HSS or CS taps and dies

Hi Adrian

For Metric threads tapping drill size deduct the pitch from the diameter, so for M6 x1 tapping drill is 5mm so perhaps the sizes you mention are for fine pitch threads.
I would not use less than 60% engagement especially in softer materials.

Emgee

Tapping compound - always a good idea to use something! I use CT-90 because it's what my local place keeps on the shelf. The modern mixtures have somewhat better heat, pressure and lubricating properties than old-school organic recipes. They are also unlikely to be a biohazard. In the old days lard etc would rot and cause painful infections via a scratch.

For tapping drill sizes, how strong do you need the connection to be? The average bolt isn't heavily loaded, and when more strength is needed it might be easier to use a bigger one than a close fitting thread. Production tends to avoid tight threads because making them wears out tools and slows down assembly.

Most of the time I use over-size tapping holes. However, rarely - when strength or safety matters - I use the recommended tight drill. Not often because I'm not bolting wings on airliners!

I don't think the depth of the hole makes much difference to choice of tapping drill diameter. Surprisingly few threads are needed to achieve full-strength. Perhaps 4. After that, additional threads in a deep hole don't add more strength. So, it's the same decision: if maximum strength is needed, drill for tight threads and accept the tap will wear faster and is more likely to break, otherwise drill larger holes to favour the tap.

Dave

Having read through this thread I will admit that I mostly have and use HSS, I am sure at some point I have broken both CS and HSS through my own fault. I might just give the spiral flute taps a go in the future, I hadn't realised that they would work as a hand tapping tool.

I had a quick look at Presto web site and they are asking to log in before seeing prices or buying taps or drills, do they only deal with companies or bulk orders?

I mostly tap dry. If lubrication is needed I use Rocol RTD; mostly on difficult materials like stainless steel and/or bigger taps, say 3/4" upwards. The RTD goo works well, but is a PITA to remove from tap and hole. On aluminium alloy I sometimes use WD40 just to stop the swarf sticking to the tap.

As for drill sizes i usually aim for a thread depth of 60-70%. In tough materials I'll be nearer 50%. For fine pitch threads I aim for 70-80% engagement.

If an internal thread is screwcut I go for 100% thread depth.

Some years ago I did some experiments on thread depth. Material was 6082 combined with a high tensile (12.9 grade) SHCS. The bolt fractured before the internal thread stripped with 50% engagement; I didn't bother testing the higher engagement percentages.

Andrew

I tend to stick with the diameter less pitch for metric a sit was what I was first taught particularly if the hole is shallow, I may go a bit larger on some thing that does not need any strength such a a screw holding parts in place that are going to be silver soldered - easier to tap and less ware on the tool. As I tend to drill in the mill it is easy to also use that to guide the tap while turning it by hand

I've a mix of both CS and HSS, tend to get CS for odd sizes that I wont use much but the commonly used ones are generally HSS both hand taps (Dormer E500 series) and the ones that I tend to reach for most often now are Spiral flute (mostly YG-1 general Purpose Combi Taps). I do have a few spiral point but don't see the need for both Spiral point and spiral flute for non production work as the spiral flute will still eject the swarf from a through hole as well as a blind one. Few spiral flute and a hand tap being powered by the SX2.7 below, details of taps if you view direct in youtube. I'm using The Rock oils monkey juice that ARC sell in the video but also use CT-90.

PS I did a similar test to Andrew and got the opposite result - a stripped thread in the hole and was even using a cheap ungraded bolt

Edited By JasonB on 22/05/2019 16:40:26

I just moved some more things from my old house to here. I found that I have a brand new set of Halfords taps and dies. Havnt a clue where it came from, but along with the Sherwood STP I just bought they cut the threads almost as easy as screwing a bolt in.

I also discovered that my old place was 1 mile from Cromwell tools. It must be the last bastion of storemen. Dont get me wrong they were totally polite and helpful. Just I got that feeling that I was only allowed in if I did not touch anything.

I asked for a 500g tub of Rocol RTD. He did glance at the terminal but did not touch it. I could see the contempt in his eye for automated stock systems. Of he stalks into the depths of the racking, a while later he returns with a bottle of Rocol RTD oil. I stick to my guns, and say "no the one in a tub". Of he goes again, a bit longer this time. He returns empty handed. Well I am prepared for this, I have the power of the Internet and have already looked up what they sell. What about Trefolex I say, and of he goes again. Even longer this time, more muffled thuds and scrapes and he returns with Sherwood STP and says what about this? I decided tweaking the tail of sleeping tigers is a bad hobby and agreed, yes that is perfect. I even got a smile which rapidly turned to a scowl as he aproached the terminal to make the sale.

My local metal vendor is like that too! I got on much better once it was established I knew what I wanted and was going to spend a decent amount of cash.

Having watched other customers while waiting for metal to be cut, I started to sympathise with the staff's off-putting attitude. Rather too many of the public arrive seeking free advice on all things metal related, or die of shock when told the cost, or otherwise dither and waste time. Much better to be the kind of customer who knows what he wants, can discuss alternatives, and then pays up cheerfully.

Last time I was in an elderly chap (ie a gent slightly older than me) was most concerned to explain why all new steel is, I quote, "crap". He lectured the assistant for about 10 minutes until he realised the queue behind was annoyed at which point he left without buying anything! I wish I'd thought to recommend the forum, what he said was misplaced in a busy shop but would have made an interesting post - his views were based on professional experience.

Dave

Castrol Variocut is a superb tapping compound very economical and easy to clean, results in very nice threads. I have about a litre left and I think it is possibly not available in UK now . As for taps /dies I much prefer HSS to CS, Emuge Franken being by far the best taps I have used [ they don't do Dies ] Dies... I would say Walter, I also have Dormer, OSG and Guhring any of those makes will give long service.