Electro-Mechanical Design Guidelines

1. Black Box (Electronic Enclosure) Packaging
   • ATR box sizes. Length, width, height, volume, weight, and center line mounting dimensions of standard Airbourne Electronics Enclosures. These dimensions are in accordance with ARINC (Aeronautical Radio Inc.) Specification 404A, air transport equipment cases and racking. For more information plus a little history, check out www.vita.com. You can also find valuable information in the Cots Journal (here is a nice 2003 article). You might also want to check out Elma Electronic, one of the leading suppliers of ATR boxes.
   • Enclosure Venting, formula for the calculation of.
   • Power Supply Sizing. Most Power Supplies can be packaged easily in a volume based on 3 to 5 watts per cubic inch.
   • Weight Estimating. A good rule of thumb for estimating the weight of a module or enclosure assembly is to multiply the volume by 1/2 the density of aluminum (.05); although, a density of .03 can be achieved for aerospace applications.
2. Cable Assembly Design should be started early in the design phase.

   Here are some basic specifications to help you through the process.

   MIL-W-5088 (s/s by SAE AS50881) wiring, Aerospace Vehicle (web indexed 5088)

   MIL-STD-454 (s/s by MIL-HDBK-454) Standard General Requirements For Electronic Equipment

   ASME Y14.24 Types and Applications of Engineering Drawings

   ANSI Y14.15 Electrical and Electronics Diagrams

   ANSI Y14.15A Interconnection Diagrams

   IEEE-STD-315 (ANSI Y32.2) Graphics Symbols for Electrical and Electronics Diagrams (Including Reference Designation Letters)

   ASME Y14.44-2008, IEEE-STD-200 (ANSI Y32.16) Standard Reference Designations for Electrical and Electronics Parts and Equipment. (ref des look-up)

   MIL-STD-12 (s/s by ASME Y14.38) Abreviations for Use on Drawings, and in Specifications, Standards and Technical Documents

   • Awg Sizes, ranging from awg 30 through 2/0.
   • Sizes ranging from awg 40 through 0000, with voltage drop calculator (web link).
   • Standard Wire and Cable Company
   • Stranded conductor data , sizes ranging from awg 36 through 4/0.(web link)
   • Solid conductor data , sizes ranging from awg 44 through 4/0.(web link)
   • Bend Radii. A good rule of thumb for determining the bend radius of multi-conductor wires is R=5xDia. Of course you should always check the manufactures specification, when available.
   • Bundle Diameter. formula for calculating.
   • Cable Lengths. In order to minimize the cost of cable fabrication it is important to assign a reasonable cable length tolerance. The following table is an excerpt from the Military Specification Cable-Cord, Standard Lengths and Standard Tolerances, MS75027.
   • Cable Tiedowns.Guidelines for locating cable tiedowns. Reference MIL-W-5088.
   • Cable & Wire Types. Listed below are a few manufacturers that can build your cables for you and mil spec wire part number breakdowns.
   • RF Coaxial. Gore can build your RF cables for you.
   • Multi-Wire. You can have Tyco build your multi wire cables and they even furnish a drawing package.
   • MIL-DTL-27500. Here is a nice part number break down of shielded twisted pair. Standard Wire and Cable has many other break downs like this on their website.
   • Standard Color code. One way to remember the standard color code is by the following phrase;

     Black Bears Run Over Yellow Grass, But Vultures Glide o'er Water.

     Key Word	Color	Color No.	Key Word	Color	Color No.	Key Word	Color	Color No.
     Black	Blk	0	Yellow	Yel	4	Glide	Gray	8
     Bears	Brn	1	Grass	Grn	5	Water	Wht	9
     Run	Red	2	But	Blu	6
     Over	Orn	3	Vultures	Vio	7

     Reference Mil-Std-681, Identification coding and application of hook up and lead wire.

   • Derating Guidelines, Reference MIL-W-5088.
   • Terminal Lugs To determine the number of wires that will fit in a terminal lug, simply add the circular mil areas of the wires in question and compare to the circular mil area of the hole in the lug.
   • Wire Diameter, circular Mil area.
3. Castings. The Aluminum Association is a good place to start; however, don't overlook the big boys, such as Dow and Dupont when looking for design specifics on plastic hinges. etc.
   • Investment Casting.
   • Mold Design.
4. Charts
   • Conversions (weights and Measures).
   • Galvanic Corrosion. Electrical current flows from + to -; therefore, no serious corrosion will result if fasteners are selected from alloys in the same group as parts to be fastened, or below. Coupling metals widely separated on the chart is most likely to cause corrosion. Avoid combinations where the area of the less noble material is relatively small. The current density is greater when it flows from the small area to the large; hence, your fastener should be lower in the galvanic series than the rest of the assembly. Reference MIL-HDBK-454, guideline 16.
     • Galvanic chart as depicted in MIL-STD-889B, Dissimilar Metals. A newer chart is located in NOTICE 3. You can also check out Table VI of MIL-F-14072 Finishes for ground based electronic equipment.
     • Galvanic chart as seen in the Assembly Engineering Master Catalog.
     • Metalmart, International Inc. another pretty good chart.
     • Engineers Edge an excellent website with a pretty good definition of galvanic corrosion.
     • Corrosion Doctors has a nice galvanic chart and much more.
5. Construction
   • Conduit trade sizes are listed here with useful information on bend radii and knock out sizes for thin wall (EMT) and thick wall (rigid).
   • Lumber Sizes, You might want to check Engineers Edge, it seems they've gotten smaller since I created this list over 20 years ago.

   Table of dressed lumber sizes.

   Designation	Actual Size	Designation	Actual Size
   2 x 4	1 5/8 x 3 5/8	2 x 10	1 5/8 x 9 1/2
   2 x 6	1 5/8 x 5 1/2	2 x 12	1 5/8 x 11 1/2
   2 x 8	1 5/8 x 7 1/2

6. Drafting.
   • Drafting Manual. The Drafting Zone is an excellent starting point for those who are not drawing to any particular standard.
   • Drawings.
   • Dimensioning. The two websites listed below provide a good overview of aircraft dimensioning and the definition of station lines (STA), water lines (WL) and butt lines (BT).
   • Aircraft Dimensioning
   • STA/Waterline/Buttline
   • Estimating. Hours required to make a drawing. These numbers where used back when work was done on the drawing board. It still doesn't appear to be to far off, for ANSI standard drawings.
   • Sizes. Drawing sizes are multiples of 8-1/2 by 11 excluding F size. For example a B size is 11 by 17, and a C size is 17 by 22, etc.
   • Standards. Mil-Std-100G along with ASME Y14.100M, establishes the essential requirements and reference documents applicable for the preparation and revision of engineering drawings and associated list.
   • Scales. Drawing scales are specified in the form of a common fraction. (The ratio of the size of the object as drawn to it's full size). DOD-STD-100C documented this best. An excerpt from that standard is listed below.

   DOD-STD-100C standard scales.

   Scale	Fraction
   Full size	1/1
   Enlarged	8/1, 4/1, 2/1
   Reduced	1/2, 1/4, 1/8, 1/10, 1/20, 1/30, 1/40, 1/50, 1/60, 1/100

   • Engineering Drawing Manual. A Manual of Engineering Drawing for Students and Draftsmen by Thomas E. French, M.E., Second Edition.
   • Complex shapes. This is particularly useful when creating sheet metal parts, Mulit Layer Insulation (MLI), Printed Cable Assemblies, etc.
   • Engineering Drawing Standards Manual. This is Nasa's 1994 manual. I did not think I would stumble across this one.
7. Formulas
   • Bend Lengths. Sheet metal bend allowances.
   • Enclousure Venting. Electro-mechanical packaging.
   • Fill factor. Pwb use area.
   • Hole Sizes. Fixed and floating clearance hole calculations.
   • Ohms law & Pwr. Ohms law and power dissipation formulas.
   • Parabola. Parabolic formula with example.
   • Pwb Holes. Spacing and pad sizes.
   • Wave Length. Here is a rule of thumb for calculating the wave length in inches. Wave length (inches) = 11.8/frequency (Ghz). For example, the wave length in inches of a 10 Ghz system would be 11.8/10 or 1.18; therefore, if an element of the design is said to be 1/4 wave length, that would be 1.18/4 or .295. If you need a more accurate number, try this frequency calculator. Here is a handy chart of the different bands, uhf, ku_band, etc., and here is a handy dandy chart.
   • Wire Bundle. Electrical wire bundle diameters.
8. Hardware
   • Captivating hardware using Truarc "E" Rings.
   • Hardware Sizes.
     • Standard. Dimensions of hardware ranging in size from No. 0 to 1/2, with the appropriate Military Specification listed.
     • Miniature. Dimensions of hardware ranging in size from .30 mm to 1.40 mm (UNM) metric and No. 0000 to no. 1.
9. Military Part Numbers and the appropriate dash numbers for ordering cres hardware in sizes #0 to 1/2 up to 3 inches long. Screws and bolts are in Table 1, washers and nuts in Table 2.
   • Look up hardware part numbers by specifing size, length and type.
10. Screw Sizes. Screw diameters advance .013 starting with #0(.060) up to 1/4. Example: #1=(.060+.013=.073), #2=(.060+.013+.013=.086), etc.
11. Screw Lengths (Inch). Lengths of pnh screws typically increase by standard increments: (.062) for .125 to .625 long, (.125) for .625 to 1.500, and (.250) for 1.500 to 3.000. Keep in mind each size has length limitations: (.086)#2=1.000 max, (.112)#4=1.500 max, (.138)#6=2.000 max, (.164)#8 through (.375)3/8=3.000 max. Reference MS51957, Screw, Machine, Pan Head, Cross-Recessed, Corrosion-Resistant Steel, UNC-2A. One exception to the rule would be AN3, where screw lengths can be furnished in 1/32 increments, starting at 1/2 inch. Reference AN3, Bolt - Machine, Aircraft (S/S By NASM3THRU20)

    NOTE: You can purchase non- standard lengths of screws and bolts by constructing your part number using the appropriate length code of the style of screw you want (For example a NAS1351N06H32 would yield a 2 inch long screw, not available, according to the specification). Work with the supplier to determine availability.

12. Thread Binding External UNJ thds iaw MIL-S-8879 are not compatible with standard UNF, UNC, and UNEF nuts or tapped holes. Reference MIL-STD-454, guideline 12, par. 4.1.1.b and MIL-STD-1515, requirement 119, par. f.
13. Threaded Inserts. Keenserts, T-serts, Helical inserts (helicoils) and Rivet nuts (Rivnuts) are covered here with sizes ranging from no. 1 (.073) to 1 1/2 diameter.
14. TorqueRequirements. Screws sizes No. 0 to 3/8 are covered.
15. Rivet upset head dimensions. Here you will find upset head dimensions for rivets ranging in size from 1/16 to 3/8 in diameter. The edge distance to the sheet or joint should not be less than two times the rivet diameter in sheet stock and three times the rivet diameter in non-ferrous materials. Generally, rivet spacing is called out on all drawings in accordance with stress requirements. Reference MIL-STD-403B, Preparation for and installation of rivets and screws , Rocket and Missile Structure.
16. Holes
    • Counterbores the hole sizes listed here are created with a single tool, machining both holes at the same time, reducing the overall cost of the part. If your parts are toleranced such that non-standard counterbore sizes are required, the cost of the part goes up. Three basic styles are listed (Standard, 1/64 oversize, and 1/32 oversize) with the recommended screw size (No. 4 through 1/2).
    • Countersink hole sizes for 100 degree rivets (MS20426) used in painted and unpainted surfaces, as well as standard machine screw countersinks for 82 and 100 degree head styles.
    • Drill Sizes and tables listed here are excerpts from the Machinery's Handbook. A chart containing smaller PCB drill sizes can be found at the Drill Technology website.
      • Fractional 1/64 (.0156) through 1 inch (1.00).
      • Numbered and Lettered No. 80 (.0135) through Y (.404).
      • Metric 1mm (.0394) through 25mm (.9843).
      • Combined List All three categories mentioned above.
      • Drill Drift Twist drills tend to drift, when the depth exceeds five times the diameter. Drawings should allow for this drift with true position tolerancing or a notation which specifies the maximum allowable drift.
      • Oversize. Twist drills have a tendency to cut oversize. This variation in size should be considered when determining minimum and maximum values of tapdrill sizes.
    • Hole sizes recommended for standard screws (using the coordinate dimensioning method), ranging from No. 2 to 1/2.
      • Feature Size. Limits of size based on Air Force-Aeronautical Design Standard AND10387, Drill Sizes and Drilled Hole Tolerances-Twist.
      • Feature Location Tolerance. Formula for calculating hole size relative to position.
      • Impedance hole sizes, for maintaining a specific impendance around a pin using an air or teflon dielectric as the constant.
17. Machining
    • Dowels and dowel pinning practices. This will give you some insight into pin clearances. Press fit, slip fit (normal and gear box pinning) and tight fit, are covered. Diameters range from 1/16 to 1/2. For a detailed description of classes of fits and aditional tables consult, Machinery's Handbook and Prefered Limits and Fits for Cylinderical Parts, ASME B4.1.
      • Pin Depth. Pins shall be pressed into the tight hole to a minimum depth of 1-1/2 times the nominal pin diameter, and a maximum of 1/2 the pin length.
      • Pin Styles.
        • When pressed into a blind hole, pins 3/16 diameter and larger may have a 1/16 flat on one side.
        • A standard slip fit in one hole and a slot in the other will prevent having to ream at assembly.
        • A diamond shaped pin used together with a round pin will permit a larger locational tolerance than two round pins.
      • Reamers. Standard reamers come in either fractional , numbered, or metric drill sizes.
      • Pre-Drilling. If neccessary the pin or hole can be sized at assembly, providing a fit of .0002 to .0008 clearance.
    • Milling
      • End Mill Depth. All end mills will deflect during the machining operation. This table will guide you in determining mazimum milling depths. If a greater depth is required consult your local Machine Shop.
    • Knurling Data. Standard diametral knurling tools are designed to be used on common fractional material sizes. Decide what diametral pitch you prefer and whether you want ornamental or press fit knurling. Additional information can also be found in the Machinery's Handbook and ASME B94.6, Knurling.
      • Design Considerations.
        • When knurling to a shoulder, provide an undercut as in threaded parts.
        • A chamfer at the end of the knurled section allows for a lead in for press fits and improves the appearance of ornamental knurling.
        • The tolerance on the length of the knurl should be as large as function and appearance will permit.
        • Diagonal and raised diamond knurling do not have a tolerance on the angle of the knurl; therefore, it should be specified as reference only.
        • Pressing knurled metal parts into plastic materials should be verfied by testing, since both the type of plastic and the size of the cross section are of considerable importance.
      • Knurling Tool Sizes. American Standard ASA B5.30 covers knurling tools with standardized diametral pitches and tolerances. Using the following table takes advantage of the most economical knurling practices.
      • Ornamental Knurling. There are three types of knurling; diamond, diagonal, and straight. A diametral pitch of 96 with a 30 degree helix angle should be considered standard for raised diamond or diagonal knurls. Diagonal knurling shall be specified as either right or left hand. Tolerances should not be applied to outside of raised knurls. Click here for sample drawing callouts.
      • Press Fit Knurling. Since the tolerances must be tightly controlled it is recommended that the after knurling dimensions be selected from the tool sizes section, the tolerances selected from these tables and the drawings be dimensioned as shown here.
18. Marking, Identification.
    • Font Sizes (Point System). When type is used as a method of printing characters or numerals, the height is designated by a dimension equal to 1/72 of an inch, called a point.
19. Printed Wiring Board
    • Capacitors
    • Chokes and diodes
    • Fill Factor
    • Resistors
    • Pwb Hole Calculations (spacing, hole and pad sizes). (Pwb use area).
20. Sheet Metal Forming
    • Aluminum Alloy.
      • Bend Radii charts. There are two tables or charts shown here one of which is an excerpt from The Aluminum Association Inc.'s data book The use of bend radii smaller than specified within should be avoided as hair-line cracks in the radius surface may appear. The sizes range from .016 to .500 and are grouped by alloy.
      • Bend Radius, Minimum. Where absoulutely necessary 5052-H32 may be bent to thickness of material, and 6061-T6 to 1 1/2 thickness of material.
      • Bending Guidelines.
        • ANSI Y14.10-1959 Section 10 of the American Drafting Standards Manual, Metal Stampings . This is cancelled, but an excellent resource and can still be ordered.
        • BendWorksThe fine art of sheet metal bending.
        • FED-STD-187B Identification of pressed bends, forms, seams, and joints (3 Jun 1963).
        • Robust design features
        • Sheet Metal Handbook
        • Wrico Stamping Co. Stamping guidelines.
      • Complex shapes, Examples how to develop different shapes, such as funnels.
      • Length of Bend, formula for the calulation of.
    • Punch sizes This section is a must for those designing sheet metal parts. Punch sizes for round, rectangular, oval, hex, and square, are typically furnished in fractional sizes. All others are considered to be special.
      • Round.
      • Rectangular and square.
      • Oval (Obround)
      • "D" Shaped, double "D", and hex.
    • Spinning. Acme Metal Spinning can spin diameters ranging from .50 to 6 foot and a thickness up to .75, using materials such as aluminum, mild steel, and stainless steel.
    • Steel Alloy, bend radii. An alloy with a carbon content of .10 or less may be bent over on itself.

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21. Wrench Sizes: Of course Snap-on Tools is always a good resource.
    • Allen Wrench sizes (actual measurements taken for Sears Craftsman Allen Wrench Set).
    • Hex Balldrivers For those hard to reach fasteners.
    • Nut Driver sizes (actual measurements taken for Sears Craftsman Nut Driver Set).
    • Standard Socket sizes with applicable nut part number.
    • Military Socket sizes in accordance with Federal Specification GGG-W-641 (CANCELLED WITH NO SUPERSEDING SPECIFICATION).

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Last updated on March 14, 2010