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How does one mill a 60" radius without a really huge rotary table?

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  • DennisCA
    replied
    I think there is some overlap between youtubers and people who keep on talking to the cashier in the supermarket line.

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  • AntonLargiader
    replied
    OMG can't that be fit into about 45 seconds?

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  • DennisCA
    replied
    Well, at any rate this looks like a clever set up, demo just using a wooden board but I can imagine using something like that, perhaps from steel or aluminum and using clamps to attach the part to be milled, then you can cut one side of the radius, move the table over along the X-axis a specified amount and cut the other radius:



    I guess the tricky part is fine tuning the radius. I need to rewatch it later with sound.

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  • Dan_the_Chemist
    replied
    Sorry about not getting back sooner... I couldn't answer some of the questions as I didn't know the tolerances. I had sort of assumed +/- 0.005 on the arcs since he had specified them to .0x So I sent an email asking for clarification.

    First of all, this thread has become totally theoretical. They found somebody with a CNC machine to do the job.

    Secondly, the part mounts on the bottom of a long swinging arm on the scope and is somewhat weight bearing. It rests against a series of bearings, and they wanted it to be +/- 0.003 (or better).

    I'd like to thank everybody for their input. I learned a bunch, and got some good ideas. My approach was going to be to make a jig that would have a series of 9 drilled and reamed holes using the DRO. Drilling and reaming 9 holes with +/- 0.001" accuracy using a DRO didn't seem terribly daunting (although I know that's near the limit of my ability). The jig would be registered onto three dowel pins on the table portion of the jig. The idea would be to move the jig carrying the steel along in small increments, and cut 2" long straight facets along my mill's X axis on the outer face (62" radius). The corner of a 2" facet will be 0.016" high compared to the desired circle. See the screen snip of a Fusion 360 design plan... blue = aluminum carrier plate on the jig, yellow = SS, and the dashed square is the "cutting line"...

    I hadn't decided if cutting down the 0.016" points to the desired line would be sander and file work, or if I'd move the jig so as to trim the tops off of those high points. That would give me a lot of little facets that would be 0.006" off of the desired circle. I was going to go file a bit of 1/2" SS and see how much bother it was going to be...

    However... it turned out that CNC won... (not that I lost anything ... it was one of those "six pack of beer" jobs for people I know).
    Attached Files

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  • kendall
    replied
    Originally posted by CalM View Post

    How would you produce the wooden pattern =/-.002?

    Same problem as making the part. Just producing the layout lines is half the battle.
    Use a router with an arm to make a slightly longer form. Then rough cut the SS as close to shape as possible, bolt 3 bearings to the table and take light cuts on the mill while keeping the form against the bearings. 2 bearings to ride on the outside curve of the form, 1 to ride the inside. MDF may be sturdy enough, but HDF would be stronger. Either should be sealed.

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  • DennisCA
    replied
    Unfortunately the OP has not responded to the question of tolerances.

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  • JRouche
    replied
    Originally posted by 754 View Post
    it could easily be done by swinging a router or circular saw , on a pivot bracket, even made of wood.. without a milling machine even.
    Glad you said it. Not sure but I think it was more of a class on lets see all the ways to do something.

    Its a very simple router job. But that doesnt work in the Machinist mind LOL

    I have 1/2" solid carbide bits that would do it. Slow going? Sure. Turtle gets it though... JR

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  • Paul Alciatore
    replied
    While this method is fairly well known and works just fine for generating a SPHERE or a part thereof, it does not generate a cylindrical shape and can not be easily modified to do so. The part in the OP was described as being 1/2" thick and the same radius will be generated across that direction as along it's 15" length where it is actually wanted. This may or may not be important for the part in question, only the designer can tell us for sure.

    According to my calculation of the sagitta of a 1/2" long segment of a 60" radius, the 1/2" dimension will be about 0.0005" deeper at it's center on the concave side and about the same amount taller at it's center on the convex one. This may or may not be OK.

    As I said before, TOLERANCES are required to actually determine a method for making this part. And a proper drawing.



    Originally posted by Denny98501 View Post
    Long radii are actually a lot easier and a whole lot faster if you don't have CNC.
    Analog is the way to go for a wonderfully smooth curve instead of a thousand tiny little steps.
    You need a manual milling machine, a fly cutter and small rotary turntable.
    Below is a photo of a 200 inch radius of curvature polishing tool that we made.
    Below that is a reference showing how mirror makers produce long radius curves.
    After 15 minutes of setup, it took about 10 minutes to rotate the turntable and produce the curve.
    The turntable is tilted the amount of the Sag or calculated as shown below.
    If tilted the other direction, a concave curve will be made.
    If you only have a small fly cutter, the curve can be generated in two passes, but the angle will have to be recalculated.
    Hope this helps,


    Click image for larger version

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    Click image for larger version

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  • 754
    replied
    The less info you give out, the more it may cost
    I notice on this one.. still no tolerance or end use stated. .. wtf.. they expect us to figure it all out ?
    help us ..to help you. .

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  • Bob La Londe
    replied
    Originally posted by Dan Dubeau View Post
    I get roped into a lot of projects like this, and sometimes it's frustrating trying to get info out of people as to the end use. They don't think it's important, but it's often the most important detail that dictates whether the item will actually work or not. Just tell me what it's for and I can come up with the required clearances, and tolerances that will produce a workable part.
    Just tell them, "I'm sure you can see the pretty picture in your head, but I can't see the pretty picture in your head." I get people all the time who want custom work done; but refuse to take the time to give me any details. Sometimes not even good working generalities. Grudging one incomplete sentence phrases typed with their thumbs on their cell phone seem to be the norm for some folks. I'm not exaggerating. I have to deal with it atleast a couple times every week.

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  • Dan Dubeau
    replied
    Final use dictates process needed. Are the radius' riding on something (i.e a trunion for a scope?) or are they simply a bracket for something and just needs to look nice and uniform? If the former, a long pivot could be setup with some tubing, or plywood etc to make an accuarate jig. If the latter, print it out on paper and transfer to material. Cut out with a bandsaw/jigsaw/plasma, and sand/file to shape. You could alternatively figure out a bunch of "plunge points" and just step it off then file to fit. Goes quicker than expected, although I haven't had to do that/generate points in many years.

    You don't need a full fledged engineering print, just the end use that will help drive the decision making process.

    I get roped into a lot of projects like this, and sometimes it's frustrating trying to get info out of people as to the end use. They don't think it's important, but it's often the most important detail that dictates whether the item will actually work or not. Just tell me what it's for and I can come up with the required clearances, and tolerances that will produce a workable part.

    Leave a comment:


  • RB211
    replied
    Draw the arc in CAD, draw a line at either 0, 90, 180, or 270. Now draw a line at both the start and ending circle centers. Get your angular start and stops and go program the Smooth Arc. OR, in the CAD package, record all of your X and Y movements every .050" and do it the really old fashioned way.

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  • Bob La Londe
    replied
    Originally posted by CalM View Post
    I thought this was a hobby machinist forum. You know, doing stuff for the pleasure in the doing.
    I never got that impression. It says home shop machinist, not hobby machinist.

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  • 754
    replied
    Ooh jeepers, missed the SS part., I apologize....was concentrating more on the technical work holding , tool guiding aspect..
    to clear up. Then use of compass is for layout, actual swinging fixture would be more like a motorcycle swingarm, much more rigid.

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  • CalM
    replied
    I thought this was a hobby machinist forum. You know, doing stuff for the pleasure in the doing.

    I don't think a 1/2 inch depth of cut into SS is going to be router work pivoting off a 5 foot compass..

    Abrasive machining maybe... swap the router for a portable belt sander.. Take it easy, It will be a while till it's done.

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