DESCRIPTION OF TAP H/D LIMITS
The basic goal of all tapping functions is to insure that the external part (bolt) correctly mates with the internal threads (nut). Not only is it important that the hole is large enough to allow the bolt to enter the hole, but it is equally important that the hole is not so large as to create a loose fit. The H-Limit's are used to adjust the size of the pitch diameter to provide for a good mating of the external parts.
The H-Limit number is an indicator, which provides information regarding the specific pitch diameter (P.D.) of the tap. This indicator shows the amount over the Basic P.D. the Actual P.D. of the tap was ground.
The metric equivalent to H-Limits is D-Limits. The Metric tolerance for D-Limits is slightly larger than the tolerance for H-Limits, but for our purposes we will assume they are the same.
You can use the following chart to determine what the H-Limits P.D. is, notice that the tolerance for any h-limit is .0005 or +/-. 00025.
PITCH DIAMETER LIMITS FOR TAPS THRU 1 DIAMETER
H1 or D1= Basic plus .0000 to basic plus .0005
H2 or D2= Basic plus .0005 to basic plus .0010
H3 or D3= Basic plus .0010 to basic plus .0015
H4 or D4= Basic plus .0015 to basic plus .0020
H5 or D5= Basic plus .0020 to basic plus .0025
H7 or D7= Basic plus .0030 to basic plus .0035
H10 or D10= Basic plus .0045 to basic plus .0050
Where: Basic = Basic P.D.
Typical reasons for needing to order a special H or D factor tap:
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