DMAjic Consulting

SHEETMETAL PRODUCTION STANDARDS

PURPOSE: This procedure specifies the criteria to be applied to metal parts manufactured for COMPANY (hereinafter referred to as COMPANY) when this criteria is not specified by the customer.
SCOPE This document specifies workmanship standards for fabricated metal parts. It is intended to supplement COMPANY drawings by providing specific information not included on all drawings.
GENERAL REQUIREMENTS: All fabricated parts shall meet or exceed all applicable requirements of this standard, except where otherwise specified.
The following sections define general product tolerances and specifications for sheet metal parts and stampings. These requirements shall be observed unless otherwise stated by customer drawings or the purchase order. Dimensions and tolerances specified apply to finished parts, not to raw materials.
Shearing: The part must be sheared square to within .015 inch per foot. The straightness of the cut shall not deviate more than .005 inch per foot. When the note “Remove Burrs & Sharp Edges or Corners” appears on the drawing, the burrs shall not exceed .005 inch and the corner shall not be radiused or chamfered more than .030.
Flatness: The flatness is checked by placing the test material on a true flat surface without clamping or weights and measuring the variation of the material surface from the plane surface
Surface Flatness Tolerance: Material thickness less than .062 thick and less than 24 inches long must be flat to within .060 inch. Material longer than 24 inches must be flat to within .125 inch. Material thicker than .062 and shorter than 24 inches must be flat to within .125 inch. Material longer than 24 inches must be flat to within .187 inch.
Bending:

Right angle bends: Angles not specified on the customer’s drawing should be assumed to be 90 degrees with a tolerance of +/- 30 minutes.


U-shaped bends: Unless otherwise specified, the dimensions should be measured 1/16 inch above the tangent point of the radius.


Bend radii: Unless otherwise specified, the bend radii should be no greater than the material thickness unless the material delaminates or the surface cracks.


Bend relief: Where flanges extend over only a portion of a part, a notch or hold bend relief should be provided to prevent tearing of the material. The minimum bend relief equals material thickness plus the bend radius.


Minimum flange dimensions: The minimum recommended flange dimensions should be at least 3.5 times the material thickness plus the bend radius.

Punching Holes:

Material thickness versus hole size: The minimum hole size is usually limited to the material thickness.


Distance from a hole to a bend: The minimum inside dimensions from a bend to the edge of a punched hole should be 1.75 times the material thickness plus the bend radius.


Burrs on punched holes: The burr should not exceed .005 inch.

Punching Holes:

Material thickness versus hole size:  The minimum hole size is usually limited to the material thickness.

Distance from a hole to a bend:  The minimum inside dimensions from a bend to the edge of a punched hole should be 1.75 times the material thickness plus the bend radius.


Burrs on punched holes:  The burr should not exceed .005 inch.


Assembly:

Riveting:  Sheets shall be pulled together tightly at the junction of the rivet shank.  The separation between sheets shall not exceed 25% of the thickness of the thinnest sheet being joined or .010 inch.  Whichever is smaller.


Head Seating:  The head of the rivet should normally sit flush to the mounting surface.


Tool marks and cracks:  Surfaces of joined material and rivet heads should be free of visible cracks or distortion.  Radial cracks on the rivet no more than .010 inch in width at the outer edges of the rivet are acceptable.  Tool marks or other indentures in excess of 25% of material thickness in depth or width are not allowed.


Inserts and Studs:

Pressed or welded inserts:  Inserts should be installed flush to the surface of the material.


Flush pressed nuts:  Flush pressed nuts must be flush +/- .010 inch and shall not protrude from the bottom side of the material.  The nut must also be perpendicular to the sheet within a tolerance of +/-5 degrees.


Pressed studs:  Flush pressed studs must be pressed in place and be flush within +/-.010 inch.  In addition, they must meet the minimum, bend test requirements.


Welded studs:  Studs shall be tested by the bend test and a visual examination.  Welded studs must not show evidence of heat distortion.  Studs must show proper weld penetration.


Bend test:  With a suitable tool, bend the stud a close to 90 degrees from perpendicular as possible.  An acceptable weld should not break.


Welding:

Electric arc welding:  Arc welding is the melting and fusion of the metal joint by heating with an electric arc between an electrode and the work.


Gas welding:  Gas is used when metals to be welded are melted by gas, such as oxyacetylene.  a filler rod may or may not be used.


Resistance Welding:  Resistance welding is when work pieces are joined by a combination of heat generated by electrical resistance and force exerted by the electrodes.


Penetration:  Weld penetration into each outer piece shall not be less than 20% of the thinner of the pieces being joined.


 Indentation:  The indentation of the spot shall not exceed 10% of the thickness of the material being joined.


Sheet Separation:  Separation between sheets shall not exceed .015 inch.


Quality and Appearance:  Resistance welds shall be sound, clean and free of foreign matter.  Welds shall be free of excessive bulging, spitting, surface burning and blown spots.


Electric Arc and Gas Welding:  Welds shall be sound, clean, and free from foreign materials and imperfections detrimental to the performance of parts.  Welds shall be free from excessive burning, cratering, undercutting, and lack of weld penetration.


Electric Arc & Gas Welding Penetration:  Penetration shall be complete and uniform and shall not show evidence of defects.


Cratering:  Cratering is allowed if it does not exceed 20% of the thickness of the parent material.

The weld should be of consistent width throughout its length.  It should be straight so that the two edges form two parallel lines.  The weld should be slightly convex and be of a consistent height throughout the weld.  The weld should have the appearance of being fused into the base metal, having a blended appearance and not having a distinct edge or undercut between it and the base metal.  The surface of the weld should have a consistently spaced ripple throughout its length.  The weld should have a clean appearance.  There should be no color spots, no scale on the weld, and no rough pitted appearance.  Lap welds and corner welds should normally show no visible penetration on the opposite side of the welded piece (commonly called burn through).  On butt welds, turn the piece over to check for penetration.  The penetration should be even along the complete length of the weld.


PAINT FINISHES:

Packaging:  Unless otherwise specified, the finished product shall be clean, neat and packaged to protect the parts during normal handling and transportation operations.  Painted parts will be placed into plastic bags or wrapped in paper.  Packaging will be of good commercial quality and designed to ensure finished part arrival in an undamaged condition.


Finishing operation marks:  Unless otherwise specified, operation marks caused by the manufacturing process are acceptable provided that they are uniform and exhibit good workmanship quality and are not in a critical appearance area.


Appearance:  When a finish is viewed from the required distance of 36 inches, the finish shall not exhibit any imperfections, blemishes or differences in color or texture that will immediately cause the viewer to focus attention upon them.


Lighting conditions:  The light source shall be a cool white fluorescent lamp with uniform illumination and intensity between 100 and 125 candlepower.


Inspection procedure:  The inspector shall scan the surface in a continuous manner at the 36 inch viewing distance.  any defects that are immediately obvious such as color mismatch or texture mismatch, shall be rejected.  It is acceptable to determine a texture and/or color match at a viewing distance other than that specified.  If determined necessary by quality control.


Threaded holes or fasteners:  Threaded holes or fasteners on painted parts do not have to be masked prior to painting unless so specified by PDS or the engineering process instructions.  However, in all cases, the painted fastener or threaded hole must accept its mating component or equivalent screw or nut under assembly torque conditions.  If required, the torque requirement will be determined by PDS.

Touch up paint:  Touching up paint is acceptable only when using a suitable matching color to blend in visually when viewed from the required viewing distance.


Paint thickness:  Paint thickness should be measured by using a suitable paint thickness gage such as the MicroTest Thickness Gage or equivalent.  Smooth paint thickness requirement is .5 to 1.5 mil.  Textured paint thickness requirement is 1.0 to 2.0 mil.


GENERAL TOLERANCES:

The following dimensional and hole size tolerances shall be observed unless otherwise specified by the customer:

 

                                                      FEATURE                                     TOLERANCE (+/-)

 1)  hole to hole                                                                        .005
 2)  bend to hole                                                                       .010
 3)  edge to hole                                                                       .010
 4)  hole to inserted hardware                                                    .010
 5)  bend to bend                                                                      .015
 6)  edge to bend                                                                      .010
 7)  bend to inserted hardware                                                  .015
 8)  edge to edge                                                                      .010
 9)  edge to inserted hardware                                                   .015
10)  inserted hardware to inserted hardware                              .015

 

Hole Diameter                                                                          Tolerance

<.250                                                                                       +.005/-.002
.251 to .500                                                                             +.008/-.003
.501 to 4.000                                                                           +.010/-.005
>4.000                                                                                     +.010/-.015

Raw Material Thickness Tolerance =  +/- 10% of Required Thickness

Clear Chromate Conversion Coatings on Aluminum:

Quality Coating:  To test for proper chromate adhesion, apply Scotch brand #600 plastic tape to the surface to be tested.  apply with uniform pressure.  within 30-60 seconds of application, remove the tape in a quick motion at 90 degrees to the surface.  For acceptance, the finish should not be removed from the base material.


Appearance Quality:  The finished should have a uniform appearance, be semi-bright, smooth and clear to slightly iridescent in color.  Visual appearance will vary between alloys and between machined, mill and cast surfaces.  Superficial stains, which are the result of liquid runoff from chromate or the rinsing process, are acceptable providing the stains are not excessive, numerous or predominately yellow.

Silk Screening & Pad Marking:

Appearance:  When viewed from a distance of 24 inches, the marking shall exhibit no imperfections which will cause the viewer to immediately focus on the affected area.  When marking over textured paint surfaces, a slight jaggedness of the edges of the masking is acceptable.


Adhesion:  To test the adhesion of silk screening and pad marking, Scotch brand #600 plastic tape should be applied over the marking with uniform pressure.  After 30-60 seconds of application, the tape should be removed in a quick motion at 90 degrees to the surface.  For acceptance, there should be no evidence of the coating pulling loose and adhering to the tape.

Zinc Plating With a Chromate Conversion Coating:

Physical requirements:  Unless otherwise specified, all cold rolled steel parts shall be zinc plated with a chromate conversion coating.  The plating thickness should be at least .00015 inch.


Quality of Plating:  The zinc deposit shall be smooth, semi-bright, adherent and free from blisters, pits, indications of burning, voids and other defects.  The thickness shall be measured only on visible surfaces which can be touched by a ball .75 inch in diameter.  All other surfaces are exempt from the minimum thickness requirement but must have plating coverage.


Supplementary Chromate Treatment:  Unless otherwise specified, zinc plating shall be given a clear bright or blue bright chromate conversion coating.


General Appearance Criteria:  Unless otherwise specified, the finish shall be a uniform appearance and be free from excessive drip marks,  coating residues, and other defects that may affect the protective value of the coating.  Ignore any slight discoloration caused by the process.  Ignore scratches covered by the zinc plate finish.  Ignore scratches showing base metal under 2 inches long.  Ignore small racking marks under ¼ inch in diameter.  Ignore areas of chromate discoloration up to 3/16 inch adjacent to spot weld seams.

Dimonte Group

This paper kindly supplied by the DiMonte Group