Thread: Which Tap & Die set to purchase?

Originally Posted by Stustoys

Metric tapping drill size is thread size minus pitch. Works for imp threads to I'm told if you can be bothered with the maths.

This method will produce holes that are generally too tight and can lead to broken taps.

The other commonly use ROT is Bolt Size - 10% of bolt size, but both are only ballpark figures

The Tubal Cain (TC) book recommends 5.35 mm holes for a 60% or loose fit down to 5.1mm for an 85% (tight) fit - the "British Standard" sizes are 5.3, 5.2 and 5.1 mm
So the -10% rule (5.4 mm) is too loose and the Size - Pitch rule (5 mm) will be too tight

M8 (1.25) TC recommendation is 7.2 mm for a 60% fit down to 6.9 mm for tight fits.
-10% rule (7.2mm) is on the loose side and Size - Pitch (6.75 mm) is too tight.

M10 (1.5) TC recommends between 9.2 and 8.6 mm. -10% rule is OK but Size - Pitch (8.5 mm) is too tight.

M10 (1.25) does not have an entry in TC but all sizes can be worked out using a formula on P92 of the TC book.

Diameter of bolt - 1.083*Pitch * % flank height/100

The % flank height is ~10% greater than the engagement, so a 60% engagement is around 50% flank height.
This will allow you to work out what ever you want.

Originally Posted by BobL

This method will produce holes that are generally too tight and can lead to broken taps.

Isnt that what I said? Though you shouldnt break taps....... it just might takes ages, no cordless drills. Likely doesnt do they taps any good either. Did I mention I can be pig headed?

The thing is most tapping drill charts use those sizes...........as we said they are really to tight for normal use on steel. But cast iron can tap like butter, as can Aluminium, plastic is anyones guess..........so I guess that maybe why the err on the tight side.

There is a lot to it, deep holes normally use larger drills........ but it all depends what you are after.

Stuart

Thank you chaps for your continued discussion and debate - very enlightening, as always.

A few Qs have come up as a result. I just had a close look at some of my threads (various common bolts) and they all have a 60° profile, as I expected once I thought about it. Is this always the case (apart from specialised threads)?

I presume that the distance between the threads is referred to as the pitch (i.e. the wavelength). What is the depth of the "V" referred to as, and what is the incline of the thread referred to as please? The depth of the "V" would be the pitch multiplied by Tan 60°. I remember back in High School Tech Drawing that the teacher explained Starter Motor threads, with multiple parallel threads for strength at the increased incline which is to enable rapid movement lateral movement per revolution.

Thinking about an M6x1 thread for example, I understand that one revolution of the nut will advance it by 1mm (very useful for jigs etc), and an M6x0.75 will advance by 0.75mm, and that lead me to something else I've noticed - backlash (specifically on hand planes ). If the nut that the thread advances though is fixed then any backlash would have to be only from the threads not mating correctly, and because the pitch must be the same for the thread to actually work, then this backlash would have to be from one of the threads not being at 60° wouldn't it? Obviously if the device that is holding the nut has some play then that will introduce it's own backlash, and I suspect that is what is responsible for it in planes, but in other nut & bolt combos I do notice it from time to time (which is where the different angle thing in the profile would have to come in). I guess that is what also creates the "chatter" that is too often found when spinning down a nut at speed.

Originally Posted by FenceFurniture

A few Qs have come up as a result. I just had a close look at some of my threads (various common bolts) and they all have a 60° profile, as I expected once I thought about it. Is this always the case (apart from specialised threads)?

No, BSW - British Standard Whithworth is 55 degrees, see here

British Standard Whitworth (BSW) Thread Form

Thanks Fred. It was an eyeballed 60° .

Originally Posted by FenceFurniture

If the nut that the thread advances though is fixed then any backlash would have to be only from the threads not mating correctly, and because the pitch must be the same for the thread to actually work, then this backlash would have to be from one of the threads not being at 60° wouldn't it?

No - if the nut thread is too large or the bolt thread too small (for example instead of a nominal 6mm major diameter it's 5.8*) then there will also be slop in the threads which will manifest itself as backlash.

Michael
* If you measured a M6 thread you would probably find that the major diameter was less than 6 anyway because of the manufacturing process

Originally Posted by BobL

This method will produce holes that are generally too tight and can lead to broken taps.

I disagree. The method "Metric tapping drill size is thread size minus pitch" does give the proper standard drill size.

However, it is up to you to correct this drill size according to the many variables of the job at hand. This requires some experience and common sense. For example:

- It is up to you to decide how strong a thread you need/want for the particular job. The standard drill size is for a 75% thread depth. But for many applications, a thread depth od 60-70% is perfectly adequate. For example, a 100% thread specification is only 5% stronger than a 75% thread specification, BUT REQUIRES 3 TIMES THE TORQUE TO PRODUCE!!!
- the material matters. Mild steel and cast Iron and and Brass etc will work best using the standard drill size (say for M6 5mm drill). Whereas tough materials such as S.S. and and better steel grades need a slightly larger drill (say for M6 5.1mm drill). And extremely tough materials like Aluminium-Bronze may need a 5.2mm drill for M6, else it will indeed be very hard to tap and if successful without breaking the tap the screw may bind in the threaded hole because the clearance is too small. Materials like soft Aluminium and some plastics tend to slightly flow when threading and need a larher drill size (say 5,1 or 5.2 for M6) - think of this a little bit like if you were using a thread forming tap instead of a thread cutting tap.
- the type of tap you use matters too. Have you noticed that the better taps come with colored rings around the shaft? These rings tell for which material the tap geometry and tap tolerance and tap coating has been optimized. You can use these taps on the material they were designed for drilling the standard size hole (here what the codes mean www.allenbenjamin.com/get_file.php?file=953). But if you are using generic all rounder taps, it is up to you to correct the drill hole.
- the quality of the tap you use matters too. Better taps are finely precision ground to a known dimensional tolerance (the tolerance is usually marked on the shaft or on the box). Cheap taps can have HUGE dimensional tolerances. A cheap, roughly finished only machined tap (possibly with some burrs left) is likely to require a lot more torque to cut than a fully ground/polished high quality tap. You are well advised to drill a larger hole than specified when using low grade taps.
- what will the threaded hole be used for? For example, a S.S. screw in an Aluminium hole exposed to the elements will want to seize, more so if the thread is a tight fit. Sure you should use an anti seize compound, but it will also help to drill a larger hole before tapping.
- are you cutting dry or with a lubricant? The hole can be choosen tighter if using a lubricant.
- a blunt/worn tap requires much more torque -- drill a larger hole or sharpen the tap or toss/replace
- are you properly countersinking the hole?
- are you using a guide or tapping freehand?
- tapping manually in a drill press or mill?
- how good are you at "feeling" whilst tapping? If inexperienced, better drill a larger hole.
etc etc etc - Chris

Originally Posted by FenceFurniture

, , , , , , If the nut that the thread advances though is fixed then any backlash would have to be only from the threads not mating correctly, and because the pitch must be the same for the thread to actually work, then this backlash would have to be from one of the threads not being at 60° wouldn't it? . . . . ..

Backlash has little to do with the angles of the thread and happens when the male thread is too small in diameter for the female thread - so even though the angles are all same the Male "V's" are not protruding far enough into their female counterparts and is why backlash increases with use/wear.

This is where having adjustable dies is really useful.
Since making taps adjustable is somewhat tricky, generally the female thread is cut/tapped first.
Then the die is adjusted to to cut a thread that is too tight and the male thread is cut and tested on the female part.
The die is then adjusted accordingly and the thread cut until a satisfactory fit with minimum backlash is obtained.
It sounds like a lot of mucking around but once the die is set it will generally cut true for repeated threads relative to the tap.

> A few Qs have come up as a result. I just had a close look at some of my threads (various common bolts) and
>they all have a 60° profile, as I expected once I thought about it. Is this always
> the case (apart from specialised threads)?

Nowdays yes. Only some antique British threads had 55 degrees. It matters where you do restoration work. Nobody would use these threads on new products.


> If the nut that the thread advances though is fixed then any backlash would have to be only from the threads not
> mating correctly, and because the pitch must be the same for the thread to actually work, then this backlash
> would have to be from one of the threads not being at 60° wouldn't it?

The amount of backlash is given by the thread tolerance. In industry, male and female threads are both made to tolerances. Very much like shafts and bearings have tolerances. For example, a nut may have been made to 6H tolerance and a bolt may have been made to 6g tolerance and they will both be a loose fit to each other. Taps and dies can be bought to different tolerances.

In the home shop one rarely works to tolerances, one usually fits parts together. This is true for shafts/bearings and for nuts/bolts too. One adjusts the bolt diameter when threading in the lathe until a test nuts fits to ones feeling. Or one adjusts a split die until the nut fits.

Originally Posted by cba_melbourne

> A few Qs have come up as a result. I just had a close look at some of my threads (various common bolts) and
>they all have a 60° profile, as I expected once I thought about it. Is this always
> the case (apart from specialised threads)?

Nowdays yes. Only some antique British threads had 55 degrees. It matters where you do restoration work. Nobody would use these threads on new products.

Whitworth may be "antique", but up until last year the majority of the nuts and bolts in Bunnings for example were Whitworth so a lot of workshops still abound with Whitworth bolts. I certainly wouldn't call them a "specialised" thread.

Originally Posted by Big Shed

No, BSW - British Standard Whithworth is 55 degrees, see here

Metric and Unified are 60 degree threads. Unified are UNC, UNF and NPT (US pipe thread)

Whitworth threads are 55 degree; these are Whit, BSW, BSC (different name, same threads), BSF and BSP (English pipe thread)

The differing angles mean that they are NOT interchangeable even though the nominal diameters and threads per inch are the same; unless you are the type of clown who also "knows" that Philips screwdrivers can also be used in Pozidrive screws and that drill bits can be accurately reground by hand. Additionally, the roots and crests on Unified are cut flat where on Whit they are rounded. Basically, when you jam in a Whit bolt into a Unified thread it is effectively slightly oversized and ends up smearing its threads. Do this with stainless and you may encounter that wonderful phenomenon known as "galling". A quality set of thread pitch gauges in Unified, Whit and Metric (Starrett or Moore & Wright are tops) will enable you to physically see the difference between thread pitches easily.

Diameter minus pitch and diameter less 10%; these are ballpark figures. Buy a set of thread tables (like Zeus) or print them off from on line sources and post them around your metal work area. The big problem you'll find is getting the correct sized drill bits; most drill bit sets come in either 0.5mm or 1/64th". Not really suitable unless you're drilling out a hole with your hand sharpened bit to tap UNC to use with a Whit machine screw with a pozidrive head that almost fits your Philips screwdriver. In which case they're absolutely perfect. Buy Dormer drills; they come in 0.1mm increments and the basic HSS used to make them is harder than you will believe; until you have seen them drill out a stainless Whit bolt that had galled in a UNC tapped hole that first blunted Sutton HSS then Sutton cobalt bits.

Whitworth nuts and bolts were discontinued in the civilised world a few decades ago; eventually Australia will be dragged kicking and screaming into the 20th century and will also stop using them. Imperial pipe threads such as BSP and NPT unfortunately will still be around for a long time though. You'll find that most combined metric/imp sets are Unified.

To summarise:
1) Buy your taps in metric and Unified threads; only buy Whit if you absolutely must. Dormer, Goliath and SKF use superior HSS steel to Sutton or P&N.
2) Buy thread gauges in all three thread forms for easy thread identification.
3) Get a set of thread tables and buy the correct sized drill for each application. Dormer and SKF for drills.

Originally Posted by cba_melbourne

I disagree. The method "Metric tapping drill size is thread size minus pitch" does give the proper standard drill size.

However, it is up to you to correct this drill size according to the many variables of the job at hand. This requires some experience and common sense. For example:

I agree, common sense and experience are extremely valuable.

Unfortunately I don't have much experience so I turn to a reliable source like the Tubal Cain book on Taps and Dies which I have found (unlike the Web) to be extremely reliable.

Poor Brett just wants some taps and dies. Very good info guys.

To make this all easier try this program Machinists Calulator

I have attached a screen shot from the last unusual threading i did, 5/8 18 UNF. It shows common drill sizes and their % fit, as well as theoretical drill sizes for any given fit. For us metal workers it will also give you measurements over wires etc plus other non thread related stuff.

Cheers,
Ew

Originally Posted by Chief Tiff

unless you are the type of clown who also "knows" drill bits can be accurately reground by hand.

Guess that makes me a clown then. Out of interest how close to size does the hole need to be? perhaps we should be reaming them before tapping?


While Dormer drills in 0.1mm steps might be great if you had a few 1000 holes to tap at say 600 an hour(when you would mosy likely be using way less that full thread depth unless you happen to have an all singing all dancing tapping machine and by that I dont mean a tapping head in a mill or a cordless drill). Its really a little over the top for your average guy in his shed. Though being a little over the top doesnt stop most of us.

Originally Posted by Ueee

Poor Brett just wants some taps and dies. Very good info guys.

Should we start on thread strenght of dia cut threads v roll formed?

Stuart