Counterboring,Spot-facing and Tapping

Counterboring,Spot-facing and Tapping are very important machining to form the holes on the crate mould making and they use different kinds of machines:



Counterboring is an operation intended to enlarge, for part of its depth, a hole previously drilled and produce a shoulder at the bottom of the enlarged portion. True seats for fillister head machine and cap screws are provided by counterboring, spot-facing, and countersinking operations

Counterboring is the same as flat-bottom drilling or end milling, except that there is a pilot in the center which fits a previously machined hole that is smaller in diameter than the counterbore diameter.

Spot-facing is the same operation as counterboring, except the cut is made only on the end to face a boss or to provide a seat on the top of a plane surface.

The pilots in the counterbore and spot-facer must fit the drilled hole, and are usually attached to the counterbore in such a manner that various sizes can be used on one or more sizes of the counterbores. Therefore, in order to avoid confusion in the shop, the designer should endeavor to standardize on counterbore and pilot diameters and their combinations. National counterbore standards have been established for the various machine screws and bolt sizes.


TAPPINGTapping is the cutting of internal threads within a hole that has been prepared by an operation such as drilling, boring, or coring. The preparation of the correct-sized inside diameter is very important in tapping operations. If the hole is too large, then only a portion of the desired thread will be produced and the resulting holding power of the mating member will be reduced.

If the hole is too small, then the tap will itself have to open the hole and this undue strain will result in excessive tap breakage.

Tap Selection. Selection of the most favorable tap must be based on the material being worked, the accuracy required, the length of thread, and the type of    tapped    hole (whether through hole    or blind    hole).

The following    general    information will assist in the    selection    of the best working taps:

  1.    Use cut-thread taps only where commercial accuracy is satisfactory. For class 2 fits, use commercial-ground taps; for class 3 fits, use precision-ground taps.
  2.    Two-fluted taps are best used for tapping deep holes where there is a tendency for    chips to    clog and break the taps.
  3.    Three-fluted    taps are    used for cutting softer materials. Used to    a large extent for cutting blind holes.
  4.    Four-fluted taps are used on materials such as cast iron where chips break up readily and are easily washed away. Used in hand tapping.

Design Factors. In order to minimize tapping costs, the following factors should be observed by the production design engineer:

  1.    Specify standard threads.
  2.    Do not specify closer tolerances than necessary. Class 2 thread tolerances are usually satisfactory for most work.
  3.    Select materials that can be easily tapped.
  4.    Provide adequate clearance on blind holes. This should be 2% to 3 * times the pitch of the thread.


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