Cut-off tool failure

Junkmaster

Junkmaster

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I've got a Smith CB1220-XL. I have never had good luck with a cut-off tools. They screams at a very high pitch, generate a lot of heat, and then usually break the tip or whole blade.
I attempted it again tonight and again the blade broke after I nursed it about 3/32nds of an inch.
The part is:
a mild mystery steel.
started out as a 1 1/4 solid bar I drilled and turned to a cylinder shape.
a cylinder 1 1/8" diameter with a wall thickness of 3/8".
sticking out about 1 1/2 inches from a 3 jaw chuck. Unsupported on the right end.
spinning at 840 RPM.


The cutoff tool is
cutting about 1/8th" from the chuck jaws.
Small. The blade is 7/16 or 1/2 inch tall 1/16 or 3/32 wide a the top, narrower at the bottom.
projecting about 1/4 to 3/8 from the holder

Here is what I double checked tonight:
cutting edge right on the center line of the part
the blade was at right angle to the axis
used plenty of oil
carefully sharpened the blade, then honed it with a stone to make sure the burr was gone.

Any advice would be greatly appreciated.

Evan R.
 
What kind of tool holder/toopost do you have? 840 is too fast. I'd bet you are dulling the tool, rounding the corners and the kerf gets narrow and grabs.

Have a picture?
 
It sounds like you're setting it up right, if indeed it is centered and squared. But it shouldn't be squealing then - that sounds like it's rubbing somewhere besides the cutting edge Are your relief angles steep enough that only the point touches?
There is a good tutorial on how to sharpen/set up at the sherline site: http://www.sherline.com/3002inst.htm
Figure 2 on that site shows the top view - putting that angle on the tool helped me make it a much easier cut than a straight chisel edge, but the biggest thing for me was getting it to cut the work right at the centerline because that made it cut instead of rub.
 
Assuming you have the tool really on centre, and square with the work. Your tool must be sharp, and the heat build up you mention leads me to think you trying to cut with a dull tool.If your blade in not sqaure to the work piece, then you going to make heat. If you hear it start to sqeel, then get out of there.

You should check your tool holder as well. Check to see if the blade is standing up straight, and not laying over on a angle. I have seen some cheap tool holders that dont hold the blade properly, and this will allways cause problems.

I find if I get deep into a cut, and the heat is building up, its time to back out and let things cool down, or turn on the flood coolant. You are using proper cutting oil, arnt you? Other problems could be loose gibs, and the sqeeling is vibration related.

The speed your trying to cut at is way to fast, parting is best done at a slower speed. I would recomend 200- 275 RPM to start with. faster speeds are possible but your over 800rpm, thats way too fast in my book. I part things all the time in the 200-300 RPM range and feed in under power with out issues.

The chips should curl off in 6 and 9s real nice without applying to much pressure. But not knowing your exact setup, make certain your setup is ridgid. Blade is mounted straight up and down. Blade is sqaure to the work. Check the gibs to make certain they are not loose, and snug them down wont hurt either. Asyou probably allready know, dont hang any more blade out of the tool holder than nessesary.

I use HSS blades all the time, and sharpen them with the end sqaured off. I do cut the end on a slight angle if Im parting parts , and dont want the nib on the end, but straight works fine for most work.

Re check the entire setup, and the blade for sharpness. Not knowing your lathe, the smaller machines seem to favor the thinner blades.
 
Many thanks for the pointers.

I did **not** check to see if the cheap tool holder was holding the blade correctly.
I will slow down the RPM's to 200-300.
I am using a 4 way tool post with the cutoff on one side and the carbide tool holder on the other. I usually leave the other sides empty.

I have the longitudinal, and cross feed axis locked during the cut.

I think the tool is ridged, but I'm a hobbyist, so I don't have any practical experience what "ridged" really means.

I'll give it another go. I'll try to take some photos, but all I have is my cell phone camera, holding it steady is tough.

Again, thanks for the help.

Evan R.
 
Evan
Yes try and get a few pics of your setup. If your using a 4 way toolpost, then with the parting blade mounted up, just swing the tool post around so you can eyeball the blade, to see if its vertical.

Also, when your having a look at the blade for being vertical, also look and see if the blade is wider at the top, and slightly narrow at the bottom.
The reason I mention this, is I had a friend tell me he could never part off with out alot of heat. I went over to his place to see what was going on. I looked at the blade, and was on a slight angle, but worst of all, the blade was upside down. with the narrow part at the top.

The blade was also ground down that way, so he assumed it was right. Long story short, over to the grinder, and regrind the rake the proper way, and it parted of perfect on the first try. As far as ridged as you mention, post a pic so we have a better idea of what you mean.

I take all my pics with the cell phone, so that will work fine. If the phone dosnt have a flash, just make sure the work area is well lit up.
 
A Smithy is not the most rigid lathe,and cutting off is problematic with a light lathe. Is there much play in your crossfeed screw(backlash)? Are the gibs of the cross feed reasonably snug,to slide,but not be entirely free? How about the compound? It can be the worst offender since it is stacked on everything else. Is your cutoff truly square into the work,and is it TRULY vertical,so it has clearance on both sides equally. Is the cut off tool Chinese(their HSS IS more brittle than USA HSS.)

I agree about the too high speed.
 
Well, I had my first successful parting-off of a disk. :drink2:

I do not believe it was the Smithy or the cheap cutoff tool. I believe it was the operator.

I double checked the list you gave me.
Reground the tool, honed it on a stone.
Made sure I put the wider edge up (yes I had made this mistake also...)
Lowered the speed down to 250 RPM (81 SPM)
Checked tool was perpendicular to the work
Checked cutting edge on center axis with my 6" rule
Put cutting oil on the tool & work
Locked carriage
Use the cross slide to gently introduce the tool to work and WHA-LA!, smooth little curls started falling.

No screaming, no bright orange sparks, no nerves a jangled. Sweet success!

Many thanks gentlemen.

Off to make more thick washers!

Evan R.
 
Great, Evan. now you can put a check mark by "Parting Off" on your list. What may seem to be a simple thing sometimes turns out far more involved than it appears. Glad you got it to work!
 
Just to add to some of the replies, if you get a chip that keeps curlingaround, it can roll large enough to lock into the groove and break a cutoff tool. If it does that, just pull the tool out of the groove and start back in only making it a tad wider, so the curl doesn't lock into the groove.

Quite a few times if you are cutting a tube off and the material starts to squeal or start making high pitched harmonics, take some paper towels and wad it up and jam it into the end of the tube where it is kind of tight. It will take all the squeal away.
 
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