Tapping your knowledge

[MedMan]

From time to time I need to drill and tap a hole for some task I am doing. The most common sizes I need are for M5 or M6 machine screws. I have a set of taps that includes both these sizes but what size holes should I drill to start with? Does the answer depend upon the substrate? (i.e. is it different in stainless and aluminium?)

I shall be doing a job shortly that involves drilling and tapping 6 holes to take M6 machine screws to secure a plate to the top of the cast aluminium Bowman Heat Exchanger on my Mercedes OM636 engine. Any hints/suggestions/precautions would be gratefully received.

P.S. Why am I doing this? I fitted a calorifier a few years back that is slightly higher then the engine. I therefore fitted an external header tank above the calorifier and put a plain cap in place of the old pressure cap on the top of the heat exchanger itself. All worked well for several years. This season water started to seep around the base of the copper fitting on which the plain/pressure cap fits. The copper fitting appears to have been silver-soldered into the cast aluminium. As the cap (plain or pressure) is now redundant I intend to replace it with a bolt-on blanking plate rather than attempt to resolder the join. The copper fitting is, in any case, bent and I would really rather not attempt to remove the entire Heat Exchanger or risk too much heat on in in situ.

 

[Guest]

the size of the hole depends on the pitch of the thread. so if the pitch of the thread is 1mm you drill the hole 1mm smaller than the finished diameter. eg. M6 tap 1mm pitch = 5mm drill. the pitch can be diiffent for each size as you can get fine and course pitch in each size. the normal is course.
so for M5 you would normally drill 4mm
and for M6 you would normally drill 5mm

on larger taps say M8 the pitch is normally 1.2mm so you drill 6.8mm you can get this size drill from an enginerring merchants or i have tapped many holes using a 7mm drill

it dosnt change for diffrent materials.

 

[Guest]

the recommended tapping drill for 6mm thread is 5mm and for 5mm is 4.2mm. the corresponding core diameters are 4.77 and 4.02. substrate makes not difference but would suggest you use a jointing compound such as duralac for stainless screws into ali to prevent electrolytic corrosion.

 

[Strathglass]

The previous posts are spot on both in diameters and sugestions.
I always use a cutting compound. Not only does it extend the life of the taps it also will pick up some of the swarf.

Iain

 

[PuffTheMagicDragon]

Screw size minus pitch gives roughly 80 per cent of the required thread and this is acceptable for most situations. Regarding substrates however I would suggest that the quality of drill and your drilling skills also play a part. With stainless steel it is essential that you use a properly sharpened bit, otherwise it would rub and the stainless would work-harden very quickly. When drilling aluminium (alloy) I personally prefer to drill somewhat smaller because this is a softer material and therefore the more thread I have the better will be the resistance to failure. If at all possible try to use all three taps as this will give you more control over the quality of the thread; the smoother the thread, the stronger and the more resistant to corrosion. Use plenty of cutting oil, if possible applied continually by a mate; this is to prevent thermal shock and premature hardening of the ss.

Wally

 

[MedMan]

Thanks for your replies.

I found some information from a Google search that came up with:

http://www.tapmatic.com/tech_manual/index.html

Most of it is way over my head, but some of the tables are useful.

Thanks again.

 

[DavidGrieves]

I'm not an engineer, but from time to time I also have to tap a thread. There is a little book that engineers use as a refferance for all sorts of tasks it's called a "Zeuse" book ( not sure if thats the correct spelling! ) It's only a small book, fits in your top pocket and would answer your question and many more. Maybe somebody else out there can tel you more about it.

 

[DavidGrieves]

I'm not an engineer, but from time to time I also have to tap a thread. There is a little book that engineers use as a referance for all sorts of tasks it's called a "Zeus" book ( not sure if that's the correct spelling! ) It's only a small book, fits in your top pocket and would answer your question and many more. Maybe somebody else out there can tell you more about it.