View Full Version : Radius cutting lathe tool problems

01-30-2006, 09:39 AM
I'm trying to cut a .125" radius in 4140HRS. 1.5" diameter. I purchased the tool from MSC upon their recommendation. Now the problem is I cannot get it to stop chattering.

I have tried all sorts of speeds and feeds, raised and lowered the tool all with no luck. I cannot even touch the 4140 without it chattering...its instantaneous. Any help would be greatly appreciated.


01-30-2006, 09:50 AM
Are you able to post a picture of the setup?

01-30-2006, 09:53 AM
Have any more specifics? One of the old timers said yesterday that tool center can be set 1/8 above or 1/8 below true center, so I guess you,ve already tried that.

01-30-2006, 09:55 AM
It must be on center for radius turning.

01-30-2006, 09:56 AM
I can post picts tonight or tomorrow. Tried up to 5/32 high and low, no luck.

From what I can see of the insert, it looks WAY to aggressive compared to other carbide inserts I use. The cutting edge just looks "sharper" than the others. Makes sense that it is taking too big of a chunk.


[This message has been edited by Jimno2506 (edited 01-30-2006).]

01-30-2006, 10:44 AM
Is this a form tool with a .125 nose radius? Are you sure this is 4140 and not 4142 pre-hard? Have you tried just grinding a form tool out of a HSS blank?

01-30-2006, 10:46 AM
Yes, I'm certain it is 4140.

I have not tried griding a .125" radius out of HSS. I spent $85 on the carbide and I'd like to get it to work.


J Tiers
01-30-2006, 11:28 AM
Every lathe chatters somewhere/somehow, even a 3 meter swing by 20 meter....

Chatter is related to the amount of tool edge in the work. That affects the forces on teh toolholder/carriage/etc.

So with a 0.125 radius, sunk in halfway, you have a cutting length of almost 0.4 inch. That's a lot, and takes a heavy machine to carry it without chatter or other hassles. I assume you have a very solid machine, not a lightweight. Big chunky Monarch, etc.

Even if you are rounding an inside corner, that's still 0.200 in contact, AND it is jammed in where side force on one side deepens the cut on the other...

And of course 4140 which machines nicely may still require more force, especially if it is prehardened.

If you can get a solid support close todirectly under the tool nose, going down to a solid part of the carriage, that might do something.......

01-30-2006, 11:33 AM
4140 is very tough stuff. It sounds like the setup is to "light" for the material. Also set the cutting tool on center. Who ever told
you to set the cutting tool above or below is
wrong. Its puts more pressure on the tool.
if the cutting tool is hight speed change to an
insert holder, use 400 rpm or less. your setup needs to be very solid.

[This message has been edited by doug47gpx (edited 01-30-2006).]

01-30-2006, 11:59 AM
I do not have a very heavy duty setup. It's a Smithy 1340, go ahead, laugh.

Ok, what can be done, I have to cut this radius.


Picts of setup

I am cutting the radius between the small diameter and large diameter. Not where the tool is.

[This message has been edited by Jimno2506 (edited 01-30-2006).]

01-30-2006, 12:06 PM
That's a honkin' big cut. No way you'll do it with a form tool on a Smithy, directly.

You might try roughing out the cut with a thin parting tool, as best you can, then try cleaning up with just the last finish cut with the form tool...but I suspect even that will chatter.

Guy Lautard describes a "step" way of cutting radii in one of his books. Basically, work out a table of position/depth values, then with a thin parting-off tool, move to a position, plunge to the specified depth, retract, move over, plunge, etc. If the steps are small enough, and the blade is narrow enough, this can work pretty well. Finish up with (maybe) a 1/4" dia. round file.

01-30-2006, 12:55 PM
done the Lautard step thingy, works well, might be tough for 1/4 though. I'd be trying JS's set up using the boring head, then you're cutting with a single point on a tangential cutter rather that a form cutter (lighter cut) albeit you may have to make a cutter slightly less than 1/4 to get in there properly.

01-30-2006, 01:15 PM
Start by chucking the part all the way in the chuck up against the jaws. You don't have much of a hold on it. If it has a through hole get a piece of ready rod and make a simple drawbar to pull it tight against the chuck.

Take small cuts and sneak up on the finish cut.

01-30-2006, 01:21 PM
From the pics, almost looks like a radius by the jaws. Maybe that's as far back as he can go to keep it concentric.

01-30-2006, 01:23 PM
It looks like in the picture you are trying to turn the part with the radius tool. Is this correct? If so you will probably never do it that way with that machine. Turn your fit to size and leave a 1/4 diameter ring 1/4 in off of the end of the fit where the radius is going to be. Then after the fit is finished put in the radius tool and set zero off of the finished diameter. Then just use small facing cuts with the side of the radius tool until you are at the finished shoulder. You will probably have to run your machine slow and polish up the fit a little after it is cut. I would use some heavy cutting oil if you got it also. If you run it too fast and it starts to chatter it will be very hard to polish it up to get it to look good.

01-30-2006, 01:55 PM
Evan and Millman,
the radius is to be cut on the intersection of the larger radius (up against the chuck) and the smaller diameter going towards tailstock.

The thickness of the larger radius is .5" so my cut is only .5" away from the chuck. My only other option is to grip on the larger diameter, but figured my .89" grip is better than .476 on the larger diameter.


01-30-2006, 02:29 PM
I think I see what you're doing, since you're only .5 away from the jaws that's no big deal. If you want to blend the radius with the shaft and shoulder; do like japcas said. I would go slow and make sure the tool height is exactly on center. Some guys here don't think tool height is very important, so that explains most of their threading, taper problems.

Mike Burdick
01-30-2006, 02:31 PM
Okay, maybe you tried this but I'll mention it anyway.

Form tools that take as much "bite" as you have are hard to work without getting chatter - at least that has been my experience. I would put the lathe in backgear and at the slowest speed it will go. Then aggressively feed the form tool into the work with lots of heavy oil (such as dark thread cutting oil). When you reach the final diameter, get the form tool out of the cut rather quickly. Attacking this with small cuts is just inviting chatter!

[This message has been edited by Mike Burdick (edited 01-30-2006).]

J Tiers
01-30-2006, 03:58 PM
Ah, a Smithy.... which has a very high center height for its weight and solidity.

That compound has a lot of joints in it, and it also has a relatively narrow base, with only two bolts holding it.

You might try something that will be useful in future as well. Two possibilities, first one is the most work.

Get the largest block of steel you can, one that you can put the toolpost onto and be able to get to center with the tool.

Put a hole in that takes your toolpost bolt same as a t-slot. Make sure the top and bottom of the block are dead smooth and sit well on the crosslide table. Get some solid way of bolting it down, either thru holes into t-nuts or hold-downs.. solid.

Pull the tool in as far as possible into the holder so it has the least lever arm.

The mass of the block should take care of the tool support side of the chatter. Afterwards, you'll still have the block for next time.

That leaves the spindle and chuck etc. Not much to do there.

Alternately, since you have the setup and nice big milling table..... move back to the next slot, if you can, and then put a machinists jack or equivalent under the tool, sitting on the table. Support the tool as close to the tip as you can.

That takes the overhang out of the picture, and directly supports the tool so it won't "nod".

Either way, better be careful, though... you do NOT want to have some other looseness allow a dig-in, because something is gonna give, and it probably won't be the support.

In second way, you also don't want the jack to get loose and fall into the chuck jaws....

01-30-2006, 04:02 PM
I'm sure no expert, but I think Mike B. has the way to go. I would also try using a center in the tailstock if you have enough room, couldn't hurt anything.


01-30-2006, 04:49 PM
You guys telling him that lathe won't even handle a 1/8" radius? If not, I would have to use dynamite on it. It would be a concern with a .5 radius; but .125???? Think about it. Looks like the insert is only .250 Diameter.

[This message has been edited by Millman (edited 01-30-2006).]

Your Old Dog
01-30-2006, 04:55 PM
<font face="Verdana, Arial" size="2">Originally posted by Millman:
Have any more specifics? One of the old timers said yesterday that tool center can be set 1/8 above or 1/8 below true center, so I guess you,ve already tried that.</font>

Actually, as I read his post it was in response to someone implying it had to be on with .001 or it would be way out in left field. It may have been a minor exageration but I have myself found quite a bit more latitude than dead nuts on.

[This message has been edited by Your Old Dog (edited 01-30-2006).]

01-30-2006, 05:51 PM
I would also use back gear, heavy cutting fluid, the dark brown threading type works well (dont breath the smoke) and definitely a tail stock with live center. JRouche

01-30-2006, 05:56 PM
Like I said; a lathe that will not cut a 1/8 radius, needs to be destroyed. No need for screw jacks, heavy coolant of ANY type.

01-30-2006, 06:50 PM
You might consider improving the rigidity of the set-up. Three jaw chucks are not famous for there rigidity. Support from the tailstock may help. I am not familiar with your lathe; but a 1/8R will probably need help. If you have a collect, that will probably reduce the distance from the spindle bearings to the cut that will help. A 4-jaw chuck will help. Lock the carriage, and tighten the gibes. 4140/4130 is tough stuff ans it will work harden easily.

01-30-2006, 07:15 PM
jimno, I think you can do it. You are just doing the corner radius and not actually turning the diameter with that tool...correct? As has been suggested...get the tailstock center in there to support the part...tool on center or verrrry little above...run about as slow as you can, yes down to 50 rpm if you can...lots of cutting oil...ease the tool into the corner, it will take a little longer because of the very slow speed...let the tool idle for a few rpm's in the corner..you should get a polished looking radius. Not sure what your insert is, but if it extremely sharp like the ones made for aluminum, you might need to stone it on top.
I just went out a cut a 1/4" radius with a home made tool (old Kennametal brazed carbide) on a 13" South Bend. 45rpm, cutting oil, no tailstock, not preturned, I just ran it in there. Only takes a minute or two.

J Tiers
01-30-2006, 09:08 PM
You guys and yer 13" S-B.... I agree those will do it.

That thing he has made out of congealed General tso's chicken and pot-stickers is really tall and spindly for its size. Most of those have the tailstock on a major riser block, compared to what you'd expect.

I'd go for the dynamite, but he's got it, and it isn't any worse than a 12" Atlas..... well not much worse. I don't think yer average 9" flat-belt S-B will do it either....

But.... LOOK at the pic of that compound, and tell me it is not flexible...... it needs major help, although its just possible that the back gear and oil might work. Might also judder like my old station wagon's clutch...

He needs to take the "wind-up" out of the set-up.

[This message has been edited by J Tiers (edited 01-30-2006).]

01-30-2006, 09:42 PM
<font face="Verdana, Arial" size="2">Originally posted by Millman:
Like I said; a lathe that will not cut a 1/8 radius, needs to be destroyed. No need for screw jacks, heavy coolant of ANY type.</font>

Not to be argumentative but.... http://bbs.homeshopmachinist.net//smile.gif
Do you think the little sherlines and taigs should be destroyed? There are numerous folks producing fine work with small, very small lathes. Sometimes they have to take a different approach than a person with a S.B. 13" or similar but they manage to get the job done. If he needs to step cut the profile or some other slow process to get it done that may be the case but it is possible.

To suggest his lathe needs to be destroyed is not a constructive statement nor helpful and not the answer he was looking for when he posted his question.

I understand you are a machinist but many of the folks visiting the forum are home shop machinists. They need to be able to post a question without apprehension do to the possibility a "real machinist" will berate them or their machine.. JRouche

[This message has been edited by JRouche (edited 01-30-2006).]

01-30-2006, 09:43 PM
sheeeesh, is a smithy really THAT bad? It does look a llittle light, but a 13" S-B isn't exactly industrial equipment either, and I've used way bigger form tools than that without much of a problem. I think he should give it a try, getting everything as rigid as possible, slow speed, and black smelly oil http://bbs.homeshopmachinist.net//wink.gif

01-30-2006, 11:16 PM
Something is moving, for sure... Have you set up an indicator against the cross slide from the bed, lean on the cutter and try to discover where it's moving/twisting? The tool post maybe? Looks like you've backed the compound out a little, need to get the cutter over the cross slide so when it takes a load it's not trying to tip, not hanging over. And lock whatever gibs you can. If you have a "standard" toolpost swap it back in and see what that does, it should move the tool closer to the post.
Could also try using the tailstock, I know it's a short piece but it might help stiffen things up.
Not familiar with your machine but maybe the headstock bearings are loose or the chuck is, or the chuck mounting. Once you get chatter it's very tough to clean it up, seems like a heavy cut is the only way to clean it off.
It was mentioned on another thread that you could try cutting in reverse, set the cutter upside down and on the other side of the work. Find the thread and see if it makes sense to try.
Definately want the cutting edge at the center. I've become convinced I need to try HSS and learn to grind the shapes I want, less load on the machine than carbide.
Hope that helps!
Mike Henry

J Tiers
01-31-2006, 12:00 AM
OK, here is a link to teh Smithy 1340 (called the 1340 CNC, for some unfathomable reason).

Note tailstock height....... No I'm not laughing (I used to use an AA/109), it's what you have, and I know people who do good work with those or their machinery-cousins from the other manufacturers. But no doubting there are limitations and drawbacks.

Now you doubters may see why added support or other means to stiffen it up might be advisable.


Bill Cook
01-31-2006, 12:03 AM

A radius cutter with a more pointed tool bit would do it, and cut through the chatter marks. You may be able to loosen the compound angle adjustment nuts, and turn the compound by hand as a radius cutter. Looks from the pix that the compound will move back far enough to get the .125” radius.

The tool bit could be set on a spacer or stack of tool bits and Shim(s) right on the compound. A lantern style toolpost would help. http://img.photobucket.com/albums/v50/skytop19/toolpost.jpg
Without one the stack could be clamped with a spacer/stack on the other side of the bolt and a strap across them.

The point of the tool bit should be pretty close to centered side to side over the compound.

The easiest way to set the radius would be to test cut on a piece of scrap and check with a radius gage or a 1/4 inch rod. If the 3 jaw chuck doesn't repeat well enough to recenter the part within the required tolerances, a 4 jaw chuck and a dial test indicator may be nessesary when putting it back in the lathe. Or match mark a jaw and the part before removing it. On rechucking, check with the DTI and if it doesn't run true, loosen and tighten the chuck until it does runs true.

Make sure that nothing will contact the chuck before starting the lathe.


01-31-2006, 01:03 AM
Sometimes when I have a similar problem, I use a round file to help clean up the chattered up cut. In your case, a 1/4 inch diameter file would be the one to use. Grip the end opposite the tang with vise grips, then go under the work, bringing the file towards the corner. Use a block of wood or something to press it into the corner while the chuck is turning. Rotate the file a bit with the vise grips, and let it ride back and forth a bit so that it doesn't get loaded up all in one spot. Realize this is getting your hands close to the chuck and can be dangerous. If a jaw catches the file, it will likely snap it and send it backwards. If you were to use the file on the top, facing forwards, it could catch a jaw, then come towards you. Mucho bad way. At any rate, if you can clean up some of the chatter, it will make it less likely to chatter instantly next time you bring the cutting tool into the corner.
Something else I've tried for difficult cuts is to put a rubbing block as close to the cutting edge as you can, and have that block about a thou behind the cutting edge. Brass or bronze would be a suitable material. The idea there is that it 'references' the cutting edge directly to the workpiece such that the cutter cannot vibrate into the work, only outwards. The cut then just cleans itself up. It's a pain to set up, but I've had some success with it.
You have two options- have it pressing towards the chuck, against the shoulder there, or have it pressing against the side of the shaft. Angle the toolpost until the rubbing block touches metal, then angle it just enough for the cutting edge to bite a bit. I used a drop of way oil to lube the contact point.
I just looked at your pics again. To use the rubbing block idea, you would have to sandwich a brass strip between the right side of the cutter's holder and the toolpost. It would be easier with a four-sided tool post which is what I use.
Well, it's an idea anyway.

[This message has been edited by darryl (edited 01-31-2006).]

01-31-2006, 01:18 AM
J Tiers...well okay...it is a little spindly looking. Sure would like to give it a try though. CAN DO spirit, ya know http://bbs.homeshopmachinist.net//biggrin.gif

01-31-2006, 01:26 AM
Maybe what you really need there is a solid spacer that will take the place of the compound assy and let you still use the toolpost. You just need a longer bolt to go through the toolpost and the spacer, directly to the t-slot in the crosslide. I bet that will smarten it up considerably. That will greatly reduce the amount that the cutter flexes towards the work under pressure from cutting forces.

01-31-2006, 01:57 AM
<font face="Verdana, Arial" size="2">Originally posted by J Tiers:
OK, here is a link to teh Smithy 1340 (called the 1340 CNC, for some unfathomable reason).

Note tailstock height....... No I'm not laughing (I used to use an AA/109), it's what you have, and I know people who do good work with those or their machinery-cousins from the other manufacturers. But no doubting there are limitations and drawbacks.

Now you doubters may see why added support or other means to stiffen it up might be advisable.


$4099! http://bbs.homeshopmachinist.net//eek.gif I only paid $2300 for my 20HP 6000# lathe. Cutting a 1/4 radius is as simple as grinding a tool bit or borrowing one from the little lathe and shimming to fit ther tool block.

Anyway I had similar trouble on my 10" Logan with a 1/4 radius bit (paid WAY less than $80 from mcmaster.com) I found a heavy feed and low speed as mentioned above did the trick nicely. I chose to feed the carriage. Use the tailstock if you have room, if you don't get an MT extension so you can use it.

-Christian D. Sokolowski

john clements
01-31-2006, 04:04 AM
Before you get involved in too much work, try running the spindle in reverse with the tool upside down - or with the tool upside down but positioned at the rear - behind the work. Worth a try - works great for cut-offs.

John C

01-31-2006, 08:46 AM
Thanks for everyone's replys.

Can someone recommend a better compound for me?

I'm going to try your suggestions and I'll report back.


J Tiers
01-31-2006, 12:21 PM
Try what Darryl and I mentioned..... going with a solid block instead of the compound....

Make it pretty wide, very flat top and bottom, and bolt it solidly down. Screw the t-nut bolt of your toolpost into it.

That will make your support as good as it gets, short of clamping the tool right on top of the block, which is also possible.

The added mass as well as rigidity should help. You will still have whatever looseness is in the saddle and crosslide, as well as headstock and spindle.

Then going slow should help also.

Short of that, since you are doing an inside corner rounding, you might try sneaking up on it. That has sometimes worked for me.

If your shaft isn't to size, work the shaft and the flange down a little at a time, trying to cut all around the radius without letting the tool have a long cutting length.

Roughing it down that way so you have a finish cut only to do with the form tool may help. OTOH, you may just get the chatter on teh finish cut.

But if you use a different and pointier cutter to rough down, you won't have the chatter marks already there. Then by going slow, you may get through the cut before the chatter starts.

Once you have chatter, it wants to keep up, because it has to cut thru the chatter marks , and they are just right to start it up again.

If the machine is tight, you CAN cut thru them. I fixed my loosey-Logan and parted off right thru the old chatter marks that had been made trying to part the same piece prior to fixing.

01-31-2006, 12:27 PM
Thanks everyone.

I think I'll make a block to replace the compound and mount my toolpost to it.

Sounds like a fun project anyway.


01-31-2006, 12:29 PM
Note the big block of 1 1/2" thick steel mounted to the cross slide in this photo. In this case I am using it to hold work for fly cutting but I can also clamp a quick change tool holder to it, and do. That's how I recently machined the sides of the mini mill spindle head, with the tool holder clamped to the block and the spindle head in the 4 jaw.