# Boring bars, lathe vs mill applications



## homebrewed (Dec 21, 2019)

While viewing a This Old Tony video on making a small boring head, there was a comment that the ones with brazed carbide (like the ones I own) are supposedly not well suited for use in a mill.  The claim is that the carbide is above the bar centerline, so the cutting geometry is not optimal.  When used in a lathe, the height of the bar can be adjusted to get the cutting edge on-center but that's not possible when used in a boring head (so it was stated).

I have to say I'm skeptical of this assertion.  I did verify that my brazed-carbide boring bars have the carbide sitting proud of the bar's centerline, but it's trivial to rotate the bar so the cutting point intersects the radius that's swept out by the tool.  I also note that these boring bars aren't categorized into ones that are good on a lathe vs a boring head.  So is this really a significant issue???

Now, in that particular video, Tony was modifying a brazed-carbide boring bar to turn it into a small dovetail cutter.  In that situation I can see that you'd really want the cutting edge to be on-center with the center of rotation.  But for use as an actual boring bar.....not convinced.

What d'yall think.


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## RobertB (Dec 21, 2019)

homebrewed said:


> While viewing a This Old Tony video on making a small boring head, there was a comment that the ones with brazed carbide (like the ones I own) are I did verify that my brazed-carbide boring bars have the carbide sitting proud of the bar's centerline, but it's trivial to rotate the bar so the cutting point intersects the radius that's swept out by the tool.  I also note that these boring bars aren't categorized into ones that are good on a lathe vs a boring head.  So is this really a significant issue???


Absolutely a problem with these. The problem with rotating the the bar to align the cutting edge with the center of the bore is you then have a significant negative rake. This makes for a huge increase in the amount of pressure needed to cut and in a thin boring bar this leads to lots of chatter. Until I saw that TOT video I struggled with a set of these cheap boring bars. They work fine in the lathe, suck in a boring head. After buying a better boring bar the difference is night and day.


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## projectnut (Dec 21, 2019)

My experience with these boring bars has been just the opposite.  I have a couple sets of the cheap 1/2" shank bars from Shars I use exclusively in the mill.  Just last week I used one to increase a bore in 1018 mild steel from 1.875" to 1.998".  I did put a slight negative rake on the cutting edge, and it worked fine.  I did get some chatter on the first pass cutting .010" running the spindle at about 1,100 rpm.  I'm not sure if the chatter was due to the speed or the fact that the original hole was cut with a hole saw.  As you might expect the original hole was not perfectly round.

In any case I lowered the speed to 575 rpm and maintained .010" per pass.  The hole turned out fine with a mirror image surface, and absolutely no chatter marks.  As an FYI rpete222 (tubalcain) has a couple videos out showing the use of brazed carbide boring bars:











He makes no comment as to the bars not being appropriate for this application, but in fact mentions the brand he preferrs (APT) can be used equally well for either a lathe or mill application.  If you watch the video you'll see he enlarges an existing hole with a brazed carbide tipped bar with no problems.


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## JimDawson (Dec 21, 2019)

The real problem seems to be that all of the brazed carbide bars are not created equal.  I have had good ones and some that are not so good.  I find that in the mill boring head, the HSS cobalt bars work the best.  But they are quite expensive compared to the brazed carbide bars.


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## Mitch Alsup (Dec 21, 2019)

The brazed carbide tools I have all have square edges (i.e., no relief) and 0.000 nose radii.

A couple of minutes on my carborundum stone gives me the relief angles and nose radius for clean work.


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## Winegrower (Dec 21, 2019)

I use these brazed carbide boring bars all the time on both the mill and the lathe.   They work fine on either.


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## Bob Korves (Dec 21, 2019)

I have had no problems with import boring bars with either lathes or mills, at least not for longer than time to do something to fix it.  The biggest problem that I have seen is a lack of an adequate main relief angle.  That means the that cutter is making contact at the bottom of the tool, not on the top corner edge.  If you look closely you can see the wear pattern in the wrong place.  I do the right thing and grind them correctly so only the main cutting edge contacts the work in the size hole that is being bored.  But I have also clocked the cutting edge down with equally good and often better results than running them square to the work.  The negative rake helps to stabilize the tool, as well as helping to get it on center.  Import boring bars often are very poorly ground, and that is a real issue that gives them many bad reviews.  With some careful grinding and honing and inspecting the cutting edge, they work just fine for me.  Plug and play?  Often not...  But very good work can be done when using them after correcting the geometry of the grind and mounting.  Really, at the prices they are selling them for, they are "kits," not ready to use tools.  Boring is the bigger problem, needs more  clearance because of limited clearance below the center of the work, especially in smaller holes. 

Hey, Mikey, how about a tutorial on mill cutting tools, and their differences from external cutting tools???


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## mmcmdl (Dec 21, 2019)

JimDawson said:


> The real problem seems to be that all of the brazed carbide bars are not created equal. I have had good ones and some that are not so good. I find that in the mill boring head, the HSS cobalt bars work the best. But they are quite expensive compared to the brazed carbide bars.



This is very true . The cheap brazed bars are not ground correctly from the factory . You always have to grind them to suit your needs . I tend to use my homemade bars more often in the mill and grind tooling to my needs or use solid carbide BBars . The lathe is less critical when boring bars are concerned , just grind your clearance or mount a hair above center .


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## mikey (Dec 21, 2019)

Bob Korves said:


> Hey, Mikey, how about a tutorial on mill cutting tools, and their differences from external cutting tools???



Yeah, this is a huge discussion that I haven't finished writing up but I can maybe give you guys my opinion, for what that's worth.

Correct me if I'm wrong but I think what Tony is referring to is how the alignment of the cutting edge impacts on tool performance. Essentially, this is tool geometry but it also impacts on the accuracy of the feed leadscrew. Have a look at this graphic from Criterion:




The ideal tool orientation is B. Note that the tip of the cutting edge is on center and there is adequate relief under the edge; this does two things. It provides adequate clearance while still providing adequate support under the cutting edge for strength. Second, it allows for maximum accuracy from the feed leadscrew; what you dial in is what you get because the point of contact is in line with the direction of the feed leadscrew.

"A" is what you have with a brazed carbide tool, or at least the Chinese versions most of us own. The carbide stands proud of the centerline of the tool so with the tool inserted into the boring head the top of the insert projects beyond the centerline. This is problematic because this essentially amounts to negative rake, which increases cutting forces and can produce chatter if this offset is extreme. It also messes with our accuracy because now we do not get what we dial in. The cutting edge is no longer aligned with the feed leadscrew so who knows what we'll get. One way to counter this is to rotate the tool in a counterclockwise direction as Bob suggests, bringing the cutting edge on center; this accurizes your feed. However, the downside is that you now have all the cutting forces in that tip and edge, with very little support underneath so your tool will be weaker and can break down quicker. In spite of this, like Bob, this is how I do it when using these bars.

In "C", the tool has to be improperly constructed or severely worn in order to get this scenario. I have not seen this alignment myself.

The question is whether using a brazed tool, as in "A", will work. Yes, it will because the offset from centerline is very small. It will be less accurate but the tool will definitely cut ... but it can cut better if you bring the tip/edge on center. Rotating it a tiny bit CCW will accurize the leadscrew and create positive rake, all at the same time. 

Personally, I rarely use cabide tools in a boring head, brazed or inserted. I prefer a HSS-cobalt tool from Borite. These tools have tapered shanks with the smaller diameter towards the cutting head; this reduces resonance and, therefore, chatter. Borite tools are also accurately ground so that the top rake surface of the tool is on the centerline of the tool shank, making it simple to align the tool accurately. These bars are easily sharpened, although they hold an edge for a long time, and they finish really well. 

I don't use inserted carbide tools anymore, mainly because I have to use too much speed to obtain an adequate finish. On small holes, this is not really a problem but once I get out to 2" or more then centripetal forces get too large and the mill vibrates. Criterion came out with a counterbalancing bar to combat this but I've been too lazy/busy to make one but it is on the list of stuff to make. 

One more thing and I'll stop. Boring on the lathe is controllable to a great extent. It takes skill but you can cut a hole dead on size on the lathe if you know what you're doing. On the mill, with a boring head, it is far less controllable. I think of boring on the mill as "cut and try". You dial in a cut and given your cutting conditions, material, cutting oil, bar, head, direction of the wind and phase of the moon, you get what you get. The reason for this uncertainty is that in addition to the same three cutting forces we always have to contend with (tangential, radial and axial), we must also contend with centripetal forces. Depending on the bar we use, the material we cut, the size of the hole and the cutting conditions we're using (speed, feed and depth of cut), it is nearly impossible to anticipate how the three cutting forces will interact with centripetal forces to allow us to predict the cut. The best we can do is set things up, make a cut and see what results. Then we dial in the next cut and see what that does. And this is made worse when using inserted carbide because the geometry and nose radius introduce their own cutting requirements and influence on cutting forces. In spite of this, you can bore pretty accurately by the cut and try method. Machinists do it all the time but you do not have the degree of control you have on the lathe; well, at least I don't.

The whole thing is a really complicated affair, which is why I haven't finished my piece on boring on the mill ...


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## mmcmdl (Dec 21, 2019)

You summed it up Mike !  With the correct bars and tooling , it's quite easy to hold .0001 when boring on a mill . With some of the stuff out there now , I wouldn't want to attempt it


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## benmychree (Dec 21, 2019)

I use  Bokum boring tools (HSS) for both lathe and mill work, they are easily set on center, and made in both shank type and a screw on style, one great advantage that they have is that they have back rake on the cutting edge, so they cut much more freely than the usual type.  They are form ground, so that they are sharpened only on top of the cutting face.


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## homebrewed (Dec 22, 2019)

Thanks for all your comments!  It sounds like the real variable here is the wide variation in quality of brazed carbide tooling.  That would explain the range of experiences with the stuff.


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## mikey (Dec 22, 2019)

If you haven't tried a cobalt boring bar in a boring head before, you should give it a try. They cut really well and definitely finish finer, and since the top of the cutting edge is on the tool centerline they tend to also be more accurate in use. Bokum and Borite are the two best brands I know of, although there are sure to be others. I think LMS sells a set for a reasonable price.


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## homebrewed (Dec 23, 2019)

Yes, LMS has a set that will fit my boring head.  The current price is just under $87.

How does one re-sharpen these?  To keep the cutting edge on the centerline you don't want to grind the top so I assume you go after the side.  But there's only so much "meat" there before you get down down to the shank of the tool.  Curious minds want to know .


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## mikey (Dec 23, 2019)

You hone the end and deburr the top. My 3/8" set is now over 20 years old and is fully functional and looks new. It takes very little honing to resharpen these things.

EDIT: I use Borite bars. Bokum bars are another story. @benmychree  can tell you about those.


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## ThinWoodsman (Dec 27, 2019)

For those awful brazed-carbide cheapos, how large a hole must be drilled beforehand? So far I've broken two of the set of six I bought a few years back. Purportedly 1/4", but they do not enlarge holes that are 1/4" or 3/8" in diameter - just rub and eventually snap.


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## mikey (Dec 27, 2019)

I prefer at least 1/8" clearance, more if possible. You need enough room to get the chips out.

EDIT: I wanted to add that a boring bar does not like to cut on two edges at the same time. For example, when you drill a starter hole for a blind bore and try to enlarge the hole, there is an inverted cone of material at the bottom. Taking a cut on the wall and then running the tip of the bar into that extra material at the bottom is usually not a good idea. That can snap a solid carbide bar in pieces due to the sudden rise in tangential forces as you hit that stuff on the bottom. Therefore, it is wise to flatten the hole to final depth first, then start cutting on the wall. I learned this little tidbit by breaking a $350.00 solid carbide boring bar off in a piece of aluminum.


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## ThinWoodsman (Dec 27, 2019)

Makes sense - the through-hole is under the boring bar shaft, after all. 

Figured out the problem I was having: the back of the cutter was rubbing, but not enough to interfere with cutting. Boring bar got about as deep as the head, then wouldn't go any further, even after being withdrawn and the hole cleared out. Turns out the rubbing gradually bent the boring bar shaft inwards, just enough so that the head was rubbing the bottom of the hole, instead of the cutter cutting it. 

So, 1/2" minimum it is!


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