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Taps and Dies

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William Harvey 113/07/2021 19:41:13
127 forum posts


I need to tap some 8mm thread for some Allen Head Bolts, so I looked at the cost of Taps and thought it was about time I invested in a set. I have a small cheapo set but doesn't go up to 8mm?

Anyway I have been looking at a couple of sets and individual taps and some come with three dies of one size, a first, second and plug tap? Some sets don't like this one from Chronos, whereas some do this one from RDG?

Some are also listed as 'coarse' and some 'fine', the RDG set is not listed as either, I didn't realise there was a coarse and fine in metric like there is UNC and UNF in imperial?

Which set do I need?

malcolm wright 313/07/2021 20:10:01
2 forum posts

Metric coarse is standard for nuts & bolts etc, if a supplier does not state coarse or fine they will be coarse.

fizzy13/07/2021 20:14:56
1809 forum posts
120 photos

You need the same pitch as your bolts - modt likely coarse

fizzy13/07/2021 20:17:49
1809 forum posts
120 photos

A very basic rule I adopt when selecting dies which applies to tap and die sets is that if the die is split it is likely to be better quality than onw which isnt. No proff other than buying them for over forty years I have found this to be the case.

Ian Johnson 113/07/2021 20:24:23
360 forum posts
101 photos

That set from RDG looks good value, I have a similar set and it cost over £60! One complaint is that the dies are not split dies, so you can't expand them a little bit in the die stock, but they cut great because they are ground HSS.

The chronos set looks like they only come with taper taps which can be a pain when tapping a blind hole. And you get a lot of taps you may never use.

So if you are tapping metric threads I would go for the RDG set.

And like the others have said, metric course is the standard metric thread.


Frances IoM13/07/2021 20:28:31
1154 forum posts
28 photos
not sure a set is a good way - if you are doing mainly thru holes a spiral tap is faster - unless you are dealing with v hard materials just a 2nd and a bottoming tap will be needed for non thru holes (tho you may need to take the bottoming tap to the grinder as many come with a tapered end).
Split dies are much easier especially on hard material.
Speedy Builder513/07/2021 20:34:41
2394 forum posts
184 photos

Why do you need taps and dies? Once you know what you are going to make and how frequently you are going to use them, then we can help you make up your mind.

For most applications, you can get away with a single taper tap for making threaded holes, (second and plug taps for blind holes) and a single die for making male threads. I prefer 'split' dies that you can adjust for tight or sloppy fit or anything in between

Personally, I have built up a big collection of second hand taps and dies over a long period. I have some cheap and rubbish sets which can only be used in an emergency when I don't have anything better.

I have and use many different thread forms when repairing old and new parts - lawn mowers post 1980 tend to be metric forms but even then you can find both fine and coarse threads. Older UK threads were mostly BSW / BSF and later UNC/UNF.

Vintage UK cars mainly BSW (coarse) and BSF (Fine) threads but even then may find the odd BSP thread or BSB.

Making a steam loco to older plans, you will find ME (Model Engineer) threads and BA for small nuts and bolts besides others.

More and more, metric thread forms are used, but be warned below 8mm they are commonly fine and coarse forms. 8 and above there are 3 or more pitches. Standard commercial nuts and bolts (B&Q and DIY stores) are Metric coarse.

Once you decide what you need Metric, BSF, BA or whatever, decide what material you will be forming your threads in. Carbon steel taps are a lower cost to HSS, but wear quicker especially in harder materials. Price is normally an indication of quality and longevity of your tools - but bargains do exist!

I appreciate that all the above is a minefield - there isn't a simple answer.

Start off deciding what you want to achieve.


JasonB13/07/2021 20:39:59
21327 forum posts
2424 photos
1 articles

As Frances says Sets are not always the best bet and spiral point taps are good for through holes though I have now taken to using Spiral flute for most things and they will go quite close to the bottom of blind holes, biggest advantage is you only need one tap per size.

Unsplit dies can also be quite high quality as the good ones will produce a thread to recognised tolerances which you can't easily do with a split one without proper measuring. Though they don't allow you to adjust the "fit" of the thread if you want a tight or loose fit.

SillyOldDuffer13/07/2021 21:05:55
7487 forum posts
1658 photos

M8 comes in standard (aka coarse) or fine pitch. Ordinary fixings are usually coarse fit. Fine threads are stronger and good for adjusters; I don't use them much.

Metric threads are measured by pitch, which is the gap between adjacent threads in millimetres. M8 coarse threads are 1.25mm apart, M8 fine are 1.0mm. For accuracy use a digital caliper to measure the gap between 10 threads. May be easier to roll the oily bolt on a sheet of paper and measure the marks rather than fiddle with it.

Taps often come in sets of three, but not always.

Taper taps fit easily into the hole and are used to start the thread, in a set of 3taps cutting it shallow. The second tap cuts deeper to finish the thread to size. Taper and second taps need a through hole because the end of the tap fouls the bottom. Plug taps are for threading blind holes.

Single tap sets are mainly for sheet metal. They look like taper taps, but produce a finished thread first time. Not so good for hard materials or deep holes, because the tap may snap. Whatever type of tap is used, Model Engineers often reduce the risk of snapping by drilling slightly oversized holes: good compromise but don't do it if strength is important.

I have complete sets of the sheet metal type for most work, but supplement my preferred sizes with taper, second and plugs, brought out when I need better cut threads. I mostly make experimental objects for which M3, M4, M8 and M12 coarse do pretty much everything.

Imperial threads are measured in Turns per inch rather than pitch, so if the number of turns over an inch is a round number, the thread probably Imperial. Double check - some Imperial and metric sizes are similar enough to confuse.

Always use lube (CT90 or similar) and take extreme care to keep taps straight, They are easily broken by going in askew.


HOWARDT13/07/2021 21:26:26
777 forum posts
28 photos

For metric coarse is standard. The only times you normally come across fine pitch is in the use of pressure fittings, and odd things like spark plugs and electrical conduit, probably many others but unless you are into industrial fittings they are the most likely. If you want to create metric pipe fittings then you may need fine pitch but depends on the pipe size.

Rob Wheatley13/07/2021 22:45:20
35 forum posts
12 photos

My tap and die set came from machine mart, I've had it years and its still going strong, it's a 3 to 12mm Clarke HSS set, taper, second and bottom of each size, die's are non split ones but the threads they cut are not loose at all.

KWIL13/07/2021 23:22:56
3414 forum posts
66 photos

I have never bought a Tap & Die set, just buy the ones you need when you need them, but you must plan ahead so you have them to hand. Spiral flute for first choice. Spiral taps feed the swarf out rather than trapping it in the flutes = less breakages.

John Baron14/07/2021 04:59:29
487 forum posts
189 photos

I've started to buy my taps and dies from

Good quality HSS UK made.

No relationship just a happy customer.

Michael Gilligan14/07/2021 06:17:06
18761 forum posts
922 photos

Thanks for that, John Baron

All looks good yes

Have you bought any screw pitch gauges from them ?

… I feel the urge to treat myself to a set.



Edited By Michael Gilligan on 14/07/2021 06:31:39

Pero14/07/2021 07:42:31
143 forum posts

I agree with Jason having been converted fairly recently to spiral point and spiral flute taps ( they do work differently! ).

As mentioned above, common 8 mm Allen head bolts are almost certainly metric coarse.

Many years ago when I first discovered metric fasteners I rashly thought there was only one standard metric thread form. Some years later I came across metric fine. Later still I found metric extra fine. I haven't found metric ridiculously fine as yet but I have no doubt they are out there somewhere.

At the other end of the scale I assume there is metric extra coarse (with only a few threads ) and metric crude - with no threads at all ( these are normally called nails or rivets ). All in all just as much complication as Imperial.

Back to the point, if you buy a set it is likely that some of the taps an dies will never be used hence the advice re only purchasing the one for the job in hand. However if you do need the odd tap or die on occasion it can be useful to have a reasonable selection on hand and a moderately priced set can be useful. Also, they usually come in a box and are just a little bit harder to lose than an individual tap.


HOWARDT14/07/2021 08:27:56
777 forum posts
28 photos

Unless you are more into general engineering then sizes up to M6 or M8 will be the norm. The only time you will use larger is if you have a larger mill then M12 may be useful. For most model engineering then M2 to M5 again unless your into bigger scale.

Michael Gilligan14/07/2021 08:59:11
18761 forum posts
922 photos
Posted by Michael Gilligan on 14/07/2021 06:17:06:


… I feel the urge to treat myself to a set.



On the subject of which … I find myself slightly confused by the listing of two ‘American’ sets

I know that there have been variations on the theme, as discussed here: **LINK**

but the list of tpi in the two Avon sets merits some further consideration, I think.

4, 4.5, 5, 5.5, 6, 7, 8, 9, 10, 11, 11.5, 12, 13, 14, 15, 16, 18, 20, 22, 24, 26, 27, 28, 30, 32, 34, 36, 38, 40, 42

4, 4.5, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 18, 20, 24, 28, 32, 40, 48, 56, 64

It looks like I need both sets to get the full range dont know


Mick B114/07/2021 12:00:52
2007 forum posts
116 photos

Both the Chronos and RDG sets look fair value, but at least the Chronos and maybe the RDG set too appear to have only solid dies. Don't know about quality of material or manufacture, but solid dies don't give the adjustment capability you get with split ones.

I'm on my second set of these in 20 years from Machine Mart:-

CHT303 set

They are of 'tungsten steel', which I guess isn't quite HSS or they'd say so, and if you use them on silver steel and tool steels they will ultimately go blunt after a lot of use - but for alli, brass and MS they'll last a good few years, and they're my default set for most work.

For the centre screw on the die holder, I found substituting a caphead H/T screw with a 30 - 40 degree point turned on it worked better for opening up the split die to get a tighter thread, or a starter thread to be followed up with a second pass for better finish.

Edited By Mick B1 on 14/07/2021 12:04:18

Roderick Jenkins14/07/2021 13:41:53
2123 forum posts
582 photos
Posted by Mick B1 on 14/07/2021 12:00:52:

They are of 'tungsten steel', which I guess isn't quite HSS...

Actually, I suggest, nowhere near HSS. Normal O1 type gauge plate has 0.5% tungsten. Record plane blades were described as Tungsten Steel and they, like gauge plate, do not have the hot hardness of HSS. Most hardenable steels have various small quantities of elements such as vanadium, chromium and tungsten to modify the behaviour of the microstructure during the heating, quenching and tempering process. I suggest that in every day model engineering terms tungsten steel is no different to carbon steel.


John Baron14/07/2021 13:48:27
487 forum posts
189 photos
Posted by Michael Gilligan on 14/07/2021 06:17:06:

Thanks for that, John Baron

All looks good yes

Have you bought any screw pitch gauges from them ?

… I feel the urge to treat myself to a set.



Edited By Michael Gilligan on 14/07/2021 06:31:39

Hi Michael,

Yes I bought a metric set from them quite some time ago ! Good quality, but I didn't pay today's price, in fact if I remember correctly only about £4.00 + vat, vat was only 15% then.

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