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turning a half-round groove

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  • turning a half-round groove

    So I'm building a jig for bending some brass tubing with a .014" wall thickness. I need it to be absolutely without kinks or ellipticity and it needs to have a certain arc. What my plan is is to make two wheels each with a half-round groove that will roll around each other and support the tubing on all sides at the point of the bend. The tubing will be filled of course. The problem is that I need a 15/32" groove (the OD of the tube). There are inserts for 1/2" and 7/16" but no 15/32" I was thinking I could turn inserts out of drill rod, maybe get fancy and put a chip breaker in there, I don't know. That way I know that it would indeed be a half-round groove, but would the drill rod really cut the steel wheel? The wheels need to be steel because I'll be using this jig a lot. What do you all think of the home made insert idea?
    Stuart de Haro

  • #2
    It would be easier to use bronze wheels to cut the groove into. Wear should not be a problem with thin walled brass tubing. Steel wheels would require a edmill type cutter. With a blank monted on a dividing head or vertical mounted rotary table you would feed the table into the wheel and then turn the table into the bottom of the center cutting endmill. Check the catalogs, you may be able to find a ready made endmill, but I doubt it.

    Garr carbide could hake you a nice endmill to do this.


    • #3
      Easier to just purchase a rolling hand bender. Available at any industrial supply. With the roller bender the fitting will slide around the bend. The rigid brand instrument tubing benders I have are the shoe type slightly crushing the tubing in the bend. You better have the fitting on there.
      Even if you decide to bend your own radius, the plan and design of the bender should be looked at for reverse engineering purposes.
      Trying to remember the brand names, but nope nothing in my noggin. yeah, I got a box full of benders and nothing upstairs, can't remember the brands except rigid.
      (french name for one?)
      I have went to sleep with one of these benders in my hands. poor I&C techs get the crap jobs.


      • #4
        To completely avoid any squashing of the tube, shouldn't your die radius be slightly less than the tube's OD? I mean, any tube in a bend will deform slightly. Maybe this deformation could be offset by giving it a prejudicial squeeze in the other direction if you know what I mean.

        Think there was an article about this in MEW a while back. Of course Hornluv said it is an arc you are after as opposed to a sharper bend, so maybe this is all reduntant.

        ibewgypsie, the only other firm I know of that makes a small machine for making perfect, non-deformed bends in thin-wall tubing (besides HERBER) is an Italian it Pedrazzoli? Small, blue, shoebox shaped jobs with a crank on one end...I'll check later.

        Edit: They're called CBC, not Pedrazzoli. maybe there is a .com as well. Also noticed that German REMS makes tube benders like Ridgid's, for on-site and field use.

        [This message has been edited by Dr. Rob (edited 05-09-2003).]


        • #5
          Get a radius gauge and grind a piece of HSS to fit. not the best way but if you take your time it can be done. If you have the right tools this is how I would make the cutter. Set up your surface grinder and radius dresser. grind the HSS down to the correct thickness about 3/4" back from the end off the tool. Use a piece of 1/2" HSS and you'll only need to take off .0312" to get your 15/32" width then dress the radius on the wheel. set up you toll in a tool maker vise at a slight angle like two to five degrees this wil grind in your end cleance angle. grind you radius(you may want to rough it out first on a snag grinder)
          drees the wheel again then grind it again just to clean up.
          now you have a parting tool type cutter for you radius. Now being that it is so big you will need to rough out your part first then use the form tool to finish and slow it down or will chatter like an SOB.
          That's how I'd do it if i had the stuff in the shop. if not I'd use a snag grinder to get the radius as close as posible. thos tube benders are not that accurate but how far off on the radius you can be I don't know. Also a trick use to bend brass tube is to plug one end and fill with water then freeze this give good suport to the tube but still alows you to bend it and this way you don't need a grove to form it in as long as you be careful with the stuff.
          Rule #1 be 10% smarter then what you\'re working on.
          Rule #2 see Rule #1


          • #6
            Yes, you can make a form tool out of drill rod. After all, that is what it is made for.
            Turn to size, reduce end to about 1/4" diameter, and slice off about 1/4" thick piece. You will end up with T shaped piece that can be mounted in mild steel holder. Mill holder to about 5* and drill hole to accept shank at same setting. Hold with set screw.
            Heat cutter to red, quench in oil, draw to light straw color, grind/hone the top and have at it.
            That is a pretty big radius, so you may want to rough it out a bit by plunge cutting with a round nosed cutter first and clean up with your radius cutter.
            Jim H.


            • #7
              Could you get a set of 7/16 inserts and open them up to 15/32"?


              • #8
                Hi Hornluv,

                My first question is, do you have experience with tube bending? What you're describing is not the easiest job to do, even with a sophisticated draw type bender. And harder to do with the type bender you're making. IMHO, your type of roller bender may not do the job. The rolls will not support the tube adjacent to the bend area.

                Do you you have a very large, rigid lathe? Plunging into steel with that large an insert takes a beefy machine. Without a rigid machine you're sure to get chatter marks which transfer to the tube during bending. How about finding someone with a CNC to machine the rollers for you?

                When you say the tube will be filled, what is the filler? This type of bending usually requires an internal mandrel. Parker recommends an internal mandrel when the wall thickness divided by OD is .07 or less, .014/.469=.03 in your case.

                I was under the impression that zero distortion bends like you've described are sometimes done by forcing a ball through the pre-bent tube while it's held in a precision jig. Or the bent pre-form could be hydro-formed as a final operation.


                • #9
                  Creating the rollers/dies is not a large problem. Rather than making a form tool I would use Guy Lautards and others method of turning a sphere. You could do the math using an Excel spreadsheet to develop your offsets ect.
                  I wold build a quick and dirty bender to see if you are going to get the shape you desire and the accuracy needed.
                  It would be a real trick to build a mandrell bender that small.
                  Have you checked for a CNC bender to outscource to? If they have the machine and the know how it might be cheaper.


                  • #10
                    Another option would be to rough out the radius and then grind with a mounted stone in a die grinder dressed to the proper radius. This would mean the axis of the grinding wheel would be tangential to the workpiece rather than paralel as in a typical tool post grinder.

                    I've seen a special bender that was used to bend tubing without kinking. It used a mandrel equal to the inside diameter of the tube. The mandrel was on the end of a long rod and sat at the tangent point of the wheel. Since the tubing wont kink after the radius is formed it only needs to be supported at the point where the curve is being formed. I've never built or used one, but I can tell you that it worked.



                    • #11
                      I've done a similar thing using a round tapered file. If you have one 1/2 in diameter, it will taper to less than 3/8 at the tip. Somewhere along the length, it will be just the right diameter. Rough out the half-circle to nearest smaller size, then file under power, rotating the file as you go, to allow filings to dislodge. Set up a jig so that the tail end of the file buts up against it, to prevent the file from opening the groove too wide. You can always wrap some sandpaper around a dowel, and use to clean up the groove, which will be left rough by the probably coarse file. I haven't seen any tapered file that is fine toothed, if there is one, I'd use that, either as the only tool, or as the second last tool, before final smoothing. Use whatever precaution is req'd to ensure that the file cannot accidently lodge in the chuck jaws. I've used this procedure many times, but not at that size, usually 1/4 in or smaller. Just another idea.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                      • #12
                        You can form the grooves in rollers in many ways, depending on how well you are equiped from single point in a cnc to roughing with manual lathe and filing in the lathe with a profile gage as a template. If you file it in, make it too deep in a groove of the correct width as a gage then turn it back to the diameter that leaves just the profile you need.

                        Consider a chain saw sharpening file to finish the groove if you file it, they are round and cheap.

                        As for the bending it can be done also. The requirement that it be perfectly round has to be qualified, there would be some deviation from round with the best methods.

                        You should look at Diarco benders, that is what they do, hand powered, with rollers. You can find them quite cheap used. You don't need to buy one , but looking will give you ideas of what works.

                        I have done miles of accurately bent tubing for medical devices with one of these units and shop made dies.
                        I have found that the best way to form the radius while maintaining the diameter of the tube is to make the stationary wheel groove less than the desired finished radius of the tube to compensate for the tube spring back. The tubing needs to be overbent. Instead of using a roller as a follower, I used a bar with the tube O.D. radius milled into it. A bar like this worked best for me.

                        The tube will have a straight end before the bend starts, it may need to be bent then trimmed to the desired length. The ends will need to be cut perpindicular to the radius also.

                        I tried filling the tubing with sand and plugging the ends, filling the tube with Cerro-bend, (a low melt point alloy) but this proved to be unnecessary once the dies were perfected.

                        Some of the things that effected the bend radius were the speed of bending, the pressure that was applied to the mandrels, Different heat lots of material always seem to bend differently and the mandrels needed to be tuned or remade.

                        I guess I wanted to point out that it can be done, sometimes the less you know about the problems the better off you are, sometimes not.

                        I would be happy to explain better how it can be done if you have any questions or do not understand my post.


                        • #13
                          reality checker wrote...."Some of the things that effected the bend radius were the speed of bending, the pressure that was applied to the mandrels, Different heat lots of material always seem to bend differently and the mandrels needed to be tuned or remade."

                          I guess I don't understand......when talking about bending "mandrels" we're usually referring to a device that fits inside the tube to prevent collapse. Is that the same device you're talking about? If so, what do you mean by applying pressure to the mandrel?


                          • #14
                            Sorry, I meant pressure between the rollers or one roller and the follower that I had sucess using. I have always used the term mandrel to descibe anything used to form the work on. Ken Bauman


                            • #15
                              How about using a template of the groove to guide a smaller round-nosed cutter into the steel mandrel? Make a follower arm with a finger ( attached to the cross slide) that rubs the template held behind or above the work? Kinda like the copy attachment on a wood lathe. Thanks--Mike.