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-Limit’s 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:
- Compensation for plating buildup
- Customer wants tighter threads
- Customer wants looser threads