Yes rpm is too slow for the small diameter, also I noticed the tool post was shaking before that?

Tony

Much too slow a feed rate and too low rpm.

Toolpost seemed to move away from the cut then back in just before it broke.

Your tool is producing dust, it should be making good curly chips like the ones seen here

From observation on your video, I would suggest you have made several, if understandable, errors. I don’t know what the material is that you are parting in the video but as has already been mentioned the feed is far too slow. In free-cutting steel the swarf should come off in a ribbon or long curls, not dust as appears in the video. Grind the tool to have about 5deg’s of top rake, do not be afraid to apply the tool to the work and once it starts cutting, keep it cutting. If you need to reposition your hand on the hand-wheel withdraw the tool slightly, reposition your hand and then start the cut again. Don’t let go of the hand-wheel momentarily to reposition your hand because it is at this point that the tool can be pulled forward and dig in to the work, thereby precipitating a broken parting blade. Also, if it is possible to have less over-hang from the chuck the parting groove should be as near the chuck jaws as possible. You also need, IMO, much more cutting lubricant/suds, preferably pumped on to swill the swarf away.

The surest way, again IMO, to achieve successful parting is by using a very rigid rear tool-post with the tool mounted upside down, a confident steady feed into the work and plenty of lubricant, preferably pumped into the groove. The speed should be somewhat slower than that used for turning. You can speed it up a little as the diameter reduces but, personally, I usually don’t as I have found in practice it makes very little difference. If you are going to part a large diameter use the independent four-jaw chuck because it will grip the work tighter. Parting is as much of a ‘feel’ as it is a skill and I have found, that by pulling the tool into the work, using a rear tool-post that ‘feel’ is more readily acquired. You can almost anticipate a ‘ping’ by listening to the cut (it should have a nice ‘hiss’ as in all turning) and take action once a ‘graunch’ starts to develop. I can only tell you what works for me.

If I remember it right, these 8% Cobalt parting blades from Paulimot are straight and not ground. For these “thick” diameter parting, you need some side clearance or the blade will get stuck in the parted groove. You can/must grind the sides to a taper or T shape.

I do all my parting at 800 RPM (max) to prevent the parting fluid being spinned of the work piece. My feed for steel is 0.02 mm / revolution using a 2 mm carbide parting insert. So parting 30 mm would take (15/0.02/800) about 56 seconds. The speed (0.03 meter * 3.14 * 800) of 75 m/min would be to fast for a HSS blade. For this HSS blade, I would use 400 RPM.

The last mm is the hardest on the parting tool because the speed at that diameter is very low. In steel I stop parting at 0.5 mm diameter to protect the parting tool. It then just takes a small twist to break that last 0.5 mm steel.

I have a not so rigid hobby lathe and place the parting tool upside down with the spindle in reverse to avoid tool dig in. It has the same advantage as placing the parting tool at the back of the work piece.

It might be my imagination but I thought the blade was deflecting a little away from the chuck, as happens if the cutting face isn’t square to the direction of feed. With a short, stiff parting blade you can get away with this – and it’s frequently used to minimise partoff pips on the component – but not with a long narrow one.

Then just before breakage there’s at least one bump that reaches the toolpost. I’d guess that looks like a dig-and-dive from a low tool, as Mark Rand mentions above.

Mystery materials aren’t ideal when experimenting with a new tool or operation type. Some quite ordinary steels can be a bu99er when parting, with work-hardening from slow feed amongst other issues – though I can’t readily see that happening here.

I think that one of the biggest problems is the increased cantilever due to the toolholder mounting method. The tool is positioned outside of the topslide & cross slide base resulting in a flexible mounting point due to this cantilever.

Yes Mr Greenwood retired several years back.

As for insert replacements – they LOOK very similar to the standard GTN.. range. These are stocked by ARC, and their web site pages give all the key dimensions which may help you determine if they are a replacement.

I use one of these, (with the ARC holder) and it’s brilliant! (rear tool post running lathe in reverse)

regards Mike

On 2 March 2024 at 10:16 Paul M Said:

Having gone through similar issues with parting off I happened to spot a YouTube video by Clough42 (worth a watch).  He was advised to run his lathe at a much higher rpm and keep feeding the tool with plenty of coolant. I have tried it and it works, however you need to be brave!!

I suggest using aluminium to practise with plenty of coolant.  Obviously make sure everything is locked apart from cross feed.  Minimum overhang of the parting tool, spot on centre height and nice and sharp. I have had a success with automatic feed but still apprehensive using it.

I well remember as a Toolroom apprentice back in the 70’s the butt clenching parting off experience and this continued throughout my career, slow and steady was the motto on manual machines, when I went on to CNC machines things were shall we say fast and furious! I too saw the video by Clough42 and gave it a go on my Warco 290V, it was an absolute revelation and a complete success on the steel I used I also upped the rpm as i went in.

Tony

I had a blade of that type break once. I switched to the T shape blades and found them much better.

https://www.chronos.ltd.uk/product/spare-chipbreaker-blade-3-32-thick-x-1-2-wide-x-4-1-2-long-m-35-hss-with-5-cobalt/

Not sure what lathe you have , but if you have power crossfeed use it. Once  the set up (speed ,tool center, etc) is correct the next issue is a constant feed. Even my puny Drummond Bs can do it.

On 2 March 2024 at 08:56 Sonic Escape Said:

The blade it not T shaped but also it is not straight. I know the T shaped blades but I don’t remember where I saw them. That is why I bought the one from Paulimot. Except that it is brittle the blade is super hard. After the last attempt you can still cut yourself on the edge if not careful!

 

PS. The Dovetail section appears to be in the holder the wrong way around in your other photograph. If this is the case it is a disaster waiting to happen, as there is only line contact on the bottom edge of the tool.

Regards

Gray,