Another broken tap thread

Edited By David Littlewood on 26/01/2012 16:32:59

Edited By Sid Herbage on 26/01/2012 22:53:39

Sid

I must apologise for having been slow in answering your question regarding possible lists on my website for the effect of producing external threads being made with reduced diameters, in short the answer is no. Its an interesting thought though as I find when producing threads on the lathe beyond M6 my three jaw is unable to withstand the torque required.

I suspect many readers of this thread have noticed a contradiction between my figures for percentage thread depth in relation to drill size compared to those as calculated using the Machinerys Handbook formula as mentioned by Hugh. Using a 5.2mm Machinerys Handbook results in a quoted thread depth of 73.9% whilst I state only 65%. The reason for this is as follows and I can only leave it with the viewer to chose which method suits the home workshop situation.

The reason lies in the quite different thread form of both Metric and Unified threads compared to earlier standards, BA, Whitworth, etc. I produced my DOS based spread sheets to calculate all the values about 20 years ago when the MEW data book was first appearing in the magazine. with this being so long ago my memory of the method was very hazy to say the least and have had to go back to view the formula that I wrote into the sheets, hence my delay in replying.

Incidentally, for anyone interested in using DOS, I have on my Vista PC a Virtual Windows 98 PC which has the facility of exiting to DOS, still back to the thread problem.

Considering first the external thread and the root of this. This conforms to the norm having sloping sides with a radius at its base. However, the external thread that mates with this has flats on its inner diameter so that there is a gap between the two. This being just like tapping any other thread form with a larger drill than the thread core diameter.

For my calculation therefore I have used the core diameter of the external thread and Machinery Handbook the larger internal diameter as produced by the flats on the tips of the internal thread. I have calculated this to prove that this method is what they are using.

As it is the norm to produce internal threads using oversize drills in the past and thread depths have been based on the core diameter of the external thread it seems illogical to use the internal diameter of the internal thread when considering tapping drill sizes.

Incidentally, using my method, Metric and Unified threads start with a thread depth of 88.2%

Whilst it has no bearing on tapping drill sizes, Metric and Unified threads also have a small flat on the outer tips of the external thread and a radius on the mating internal diameter. Because of this you will find that the outer diameter of a tap is larger than the thread's quoted size. Typically, an M6 tap will have a diameter in the region of 6.1mm (amount quoted from memory).

Even though the standard quotes flats, the tap and die manufactures are permitted to add a radius to ease manufacture though these must stay within the quoted dimension and not be on the top of the flat.

I intend to stay with my method of defining thread depth as I consider it more logical, also noting that Tubal Cain appears to have taken this approach.

Harold

Sorry to resurrect this but the acid hasn't worked (been at it three days) so I'm going down the cobalt drill route (set ordered) and need to know what speed to run the drill at. The set I've bought goes up to 6.5mm from 1.5mm in 0.5mm increments so which should I use (original hole was 5mm for the 6mm tap)?

Cheers folks!

John