Tapping cast iron

[rkepler]

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).

 

[JackG]

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

 

[Blogwitch]

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

 

[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.

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.

 

[Blogwitch]

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

 

[peatoluser]

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

 

[tomol409]

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.

 

[tomol409]

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.

 

[MachineTom]

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.