surface Finishing Technical Case Studies
Typical Surface Finishing Problems & How to Fix
Let's say that you don't have any lines in the workpiece, and you have a very high finish on it, but the thing looks like a piece of skin from a chrome plated orange! You have just been introduced to the biggest problem. This orange peel condition is almost always caused by over-stressing the steel during polishing, that is to say over-polishing.
Many years ago most of the high polishes were applied by hand lapping with wooden sticks and felts. The danger of over stressing the steel was negligible. Today no shop can afford the time it takes to hand lap and so we must go to mechanical polishing. While using polishing machines it is very easy to apply pressure and too much speed and over stress the surface of the steel.
When a polisher comes up with a bad case of orange peel, one of the first reactions is to blame the steel. If he has some pitting also this reinforces the suspicion of bad steel. As a result of this, the steel companies have done a lot of research on the subject. From the results of some research and from actual experience with various mould steels, the following information about the causes of orange peel has been put together.
Almost any work you do to a block of steel stresses it. Conventional machining puts deep stresses in it. EDM burning puts stresses in it, grinding stresses it. Carbide and ceramic machining can put enormous stresses in it. Under actual working conditions that block is hardly ever stress relieved before you get it for polishing so you start out under a handicap.
The surface of each type of mould steel has a certain yield point. This surface moves like plastic under your polishing tools. If you do not move it beyond its yield point it will go back to its original position. If you stress it beyond the yield point, the steel will remain in that position and you have produced what amounts to little ripples on the surface. If you continue to work the steel you will tear the tops off these ripples and produce pits. Even if you do not see pits when you finish polishing they may appear while the mould is running, because of the stresses involved there and they will surely show up in repolishing after production. Of course, if the orange peel is not too serious, the pitting may not occur.
Now in actual production, it is almost impossible to mechanically polish a mould and not get some orange peel. Remember that it shows up worse when you get in a hurry and start using too much pressure and too much speed on your polishing tools. You will notice as you polish that the harder the steel you are polishing, the less chances are that it will orange peel. The harder the steel, the higher the yield point and the more polishing pressure it can take before rippling and tearing.
Mould steel with a Rockwell in the low thirties will not take as much polishing abuse as a steel with a Rockwell in the middle fifties. When polishing soft steel one must keep a close watch on ones brushes and felts to make sure the speed and pressure is kept down. The polisher must also keep close watch on the surface and move on to the next step as soon as possible so that the rotary traffic is kept to a minimum.
When stainless steels started gaining popularity in the mould industry, the steel companies had a lot of complaints about the steel pitting during polishing. The customers thought they were getting bad steel. It invariably turned out to be caused by over-polishing. Stainless steel is very sensitive to polishing pressures. The surface moves and pits easily. If real care is taken, however, both will polish up quickly and beautifully.
Another cause of orange peel
Another cause of orange peel that should be mentioned is over-heating during heat treating. The information that Crucible Steel Company puts out says that steel that is overheated in heat treating will retain some austenite in its structure instead of converting all to martensite. Austenite is softer than martensite so you have a surface that has hard and soft areas, ready-made to ripple or orange peel.
I think this explains why once in a while, as a polisher, you come across a piece of steel that will orange peel no matter how careful you are in polishing. Sometimes you can see it forming even in the stoning of the block. Apparently the soft areas wear away quicker under the stone and the ripples start to show early. You can hand-work this block and maybe salvage but, it will never be very good.
What to do about orange peel
There are a couple of things you can do to try and remedy the situation. You can go back to the 600 stone and work out the distortion. Just a superficial stoning will not do it. You will have to put enough time into the stoning to really get below the ripples. Then re-polish it, being more careful this time about pressure, speed and the actual amount of traffic put on the block. You may improve the surface, but you will hardly ever eliminate it. Sometimes it gets worse. The best thing you can do if you are really serious about eliminating the orange peel is to carefully re-stone it with the 600 stone, making very sure that no pits get overlooked, then stress relieve the steel. This should be done at about 100oF. below the tempering temperature of the steel so that the block does not anneal.
Then re-stone the block with the 600 grit stone, remove these lines with 600 wet paper, and polish with diamond compound as described earlier, this time being more careful to keep polishing pressure to a minimum. Do not skip any steps, and be careful not to spend any more time than necessary in each step, thereby reducing the overall amount of polishing stress on the block.
Plucking of minute particles from the surface, causing cavities and 'comet tails'
Uneven flatness of between adjacent metals sometimes called 'relief'
Surface flow or smearing of metals leading to indistinct boundaries of adjacent metals