[nickcnz]

Hi all,
I was hoping to get some advice on how to effectively bore a deep hole in 17-4PH. I'm having an awful time eliminating vibration - my hole depth is approx 5.3" and my boring bar is 0.5". From what I understand L/D > 10 is quite prone to vibration...
Having said that I have successfully turned 3 great parts with this setup in the past... all in one day... problem is ever since then I haven't been able to recreate them. The only thing I can see that could have changed since then is the angle of attack of the insert (my boring bar is carbide and completely round so I'm never 100% sure at what angle the insert should sit at).
I'm using CPGM 2150 KC730 Kennatmetal inserts and a 1/2" circle carbide boring bar.
From my notes on the day I had success, my feeds and speeds were 0.013in/rev and 815ft/min in CAM, but to be honest I probably slowed that down manually on the lathe. Step down was 5 thou.
I wish I had taken more comprehensive notes on the parts that came out so well, it was just so easy at the time I didn't think it would be an issue.

Any help is greatly appreciated, I'm not a machinist and I feel like I'm well over my head!

Thanks,
Nick

[handlewanker]

Hi, just for a start....you CANNOT bore a hole that deep with a .5" boring bar.......it's physically IMPOSSIBLE!.....I don't care if you state you have done it before, you were dreaming or high as a kite....LOL....joking.

The most you can bore with a boring bar without having deflection is 4 times the diam, and you can just squeeze 6 X the diam for a finish with a light cut, and using a round carbide insert is not the way to bore a hole.....the thrust against the side of the tool point is like a wedge action.

For a solution to your problem, go on Ebay and buy a .5" carbide tipped reamer......it cuts on at least 5 points and is a balanced deep hole tool for parallel holes with a good finish.....speeds and feeds must be right and coolant too.

For a reaming solution, you drill the hole with first a centre drill then several drills to leave you with .005" to .007" for the reamer to clean up on......a 12.5mm drill will leave you with about .008" to ream a .500" hole.

Speed is about 100 rpm and a slow feed with lots of coolant.....don't bottom the reamer (if the hole is blind and not straight through) or it'll seize up and break off.

BTW, why do you need a .5" bored hole?.....it's unimaginable for what useage you'd put it.
Ian.

[RICHARD ZASTROW]

Personally, I'd drill undersize then bore a pilot hole. The pilot hole will function as a guide bushing. From that point a size gundrill will drill/bore a straight hole to size and burnish the I.D. to depth. That should provide a straight, sized hole with a smooth finish. All you need is enough rpm, coolant supply and controlled feed.LOL

Alternately, after an undersize pilot hole you could use a same undersize double ribbon drill to finish the drilling then ream for size and finish.

Dick Z

[handlewanker]

I quite agree Rich, after the final drill you do need to start the bore with a short boring bar to get the reamer, whatever, running true, or the pre-drilling will just make the reamer follow any deviation and out of concentricity that has happened......most people I know don't bother.......slackers!

If the hole is a standard size you can also use an end mill, 4 flute, (quicker than setting up a boring bar or on a mill a boring head), to get the hole true for a start with the reamer......mostly the end mill won't follow an out of concentric hole if fed in carefully.....slot drills (2 flute) don't work in this situation.
Ian.

[RICHARD ZASTROW]

Another chance for my favorite complaint; Doesn't anybody use jig borer reamers anymore?????????

ADD: Jig borer reamers on page 91 of my 1998 Weldon catalog.

Dick Z

[handlewanker]

......and what is a jig borer reamer?

I have machine reamers, that is Morse taper shank, parallel shank reamers with carbide tips or just plain HSS.

I also had some Herbert reamers with the end having a taper screw to expand the end for regrinds etc, and a few David Brown reamers with the sliding micrometer adjustable cutter.

When I worked on jigs and fixtures we always bored our holes.....reamers don't hold positional tolerance without a hardened guide bush......ever.

Reamers will give you a round parallel hole to size, but it could be anywhere.
Ian.

[RICHARD ZASTROW]

Ian, no jig borer reamers in Oz??

A jig borer reamer looks a lot like a multi flute end mill. They are accurately ground to size on the O.D. The O.D. is concentric with the cylindrical shank. They have no chamfer and are end cutting (similar to the end mill solution).

They do NOT follow the drilled hole. The tool holder/spindle MUST RUN TRUE. In the past, that meant jig borer. Nowadays, machines and tooling in general is more accurate/precise.

I like to use them in applications as described here, starting bores for deep holes such as oil galleries or coolant passages in molds (moulds in Oz?)

Check out the internet, there's quite a few suppliers in the USA. Some have good descriptions of the tool. Might "google" jig borer reamer.

Dick Z

[handlewanker]

Hmmmm, I imagine the reamer is held rigid and if the hole is slightly off centre it would cut into the one side more than the other by having the end cutting ability like an end mill, something like a boring bar but with multiple cutting ends.

I've never used one of those reamers.......anything over 4 times the diam will deflect if the hole is not dead concentric with it....I may be wrong on that score......I can imagine an end mill cutting it's own circle due to it being short and stiff, but anything longer and I'd be a bit worried.

I used to have a set of specially ground 1 flute slot drills.....one flute was ground back and the lands of the other flute were "touched" with an oil stone to stop side cutting , and they acted like a short stiff single point boring bar, specially made for starting a hole for a reamer to follow......they ran dead true, easy to make.
Ian.

[RICHARD ZASTROW]

Ian, I did the same single flute end mill /boring bar thing,worked fine. Try "googling" jig borer reamer, should be descriptive and enlightening. LOL

Dick Z