making threads: tap bit size vs drill bit size

[neskusen]

I know this info is "out there" but when you don't know the terms to search for and english is not the first language simple things like this feel confusing.
There are charts listing tap bit sizes and corresponding drill bit sizes but there are more numbers in the charts I am not sure what mean. I'd be thankful if someone would help me with this.
So this is the first chart that comes up in my search results: http://www.littlemachineshop.com/ref...drillsizes.pdf

First thing, does "Tap Drill" mean a drill bit that makes a hole which then an appropriate screw size tap bit can be used on, and "Clearance Drill" is the diameter of a drill bit which will create a hole through which an appropriate screw will simply pass through?
Second question, what is the purpose of "close fit" vs "standard fit" sizes for clearance drills?
"75% Thread for Aluminum, Brass,& Plastics", "50% Thread for Steel, Stainless,& Iron " sub-columns for the tap drill column, what do they mean? Different tap bits should be used on same screw size holes depending on the material? I would like to understand the point.
Finally, some metric screw sizes appear to have several "thread pitch" sizes. Should the same size tap be used in this case? For example should the same M5 tap bit be used for making threads for 1.0, 0.9 as well as 0.8 mm thread pitch M5 screws?

And an unrelated question, why do tap bits come in pairs, one with longer "blades"? I was told that the shorter one should be used first and then finished with the longer one. What does having two separate tap help to achieve which wouldn't be witha single tap bit?

[NobleGas]

With metric taps, a rule of thumb is to deduct the thread pitch from the thread diameter, that gives you the minor diameter of the thread. In softer metals, you can drill this size then tap; in ferrous metals, go slightly larger. For example, M6 is typically 6mm x 1mm pitch, so the minor diameter is 5.0mm. In aluminium, I would drill 5.0mm, in steel I would drill 5.2mm.
Taps typically come in 3 tapers, 1st, 2nd, & plug or finishing tap. In a thin plate with through hole, you could use the 1st taper tap to make the thread providing that you run the tap to its full depth; in a blind hole, you can't do that, so you need to use a taper, followed by the plug (finishing tap); if the blind hole is especially shallow, you would use the 1st, then the 2nd, followed by the plug tap to get a full sized tapped hole.
Don't bother with cheap sets of taps for anything other than plastic, buy only the sizes you require in a good quality steel, high speed steel (HSS) for tough metals such as steel or carbon steel (cheaper) for softer metals.
Lubrication is critical to a good tapped thread, I use Rocol ASP for tapping in steel and paraffin (kerosene) for tapping in aluminium (WD40 works if you don't have paraffin).

[lasersbee]

How about doing a Google search for "tap bit size vs drill bit size chart" ??
I just did and got 221,000 hits.

Jerry

[neskusen]

How about doing a Google search for "tap bit size vs drill bit size chart" ??

How about reading past the topic title?

[VJ AIWAZ]

or you could use these and then you dont have to worry

https://www.zoro.com/dewalt-drill-an...Q&gclsrc=aw.ds

[masterpj]

You need the tap and drill chart or else you will be having a hard time.

[neskusen]

You need the tap and drill chart or else you will be having a hard time.

I posted one. I'm just asking for some clarification on some things listed in these charts.

[neskusen]

With metric taps, a rule of thumb is to deduct the thread pitch from the thread diameter. that gives you the minor diameter of the thread.

I see, thanks.

in a blind hole, you can't do that, so you need to use a taper, followed by the plug (finishing tap); if the blind hole is especially shallow, you would use the 1st, then the 2nd, followed by the plug tap to get a full sized tapped hole.

Ah, I understand now thanks for explaining. But why not go directly with the finishing tap? Too much pressure on the taper bit?

Don't bother with cheap sets of taps for anything other than plastic, buy only the sizes you require in a good quality steel, high speed steel (HSS) for tough metals such as steel or carbon steel (cheaper) for softer metals.

Thanks.

Lubrication is critical to a good tapped thread, I use Rocol ASP for tapping in steel and paraffin (kerosene) for tapping in aluminium (WD40 works if you don't have paraffin).

Some praise ordinary dishwashing liquid. Have you tried it? I wouldn't mind trying on a test block , its less messy.
And just for knowing the reason behind it, does lubricant help by preventing the metal pieces removed by the taper to cause too much friction, or is there something else going on when using a lubricant?

[lasersbee]

How about reading past the topic title?

I read your entire Post....

I looked at the Chart you linked to....

I understand that simple chart....

If you don't understand that chart then search for something easier for
YOU to understand rather that being a jerk....


Jerry

[NobleGas]

I see, thanks.


Ah, I understand now thanks for explaining. But why not go directly with the finishing tap? Too much pressure on the taper bit?

Yes, that's exactly it.

Some praise ordinary dishwashing liquid. Have you tried it? I wouldn't mind trying on a test block , its less messy.
And just for knowing the reason behind it, does lubricant help by preventing the metal pieces removed by the taper to cause too much friction, or is there something else going on when using a lubricant?

No, never tried it! The lubricant is principally to prevent the swarf from sticking to the tap, although the Rocol ASP also prevents the tap from seizing in steel.