Tapping Stainless Steel.

I want to tap a 5 X 0.8 mm hole in a 12.5 mm Stainless Steel bar.

My Black book tells me the tapping drill size is 4.20 mm, done this and when I tried to tap the hole the tap started then stopped and I am afraid to turn it any more as it may break the tap.

What size hole is recommended?

Hi Mark,

You could try 4.4mm or even a 4.5mm drill. I would only use a 4.2mm tapping drill for materials that are easy to cut. Does the tapped part have to be 12.5mm deep? Could you drill the hole 5mm for part of the bar?
When tapping difficult materials I use serial taps, ARC sell them.

In Tubal Cain's book: Drills, Taps and Dies (Workshop Practice Series #12) you will find tables of tapping drill sizes for different thread engagement.

Thor

For stainless steels I aim for a thread engagement of about 50%, so for M5 I'd drill 4.6mm. For other materials I use a thread engagement of about 70%. For fine pitch (mostly ME 40tpi and 32tpi) I use an engagement of >80%.

Andrew

The metric tapping drill being diameter - pitch is fine but if you look at charts that give full thread data I think you will find that this will mean cutting part of the radii that is specified for the root and doesn't really account for any tolerance on the tap itself or the drill. I never have any problems with free cutting stainless but the real thing is a lot more difficult and having 1st and 2nd taps becomes more important. The free cutting one is slightly magnetic. I always check as sometime when buying small pieces the suppliers might pick the wrong type up.

To be sure of clearing rads and drills being slightly undersized etc 4.4mm should do it or even 4.3 really but if you only have a plug tap you might still struggle. Scraping a bit of hand bar soap onto taps makes a fairly decent lubricant.

I did have problems with one small piece I made. M4. Turned out that the die was well under sized. Initially I couldn't understand what was going on. It's might be worth checking the diameter of the taps you are using.

John

-

I have always found that SF (spiral flute) taps are superior for tapping all materials. You do not get the accumulation of cut material cloging up the flutes that occurs with "conventional" straight fluted taps including serial taps, as the swarf spirals out away from the thread!

Accumulated swarf = broken taps

Those serial taps sound like they might be a good idea - reduce back cutting.

With M5 causing problems though I wonder about the tapped depth. There is little gain and then a fall off in strength as thread engagements increases. Even with precision fixing screws it's pretty common TDO practice to limit the engagement length to 1 to 2 diameters of the thread. Some reckon less than 2.

John

-

Mark,

I work with stainless 316 on a daily basis with my job and I find that standard taps with lots of tapping compound and one turn then back off, one turn then back off works well for me. Every time you back off another dip into the tapping compound.

Martin P

Thanks for your replies.

I finally succeeded, I used 4.5 mm then 4.8 mm drill, I was using Wurth Cut+Cool Cutting Oil,then swapped to Diesel engine oil which is a 100% better lubrication.

http://www.louisandcompany.com/storefront/promotions/p-l-e-a-s-e-e-m-p-t-y-m-e/cut-cool-cutting-oil/prodWR893050004.html

I am making a new chisel that will take carbide tips for wood turning,

I'm old fashioned. If a tapped hole matters I go 1/3 of a turn in, around the same out to break off the cut and so on. I understand that the usual straight flute tap is designed to be used like that. I don't use a protractor to measure the angle turned and would guess it might be less than 1/3, just what's comfortable. I also make an effort to ensure the tap is aligned with the work and to keep it like that - going in at an angle is a certain way of breaking smaller taps or having them jam up, Too much of a turn will break very slender taps as well.

John

-

Hi Mark,

Don't want to be fussy but you have only got .1mm or less depth of thread now

Tony

For proper looking serial taps, and a good illustration showing the rings on the shanks, key this into 'the well known auction site' -

Metric Coarse M 5 x 0.8 5 mm Hand Taps Serial Form

Too late for now but ready for next time and all the others who read these comments.

Tim

How are you taping?

assuming a mill or lathe then after you have drilled the hole then the chuck should be exactly aligned with the hole you made and therefore a perfect support for taping perfectly straight.

I use this (picture later) to tap by hand, the machine isn't turned on for this! The chuck is holding the tap just enough to keep it on axis but allowing it to turn freely.

Tap is loose in chuck, it can rotate and move in and out freely, the tap handle is tightened onto the shank of the tap just above the threads.

Or you can use a tap stand that the usual model engineer places sell.

I use metal working oil on small taps as I believe it helps prevent clogging in the flutes compared to thicker oil.

I tap stainless 304 with a 4.2mm hole with HSS taps.

Chris I did it as in your photo

I am ordering some sets of taps from Ausee,, I believe they are the same as you get from Arceuro Trade.

Had another look this morning at the thread, very shallow, I will see how it goes, I can always cut it off and start again.

This is the first time I used a mill so all a learning experience.

What is metal working oil, Google is giving me conflicting views.

I use this:

http://www.molyslip.co.uk/products.php?prd=48

Thanks Chris,

Found some in Aust,

$30.20 plus $44.61 postage

From the UK

£16.01 plus £3.50 postage.

Work that one out.

Edited By Mark Eisen on 14/06/2015 04:10:06

Edited By Mark Eisen on 14/06/2015 04:11:45