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Thread: deep cut advice

  1. #1

    deep cut advice

    I'm very new at CNC routing. Lots to learn. I am trying to machine a full scale replica of a pre WWI propeller. See attached picture (a couple of support ideas shown). The thickness is a little over 5". Prop length around 8' and 14" wide. I'll prototype at probably 1/2 scale but am designing for full scale. Because of the blade pitch, I will need to do a cut depth of over 4" in the roughing pass. I'm thinking I'll need to buy some expensive 6" end mills and ball end cutters with a reduced shaft diameter for this project. Is there any other way to do this with shorter bits? If I go with long bits, what diameter would you recommend? Considering the length, I was thinking 3/8 or 1/2"
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  2. #2
    You could machine the parts in layers then stack laminate them and clean up the joints by hand.

  3. #3
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    You could slice the model and as noted by Kevin and build the assembly from layers. For longer tooling, the larger diameter will be more stable. I specifically bought a longer .375" end mill for certain things I do that require a nearly 2" deep cut as the .25" reduced shank bit I originally bought for the purpose "chatters" too much and can't be used when a final, full depth pass is desired.

    Does your machine have the Z-axis capacity to cut 5" thick material to full depth? If so, it also likely has the collet capacity to support larger tooling.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  4. #4
    Quote Originally Posted by Jim Becker View Post
    You could slice the model and as noted by Kevin and build the assembly from layers. For longer tooling, the larger diameter will be more stable. I specifically bought a longer .375" end mill for certain things I do that require a nearly 2" deep cut as the .25" reduced shank bit I originally bought for the purpose "chatters" too much and can't be used when a final, full depth pass is desired.

    Does your machine have the Z-axis capacity to cut 5" thick material to full depth? If so, it also likely has the collet capacity to support larger tooling.

    I'll explore slicing it. May get tricky due to shape.

    I have 8" z capability with a 12" z travel. It can handle up to 1/2" collet.

    To be able to machine the vertical plunge around the prop hub, I'll need a tool that has about a 6" overall length. It needs to have a huge cutter length or one that has a smaller LOC and a reduced shank. This will allow the cutter OD to be larger than the shank so the shank won't interfere with the wood as the tool goes deeper than the LOC. The only bit I've found that will do this is the Amana 46593 (1/2" End Mill) or 46591 (3/8" end mill). They also carry it in a 1/2" ball end. Lots of bucks but maybe OK if it allows me to take advantage of the machine's capability. Again - I'm new to the CNC world and there's a lot I don't know. Am I thinking about this correctly?

  5. #5
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    Tom, given your machine's capacities I'd absolutely try to use .5" tooling for this because of the lengths. In addition to the Amana you're exploring, there are some longer bits at more reasonable cost available via Amazon and that auction site. the .375" cutter I mentioned above came from Amazon and is branded "Cobra Carbide". They have a .5" available in 6" length, although it only has a 3" cutting length. A .5" reduced shank would be ideal for the cutouts, but...big bucks. I think you need to go to Vortex for that.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  6. #6
    Just be aware that vibration/deflection increase rapidly with the length of the bit. You will have to experiment with depth of cut and feed rates, Do as much stock removal as possible with shorter length bits.

  7. #7
    Thanks for the inputs. I'll look at all suggestions.

    I am concerned about vibration. I calculate the lateral stiffness of a 1/4" diameter bit with a unsupported length of 2" is almost the same as a 1/2" diameter bit with a 5" length. That means, for the same side load, these bits will deflect the same amount. The problem is that the lateral load is larger in the 1/2" bit since it is likely taking a larger cut of material. I notice the big Amana bits are 3 flute. Probably to reduce load. I don't think I'd want to chance it with less than 1/2" diameter.

    This will be fun.

  8. #8
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    For optimum chip load, you might be running at x speed for a given RPM, but you may compromise on chip load and slow feed down to reduce stress on the cutter when deflection is a factor like it might be here. Part of the solution here is to get rid of the extra material in an organized way with your rough 3D toolpath(s) so that your final finishing toolpath is manipulating the minimum amount of material possible. If you reduce your depth per pass for the rough cut, it will take longer but greatly reduce the load on the cutter and in turn, reduce deflection, even at full speed. It's a dance...
    Last edited by Jim Becker; 07-05-2020 at 9:31 AM.
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    The most expensive tool is the one you buy "cheaply" and often...

  9. #9
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    Your not going to have much trouble on your final pass. Ball end mill, as long as you want and as large a diameter as your machine will allow. Conservative depth of cut and stepover and you will be fine as long as your machine has the rigidity. The hogging passes will be the same, you will just have to adjust your depth of cut and stepover to what your machine will handle.

    Ive run 5" of tooling several times and if you need/want a nice finish its just a matter of a very light pass and shallow stepover. Who cares about a little bit of poor cut on the hogging pass.

    If you have the clearance on the part you can run shorter tools with a tool extension and you will have the extension to keep for future work.
    Sometimes I just want to look at pretty pictures,... Thats when I go to the Turners Forum

  10. #10
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    PS,
    Your second option, I would just ditch all the tabs but the two on the tips, and a center through bold in a fixture (T-nut or insert whatever you choose) and either band saw off all that waste material or if you have the time just waste it away in chips. If that thing is full scale you dont need 6 tabs and after your roughing toolpath any finishing toolpaths are going to be so light/tight stepover you wont need the extra tabs.
    Sometimes I just want to look at pretty pictures,... Thats when I go to the Turners Forum

  11. #11
    Are extensions pretty stable? Seems like more mass than a standard tool at a bigger diameter thus more chance for vibration. I ordered the longer bits from Amana but the extensions seem like a handy option. I'll look into it.
    Thanks for the input on the tabs.

  12. #12
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    Personally, I'd be uncomfortable with a bit extension on the CNC...but that's me.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  13. #13
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    You don't need a long bit to do what you want. Here is a training video that TJ from Shopbot did of a propeller

    http://www.talkshopbot.com/forum/sho...ining+training

  14. #14
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    Quote Originally Posted by Jim Becker View Post
    Personally, I'd be uncomfortable with a bit extension on the CNC...but that's me.

    Sure. Personal preference, machine rigidity, type of work, on and on. I run a 1/2" extension rarely and without a doubt cuts get very conservative but its of no issue if its a good quality extension.
    Sometimes I just want to look at pretty pictures,... Thats when I go to the Turners Forum

  15. #15
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    Here is a video of someone doing a fairly good sized prop on a CNC. https://www.youtube.com/watch?v=MsdZ_eYe9i4

    They leave out the hub steps. I would think about doing shallow cuts for the hub parts and then finishing the holes on another machine.

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