Parting cast iron

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Wino1442

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Hi all...I was parting a piece of 1.375 diameter grey cast iron at approx. 75 rpm using the lathe's power cross-feed, which is set to a .0024/rev....the slowest setting I can get, about 2/3's of the way through the parting blade grabbed and the lathe stalled. No damage to anything, but I started thinking, which may be my 1st mistake. Just about everything I have read or seen in video has stated that one should part at a slower rpm. My question is: Is it absolutely wrong to part at the recommended turning speed for a particular material?
I have made sure that the parting blade is on center and square to the work so I'm wondering if maybe I'm turning too slow which is allowing the parting blade to grab.
Any suggestions and/or opinions would be greatly appreciated.....Thanks.
 
What size/type of blade are you using? Is it ridiculously sharp?
I have found that sometimes even the slowest infeed is still to fast when parting. By hand feeding I can feel it the pressure is getting high and stop to let the spring catch up. remember that as you get closer to center the SFM keeps getting slower and your infeed will have to slow accordingly to control the chip size. There have been times when I had to stop and speed up the RPM as I got closer to center to help get a better SFM.
 
As your cut depth gets deeper, your RPMs may need to go up to get your surface speed into a more workable value.
Once you have less than a 1/2 inch of material, 75 RPMs seems super low.
 
I had a parting blade grab in a 3" aluminium round. Twisted the blade and snapped the tip right off. That was HSS - with the carbide inserts, I find the inserts get pulled out before any damage is done.

As for suggestions, well, hand-feed on large diameters (1"+) because you will need the tactile feedback as the diameter changes. start the first half of the parting operation about .002 into the waste material, then when more than halfway through, move back to the original, correct cutting position. This will give some room for the tool and the chips to grow into. Might need to do this two or three times on large stock; basically, keep cutting relief into the waste so that you can run the final parting cut in cleanly.

I have also heard there is no "too slow" for parting spindle speed. There is, however, "too slow" for parting infeed, and often parting problems are resolved by feeding in faster. I suppose this has to do with the changing diameter, as mentioned above: faster feed required for smaller diameters to keep the tool operating correctly.
 
I run my parting blade upside down and the lathe in reverse and part at normal turning speeds without problem. I find you need to be a bit aggressive when parting on most materials.
 
Sometimes you have to part at a faster speed. And the use of a lubricant is required.
 
NortonDommi said:
I run my parting blade upside down and the lathe in reverse and part at normal turning speeds without problem. I find you need to be a bit aggressive when parting on most materials.
Click to expand...

Ok, why do you do this? What am I missing?
 
7milesup said:
Ok, why do you do this? What am I missing?
Click to expand...
Any movement of the blade due to work pressure is away from the work so it eases up the pressure and saves the blade from binding and busting. When the blade is upright any movement of the blade pushes it into the workpiece farther which is certain to cause breakage and workpiece gouging.
 
7milesup said:
Ok, why do you do this? What am I missing?
Click to expand...
What MontanaLon said. Rear toolpost works the same way. I run an Aloris style toolpost and twice have hit super hard spots on interrupted cuts and the tool has just shifted up out of contact. Another bonus is that the chips /swarf all exits down.
 
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