The Science Of Flatness

what does a granite countertop a sheet of glass and this table all have in common well they're all surfaces we consider flat but are they actually flat and how do we even test them for flatness flatness is an often misrepresented property of our own intuition many of the objects we consider flat pale in comparison to surfaces manufactured to actually be flat it's also a property that our industrialized world relies on to function while most of us experience flatness as a part of aesthetics and ergonomics flatness and manufacturing is a critical property of positioning mating and sealing parts together in fact some of the most common examples of flat surfaces at work in our daily lives can be found within car engines the high pressures produced by combustion are contained by two mating flat surfaces aided by a gasket even fluid such as air fuel coolant and oil that are moved around an engine bay are sealed at mating points by a combination of or rings and gaskets all of which require flat surfaces to function properly what exactly defines the flatness of a surface and how is it measured let's look at a sheet of flow glass found in virtually all modern windows and doors flow glass is formed by floating molten glass on a bed of molten metal until it cools the floating process self levels the glass giving it a relatively flat uniform thickness let's say a manufacturer specifications call for a three millimeter thick sheet of glass for a sheet to pass a quality check its thickness is sampled at various points along its length and as long as it is three millimeters thick plus or minus the specified tolerance the sheet passes but what if during the process of moving the floating ribbon of molten glass a subtle disturbance is introduced to the molten metal let's say this disturbance imparts a quarter of a millimeter wave like undulation throughout the entire ribbon not to the eye the cut sheets would appear flat and they would pass quality checks for thickness but the surface of those sheets of glass are far from flat flatness isn't derived from how closely apart matches its specified dimensions it's a property completely independent of the parts gross shape parts can be made completely dimensionally compliant yet still like flatness on its features because of this the flatness of a surface is specified independently of other dimensions if we take a surface and sandwich it between two imaginary parallel planes the gap between the planes that encompassed the surface is known as a tolerance zone the distance between planes in this tolerance zone is how flatness is specified the smaller this distance the flatter the specification if the surfaces irregularities can't fit within the tolerance zone it's not flat enough for the specification flatness is in effect the difference between the highest and lowest point of a surface and its relationship to a specified tolerance zone it is possible to force flatness on a surface without explicitly specifying it by tightening the dimensional tolerances of a part for example if we mean the thickness tolerance of our float glass to be plus or minus 0.1 millimeters it would cause the sheets affected by the undulations to be rejected your in quality control however this practice can create dimensional specifications that may be impractical or cost prohibitive to meet for some parts much like flow glass many objects appear to be flat because of their dimensional tolerance but are actually not designed explicitly to be flat a flat wooden tabletop for example may vary up to several millimeters yet still fit at specified tolerances even a large polished granite kitchen countertop generally known for being a flat solid surface can vary up to three hundredths of a millimeter along its surface on parts that do explicitly define flatness the method of both measuring and producing flatness is determined by how tight of a tolerant zone is required adding to this some specifications may even specify a maximum allowed change per area of surface preventing abrupt changes to the overall flatness of the entire surface flatness specifications down to around 10 microns or about 4 10 thousandths of an inch are quite common in machinery found predominantly on part mating surfaces at these tolerances traditional milling can be used for larger faces tooling designed for milling flat faces such as face mills and fly cutters are common those mating and sealing surfaces found in car engines can be found at this level of flatness sealing in fluids at this level of thoughtless require the use of a gasket a gasket is a mechanical seal which fills the space between mating surfaces while under compression gaskets allow for less than perfect mating surfaces on machine parts where they can fill irregularities they are usually made out of compressible materials such as paper rubber silicone or metal field testing flatness of this level is done with a known precise flat edge and the clearance probing tools such as feeler gauges the flat edge is placed at several positions spanning the surface to be tested and a feeler gauge at or above a maximum allowed deviation for flatness is used to probe the clearance between the surface and the flat edge offering a simple part usability flatness test actually measuring the flatness of a surface is a lot more complicated an obvious solution would be to measure the surface against a flat reference for example if a part has a surface parallel to the surface to be measured it can be placed on a surface plate a surface plate is a flat plate used as the main horizontal reference plane for precision inspection a height gauge can be used to probe the top of the surface for flatness relative to the surface plate however the problem with this method is that you're really only testing how parallel surfaces to the surface plate and not actually testing for flatness furthermore not every part can be fixtured in this method because flatness is a property that's independent of other dimensions it must be measured in a similar manner if we first place the part to be measured upon three columns with adjustable Heights then with a height gauge run the probe along the surface while looking at the amplitude of the probe needle we get a snapshot of the differences between the highest and lowest point on that surface we then adjust the three columns to minimize the amplitude of the needle the goal is to adjust the orientation of the plane in order to come up with the smallest difference between the high and low spots on the surface this smallest possible range of travel is the measure of the flatness of our surface in effect we're reconstructing the tolerance zone sandwich of the surface measuring flatness manually requires significant training and experience and it can be a very tedious procedure automating the process with the use of a coordinate measuring machine or a CMM is a common practice a CMM is a device that measures the geometry of a physical object by sensing discrete points on the surface of the object with a probe CMMS are typically computer controlled and can be programmed to perform the tedious repetitive measurements needed to determine surface flatness the probe is programmed to sample the location of the surface within the CMMS working space at numerous points the resulting data known as a point cloud is then processed through an algorithm known as the minimum zone method to theoretical planes are calculated that best sandwich the point cloud effectively determining the allowances own of the surface because the aligning of the surface is done in software measuring flatness with a CMM can be done very efficiently with minimum fixturing CMMS are generally accurate down to about 2.5 microns or one ten thousandth of an inch making them ideal for use with milled flat surfaces going beyond the 10 micron level of flatness requires the use of surface grinding it's a widely used abrasive machining process in which a spinning wheel covered in rough particles cut chips of metallic or nonmetallic substances from a workpiece making it flat grinding is commonly used on cast iron and various types of steel these materials lend themselves to grinding because they can be helped by the magnetic Chuck commonly used on grinding machines and they do not melt into the wheel clogging it and preventing it from cutting surface grinders can easily produce flat finishes down to about two microns of surface variance this process is typically used to produce precision parts precision fixtures measurement equipment and tooling similar to grinding face honing is a process in which a large working wheel with a thin layer for brosef is used to grind work pieces held in a carrier disc face honing allows for the grinding of multiple small parts at once and is commonly used for ceramic tooling inserts watch windows led wafers bearings gears cutting knives and carbide tooling when extremely flat surfaces are required beyond what grinding is capable of lapping is used lapping is the process of rubbing two surfaces together with an abrasive between them in order to remove material in a highly controlled manner in lapping a softer material known as a lap is charged with an abrasive the lap is then used to cut a harder material the abrasive embeds within the softer material which holds it and permits it to score across and cut the harder working material lapping can be used in a range of materials from metals - ceramics and glasses and at its limit can produce surfaces that varied by only 30 nanometers or about the height of 300 hydrogen atoms it can be performed by specialized machines or by hand and the process can accommodate sizes from tiny handheld parts all the way up to large optical pieces found in telescopes that can take months to produce a curious phenomenon that occurs when surface flatness approaches one tenth of a micron is called ringing ringing is the sliding of two ultra flat faces together so that their face is lightly bond when rung the faces will adhere tightly to each other requiring a significant amount of force to separate them the exact mechanism of why this occurs is unknown though it's believed that molecular attraction starts to occur when the two very flat surfaces are brought into contact as flatness gets into the nanometer scale the force reducing effects of surface roughness are removed and full contact bonding actually becomes possible purely by inter molecule forces such as van der Wolfe is hydrogen bonds and dipole interactions this technique is used in an optics manufacturing process known as optical contact bonding measuring the incredibly small surface variances at this level of flatness are near impossible to do with direct surface contact so optical methods are used in order to accomplish this the interference pattern created by light reflecting off the surface to be measured is used this is done by using a special precisely polished flat lens known as an optical flat and a monochromatic light source of a known wavelength common sources are low pressure sodium lamps or a helium neon laser when an optical flats polished surface is placed in contact with the surface to be tested dark and light bands are formed when viewed with monochromatic light these bands are known as interference fringes and their shape gives a visual representation of the flatness of the surface being tested the surface flatness is indicated by the amount of curve and spacing between the interference fringes if an imaginary line is run through the optical flat each band of curved interference fringe it intersects represents a half wavelength of variance in the surface at that region for example if the light source has a wavelength of 600 nanometers and the imaginary line intersects three bands the surface flatness for that area varies by 900 nanometers furthermore the direction of the curves represent if the surface is concave or convex straight parallel and evenly spaced interference fringes indicate that the work surface flatness is equal to or higher than that of the reference surface some optical flats can even create band divisions anywhere from 1/4 for wavelength down to one twentieth of a wavelength allowing for flatness resolution down to about 30 nanometers both grinding and lapping can be used to achieve a subtly different property of surfaces known as surface roughness surface roughness like flatness is based off the peaks and valleys of a surface surface roughness however measures how much the average of a surfaces variance deviates from the intended surface surface roughness can also be specified for any shape not just flat surfaces in general surface roughness specifications are far smaller than those for flatness they typically exist in the submicron range but as flatness variance approaches extremely small levels the two properties may begin to converge perfect flatness near or at one atom of variance are possible but are beyond the scope of mechanical means at these levels mechanical flatness give way to chemical deposition and thin film technologies direct manipulation of atoms with scanning tunneling microscopes can even achieve perfect surfaces by manipulating them one atom at a time at these scales the definition of flatness quickly become

Channel: New Mind

Views: 658,419

Rating: 4.8584185 out of 5

Keywords: science off flatness, flatness, flat honing machine, flatness measurement, flatness measurement methods, flatness measurement using dial indicator, surface grinding, surface grinder, cmm flatness measurement, wringing gauge blocks, lapping machine, lapping glass plate, gauge blocks, gauge blocks stick together, gauge blocks wringing, face milling, fly cutter milling machine, surface plate lapping, surface plate calibration, surface plate how it's made, flatness gd\&t, cnc

Id: OWa3F4bKJsE

Length: 13min 1sec (781 seconds)

Published: Fri Jun 21 2019

Reddit Comments

[u/TheDarkSinghRises]

Wanna experience true flatness?

[u/DontMakeMeDownvote]

Fuck all the ads on this otherwise interesting video. Also, the video abruptly ends midsentence.

[u/OllieSDdog]

Quickly become what?!?

[u/sethamphetamine]

Quickly become.....?

[u/The48thAmerican]

Interesting video. The ending is messed up though, cuts off miss sentence

[u/blobber5678]

I see he has fell upon r/animemes

[u/InterspersedMangoMan]

My ex used to be a professor of flatness at Aint Got No Titties University

[u/iIK0D3RIi]

Morty has experienced true flatness

[u/jmule34]

Nothing we own is flat!!!! Lol