View Full Version : Tapping Stainless,should I buy a lotto ticket?

08-05-2015, 12:04 AM
Tonight at work was fun,last operation on some stainless parts I am making was tapping eight 1/4-20 holes 3/4" deep,through holes luckily.

I keep a couple EM taps on hand just for tapping stainless and normally they work great and tap many holes before they dull and get tossed.Tonight was the exception,third hole in and suddenly it started to be hard going.Since I had a lot of time invested in these parts,experience kicked in and I back off and looked things over before making another run at it.

Checked the tap over and sure enough a couple small chips right near the start.Okay,new tap time,went to the toolroom and nada,no more 1/4-20 ER taps to be had.Parts have to leave tomorrow so what to do?Looking around all I find are hand taps,those would be slow going and much more fragile,then I remembered the last batch of cheap spiral point taps I bought for general knockaround work.

Found those and finished the remaining five holes no sweat.It even went easy compared to the EM tap.Now the shocker,the tap in question? $1.95 Chinese 2-flute gun tap.The threads are even clean and tight:eek:

08-05-2015, 12:11 AM
Buy a lottery ticket

08-05-2015, 12:17 AM
If you run the drill at about 1/2 the normal rpm you will tap stainless easier. Running the drill at too high an rpm heat treats the hole, making it wear the taps faster. Some alloys are more sensitive to this than others.

08-05-2015, 04:45 AM
No point buying the lotto ticket, you just used up all the luck that you had been accumulating for the last year or so.


It is always fun to finish up a job and send it on it's way.


08-05-2015, 05:02 AM
I don't ever tap stainless so what I was told may be BS, the guy showing me years ago said that problems tapping stainless happen before the tap ever gets in the hole, he reckoned that the drilling bit was done wrong then tapping was going to go to hell, it was his theory that drilling with a blunt drill or too slow would leave a work hardened hole (working girls as he put it but he was a bit of a character), he reckoned that sharp drill coolant and heavy feed was the solution, not allowing local hardening, was or is this sensible, it sounds right as I have seen a blunt drill in stainless glow.

08-05-2015, 09:49 AM
Consider how much depth of engagement you need. Standard tapping charts are for 75% (as I recall), but you don't lose much strength going to a slightly larger tapping drill.

08-05-2015, 11:11 AM
I drilled the holes 3/16" through first,then drilled them out to #6 .204" rather than the usual #7 .201.
I've done hundreds of holes like these over the years in Stainless and always drilled in two steps to counter the possible work hardening.That's why I didn't expect the tap to fail like it did,course I was unknowingly down to the last tap which means of course something would go wrong LOL ;)

08-05-2015, 11:17 AM
A dull drill or too slow on the feedrate will harden the hole. When you slow down the rpm you can still take almost the same feedrate, you're just getting a thicker chip. Anything that reduces friction (and heat) and keeps the metal and cutting tool cooled down is what you're after. A sharp drill at low rpm with cutting oil or coolant will walk right through, where a higher rpm will get harder and harder to drill, in some cases almost impossible to tap.

08-05-2015, 01:06 PM
Buy a lotto tik then some Moly-Dee :) Stuff loves stainless. JR

08-05-2015, 01:19 PM
ER tap? EM tap?

What's with the lingo??


Rich Carlstedt
08-05-2015, 05:46 PM
Paul is right, standard drill charts are 70-75 % and do not apply to Stainless Steels or Hardened Steels.
This was determined by the machine tool industry back during WW II .
For these metals you go with 55 to 60 % as thread strength is far superior to milder steels.
Also SS will work harden in the threads and the yield point increases with load as deformation occurs.

Another concern many do not consider is the ratio of the thread root diameter to the major diameter
The lowest ratio (very weak) is a 10-24 thread, followed by a 1/4-20 (2 nd)
This means the tap is asked to cut a large amount of metal with a minimum body diameter .
It also means these fasteners fail quickly when stressed ( Like a head shearing off a torqued 1/4-20 screw )


Don't know what the use is Weird , but with a 3/4" deep hole in SS, a "standard" tapped hole is way overkill
Glad you opened it a bit from the #8 (.199) which is 75 %
I would have gone all the way to .209 on the tap drill and gotten 60 %.
Most engineers I have run into have no idea of the above concerns in manufacturing
As a manufacturing engineer, I have had my battles over this with designers ( and overkill....or worse planned failure !)

08-05-2015, 06:51 PM
This was discussed a while ago in reference to my shop project of a wobbler air engine, that called for 1/4-20 tapped holes 3/4" deep. It was supposed to be 1018 mild steel but I accidentally cut off a chunk of SS and used it, not realizing my mistake until I tried to use the magnetic chuck on the surface grinder. I drilled it for the standard 75% tap (0.201") and was able to get at least 1/2" depth using my cheap taps, and then I pecked my way to nearly the full depth by alternating between an alloy steel and carbon steel tap that I ground as a semi-bottoming type. I also finally "cheated" by drilling a bit larger, perhaps 0.205".

Here is some information on thread strength:


Seems like 4% to 5% oversize causes little change in strength. 4% over 0.201 is 0.209.

Mike Burch
08-05-2015, 07:04 PM
Boslab, I've tapped a lot of holes in 316 stainless (horrible stuff, but necessary on a boat), and I would say that your informant could well be right. Stainless generally, and 316 in particular, work-hardens at the drop of a hat, and poor drilling could easily do it, making the subsequent tapping difficult.

08-05-2015, 08:09 PM
ER tap? EM tap?

What's with the lingo??


Doozer: I'm not the O.P., but he is probably talking about E-muge taps (sic).


08-06-2015, 12:21 AM
Doozer: I'm not the O.P., but he is probably talking about E-muge taps (sic).


Brain fart on my part,these are the taps-


08-06-2015, 12:28 AM
Seems like 4% to 5% oversize causes little change in strength. 4% over 0.201 is 0.209.

I could have opened it up larger,but they started out taking about the same torque as mild steel,so I figured good enough.

It might have been the material too.It was some 1" 304 plate that we had in stock,no telling where it came from,it just had the alloy and mill number on it,no country of origin.

08-06-2015, 07:36 AM
I stopped using Greenfield (now Widia) taps a LONG time ago. When GTD got bought out and moved production out of Greenfield MA, quality went bye bye. Get an OSG, Prototyp, Emuge or other quality tap and this job is cake.

That tap, by the way, is completely the wrong style for a through hole. You want a spiral point tap, not a spiral flute.

I have never needed to drill oversized holes to tap successfully.

Here's a tap I bought off eBay for under $15 that will tap hundreds of holes in 300 series stainless. It's a Prototyp (brand) "Inox" spiral point tap meade specifically for stainless steels, and it ROCKS.


08-07-2015, 07:44 AM
Hi I use red stripe (not the beer!) Sowa Taps, They work well are cheap. Also drill the hole .005 larger than Tap Drill Chart. I've tapped many holes in S S . On the larger Stainless Tapped Holes Specifically The Pipe Taps I use a tap reamer to shape the hole for the Tap what a huge difference when tapping afterwards, Hope this helps OH also fill the hole with tapping fluid, Good Luck Mike