drills for stainless

[JonE]

I have some 'cobalt' badged drills which dont seem to get through a 2mm-ish stainless plate (an old wide spreader blade) I was trying to make into a coverplate for the fuel pump aperture.
I did research last night and notice that cobalt comes in 5% (M35?) and 8% (M42) but that also Cobalt introduces instability - i.e. more chance of snapping.
Any advice on drills generally? I'd like to buy a set which work on everything, but now confused. I'm also confused by the badging of drills. Do people use imperial which seem to be pricier, or just work with metric sets which rise in 0.1 intervals, and convert?

[Howard Wright]

Hi Jon

I use standard HS drills (Bosch as it happens).  I have a metric set in 0.5mm increments and an Imperial set (not Bosch).  Almost all the time the metric in 0.5 increments are OK and if you need more accuracy then I go to the nearest size then use a reamer (by hand).

Hope I’m not teaching Granny etc…. But sharp drills are vital!  If you don’t have one, buy a bench grinder and learn how to set the right angle. Also punch the hole first and use progressively bigger drills starting small.  A drill press is invaluable and I set mine to a low speed which seems to work best. 

Hope this helps

Cheers

Howard

[Charles P]

Stainless work hardens easily, especially if it gets hot from cutting, so go very slow, very steady and gentle on the feed rate. And use lubrication.
Trying to redrill a "failed" hole is a fools errand

[Austin in the Shed]

Although I'm an old school engineer I tend to avoid using stainless if I can.In bolts it's A2 and A4.A4 being the marine quality and don't rust.
Steel off the top of my head ,might be wrong but think there's 303 and 316,303 the much easier to work.
S/S tends to work harden,especially if you get it too hot,try cutting it too fast,blunt tools.
I again as old school have a set of imperial and metric drills but really with the 0.1 mm increments on metric sets they cover sizes better than the old fractional ones.
You may be lucky with a cheap set re quality but unlikely I think.Run them slow and clamp your job,especially in a drill press and use a cutting fluid.

[JonE]

ok thanks both - so it may be that Charles' point is also most relevant too. I have two failed holes! But what do I do about it?

Howard. I must admit my drills normally get lost or break before getting sharpened, most likely as I know they are cheap jobber ones. Do you have lots of small radial files for taking off the rough stuff of the flat face after grinding?? I sharpen gouge chisels, which I presume are pretty much the same as drills.

[Stuart Giles]

Re your failed holes, drill from the other side of the plate on the same hole centres; the work hardening will only be on the drilled surface in my experience. As all have said, you'll be wasting your time and effort unless the drill is sharp. Use a cutting fluid NOT motor oil. Maintain constant pressure when drilling. The moment the drill 'rides' on stainless, the Nickel content will work harden it.

[Howard Wright]

Hi Jon

I’ve never needed to tidy up a drill after grinding. I have a fine grit wheel which presumably helps.

If you’ve made a hole in the steel then laborious use of a small round file could tidy it up sufficiently?

Cheers

Howard

[Hedd_Jones]

The biggest help I've found to assist drilling stainless, particularly thin plate (which is tue worst stuff to drill) is having a wide point angle on the drill (a nice flat end) by grinding it myself. 

Yes slow and sharp helps, the point angle makes the most difference IMO. 

I have zillions of used drill bits in a tray that weighs about half a hundredweight so it never worries me doctoring one to suit the job in hand, I guess If you had a nice case of swanky bits you would be less keen to do such things.

[Charles P]

If you have to a redrill a failed hole the careful application of grinding bit in something like a dremel can grind away the work hardened material. Easier on bigger holes. Don't ask how I know.
Like Dave I tend to avoid stainless unless the application really requires it. This job appears to not need it.

[Austin in the Shed]

For the fuel pump cover plate I'd use something like 6mm alum,2mm steel over the two holes is likely to leak maybe.