Parting off help

[545Kurt]

Hey there guy's
Im having difficulty with parting off. Not sure what is wrong but I have a blade type parting tool. The blade is 1/16th by 5/16th HSS. I have it sharpened very well (sticky sharp on the finger nail). I have the blade point at or a hair below center. All I get is a lot of vibration and a very slow cut. I could cut it way quicker with a hack saw. If I try cutting to quick the blade digs in and stalls the lathe. The blade goes dull very quickly (probably from the vibration). Any help would be appreciated.
Kurt

[HuFlungDung]

On a flimsy setup on a tiny lathe, you might have grief due to the neutral rake of the top face of the tool: it is not free cutting. Cutting metal is not like cutting cheese: the tool basically bulldozes the material off, the workpiece material builds up then fractures along shear planes, over and over again at high speed. The effect of this fracturing of shear planes equals vibration on a flimsy setup.

If you grind a more acute angle on the top of the tool (like a hook), this lessens the pressure required to break the material along its shear planes, in other words it flows over the tool better because the hook type tool acts more like a lever prying the material off more in line with the natural shear planes. However, a hook type point takes more abuse and may dull more quickly of the surface speed is too high. Coolant of some sort should be applied continuously during the part-off when using HSS.

The front clearance of the tool also must be kept minimal to control hogging in (which contributes to vibration). The front of the tool should have only maybe 3 degrees of clearance (from the vertical) provided that the cutting edge is exactly on center height. Lowering the edge below center effectively (and rapidly) increases the clearance of the tool, which is exactly what you do not want.

[545Kurt]

Thanks for the advice. I will give your suggestions a try, I have been grinding a lot more than 3 degrees of front clearance more like 30 deg. What speed should I be using for mild steel, normal cutting speeds? I have read parting in reverse may help, what is the difference?
Kurt

[HuFlungDung]

For HSS, try between 75 to 100 sfm.

Parting off with the tool upside down behind the work changes the way the forces act on the lathe. Essentially, the tool is lifting against the rather meagre hold downs that keep the carriage from lifting off the bedways. Since a small machine has such a tiny amount of mass, the weight of the carriage does not help counteract the tool forces, and you could possibly lift and bend something. Because lifting the carriage off the bed is a non-elastic movement, this may kill vibration tendencies, thus seeming to be smoother. You judge whether your machine is beefy enough to handle this.

Try decreasing the front clearance of the tool first. The front clearance only needs to be sufficient to allow the tool to progress into the cut, hence 3° is ample.

[TroyO]

Also, check to make sure your parting tool is at 90 degrees to your lathe centerline.

I usually do that by advancing the crosslide so the blade is next to a faced off surface, then loosening the tool holder and sliding the blade up alongside the faced surface... once you have your cusoff blade flat against the faced off surface lock the toolpost down. (OK.. that probably made no sense, LOL.)

Also, I ended up overtightening my cutoff blade in the holder and it cut off the bottom of the tool holder, LOL. Anyway, desperation led me to grind a 1/4 HSS steel blank for the job, and lo and behold it works better than I ever got the official cutoff blade to. It's a bit less than 1/8" wide and about an inch long.

But, I'm a newb so it could have been any number of problems that led to that one working better by pure happenstance.

I've been mostly meddling with aluminium, but I have noticed that sometimes the fix when it starts "getting whiney" and making noises is to actually increase the feed rate a bit too. (As anti-inuitive as that sounds.)

[545Kurt]

Ok guy's I am still having problems but making some headway. I could not get the parting blade to work regardless of how I ground it. I did take a 1/4 X 4" HSS tool bit and grind a .062" wide parting blade on it with 0 deg front rake and a top hook. I was quite carefull to get the bit narrower on the botom than the top the same as the blade. This actually shaves off metal quite nicely but I can only get about a 1/16th deep cut before the tool is to dull to do anything with. The stuff that I am parting off is 1 1/4" so it takes a lot of tool grinding to get one piece parted off. I am still quicker to just use a hack saw so far. Does any one have some pics of a tool bit profile that I colud have alook at? Any links to parting bit grinding?
Kurt

[HuFlungDung]

Can you take a good photo of your setup? There is not a lot of mystery to the part-off blade, but a look at how sturdy your setup is might be helpful.

If you are burning the point off the tool, then you are running too fast.