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View Full Version : lathe cutting tool; above or below axis, which is better?



Elninio
03-18-2011, 10:47 PM
Tonight, I didn't have my QTCP ready, so I was forced to using an HSS chinese boring bar to turn OD's on some some arbors I needed (to finish the QCTP with). After adjusting the tool heigh a bunch of times (and angles), I managed to produce a configuration that cut surprisingly smooth. I pushed my luck and turned the compound so that the boring bar was nearly parallel, with the bar I was cutting. RPM was around 1000 and the part was 0.800" OD. I kept pushing the compound angle closer and closer to 0 - I think I must have gotten within less than a degree. The finish was absolutely great, about as good as ground, nice and consistent throughout. Due to my curiosity and negligence, I pushed it in shoulder and broke the bit. The bit is was about 3/16ths between the cutting tip and the shank, 1/4" at the shank. The kind of swarf that was being produced was similar to dust; mixed with WD40 it produced a slurry that dripped off the cutter.

...

as hard as I tried with another boring bar, I could never get another one of these boring bars to produce the same kind of finish. I've tried various angles in all three axes, and various tool heights.

...

It got me thinking about something I read here once; either above or below the cutting axes, one is better than the other. Which was it? I think it was, you're better off being slightly above axis than slightly below axis. My second setting was making horrible noises ...

RenoF250
03-18-2011, 10:56 PM
I have always gone for slightly below but I am a rookie. If you are slightly above and the bit grabs as it gets pulled down it also gets pushed out digging it in deeper. Slightly below center it just gets pulled down a bit gets unhooked and moves on.

Carld
03-18-2011, 11:00 PM
I don't understand why your using a boring bar to turn the OD of something. You should be using a lathe tool which would be anything from 1/4" square to maybe 1/2" square depending on the size of your lathe.

You should have the tip of the cutter at the axis of the lathe spindle. That is where you will get the best cut. DO NOT set the tip above the axis of the spindle, the tool will rub on the front. Sometimes you can set the tool a few thousandths low.

If a boring bar inside of a bore it needs to be on the center line (axis) to cut correctly.

mechanicalmagic
03-18-2011, 11:02 PM
I was taught to place a lathe tool ON CENTER, not above or below. That has worked for the last 45 years, for me.

DJ

Elninio
03-18-2011, 11:10 PM
I was taught to place a lathe tool ON CENTER, not above or below. That has worked for the last 45 years, for me.

DJ
That's an ideal scenario. Let me rephrase the question; would you approach from above or from below?

Carld
03-18-2011, 11:15 PM
Ok, If you always keep the tip of the cutter on the center line of the work you will not have any problems.

Is that clear enough? Yes, that is an ideal situation and is what should be done.

airsmith282
03-18-2011, 11:34 PM
id have to say that well over 90% of the time i set my cutter just a slight hair below center when turning an OD when boring an ID its above and the cutter is just a hair twisted down ward makes a nice finish, and when parting i now have adopted going just a hair above center ..

mechanicalmagic
03-18-2011, 11:47 PM
That's an ideal scenario. Let me rephrase the question; would you approach from above or from below?
I don't approach. I have a single purpose height gauge, it's fixed ON CENTER. My cross slide is flat, the height gauge is used to set the tool once, then I cut. (OK, to get picky, it may be +-.002".) But I never intentionally set high or low.

DJ

jkilroy
03-19-2011, 12:07 AM
On the OD set your tool on center or very VERY slightly below NEVER above. On the ID it is opposite. This is because when on the OD, being above would mean that if your tool dug in, the DOC (depth of cut) would get deeper, not what you want. If you set it on center or below, the DOC would reduce, which is a good thing. You can probably figure out why being above is ok when cutting on the ID.

TClarke
03-19-2011, 03:26 AM
Below - your rake angle is slightly negative, but you'll have more than adequate relief (clearance).

Above - more positive rake angle, but likely inadequate relief, causing the cutter to rub and chatter.

Elninio
03-19-2011, 03:30 AM
On the OD set your tool on center or very VERY slightly below NEVER above. On the ID it is opposite. This is because when on the OD, being above would mean that if your tool dug in, the DOC (depth of cut) would get deeper, not what you want. If you set it on center or below, the DOC would reduce, which is a good thing. You can probably figure out why being above is ok when cutting on the ID.

It's good to hear that some of you know this from experience. I would have reasoned that having it only slightly above on the OD would put greater load on the cutter, reducing vibrations. I'd rather reap the advantage of undercutting a part over improved finish, than machining a part twice because I cut too much ...

JCD
03-19-2011, 12:29 PM
According to the south Bend lathe works publication; How to Run A lathe, "the cutting edge of the cutter bit should be about 5 deg. above center". Form tools, like threading tools should be placed with the cutting edge on center. This also applies to taper turning.
I have used this rule for about 45 years, it has served me well.

GadgetBuilder
03-19-2011, 08:06 PM
Leo St. Clair in "Design and Use of Cutting Tools" suggests the tool tip be 1% of the cutting diameter above center; he assumes front relief of 8 degrees. The idea is that if you attempt too great an infeed the tool will rub just under the cutting edge, making excessive infeed difficult due to the increased pressure needed. Also, if the tool nods downward the tool will rub, helping to support the tool which reduces the tendency to chatter. When the tool tip is below center, chatter can be a problem because this effect is lost.

I've been using this approach since reading the book over a year ago and it has worked well for me. I set the tool about on center and adjust the tool up if it chatters and down if it takes more infeed pressure than expected to cut.

Parting sometimes requires pausing to adjust tool height as the diameter decreases so this technique works best with a QCTP to make changing tool height easy.

John

philbur
03-19-2011, 09:04 PM
5 degrees is 0.5 mm above center on a 12mm workpiece!!!!!

Phil:)


According to the south Bend lathe works publication; How to Run A lathe, "the cutting edge of the cutter bit should be about 5 deg. above center".

Elninio
03-19-2011, 10:53 PM
Here's the piece;
http://ompldr.org/vN3Z3aQ/cats.jpeg
The taper was turned with carbide, on center aligned with a magnifying glass. The two oval patched on the shiny part are the jaw marks, I didn't tighten it very hard. The piece is about 3/4" diameter at the largest part. The white 'tube' on the side in the background is the wire from my earbuds (headphones), to give you an idea of the size. The finish isn't exagerated, I guess I should have compared it along side an HSS bit ...

PeteF
03-19-2011, 11:12 PM
Did I understand correctly, the taper was turned with carbide, while the part with the fine finish was turned with HSS? If that's the case I'm afraid you're not really comparing apples with apples, as many (most?) experience a better finish with HSS on typical home shop style lathes than they do with carbide. That's most definitely been my experience anyway.

With regard height, I wouldn't sweat it too much. Most of the time I set my tool height with a centre and 6" steel rule, so it could be a few thou above or below in all honesty. No children with 2 heads were born as a result, and I think some may get wound up a little too tightly in the theory rather than the practice. Personally I aim for smack on centre, and with the standard clearance angles I grind in, I'd have thought the bit would need to be a fair way above for rubbing to be a major concern, but having never deliberately set a bit that way I could be mistaken.

Pete

Elninio
03-19-2011, 11:19 PM
You're right Pete, I am comparing apples with oranges, but even apples and oranges are both round and approximately the same size. There shouldn't be that great of a difference. You can barely see the shank of this bar in my photo, the shank was turned with HSS, in fact it was turned with the same bit that did the nice finish, before it broke, and before I adjusted it to cut really smoothly. The finish is almost the same as the taper.

J. Randall
03-20-2011, 12:36 AM
Elninio, without seeing a pic of the setup, I am going to guess your cutting edge was working more as a shear tool. Check some of Carld's experiments with them on this site. I am also in the camp of setting exactly on center for an outside cut.
James

Elninio
03-20-2011, 01:08 AM
Elninio, without seeing a pic of the setup, I am going to guess your cutting edge was working more as a shear tool. Check some of Carld's experiments with them on this site. I am also in the camp of setting exactly on center for an outside cut.
James
The reason I asked was for troubleshooting purposes; I figured if the tool was above axis it would make a distinct sound or pattern on the work piece from a tool below axis.

philbur
03-20-2011, 08:21 AM
This is an assumption that you should not make.

Saying you used carbide leaves a whole world of good and bad choices.

For example down at that diameter you almost certainly don't have anywhere near enough rpm for carbide.

Phil:)


There shouldn't be that great of a difference.

tsmartin_98
03-20-2011, 09:17 PM
I don't understand why your using a boring bar to turn the OD of something. You should be using a lathe tool which would be anything from 1/4" square to maybe 1/2" square depending on the size of your lathe.

You should have the tip of the cutter at the axis of the lathe spindle. That is where you will get the best cut. DO NOT set the tip above the axis of the spindle, the tool will rub on the front. Sometimes you can set the tool a few thousandths low.

If a boring bar inside of a bore it needs to be on the center line (axis) to cut correctly.

You sometimes need to use a boring bar to turn an OD. When the OD you are turning exceeds the max you can adjust the compound, cross slide and QCTP.

I ran into this very problem trying to make the extended chuck I posted in the shop made tools thread. I used every boring bar and tool post combination I had and still couldn't make it work. I ended up fabricating an Armstrong type boring bar holder to get the clearance I needed.

This is one of those cases where I had to make a tool in order to make a tool.

TS

Elninio
03-20-2011, 09:22 PM
Elninio, without seeing a pic of the setup, I am going to guess your cutting edge was working more as a shear tool. Check some of Carld's experiments with them on this site. I am also in the camp of setting exactly on center for an outside cut.
James
This may be possible, but I don't think it was the case. I've had poor experience with shear tools on my shaper, so as I remember it, I made sure not to do a shearing effect, knowing how shakey things got on the shaper.

gwilson
03-20-2011, 10:31 PM
It is necessary to put the tool above the center line when boring smaller bores,to keep the tool from rubbing at some point below the cutting edge. I never put a boring tool below the center line in any event. As some have mentioned,if it digs in,the tool will be broken.

J. Randall
03-22-2011, 12:06 AM
This may be possible, but I don't think it was the case. I've had poor experience with shear tools on my shaper, so as I remember it, I made sure not to do a shearing effect, knowing how shakey things got on the shaper.

Elninio, I have zero experience with a shaper, so I have no comparison there. However I have played with a shear tool on the lathe, and that good finish and that spooge you had dripping off sure sounds like the results of a shear tool.
James

Elninio
03-22-2011, 01:34 AM
What leads me to think that it wasn't a shear effect, is the ease i could take a deeper cut with. Another possibility I've thought of is that a originally the bit was perhaps slightly higher than center, and instead of chipping, a small radius got polished into the cutting tool, so that after the chip meets the top rake, the slight radius below that rubs to produce a nice finish.

J. Randall
03-22-2011, 05:13 AM
What leads me to think that it wasn't a shear effect, is the ease i could take a deeper cut with. Another possibility I've thought of is that a originally the bit was perhaps slightly higher than center, and instead of chipping, a small radius got polished into the cutting tool, so that after the chip meets the top rake, the slight radius below that rubs to produce a nice finish.

Burnishing is a possibility, may never know unless you can duplicate the setup and results.
James

Carld
03-22-2011, 10:13 AM
tsmartin_98, using a boring bar to turn an oversize work piece is something I have done for, ummm, at least 30 years so it's nothing new to me but I still question why he was using a boring bar to do what he was.

As to the cut working like a shear tool, not in this life, the surface finish with a shear tool would look like it was ground, not gouged out as the photo shows.

I take issue with the notion that you HAVE to run carbide fast to make it work. I have used brazed carbide cutters and insert carbide cutters for over 40 years. With higher speed and the right feed the finish is quite good with carbide but you can get good results at the same speeds with carbide that you would use for HSS. The trick is you have to learn how to do it. Just trying it once and it don't suit you is not experimenting, many, many tries are required to really learn.

Elninio, I think your feed is to fast, it looks like your cutting a thread. The tip of the tool needs a very slight radius to get a better finish. By slight radius I mean just stoning the tip, not grinding it.

The look of the cut tells me you may be above center and to fast a feed. Here's something to think about. When the cut starts the force on the cutter pushes it down so now it is under the center line if it started on the center line and that won't hurt anything.

From reading your posts I assume your using the compound to feed the cutter along the work, is that so? If so, turn the handwheel slower and smoother and put a radius on the cutter tip. When your hand feeding you have to be smooth and steady with the feed.

If you have to use a boring bar to get the clearance to cut the work then keep in mind the bar flexes and as it cuts will bounce up and down and cause a crappy finish. Any slop in the crossfeed and compound ways will cause a lot of issues as well.

Having the cutter above the center line by more than a few thousandths can cause rubbing on the front of the cutter against the work. That will cause a crappy looking finish too.

Alistair Hosie
03-22-2011, 10:46 AM
All the good books propose dead on centre to be correct as far as I know that's how it should be done and how I do it always.Alistair

Boucher
03-22-2011, 11:45 AM
This thread has taken off in several different directions.

A picture of the setup showing the work and the boring bar would help.

A 3/16 boring bar with a 1/4" shank is going to be pretty limited in its reach without flexing and chatter. The picture of the part really doesn’t show chatter. I agree with Carl’s observation that the feed rate is too high. The finish looks more like threads than a turned surface. Cheap carbide boring bars are not very useful. They do need to be sharp and have the proper relief below the cutting edge when used. The Criterion carbide boring bars are far better than any others that I have used. One good cobalt boring bar is better than ten of those cheap carbide imports. The tool should be set on center as close as possible. Plus or minus 0.005 is probably realistic and should be confirmed with a facing cut to look at the nubbin.

A properly oriented turning tool is fundamentally better than a boring bar for OD turning operations.

A sharp cutting tool fed slowly may cut very small shavings but it does not make dust.

Elninio
03-22-2011, 03:08 PM
is it not recommened to design round profile HSS tools that cut on the front and the size, since the height of the tool will vary due to the progression between the rakes?

Carld
03-22-2011, 07:23 PM
Oh oh, Elninio, I guess I will have to stop doing that because I have been doing just what you described for years. I had no idea that it would not work while I was doing it. It always seemed to do just fine but I guess I will have to stop doing it now since you said it won't work. :eek:

EDIT: I have to add that when you do a profile cut the cutter HAS to be 0 rake and on center line.

steverice
03-22-2011, 07:31 PM
Tool On Center

J. Randall
03-23-2011, 12:54 AM
As to the cut working like a shear tool, not in this life, the surface finish with a shear tool would look like it was ground, not gouged out as the photo shows.


Carld, I think he was talking about the finish on the parallel part of the piece, are you calling that a gouged looking cut, or are you looking at the taper that he said he cut with carbide?
James

Carld
03-23-2011, 09:16 AM
No, he said that is where he had it in the chuck. You can see the jaw marks on the smooth area of the rod. He even said those are jaw marks.

Elninio
03-23-2011, 06:57 PM
No, he said that is where he had it in the chuck. You can see the jaw marks on the smooth area of the rod. He even said those are jaw marks.

Those are jaw marks, but I cut that nice finish while clamping it on the other side, with the boring bar crappy setup.

J. Randall
03-26-2011, 01:05 AM
Those are jaw marks, but I cut that nice finish while clamping it on the other side, with the boring bar crappy setup.

Elninio, Thanks for clarifying that, and seriously if you are interested in duplicating that finish, Carld's previous posts on the shear tool are worth a look if you missed them the first time around. If not, carry on.
James

Mr Ron
05-01-2011, 06:39 PM
To be able to cut a specific amount of metal using the cross slide micrometer dial, the cutting bit MUST be exactly on center. If you feed the cutting tool in .010", the diameter should be reduced by .020"; otherwise the diameter will be around .015"± less if not set on center.

IdahoJim
05-01-2011, 08:19 PM
Leo St. Clair in "Design and Use of Cutting Tools" suggests the tool tip be 1% of the cutting diameter above center; he assumes front relief of 8 degrees. The idea is that if you attempt too great an infeed the tool will rub just under the cutting edge, making excessive infeed difficult due to the increased pressure needed. Also, if the tool nods downward the tool will rub, helping to support the tool which reduces the tendency to chatter. When the tool tip is below center, chatter can be a problem because this effect is lost.

I've been using this approach since reading the book over a year ago and it has worked well for me. I set the tool about on center and adjust the tool up if it chatters and down if it takes more infeed pressure than expected to cut.

Parting sometimes requires pausing to adjust tool height as the diameter decreases so this technique works best with a QCTP to make changing tool height easy.

John
Good post, John. I'm fairly new to machining, but found this to be especially true when parting. It really helped me eliminate the chatter.
Jim

noah katz
05-04-2011, 01:20 AM
To be able to cut a specific amount of metal using the cross slide micrometer dial, the cutting bit MUST be exactly on center. If you feed the cutting tool in .010", the diameter should be reduced by .020"; otherwise the diameter will be around .015"± less if not set on center.

How can you quantify the amount the dia. will be off without quantifying how far from center the bit is?

philbur
05-04-2011, 03:30 AM
The part diameter will also have a big impact.

Phil:)


How can you quantify the amount the dia. will be off without quantifying how far from center the bit is?