9 Replies Latest reply on Jul 8, 2014 3:15 AM by Andrea Frezza

Adding machining allowance to a 3D model

Andrea Frezza Jul 6, 2014 1:27 PM

Hello,

I've a project about machine tools working of a Pelton bucket. I've only knowledge of traditional machining, not CNC. Thus I thought to add allowances in the lower part of it, where occurs the connection and where there are the holes. I thought also to add machinig allowances to the superior knife and elliptical shapes. I'd like to know if you would add machining allowances in the internal elliptical shapes and over the knife of the bucket and if, in you opinion, it's possible to machine them without CNC tools.

There is an image of the bucket attached.

Thank you all

Functional surfaces.PNG 24.0 K

Re: Adding machining allowance to a 3D model

Jamil Snead Jul 7, 2014 12:13 PM (in response to Andrea Frezza)

I do not believe the inside of the bucket or the entire knife could be machined without CNC. If the knife edge needs to be sharp you could add allowance only close to the edge and that portion could be machined with a tapered tool, although you wouldn't be able to go all the way over to where it meets the ovals.
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- Re: Adding machining allowance to a 3D model
  
  Andrea Frezza Jul 7, 2014 1:07 PM (in response to Jamil Snead)
  
  Estabilished the impossibility to use traditional tools, do you think it would be done manually? In this case, which suggestion can you give to me?
  
  Thank you very much
  
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  - Re: Adding machining allowance to a 3D model
    
    Jamil Snead Jul 7, 2014 1:57 PM (in response to Andrea Frezza)
    
    You can very easily find a tapered end mill to use, for example here is one from McMaster Carr with a 20 degree taper. Also the cutting path would just be a straight line, so it could be done manually. But whether or not you decide to mill it will really be an engineering decision about how important it is to have the edge sharp. The cutout at the end of your bucket is a very strange shape, is almost looks like it is designed for use with 2 smaller jets of water rather than one large jet that gets split. If this is the case and it will be used with two side-by-side jets of water then the blade sharpness will not be important at all.
    
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    - Re: Adding machining allowance to a 3D model
      
      Andrea Frezza Jul 7, 2014 3:21 PM (in response to Jamil Snead)
      
      Thank you for your suggestion which confirms most what we had thought; we take the opportunity of your kindness to ask you something else: in your opinion which parameters can be significant in handwork as what you might do on the bucket?
      
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      - Re: Adding machining allowance to a 3D model
        
        Jamil Snead Jul 7, 2014 3:34 PM (in response to Andrea Frezza)
        
        In full disclosure I don't have any experience with these devices, but from what I understand about how they work I can't image that any of the surfaces are particularly critical for the function. I think the only features that might be worth machining by hand are the inner surfaces of the tabs with the holes in them (because that may need to fit nicely around the wheel) and the holes themselves (because those will locate the bucket). It might be best to omit the holes completely in the casting rather than just reducing their size for machining allowance, because if there are sloppy holes in the casting then it could cause your finishing drill to wander. Also it leaves you the option to change the hole location later on.
        
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        Re: Adding machining allowance to a 3D model
        
        Andrea Frezza Jul 7, 2014 4:15 PM (in response to Jamil Snead)
        
        Also we had the same ideas about what you told us; you've been helpful to have confirmations.
        
        Thank you!
        
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Re: Adding machining allowance to a 3D model

Chase Draper Jul 7, 2014 3:57 PM (in response to Andrea Frezza)

That domed surface will be nearly impossible to do manually.
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Re: Adding machining allowance to a 3D model

Jerry Steiger Jul 7, 2014 8:54 PM (in response to Andrea Frezza)

Andrea,

I'm a really lousy machinist, so I couldn't machine your part if I spent the rest of my life trying, but I find it interesting that people are saying that it can't be done manually. Pelton wheels were very popular in the late 19th and early 20th century. All machining was manual then!

Jerry S.
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- Re: Adding machining allowance to a 3D model
  
  Andrea Frezza Jul 8, 2014 3:15 AM (in response to Jerry Steiger)
  
  Obviously working with the machine allows you to have better tolerances and surface roughness, but also we thought that you could accomplish any work manually; therefore, we focused on milling the inner surfaces of the tabs with not CNC machines, drilling them to realize the holes and machining manually the domed surfaces.
  
  Thank you all for the advice!!
  
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