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Long standing question: carbide for boring bars

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  • Long standing question: carbide for boring bars

    This has been in the back of my mind for awhile. I might as well share it A few things, first, that I have been given the impression or perhaps "learned":

    1) Carbide needs a decent DOC or else it rubs.
    2) The reason carbide cannot be ground to a very, very sharp edge like HSS is because it is brittle. When ground to such an edge, it tends to break off.
    3) HSS can be ground to a very, very fine edge. This allows an exceptionally low DOC to be possible.
    4) The overwhelming vast majority of commercial boring bars available are solid carbide, brazed carbide tip, or carbide insert type.
    5) The majority of boring heads are graduated in .001" on bore diameter increments or finer. A whole category of boring heads are graduated down to tenth adjustments and are considered worthy of a jig-borer.

    For these relatively high-precision, highly-graduated tool operations isn't carbide a little out of place? Would it not make more sense to use a tool which is better suited to small DOC's and adjustments? I never hear, though, of anyone every using a HSS boring bar. The only rebuttal I have is that carbide is inherently stiffer. As a boring bar has a large projection, a stiffer tool should be more accurate. Thoughts?

  • #2
    Well, First off, to be 'stiffer' the body of the tool needs to be solid carbide

    Those are Definately superiour due to the rigidity they offer. And they cost an arm and a leg. Basicly they are what you want for extream depth to diamiter ratios, And for very small holes as they are a little more affordable in the small sizes.

    Everything else thats steel bodyed has aprox the exact same overall rigidity.
    Basic boring bars tend to hold small square bits that can be tiny brazed carbide bits OR more often, HSS! You can get a whole set of em for $20, thats why you don't see em talked about often, theres just not much to say.. other then buy them!

    As for the brazed carbide bar sets.. They are just damn convienant and do tend to last a long time for roughing. They can be honed with diamond laps to be damn sharp too for finishing! But being sharp, they still last a long time because in boring applications you tend to take a smaller DOC due to chip buildup and deflection in the tool.
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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    • #3
      Poor wording on my part. Yes, it would have been better to say "solid carbide" or "carbide shank" for the last statement. I tend to use the "old standard" type of bar with a square, broached hole to fit a HSS bit. In all these catalogs I get, the overwhelming majority show pages and pages of brazed carbide and carbide insert type boring bar sets. It seems misplaced to me---who finds a HSS set like THESE to be more useful and appropriate. There are plenty of tooling suppliers that seem to only carry the carbide bars. Perhaps that has just fostered a wrong impression, and HSS is still used to a greater extent than I realized for this work.
      Last edited by Arthur.Marks; 05-31-2011, 10:32 PM.

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      • #4
        I use what you use. Tool companies do not make very much money on that though. I don't like the ones you reference because they are more inconvenient to sharpen and offer no advantage (to me).

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        • #5
          The Borite bars are nice, but very pricey. I've got some hand ground from round HSS that work quite well for smaller holes (up to 1/2 or so). I don't care to grind them from square, too much work. This along with some tiny micro-grain solid carbide bars cover my small/shallow needs just fine.

          Above 1/2" or so, or deep, I switch to one of 2 options. Either the cheap old-school bars that take HSS, or carbide inserts, the latter on carbide bars if I have it. The HSS bits get the nod when low pressures are required (small cuts, etc). They work well, and the flex isn't so much an issue, plus you can get great finish even on very small cuts. But if more hang out (boring a 1/2 hole in tool steel 4" deep!), higher forces, or harsh material (cast iron, hard steel, some bronze) I grab the solid carbide bars. Mine start with a 3/8" x 8" solid carbide Kennametal and go up to a 3/4" x 10" Kennametal, with some Polish and other makers taking up the slack in between. All bought fairly (relatively) cheap, including some that were broken off that I got VERY cheap. Even broken off, my 5/8" x ~6" bar won't go deep, but it will hog like no tomorrow (when otherwise suited), and doesn't have the excess back length in the way on shallower holes.

          But beyond that, I have no issues with any of the standard options. You can get carbide inserts that will easily take a fairly shallow cut with low pressure. Or you can step up to Cermet and beyond for some really amazing performance (and speeds!) using the same solid carbide insert boring bar. I've got a cermet insert (gifted by a friend) that loves taking 0.002 DOC in steel at over 1000 sfpm and leaves a mirror finish in the process.

          So to summarize. If I want fine finish in a shallow cut, I can generally use HSS in a standard bar, and the low forces don't generally cause a problem even with fairly significant hang out. If it does, then I can use solid carbide insert bars with *appropriate* insert to achieve what I require.
          Last edited by BadDog; 05-31-2011, 11:19 PM.
          Russ
          Master Floor Sweeper

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          • #6
            From the listing of the Borite bars I will assume your using them on a mill in a boring head. I buy the import brand bars just like the Borite. You can get them in C2 and C6 and I also use many bars that I have made over the years. My home made bars take square cutters and I use HSS, HSS Cobalt and brazed carbide in them.

            The Borite type bars are expendable and I don't see any reason to spend a lot of money on something that a few sharpenings makes it scrap. You can only sharpen those type bars so many times.
            It's only ink and paper

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            • #7
              Originally posted by Arthur.Marks
              This has been in the back of my mind for awhile. I might as well share it A few things, first, that I have been given the impression or perhaps "learned":

              1) Carbide needs a decent DOC or else it rubs.
              2) The reason carbide cannot be ground to a very, very sharp edge like HSS is because it is brittle. When ground to such an edge, it tends to break off.
              3) HSS can be ground to a very, very fine edge. This allows an exceptionally low DOC to be possible.
              These points aren't as true these days. The sharpness of carbide is limited by the tiny grains inside it. The carbide grains are pretty small these days, which means very sharp edges that can take light cuts. Crappy carbide like in the Chinese tools has larger grains and so you can't sharpen them as much.

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              • #8
                One of my favorite cutting tools is a 1/4 inch diameter micro-grain carbide rod that I took the time to grind to proper angles and cutting edge height. I mount it in a steel holder that I first fitted to the toolpost, then drilled for the rod by chucking a drill bit in the chuck and bringing the carriage along. With the hole being exactly on center and the cutter ground to put the edge also on center, I figured out and ground the relief angles. Two set screws hold the rod in the hole, and I can spin it one way or the other- handy on some boring jobs.

                This is the tool that I put through hell when I know no other cutter will do the job, or last long enough to finish the job. Cutting rusted surfaces, mill scale, case hardening, etc.

                A large part of my success with this tool must be due to the quality of the carbide. The same is true for hss- some of the hss cutters I buy just aren't that great. I only really come to know this when I get some other piece of hss and see the difference in cutting action, most particularly how long the edge will last.

                I have a few orphan carbide inserts, which I've mounted and used, and they work great. To be honest, I don't know the grade or anything about them, but what I do know is that the inserts I've bought lately don't seem to last or even cut very well. I wonder which foreign country they were made in-
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Originally posted by beanbag
                  These points aren't as true these days. The sharpness of carbide is limited by the tiny grains inside it. The carbide grains are pretty small these days, which means very sharp edges that can take light cuts. Crappy carbide like in the Chinese tools has larger grains and so you can't sharpen them as much.

                  Agreed. For small-diameter bores (usually those under 1/2" - 12.7mm), I use Micro 100 brand solid carbide boring bars. I can resharpen them if they get chipped or worn (rare), they can be dead sharp, and they can take a very light cut at low feed to leave a smooth finish.

                  For larger bores I have insert boring bars, both steel and solid carbide, with a good choice of carbide and cermet inserts. I have lots of HSS tools too, and find HSS works fine on one-piece jobs as long as it's not tough stuff. It wears so quickly unless you slow down to a crawl. My machine just doesn't have speeds that slow, and neither do I.

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                  • #10
                    I think that there is a lot of HSM mis-information regarding the use of carbide as cutting tools.

                    I used to service a stamping die that punched out flat discs from blue hard spring steel, 7 at a time at about 500 hits per minute.The die used all carbide tooling.

                    When I started sharpening the punches I would get some punches that lasted for several days and some that would chip out in no time.

                    I started to inspect my work under a microscope and found that some punches had chips where the face of the punch met the OD. Those chips were not visible without magnification. I started inspecting everything under a microscope and re-grinding those with chips. Ended up getting long life out of all of them. I do realize that a punch is not a tool bit or an end mill but the same things apply.

                    I suspect that a lot of the problems HSM'ists get is trying to sharpen carbide with silicon carbide wheels. They are OK for roughing but very poor for sharpening things. I don't own any.

                    Most commercial shops that I am familiar with use ALL or almost all carbide. very little HSS.

                    Brian
                    OPEN EYES, OPEN EARS, OPEN MIND

                    THINK HARDER

                    BETTER TO HAVE TOOLS YOU DON'T NEED THAN TO NEED TOOLS YOU DON'T HAVE

                    MY NAME IS BRIAN AND I AM A TOOLOHOLIC

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                    • #11
                      Thank you for such detailed responses. They have chased out some misinformation in my understanding of carbide. beanbag's point on grain size is very eye-opening and interesting to hear. Brian's note on edge consistency is equally so; furthermore, the recommendation of Micro100 is just as helpful. I wonder if the advancement in grain structure is what has allowed the (recent?) introduction of highly polished inserts specifically for aluminum?

                      I am now getting the impression from this thread that the DOC/finish gap between carbide and HSS is rescinding.

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