Size Tolerances

A basic step in order preparation is specifying size and tolerances. Every application is unique in that each has an optimum strip size with a compatible tolerance range. It is essential that tolerances are expressed as equal values above and below a nominal (target) dimension. In the use of Statistical Process Control (SPC) techniques, it is always beneficial to aim for the center of the specified range so the variations realized will be centered at the nominal and be maintained well within limits.

Strip thickness and width should be given in decimal inches or millimeters. When cut lengths are required instead of typical continuous coils, the length should be expressed in inches or feet (centimeters or meters). The use of gauge numbers or fractional dimensions is less accurate, and therefore undesirable.

In general, as strip width, thickness, or length increases, it is more difficult to hold close tolerances. ASTM has published widely accepted tolerances for different widths and thicknesses. The following table lists Theis Precision Steel thickness tolerances which are considerably tighter than ASTM standards.

Width Tolerances for cold rolled steel
Edge Width Thickness Tolerances
no. in. (mm) in. (mm) +/- in. (mm)
1 To 0.75 (19) incl <0.0938 (2.4) 0.003 (0.08)
  Over 0.75    
1 to 9.5 <0.125 (3.2) 0.004 (0.10)
  (19.1 to 241) incl.    
3 0.094 to 14.0 To .025  
  (2.4 to 355) incl. (0.64) incl. 0.004 (0.10)
  0.375 to 14.0 Over .025  
3 (9.5 to 355) incl. (0.64) to .068 0.005 (0.13)
    (1.7) incl.  
    Over .068  
3 0.5 to 14.0 (1.7) to .099 0.005 (0.13)
  (12.7 to 355) incl. (2.5) incl.  
5 To 0.75 (1.90) incl. <0.0938 (0.24) 0.004 (0.10)
5 Over 0.75 to 5.0    
  (19 to 127) incl. <0.125 (3.2) 0.005 (0.13)
5 Over 5.0 to 9.0 .008 (0.02) to  
  (127 to 228) incl. .125 (3.2) incl. 0.005 (0.13)
5 Over 9.0 to 14.0 .015 (038) to  
  (228 to 355) incl. .105 (2.7) incl. 0.005 (0.13)

Tighter tolerances can be achieved. Consult your Theis representative.


Thickness Tolerances for cold rolled steel A,B
  Width, inches(mm)
   Thickness tolerances in inches (mm) over and under
Specified thickness Up to 3" (85 mm) 3" to 6" (85 to 170 mm) Over 6" to 14"
  excl incl (170 to 397 mm) incl
       
Under 0.01(0.254) 0.0003 (0.008) 0.0003 (0.008) 0.0003 (0.008)
0.010 to 0.015 (0.25 to 0.38) incl. 0.0004 (0.010) 0.0004 (0.010) 0.0004 (0.010)
Over 0.015 to 0.020 (0.38 to 0.51) incl. 0.0005 (0.013) 0.0005 (0.013) 0.0005 (0.013)
Over 0.020 to 0.030 (0.51 to 0.76) incl. 0.0005 (0.013) 0.0005 (0.013) 0.0005 (0.013)
Over 0.030 to 0.040 (0.76 to 1.0) incl. 0.0008 (0.020) 0.0008 (0.020) 0.0010 (0.025)
Over 0.040 to 0.070 (1.0 to 1.78) incl. 0.0010 (0.025) 0.0010 (0.025) 0.0012 (0.030)
Over 0.70 to 0.100 (1.78 to 2.54) incl. 0.0012 (0.030) 0.0012 (0.030) 0.0015 (0.038)
Over 0.100 to 0.125 (2.54 to 3.18) incl. 0.0015 (0.038) 0.0018 (0.046) 0.0020 (0.051)

When extra-close tolerances are required, depending on the size and condition, additional processing can be performed to achieve these tighter tolerances (i.e. thickness tolerances of +/- .0001" [0.0025 mm] or width tolerances of +/- .0005" [0.0127 mm]).

Computer-assisted rolling mills with automated gauging

Every major rolling mill in operation at Theis Precision Steel utilizes state-of-the-art computer controls. The systems are closed-loop, feed forward mass-flow control using non-contact gamma-ray gauges at the entry and exit of the mills. The computer automatically adjusts the roll pressure to assure uniform thickness throughout the entire length of the coil and maintain the tightest controls in the industry. Every coil which we produce off our mills requires a minimum Cpk value. Each mill can produce an SPC printout showing statistical and operational results (see Control Chart below). These records are maintained on file within our computer database and can be retrieved at any time.

Control Chart Characteristics: Strip thickness

Exit thickness distribution


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