Manipulate too slow for rigid transformation of 3D object

Question

I am trying to write a code that moves a cylinder inside another (a prismatic joint). Manipulate is very slow and I suspect that this is because the position of the movable part is being computed and rendered everytime. I know you can do this with Dynamic but I haven't really learn how to use this properly. Could someone give me some help? Here is the simple code I use:

cylJoints[R_, r_, {z1_, z2_}] := 
  Module[{b, cyl, d}, 
   b = BoundaryDiscretizeRegion[
     RegionDifference[Cylinder[{{0, 0, z1}, {0, 0, z2}}, R], 
      Cylinder[{{0, 0, z1}, {0, 0, z2}}, r]], 
     BaseStyle -> {Opacity[0.3]}];
   d = (z2 - z1)/3;
   cyl = Cylinder[{{0, 0, z1 - d}, {0, 0, z2 + d}}, r];
   Manipulate[
    Show[b, Graphics3D@Translate[cyl, {0, 0, t*d}]], {t, -1, 1}]];

cylJoints[6, 3, {-5, 5}]

Answer 1

Dynamic[t]*d is a quick fix but there is another problem.

By using outer Module variables in Manipulate you may end up with broken Manipulate if you intend to save it as a cdf etc. Module variables definitions will not survive kernel reset. You can rewrite your example a little:

cylJoints[R_, r_, {z1_, z2_}] := Manipulate[
   Module[{b, cyl, d}, 
    b = BoundaryDiscretizeRegion[
      RegionDifference[Cylinder[{{0, 0, z1}, {0, 0, z2}}, R], 
       Cylinder[{{0, 0, z1}, {0, 0, z2}}, r]], 
      BaseStyle -> {Opacity[0.3]}];

    d = (z2 - z1)/3;

    cyl = Cylinder[{{0, 0, z1 - d}, {0, 0, z2 + d}}, r];

    Show[b, Graphics3D@Translate[cyl, {0, 0, Dynamic[t]*d}]]
    ]
   , {t, -1, 1}];

cylJoints[6, 3, {-5, 5}]