Drill size for tap?

[SR_matt]

hey guys quick question, for a 1/8 npt tap what drill bit shoudl i use to drill the hole before i tap it out. i remember it was some weird letered bit as apposed to a number size, is that bit required or can i jsut use a standard sized bit?

thanks
-matt

[chettacheez]

hey guys quick question, for a 1/8 npt tap what drill bit shoudl i use to drill the hole before i tap it out. i remember it was some weird letered bit as apposed to a number size, is that bit required or can i jsut use a standard sized bit?

thanks
-matt
Use a lathe and drill out to 23/64, or an 11/32 drill bit would work just fine too.

Make sure you use a 1/8 npt tap instead of a regular 1/8 tap.

[SR_matt]

ya my tap set has a specific 1/8 npt tap, i found the drill bit-tap match up sheet and it said an 11/32 bit.

so what ws the "r" bit used for them
-matt

[chettacheez]

Roundover Bits are used to make quarter rounds of various radii

[SR_matt]

ok still slightly confused. im new at metal working, got a little lost on the quarter rounds part.

but so then what are the other letered bits for (such as s)

thanks
-matt

[chettacheez]

Oh, I get ya. Ignore that last post.

Lettered bits are .003-inch undersize relative to their nearest fractional counterparts.

This should tell you everything you need
http://www3.sympatico.ca/fhofmann/art_round_hole.html

[SR_matt]

^^ thank you, thats exactly what i was wondering... but you still have me confused om the quarter rounds thing

thanks
-matt

[cyberave68]

Quarter round is taking a full circle and cutting into 4 equal parts being quarter sections hence quarter round. Just like a half round would be half of a complete circle...

Someone correct me if i got that wrong...

[SR_matt]

ya i get that part but how it relates to bits and drilling is where im getting lost
-matt

[cyberave68]

Oh you mean the "R" type drill bit? If so thats just another bit labeled all funky. There are 4 different labels for bit as i'm sure you know. Metric, Standard, Numbers and letters. Its just anohter way to have you going crazy trying to figure out if you have the right bit or not if you ask me... Thay are all measured by decimal point anyways(Not metric) so really i just always find the closest size by looking at drill charts. Hope i got it right this time?

[SR_matt]

ya ya, i got that but im confuzeled by the quarter rounds of different radi part
-matt

[chettacheez]

ya ya, i got that but im confuzeled by the quarter rounds of different radi part
-matt

Dont worry about it, I was talking about router bits. Has nothing to do with this.

[SR_matt]

o oki get what your taking about now.

thanks
-matt

[p8ntbal4me]

Im using my buddies Tapand Die set e purchaced from Matco Tools.

I didnt know anything about how it works,... but this kit came with a nice easy to read sheet with what size threads you wanted, what type (corse or fine) and then tells you what drill bit size to use based on what friction u need.

I ordered the same one as his,...... VERY simple to use!

[SR_matt]

mine has a chart as well but it has a 60% thread sixe and 75% thread size collum so i havent goetten a chance to understand it exactly yet

-matt

[p8ntbal4me]

mine has a chart as well but it has a 60% thread sixe and 75% thread size collum so i havent goetten a chance to understand it exactly yet

-matt

I was told to think of it this way: use 60% when the threading and set needs to be critical. Such as a barrel thread, trigger travel limit screw, grunt screws that hold the safety retaining spring in. Use on parts that have alot of vibration.

Use the larger tap sizes for things that can have some play and the screws could have play in them or have another force applied to them to hold them in place. (IE: screws that hold the sticky grips on the intelli frame screw in smooth but dont come out because when tightened up, the grip actually pushes between the head and the frame causing it to stay snug.

I dont know if all thats perfectly true,... but Im using 60% all the way so far at its holding on everything I've milled.

[SR_matt]

i went and looked at my guide and the way it is set out it says the 75 is to be used on very shallow screws buut that it doesnt hold as well (even though the bit size to be used with it is smaller so im nto sure)

either way i dont think im gona be worrying about that for now and ill see what works when i need to start doing stuff alot.

thanks

-matt