There are six main steps to consider as you prepare to order a tap for any application.
 
1. Determine just what kind of tool you need to satisfy the requirements for your application.   
 

      a. Are you just trying to put in a few threaded holes in a piece of relatively easy to machine material? If so, the general-purpose tools that are found in most catalogs are most likely adequate for your application.  
      b. Maybe you have many thousands of threaded holes to complete on very expensive parts that cannot have any rough or poor quality threads. In this case consider tweaking the application by selecting High Performance Taps that are material specific, and made to give long life as well as a high quality finish. The costs will be higher for each tool, but the results obtained will in most cases give a lower cost per hole, and superior results.   
 

2. Determine the style of tool that will be required  
      
 
        a. Hand Tap: Straight flute taps that store the cut chips in the flutes. 
        b. Spiral Point Tap: Straight flutes with an angular left hand grind near the front of the tap that shoot the chips forward in front of the cutting action in thru holes. 
       c. Spiral Flute Tap:  A tap having a right hand spiral in its flutes and tend to lift the chips out of the predrilled hole. These work best in blind holes.
       d.  Form Tap:  A tap made to extrude the thread into the predrilled hole without making any chips in the hole. These provide a very fine finish in the hole and work particularly well in softer ductile materials as well as deep hole application. 
 
3. Determine the chamfer required on the tap style you have chosen. 
       
      a. Taper: This is a chamfer length of 7-10 incomplete threads on the front end of the tap. This chamfer is used in thru hole applications where the chamfer length is not objectionable. 
      b. Plug:  This is a chamfer length of 3-5 incomplete threads on the front end of the tap and is the most commonly selected of all chamfer lengths.
      c.  Semi-Bottoming:  This chamfer length is 3-4 incomplete threads of the front end of the taps and is used where it is necessary to thread closer to the bottom of a blind hole, but still have a lot of cutting action. 
      d. Bottoming:  This chamfer is the least desirable and is 1-2 threads in length of incomplete threads on the front end of the tap. This is used only were it is important to get very close to the bottom of the predrilled hole with full thread. This chamfer should be avoided where possible, as it requires a very large chip load per tooth and can give much shorter wear life than the longer chamfers.  
4. Determine Class of Fit required by the part manufacturer on the print .
     
      a. There are three Class of fit for all internal unified series threads in the USA   
         i.      Class 1B: Loose Fit
        ii.      Class 2B: Medium Fit
       iii.      Class 3B: Tight Fit

      b. There are three class of fit for all external unified series threads in the USA   
         i.      Class 1A: Loose Fit
        ii.      Class 2A: Medium Fit
       iii.      Class 3A: Tight Fit
 
5. Determine H-limit required on the tap (Tap Tolerance)  
      
      a. H-for high above basic Pitch Diameter 
      b. L-for low below basic Pitch Diameter 
      c. H- Limits are in .0005 increments 
      d. Place the H -Limit at approximately 40% of the total tolerance of the required Class of Fit  

6. Determine the Surface Treatment, or Thin Film Coating
  
       a. Surface Treatments
           i.      Black Oxide: Works best in ferrous (iron based) materials
          ii.      Nitride: Use in non-ferrous materials such as aluminum
         iii.      Oxide over Nitride: Works well in steels, stainless, and nickel alloys
      b. Thin Films 
           i.      TiN: Woks in broad range of materials such as steels, irons, and plastics
          ii.      TiCN: Great in steels and irons
         iii.      TiAlN: Use in PH stainless and inconels
         iv.      CrN: For use in brass, bronze, zinc, and magnesium, replaces chrome
          v.      TiN + CrC/C: Combines wear resistance and lubricity. Use in 300 series stainless, aluminum, and titanium

 
 
 
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Kennametal-Greenfield Round Tools