[How do I?] Parting Blade Help

[T Bredehoft]

Upside down tool...... it's just like using one behind the work, only in front, the tool geometry remains the same. the bottom of the tool in the back is the same as the top of the tool in front. If you put it upside down in front, the bottom must be on center, and the spindle reversed.

Technically it's a Atlas Mk2 3950, but that's more-or-less exactly the same as a 101.21200

That's what my lathe is, I've removed the compound, put my four tool holder directly on the cross slide. . I get a bunch of chatter when cutting off, but I haven't broken a blade yet. I find it works best if I clamp down on the gibs, both on the cross slide and the ways. I have a cutoff holder built into a holder that fits in one of the slots on the four way.

 

[Dan_S]

Running any tool upside down on a small manual machine is questionable. The cutting forces, will be trying to pick the carriage up off the bed, instead of pushing it down against. You might be able to get away with it in softer materials if you run all the gibs super tight.

Machines that where designed to do it, usually had special carriages & beds.

 

[joshua43214]

Ok, every one, lets back up and take a deep breath.

Parting is not rocket science, it just requires care be taken to ensure that everything is as rigid as possible, and the tool is aligned as perfectly as possible.
The tool holder design is not flawed or a bad design. This design has been in use for as long as there has been lathes. Even the old fashioned rocker posts used a similar clamping arrangement. It is a time tested and proven method that works on everything from low speed, low HP manual lathes to high speed CNC turing centers.
The tool MUST be clamped vertically, not on it's sides. This is to prevent vertical vibration. Horizontal vibration is not near as much of an issue, it just gives a bad surface finish.
If you do as I described in my first post, you will not have an issue getting the tool clamped properly in the holder. just lightly tighten the wedge, slide the tool in and out, tighten a bit more, slide, then snug it down. This will ensure the flat faces of the top and bottom are flat in the holder.

The tool is not too thin. Light weight lathes requires a thin tool, they simply do not have the horsepower to run a wide blade. On my old lathe, my parting problems disappeared when I went to a thinner tool. A thick tool on a light machine will be nothing but problems because the machine is not rigid enough to support a heavy cut.

Just to prove the point. I dug up my old parting tool from my old lathe. It is even thinner than the one the OP is using, and it is not tall enough to fit in my BXA holder. I had to put a narrow parallel under the tool to get the wedge to clamp it up. The tool is a cheap Shars parting tool. I touched up the edge on the grinder, I did not hone it. In fact I even ground it slightly off square just to prove that it does not matter.
My QCTP came from Quality Machine tools. It is a no-name Chinese thing, that is quite frankly a piece of junk. My old Bostar was a far better post.

I then parted off a piece of 2" 4140 cold roll. The work is only centered by eye, and is probably about 0.005" off center. I did this just to make it more interesting because making the initial plunge can be hard on the tool if you are cutting into the mill surface.
I have 1 1/8" stick out on the tool. I ran the lathe at 135RPM because this is the typical low speed for many lathes.




You can see the crappy surface that comes from the tool being ground out of square.
You can see the nice quality chip I get. I just keep pushing the tool into the work until I get this chip.
You can also make out the parallel under the tool in some of the pics.
I even stopped the part to answer the phone about half way through.

 

[zimm0who0net]

Thanks Joshua. Quick question. From your first image it looks like your blade is 0.099" thick? Can't tell if that's the top or the bottom thickness. The blade I broke was 0.040" at the top and 0.012" at the bottom.

One other question. You ran at 135RPM. I was running at 1500RPM. Granted you're turning 2" stock and I was turning 1/2", but that's only a factor of 4. My speeds/feeds chart seemed to indicate 1500 was OK. Am I too fast for parting?

 

[Dan_S]

The tool is not too thin. Light weight lathes requires a thin tool, they simply do not have the horsepower to run a wide blade. On my old lathe, my parting problems disappeared when I went to a thinner tool. A thick tool on a light machine will be nothing but problems because the machine is not rigid enough to support a heavy cut.

Just to prove the point. I dug up my old parting tool from my old lathe. It is even thinner than the one the OP is using,

It doesn't look like it!

He is running a P-1-N parting tool that's 0.040" wide, from your picture, it looks like you are running a fat P-2 that's supposed to be 3/32" (~0.093) wide. The blade you ran is over twice as wide as what he was running.

 

[joshua43214]

The thickness is at the bottom.

1500 is way to fast. I am not sure even the old turret lathes parted at that speed, and they used inverted parting tools for the most part (gives better chip ejection). At 1500 RPM the chips do not have a chance to eject and will wedge up in the kerf. This is CNC territory where there is coolant being flooded into the kerf to force out the chips and the feed can be optimized. I doubt I could turn my cross slide fast enough to part at that speed.

I suggest parting between 100 and 200 RPM. Go as fast as you can and still get a full chip. I will part up to about 500RPM, but I really have to crank pretty fast. At slow speed it is easier to adjust your feed rate so you get a nice chip. It will make a very distinctive sound when you are doing it right, and the hand wheel will have a distinctive feel to it. Once you feel and hear it, you will know you are on it - it just feels right. Start at a lower RPM, and if you get chatter or banging, push harder until it goes away. I find parting under 100RPM difficult, I usually have the best luck around 150ish for about all diameters.. If you have variable speed, increase the speed as you near the center. If you must stop the lathe, make sure the tool is in the kerf before you restart it. Sometimes the part being parted off sags a bit and if you try to go back into the kerf with the lathe running it will catch.

I find Tom's Techniques to be just about the best channel on YouTube for basic machining operations. He gets right to the point, makes even the most complicated things simple.

I do just about exactly the same thing he does.
You can hear the distinctive sound in the video.

 

[Dan_S]

My speeds/feeds chart seemed to indicate 1500 was OK. Am I too fast for parting?

That's 196 sfpm at the outer diameter, way to fast for a hss parting blade. What type of steel are you using? off the cuff, I'd recommend 60 to 80 sfpm.

 

[Dan_S]

The thickness is at the bottom.

So you are running a P-3-S? That's 3 times as wide as his blade at the cutting edge.

 

[projectnut]

I agree with the observation that the tool hasn't been sharpened to provide a good cutting edge and sufficient relief to allow the swarf to escape. In addition I would also check the following. Was the tool perfectly centered top to bottom? It looks like the tool was either slightly below center and/or the tool post wasn't rigid enough to keep the tool from diving under the cut. First make sure you're on center by putting a 6" rule between the tool and the work. Then move the tool in enough to trap the rule between the cutting point and the work. If it's perfectly centered the rule will be straight up and down. If the cutting point of the tool is low the rule will lean back toward the tool post. If it's high the rule will lean forward toward the work piece.

Next be sure all the gib adjustments are tight on both the cross slide and the carriage. Then reduce the spindle speed to less than half (preferably closer to 1/4) the normal cutting speed. Parting with a HSS tool is generally considered a 1/4 speed operation. Be sure to feed slow and steady with plenty of lube being brushed into the cut. If you don't put enough lube on the cutting surface swarf can get trapped between the tool and the work and force the cutting edge down. If the tool dives down it will almost always break.

 

[joshua43214]

So you are running a P-3-S? That's 3 times as wide as his blade at the cutting edge.

I stand corrected. for some reason I though his was thicker.
The principles are the same regardless.
He is parting 1/4 the diameter with a tool 1/3 the thickness. If I had a small lathe, I would probably also being using a super thin tool on 1/2" stock. The thinner the to0l, the less HP and effort parting requires. The OP's image shows a 250-xxx holder which is a BXA if I'm not mistaken. Should be plenty rigid enough, not sure how it fits on a 6" lathe though...

Put another way, the width of a parting tool is the equivalent of depth of cut when turning. Small lathe require thin tools because they can only make light cuts, and parting requires even more HP than turning. If the lathe can only make a 0.050" deep cut when turning, it would be asking too much of it to run a 0.100" thick parting tool. On my old Seig SC8, I would come close to stalling the motor with that blade I used in the photos above - it was the thickest blade the lathe could run. The lathe could make 0.150" deep cuts when turning steel. Not sure what my new lathe can do, I have only made 0.200" deep on steel, and 0.400 deep on aluminum so far.