Cutoff tool

[jroma1]

I have a Far East type 9 x 20 lathe and I am having a problem using the cutoff tool.

There are four attachments,photos,that may help in understanding the problem. Any help ,comments and recommendations greatly appreciated.

The cutoff tool is shown in PICT0001 it has a shank dimension of .290 x .750, also included is a steel wedge used to bring the cutters tip in line with the lathe's center see PICT0002.

PICT0003 is a fronf view of this assembly. The cutting tip of the cutter was alligned with the tip of the live center in the tailstock. The cutter was then adjusted to be,in my opinion,perpendicular with the workpiece see PICT0004.

There is a loud chatter as is evident in PICT0004 by the ripples at the bottom of the undercut. I did not use ant fluids,anybody got any comments.

THANKS
John R

[Switcher]

Looks like you need to push the OD back the thickness of the C/O ?

Is the C/O hitting rapid?

[Geof]

I think you have done everything correctly and discovered that on small machines parting can be a difficult operation. Part of the reason is that there is a very heavy load on a parting tool cutting across a wide face and this cause a lot of deflection. Unfortunately this deflection many times cause the parting tool to dig in and then spring back creating the chatter.

There are two approaches to combat this; mount the parting tool upside down at the back of the work in a dedicated custom tool holder fastened to the cross slide, or mount it upside down at the front and run the machine in reverse. Some machines have tee slots at the back of the carriage to allow mounting a toolpost there.

If neither of these options are feasible a crude fix is to set the parting tool slightly below center and this may reduce the tendency for it to dig in when things deflect.

Yet a third option, a bit mickey mouse, is to devise a little screw jack affair directly under the front end of the parting tool holder from a nut and bolt down to the cross slide; this removes some of the deflection.

[HuFlungDung]

HSS cutoff tools are for cast iron and brass. Other than that, they suck.

The reason is the poor geometry of the top of the tool on ductile metals does not help reduce the width of the chip, nor does it help roll the chip up. Because the chip is not flowing back along the face of the tool reliably, you get a pile of built up material on the tool, then it moves, then you get another pile, all happening very quickly, and this creates your vibration.

Back when I used those HSS blades, of course, I was reluctant to grind the top of the blade into positive rake, because then as you grind back the end of the blade in normal resharpenings, the top of the tool gets lower and lower below centerline. It needs not just a couple of degrees of positive rake to improve chip flow, but maybe 15 degrees, so this is quite an angle to grind on top of the tool. If you do this much grinding, then you've got room for chips to wedge in between the sidewall of the groove and the tool because the tool is wedge shaped, and grinding positive rake puts the back gullet of the rake face down into the narrower part of the blade.

I recommend to all who will listen, to get a carbide insert type part off tool and be done with the grief. The insert has a complex geometry that provides positive rake chip flow, and narrows the chip so it comes cleanly out of the groove. Coolant is still necessary while using it. Seemingly aggressive, steady feed is also necessary. When you are accustomed to using it properly, it doesn't seem aggressive. You might even try power cross feed at .004" per revolution to get an idea of how fast a correct feed is. Just be ready to throw out the feed clutch and retract the tool quickly if you hear it starting to bump and grind or vibrate. Be aware that a normal partoff requires significant pressure of the tool against the work. It is not a tool to be babied.

Insert carbides are available as solid shank tools, in smaller sizes, with insert widths of maybe 1/16" or 2mm. Sandvik makes a cute one.

But my standby favourite is a 1/8" width part off tool because the insert and blade is rugged enough to handle manual cutoffs. Narrower blades and inserts never seemed to have as long a life in my hands, anyway But the 1/8" blade and toolblock might look out of place on a small lathe

If you are parting off large diameter stock (large relative to your lathe swing), use a center hole and tailstock to steady the piece for the initial parting cut. Remove the tailstock from the center and ease off on the cross feed pressure when you get below the 1/2" diameter mark in the partoff.

[wildwestpat]

Hi John

Your second picture has be bothered about your tool holder ridgidity. This considerable force exerted by the parting off tool as the above members have indicated. The advice about replaceable tip tools is good if you can afford the outlay. However repalaceable tipped tooling is getting cheaper and with the recession there are some bargains appearing in the way of tips.

My concern is that the HSS holder is not clamped firmly to the base of the tool post but has been wedged up at the outboard end. You need to remove the wedge - adjust the top of the HSS tool to the center height - I suggest taking a light cut off a scrap bar and adjusting the height with suitable shims. Drinks cans are a source of thin shim material and other scrap should furnish you with the necessary build up to bring the tool to centre height. If you tilted the tool downwards with that wedge because the tool came in at above centre height then you should machine the bottom of the tool holder away to bring it down to the correct height.

I have found that parting off is best done with the tool mounted upside down with the motor running in reverse. This assumes the lathe motor can be reversed and that the chuck is held on with bolts not a single screw thread. For those lathes with a screw attached chuck reversing causes the chuck to fall off and these machines often have provision for a rear tool post which achieves the same effect.

Running upside down means that as the tool is advanced and the material being remove gets jammed on the tool face the tool dives away from the work releaving the pressure. With the tool cutting conventionaly the tool starts to dig in tips down and digsin even further resuling in disaster as you have found out.

Try it for your self - with my lathe I need a simple spacer under the standard too post - think square washer the size of the tool post across the flats brings the standard HSS parting off tool in on dead center. so the only adjustment required is to get the cutter at 90 degrees to the headstock center line.

Hope this gives you the smooth cuttof you seak.

Regards

Pat

PS assume the gibs are tight.

[PixMan]

+1 to what HuFlungDung wrote. Your lathe has the capacity to hold a 3/4" high VGTB-style blade holder, get one. Get a double-ended blade, 3mm thick, and 1/8" wide cutoff inserts.

I was watching an episode of American Choppers recently (I needed some chuckles), and saw those hacks using one successfully. If those Bozo's can cutoff steel with one and not screw it up, it's gotta be a good system. I recognized the lettering of the insert when for a brief second the camera had a close-up of it. It read "5735", so I knew they had Valenite tooling.

Myself, I have the older Valenite VC cutoff system so I will probably have to buy a newer-style blade once my supply of inserts is gone. I love it though. Just as HuFlungDung says, I run it fast & hard. His recommendation of using power feed in @ .004" per rev is spot-on. On a CNC machine you could run even a little harder than that. Beautiful clockspring chips falling out of the groove as it cuts off, and not a hint of grabbing, chattering or crunching.

I'll be able to use the same VGTB 12 25 block with a new VG101 double-ended blade and their newer VSG inserts. Those are nice because you can also buy inserts in precision groove widths full-radius styles. Now the cutoff tool is also a versatile groove tool.

[wildwestpat]

Hi John

Just to make it clear here is the HSS parting off tool holder held upside down. Must be run with motor in reverse. Give it a try it might save you some cash for some thing else.

The bits in the background are the shims to bring another tool back to center line. Please note I have cheated in the phot as I have not shown the square packing washer under the main body of the tool holder as I did not want to get grease on my cammera.

Regards

Pat