Chipping HSS

I have some 1/4" square HSS that seems to have a propensity to chip on the leading edge when taking heavy cuts (using it in a tangential holder). I have only done this very rarely before but the edge has just broken away on me four times on this one piece.

I am working the tools much harder now I have a hefty motor behind my lathe, but even so I'm surprised to chip HSS so easily. is this unusual?

Neil

Neil,

That does seem very surprising;

Three hypotheses [with so supporting evidence]:

1. in a tangential holder, the loads are "along the grain" of the tool
2. the tool clamping may not be good enough ... causing chatter/dig-ins
3. a bad batch of HSS

MichaelG.

What material are you cutting? could be a hard spot in a casting or cheap HSS

Neil.

I have managed to chip HSS due to bringing the tool in accidentally and violently.Its certainly possible to break a parting off tool as I'm sure others will agree! There are, however several grades of HSS some of which are harder than others so that is probably the reason for your problem. Colin

This is EN1A, so it should cut nicely enough. I have been 'forcing' the cuts a bit in proportion to my lathe, but the cut depth is nothing for 1/4" HSS on a bigger machine - it went on 30-40 thou cut last night, admittedly an interrupted facing cut.

Neil

Hi Neil, It sounds like you have a particularly brittle tool, are you sure it is HSS and not, say, Stellite?

If you suffer chips on the cutting corner, I found that grinding a larger radius helps, this is definitely required when using a solid carbide tool in a tangential holder.

chriStephens

Neil,

Are all your failures associated with intermittent cuts? Check your overall tool clamping and stiffness as you could see at least twice the cutting load applied at the moment of restarting the intermittent cut and this may flex your whole mounting structure (toolholder , toolpost, topslide etc) If this flex leads to a digging in of the tool you could expect occasional chipping and tool failures.

Not withstanding this, the other suggestions are all relevant,

Bob D

Just to expand on this when I first started using a tangential holder I only gave the front corner if the tool a slight bevel, and often found the sharp point would break off. It's very annoying because you have to grind off a lot of hss to re-form the point. I now round the edge quite a lot and have had no more problems except when taking heavy intermittent cuts. I think it's also useful to run the spindle faster, I've stopped pussy footing with the lathe and find everything works better when you wind up the speed.

I agree John,

I've found that faster spindle and a much heavier 4" chuck compared to the 80mm one makes a difference too - the extra momentum keeps the cut going.

I'm not asking this tool top do anything I didn't ask the previous piece of HSS to do. Methinks it's either brittle or an unsuitable grade of HSS for this task.

It's marked HSS 1/4, nothing else, although the machining is less fine than most HSS I've seen before.

Neil

This steel was developed by Fredrick Taylor at the Bethlehem Steel work in the US in 1900 and given the designation HSS for High Speed Steel.

Unfortunately the Indians in a cock up of translation designated it HSS Highly Soft Steel.

Sounds like you have the latter.

It is possible, if you need to grind quite a bit off the tool, that it could be getting too hot. Cooling it quickly in water while grinding,could be changing the nature of the tool bit at the cutting edge,thus making it harder but more brittle.

I think your technique needs investigating before the tool bit is.

I've had a similar problem before and think I had two issues one was getting it to hot on the grinder and not hardly any radius on the corner sorted those two out and touch wood it's been ok since and it the same Hss

Al.

> I think your technique needs investigating before the tool bit is.

Maybe, but I've only ever chipped end mills and parting tools before in fifteen years.

I've been using the tangential tool for about a year now, but I got over-enthusiastic rounding the corner of the old bit of HSS I was using. I'll turn it end for end and put an edge on it and see how it compares.

What was interesting was when I shortened the new HSS. I followed my usual route of grinding a groove all round, putting it in the vice, covering with a rag and popping it with a hammer. Unlike the usual clean break, teh break actually left the groove.

I'm wondering if that was a clue that it's rather more brittle than normal.

Neil

I wonder if the grain structure of the tool bit is the problem? If it is in a tangential tool holder then the stress is down the length of the tool and if the grain structure is along the tools length then it will chip. If it is used in the normal fashion then the stress is across the grain, just something to think about.

Clive

Neil, I found out how to chip the end of A HSS tool today, I had a 1" high tensile bolt that required it's heat bought down in diameter, so I deliberately pushed it until I stalled the lathe (belt head), and then backed out, then a little click, and a sliver about a mm wide broke off the tip. The tool was not very sharp to start with, it would have done the job if I had done it properly, but I just wanted to see what would happen. The tool was 1/4" sq. Ian S C

Sounds like Chinese HSS to me. I have a few nice pieces of 30+ year old UK, Australian and Swedish made HSS and it is noticeably tougher stuff than the new HSS I buy from model engineering suppliers today.

I have some tool bits about 6" long named Deloro 100 and it is hard to grind but tough as old boots when working. Trying to snap a bit off by grinding all round is dangerous and I wrap a cloth around it for safety. Then again I have some 6mm bits for the boring tool that lose their edge in minutes!

Clive