Cold Forming Internal Threads With Taps:
Internal threads can be produced by a cold forming or swaging process. The desired thread is formed in the metal under pressure and the grain fibers, as in good forging, follow the contour of the thread. These grain fibers are not cut away as in conventional tapping. The cold forming tap has neither flutes nor cutting edges and therefore, it produces no chips and cannot create a chip problem. The resulting thread has a burnished surface.
Care must be taken to minimize surface damage to the hole when tapping materials which are prone to work harden. This may be accomplished by using sharp drills, correct speed and feeds. Surface damage may cause torque to increase to a point of stopping the machine or breaking the tap.
Cold forming taps have been recommended for threading ductile materials. Examples of material classes which have been tapped are:
- Low carbon steels
- Leaded steels
- Austenitic stainless steels
- Aluminum die casting alloys (low silicon)
- Wrought aluminum alloys (ductile)
- Zinc die casting alloys
- Copper and copper alloys (ductile brasses)
Cold Forming Tap Application Information
Tapping Application The Same:
Except for changes in hole size, the application of cold forming taps differs in no way from conventional cutting taps.
Blind Hole Tapping Possible:
Whenever possible, in blind holes, drill or core deep enough to permit the use of the plug style taps. These tools, with four threads of taper, will require less torque, will produce less burr upon entering the hole, and will give greater life.
Where the operation calls for 75% of thread or less, the torque required varies with the material from no additional torque to 50% additional torque. On most applications, therefore, conventional equipment is suitable for driving cold forming taps.
No Lead Screw Necessary:
These taps work equally well when used in a standard tapping head, automatic screw machine, or lead screw tapper. It is unnecessary to have lead screw tapping equipment in order to run the cold forming tap because the tool will pick up its own lead upon entering the hole.
In general it is best to use a good cutting oil or lubricant rather than a coolant for cold forming taps. We recommend MQL Systems Dry-Cut Cutting Fluid.
For most materials, spindle speeds may be increased over those recommended for conventional cutting type taps. Generally, the tap extrudes with greater efficiency at higher RPMs but it is also possible to run the tap at lower speeds with satisfactory results.
Counter Sinking or Chamfering Helpful:
Because these taps displace metal, some metal will be displaced above the mouth of the hole during tapping. For this reason it is best to countersink or chamfer the hole prior to tapping, so that the extrusion will raise within the countersink and not interfere with the mating part.
Tapping Cored Holes Possible:
Cored holes may be tapped with these taps provided the core pins are first changed to form the proper hole size. Because core pins have a draft or are slightly tapered the theoretical hole size should be at a point on the pin that is one-half the required length of engagement of the thread to be formed. In designing core pins for use with these taps, a chamfer should be included on the pin to accept the vertical extrusion.
Drill Selector Chart:
The chart shown previously is based upon a formula derived from research statistical data and is designed to reflect the flow characteristics of all ductile materials. Laboratory experiment proved that there are only slight differences in the flow characteristics of the different metals as related to internal threading. It will be necessary to deviate slightly from the recommended hole size when tapping extremely ductile or extra hard metals.
The formula for these theoretical hole size determinations is as follows:
Theoretical Hole Size
To determine the proper drill size to form 65% of thread with a 1/4-20 cold form tap.