Profile and Path

[laverne3ca]

Is there anything or any way to produce the results like in this Solidworks link in Geomagic? http://help.solidworks.com/2010/english ... 22c440#Pg0
I need the CAD for an almost identical part to fourth example in the link, a cylinder offset from the axis of another and perpendicular to that axis and revolved around that axis with a variable pitch.

 

[JST]

I think not, at least not directly.

You would need to create the varying cut profile. The cutter when laid across, cuts a different profile than its cross-section. and the profile that is cut will vary with the variation of pitch.*

I do not know what that variable profile is useful for (maybe the pellet screw for a molding machine?), but I believe you can only cut a fixed profile at present. And the profile is 2D not 3D as with that.

With a radial cutter, you can make a cut like that, although the cut will vary in profile with pitch which does not happen using a round cutter. With the off-radial cutter, I don't think that facility is present at all in GMD.

Not having it is OK so long as the pitch is not varied, but you CAN vary the pitch in GMD. That will change things. I see the issue.


*
At a pitch of zero advance per turn, the cutter (as an end mill) cuts a profile that is round. At infinite pitch the cutter cuts a flat along the axial direction, with intermediate profile at pitches between those. There is no model for that behavior in GMD that I am aware of.

Likewise, a round cutter oriented radially cuts its own profile at any pitch, both zero and infinite. But a 2D profile cuts correctly at zero pitch, and cuts nothing at infinite pitch, varying in between.

So you CAN do this, but it is not quite what you mean



Starting with this

 

[laverne3ca]

Yes, that is the problem. This screw feeds a bottle (standing upright) through a labeler, when the forward motion is stopped, the profile is circular, when it's moving quickly, the profile is nearly flat, sort of a twisted almost flat surface.

 

[Oldbelt]

 

[JST]

Unfortunately, that is no different from my example.

Neither of them is like the solidworks example, nor is either of them what the original poster wants.

Cutting with a 2 dimensional outline is fundamentally different from cutting with a 3D outline. Cutting with a cylinder, like an end mill makes a cut that is the same no matter what pitch the helix is. But cutting with a flat profile varies the shape depending on the pitch because the projected extents of the flat profile are zero looking along the plane of the profile, and only full size when looking at the profile at 90 deg to the plane. In between they are at some dimension depending on the sin of the angle to the plane.

The poster wants the cut that is made if an end mill is laid across the cylinder, and cuts while the cylinder is rotated and advanced. That always has a circular profile cut visible when viewed wt 90 deg to the axis. The flat profile cuts only in the plane of the axis and when viewed does NOT show a circular profile.

The ACTUAL cut in the cylinder will vary with the pitch. Circular if no advance (zero pitch) and a flat cut if infinite pitch.

I do not believe there is a good way to do this in GMD.

 

[albie0803]

While far from exact , is this the sort of thing you want? I used overlapping helical cuts with increasing variable pitch. I had to give each cut a minuscule diameter variance or only the first cut would work.

 

[Oldbelt]

Jst you are right, I havn't understud the problem.
Albie0803 have the same ide as I was thinking at, its only one flank in the grove who transport the bottle, so why not make the guide pin smaller in dia.
But if the true grove is wanted then see this solution ;
A helical cut is made in the cylinder surface, (a V- grove), one edge of this grove is projected to a 3Dsketch.
A plane is created at the end of the 3d spiral sketch and used for a sweep cut with a cirkel sketch.
Sweeps are normal to the path.
I can't stand when poeble want to use other SW than the god old Alibre, there is alwaes a way.

 

[JST]

Even that does not make the cut that is wanted.

Look at the picture.



The cut that is wanted will have the profile of a half circle all along the length when viewed from this angle (normal to axis). But as you can see, the cut in the picture becomes a different shape as the helix changes. It is the best with close-spaced turns, and as the turns become wider spaced (and helix slope changes) the shape is no longer a half circle.

There seems to be no direct way to cut the shape that is wanted. I think it could be done with a loft cut or loft and boolean, but one would have to come up with a formula for the change of shape with the change of helix slope.

Of course, with Solidworks, the developers seem to have made a command that is intended to work just like a machining operation. So it does the cut directly.

Here is their picture



You see that with the cutter oriented that way, it always cuts the shape of a half circle when viewed normal to the part axis, but the true shape of the cut changes with helix angle. This directly emulates the action of a rotary cutter such as an end mill.

Because there is no cut method in GMD that uses a 3 dimensional cutting device, it cannot be done directly in GMD.

 

[DavidJ]

If you really have to achieve this, there is a brute force method - but by no means perfect. The image shows the effect of offsetting the profile sketch for each of multiple cuts by a small amount. The axis is common for all cuts. Effectively you simulate a cylindrical tool, by using stacked circles.

I've only stacked the circles above the axis of the screw - needs to be repeated below. I don't really feel this is a sensible method, but it can be done.

A proper option to use a solid as a cutting tool would be great - for example when modelling fluted drills, milling cutters etc. - where the form initially looks simple, but is actually formed by moving a profiled grinding wheel along a helical path.

 

[JST]

I also tried using an elliptical cut tool, set at an angle. That did not really work as expected, for reasons I am not yet sure of.

It presumably has to do with how GMD uses the form to make the cut. The true way is to use the form in its 3D space actuality, so the cut occurs in 3D space. GMD may project it onto a plane, and do all the cutting that way.

 

[Dave H]

The problem is that the cut profile follows the helical path around the core.
What the OP really wants is to keep the cutter profile (whatever shape/size it needs to be) on a set plane, keep the cutter shape parallel with the axis of the core, and move it along the axis of the core at a varying pitch as the core rotates. My 2 cents... :?:

What about making a varying pitch helical solid of an oblong or oval cross sectional shape, and then doing a boolean subtract from the core? Just another fleeting thought. (Yes, I am bored at work today)

 

[JST]

I think you mean "perpendicular to". but both descriptions fall short of what he wants.

He really wants exactly what the solidworks example shows..... A particular unchanging 3D object used as a cutter for a varying pitch (and slope) helix.

Unfortunately, that is not reproducible by ANY unchanging 2D object. A loft could do it, loft cut or loft and boolean, but no single helical cut can reproduce it. A series of helical cuts can approximate it, but GMD offers no direct and elegant method such as what SWX has.

 

[Dave H]

I think you mean "perpendicular to". but both descriptions fall short of what he wants.

He really wants exactly what the solidworks example shows..... A particular unchanging 3D object used as a cutter for a varying pitch (and slope) helix.

Unfortunately, that is not reproducible by ANY unchanging 2D object. A loft could do it, loft cut or loft and boolean, but no single helical cut can reproduce it. A series of helical cuts can approximate it, but GMD offers no direct and elegant method such as what SWX has.

I think the cutter shape would stay parallel to the core axis. The cutter axis would be perpendicular to the core axis.

In either case, I am thinking you are right.

 

[idslk]

Hello all,

this is only to show what was already described by JST.
I've used a "10mm milling tool" and cut only every 5 degrees (the pitch is constant). Only to show the geometry result...
So far i have no good idea to do this automaticly and seamless.




Regards
Stefan

 

[Oldbelt]

The profile is an exact cirkel, as cut in a 4-axis millingmachine with a ball nosecutter,
a end mill cutter can be used for a Square grove.
For transport of the item a guide pin like the ballnose cutter can be used.
But we don't know the design, so its dificult to give advice.

 

[JST]

Ah... I understand the confusion.

The original poster wants the open profile to be a circle completely across the cylindrical part. So that a rod could be laid in place and fit, with the rod perpendicular to the part axis. It is not only the profile in a section, but the complete open profile

In the picture, I have circled some of the areas which are a problem.



Also look at the part that idslk shows. That part you can see has a profile that appears to be a half circle when viewed from the side, but the actual material removed is a quite different shape. That different shape will change with the angle of the helix.

 

[Oldbelt]

JST, yes I am aware of this, I only succest that the design is changed.
With a guide pin placed as the ball nose cutter, the item can still be moved,
but that is maybe an impossible redesign.!

 

[ajayre]

I don't know if this is right, but I had fun creating it anyway.





The key parameters are at the start of the script. Decrease StepAngle for more detail. The relationship between the angle and the distance along the cylinder is given by:

XStep += (Angle * 0.001)

Here is the script:

# cylinder dimensions
Diameter = 20
Length = 100

# cutter dimensions
CutterDiameter = 5

# angle to increase by on each pass of the cutter, in degrees
# must be a whole divisor of 180
StepAngle = 10

# total angle of cutting around the cylinder
TotalAngle = 1440

# starting distance from end of cylinder
StartX = 10

# create the cylinder
P = Part("Cylinder")
CylPlane = P.GetPlane("XY-Plane")
CrossSection = P.AddSketch("Cross-Section", CylPlane)
CrossSection.AddCircle(0,0, Diameter, False)
P.AddExtrudeBoss("Cylinder", CrossSection, Length, False)

# create the planes
Planes = []
NumPlanes = 180 / StepAngle
for PlaneIndex in range(0, NumPlanes):
  Angle = PlaneIndex * StepAngle
  Pl = P.AddPlane("P" + str(Angle), P.GetPlane("YZ-Plane"), P.GetAxis("Z-Axis"), Angle)
  Planes.append(Pl)
for PlaneIndex in range(0, NumPlanes):
  Planes.append(Planes[PlaneIndex])
NumPlanes = NumPlanes * 2

# start of helix has no offset along cylinder
XStep = 0

# create circle sketches then extrude cut 'through all'
for Step in range(0, TotalAngle / StepAngle):
  Angle = Step * StepAngle
  NormalizedAngle = Angle % 360
  XStep += (Angle * 0.001)
  if NormalizedAngle < 90:
    X = -(StartX + XStep)
    Y = Diameter / 2.0
  elif NormalizedAngle == 90:
    X = -(Diameter / 2.0)
    Y = -(StartX + XStep)
  elif NormalizedAngle < 180:
    X = (StartX + XStep)
    Y = -(Diameter / 2.0)
  elif NormalizedAngle < 270:
    X = -(StartX + XStep)
    Y = -(Diameter / 2.0)
  elif NormalizedAngle == 270:
    X = (Diameter / 2.0)
    Y = -(StartX + XStep)
  else:
    X = (StartX + XStep)
    Y = Diameter / 2.0
  Sk = P.AddSketch("S" + str(Angle), Planes[Step % NumPlanes])
  Sk.AddCircle(X, Y, CutterDiameter, False)
  P.AddExtrudeCut("S" + str(Angle), Sk, 0, False, Part.EndCondition.ThroughAll, None, 0, Part.DirectionType.Normal, None, 0, False)

Andy