I assume the holes are blind and a rotabroach is unsuitable.

What about a pilot followed by a 20 mm core drill?

The blacksmith's drill is perhaps not as high quality as desirable here.

What RPM are you running? What size pilot drills are you using? How sharp are you keeping your drill bits?

Stainless steel, including so-called "free machining" 303 are tricky to machine. They tend to work harden as you machine them, which then bluntens the tool, which then rubs on the stainless steel and makes it even harder.

But you should not have too much trouble drilling and boring those size holes if you take care. Don't run the lathe too fast. 500rpm for the larger diamter drill bit and the boring might be enough. Faster for centre drill and pilot drill/s.

Keep the feed up to the job. Don't slack off and let the drill bit or boring bar bit rub on the job. Keep the cutting edge cutting into the job without rubbing.

Keep your drill bits and boring bit sharp. Touch them up between each job if necessary.

And use the coolant pump if you have one.

You should be able to drill one hole in a matter of minutes, not hours. Ditto for boring it.

With any drill that size, you are going to need a pilot hole.

Going in "one slow stage" in stainless may be a recipe for work hardening and very slow progress.

With stainless, it's better to go in a number of quick stages and keep the bits cutting, not rubbing.

Like I said, what you are doing should take only a matter of minutes per hole, not hours, so something drastic needs changing.

Edited By Hopper on 08/05/2017 11:54:39

Hi Alan,

often have to the same job myself and in st/st…….I use a small pilot drill then hit it with say a13mm to within 5mm of the required depth…….once that's done just open it out with rotorbroch's as close as poss to the finish size….then clean up with a boring bar…..

I was lucky to buy a large job lot of rotorbroaches and an er42 collet chuck….it really doesn't take long…..slow chuck speed and loads of coolant, you soon learn to feed in at a firm steady pace….just watch out for long razor sharp waste……

Clogs

I suggest starting the hole with a large pilot point drill, say 12 mm, then use a core drill. I have a couple of these on MT2 shanks from Tracy Tools and they are very useful for drilling large holes – they have 3 flutes and cutting edges but need a hole slightly larger than their core diameter to start.

I do the same with a few grotty large drills that I use for opening up holes before boring.

Ian P

Alan W – As asked before – is this a commercial venture?