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  • Form tool grinding question

    I'm grinding a lathe form tool from a 3/8" stick of HSS and am having trouble getting my head around how to position it as I follow the profile shown. The tool will be plunged into a 9 mm hole in 6061 to form the ID of a venturi.

    So far I've kept it parallel to the wheel as I roughed it but now need to properly shape the tighter radius of the quarter elliptic profile. (The template was drawn in CAD and glued to the blank with CYA glue.)

    The table is clamped at 30 deg. The grind so far has been on the left side of the wheel which is spinning c/cw. I'm thinking I need to reverse the wheel rotation and approach it from the right side with the blank rotated about 45 deg c/cw, rotating the blank c/cw as I follow the profile to where it meets the edge, keeping the template line parallel to the face of the wheel as I rotate the blank. Does that make sense?

    Edit: Oops, the picture fool.

    Last edited by DICKEYBIRD; 08-15-2010, 11:01 AM.
    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  • #2
    The fool posted the picture finally

    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

    Comment


    • #3
      Can you raise the rest so that the small radius is presented to the wheel somewhat above wheel center height? This means the wheel will be undercutting the tool and providing the clearance you're looking for below the cutting edge. There are formulas for determining how much height above center is an approximation of a specific clearance angle but you may be able to eyeball it satisfactorily. You'll have a straight clearance surface transitioning to a concave one but that shouldn't be a problem in use.
      .
      "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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      • #4
        Never thought o'that! But no, it's a standard HF Baldor clone and it's rest only pivots, won't raise lower. I'm now trying the method I posted above and it looks pretty close. I'm sure I'll have to blend in the final shape with a Dremel tool. It should work...but the proof'll be in the chip. Shiny & slick hopefully; not dusty & smeared.
        Milton

        "Accuracy is the sum total of your compensating mistakes."

        "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

        Comment


        • #5
          Form tool

          The method of grinding the contour you propose will work fine, but you'll have to grind a lot off the bottom to get it into a 9mm hole. 3/8 is about 10mm and you need the cutting edge to be at half of 9mm. Also, you will need additional back clearance along the skinny part due to the curvature of the hole below the cutting edge. That's a 2 part operation. Get the contour right, then grind until you get enough back clearance behind the cutting edge.
          Last edited by Toolguy; 08-15-2010, 01:35 PM.
          Kansas City area

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          • #6
            You can clamp a wedge shaped riser to the grinder table to get the angles you need. I've used hardwood for this because the smoke smells better than yellow pine

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            • #7
              It works! I had drawn the thing in CAD to get the angle and dimensions right to make sure it would go in the hole and had enough relief to keep it from dragging. After getting the basic shape pretty close, I smoothed it up with a Dremel with the little sanding drum and polished it up by hand with a diamond lap. It peeled off nice shiny ribbons of swarf and left a great finish. Hoo-yah!





              Milton

              "Accuracy is the sum total of your compensating mistakes."

              "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

              Comment


              • #8
                Great Job!
                Kansas City area

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                • #9
                  Great job on the cutting tool. However, leave the gloves on the bench or in the tool box when using a grinder. Even the disposable gloves increase the chance of getting tangled up in the wheel and that's not a pretty sight.

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                  • #10
                    Thanks Doc, you are, of course, right. I forget sometimes.

                    Here's the finished product it came out real nice:



                    Goes here:



                    It's a custom venturi that goes on a .60 cu. in. high power Italian R/C model engine. The venturi goes in place of the R/C carburetor since the engine will be used in a control line model. The fellow that will fly it must have gorilla arms 'cuz this thing has some serious Italian (not Chinese) horsepower.
                    Milton

                    "Accuracy is the sum total of your compensating mistakes."

                    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                    Comment


                    • #11
                      Originally posted by DICKEYBIRD
                      Thanks Doc, you are, of course, right. I forget sometimes.
                      Very nice work. BTW, I had to take a student for medical attention due to his leaving gloves on when using the grinder. Pulled his thumb between the wheel and the inside of the wheel guard. No fun, especially for him.

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                      • #12
                        Great final result on the grind job, was going to suggest a dremel for finishing but you beat me to it. I concur with the glove danger. Don't even forget once, that is when you are digging your thumb from between the wheel and the rest. Repeat kudos on the finished tool. Peter
                        The difficult done right away. the impossible takes a little time.

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                        • #13
                          Originally posted by DICKEYBIRD
                          Goes here:

                          [IMG
                          http://i57.photobucket.com/albums/g227/DBAviation/NR60.jpg[/IMG]

                          It's a custom venturi that goes on a .60 cu. in. high power Italian R/C model engine. The venturi goes in place of the R/C carburetor since the engine will be used in a control line model. The fellow that will fly it must have gorilla arms 'cuz this thing has some serious Italian (not Chinese) horsepower.
                          DB,

                          Is that a Rossi? I used to muck around with the Rossi .40's which put out damn near as much power as most of the regular .60's. A Rossi .60 is definitely a brute to be reckoned with.

                          Mark

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                          • #14
                            nice work DB, I was a little concerned with the DOC of the tool and how unsupported it was but that came out very well
                            in Toronto Ontario - where are you?

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                            • #15
                              Great job. I make similar form tools from HSS and it's amazing how well they work even with very long unsupported shapes.
                              Free software for calculating bolt circles and similar: Click Here

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