Thread: Parting off blades

I'm thinking 350 rpm is too fast, especially under power feed on an AL320G. Were you using oil or coolant?

I've parted larger diameters using the same size blade on a 320 and later on the AL960B but running at much lower revs (70-110) and hand-feeding so I could feel how the blade was biting.

I was hand dripping cutting oil to the blade, in the past I've had trouble parting off using slower RPM and hand feeding,broke a few blades, started using power feed and higher RPM and all my troubles went away until I broke the blade on this piece which made me think I was asking too much from this size blade, also this is the first time I've tried to cut 50mm.
Leroy.

Hi Lenroy,
Just to get it out the way.
You did reset your tool height?

Stuart

Tried three times to post this. Bloody Internet Explorer , on Firefox now.

Hi Leroy

Bugger about busting a blade.
There's not really an exact maximum diameter that the blade can cut, but the bigger the diameter you go, the less the margin for error you will have.

For 2" dia you really need the cut to be as close to the chuck jaws as possible, wind the topslide back so there's no overhang on the dovetail, lock the saddle, use plenty of coolant, make sure the blade is dead square to the mandrel, sharp tool edge etc, and as the cut is so deep it's a good idea to make the groove wider than the blade so that swarf can be evacuated easier.

If you are only parting off small diameters, you can probably get away without a couple of those points, but you really need to maximise rigidity the larger you go.

As far as hand/power feed, my mate Des swears by power feed, but I usually prefer hand feeding, they both have pros and cons I guess.

I was going to ask if you had reset the tool edge on centre when you extended the blade, but in between explorer crashing Stuart asked the same thing.
Were you using a decent blade ?.

Gary
Eccentric Eng

Hi Stuart and Gary, yes I did reset the height and had the saddle locked, squared the tool post to the work, didn't widen the cut but will try that next time,
had plenty of lube in the cut, it just had me wondering about the blade(they come from CTC tools) as it was cutting so smoothly until I extended the length,
Leroy.

Originally Posted by leroy

Hi Stuart and Gary, yes I did reset the height and had the saddle locked, squared the tool post to the work, didn't widen the cut but will try that next time,
had plenty of lube in the cut, it just had me wondering about the blade(they come from CTC tools) as it was cutting so smoothly until I extended the length,
Leroy.

Hi Leroy,
did you increase the spindle speed when you reset the blade. The feed will be the same rate but the work will have less surface speed the deeper you go in when using power feed.

Phil

It could be flex in the shaft of the cutter under power feed it has no chance to spring back which means the tool tip gets further under height. If you do it by hand and back off ever so often flimsier tools get a chance to do heavier work.

The speed of the cutting is not the speed set on the lathe, ie the outside of the material on a large item spins faster than the smaller diameter of the centre.

The other point can be the cutter didn't have enough side clearance and the end being parted off is getting a bit of centrifugal force and closing in and binding the tool.


Few other possibilities that may not be the obvious or easy answers???

OK thanks for all the suggestions I guess it could be all or any of the above, I just watched the parting off video at eccentric engineering and the last cut is 50mm dia. with the blade fully extended from the start so that answers my question about the blade being capable of cutting that depth, I shall keep going until I get it right, watch this space!
LEROY.

Parting tools come in many sizes and designs I have at least 8-9 probably I lot more. I have silver soldered tips, of diff sizes and grinds and a few blade type as well as fully ground tool steel for example. There are also carbide tipped options.

The tool steel blade type that go into (again various design) holders come in different width and depths, they still need to have the back of the top edge ground correctly.

I don't know what type you have. But when watching the video as a basis of what to do make sure your comparing apples with apples. Also in the video does it state the cutting speed and feed rate along with the material spec? All these factors are very relevant.

Just had a look at the video and three points to consider are the he changes speed slower for the large diameter and as the diameter reduces he speeds up. This relates to as I said earlier the diameter x lathe speed = cutting speed (in very basic terms there is a ratio to calculate this and tables can be found).

There is a very big differance to parting of a thin slice from the end which is rigid to parting the same material in say the middle of the stock. A longer length being parted off behind the turned length is unsupported even of running on a centre. as the material gets close to the centre it can sag, add centrifugal force and you can bind the tool.


Also that video is for a specific upside down tool holder and as it states this has benefits over a standard fitted cutter. Are you using the same as in the video?? That holder has a toolsteel blade, which isn't used as supplied but must be ground correctly for the material to be cut, steel, alloy & brass all have different grinds for best results.

I am no expert just learn't along the way, and passing on the things I have gleened. Sorry I don't have answers just more questions for you to look at.

RPM for 50mm bar with HSS is 190.
Parting off would go about 120 and see how it cuts.
cutting action can be affected by your tools top rake and front clearence so maybe a pic of your tool would be good.
maybe some slight side cutting to clear the blade may of also helped.
so many variables.