Switch to carbide drill = Destroy boring bars??

[PoiToi]

Hi all, I have a strange thing happening on my turret Lathe.

We are making a part out of 303 Stainless ( the Ugima 303XL super-duper stuff)
and it usually runs wonderfully.

We got a much larger order in, and I convinced my boss to try a solid carbide drill to drill a .5" hole that's .8 deep into the part.

After drilling, the part is bored and a thread relief is put in, and then threaded in the ID.

After I switched to the carbide drill, the boring bars have horrible tool life, and the failure is VERY sudden. it will run 50-100 perfect parts, then just snap off in the part, taking a bunch of other tools with it.

I'm very happy with the SC drill, but I need to know why it would affect the boring? With the HSS drill, the material got plenty hot too, so i dont think the problem is work hardening.

I'm running the drill at 2200 RPM at .010/rev.
Any thoughts would be appreciated!
thanks!

[HuFlungDung]

If your boring tool still bores between 50 and 100 parts perfectly, then I would think that work hardening is not the real issue. Hardening of the part would degrade the cutting edge with each part, and should (I would think) cause more and more bar deflection with each part, which should be detectable as an error increasingly departing from perfection.

What is the exact hole diameter that the drill produces? The point of this question would be to determine if the drill is perfectly on center when it is working, or whether it would be bearing hard on one side of the spindle centerline.

Does the boring bar have to face the bottom of the hole flat?

[PoiToi]

The drill is right on center, I thought about that too. If i cram a .501 pin in the part, I have to use pliers to get it out.

and yes, it has to be a flat bottom. at least the diameter at the flat bottom is .510, so not much material to take off. also, we have run 10,000's of these parts with little problems, and that was with a 118 degree HSS drill. So a 140 degree drill would mean less material to take off at the bottom of the hole, right?

[HuFlungDung]

If the drill is still in good condition, then I would not expect that the occasional bit of carbide is getting embedded in the bottom of the hole and taking the bar out.

Is there a possibility of a chip adhering to the hole? Maybe you need a fraction of a second of dwell at the bottom of the hole just to ensure that the hole gets cleaned at full depth. At 2200 rpm, the in and out might be so fast that the depth is inexact, particularly if the part has some flex.

Have you tried pecking to create the flat at the bottom? I personally hate facing holes flat in the lathe, its always an aggravation.

[Glacern]

We always use cobalt drills in stainless.

The sudden failure could be caused by work hardening on random pieces.

[FastBusa]

We had a similar problem like this with 1 1/2" carbide drill bits, we never had any problems until one of the "bosses" decided to save some money and buy some cheaper bits. They some times would break after the 4th or 5th use so I did some research and found out that they were not the USA made ones we used to get they were made in China, fact is that China does not do well in making any kind of tooling so I would check and see where they came from

[PoiToi]

We have had that same sort of problem here in the past, so we are diligent about buying quality stuff.

of course, getting this drill was all in the name of saving time and $, and look where i've ended up. i'm sure i've broken more tooling than saved machine time. >_<

the drill is an MA Ford Twister drill, with a 145 degree point.
and yes, the drill still looks like new at the tip.

and Hu, that's a good point. though none of the 20 or so parts that were made wilth just the drill have any chip, i'll try putting a quick dwell at the bottom to see if that helps, once i get more boring bars.

there is also a small (.150) thru hole that goes in before the bore, i figured that this would help clear any chips that would get in the way.

[MDLang]

I am curious to know the difference in hole diameter between the HSS/Cobalt drill and the carbide drill. The carbide drill as you have discovered drills on size, in my experience the other will not.

The point here is how is the DOC for the first boring pass affected. I know it sounds over simplified but I can't think of any other good reason for your problem if all you changed was the drill. I'm sure you have already checked to ensure the hole depth at the drill point is the same.

Good luck

BTW we have had very good success using carbide drills in 316 as long as they are 1/4" or larger. smaller drills have had a tendency to break.

Mike