Tapping Stainless

wooslehunter

wooslehunter

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I need to fit a cable gland to the top of my mast. There's a 6mm stainless steel plate bolted over the top & I need to tap this.

I have cobalt drills to drill the holes but I'm assuming I can't use my normal taps since they'll be too soft & I don't want to risk beaking one inside the hole.

Any ideas on what taps to get & where to get them? Can you get cobalt taps for instance.

Or, for that metter, would a stainless self tapper cut into a stainless plate?
 
'Normal' taps will cut threads in stainless steel without any problem. 300 series stainless steel is as soft as butter, the only problem with it is that it work hardens rather easily, such as by drilling with low loads and blunt drills.

Taps are hardened to a very high hardness and provided they are in reasonable condition will cut threads perfectly well. I have just finished tapping about 30 holes in stainless steel, using bog standard taps. Use plenty of oil, for both the drilling and tapping.
 
[ QUOTE ]
Use plenty of oil,

[/ QUOTE ] Preferably soluble oil? Or will any light oil be just as good.

Centre punching the holes over enthusiastically can cause problems due to work hardening I believe.
 
I also found that it makes it easier to clean the thread a little more often i.e. 1/2 turm clockwise followed by full turn or turn and half backwards gets rid of swarf .
 
Soluble cutting oil is preferable, as being water based it has a greater cooling effect. However, I don't have any and only use engine oil.

I don't understand the paraffin statement, why would oil boil before paraffin? Boiling point of paraffin is 300C, boiling point of machine oil is 380 - 400 C.
 
Be sure not to harden the stainless when drilling it. Then use a proper set of taps - ie 1st and second taper and plug. If going right through you wont need the plug but using the two different tapers mkes much lighter work of it and hugely reduces the risk of breaking a tap.
 
parafin is good for alumin not stainless. Any oil is ok, proper stainless cutting fluid is best but expensive for one off job. Use a good quality HSS tap not a HCS. 1/2 turn in and 1 turn out will remove tap! Good to clear swarf if hole is deep and blind. Drill hole square and tap square.
 
If you are buying taps specially, and don't mind the extra cost, go for high speed steel, (HSS) ground thread taps from an Engineers' Suppliers, rather than carbon steel from DIY outlets etc.. They work much better in stainless steel, but perhaps a bit extravagant for a "one-off". Also, drill the hole a few "thou" over the recommended tapping size. Your suggestion of a self tapper doesn't sound very good for 6mm plate if you want any strength.
 
The right drilling and cutting fluid is tepmatic or similar. Your success depends on drilling. If you work harden the SS while drilling, you can forget cutting thread.

Paraffin is used for aluminium.

All special thread cutting fluid consists mostly of acetone with some oils added. Acetone evaporates and so cools the work, no work hardening can take place.

I make my own mix, but have used pure acetone for cooling in a fix.
 
I personally have not come across cutting fluid containing acetone, sounds rather flammable. Stainless steel does not harden through heating, but due to very localised distortion of the metal caused by rubbing rather than cutting from the tool. Hence the need for sharp tools, low speed,and heavy pressure, (high feed), on the drill. Don't be tempted to open up a hole in stainless in very small stages.
 
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