Announcement

Collapse
No announcement yet.

Manual milling offset calculations

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Manual milling offset calculations

    OK, here we go. Ready?

    I have Machinery's Handbook and it shows briefly how to calculate offsets when milling. I m looking specifically for offsets when milling at an angle, say 40, 50 even 5 degrees.

    Looking online, there are very few explanations as to how it is done. I have made my own calculations using my own trig formula, but I come up with different answers than the Machinery's Handbook formula gives me. I assume I'm wrong and it is right, but for the life of me, I can't figure out how my calculations are wrong.

    Simply, using the radius of the cutter, right angle triangles drawn inside the mill extending to the tangent contact of the workpiece, at the correct angle, I get to MY offset in 2 steps. The Handbook formula does it in one, but is a different result.



    Thanks!
    Last edited by Tim-Bob; 03-07-2019, 12:55 PM.

  • #2
    I'm not sure I quite understand what you're looking for, but I'm good at trig and have a 22nd edition in front of me. I can't locate milling offsets in the index. What page are you on or section header.

    Comment


    • #3
      I'm pretty good at trig etc but don't understand what "Offset" youre talking about. I do use a mill so describe the set -up first.
      ...lew...

      Comment


      • #4
        Are you talking about stepping off an angle with a ball endmill?

        Comment


        • #5
          Is this a CNC issue or a manual machine ?
          What are you doing ? give an example please with real numbers.

          Rich

          Comment


          • #6
            Originally posted by Tim-Bob View Post

            I draw all this crap by hand, so a pretty picture explanation is out. Does anybody have a link with a good explanation?

            Thanks!
            Even just a picture of a hand drawn example would help. don't worry about making it nice.

            Comment


            • #7
              Originally posted by JCByrd24 View Post
              I'm not sure I quite understand what you're looking for, but I'm good at trig and have a 22nd edition in front of me. I can't locate milling offsets in the index. What page are you on or section header.
              I'm in the CNC milling offsets section, subsection Cutter Radius Offset(G41, G42). A little drawing of a rectangle with the corner cut off at an angle. Figure 13. Manually Calculated Radius Offset.

              I want to mill insert pockets without "sneaking up" on the right dimensions. Since the pockets are at different angles meeting up at one point, a simple radius offset doesn't work. Or, am I overthinking this?

              The mill is CNC, but I am using it "manually", that is give it commands where to go. I've done some coding, very simple, but I want to be able to put in offsets manually. Straight cuts are not an issue, but angles are.

              How do I attach a picture?





              Oooooh!! Like that!!!!
              Last edited by Tim-Bob; 03-07-2019, 12:55 PM. Reason: Brain farts.

              Comment


              • #8
                I think you are way overthinking this. I used G41, G42 for years programming manually and its value is that you can see your code is correct with no offset applied. You will find it difficult if you program in absolute mode though, works much better in incremental. The CNC will handle the issue you are dealing with without your help unless the tool won't fit and then you get crop circles that gouge your part! Here is how G41, G42 works that is bulletproof. Add a tiny extra move line to the beginning of your tool path, 0.010" is plenty. With the tool at the safe plane move to the end of that line and call G41, G42, then move to the beginning of the part line again at safe height. You will see the tool move at right angle to the tool path as compensation gets applied. Now lower the Z into the work and start cutting. Always cancel compensation immediately after retracting the Z and before doing another move or cut as you may get a nasty surprise where the tool goes.

                I don't program by hand anymore, life is too short and that is a job for computers. I use CamBam and get the job done in seconds with much better results, even on very complex cuts. Thousands of lines of code with a couple of clicks, you gotta love it!

                Comment


                • #9
                  The Op said he was doing manual moves (MDI ?)
                  You want the sine of the angle and then subtract the radius
                  In your sketch, lets say that the angle is 60 degrees (Where you have the red line.)
                  So sine of 60 degrees is .866 .
                  You need to invert that which means dividing into 1.00 which becomes 1.154 .( This is your Hypotenuse )

                  Multiply your Radius by this number..( say it is a 1/4" endmill) so .125 x 1.154= .14434.
                  Now subtract the Radius from your answer and you get .14434-.125= .01934.
                  That is the red line
                  So you have 1.0 divided by the Sine of the angle, times the radius , then subtract the radius

                  Rich

                  https://www.rapidtables.com/calc/mat...alculator.html

                  Comment


                  • #10
                    Originally posted by Tim-Bob View Post
                    I'm in the CNC milling offsets section, subsection Cutter Radius Offset(G41, G42). A little drawing of a rectangle with the corner cut off at an angle. Figure 13. Manually Calculated Radius Offset.

                    I want to mill insert pockets without "sneaking up" on the right dimensions. Since the pockets are at different angles meeting up at one point, a simple radius offset doesn't work. Or, am I overthinking this?

                    The mill is CNC, but I am using it "manually", that is give it commands where to go. I've done some coding, very simple, but I want to be able to put in offsets manually. Straight cuts are not an issue, but angles are.

                    How do I attach a picture?





                    Oooooh!! Like that!!!!
                    Please be kind since I am doing this in my head with no paper (but a calculator).

                    The unknown "x" you are looking for is (R/cos A) - R =X

                    or with numbers (1/cos 22.5) - 1 = 0.082

                    I'm sure there are alternate ways to trig it.
                    Bill Pendergrass
                    Rotec RM-1 w/Rusnok head
                    Atlas TH42 QC10

                    Comment


                    • #11
                      Originally posted by rzbill View Post
                      Please be kind since I am doing this in my head with no paper (but a calculator).

                      The unknown "x" you are looking for is (R/cos A) - R =X

                      or with numbers (1/cos 22.5) - 1 = 0.082

                      I'm sure there are alternate ways to trig it.
                      So what you calculated is the effective radius? The hypoteneuse of the triangle created by extending the radius at 22.5 degrees, adjacent being R?

                      I had done that already, but it seemed too simple! I overthink things, and have forgotten some basic trig/geometry.

                      Eventually, I will use G41 and G42, but I like to understand at a basic level.

                      Any more input is appreciated! Makes me wish I'd payed closer attention in school, but figuring it out is good mental exercise.

                      Tim

                      Comment


                      • #12
                        Don't feel bad. I did badly in trigonometry in college. Engineering even. It was just a bunch of stuff that made no sense. Just memorize that the THIS is defined as the THAT divided by the SOMETHING-ELSE. It's also equal to 1 divided by the WHATEVER. It was just a slog and nothing stuck. Many years later I wind up in tool design working on molds. They've got draft all over them and you're always figuring out what an insert dimension should be. Realizing that it was just the relationships of right triangles finally made complete sense and it's almost fun sorting through the geometry.
                        .
                        "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

                        Comment


                        • #13
                          My take on this trig problem is: Do you want to learn or just like to bitch?? If youre interested in learning I'll be happy to teach but I need to know the problem. A simple sketch is a good starting point. There are several places to store a pic that can be linked here easily. The ball in in your court now.
                          ...lew...

                          Comment


                          • #14
                            Without a picture I'm not even going to try to guess at what you're doing in the totality of the situation. But I'd point you to the You Tube channel of Joe Pieczynski. He's put out a few videos related to drawing out and analyzing various geometric problems. And the TLDR version is triangles.... lots and lots of right angle triangles. But they all link up and each right angle triangle is another rung on the ladder you need to climb up out of the hole you're in.

                            Here is the link to his channel

                            And here is a couple of links to videos where he triangles his way to happiness and fulfilment. But I'm sure there's another one or two. It's a pet topic of his and worth digging through all the videos to find the Meaning of Life for happiness in the shop... or at least to untangle offsets....

                            https://www.youtube.com/watch?v=wS1043KWm-M
                            https://www.youtube.com/watch?v=cUL9ihCxlCQ&t=1250s

                            He does this geometry stuff in a few more but they aren't leaping out at me at the moment. But these two should help you to help yourself.

                            Comment


                            • #15
                              Originally posted by Tim-Bob View Post
                              So what you calculated is the effective radius? The hypoteneuse of the triangle created by extending the radius at 22.5 degrees, adjacent being R?

                              I had done that already, but it seemed too simple! I overthink things, and have forgotten some basic trig/geometry.

                              Eventually, I will use G41 and G42, but I like to understand at a basic level.

                              Any more input is appreciated! Makes me wish I'd payed closer attention in school, but figuring it out is good mental exercise.

                              Tim
                              First off, you did not ask for the effective radius. You asked for the extension bit. Second it is easy. From the formula I showed it is simply R+X or 1.082 using the 1" radius and 22.5 degree example. The alternative calc is simply R/cos A = eff radius or 1/cos 22.5= 1.082

                              Please note that Rich C's post came in while I was typing and I did not see it. Both formulas are of similar form R/sin - R = x but differ by cosine vs sine. This is simply because the two of us are using angles from different corners of the trig triangle. I used the angle placement from your example (at the center of the cutter). Rich used the angle at intersection of part and effective radius which is 90 minus 22.5 using your example dims.. The math results are the same, given the appropriate numeric inputs.

                              Using your example dimensions and Riches formula.... 1/sin 67.5 = 1.082
                              Bill Pendergrass
                              Rotec RM-1 w/Rusnok head
                              Atlas TH42 QC10

                              Comment

                              Working...
                              X