# Tapping cast iron



## smfr (Mar 25, 2012)

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


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## cfellows (Mar 25, 2012)

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


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## steamer (Mar 25, 2012)

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


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## Jasonb (Mar 26, 2012)

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


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## Ken I (Mar 26, 2012)

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


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## tel (Mar 26, 2012)

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!


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## rhankey (Mar 26, 2012)

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


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## Stan (Mar 26, 2012)

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.


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## JackG (Mar 26, 2012)

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


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## smfr (Mar 26, 2012)

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


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## Swede (Mar 26, 2012)

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.


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## Lew Hartswick (Mar 26, 2012)

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


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## rkepler (Mar 26, 2012)

Lew Hartswick  said:
			
		

> 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.


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## Lew Hartswick (Mar 27, 2012)

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...


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## JackG (Mar 27, 2012)

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

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


Jack


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## lensman57 (Mar 27, 2012)

Lew Hartswick  said:
			
		

> 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


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## rkepler (Mar 27, 2012)

lensman57  said:
			
		

> 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.


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## Blogwitch (Mar 28, 2012)

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


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## Jasonb (Mar 28, 2012)

rkepler  said:
			
		

> 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


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## lensman57 (Mar 28, 2012)

rkepler  said:
			
		

> 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


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## rkepler (Mar 28, 2012)

lensman57  said:
			
		

> 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.



They're quite uncommon on this side of the pond and 20 years ago I couldn't find a supplier. I ended up buying a set from the Tap & Die company when I was in London on business. Paid for HSS and they supplied high carbon (calling it "HQS") and it took a lot of pressure to make them supply a replacement. I ended up building a Stuart Sun and used them, still have the other casting sets from Stuart that will take BA fasteners (what can I say, I'm a traditionalist).


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## JackG (Mar 28, 2012)

I Bought some BA taps from Coles Power Models several years ago, and assume that they still carry sets of BA taps and dies.

Here's a link: http://www.colespowermodels.com


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## Blogwitch (Mar 29, 2012)

Rkepler,

I have been using those HQS taps and dies for a fair few years now, in fact I have many full sets of them, costing lots of pennies.

You should have kept them.

People are under the impression that carbon taps and dies are not as good as HSS.

If they are made and heat treated correctly, as the Tap & Dies ones are, they will stay sharper and last much longer than your normal HSS ones.

I swear by mine for cutting stainless steel, which normally HSS will very quickly go blunt on. Carbon taps and dies hold a very keen edge, and in my experience far outlast run of the mill HSS tapping tools.

I have BA and metric sets in carbon that are well over 30 years old, and cut as well today as they have ever done, despite being the only sets I used for about 30 years. They were made by a UK company called Presto.

Have a look at the Apex brand on these sets, all carbon, but much more expensive than all the others, why?
Because they have been made to top specifications, and you would find that they will far outlast and cut better than the cheaper HSS ones.

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

Carbon taps and dies have a bad reputation, purely because of the very badly made cheap import sets that are knocking about, I should know, I have tried them, and they really are rubbish. But in my honest opinion, as I have stated before, if you want a good cutting tap or die, you can't go far wrong with a well made carbon one.


John


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## rkepler (Mar 29, 2012)

Bogstandard  said:
			
		

> I have been using those HQS taps and dies for a fair few years now, in fact I have many full sets of them, costing lots of pennies.
> 
> You should have kept them.



If I has asked for and paid the lower price for the carbon steel set I may well have been satisfied, but after having paid for HSS and getting a less expensive carbon steel set I think my annoyance was justified.



> People are under the impression that carbon taps and dies are not as good as HSS.
> 
> If they are made and heat treated correctly, as the Tap & Dies ones are, they will stay sharper and last much longer than your normal HSS ones.



It's been my experience that a new carbon steel tap will outperform a new HSS tap, but after some use the CS tap will lose quite a bit of its original performance while the HSS one will retain most of its performance. I've always thought that it was because you could get a keener edge in the CS but that the edge had a higher tendency to break down. But our difference in experience could be in our cutting different materials.

But my objection was not against CS taps but their replacing a less expensive set for the more expensive set I had paid for without mentioning that replacement or offering a refund of the excess funds to me. In fact, until I enlisted the help of a local (and imposing) friend they were anything but helpful and then it was more grudging than anything.


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## Blogwitch (Mar 29, 2012)

Rk,

It is certainly a personal choice, but as I have already shown, good quality high carbon taps and dies are a lot more expensive than HSS. I think maybe because the heat treatment of them is so critical to get a long lasting tool, so pushing the price up.

As I have said, I use mine mainly for cutting stainless, and normally, even good quality HSS won't even touch it without binding (a bad trait of stainless). The carbon ones that I have (Tap & Die, HQS), cut it as though it was cast iron, silky smooth and no binding at all, in fact, above 3mm, I normally power tap in the lathe and mill, and up to now, haven't had a breakage.

But as you say, things like this are personal choices, just like what machines you have, and what tooling you use in them.


John


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## peatoluser (Mar 29, 2012)

There is one other reason why carbon maybe more preferable to HSS, and not unrelated to another topic on this forum, and that's when (not if!) you break a tap in mild steel -oh! and of all the metal working arts, the breaking of the tap is the one I seem to have mastered the quickest - you at least might have the option of annealing the tap and then drilling it out.

as Bogs said, the problem with a lot of carbon taps and dies is that they are badly made - I've seen some with swarf still attached from the manufacturing process. that's why I tend to buy HSS sets - the threads are at least ground to size. and if you can't justify buying another boxed set, I've noticed that GLR supplies sell the APEX brand individualy. might just treat myself to a few taper taps - i seem to have an ever growing supply of - erm - newly ground plug taps, if you get my drift.

to answer one of the OPs original questions, there are no standards as such, but one of my pocket references gives the following recommendations for depth of tapped holes (I assume the minimum)

M3 drill depth 8mm  tap depth 5mm
M4 drill      9mm  tap      5.5mm
M5          11mm         7mm 
M6          12mm         8mm      

these are for mild steel , the figures for bronze are between 1.5 and 3mm deeper


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## tomol409 (Apr 6, 2012)

The point on taps is there to mount for the fluting in the manufacturing process.  It can be ground off with impunity to bottom out a thread. Some taps have a centre hole for the same purpose. Use a chuck type tap wrench when tapping small sizes, which have a short T-handle and impart less torque. You can get a better feel. Using a normal type tap wrench and applying force at the ends is courting disaster with the smaller sizes.  If you can detect the smallest degree of twist, back off the pressure, as no tap will twist very far before breaking. Taps and dies can easily be sharpened with a Dremel tool, and suitable carborundum or even diamond burrs. I have used conical burrs to suit the flute in a tap and run the burr towards the tip because if the other way and the burr jumps, it will immediately take off the cutting edge. Dies are simple. Just choose a cylindrical burr as big as will pass through the holes in a split die and run it back and forth a few times. Visual inspection will easily show when the die is sharp.


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## tomol409 (Apr 6, 2012)

BA is short for the British Association of Scientists and is a metric thread derived from a Swiss thread for scientific instruments in the beginning.  Apart from the angle OBA is 6mm in size and pitch. Each successive size down is 0.9 0f the previous one hence the peculiar decimal TPI. 
  To compare UNF/UNC sizes to BA the are the converse in size. There are no 7 or 9 sizes in UNF/UNC series. 
   So a 2UNF = 8BA in size (Near enough).  5UNF = 5BA. 4UNF = 6BA etc.


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## MachineTom (Apr 11, 2012)

Someone mentioned using Spiral Point taps, These are great designs, but only for through holes as they push the chip ahead of the cut. They are also NOT recomended for CI and non-ferous materials. Spiral flute are great for blind holes, but again not recomended for CI and non-ferous.

Another item often forgotten is threaded depth, a 10-24 the chart says use a #25 tap drill .1495, this size is good for a threaded lenght of D* 2/3, if threaded lenght is over 2/3 then the tap can be ..154 up to D1.5, then a .159 up to D3. A larger hole is of course easier to tap. The MH is the guide for this information.


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