# Simple Question Re: Tapping Drill size



## CrewCab (Aug 2, 2008)

OK, a while back, after breaking a 4mm tap in a piece of steel I received some dam good advice :bow: (Thank you Chaps) about the "correct" size of a Tapping hole, ie 70% thread is usually OK, so 3.5mm hole rather than 3.2mm for a 4mm thread etc ... 

Since then I've tapped a lot of 3, 4 and 5mm threads with no real problem ........... apart from getting them straight  ........... must start a thread about a simple hand tapper ..... : ........ but for now ......

What's the general consensus about tapping hole sizes in different materials, in soft material, brass aluminium etc is it best to stick to the recommended size ......... ie: thread size - (minus) 2 x thread depth or is it still better to add a little tolerance, 

So far I've stuck to recommended sizes in brass and aluminium but in steel and CI I've drilled a nad's bigger ............. It's worked for me so far ........ but am I doing it right ???

CC


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## rickharris (Aug 2, 2008)

I doubt this is an approved method but it works for me - assuming the item to be tapped is in the chuck - I put the tap in the chuck in the tail stock - leaving the tail stock loose on the bed I present the tap to the hole - turning the chuck by hand I press lightly against the tail stock to star the thread. 

Then loosen the chuck to leave tap in the hole and transfer to the bench where a standard tap wrench can be used.

Alternative is to make a jig - a suitable hole in a block to accept the tap will keep it perpendicular whilst the thread is started.

You can do the same trick with a die stock using the flat face of the tail stock as a guide to get the die started.


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## sparky961 (Aug 2, 2008)

Approved or not, I've been using this method successfully since soon after getting my machine. I haven't tried it for dies though - will do that next time. I find it much harder to get outside threads cut straight with a die than inside ones with a tap.

It works great for parts that are easily held in a chuck, but for odd-ball parts it would be nice to have something to help align the tap.

-Sparky


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## Lew Hartswick (Aug 2, 2008)

CrewCab  said:
			
		

> OK, a while back, after breaking a 4mm tap in a piece of steel I received some dam good advice :bow: (Thank you Chaps) about the "correct" size of a Tapping hole, ie 70% thread is usually OK, so 3.5mm hole rather than 3.2mm for a 4mm thread etc ...



Well! you cant make quite that broad a statement. It depends on the thread pitch what sice the 
hole should be drilled to tap. 
For the "70 pct thread" answer it should be the screw dia minus the pitch of the thread. 
 ie. For your 4mm screw if the thread is 0.7 then the hole should be 3.3 .  [4-0.7=3.3]
but if the pitch is coarser then the hole should be larger .
  NOW all that being said it is often that you don't need a 70 pct thread so a larger hole is a LOT
easier to tap (and safer on the taps). 
  ...lew...


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## tmuir (Aug 2, 2008)

I bought myself a book a little bit back called.
'Drills, Taps and Dies' by Tubal Cain
book 12 in the Workshop Practice Series and found it to be very useful for tapping.
Its a pretty cheap book and worth having it you can find it just for the charts in it.

I've attached a tapping chart I made for ISO Metric threads that tells you what drill size to use for 60 to 85% thread engagement.
Sorry haven't made a chart for other threads. 

View attachment tapping Chart.xls


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## mklotz (Aug 2, 2008)

Download the DRILL program from my web site, select the "T" option, answer the questions and all will be revealed.


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## greenie (Aug 2, 2008)

For most materials, you can use the recommended tapping drill sizes for the correct thread percentage.
If you are not game to use these, then by all means go a tad bigger on the drill size, it's your stuff, isn't it?

The only material you should ALWAYS go a size or two bigger with the drill, is stainless steel, this stuff will break taps quicker than you can break wind, always go over size on the drill holes for your own piece of mind.

If the drawing for a part made from stainless, calls for a certain percentage of depth of thread, like 70% or above, then use plenty of lubrication to assist with the tapping process and be ready to dig out the broken tap. 
The way I see it, stainless is a ***** to tap!

regards greenie


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## Circlip (Aug 3, 2008)

Made a rotary table to round the ends of links to the MEW design, and cos I could, I made it all from Stainless. About forty M4 holes drilled and tapped in table,used two taps, no, I didn't break one, when it started "squeaking" I changed it. The 300 series that we use shouldn't be much of a problem if you go for the "L" grades, ie. 304L, 303L (L = low carbon content) 
 Tony thing to be aware of when screwing stainless bits to each other is "Galling" This is when the nut picks up on the thread and gets tighter and tighter and tighter till it locks. You then have to cut them apart to disassemble, (and got the tee shirt) We use "Copperslip" on this side of the pond to ensure this doesn't happen.
 Regards Ian.


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## Mcgyver (Aug 3, 2008)

sparky961  said:
			
		

> Approved or not, I've been using this method successfully since soon after getting my machine. I haven't tried it for dies though - will do that next time. I find it much harder to get outside threads cut straight with a die than inside ones with a tap.
> 
> It works great for parts that are easily held in a chuck, but for odd-ball parts it would be nice to have something to help align the tap.
> 
> -Sparky



just take a small slug of whatever, aluminum works well, face on side and drill through the tap's OD. Voila! tap guide is done. hold over hole while you start a tap. in no time you have a small drawer full of the common sizes and tap breakage will approach zero.

As for dies, except to chase a thread, I can't remember the the last time i put a thread on with a die stock. since why you're threading is inevitably round, it can almost always be done in the lathe. single point the thread or use a tailstock die holder. The later is super handy and one of the first projects that should be made for a lathe. I'll try to take some pics, but the are simply a round section of metal that holds the die on one end, and a morse taper stub to fit the tailstock on the other with some set screws to hold the die. make three for the three common die sizes while you're at it and threading will get a lot easier


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## sparky961 (Aug 3, 2008)

Mcgyver  said:
			
		

> just take a small slug of whatever, aluminum works well, face on side and drill through the tap's OD. Voila! tap guide is done. hold over hole while you start a tap. in no time you have a small drawer full of the common sizes and tap breakage will approach zero.
> 
> As for dies, except to chase a thread, I can't remember the the last time i put a thread on with a die stock. since why you're threading is inevitably round, it can almost always be done in the lathe. single point the thread or use a tailstock die holder. The later is super handy and one of the first projects that should be made for a lathe. I'll try to take some pics, but the are simply a round section of metal that holds the die on one end, and a morse taper stub to fit the tailstock on the other with some set screws to hold the die. make three for the three common die sizes while you're at it and threading will get a lot easier



Seeing as I've broken more taps (2 or 3) in the past few months than in my entire life, the time taken to whip up a tapping block the next time I go to cut some threads sounds well worth the effort. I think I've just been rushing things a bit too much lately.

I've heard of the idea for a tailstock die holder before, but I was hesitant to do this because the dies I have right now are hexagonal. Any good ideas on how to hold these?

-Sparky


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## lugnut (Aug 4, 2008)

I made my 1" hexagonal die holder from a 1" deep socket. works great
Mel


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## sparky961 (Aug 4, 2008)

lugnut  said:
			
		

> I made my 1" hexagonal die holder from a 1" deep socket. works great
> Mel



Well that was so obvious it was painful... thanks for pointing that out as an option!

-Sparky


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