Making big Long Holes

I'm machining 35mm bars of 303 stainless in a Chinese CV8VS lathe. I need 20mm wide holes, 50 - 60mm deep. Currently I'm using a 17.5mm "Blacksmith's drill" first, then finishing with a boring bar. It takes a couple of hours, and the Blacksmiths drill wear out fast, and have a tendency to bind inside the hole high up the shank as the hole gets deeper.

Does anyone know of a better way to do this? If so, I'd really appreciate your advice.

Good quality drill with a morse taper perhaps best

centre drill then perhaps 6mm then 17-19 mm big drill ...use lots of cutting and or coolant

the bore to final size

How many you got to do ?

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.

Main thing is to keep the drill cutting. I would use lowish speed and sharp drill/drills. Once centred use an intermediate drill size that the motor power will cope with easily (say about 10 or 12mm) and drill to full depth. Then change to the final diameter (allowance for boring or whatever) at a lower speed.

Its essential for the drill to keep cutting, once there is a bit of wear especially on the flute lands the SS will play havoc and the drill will have a tendency to bind.

I find 303 easier to machine than free cutting mild steel but sharp good quality tools will help.

Ian P

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.

Thanks gents. I have an unknown number to drill - I'm making plastic injection moulding machines to order.

I've heard 2 schools on big drills - one saying go u pin stages, the other saying to it in one slow stage. My personal experience with Blacksmiths drills has been that one stage is better, though still not without misery.

Yes, the hole is blind. Well there's a 6mm exit for the molten plastic at the "other end". I'd never heard of Rotabroach before, but I just checked out a video. If it works by pushing a central slug out the far side, then it's no good for this.

I did wonder about a zonking great end mill, but I suppose that's no good in a lathe.

However, I never - oddly - considered using a morse tapered drill. So I'll but one or more - good quality large drills - and see how they do.

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

This sounds like a commercial venture. Is it?

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

Hopper / Clogs - thanks for the useful feedback. SO many possible ways to go.

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.

Most flute wear/binding is from running at to high an RPM. I grind my drills slightly off equal to make them cut oversize to prevent any rubbing.

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

Ian P

That;'s the thing, there is always more than one way of doing any job so you just have to try stuff out to find what works best for you. These are not particularly large holes, nor particularly deep, so I would keep trying with the drill bits you have on hand first, using suitable techniques as discussed. Ordinary drills should be able to do the job, with care.

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