What size drill bit for hole to be threaded M3?

[Phylodog]

I've broken two taps in the last five days. They were part of a cheap $10 kit from Harbor Freight and were a drill bit/tap combo. I'm not buying another kit and just saw that I can get an M3-.50 tap from Snap On for $5.35 plus shipping. What size drill bit do I need to use to follow up with this tap?

[ol' gravy leg]

I use a #39 but a #40 works too.

[Phylodog]

Thank you

[SaxtonSlaughter]

I use the Harbor Fright ones all of the time, and I've never broke one. Make sure you are using oil on the Tap and taking an air hose to it, before and after each turn of the tap.

[rmdesignworks]

a big thing too is dont try to cut all of the thread in a sngle turn,,turn the tap in a bit then back out a tad then back in,,let it actually cut the threads then clean them as you run the tap in...the shavings are usually what cause a tap to bust as they build up on the lead cutting edge of the tap..backing it out just a little bit after a bit of turning it in will break the shavings off the leading edge and allow the tap to work better and easier..ive been running the same same set of misc. taps for about 5 maybe 6 years

[Mad-Man]

A 2.5mm drill would be ideal. The best way to figure metric sizes is to take the tap size (3mm) minus the pitch (.5) to achieve the correct drill size.

[sbarnesvta]

wikipedia has a good sizing chart for drill size compared to tap size. If you click on the chart you get a full machinists charts of every size possible
http://en.wikipedia.org/wiki/Tap_and_die

[vw_thing]

Get some proper cutting fluid. That stuff is way cheaper than constantly breaking taps and drill bits in steel. You will want to take care of the snap-on tap, clean it out after use and don't ever force it. A tap that small has very little resistance against twisting, so back it out anytime it stops.

And if you have a choice between two bits that aren't exactly labeled on the tap, consider your material. Steel will work fine with a drill bit slightly larger than labeled, a material like brass or aluminum will do better with a slightly smaller bit. Good if your drill set doesn't include all of the numbered sizes (mine goes by 1/64in increments).

[Phylodog]

Thanks for all of the help. I'd admittedly a complete newb to metal work and I'm learning as I go. This type of information is very helpful.

[roksloski]

2.5 mm

or #39 or 40

sometimes depends on what you are tappimg...

[friscokidd]

Originally Posted by sbarnesvta

http://en.wikipedia.org/wiki/Tap_and_die

thanks for this one, i have been trying to find out this info for a week now

as far as the taps go, i too have the harbor freight ones and i have only broken the 4-40 one. Be easy and gentle and add lube kinda like the first time...haha

[Red-F150]

Originally Posted by friscokidd

Be easy and gentle and add lube kinda like the first time...haha

Way to much info :-P

[Joat]

1 turn in 1/2 turn out if shes tight and flaky a little lube/liquid courage always makes nice and smooth

[Reflection]

Originally Posted by Phylodog

I've broken two taps in the last five days. They were part of a cheap $10 kit from Harbor Freight and were a drill bit/tap combo. I'm not buying another kit and just saw that I can get an M3-.50 tap from Snap On for $5.35 plus shipping. What size drill bit do I need to use to follow up with this tap?

One thing to keep in mind,it depends on the style of tap as well. Cutting taps and forming taps will require different size drills. With smaller taps,cutting taps are typically weaker than forming taps. Cutting taps usually have 2-4 big open flutes and form taps usually only have 1 little tiny flute. The form taps having more material in the tap itself,naturally they are stronger and resist breaking better.

Cutting taps do exactly that,they actually cut the threads. Forming taps,they roll the chips and form the threads. Depending on material and the application,they both have their place. Believe it or not,the threads from a form tap are allot stronger than you would imagine.

Good cutting oil and the proper sized hole is key. Hand tapping sucks...to dang slow. At work,naturally I use my machine and power feed everything. At home,when fooling with these little taps,I use my cordless drill and power feed the tap with it. Much faster and easier.

[tom@vp]

Originally Posted by Phylodog

I've broken two taps in the last five days. They were part of a cheap $10 kit from Harbor Freight and were a drill bit/tap combo. I'm not buying another kit and just saw that I can get an M3-.50 tap from Snap On for $5.35 plus shipping. What size drill bit do I need to use to follow up with this tap?

with a metric cut tap, you can subtract the pitch from the major diameter to get the drill size. This works well for general use and put you at about 70 percent thread.

so a m3x.5 cut tap is 3-.5=2.5

convert to english is 2.5/25.4 = .098 or #40

i know im way to late but, i think it still good info...

[tom@vp]

Originally Posted by Reflection

Forming taps,they roll the chips and form the threads.

Believe it or not,the threads from a form tap are allot stronger than you would imagine.

very close on your description, roll taps actually do not form a chip at all, they cold form the thread, and yes cold form threads are stronger than cut threads.

[Joat]

Originally Posted by tom@vp

very close on your description, roll taps actually do not form a chip at all, they cold form the thread, and yes cold form threads are stronger than cut threads.

Compression of material verse removing of material correct?

[tom@vp]

Originally Posted by Joat

Compression of material verse removing of material correct?

basically yea..

[Reflection]

Originally Posted by Joat

Compression of material verse removing of material correct?

That's the way it was explained to me.

[rex cars]

enough information about taps to make your head explode!