View Full Version : Boring bars 101

02-15-2013, 01:08 AM
I've been working on boring out a steel ring, a little at a time. This should be soft steel, but it sure seems tough. Tried different angles, sharpened the tool, etc. The piece gets pretty hot, so I'm taking a few cuts then leaving it to cool awhile, etc. But the project got me to thinking about my boring bars.

The better results are had with my slide-mounted bar holder, ie I remove the compound and mount the holder direct to the cross slide. It's a custom mount to allow this of course. On this particular job I discovered that light finger pressure on the bar makes a lot of difference as to how well it cuts. The bar is not singing, at least not at a frequency I can hear. It's probably got every dog in the neighborhood howling though- my finger pressure isn't enough to affect anything other than a resonance. I know all about damping, etc, and this isn't the focus of my topic at this point.

My cross slide is a flat table with two T-slots. What I'm thinking to do is make up a mount for a 1 inch diameter bar, which would be turned down to a smaller diameter for the portion holding the cutter. Probably that would be slightly tapered as well, but the focus would be on the bar mounting being on the right side of the cross slide. The mount would be about half the width of the table, and it would be secured using the T-slots. A portion of the mount would extend towards the left T-slot to give added stability to the mount. The part where the bar mounts would only go about half way across. Some of the mount might extent towards the right, giving a longer seating area for the bar. For the moment I'm not concerned with how to mount the bar.

Some of the bars would be turned down for the portion that holds the cutter, as is typical from what I've seen. I would probably end up with several versions of it- some narrower for smaller bores, some larger, and at least one that doesn't get turned down at all, or maybe just tapered from 1 inch down to about 7/8 inch at the cutter end. Might make some shorter, some longer.

In large part the advantage would be more steel, more rigidity, and more often the cutter would be above the table rather than overhanging off to the left. This would put the pressure from cutting over the saddle, which should help a fair bit.

It might be best to bore the holder to take the bar, then slit the side of the bore and arrange to pinch it closed over the bar. This would be better than pinching down on the bar with a set screw or anything similar. I'm thinking also that when the bore I'm turning is longer, so will the boring bar be longer, and thus there should be little problem with the crosslide interfering with the chuck. Only when the bore is going to be deeper than 2 inches will the cutter begin to overhang the slide.

I'm just feeling for comments, intuitions, etc. A good question might be what material would be best to make the bars from. I can rule out carbide since I'd have to sell my arms to get a 1 inch diameter bar of any length, and then I wouldn't be able to machine anyway. I could, and would set a smaller carbide bar into the 1 inch bar so it fits the holder- I can see some version of that making sense. I've thought about making the right end of the bar a morse taper- using a drawbar to suck it tightly into the holder. I'm not sure which would be the most rigid holding- my first method or the taper method. The first method would allow me to make one bar hollow and put a spindle within it- the drive would be to the right of the mount and the spindle made to hold 1/4 inch shank tooling. I would find that to be a worthwhile addition to my tool post grinding arsenal of machines.

My ring must have cooled by now- time to put a few more minutes into it-

02-15-2013, 01:46 AM
If you use carbide, you can get 10x diameter per length - i.e 5 inches for a 1/2 inch bar. You can buy cheap old carbide bars with messed up insert mounts. No machining.. take off the old end (heat) and just braze on any tip etc you want.

On my small lathe I tossed my compound for most boring operations a long time ago. Made a cross slide mounted block and bored (with the headstock) a few smaller blocks that mounted on top with a single post. Made a shims to account for a few different cutter offsets.

Steel for bars... really doesn't matter. it all flexes about the same.

02-15-2013, 04:13 AM
i have just ordered a carbon bar to try and make a boring bar. has anybody done that? another idea i have, is to wrap (epoxy) a hollow steel bar with carbon and fill it with lead shot or oily sand. another idea i havent tryied yet is to wrap the boring bar with a rubber bad with some lead glued to it. seems to work for brake discs.

02-15-2013, 08:51 AM
A split cotter is a great way to hang on to round stuff w/o damaging the surface. Lots of clamping action without much force on the screws; split rings take some serious torquing sometimes. Whenever I have the room and the forethought, I'll use a split cotter.

Jaakko Fagerlund
02-15-2013, 09:00 AM
Just use a piece of steel for the boring bar, doesn't make much of a difference in your situation. If going with HSS, I would just tack weld the HSS piece in the end of the bar or if the bar dimensions allows, drill a hole in 45 degree angle or make a rectangular slot and use screws.

I would like to hear more of your boring operation that you mentioned, like pictures of the tool, setup and more in detail. Has your tool enough clearance under it? Is it a little bit over the center height so that if it tries to dig in it will ease off of the material and thus reduces tendency to ring and snap the tool. Mystery metal? Speed? Feed? DOC? Machine model?

02-15-2013, 10:15 AM
Interesting thread darryl,

I have a project going that requires some boring also. I am trying to bore a piece of 2.5" X 6" A36. The first inch neeIds to be bored out to 1.004", the next 1/2" to 1.25" and the rest to 1.5077".

The bar I am using is home made from mystery metal (it's pretty soft), about .8150 diameter and 14" long. I drilled and tapped a 1/4"-20 hole in the end of it to capture the tool bit. Then crossed drilled a 1/4" hole about 3/4" from the end of the bar I then drove a 1/4" HSS tool bit through it (poor mans broach). I have about 7.5" of bar hanging out of the tool holder.

I have tried HSS and cheapy carbide bits and still the surface finish is terrible. I started with a 7/8 hole and so far I have got it out to .975". I have got figure out how to get a better finish on the bore before I get it out to 1". From 1" I will hone it out to the final 1.004". I think my bar has too much flex in it for the stick-out that I need. It starts to chatter about midway through the bore.

Lakeside, you refer to a "10 X the diameter of the bar" as a stick-out limit for the carbide bars. What is the limit for a steel bar? I was looking at a 3/4"X10 oal hardened steel bar from enco model #377-1016 that uses a triangular carbide tool bit. would this work any better than what I have?

What is meant by the term "lead angle". The Enco bars I was looking at list one bar at a -5deg lead and the other at a -1deg lead.

Sorry darryl, not meaning to hijack your thread. Hopefully these questions will yield some helpful information about boring bars in general.


02-15-2013, 12:40 PM
Steel is about 3X.. but at all depends on the depth of cut etc. I've hung them out way more on lighter cuts, but with carbide inserts you quickly run up against minimum DOC.

Terrible surface finish on A36? lol.. yep.. although if you turn it really fast, take a decent DOC and use finishing grade positive rake carbide, it can often look pretty good. A36 can really suck, and is variable from piece to piece. Keep it for projects where it doesn't matter, or for welding, and buy decent material.

Emco insert boring bars? Will they work better then your homemade bar with HSS - probably not. They will work fine as a bar, but toss their carbide in the garbage and buy some quaity brands. The difference is night and day. I have dozens of inherited bars.. many from the 70's when the old guys made their own from 3/4 to 1 1/4 round mild steel; HSS attached to the ends often with a slot and set screws. They still work fine for bigger work. I also have some carbide 5/32 diameter that takes inserts!. My "go to" bars are a couple of 1/2 inch carbide.

Lead angle is the angle the insert makes wrt the work. 5 is typical as it allows the swarf to clear without rubbing. On the other hand you won't be able to make a flat bottom cut unless you clean up the end with a cross feed cut.

You won't cure a floppy machine with a stiffer bar. Ditching the compound on a small machine goes along way to "cure".

Boring block... on compound (takes two bars).


AXA on riser block. Boring block also fits on top. And.. all made from A36;)


02-15-2013, 01:45 PM
When I worked in Auto Repair....we would wrap brake drums and rotors in a big glorified rubber band to absorb harmonics when machining them. I applied this same theory to my boring bar a few times and it worked. I wrapped the bar with rubber bands...as many as I could fit around it, and also wrap the workpiece with rubber bands. All but one time, it has made an improvement. Now that I've got a fairly rigid machine, I rarely find the need any more.

02-15-2013, 03:49 PM
Been working my way through the "Turret Operators Manual" and for me there maybe a few interesting (odd?) items coming from that...there is a fair bit on, shall we say, "efficiency" of doing the work, stuff like multiple cutters, large DOC etc. so one of the other topics is of course rigidity.
The solution in the turret situation regarding boring appears to be [a] use of a secondary support arm (I always think "overhead". I guess it does not have to be but is most of the time), use of a bushed boring bar. More or less what it amounts to is getting the boring bar as rigid as is possible, often more to increase the depth of cut than improvement of finish (though both certainly apply).
In the case of the overhead support, the turret becomes more connected to the lathe headstock and in the case of a bushed boring bar both ends of the bar have support.
I suppose the other thing to mention is the option of "pull boring" where, rather than pushing the cutter in, you put it to the far end of the hole already existing, dial in the cut and pull the bar out. My understanding of the latter is the different dynamic of the forces involved since you are pulling.

"Odd" since I am seriously considering a sound method to attach some sort of over arm support.

[B]Edit: http://www.practicalmachinist.com/vb/antique-machinery-history/warner-swasey-2a-info-needed-201605/ post #14 shows how such an overhead bar looks/works

02-15-2013, 04:41 PM
Your thoughts on boring bar holders in the OP mirror what I found last fall when poking around on the net for hints on how to build one. One point that made sense to me was that the bar is most securely held by a well fit split clamp arrangement. Most of my boring bars are Everede with an accurate 3/8" shank so I reamed the holder to that size and use 3 screws to clamp bars in place. This is very solid and doesn't chatter despite the flip up capability, see: http://www.gadgetbuilder.com/ToolHolders.html#Round_QCTP

I can't suggest much on making boring go faster - on my little lathe patience while boring is a virtue.

On surface finish, the vertical shear bit often works well. I use it on external surfaces but BryceGTX really got into it and found it works well for improving finish while boring too, more info if you look through this thread:

It's a finishing tool - only good for very light cuts, one or two thou, but seems to work well on many different materials, may be worth a try.

02-15-2013, 05:11 PM
Centre drill and bore a hole down the bar, fill with lead shot, use upside down at the back of the work so the chips clear better and better visibility
The lead shot will damp the harmonics of the bar (plug with Allen grub screw)

02-15-2013, 08:23 PM
You can buy cheap old carbide bars with messed up insert mounts. No machining.. take off the old end (heat) and just braze on any tip etc you want.
I've acquired a fair selection like that. But in my case the damage was TIG welded and then machined back to form a good pocket. And I've also gotten spoiled, I don't like using any boring bar that is NOT solid carbide, they are so much nicer.

02-15-2013, 09:14 PM
My favorite boring tool is a carbide rod 1/4 inch diameter, held in a custom holder. I've shaped the end into sort of a D bit, and ground some front and side relief on it. It has to be angled for the cutting edge to work and to keep the shank away from the bored hole, but that's fine. I can only go about 3/4 deep with it because it's too short. I think I will look for some longer pieces, wider also.

The one I've been using for this latest job is brazed carbide, some cheap set made in India probably. The cutting edge is not on center, so I have compensated by turning it in the holder so the back rake is 0 to positive some degrees. The cutting force is not directly down, but more towards the front because of this. It's not a very ideal setup, but it is the most solid holder I have currently. I can usually get it to work fairly well, but it's not ideal.

I got my bore chewed out to within 40 thou of target (about 1.5 inches) and then sharpened the cutter. It did pretty good until I got to about 10 thou from target, then the going was tough again. I repositioned it so I could use a slightly different portion of the nose radius, and it cut well until I was almost there. About 3 thou short of goal it got tough again. Seems to me the steel is eating the carbide. It's not chipping it, just wearing it down . You can see a bright polished spot developing which is erasing the relief. At that point there's a burnishing action going on, which by the way is giving a great looking finish, almost mirror and beautifully smooth. The metal I'm turning is from the magnet assembly of a speaker. I'm not turning the magnet, just the center pole piece. It's probably a high silicon content alloy, and I'm thinking that the silicon and the silicon carbide cutter aren't mixing too well.

But all that is beyond my quest here, which is to develop the best boring setup I can manage with my 8x18 lathe. I have run into it's limitations a few times lately-

02-16-2013, 08:40 AM
I made up a couple different bars now out of old cummins valves. I would like to make one into a holder for inserts of some sort. Not sure how though since they are so hard and impossible to drill and/or tap.



loose nut
02-16-2013, 12:52 PM
I use the HSS boring bars that are made for boring heads as much as possible, they work well with little to no chatter and are very easy to sharpen. A two flute HSS end mill with one ground off works well also, easy to sharpen and cheap. These seem to be stiffer (if that is the right term) then straight steel boring bars, for tool inserts, so less chatter.

02-16-2013, 09:29 PM
I've toyed with the idea of making the bar from some normal steel, making a bit holder in the end of it, then wrapping the bar in carbon fiber/epoxy. You would get both a stiffening and some damping effect, and you'd be able to machine the bit holder because the core material is just plain steel.

Here's one idea I had for a bit holder- drill the bar out lengthwise to fit a bolt. Using a grade 8 bolt, chop the head off and machine a slot exactly halfway into the diameter, near the end. You might leave say 1/8 inch of meat on the outside of the slot at the end. The slot width would be to suit the diameter of the cutter. In my case, 3/16 would work well because I could then use a 3/8 bolt, and probably a 5/8 minimum diameter bar. You would also mill the end of the bar so with the bolt inserted you would see a 3/16 square hole in which to insert the cutter.

If all fits were perfect, the top side of the cutter would be at central height. You could leave slightly more than half the meat on the bolt when machining the slot in it to compensate for play of the bolt in the hole, and you would mill the end of the bar to suit the slop-free fitting of the cutter into the square hole. There would be no setscrew needed as the clamping force is from the right end of the boring bar.

In a variation on this idea you could turn the bolt head down to a taper instead of cutting it off. In the end of the bar you open up a short length of the hole to the same taper. You mill the slot in the bolt in the same way, but now when you tighten the bar you are slightly pinching downwards on the cutting bit as well as pinching it from the sides.

I've seen the type of bar where the drawbolt has the square hole across it, and the end is slitted so it pinches the cutter as it's drawn in, but those are subject to the bolt turning in the hole under cutting pressures. I have one of those- it works but it's not ideal.

For anything less than a 5/8 diameter bar you'd probably want to braze on the cutter instead of having it removable.

02-17-2013, 12:39 AM
I've toyed with the idea of making the bar from some normal steel, making a bit holder in the end of it, then wrapping the bar in carbon fiber/epoxy. You would get both a stiffening and some damping effect, and you'd be able to machine the bit holder because the core material is just plain steel.

I suspect that will not have the desired effect. As I understand it, carbon fiber/epoxy is not particularly stiff when compared to a steel bar of the same dimensions.

The problems you are experiencing when boring that ring are likely to be caused by the grade of carbide that you are using. Carbide inserts range from tough (but soft) to very hard (but brittle). The boring bars with brazed on carbide cutting bits should indicate the grade somewhere.


02-17-2013, 02:42 AM
I have to suspect that this boring bar set I have is nothing but crap- same for the carbide inserts I bought. I've seen the difference between HSS and HSS- it's not all good.

I had a thought today about the ring I'm turning. After having gotten the bore done, I switched to HSS and am turning off the face to get to the correct length, or thickness for this part. The HSS is cutting it better than the carbide did, but to be fair the setup is different- it's now a turning operation. But it is still not easy cutting, though going well enough. The heat generated is going into the part and not out with the chips. It's kind of like soft aluminum. The characteristics are not what I'm used to- then I realize that this piece I'm working on is cast. Not cast iron, cast steel. It is the metal from a large loudspeaker, the back plate and central core. It's probably a particular alloy to allow high magnetic field strength. When I got it, the core was 3/4 sawn off, so I finished the job and had the piece to use for some project. It is mystery metal :)

02-17-2013, 07:47 AM
carbon fibre has a modulus of 77 10^6 psi.

Jaakko Fagerlund
02-17-2013, 10:37 AM
carbon fibre has a modulus of 77 10^6 psi.
It still doesn't mean it is any good in the given application.

02-17-2013, 02:24 PM
The forces are torsional as well as deflection. It looks like epoxied carbon fiber has about 20% lower youngs modulus and it's strongest when forces are parallel to the fibers. That does not sound very boring-barish to me.

A good solid carbide bar is usually the way to go.


02-18-2013, 04:22 AM
It still doesn't mean it is any good in the given application.

why not? a lot of stuff is made out of carbon, drive shafts for example.

one of these days i am going to try it.

02-18-2013, 08:03 AM
I think it's worth a shot anyway. It will build up the diameter of the bar, so you might have to start with a smaller bar. In itself, that may or may not be a good thing.

I suspect that the effect is going to be to concentrate more stress at the point where the carbon fiber wrapping ends- probably just before the bar enters the holder. If the bar is tapered, it will be fatter at this point, so maybe it will be of no consequence. I don't think it will be found advantageous to make the entire bar from carbon fiber and epoxy as you do need an attachment point for the cutter- I can't see a way to create this properly. Also, the mount portion of the bar needs to fit the holder very well, so it would have to be very precisely formed. Possible I suppose. Personally I would make the bar from steel, then just wrap a covering over the extended portion of the bar. A gut feeling says it should be about 75 thou in thickness. That might be made up of one layer spirally wrapped one direction, a spiral layer in the other direction, a layer or two inline, then repeat the spiral layers on top of that. The spiral layers might be at about a 30 degree angle to the shank. At both ends of the layup, I'd wrap a bundle of fibers around as in whipping. If I do it, then that will be my procedure.

02-18-2013, 08:23 AM
I went through some boring bar situations a while back and finally bought a 5/8 carbide insert holder from mesa and I was able to complete my work much faster. This was for cast iron but any tough material would benefit from a good insert holder.

02-18-2013, 08:40 AM
I don't know how to work this out, but just out of interest- I wonder how a solid carbon fiber/epoxy bar would compare to a carbide bar in stiffness?

02-23-2013, 03:11 AM
carbide seems to have exactly the same modulus as carbon fibre. a carbon composite would be somewhat lower. as i can get a 10 mm x 500 carbon rod for $30, its worth a try.

02-23-2013, 08:48 AM
How come no tungsten bars out there?

04-10-2013, 01:08 PM
so i was wrong. the readily available carbon bars are considerably less stiff than steel:


the deflection is 8 mm on the carbon bar versus 5 mm on the steel rod.

how come? one of these days i will get micro tubes and make a bar. maybe even around a lead rod.

04-10-2013, 02:23 PM
so i was wrong. the readily available carbon bars are considerably less stiff than steel:

the deflection is 8 mm on the carbon bar versus 5 mm on the steel rod.

how come? one of these days i will get micro tubes and make a bar. maybe even around a lead rod.

The carbon rod is anisotropic. The carbon fibres are very stiff in tension, however you are loading this bar in bending. In that case you're just using the stiffness of the epoxy matrix to support your load and that isn't nearly as stiff.


04-10-2013, 03:21 PM
Not a fair comparison. Try it with a steel tube the same dimensions as the composite tube. Then multiply the steel droop by the ratio of weights.

Or, use a steel tube the same weight as the composite tube.

To damp out a steel boring bar just wrap it with lead solder, if you have any.

04-11-2013, 04:45 AM
anisotropic? you mean they are short fibers mixed into epoxy? that would explain it. (i have no idea how these rods are made.)

04-11-2013, 02:24 PM
There are various types of carbon composite rods and they have very different properties. The strongest and stiffest are spiral wound in opposites directions with long fibres and equal amounts in each direction. The resin is low temp COE epoxy. This produces a rod with close to zero COE in both radial and axial directions It also gives very high torsion moment as well as bending moment. There are many much cheaper rods on the market including chopped fibre randomly oriented in polyester resin. While still stronger and stiffer than a fiberglass/poly resin rod they are not nearly as stiff or strong as the high end rods.

The best place to find the really high quality rods is from a real kite shop. You will pay accordingly.

04-11-2013, 02:30 PM
Very interesting Thread fellows. I am learning a lot following some of these. I have a question - I haven't seen Drill Rod mentioned here yet, I was wondering if a boring bar could be made from it then hardened and tempered to the desired hardness?
I would think that would make a reasonably rigid bar.

04-11-2013, 02:32 PM
All steels have the same bending moment regardless of hardness within less than 10%. Hardness only makes it possible to bend it further without plastic deformation. It does not make it harder to bend.

Note that in this context "bend" means bending without producing any permanent deformation. Once you deform the material the concept of bending moment changes dramatically.

04-11-2013, 02:57 PM
Thanks Evan - There, I learned something else today.