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  • Machining a large radius...

    Hi all,

    I am seeking advice on a small problem, here's the scenario...

    I have a Bridgeport 9x48 varispeed, and a Colchester Student lathe.
    I need to machine a 6" radius into the bottom of a traction engine cylinder block so it sits on the boiler. Length of block is about 7". I have a 2" R8 boring head.

    Whats the best way of doing the job on the given equipment? I dont mind making new tools or attachments etc or getting hold of the right stuff if needed. I know the length is beyond the quill travel but i could lift the knee i guess.

    Material is good quality cast iron.

    Anyone got any ideas?

    As usual, all ideas are much appreciated.

    Dave
    If it does'nt fit, hit it.
    https://ddmetalproducts.co.uk
    http://www.davekearley.co.uk

  • #2
    Do you have a band saw?

    but i could lift the knee i guess.
    Or go at it from both sides.

    Comment


    • #3
      so the boiler is 12 inch diameter and the cylinder sits on it horizontally.

      i would take as much off with an endmill..with it horizontal on the milling machine
      then make a big fly cutter about five inches in diameter, to take the rest off with it vertical on the milling machine..will take many many passes.

      all the best.markj

      Comment


      • #4
        One way to do it would be to figure out incremental depth steps and get it close that way, then finish with files.

        That is, clamp the block down flat on the table. Locate the centerline. Put say, a 1/4" dia. end mill in the spindle. Figure out what the depth needs to be for a given Y offset so the corner of the end mill *just* goes deep enough to touch the desired final radius. This will be symmetric across the centerline. You'll end up with a series of small steps, but depending on your patience and how small your Y increments are the ridges that get left can be pretty small.
        ----------
        Try to make a living, not a killing. -- Utah Phillips
        Don't believe everything you know. -- Bumper sticker
        Everybody is ignorant, only on different subjects. -- Will Rogers
        There are lots of people who mistake their imagination for their memory. - Josh Billings
        Law of Logical Argument - Anything is possible if you don't know what you are talking about.
        Don't own anything you have to feed or paint. - Hood River Blackie

        Comment


        • #5
          Why machine a full-contact surface? Not likely to be a perfect cylinder anyway. Make 3 ribs that contact the boiler, center and two edges. Use CAD to figure the depth, relieve the non-contact areas stepwise. Use the largest radius ball mill you can find. If you want the ribs to contact better, simply stick some SiC sandpaper onto the boiler with tape and rub it back & forth a bit. If it were a heavy item, I'd do otherwise but this is the easy way.

          Comment


          • #6
            Dave,
            Not got time to work out the maths, hopefully someone else will chime in but it can be done easily with a flycutter set in the boring head and the head of the Bridgy tilted.

            Think about this, with the head vertical the tool describes a radius of infinity on the work, i.e. flat. in the horizontal position the radius is equal to the tool radius.

            From horizontal to vertical the radius will go from R to infinity so it's
            possible to cut a radius larger than the tool radii

            A couple of pictures that may show the principle is the following one of radiusing the back of a rack so it can lie on a tubular column.









            [edit] from reading some of the other posts, this radius needs to be a good fit on the boiler as it's a steam fit face .
            Last edited by John Stevenson; 01-27-2009, 04:48 PM.
            .

            Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



            Comment


            • #7
              Originally posted by Just Bob Again
              Why machine a full-contact surface? Not likely to be a perfect cylinder anyway. Make 3 ribs that contact the boiler, center and two edges. Use CAD to figure the depth, relieve the non-contact areas stepwise. Use the largest radius ball mill you can find. If you want the ribs to contact better, simply stick some SiC sandpaper onto the boiler with tape and rub it back & forth a bit. If it were a heavy item, I'd do otherwise but this is the easy way.
              I see where your'e coming from but it needs to be a steam tight joint to 180psi. There is a gasket but only a thin one.

              I think the large flycutter is the best option and was top of my 'possibles' list. Something like a bar welded to an R8 arbour with a tipped tool in it?

              The curve is already cast in roughly, i just need to remove about 3mm of allowance to give a good fit on the boiler.

              Dave
              If it does'nt fit, hit it.
              https://ddmetalproducts.co.uk
              http://www.davekearley.co.uk

              Comment


              • #8
                Originally posted by John Stevenson
                Dave,
                Not got time to work out the maths, hopefully someone else will chime in but it can be done easily with a flycutter set in the boring head and the head of the Bridgy tilted.

                Think about this, with the head vertical the tool describes a radius of infinity on the work, i.e. flat. in the horizontal position the radius is equal to the tool radius.

                From horizontal to vertical the radius will go from R to infinity so it's
                possible to cut a radius larger than the tool radii

                A couple of pictures that may show the principle is the following one of radiusing the back of a rack so it can lie on a tubular column.









                [edit] from reading some of the other posts, this radius needs to be a good fit on the boiler as it's a steam fit face .
                I like the theory of that, where would i look for the angles and tool radius etc, i've never seen that in a book or anywhere else come to think of it?

                Thanks
                Dave
                If it does'nt fit, hit it.
                https://ddmetalproducts.co.uk
                http://www.davekearley.co.uk

                Comment


                • #9
                  Keep in mind thaqt the technique John suggests won't make a true circular cut but instead an elliptical cut. The side projection of a tilted circle is an ellipse so for a particular desired depth of cut the edges will be too high/low. The amount of error is dependent on the ratio of the fly cutter size to the desired radius and the depth of cut. The closer the fly cutter is to the actual diameter of cut and the shallower the cut the less error in the shape.
                  Last edited by Evan; 01-27-2009, 05:00 PM.
                  Free software for calculating bolt circles and similar: Click Here

                  Comment


                  • #10
                    Originally posted by Evan
                    Keep in mind thaqt the technique John suggests won't make a true circular cut but instead an elliptical cut. The side projection of a tilted circle is an ellipse so for a particular desired depth of cut the edges will be too high/low. The amount of error is dependent on the ratio of the fly cutter size to the desired radius and the depth of cut. The closer the fly cutter is to the actual diameter of cut and the shallower the cut the less error in the shape.
                    hmmm dunno

                    just imagine you slice off a bar at 45 degrees ....that slice will fit inside a tube and block it at 45 degrees ...like throttle butterfly.

                    so johns flycutter is following path of circle ..

                    then again i could be wrong ..its very hard to imagine without actually doing it .

                    all the best.markj

                    Comment


                    • #11
                      To see the shape produced draw a circle of the same diameter of the fly cutter or boring head and then scale it in the vertical axis so the minor diameter is the same as two times the depth of cut.

                      Here are examples of the shape made as you move from horizontal to vertical.

                      Last edited by Evan; 01-27-2009, 05:18 PM.
                      Free software for calculating bolt circles and similar: Click Here

                      Comment


                      • #12
                        Dave,

                        Bridgeport boring heads like mine have a horizontal hole for a boring bar, as well as the vertical holes, in other words you can fit tooling to bore any radius. Great because you can adjust the radius in the normal way. Maybe your one doesn't...

                        I have used a large capacity fly cutter on a BP - it was a piece of flat bar, say 50mm x 15-20mm flat bar, about 300mm long, drilled in the middle to attach to an R8 arbor, and made to take a piece of HHS toolsteel on the end. (Intended for very light clean-up work on aluminum plates etc). Downside is no radius adjustment, so how about making something to fit your boring head so you get the benefits of adjustment? Could be double-ended for balance as well?

                        Comment


                        • #13
                          If I read the specs right, that lathe of yours will handle 13" or so diameter... so can you make a suitable boring bar to run between centers, and bolt the cylinder to a bracket of some sort bolted to the top slide (compound removed...)? This would be much stronger than than the BP, I think.

                          - Bart
                          Bart Smaalders
                          http://smaalders.net/barts

                          Comment


                          • #14
                            Hi, could you hold the block on a face plate and machine the radius in like this,

                            http://i156.photobucket.com/albums/t...generation.jpg

                            http://i156.photobucket.com/albums/t...eration001.jpg

                            http://i156.photobucket.com/albums/t...eration002.jpg


                            Lee.
                            I no longer own tools I had.

                            Comment


                            • #15
                              Wow! Thanks all,
                              theres some food for thought there.

                              I'll look into a custom flycutter, with carbide tips i think. I actually need two radius jobs on this, one at 6" and one at 6.5" so ab bar with a small amount of adjustment will do both.

                              The boring head does have the cross-hole but its only a 2" head so that will mean a 5" tool, sounds a bit springy to me.

                              I'll get the thinking cap on.

                              Dave
                              If it does'nt fit, hit it.
                              https://ddmetalproducts.co.uk
                              http://www.davekearley.co.uk

                              Comment

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