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A How to calculate twist rate for a cycloid on a cylinder

  1. Jan 23, 2017 #1
    Hi all this is my first post.

    Okay I can have a rifle cut into a barrel at any twist rate I want, so I want to see if a cycloid can work for what I want. Now most twist rates are static so as an example one barrel may be 1 turn in 12 inches for 26 inches an example of what is called a gain twist or as I refer to it as a progressive twist rate the barrel might start or breech at 1 turn in 20 inches and finish at 1 turn in 8 inches at the the end or muzzle. Now I was trying tho think how would I calculate a cycloid along say a 30 inch barrel? So If I want one the barrel maker will want the parameters to input into the CNC lathe I just don't know how to calculate such a parameter.

    Also the barrel will be 7.62mm by 762mm long so 30 inch by .30 inch.

    I would be grateful for any assistance. The barrel design is for competition use I'm trying to see if a cycloid will be too aggressive on the bullet.

    Thanks.
    Shane.

    Edit I think I put this in the wrong section sorry.
     
    Last edited: Jan 23, 2017
  2. jcsd
  3. Jan 24, 2017 #2

    Nidum

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    Gold Member

    It's a helix not a cycloid .

    Think of your rifling groove as being the same as one start of a multi start screw thread but with a very long pitch relative to diameter and only one or a small number of turns .

    Cutting such a coarse pitch groove in a small bore and over a long length may not be possible on a standard CNC lathe even with special tooling .

    Generally this type of groove is cut by broaching . There are CNC broaching machines which could do this task quite easily .

    Modern CAM suites can generate the tool paths to cut a variable pitch groove on a suitable machine from just a few basic dimensions - there is no need to input any complex formulae .
     
    Last edited: Jan 24, 2017
  4. Jan 24, 2017 #3
    I understand what you are saying. What I was wanting to know say if you lay a progressive twist rate flat on a 2d surface it's going to make a curve. what I want to know is if you have a cycloid lying flat on a 2d surface how do you twist it into a helix and then get the start twis rate of the cylinder and the finish twist rate?
     
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