Tap advice

[jgarrett]

I am trying to tap multiple 1/4"-20 holes in a piece of 1/2" mild steel. I am using a #7 pilot drill and a 4 flute taper tap. I have broken 3 so far. I am unable to go more than 1/4 of a turn at a time. I have tried cutting oils and even the next size larger bit..
Would a spiral flute tap be any better. I don't understand why it should be so hard to tap in mild steel!!!
Using Irwin brand taps.
Thanks,
Julian

 

[abby]

Are you tapping by hand or machine ? ordinary cutting oils are not recommended for thread tapping , you would be better off with a specialised tapping fluid , Trefolex for example.
Spiral flute taps are designed for machine tapping and do not require reversing to break the swarf , these are the only type that I buy now and , touch wood , I have had very few breakages using them by hand.
The quality of the steel you are using may be suspect , "mild steel" can cover a wide range of products not all of which are easy to cut or machine.
The angle iron used in old bed frames is a good example , I have burned HSS bits trying to drill holes in some examples!
Dan.

 

[mechman48]

+1 for Abby's comments, another tapping compound used by many here in the UK is RTD. 1/4 x 20 is a Whitworth /UNC thread; so the BSI recommended tapping drills are 5.1 / 5.2 mm...0.2008" / 0.2047" so your #7 drill should be ok, I suspect the 'mild steel' ... you could try #4/5 drill - 5.22/5.30 to give you that bit extra tapping dia.

 

[goldstar31]

Surprise, surprise but I have actually been tapping 1/4" BSF and half an inch deep- and 5 of them so far. They are going to be part of a 4 way turret for my little lathe. OK, it all follows the initial criteria of holding two parts together so that they can be disassembled at some time or in my case- regularly.

But why a whole half inch to hold nothing more than 3/8th lathe tool driven along by a 1/2HP motor?

I spent rather a lot of time messing about around with these 12" to the foot proper steel ships and my mill drill was welded up from ship's plate which was 1/4" thick. It was an off cut out of Swan Hunter's ship yard.

So why or why do I need a full 1/2" deep thread?

Maybe I should rethink- or maybe suggest to our poster, rethink the same question?

Norm

 

[kvom]

Irwin taps are your problem. Get some good quality HSS taps.

 

[ShopShoe]

I did not see that you said you are tapping a through hole. If you are only going part way through your metal a taper tap will hit the bottom of the hole and jam.

I agree that a higher-quality tap may be what you are looking for. The "Irwin" taps sold at home centers in the USA have been very low quality, in my experience.

Check out the taps sold by the major national machinist suppliers: MSC, KBC, Travers, etc. (No connection to them, but they spring to mind.)

I tend to order different brands and types of taps for experimentation. If things work well, I order more of the same the next time.

I agree that a lot of "found" steel is variable in quality. I have used the bed-frame angle iron for various things and the same piece may drill very easily in one place and then break three drills two inches away. Steel purchased from a steel supplier I have been able to drill and tap from 1/2" UNC down to 6-32 with no problems.

BTW, A P.S. thought: Drilling Mystery Metal may work-harden the hole, making tapping the hole difficult to impossible. Also, drills can be dull or poorly ground and not cut well, leading to work-hardening, etc.

--ShopShoe

 

[mcostello]

Gee I have been using ordinary black cutting oil for tapping for 30 some years, glad to find out it isn't working. Try a different tap as suggested, maybe different material if necessary.

 

[jgarrett]

I ditched the "Irwin" taps and got a couple of HS quality taps. Have done 5 holes with no problem breaking one. Still can only go 1/2 turn at a time but thats OK,,its getting the job done.
I am always on the lookout for bed rails at garage sales. I can always find a use for the stuff.
Cheers,
Julian

 

[RonGinger]

Bed rails are the most miserable steel ever made. Its not even good for welding practice. Nearly impossible for any machining operation.

 

[mcostello]

Bed rails harden almost instantly,the only thing worse I have found was a piece if railroad plate, Carbide would not penetrate,just end up breaking.

 

[Wizard69]

I ditched the "Irwin" taps and got a couple of HS quality taps. Have done 5 holes with no problem breaking one. Still can only go 1/2 turn at a time but thats OK,,its getting the job done.
I am always on the lookout for bed rails at garage sales. I can always find a use for the stuff.
Cheers,
Julian

Ditching Irwin is a good move.

Your taps will be stronger if you reduce the number of flutes. That is something to consider.

Make sure you are getting taps made of HSS. For the most part plain carbon steel taps are of low quality.

Work on your tapping technique. Realistically new taps are broken by poor technique. Also when ever possible use a tapping aid. The primary function of a tapping aid is to make sure the tap is coaxial to the hole. In the case of your 1/4" x 20 tapping a half turn is just about all you can expect and that half turn must be followed by a quarter turn in reverse v

Use lubrication. TapFree beats plain old El Dorado ool.

 

[RonGinger]

Another bad steel is the stuff made as wrought iron for porch railings. I found some square pieces, about 1" square. I was happy to find a source of square stock, but they were terrible to machine. An old friend of mine told me that as railroads were being torn up in the 79's and 80' the rails were turned into lots of things, including this square stuff.

Another 'free' material not worth the cost.

 

[goldstar31]

Bed rails harden almost instantly,the only thing worse I have found was a piece if railroad plate, Carbide would not penetrate,just end up breaking.

As far as I am aware, bed rails were made from scrap road tramline lines- which pretty agrees with the above.

These were welded originally with ferrite which was the constituent of German incendiary bombs.

I suppose one pays one's money and gave John Ruskin his famous discourse on Price. He gave his name to an Oxford UK university which, I suppose is a sort of recognised qualification. :hDe:
Oops,

Norm

 

[GLCarlson]

Threading is not a one-size-fits-all deal. One should consider material and % of theoretical thread being cut.

1/4-20 and #7 drill is a standard what-size-drill-do-I-use chart seletion, which will give around 75% depth threads. 75% for hand tapping in steel is pushing the limit, especially with carbon steel taps. Doable, but only if everything is right.

Get a better chart. Harvey Tool has one. So does Little Machine Shop, and theirs specifically makes recommendations for steel vs aluminum. One can use anything from a #9 to a #1 drill for 1/4-20, depending on % of thread desired. A #1 drill will give about 50% thread, which is the LMS reco for steel/stainless.

And, yes, ditch the hardware store taps, use decent lube, and learn good technique (starting square with hole, tap and reverse, choosing the right tap style, etc).

 

[kwoodhands]

#7 drill is fine up to 1/4" thick steel. Drill with #3 for 60% and the tapping goes easier. You will still have plenty of thread depth. Irwin/ Hanson drills and taps are of good quality, not the highest but fine for most applications. I usually buy Cleveland taps as replacements. Greenlee is another good quality tap. I use Castro Moly Dee which is no longer available for ferros metal tapping. Plumbers lard based cutting oil is a good substitute for steel.
mike

 

[Wizard69]

I must admit i prefer the fuller thread forms and haven't had much in the way of issues staying at around 75%. We use a lot of stainless, Aluminum and structural steel at work, some of the worse and some of the easiest. One can break a tap in any of these.

A few more points that have come to mind:

1. If you are using traditional hand tap forms, that is taper and plug taps, It often pays to completely remove the tap from the hole to clean it off. This especially on 4 flute taps that plug quickly with chips.

2. It might be my imagination but coated taps seem to suffer less from sticking. TiN coating seems to be good enough to reduce stocking when hand tapping. Of course we would be using lube here. Im often reluctant to buy uncoated taps these days.


3. Lubrication is very important which everyone has mentioned. Something is better than nothing and in many cases I've used IPA alcohol when the possibility of contamination couldn't be tolerated. IPA is a poor lubricant but it is a noticeable improvement over dry.

Sometimes the unusual is sworn by the user. I know of one guy that swore by Crisco and used it in his heavy fabrication work that he was involved in. He would literally climb around the structure with a can of Crisco and lube up the drilled holes to make ready for tapping. The follow up was to run around with a large drill motor and run the taps into each hole.

In other words lube is good even if it isn't made for the task.

4. Throw out dull taps. This might be obvious to many here, but dull taps lead to heartbreak. Well maybe don't throw them out literally, instead put them in a container of scrap tool steel. You never know when you might need to make a special cutter or just need a hard piece of steel.

5. Being in a hurry never helps with tapping. Actually this is the case with lots of things. The problem is being in a hurry when tapping leads to a broken tap in the worse possible place.

6. Whenever possible use the machine that drilled the hole to start the tap. Assuming everything is clamped down this leads to the best possible tap alignment.

 

[MachineTom]

All great suggestions, here some more. A Tapered tap is a starting tap, those long partial teeth at the nose guide the tap to be centered in the bore. The down side is that all those teeth engaged in cutting take lots of torque, and result breakage if tapping deep. Use a tapered tap for starting, then tap the hole with a plug tap.

A great tool to make is a tapping block, at its simplist its a 1/4+ thick steel, with pretapped holes done with a mill or DP, so you know they are straight.
Put a chamfer on each hole you intend to tap, thread the tap through the tapping block, it centers over the hole, keep hand pressure on the block until started and remove it after a few turns and finish the job.

With a tapping block no need for a starter tap, a plug just feeds in.

 

[Herbiev]

Great idea Tom. I must remember that one.

 

[GLCarlson]

Sometimes the unusual is sworn by the user. I know of one guy that swore by Crisco and used it in his heavy fabrication work that he was involved in. He would literally climb around the structure with a can of Crisco and lube up the drilled holes to make ready for tapping. The follow up was to run around with a large drill motor and run the taps into each hole.

Hard advice for most of us to take, but YES: toss taps (or resharpen) as soon as they seem even slightly dull. A few bucks saves a few hours, or a part.

And re Crisco: I'm one of those guys. Taught to use it for reamers 50+ years ago (fill the flutes solid). I use A9 or chlorosulfunated oil for tapping, but for reaming, Crisco is the lube to use- the full flutes float the reamer in, and chatter just disappears. Especially good with hand reaming where the setup is not so rigid.

 

[BaronJ]

I have used ordinary hand soap before now, particularly if there isn't any thing else handy.