Help with 6-32 tap drill size

[blockmanjohn]

Hi, I have been trying to tap some holes 6-32 in a cast iron casting. I'm using the #36 drill which is supposed to leave a 75% thread. As soon as the taper tap reaches the full thread form the tap will go no further. It is like it bottomed out on a blind hole. I know if I pushed it, the tap would break.

I switched to a scrap 3/8 inch piece of cast iron and tried a #35 drill. Same thing. No go after the full thread was reached. The taps are HSS and brand new. I even tried some cutting oil but to no avail. I'm wondering how large a drill I can use and still have serviceable threads. The holes are for the main bearing caps on a 1/4 scale hit or miss engine.

Thanks, John.

 

[BaronJ]

Hi John,

I wonder if 6-32 isn't a bit fine for cast iron ! Is that the size specified for the bearing caps ?
In any case I would do the same as you and try a larger drill size.

 

[Brian Rupnow]

Perhaps the problem may be the new tap itself. I just checked, and I always use a 0.106" diameter drill before tapping #6-32. I haven't experienced any problems with tapping aluminum, steel, or cast iron.---Brian

 

[blockmanjohn]

Thanks for the reply Brian. It is indeed the size specified for the bearing caps. I am going to order a new set of 6-32 taps and see what happens. Faulty taps seem to be the most logical problem, as I have always used the specified tap drill size on cast iron with no problems, and I have never used these taps before. I hate to just keep using a larger drill. Thanks, John.

 

[xpylonracer]

I always 2.8mm for tapping 6-32 in aluminium using spiral point hss Titex taps in a Tapmatic 3 head.
xpylonracer

 

[Wizard69]

Thanks for the reply Brian. It is indeed the size specified for the bearing caps. I am going to order a new set of 6-32 taps and see what happens. Faulty taps seem to be the most logical problem, as I have always used the specified tap drill size on cast iron with no problems, and I have never used these taps before. I hate to just keep using a larger drill. Thanks, John.

It kinda sounds like bad taps. The question then becomes where did you get the taps? You might want to visually inspect the thread form to see if anything is obviously wrong with the tap.

Other possibilities:

I always use a Lube when tapping, even in cast-iron. The lube is to help with chip flow and in other materials can help with sticking.

A hard spot in the cast iron. Of course you should feel this when drilling.

Maybe you aren't giving it enough. Obviously you don't want to break the tap in the work piece so if possible try running the tap into a piece of scrap. It doesn't have to be cast iron or even especially thick. The idea is to run the tap in, to the same point you feel resistance in the cast iron and then try to work it through. if good tapping practice doesn't run the tap through then you have a tap problem.

 

[Edke6bnl]

I had to drill a lot of holes for my dro on the mills cast iron. It was a piece of cake. My guess bad taps

 

[bazmak]

My first thought would be a hard spot in the iron
However it is bad to go too deep with a taper tap
When it goes to full depth of taper it is cutting on
on too large an area.I would switch to a plug tap
and cut the taper section the go back to the taper
tap for a couple of turns then the plug again etc etc

 

[goldstar31]

Two other possibilities arise. Much has been written in the past about such matters and my guess is that if the cast iron is decent and who knows about hitting hard spots, the other thing that comes to mind is that the experts were going down to tap 12BA without difficulty. However, like me, they used a universal pillar tool which provided for things like staking and tapping.

I saw a couple advertised recently- one for £80. It tends to answer part of the other thing about inaccurate chucks!

My lap top is non the worse for its immersion in red wine although I cannot say that for myself.

UPT- UPT! There's books on the subject.

Somewhat hazily

Norm

 

[DJP]

If the tap didn't start straight it will bind at full diameter, in my experience.

 

[kwoodhands]

If the cast iron is thicker than 1/4" then use a drill that bores 60% of the thread. Look carefully at your tap, do the threads look the same at the bottom of the tap as the beginning of the full threads. It is possible to get a new tap that is junk. Also use plumbers cutting oil or another approved oil for cast iron.

 

[blockmanjohn]

It's always nice to have a happy ending. The new set of taps arrived today and they work fine with no issues. The only thing that seems a bit different between the two sets is that the new ones feel to be a bit sharper if any thing. Thanks for all the great advice, John.

 

[Wizard69]

That is good news!

One question, was the problematic tap new and if so which vendor do we stay away from?

It's always nice to have a happy ending. The new set of taps arrived today and they work fine with no issues. The only thing that seems a bit different between the two sets is that the new ones feel to be a bit sharper if any thing. Thanks for all the great advice, John.

 

[blockmanjohn]

The tap was brand new. It was a Qualtech brand from "Drills and Cutters.com". In all fairness, I have bought many tools from them and this is the first one that was defective. I plan on buying from them in the future. John.

 

[Jhawk]

Use a 7/64 bit for a 70% thread. Less chances of breaking a tap.