Tandem, originates from the Latin meaning “at length”, first seen in modern language in the late 18th Century to describe any two things working together, “in tandem”.
Close your eyes and say the word “Tandem”. What do you see? A bicycle built for two? A stroller made for two children? What about a semi-truck pulling two trailers down the highway? The more adventurous might envision a tandem sky-dive or a 2-person kayak.
For this blog, I will concentrate on Tandem Taps. This may not sound as fascinating as the examples already shared, but some knowledge is worth the pain. Tandem taps are a special type of tap that includes two or more threaded sections for creating internal threads.
The vast majority of these tools will be employed in the manufacture of “Acme” type thread-forms, as thread-forms that require the removal of a lot of material might require the use of sets of taps. The sets are comprised of “roughers and finishers” when provided in a set of 2 or more. The first tap produces a partial thread-form, and the second tap removes additional material necessary to complete the final form. A tandem tap can serve the same function, without using multiple taps, by combining a “rougher” and “finisher” on the same blank.
Although longer than a “single” tap, the tandem tap eliminates an additional tool change when sets of taps are required. Clearance for the extra length must be considered. Unless lead control on the machine is very accurate, this type of tap should not be reversed to retrieve from the finished part. As the “rougher” section of the tandem tap is not at full form like the “finisher”, it loses engagement in the part once the finisher clears. This allows a risk of damaging the finished threads as the tap is backed out. In the days before CNC control, tandem taps were usually removed from the holder once the finisher-section cleared the tapped hole, to avoid damaging the finished thread. Rigid set-ups are strongly recommended.
Tandem taps may also be an option when faced with machining a threaded hole with different diameters on the same axis, as long as the thread pitch of each is the same. By definition, combination drill/taps, like North American’s “Cost Cutter” are tandem tools. Although the potential benefit of using “combination” tools is apparent, these taps should definitely be considered “Application Specific”.