Technohydra Posted May 29, 2007 Posted May 29, 2007 Welcome to Technohydra's Lapping Tutorial! Ok, finally ready to begin the lapping tutorial! This tutorial is a little different, as I'll be handling it in 3 separate parts, a 101, 201 and 301 session. Each is beneficial on its own as a mini-course, so to speak, but as a whole, it'll get you to being able to identify, lap and polish damned near anything, with a lot less hassle than you'd think. Session 101 Intro to Lapping Terminology and Nomenclature So, you wanna take something dull and make it shine. What does it take? What process are you really using to get to your end result, and what do you need to get there? These are a few of the things we'll be covering in this 101. So, let's start with some terms. Machining: Any process which by repetition and combination of motion, force, and energy, causes a piece of material to be altered from it's original state and shape to a desired state and shape. Examples include milling grinding, sanding, lapping, turning, cutting, electro-chemical, and CNC(computer-numeric-controlled) processes. Grinding: Grinding is the process of machining a part by friction of a rapidly rotating abrasive surface against a moving or stationary base material. Typically, this is done by hand, and involves a machine having either a wheel of some form of abrasive, or a belt with a resin-bonded abrasive. The process generally involves removing large amounts of material in a semi-precise fashion. Sanding: The process of machining a part by friction of a moving abrasive against a stationary part. The abrasive takes the form of hard oxide crystals and composites adhered to a paper or cloth surface. The process removes a moderate amount of material in a semi-precise fashion by applying force to the surface of the material being sanded. Lapping: The process of machining a part by the movement of said part against a stationary abrasive surface. Almost always performed by hand, except in the case of lapping wheels. The process removes very small amounts of material in a precise fashion, and is achieved by applying force at a 90-degree angle to the abrasive surface. The only downward force is that of the weight of the part resting on the abrasive. This process is used to produce both flatness and surface finish. Polishing: The process of machining a part by the friction of a non-abrasive surface against a part, either moving or stationary. The actual amount of material removed is almost none, and is representative of a very precise process. Removal of the material is facilitated by coating, or loading, the non-abrasive surface with a mild abrasive compound, such as diamond paste or pumice. All force applied in this process is rotational, performed parallel to the part surface. The only force on the part surface is that of the weight of the non-abrasive surface resting on the part. When performed along with lapping, the total process is referred to as finishing. Hardness: A rating of the resistance of a substance to being physically machined or altered. This property is reflected in two ways, by trade rating(i.e.; hardened, half-hard, quarter-hard, normal, annealed) and by ASTM scales such as the Rockwell hardness scale(which is beyond the scope of our interest hear). Generally speaking, the harder a material is, the more difficult it will be to machine. Abrasiveness: The rating of the level of abrasiveness of a given machining substance or surface. The scale is general, and ranges from extra-mild to extra-aggressive. The more aggressive the surface, the more material removed in one pass of the material over the part. The more aggressive the material or surface, the more material removed in a single pass. Grit: The frequency of repetition of abrasive particles on a machining surface. The rating is expressed as grits per square inch. Also classified from very course to ultra-fine. The lower the grit rating, the coarser the surface is, and the more material that will be removed in a single pass. Generally speaking, the process of grinding uses extra-coarse materials of high aggressiveness, sanding uses from coarse to fine in aggressive to normal, and lapping uses semi-coarse to extra-fine in semi-aggressive to extra-mild materials. Polishing uses only ultra-fine materials of ultra mild abrasiveness. Finish: The degree of light refraction off of a parts surface, expressed in grits. The rating refers to the last grit of abrasive used to machine the part. Thus, a part properly sanded with 220 grit abrasive will be said to have a 220 grit finish, and a part polished with a 3000 grit compound will have a 3000 grit finish. The higher the finish rating, the more shiny and reflective the part will be Flatness: The degree of deviation of a surface from that of a perfect plane, expressed as amount of deviation over a given distance(i.e. deviation in inches divided by the distance of one inch) Thus if a part deviates from a plane by .10" over a distance of 1", the flatness of the part is .10". The lower this rating, the closer the surface is to being a perfect plane. So, what does all this boring stuff mean to you? First off, since we are lapping parts to achieve flatness, we need to be sure not to press the part into the sand paper. That's not lapping, it's sanding and it does not produce either flatness or finish, which is what we are after. Secondly, this should help you a bit in selecting your sandpaper. There is a big difference between the types out there, and getting the right one makes your job a lot easier. Trying to lap through hard nickel-plating on an IHS is a ball-breaking process unless you have the right paper. Third, I can now deliver the rest of the guide as I would normally speak, and you'll totally understand what I'm saying about the processes and materials, so you can concentrate on the how to, not the what. That's all for the 101 session! Boring, I realize, but it's important stuff that's good to know anyways. Coming up next is the 201 session, which will cover material selection, what you need, and how to do the actual finishing operation from beginning to end. Stay tuned, we'll be back! Quote Share this post Link to post Share on other sites More sharing options...
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