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Shewhart X-bar and R limits are built from the average subgroup range rather than a computed standard deviation. The A2, D3 and D4 constants depend only on subgroup size and convert R-bar into three-sigma limits, which is why the method survives on a shop floor with no statistics software.
X-bar limits
UCL = X-double-bar + A2 x R-bar; LCL = X-double-bar - A2 x R-bar
R chart limits
UCL_R = D4 x R-bar; LCL_R = D3 x R-bar
Control limits describe what the process does; specification limits describe what the customer wants. Mixing them hides the difference between an unstable process and an incapable one.
Because D3 is zero for subgroups of six or fewer — with so few readings a range of zero is not statistically surprising.