MAS Facet Descriptions

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3/30/99

: Facet Descriptions

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The following is a list of all of the facets used to describe a part in the manufacturing analysis service, along with descriptions of the basic and advanced modes for entering a given facets data

Process Facets	Material Facets
Batch Size Shape Bounding Box Compatible Materials Dimensional Tolerance Surface Roughness Wall Thickness Per Part Cost Setup Cost Production Rate Setup Time	Compatible Processes Cost Per Pound Density Yield Strength Elastic Modulus Thermal Distortion Hardness

Batch Size

Basic: One of the single most important design attributes, geometry being another, is the final batch size expected for the part. The cost effectiveness of all manufacturing processes is a function of the total number of parts that will be created.

Shape

NOTE: If your shape is both a 'surface of revolution' and has a constant cross section, constant cross section should be selected instead! Tubes and cylinders fit into the category. A surface of revolution with cross sections that constist of circles with different diameters does not have a constant cross section! Also, a soda can or two liter bottle is a THIN WALL part!

Basic: The shape category is used to define the primary shape of the part. Each category is briefly described and illustrated below:

Freeform Drape -- A free form drape shape is like a waffle, where the geometry is wholely determined by a surface with NO UNDERHANGS. For our purposes, the waffle iron surfaces can be curvy as long as the entire surface is visible in the z-direction. For the more mathematically inclined The part volume can be desribed as 2 z=f(x,y) functions, one for the top surface and one for the bottom surface. The volume can contain holes, but only in the z-direction.

Pictured above is a water valve knob, hammerhead, wrench, and a penny, all free-form drape surfaces. A kitchen sink and a bathtub are better described by the thinwall shape below.

Freeform General -- This applies to any surface, usually very complicated ones, that can't fit into any of the other categories.

Planar -- This category is for all 'flat' parts. Given a sheet of plywood and a jigsaw, all you can make is planar parts.

Pictured above is a a simple brace, and a house key.

Prismatic -- This means that most of your geometry can be described by extruded polygons and circles being removed from a piece of material. If you are familiar with milling machines, any part which is made on a standard milling machine is considered 'prismatic.'

Surface of Revolution -- Given a profile and an axis of revolution, a surface of revolution is the surface created when the profile spins through space 360 degrees. The candlestick in the 'Clue' boardgame is a perfect example of this. Screws and bolts also fall into this category. Remember that tubes and cylinders are classified as Constant Cross Section shapes!

Pictured above is a machine screw, a poorly designed spinning top, and something vaguely resembling a disk from an automotive brake assembly.

Thinwall -- Any part in which the thickness of the walls is predominately an order of magnitude less than any of the parts other dimensions can be categorized as 'thin walled.'

Pictured above is a soda can, plastic soda bottle, metal support cage, and an dial phone enclosure. Other parts that would fit this category are eyeglass frames, kitchen sinks, 'built-in' bathtubs, computer monitor, keyboard, or tower housings.

Constant cross section -- To generate this shape, take a two dimensional profile drawn on a piece of paper, and extrude it perpendicular to the paper.

Pictured above is a rod, a tube, and a widget.

Constant Cross Section that you can draw a shape with holes on a piece of paper and extrude the shape along a line perpendicular to the paper. IMPORTANT

Advanced: You can define a part with a primary shape / tolerance / surface roughness, as well as secondary features with different tolerances. This secondary information is not entered in the main process search mode, but is accessible in the multi-process chain section found in the results "survey mode."

Bounding Box

Basic: How big is the box that completely encompases your part? Enter the volume of the bounding box in cubic inches.

Advanced: Instead of entering a single number for the volume of the bouding box, you now get to specify the length, width, and height of the part. The length should be greater than the width, and the width greater than the height.

Material

THIS FACET IS IGNORED IN THE RESULTS SURVEY! Instead, each process is compared with every possible viable material, and then ranked.

NOTE: If you are only doing a process search, you should IGNORE this facet if you are interested in look/feel prototyping, as you will elimate many rapid prototyping processes if you specify a material during a process search.

Basic: If you are only doing a process search, selecting this facet allows you to consider the compatibility of the selected material with the candidate processes.

Dimension Tolerance

Basic: This is a single number for the needed dimensional tolerance that will govern most of the parts dimensions. If you have a small region of the part that needs higher tolerances, say the threads at either end of a copper pipe, DO NOT USE this tighter tolerance! Instead, use the looser tolerance that the applies to the bulk of thepart. An additional module will allow the MAS to generate multi-procss chains.

For most processes, given 'standard' sized parts, achievable tolerances scale linearly with the size of the part. Thus, the tolerance you enter is measured in inches per inch. So if you specify a tolerance of 10 mills (0.01"), and you have a part that is eight inches long, the actual length of the bar will be 8 +/- 0.08". This reflects the fact that it is much easier to make an accurate 1" cube than an accurate 10" cube. Below is a table with some common products and tolerances. Remember that the MAS is designed for 'standard' manufacturing processes, and that optics from an observatory are made using special processes.

Product	Dimensional Tol (thousandths of an inch)
Plastic Soda Bottle	25
Outside Surface of Auto Engine Block	20
Plastic Laptop Computer Housing	12
Metal Gears	7
Pistons from Auto Engine	2
Optics from Observatory	0.002 (Yes, that's 0.000002")

Surface Rough

Basic: The following table is taken from "An Introduction to Process Planning Systems," the rough column is the centerline average roughness measured in microinches.

Rough	Adjective	Purpose - Parts	Relative Cost*
1000	Extremely rough	Used for clearance surfaces only, where good appearance is not required
500	Rough	Used where vibration, fatigue, or stress concentration are not critical and close tolerances are not required
250	Medium	Most popular for general use where stress requirements and appearance are esserntial -	100
125	Average - smooth	Suitable for mating surfaces of parts held together by bolts and rivets with no motion between them -	200
63	Better than average finish	For close fits or stressed parts except rotating shafts, acles, and parts subject to extreme vibration - ratchet teeth,	440
32	Fine finish	Used where stress concentration is high and for such applications as bearings - push fits, general journal bearings, general machine sliding surfaces	720
16	Very fine finish	Used where smoothness is of primary importance, such as high-speed shaft bearings, heavily loaded bearings, and extreme tension members. - pistons, cam lobs, piston rods, heavy load gear teeth, precision journal bearings, precision sliding surfaces, valve stems, brake drums, clutch plates	1400
8	Extremely fine finish	Produced by cylindrical grinding, honing, lapping, or butting. Used for parts as surfaces on cylinders - crank pins, valave seats, rolled threads	2400
2-4	Superfine finish	Produced by honing, lapping, buffing, or polishing. Used on areas where packings and rings must slide across the surface where lubrication is not dependable. - piston pins, pressure-lubricated bearings	4500

The LODTM can turn a 0.2 microinch RMS sruface finish!

Wall Thickness

Basic: The wall thickness number here is the average wall thickness present in a part.

Advanced: Wall thickness is now represented by three numbers: The thinnest section, the thickest section, and the distance the thinnest section must span (the web). If you have a rectangular thin section, the shorter value of the length and width is the web distance. For thin parts such as a soda can or ball point pen case, use the diameter of the part for the web length. Many processes tailored to producing parts with uniform walls will have their wall thickness facet ranking drop dramatically if the thinnest and thickest sections differ drastically!

As an aside, it should be mentioned that in basic mode, if the bounding box volume is greater than zero, the web thickness is automatically set to the cube root of the volume. If the volume is less than or equal to zero, the web thickness is set to two inches.

Production Rate

Basic: The production rate is measured in parts per hour, which can be less than one for extremely slow processes.

Setup Time

Basic: The setup time consists of designing and fabricating the tooling needed to create the parts (i.e. casting patterns), finding which capital equipment to use (i.e. stamping press), as well as working the bugs out of the system. The setup time is selected on a rough time basis, with the user selecting hours, days, weeks, or months.

Setup Cost

Basic: Setup cost is the rough investment needed to design and fabricate all of the permanent tooling needed. It does not include factory overhead, rent on floor space, etc.

It may be intuitive to enter a dollar value, but this is impossible due to the cost variation within a single process. For example, the stamping die cost for a small metal thimble will be orders of magnitude less than for a metal trash can. However, it is possible to say that the final per part cost for stamping the trash will be much cheaper with than an EDM trash can. Thus, the cost facets are used to compare the relative abilities of the processes, as the absolute costs are strongly dependent on a given design.

Instead, select from one of five cost categories for each of the production cost facets, from "Very Low" to "Very High." These qualitative groups establish relative costs between the processes.

Per Part Cost

Basic: The per part cost represents the expense of running the machine, paying the operator, and a rough estimate of material costs for one complete production cycle. This does not include any of the overhead or tooling costs.

Instead, select from one of five cost categories for each of the production cost facets, from "Very Low" to "Very High." These qualitative groups establish relative costs between the processes.

Process (as Material Search Facet)

THIS FACET IS IGNORED IN THE RESULTS SURVEY! Instead, each process is compared with every possible viable material, and then ranked.

Basic: If you are only doing a material search, selecting this facet allows you to consider the compatibility of the selected process with the candidate materials.

Cost Per Pound

Basic: An estimate of the cost of a single pound of the material. This number is an approximation, as the cost per weight of a material is usually dependent on the geometry of the raw material. Many materials come in standard sizes, and ordering a non-standard size will cost addition money. Also, certain geometries, due to demand, are cheaper than other geometries for the same material. For example, steel rod is usually less expensive than hexagonal stock.

Density

Basic: The mass per volume of the part.

Yield Strength

Basic: In the basic mode, the value for yield strength corresponds the the desired yield strength for the material, in ksi (kilopounds per square inch), for a uni-axial tension test. Shigley defines the the yield strength as the point in the tension test in which the specimen has stretched to 0.2 % of its original length. This often corresponds to the yield point: The point at which "the strain [stretching] begins to increase very rapidly without a corresponding increase in the stress."

Advanced: In many industries, the strenght to weight ratio is very important. When in advanced mode, the numbers used in this facet change to specific yield strength, or the yield strength / density.

Thermal Expansion Coefficient

Basic: The linear coefficient of thermal expansion is a measure of how most materials expand when heated and contract when cooled. For materials that are anisotropic in regard to this parameter, the MAS uses the average value. In some applications, such as machining, it is critical for the size or shape of the component to remain nearly constant while operating at different temperatures. Thus, it is important that the material used has a low coefficient of thermal expansion.

Measured in 10^-6 cm / (cm deg C). This means for each deg increase in temperature, a one cm cube will expand a number of centimeters equal to the coefficient. Sample values: iron/steel-10 to 15, aluminum-10, magnesium-6.5, plastics-24 to 250.

Elastic Modulus

Basic: Elastic modulus is the stiffness of the material. More precisely, it is the ratio of stress (force/area) over strain (deformation) for a given material. Thus, the higher the elastic modulus, the more stress is needed for the same deformation, the stiffer the material. Stiffness is important when a component must maintain its shape while under changing loads - such pistons in a combustion engine. Some representative values for elastic modulus, measured in 10^6 psi, are: iron/steel-30, aluminum-10, plastics-0.025 to 0.6.

Hardness

Basic: Hardness is a measure of a material’s resistance to mechanical penetration. The hardness of a material correlates with the yield strength; higher yield strengths means a higher hardness. However, the testing for yield strength requires the destruction of a sample, while hardness can be tested non-destructively in many cases - including testing finished parts coming off of an assembly line.

The MAS uses the Brinell scale to measure hardness as it covers the entire range of material possibilities. Some represetative values for hardness are: diamond-7000, tool steel-700, Aluminum-50, most plastics-15.