# Thread tapping.



## Fitzroy (18 May 2022)

Go gentle my first metalwork post! For my workbench I plan to be able to knock it down if required to move in the future. To enable this I will bolt the stretchers into the leg assembly. You can buy ready made hardware for this but it’s expensive and not as long as I want. So hey let’s have a crack at making it myself!

I bought a piece of 35mm mild steel and plan to cut it into 4x60mm lengths, then to drill and tap an M12 hole. 

The cutting to length and drilling a pilot hole has gone well. I was amazed at how easy it was to cut with a hacksaw and the drill the 4mm pilot. 

I’m now wondering if once I’ve drilled the main hole and tapped the thread do I need to harden it? I only ask as I was amazed at how soft the metal seemed and am worried the thread won’t be very strong. 

Thanks

Fitz


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## Ttrees (18 May 2022)

I'd say it would be plenty sufficient, plenty of those barrel nuts are often made of brass.
Bit of a pain lining things up, so unlikely it will get disassembled too often.
Biggest lesson I've had so far is to have sufficient force on the tap to get a thread established,
say when tapping an M10 thread the case, which is a big difference to say an M6 thread.
Make sure the work wont move if the former, say a vise on the end of a table which might be deflecting,
and indeed more so the case if going to M12 which some are.
Some folks use their drills for force and alignment, hand turning the chuck with tap in jaws, differing flavours include some making a centre pin for chucking up, and locating pin on the end of a tap, and spin the wrench, likely a better idea, never tried either method myself.


Metric drill sizes for tapping are conveniently found on the back of a ruler, but I've seen mistakes on one I have.
Best double check with another rule or two to be sure.
Seen some odd looking charts on the net.

All the best
Tom


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## Lorenzl (18 May 2022)

Couple of tips:
Tapping drill size for metric threads = Thread diameter - pitch
M12 will have a lot of friction so a interrupted thread tap will help especially if tapping a deep hole
Use tapping fluid - the brown one
Reverse the tap approx a third each full turn to break off the swarf
You could probably drill your hole a bit larger say 0.2 if you are having problems

I see no point in hardening it

I was told a M8 thread in steel would support 1 ton

A taper tap is good to get started and I like to use spiral point tap as it has a slightly different geometry


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## Fitzroy (18 May 2022)

Thanks folks. I may have bitten off more than I can chew again, but that’s the spice of life. I report back on how I get on with my £4.25 matched drill and tap set from eBay!


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## Vann (19 May 2022)

Be careful cutting the threads - don't force the tap too much. They snap off in the hole and then you might as well throw the mild steel piece away and start again. A cheap tap will most likely be less forgiving than a quality tap. Some old tapping sets say "cutting oil is useless" and recommend the use of dripping or similar - so that's an option. 

A thought, 35mm it a LOT of depth. Apparently just three full turns is supposed to be maximum strength. Personally I'd at least double that, but it may be worth drilling say 20mm at full diameter and only tapping the remaining 15mm. That would almost certainly save you a broken tap or two.

My tuppence worth.

Cheers, Vann.


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## Fitzroy (19 May 2022)

Vann said:


> Be careful cutting the threads - don't force the tap too much. They snap off in the hole and then you might as well throw the mild steel piece away and start again. A cheap tap will most likely be less forgiving than a quality tap. Some old tapping sets say "cutting oil is useless" and recommend the use of dripping or similar - so that's an option.
> 
> A thought, 35mm it a LOT of depth. Apparently just three full turns is supposed to be maximum strength. Personally I'd at least double that, but it may be worth drilling say 20mm at full diameter and only tapping the remaining 15mm. That would almost certainly save you a broken tap or two.
> 
> ...


That’s a very interesting thought. One of the issues with barrel nuts is lining the up. If I drill say 10mm in from each side at full diameter and only tap the centre 15mm it will make tapping and alignment easier. Cool plan!


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## gcusick (19 May 2022)

M12 is awfully big!


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## --Tom-- (19 May 2022)

Being mild steel it doesn’t have the carbon in it to harden, but the heat may distort the threads. So leave as is.

Take your time with the tapping, use tapping fluid and back out regularly. A drill press does help to start it square as described above. Mild is nicer to tap than a lot of other metals.

At last the pieces are fairly straightforward to remake if the tap snaps. That’s the least fun


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## J-G (19 May 2022)

Vann said:


> Be careful cutting the threads - don't force the tap too much. They snap off in the hole and then you might as well throw the mild steel piece away and start again. A cheap tap will most likely be less forgiving than a quality tap. Some old tapping sets say "cutting oil is useless" and recommend the use of dripping or similar - so that's an option.


!! If the OP can break an M12 Tap 'by hand' he's had more Spinach than Pop Eye !!

I have broken taps when tapping by hand but never larger than M4 and even that was only due to carelessness.

M12 is 1.75mm pitch and that's a deep thread (0.947mm) the 'Nominal' bore is 10.1 mm but, for non-critical applications, it is common practice to make the thread depth in the 'nut' only 5/8ths of the Nominal so although (as has been said) the 'Tapping drill size' can be considered [ OD - Pitch ] - making the tapping drill for M12, 10.25mm, you may well get away with 11mm. This will ease the load considerably.

There are also sets of 'Serial Taps' available. These go a stage further than the 'Taper', 'Second', 'Plug' sets - though they still do have the differing 'lead' - the OD on the 'First' tap (single groove on the shank) is smaller than that of the 'Second' (two groves) and that is smaller than the 'Finishing' Tap (no grooves). These make larger threads much easier to cut 'by hand' since they effectively don't cut the full depth in one go like the Taper/Second/Plug sets do.

Drilling some length at the OD would aid in getting the thread started and make sure that the Tap was 'Square' to the hole - - - there is little worse than getting 4 or 5 turns into a thread and finding that it's getting harder to turn the Tap because it's not 'Square'


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## Fitzroy (19 May 2022)

With all the fab advice I cracked on as the tap and drill bit had arrived and I couldn’t resist. Turned out nice like.


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## --Tom-- (19 May 2022)

Happy days


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## stuart little (20 May 2022)

Lorenzl said:


> Couple of tips:
> Tapping drill size for metric threads = Thread diameter - pitch
> M12 will have a lot of friction so a interrupted thread tap will help especially if tapping a deep hole
> Use tapping fluid - the brown one
> ...


I use 'Trefolux' cutting paste for thread cutting - works like 'magic'& mine must be at least 55 yrs. old now.


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## Digger58 (20 May 2022)

I always use a cutting fluid and back off to break chips, most important is to get it started square into the hole. I often use the drill press and turn it by hand then remove it to a vice when the tap is well established. That looks to be a fine job. As said above for non critical jobs a slight oversize hole makes the thread cutting a lot easier without compromising the strength. I often do it in Stainless Steel when making BBQ bits and pieces.


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## morqthana (21 May 2022)

stuart little said:


> I use 'Trefolux' cutting paste for thread cutting - works like 'magic'& mine must be at least 55 yrs. old now.


In reply to this:


Vann said:


> Some old tapping sets say "cutting oil is useless" and recommend the use of dripping or similar


I was just going to say "Trefolex".


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## raythompson102 (21 May 2022)

stuart little said:


> I use 'Trefolux' cutting paste for thread cutting - works like 'magic'& mine must be at least 55 yrs. old now.


I used to have an old tobacco tin with some Trefolux in it I got from work when I was an apprentice. When that finally got used up I bought a tin of Rocal cutting compound. I am now in my 70’s so you can guess how old the trefolux was and I must have had the Rocol a Good 40 years.


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## stuart little (21 May 2022)

raythompson102 said:


> I used to have an old tobacco tin with some Trefolux in it I got from work when I was an apprentice. When that finally got used up I bought a tin of Rocal cutting compound. I am now in my 70’s so you can guess how old the trefolux was and I must have had the Rocol a Good 40 years.


The stuff lasts years. I finished my apprenticeship in '66 (19 NOT 10) which wasabout the time I got my Tref. (red tin then), lasted all through my engineering, DIY motor car maintenance & 34 yrs self employed vehicle repair.


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## Vann (29 May 2022)

Vann said:


> ...Some old tapping sets say "cutting oil is useless" and recommend the use of dripping or similar...


As that statement sounds outrageous - I finally found it.






Just left of the "Guarantee" - "Lubricating Oil is useless" (I nearly got it right 

Cheers, Vann.


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## Scarlet Lancer (21 Jul 2022)

Vann said:


> As that statement sounds outrageous - I finally found it.
> 
> View attachment 136660
> 
> ...


Lard is also a good flux for soldering and brazing brass.


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## Sandyn (21 Jul 2022)

Fitzroy said:


> With all the fab advice I cracked on as the tap and drill bit had arrived and I couldn’t resist. Turned out nice like.


Easy!!!!
When I am drilling/tapping holes, I set the piece in the drill vice, drill the hole to the required size, then fit the tap in the chuck without disturbing the piece. I do the first bit of the thread just turning the chuck by hand, or using the chuck key for a bit of leverage. This ensures the tap is square to the hole. That's the bit that always gets me. I should get a proper set of taps for the common sizes I use.

Edit: I should have said, my drill vice is bolted to the drill table, so nothing moves when I change from drill to tap


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## Trainee neophyte (22 Jul 2022)

I've just done a lot of m10 threads in mild steel - not a happy experience. The cheap handles on cheap(Parkside) sets are made of pot metal and _will_ break if you look at them the wrong way. Actually using them is a guarantee for a mishap. I bought an "expensive" set just to get a better handle, but ended up using a 1/4 inch socket which happened to fit the m10 tap.

Go backwards at least as much as you go forwards, and don't force anything. Good luck.


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## clogs (22 Jul 2022)

I'd be using brass if u can afford it.....it's just as strong but a puppy to hand thread....even with the correct brass taps of old......
best bit about brass is the lack of corrosion....good luck with the steel and wood reactions....
I have a load of outside hardwood furniture that need ur kinda modification as the standard screws (large flat head direct into the wood) are useless after 1 season...
Cant seem to find brass or stainless stocked here.....
will be using ally for the threaded plugs and brass metric bolts lubed with a copperslip and oil mix.....


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## Lard (22 Jul 2022)

Scarlet Lancer said:


> Lard is also a good flux for soldering and brazing brass.


I take offence at that


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## J-G (22 Jul 2022)

clogs said:


> I'd be using brass if u can afford it.....it's just as strong but a puppy to hand thread.


Not quite true - the "it's just as strong" bit - Tensile strength of steels are in the range 58-90k psi whereas Brass is around 45 - 50k - the yield strengths are similar though. The main reason for Brass seeming to be 'easier' (which is what you imply) is that Steel doesn't break up into 'chips' in the way that Brass does so reversing the tap to break the swarf is more necessary when tapping steel.


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## Lorenzl (22 Jul 2022)

The quality of the tap can make a difference. The electrician has a RS set and the lab manager brought a Drapper set; I have tried both in emergencies and they are rubbish.


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## Scarlet Lancer (30 Jul 2022)

Sandyn said:


> Easy!!!!
> When I am drilling/tapping holes, I set the piece in the drill vice, drill the hole to the required size, then fit the tap in the chuck without disturbing the piece. I do the first bit of the thread just turning the chuck by hand, or using the chuck key for a bit of leverage. This ensures the tap is square to the hole. That's the bit that always gets me. I should get a proper set of taps for the common sizes I use.
> 
> Edit: I should have said, my drill vice is bolted to the drill table, so nothing moves when I change from drill to tap


I certainly your methodologies thanks for sharing.


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