Tapping cast iron

[smfr]

I know that this is probably a FAQ, but how do folks go about tapping smallish holes in cast iron?

I was tapping 4BA into a casting today, and the tap got really tight in the first hole; so tight that I was afraid I'd break it, and wasn't able to tap to depth. I'd drilled #33, started with the taper tap, then was finishing with the bottoming tap. I was using Re-li-on cutting fluid (though I did the rest of the holes dry, and that seemed better). I started the tap in the mill chuck, turned by hand (allowing the feed to move as the tap screwed in), so alignment wasn't an issue.

I did manage to tap the 4 holes to depth in the end, but it sure was nerve-wracking! I think the tricks were:
* did it dry, no cutting fluid
* went one size up on the drill (#32, rather than #33)
* made sure the hole is deep enough
* went as far as I could with the taper tap first

So I have the following questions:
1. Is there a rule of thumb for how deep to drill the hole so that you can tap to the required depth?
2. Do you go as far as you can with the taper tap, then only switch to the bottoming tap at the end?
3. Do you use cutting fluid in cast iron?
4. How much flex should I expect the tap to handle? If I can see it twisting, should I stop?
5. Do you grind the points off the end of your taps so that they don't bottom-out in the hole?

Thanks!
Simon

 

[cfellows]

Well, I can answer a couple of those questions. First, I always tap cast iron dry, never use fluid. Second, I usually try to drill the hole deeper than I plan to tap if possible... otherwise I use a plug tap to the bottom of the hole then follow up with a bottoming tap. In addition, I always use a tapping guide which removes any side to side flexing in the tap. Finally, you need to remove the tap entirely from the hole from time to time to blow accumulated dust out of the hole and blow off the tap. Never had a problem breaking of a tap in cast iron. Steel seems to be the tough one for me.

Taps will tolerate some twisting flex but a combination of twisting and sideways flex will cause it to fail in a hurry. As far as how much flex... well that's pretty much a matter of feel. And I sometimes get it wrong!

Chuck

 

[steamer]

Hey Simon,

What Chuck said for sure. I've not had too many problems with CI doing the way he describes, but if your running into a hard spot, well it can get nasty!

Always make sure your taps start in square to the hole. Making a tapping block is a good idea.

A tapping block is a block of steel with carefully drilled dead perpendicular to the bottom face with holes sized such that all your usual taps are a slip fit. In use you hold the block up tight against the surface as you start the tap while inserted in the block.

It works very well and is well worth the effort.....If I'm telling you something you already know ...well sorry, but I use my block all the time and swear by it.

Hope that helps!

Dave

 

[Jasonb]

No 33 will give close to full thread depth, I would drill 3.1mm which is somewhere between 30 & 31.

I also grind the slight point off all my taps that way you can get further down with each. Another thing you can do is when the taper starts to get tight run the 2nd in for a couple of cutting turns and then go back with the taper, this saves the taper having to cut along its full length.

J

 

[Ken I]

As per advice above.

Use a tapping guide or floating holder.

http://www.homemodelenginemachinist.com/index.php?topic=16541.msg168701#msg168701


If you ever break a tap (yeah right ...IF) and it still has some useful thread, grind it off square and chamfer 1 x pitch @ 45° and keep it as a "bottoming" tap for those applications where you can't go any deeper but want as much thread as possible - obviously this only gets used after all the other tap stages).

Some metric sets the No I & II taps are undersize / truncated, only the III tap is size.

Ken

 

[tel]

All good advice above, but the most important TAP DRY and 0.1mm over the nominal tapping drill - given those and a decent bit of iron it is a joy to work with!

 

[rhankey]

I find 7BA way more nerve wracking than 5BA or 4BA, and SS has proven to be much scarier for me than CI. I had a bunch of rather deep 7BA holes to tap in SS yesterday, and even with brand new taps, it was still a slow and tedious process. As others have said, CI seems to cut easier dry. Where possible, I drill non-through holes much deeper than what I need to thread so the taper tap can cut much of the threads. When tapping non-through holes, I cycle through taper, second and bottoming until each is giving too much resistance for my comfort. I keep repeating the cycle until I reach the desired threading depth. It can be a slow process. To date with CI, I’ve been able to reach desired depth in a single pass, even with 7BA taps.

Robin

 

[Stan]

More points to consider. Cast iron is notorious for hard occlusions that are almost impossible to thread with small taps.

Taps that are not sharp will bind mush worse than sharp ones.

The amount of flex a tap will tolerate is determined by the manufacturer when the tap is tempered. Too hard and they snap instantly. Too soft and they flex well but wear out quickly.

 

[JackG]

In my experience, the first rule is: keep it clean. When the tap feels like it's starting to bind, I back it out, brush it off and blow out the hole with air to remove any debris.

Jack

 

[smfr]

Thanks for all the excellent advice, guys! I need to make myself a floating holder.

I think my main error was trying to go for full thread depth. I have plenty more holes on this model to practice on!

Simon

 

[Swede]

If tapping scary material, another technique is to alternate taper and plug taps. Run the taper tap in until the forces become a little scary, then back it out. Follow with the plug tap, again until forces get a bit high for the tap in question. Repeat the process with the taper tap.

The two profiles complement each other nicely, with the added bonus of clearing chips frequently and well. It's tedious, but beats a broken tap in SS and the like.

 

[Lew Hartswick]

If you have the material depth just drill deeper and use a spiral point tap.
Piece of cake.
...Lew...

 

[rkepler]

If you have the material depth just drill deeper and use a spiral point tap.

A 4BA spiral point tap? I've got to get the name of your supplier! Maybe they have spiral flute too!

Another thing you can try is that if you've got a lot of threads in the assembly you can really reduce the tapping effort by reducing the % thread. If you have 3x diameter in the engagement you could go as little as 50% thread.

I also tap in the same setup as I drill - spot, drill to depth, tap. That helps a lot - a misaligned tap will almost alway break at depth (no matter what that might be - they *know* when the point of max inconvenience is reached.

If there isn't room for the tapping chips in the bottom of the hole I'll break a shop rule and pop 'em out with the air gun - usually by sneaking up on it from the side. Pretty much the only way to clear the hole short of lifting up the mill and shaking it when upside down, and that's just too much trouble.

 

[Lew Hartswick]

Russ, I haven't any idea what a 4BA tap is. I have a 4-40 or is it a 5-40 spiral point
tap and it woks fine.
...lew...

 

[JackG]

BA is a British thread standard. Check here for more info.

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


Jack

 

[lensman57]

Russ, I haven't any idea what a 4BA tap is. I have a 4-40 or is it a 5-40 spiral point
tap and it woks fine.
...lew...

Hi,

BA is the old British standard of threads , as old as the industrial revolution that started from England and mostly from Manchester, it is 55 degrees inclusive and bloody hard to come by these days cheaply, even in England. The smallest that I have are 10BA then 8BA, 6BA,4BA and it goes on. These days almost nearly, we use the metric system in the UK, much more logical and a lot cheaper.

Regards,

A.G

 

[rkepler]

Hi,

BA is the old British standard of threads , as old as the industrial revolution that started from England and mostly from Manchester, it is 55 degrees inclusive and bloody hard to come by these days cheaply, even in England. The smallest that I have are 10BA then 8BA, 6BA,4BA and it goes on. These days almost nearly, we use the metric system in the UK, much more logical and a lot cheaper.

It's actually 47 1/2 degree but the rounded crest and valley of the Whitworth form. Not really that old, it was proposed near the end of the 1800's and accepted in 1903. Sort of a odd thread as the pitch and nominal diameter is metric(ish) - no real relationship that I can find (maybe related to a wire standard or something?).

For Lew: a 4BA is somewhere between 6-32 and 5-40 at .142-38.5 (metric 3.6x.66) I wouldn't want to have to single point a BA thread.

 

[Blogwitch]

There are very few lathes that can single point BA threads, usually only very expensive toolroom machines, mainly because every size has a special non standard pitch, neither Imperial or metric standard. So for cutting BA threads, most people use tap & die sets.

BA was used as standard throughout the aviation industry by the electrical trades until maybe the late 80's early 90's, before converting over to the metric system, it was also used by certain electrical trades in industry as well, but now has been superceded by metric.
Because of the longevity of life of aircraft, there are most probably still hundreds knocking about that still use a lot of BA electrical fixings.

BA, in the UK are just as easily available as metric or imperial thread cutting tools, and no more expensive than the others. I myself have 3 or 4 full boxed sets up to 12 BA, and the last set that I purchased was only a couple of months ago.

Most engineering suppliers have them in stock

Second set down

http://rdgtools.co.uk/acatalog/TAP_&_DIE_SET_S.html

Sets 6, 7, 8, 10, 12 & 14 on here

http://www.chronos.ltd.uk/acatalog/Engineering_Menu_Taps___Dies_Sets_182.html

Pay your money, make your choice


John

 

[Jasonb]

A 4BA spiral point tap? I've got to get the name of your supplier! Maybe they have spiral flute too!

MSC / J&L over here have selected commonly used BA sizes in both spiral point and also spiral flute, so yes they are available.

I now get my BA taps from one of the ME suppliers who also does all sizes in spiral point, but their web pictures show otherwise.

As mentioned in Jack's link the pitch is a mathmatical progression startion from 0BA which is 6.0mm dia hence the rather odd pitches.

J

 

[lensman57]

It's actually 47 1/2 degree but the rounded crest and valley of the Whitworth form. Not really that old, it was proposed near the end of the 1800's and accepted in 1903. Sort of a odd thread as the pitch and nominal diameter is metric(ish) - no real relationship that I can find (maybe related to a wire standard or something?).

For Lew: a 4BA is somewhere between 6-32 and 5-40 at .142-38.5 (metric 3.6x.66) I wouldn't want to have to single point a BA thread.

Thank you for correcting my mistake, I must have been thinking about the Withworth screw.
Apparently these were influenced by the Swiss system ( metric?) and they are supposed to be high precision screw system, 0BA has 6mm diameter and 1mm pitch. I have some of them in 8BA, 6BA and 4BA sizes mainly for my model engine making hobby but I am gradually going Metric.

Thanks again,

A.G