[Jensen3127]

We will be using a 1/4-20, 5/16-18, or a 3/8-16 tap (we aren't sure yet because the job is still in the works). The catch here is that we can only passivate the surface of the stainless from were we work with a handheld device - the parts are about 17ft long and we have already determined that dipping these in passivation tanks would place us out of budget. That said, how can we use either one of these thread diameters (approx 5/8" of thread depth) without the use of lubricants on 304 stainless steel? (we are allowed to have an additional 3/4" of hole depth clearance, but cannot break thru.) The thought of not using tapping oil screams nightmare, but then again I've never tried anything like this.

I have tossed around the idea of using some exotic taps, but my knowledge of taps isn't that extensive. Would this help at all? What types?

Thanks!!!

[txcncman]

Probably powder metal roll form taps is the way to go. I am curious about your passivation process and standard. My experience with passivation is you put the part in an passivation chamber (atmosphere and pressure controlled) and apply a vacuum then apply nitrogen gas and then heat to a specified temperature and then hold for a specified time.

[harryn]

In the semiconductor industry, people sometimes use semiconductor grade isopropanol (2 propanol / rubbing alcohol) as the machining lubricant. Perhaps this will be considered acceptable.

I use it at home myself just because I hate cleaning oil off of parts, but mostly on Al. SS is outside of my home capabilities.

The trick is that 90% IPA has a very low flash point, so it has to be done by hand and away from any electrical sources.

If you dilute it 1:1 with DI water, the flashpoint is much higher. Good ventilation is a must for either approach.

[jeffrey001]

You could try solid carbide thread mill and just go slower, or keep SFM low, for a couple of holes, it will do the job.


Jeff

[Jensen3127]

Thanks for the tips guys. I will do some tests and keep you posted.

@txcncman
The passivation process we would be doing is the "wet" form. Basically a caustic soda bath at 150F. We wanted to have some sort of actual film/coating on the metal as this will be exposed to some level of humidity.

[Geof]

I am puzzled why you must not use lubricants for tapping stainless parts when they are going to passivated.

Passivating is a process that is performed in a bath of strong oxidizing acid. The longest established form of passivating uses a mixture of sulphuric, nitric and chromic acids which are really nasty. A newer passivating mixture uses citric acid which is less nasty and easier to dispose of.

Before passivating any stainless part must be thoroughly cleaned, and this is the function of the sodium hydroxide bath, to remove grease, oil, etc.; including natural oils from skin. Any oily contaminants on the stainless will react in the acid bath and produce gas bubbles which prevent the formation of the correct passivated layer.

Free machining grades of stainless also need a sodium hydroxide bath after the acid passivation to neutralize any residual acid on the surface as this can react with the sulphur in these grades and lead to a break down of the passivated film.

What passivation does is remove any traces of non-stainless metal fragments from the surface of the stainless steel. These can occur as wear particles from HSS tooling and also from contact with vices jaws, wrenches, wire brushes, saw blades, or just from airborne particles settling on the surface of the stainless. If these particles are not removed they can cause catalytic corrosion of the stainless discoloring the surface and eventually leading to pitting. The oxidizing acid in the passivating baths dissolves these non-stainless particles away. At the same time it creates a strongly oxidized layer on the surface of the stainless which makes it extremely resistant to further chemical attack. Passivating does not add anything to the surface of the stainless it chemically modifies it to an oxide form that strongly adheres to the surface.

[txcncman]

Thanks for the tips guys. I will do some tests and keep you posted.

@txcncman
The passivation process we would be doing is the "wet" form. Basically a caustic soda bath at 150F. We wanted to have some sort of actual film/coating on the metal as this will be exposed to some level of humidity.

Thanks. I just increased my ejumakation.