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  • Flat Bottom Boring

    I've been doing a small dia flat bottomed bores with my little Everede without drama. I've used my larger boring bars for thru holes with success. But today I needed a 1.700 dia hole an inch deep and what a disgusting experience that was! LOL! Sometimes I wonder if I'll ever learn ...

    It was the usual chatter issue and I fussed with the bit, height, rake, and pretty much got myself all fussed up for my trouble! Couldn't get a .015 cut without major surface eff ups no matter what I did. Even on a 2 or 3 thou cut I could hear the underlying vibration.

    My latest brainwave is I think I need to face the end of my boring bars off some. For a flat bottomed hole I need to have the tip extending past the end of the bar. That gets the bit pretty far out there. I'm thinking too far ....

    The little Everede on the other hand has the bit right out there at the tip. Minimal overhang. If I take the ends of the larger bars down right next to where the bit sticks out I'll need less overhang on the bit. Rigidity will go up.

    But before I start cutting up my cutting tools I thought I'd better check with the experts. Thanx for any advice.

    SP

  • #2
    Well i'm pretty much an expert so i will chip in

    Not sure about the answer to your specific question but i've had to do alot of similiar holes making chrome-moly "buckets" for the FSAE car. I drill the major diameter or as close to it as i can find in a center cutting end-mill. After drilling the hole, i clean up the bottom of it with the end mill and then, if it was undersized, i will take one or two passes with a boring bar. Usually i end up pretty close so i only need one or two light passes with the boring bar and the bottom of the "bucket" is nice and clean except for the last .01 or so that might have some marks from the boring bar.

    I'm sure you already knew that and your issue was probably that last .02 causing problems...

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    • #3
      Try using solid carbide boring bars. They are so much stiffer than steel shafted bars. They do cost a few bucks but well worth it.
      Keep them sharp with a diamond hone or file. You can get beautiful accurate bores with solid carbide boring bars.
      Steve

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      • #4
        Chatter?

        If you get chatter, keep increase the feed until you get to the maximum chip size per tooth usually abou 0.004" or 0.1mm, then with same feed per rev, keep decreasing the speed. Make job stiff as possible. Hold it with your hand? Bag of sand, anything to kill the resonance. Keep cutter overhang to a minimum.
        It is important that the tool geometry is such, that as the cut load increases, the cutter does not try to cut deeper as it bends. It must bend so that an increase in load will bend tool parallel to cutter path or away from the job.
        If it is tough material carbide is a must.

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        • #5
          what are you cutting and at what speed? sometimes you can change the resonance by changing the feed, speed, amount of overhang etc but I've never had great success with this....

          are you using hss? the more acute angle possible with hss creates a short shear plane in the work resulting in less cutting force and therefor for less deflection of the bar.

          what dia is the boring bar? A 1.7" dia hole only 1" deep should allow you get a monster sized bar in there. I'd bet your chatter problems come from to high a ratio of boring bar overhang:dia. make or get a bigger dia bar
          Last edited by Mcgyver; 12-09-2007, 12:30 PM.
          .

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          • #6
            More to the point is, are you boring in a mill or lathe????

            It does make a difference as to the solution to use.
            It's only ink and paper

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            • #7
              Sounds like your using the old style boring bars with square HSS bits? One at 90, the other end at 45*? Those aren't really good for facing (or so I think). Not only does it flex, but hanging way out the side provides torque, and then there is the bit hanging out too. If I did try that, it would be holding the biggest bar possible, as short as possible, and with the bit sharpened and shaped to be as short as possible while only barely extending one little positively raked point past the end of the bar.

              For that, I would use the heaviest brazed or inserted carbide bar I have. If you're facing the full bottom, then that would be a bar slightly smaller than the radius from point to heel. I think it should be doable even with the little 1/2 boring head bars, as long as the end is properly sharpened and cleared (not rubbing).
              Russ
              Master Floor Sweeper

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              • #8
                Thanx for all the replies. I'm actually making a threaded collar on my lathe. The reason for the flat bottom hole was just to save stock. In addition to being of Scottish ancestry there must have some jewish people back there too!

                In the end I opted to whack the thing off with the bandsaw and then re-mounted in my 4 jaw. With a gouged up thru hole leaving room for the tip of the bar I was able to pull the bit back in where it belongs and we were good to go. .035 bites and I was thinking of trying a .040, , but it was already time to slow down ....

                So, the lesson from this episode is if I ever really need to do a flat bottomed bore .... these bars ain't gonna work.

                Thanx again.

                SP

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                • #9
                  Gotta go solid carbide like SJH mentions and with a "real" brand. My Circle boring bars work so much better than the Chinese knock-offs that I mostly use the latter with the boring head in the mill--that doesn't seem as chatter prone as the lathe.

                  With a hole that large, try angling the boring bar. Trig tells you this reduces the load by angling it into a vector sum that doesn't have the whole thing acting at right angles to the skinny bar. This was a tip from Machine Shop Trade Secrets and it has been helpful to me. You may have to set it up carefully to keep a flat bottom in the bore when using an angled bar, but your description indicates the flatness need not be precise.

                  Minimize overhang as mentioned.

                  Maximize boring bar diameter--again, you have room with a hole that size. I forget the ratio of bar diameter to hole depth but it is a lot less than you'd think. I big fat bar will work wonders on this hole. You may have to start with a smaller bar until you have enlarged enough to go larger.

                  One last from Machine Shop Trade Secrets: he says you can never own too many boring bars. With the proviso that cheap bars have been useless to me, I think he's right!

                  Cheers,

                  BW
                  ---------------------------------------------------

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                  http://www.cnccookbook.com/CCGWizard.html

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                  • #10
                    Your boring it on the lathe. He's what I do.

                    Drill a hole that is big enough for the bar to start easily.

                    Don't use a flat on the cutter, have a slight angle where it bottoms out. 20 deg will do.

                    Bore the hole out using a dial ind. on the way so you can stop at "0" each time. Have the bottom of the hole about .040" short of the depth you want.

                    Bore the hole to the dia. you want.

                    Run the bar to the bottom of the hole so the cutting tip is in the dimple in the center of the hole. Feed the carriage .010" in. By hand, feed the crossfeed out untill it is to the finish dia of the bore. Do this untill the hole it to depth. There should still be a dimple in the center from drilling at the start. You will have to finish the bore by backing the carriage out with the cutter at the correct dia. to finish the bore. You need only to do that at the very bottom to clean the sides up.

                    It is important to drill deeper by about .060"-.080" than the finished depth so you have a dimple for the cutter tip to go into.

                    It will give a perfect bore and flat bottom everytime.

                    Don't ever try to bore a flat bottom with a cutter ground 90 deg to the bore. All you will end up with is chatter marks on the flat at the bottom.
                    Last edited by Carld; 12-09-2007, 04:48 PM.
                    It's only ink and paper

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                    • #11
                      Pics always help. This is a 1/2" shank solid carbide bar (circle brand) with a 2" reach. I have modified the tip as you can see.


                      Carbide is 2-3 times stiffer than steel so it does not deflect near as much as steel! The reason I modified the tip was for a certain project. However since that time I have used it extensively for normal boring. The small tip and generous clearance makes chip control easy and it bores even better than new. It also make it easy to keep razor sharp since the cutting edge is small. I can easily take off .0002" cuts with this tool and yet rough out .1" DOC in leaded steel like nothing. It can be used for a right hand cut towards the tailstock and gives a awesome finish with the angled leading edge.
                      Here is a 303 stainless part bored with it and a flat back and it is only .040" thick.

                      No ringing was ever experienced. I used this on the bronze bearings I just made for my S/B headstock. .070" thick and 1.81" ID and about 2" deep.
                      A little rubber tape wrapped around the exterior can help with ringing. I did not even bother with it on the next bearing as it never went into a resonance.
                      Give one a try.
                      Steve

                      Comment


                      • #12
                        Drum brakes

                        Originally posted by S_J_H
                        Pics always help. This is a 1/2" shank solid carbide bar (circle brand) with a 2" reach. I have modified the tip as you can see.
                        ..........................
                        ...........................
                        Carbide is 2-3 times stiffer than steel so it does not deflect near as much as steel! The reason I modified the tip was for a certain project. However since that time I have used it extensively for normal boring. The small tip and generous clearance makes chip control easy and it bores even better than new. It also make it easy to keep razor sharp since the cutting edge is small. I can easily take off .0002" cuts with this tool and yet rough out .1" DOC in leaded steel like nothing. It can be used for a right hand cut towards the tailstock and gives a awesome finish with the angled leading edge.
                        Here is a 303 stainless part bored with it and a flat back and it is only .040" thick.
                        .............................
                        ..............................
                        No ringing was ever experienced. I used this on the bronze bearings I just made for my S/B headstock. .070" thick and 1.81" ID and about 2" deep.
                        A little rubber tape wrapped around the exterior can help with ringing. I did not even bother with it on the next bearing as it never went into a resonance.
                        Give one a try.
                        Steve
                        Thanks Steve for the fine example and the pics.

                        The last pic (I've edited the first 2 out) is a classic solution to "ringing" as is the solid "as big and as stubby as can be" carbide boring bar.

                        Good example is car brake drum being machined on a specialist lathe. They sometimes use leather or elastic material "wrap-arounds" for the same reason as you have done.

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                        • #13
                          Carbide boring bars. On the list! LOL! It's never gonna end is it?

                          SP

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                          • #14
                            Flat Bottom Boring

                            Don't know if they are any good. They sound American made.
                            I found an outfit called carbideboringbars.com,
                            They show solid as well as "brazed" carbide tip types.
                            The prices don't look imported. Just another place to look.
                            Good luck. - Fred.

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                            • #15
                              You don't need solid carbide boring bars, you need to change the method your using.
                              It's only ink and paper

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