Tapping a thread

[David Marks 2Participant @davidmarks2]

For anyone that has read the excellent article on tapping threads by Richard Simpson in the current edition (October 2015) of Model Boats. The following extra tips/comments may be of interest.

As Richard states, a common problem when producing a tapped hole is failing to keep the tap upright, resulting in a drunken thread. The tapping method detailed by Richard is fine, but if you find that the tap you are about to use has been provided with a `centre’ in the squared off stock end, then you can make use of it. Taps are often (but not always) `centred’ at both ends as part the production process. The centre is normally female but sometimes (on smaller taps) it may be a male centre. You will need the use of a lathe (or have a friend with a lathe) to make a Centring Pin. This is simply a short length of round bar say 6 or 8 mm dia., turned to a point with a 60 degree included angle. At the same time centre drill the opposite end of the bar to accommodate any taps with male centres. Place the Centring Pin in the chuck of the drill, engage the point (or centre drilled hole) with the tap and follow the tap down as it cuts the thread. Ha-presto – one `sober thread’.

Richard mentions that brass can be tapped dry i.e. it does not require a lubricant. Cast Iron, (although not a material normally encountered by boat modellers), falls into the same category, as it contains `graphite’ and therefore is self-lubricating. If you are tapping aluminium or Perspex, then paraffin is a good lubricant. There are proprietary tapping compounds available, supplied in `squinty tins’. The one I use is Screwfix own brand but I cannot find it in their current catalogue. However, I note that Toolstation market a Cutting & Drilling Cooling Lubricant – Stock Code 75018. This is probably not as good as a true `tapping compound’ but far better than cutting dry.

If you need to purchase taps and tapping drills, then in my opinion there is only one supplier to use….Tracy Tools of Torquay. They have a good web-site http://www.tracytools.com and a fast mail order service. They attend many model engineering shows and if you see their stand then get one of their free catalogues as it contains a page full of thread data i.e. threads pitches, tapping drill sizes etc.

Finally, due to a fluke of trigonometry, identifying the tapping drill for a metric thread is very easy. Simply subtract the `pitch’ from the `nominal diameter’ e.g. for an M10 X 1.5 thread, the tapping drill size is 8.5 mm.

[David Marks 2Participant @davidmarks2]

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