Metric hole tapping

[griffdo]

I have a threaded shaft, .2770 O.D. The threads appear to be metric with 6.5 mm threads (measured with a metric thread gauge). Question: What size drill for the hole do I use and what size tap do I use? Looking at a drill chart, would I use a "B" drill (.238) and a M7 X 1 tap as the chart shows? Is the M7 tap a .2756 diameter that will produce the hole required? Thanks.

DLG

 

[diamondjim]

Metric threads are quite simple.
If you know the thread diameter and pitch
as in this case the tap drill is simply
7 - 1 = 6mm drill (.236&quot, i.e. b drill
looks good. The thread size is the od
of the bolt or shaft and is 7mm (.2756&quot.

 

[Drazen]

Hi,
Tell me, what is 6,5 mm thread? Where did you take that dimension?
For M7 root diamtar is 5,9 mm. You must check the pitch - it should be 1 mm for normal M7 (M7 fine will have 0,75 mm pitch but it is not expectable that you will find tap for this one - hope it's normal M7).

On the other hand, for M8 thread root diametar is 6,65 (pitch should be 1,25 mm for normal M8.

It is critical how did you find 6,5 mm. If you have thread gauge that gives you pitch diameter (not root diameter that you can measure with vernier caliper), than 6,5 pitch diameter points to fine M7x0,75 thread.

That is crutial to discover. (I know that you checked shaft o.d.- check it once more!) When you find it, you can use formula for drill size:
Drill Size [mm]= Outside Diam - (.974 x Pitch in mm).
For M7 and M8 it is obviously outside diam. - 1mm.
Regards

 

[electricuwe]

M7 isn't a commonly used dimension ! Are you sure that it is a metric thread ?

 

[site]

Usually for general purpose thread the tap size drill will be outside diameter of the thread minus pitch of the thread ,so you have to measure the pitch of the thread and find out side dia of the thread.

 

[pvdvyve]

Hay,
Looks rather strange to me as an European, see you talking about M7, because as 'electricuwe' already said that's not a common dimension. We have M6 (normal-M6 and fine-M6x0,75), we have M8 (normal-M8 and fine-M8x1 tol classes 6g or 4H for example) as common dimensions.
See DIN 13, where a M7 is defined.
This is just a note in the forum, the data you requested is in the other replies.
Succes. Greetings from Belgium (Europe).