Hard spot in a 70mm steel bar

I have had this round 70mm bar for many years sitting in the shed and I used it to make a flywheel, as I center drilled, the center drill hit a hard spot. I faced it of with no problems and it left a nice finish, I used a 4mm drill but would not pass the hard spot, so then used a masonry drill and the same happened. I used the other end and had no problems with drilling a 8mm hole, any ideas what could of caused this.

Hello Robert,

​It is unusual, but it could be a central inclusion in the bar, it is where they tend to collect, although I would have thought a drill would fetch that out.

​You might find the other end of it eventually working back towards it.

Regards
Brian

I'm wonder if it's work hardening? You could test for that by forcing a deliberately blunted drill into a fresh part of the bar to see if you can create a hard spot. Then use a new drill in the same spot to see if it cuts.

Dave

You didn't break the end off the first centre drill and leave it embedded in the steel bar did you? You-ll have trouble drilling through that with anything short of a carbide bit.

But you can use a crafty old-timers trick and take the remainder of the broken centre drill and Dremel a small slot into the body in line with where the broken off central tit used to be. Then you can drill out the steel around the embedded HSS tip until it comes out, then redrill with a new centredrill.

Could it be a ball bearing in the original melt?

Never come accross it but I have heard people talk about it. You did say it was old steel.

regards Martin

If I'm going to use a masonry drill on hardened steel I first sharpen the carbide tip up on the grinder (they are usually a bit blunt straight out the box), then I use it on the backgear on the lathe, torque and pressure being more important than outright speed.

Take your time and let the tip cool off when it starts to get hot and use a bit of oil to help with the cutting, maintain a good solid constant pressure on the job while it's working, a 4mm will heat up pretty fast so you'll need to be patient but you should hear it cutting as it progresses

DO NOT use water to cool the tip, this will cause microscopic cracks to form on the carbide cutting edges, it needs to cool naturally

Higher speeds will simply burn the drillbit up when you do a really tuff job, it does sound like you have something in there like a bit of slag or a ball bearing

GL

Edited By Ady1 on 18/10/2016 14:52:42

The OP faced up the end with no problems!

Please have an explantion why it cannot be bored

Yep, my suspicion entirely, it would explain why it wouldn't drill, even if you used a carbide drill you'd need really high speed to work your way through a HSS bit without it just snagging and snapping the drill. At worst you'll just end up pushing the bit even deeper into the work, so i think removal rather than cutting through it is a safer solution.  Your mentioned method of drilling a hole next to the broken tip would be a good way to remove a stubborn one.

You can sometimes pick out the centre point with a scribe, They do have a tendency to break every once in a while but i frequently back off the centre drill, it seems to work better than just plunging straight in it on harder materials. 

Michael W

Edited By Michael Walters on 18/10/2016 15:59:16

Melting scrap to re-roll I have seen bicycles complete with ball bearings pushed in. S an unmelted one is quite possible.

I had a mild steel bar with a hard inclusion, an off centre bump came up when I faced it, very odd! I fixed it by throwing it out.

I've had hard inclusions in ground flat stock.

Thanks for these replies,I checked my centre drills and they are all ok. I will have a go at cutting it out and see what it could be this week and relay what I find.

How long is this item? If sufficiently long, just cut off a section and try from there. Or turn around and use from the softer end. Scrap the rest if the softer end 'runs out'. Life is too short to sort out imponderables.