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- Dec 20, 2012
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The first one was too much rake on the parting tool. ... Also, tightening the gib screws for both cross slide and compound helps.
A P-type parting tool is flat on top so it has zero rake. Are you using a different tool in the above statement or are you talking about the relief angle, the angle under the tool tip? Also, are you using a QCTP with the tool holder that holds the blade at an upward angle? If so, that gives you positive rake and can definitely dig in if the lathe is not rigid.
Another problem I had was not feeding aggressively enough. ... You have to listen to what the machine is telling you and "do the right thing."
The feed when parting manually is actually about feel rather than sound. You want to feel a slight resistance to the feed. When you get it right you have to maintain it all the way in. This applies to all materials. The resistance is lighter at higher speeds and harder at lower speeds.
So, the time came to part a larger steel bar (about 1.5"). Slow feed caused chatter, so time to crank it faster! This didn't work. The blade jammed, and in a scary way. Finally, the belt slipped. It appears that there wasn't enough torque, so I used the back gear. This gave plenty of torque, but the feed became very fiddly. I could only feed slowly, else the blade would catch and stall the lathe. I was afraid of breaking something! This problem was probably due to insufficient power. I could part by feeding very slowly with little crumb like chips. This was no good, but as soon as I tried feeding faster it would jam again. The job could be done this way, but it would take forever, and also be risky. I didn't want to damage anything.
This experience reminded me of a parting lesson that I saw at Techshop. The instructor told the students to part at minimum speed, which on the 14x40 lathe is pretty slow. Feed slowly and carefully, barely getting a chip. This took a long time to part, but at least he said it was safe. These 14x40 lathes had a lot of power, and parting was easy at almost full speed for the material, but this was an intriguing technique. It seems that it does have its place with an underpowered lathe.
You should be able to part most materials at 1/2 - 2/3 turning speed. For your 1.5" bar, that would be between 125-200 rpm or so. As I mentioned, feed gets easier at higher speeds. My Super 11 will part mild steel at 200 rpm with no issues, and my little Sherline lathe with a rear mounted parting tool will part a 1" mild steel bar at about 1200 rpm. The speed helps.
Could my parting blade be too thick?
All else being good, a P2 blade should part most materials on a 9" lathe. Parting tools are form tools; the wider they are, the more resistance you will have and the more power and rigidity the tool will require to function for any given material. In my opinion, it is best to use the narrowest blade that will work with your machine. I use a P1-N blade (0.040" wide)on my Sherline to part almost anything that fits in the lathe and have no problems parting at speeds that are usually 2-3 times normal turning speeds, and that includes steel up to about 1.25" OD.
I do not like the tool holders that angle the tool upwards. In my opinion, a horizontal tool holder is better because it allows the tool to perform like what it is - a form tool with zero rake.
I suspect the issues you're having may be due to multiple factors. The gibs and fits of the cross slide and compound are always suspect. This is why guys with plinths have fewer issues when parting - no gibs, just solid material under the tool post. Getting the tip of the tool exactly on center is important; not close but dead on. The tool aligned perpendicular to the work is important, too. What is not as critical is overhang; a P-type blade can stick out and still maintain rigidity if all other factors are good.
I would look really hard at movement at the tool due to movement of the cross slide and compound. Put a turning tool in your tool post and lock it down. Put a dial indicator on top of the tool bit and try moving the cross slide and compound up and down with your hands to see how much movement there is. If it is excessive, like more than a thou, then you have some issues to sort out.