Tooling & Production December 2006

"Shop Talk with Steve Rose"

The Author, Steve Rose

Back to basics: taps


Once again we left you thinking about threads and taps.  Last month we ask for the difference between a cut tap and a roll tap. Well, just like the name says, a cut tap cuts the material to form the thread.  A roll tap forms the thread through metal displacement; it pushes the material into the thread shape.


Both taps create an ID thread, but a roll tap is a stronger tool and has the advantage of not producing chips.


No matter which type of tap you select, the ease of use depends a lot on your pre-machining planning.  When you plan ahead and select the most appropriate drill size first, you give yourself a head start to successful tapping.


Look around your shop do you see a drill and tap wall chart?  Most companies have these charts that advise a drill size to be used when tapping a specific thread. The drill sizes recommended on these charts are a starting point -- as with most things we may need to investigate further to obtain the optimum drill size.


Why does the drill size matter? Well if you drill the hole too small you can expect to break taps.  If the hole is too big your thread form will be incorrect.  If the wall charts are just starting recommendations how can you find the best drill size?


Recall from our past discussions there are three diameters on a thread: the major, minor and the pitch diameter.  Look up the thread size in your Machinist Handbook.  Each of these diameters has a tolerance range.  For an internal thread, the minor diameter represents the largest drilled hole allowable inside the part.  Use the tolerance of the minor diameter to determine the best pre-tap drill size.  Dont be surprised if this drill size is not the same as that listed on the wall chart.


Selecting a more accurate drill size in the beginning makes the tapping process much easier. Be aware that as you increase the size on the drilled hole you must be sure to gauge the resulting thread to ensure the minor diameter is within tolerance.


The larger the initial hole, the less material will be removed by the tap resulting in a reduction in potential tap breakage.


Another aspect of pre-machining planning is selecting the style of tap chamfer.  In tooling catalogs, these styles are called bottoming, plug and taper style taps.  Longer chamfers (taper and plug taps) help to distribute the cutting force along the length of the tap.  The shorter chamfer means there is greater force as the tap enters the drilled hole.


The length of each individual chamfer is determined by the thread size.  Each tap chamfer style length is based on the pitch of the thread being made with the tap.

Chamfer lengths


*bottoming tap chamfer length = 1-2 times pitch length

*plug tap chamfer length = 3-5 times pitch length

*taper tap chamfer length = 7-10 times pitch length


Using a 1/4-20 UNC tap as an example, the pitch length of this thread is 1/20 = 0.050.  For each type of tap the chamfer lengths are:


Bottoming tap:  0.050 0.010

Plug tap:  0.150 0.250

Taper tap:  0.350 0.500


Taper taps are rarely my first choice.  I tend to select a plug tap, depending on the clearance allowance specified on the print.  Bottoming taps are good choices when there is minimum clearance at the bottom of the hole.


A blind hole with insufficient clearance can be frustrating.  Think about the problems caused by this situation and possible solutions.  Well meet back here next month to continue the topic.


As an aside, it seems that some design engineers dont give enough consideration to the blind hole clearance.  When you have a blind hole with minimum clearance between the drilled hole and the tapped hole, its not just the machinists who suffer.  Tap breakage due to insufficient clearance is more then just frustrating to the machinist, it increases production costs in terms of material, tooling and time.


A basic tap has a straight flute with a spiral point.  This type of tool forces the chips into the drilled hole.  Again, with minimum clearance, forcing the chips into the bottom of the hole can promote tap breakage.


An alternative is a turbo tap.  This tool uses a high spiral flute to force the chips out of the hole.  In my experience a turbo tap requires more force to feed forward but it does evacuate the chips.


When chips are a problem, a good alternative is a roll tap.