Threading: Drill Size Versus Tap Size

[GreyShark]

I couldn't think of a way to phrase this question that was yielding the right results either on the net or on the forums so here it is...

If I'm drilling a new hole to tap how do I know what size hole to drill?

The funny thing is I own a tap and die set and I have in fact successfully tapped a few holes but the hole was always pre-existing and I always knew exactly which tap to use because I was following instructions for a specific project. Now I have a couple projects that require drilling and tapping but I realize I don't understand the fundamental principles behind the process.

 

[Mick]

Usually your tap and die set has the correct drill size listed in the box that it came in. If not google "tap drill size"

 

[Th232]

Personally, I just look up a table. Here's one for metric, and one for UTS.

 

[mahoney]

Google "tap drill chart". There's a bunch online, just look for the thread you are tapping and the correct tap drill should be listed, as either fractional, number, letter, or metric, depending on the thread. Most charts are calibrated for 75-80% thread, so you can use a drill that's close in size a few thousandths either way and usually still get OK threads if you don't have the exact size suggested.

 

[GreyShark]

Awesome! Thanks!

Now one more question. One of the projects I'm working on is a custom light I want to have machined by somebody else as I noted in my thread about drawings. If I want a given thread on the tail cap how would I specify it because I'm pretty sure a drill bit would tend to drill a hole slightly larger than its stated size.

 

[LLCoolBeans]

Awesome! Thanks!

Now one more question. One of the projects I'm working on is a custom light I want to have machined by somebody else as I noted in my thread about drawings. If I want a given thread on the tail cap how would I specify it because I'm pretty sure a drill bit would tend to drill a hole slightly larger than its stated size.

The machine shop will know how to drill and tap a hole. You won't need to tell them what drill to use, it's their business to know what drill to use. What you can do is specify the percentage of thread engagement. The usual is 75% and that is, I believe what most of the charts are based on. That's probably what you want anyway, unless you have a specialized purpose. All you need to do is tell them what thread you want.

If you pick up a copy of the machinery's handbook, it will have far more extensive and detailed charts.

 

[65535]

You specify the major diameter and pitch. I doubt a drill will be used for large threads, usually it is turned then threaded on a lathe with indexed inserts.

 

[KC2IXE]

You specify the major diameter and pitch. I doubt a drill will be used for large threads, usually it is turned then threaded on a lathe with indexed inserts.

abd sometimes - usually "fit class" - usually -2b

 

[modamag]

GreyShark, try out this post.
The # and metric taps size does not show up anymore in the pictures, but you can download it from RapidShare