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Face to Face Loft Examples


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Menu: Insert / 3D Object / Face2Face Lofting
Insert a 3D object by connecting 2 faces.


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Face to face lofting is carried out on Acis objects but although it will work on Acis surface objects the results may not be what was expected (explained later), Acis starts to loft tangency through the edges of adjacent faces (Thanks to Murray Dickinson for that information) and produces a smooth transition, which means if you wish the loft to go directly at 45 degrees to the face you would need to use standard lofting techniques.


Fig 1


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One thing to note when using face to face loft to create the bend for a pipe, is that it may not create the desired curvature on the bend, this is best explained in a picture, the two red circles in the image below are what one could expect the curvature of the pipe to be at an elbow, however as can be seen Acis takes a slightly different path, whilst in most cases this is totally acceptable, if one is producing accurate 2D drawings (by using the drafting palette) then standard lofting techniques may be preferable.


Fig 2


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Another example of how Acis creates the loft can be seen below in Fig 3. the image on the left shows one of the objects has been chamfered, this prevent Acis from carrying out the loft, one cure is to ‘pull’ out the face slightly giving the desired edge for the loft to be completed.


Fig 3


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In this example we will draw a drawer handle, however there is one point that needs to be taken into account, in that Acis can’t loft ‘face to face’ if the said faces are in the same plane, or opposed to each other, to get round this they are angled towards each other, however simply angling the top face can also produce an error, angling one or both of the objects allows it to loft.


Fig 4 and 5


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In this example a circle and polygon (hexagonal) was extruded to the desired height, but it could just as easily be a cylinder and polygon prism. As the faces are in the same plane they need to be rotated slightly to prevent an error occurring, therefore rotate ‘Y’ axis in this case, for the circle 1 degree, and the hexagon -1 degree (or whatever figures you prefer), the corners are blended to produce a smooth appearance with the inside edge given a larger blend, Fig 6 right.


Fig 6


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Activate ‘face to face’ loft tool and click the two top faces to create the loft fig 7, TC will automatically create the loft after the second face is clicked, and the result will be a a complete single acis object consisting of the two original objects and the loft.


Fig 7


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Fig 8


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For variation, A copy of the profiles were lofted face to face, then a centre section was sliced out. This centre section was then enlarged and used to create an intermediate object, and two face to face lofts carried out.


Fig 9


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To get a flat face to the bottom it can be simply sliced flat, an alternative would be to do multiple lofts with only the top two angled, and of course one isn’t restricted to the original shape for the intermittent piece, more on that later.


Fig 10


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Acis surface objects from profile


Lofting face to face on Acis surface objects which are a result of using the 2D profiles and selecting the ‘surface from profile’ tool does actually work, but the results are probably not what is expected, this is because Acis produces a loft from the edges, but a surface object created from profile has no thickness and thus no edges.

Fig 11. below shows the direction that the loft would be created from the object.


Fig 11


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The resultant loft created when lofted face to face is shown below, fig 12 left. Shows the 2d profiles which are converted to Acis surface objects, fig 12 centre. Shows the resultant loft, and rendered in fig 12 right. As can be seen that because the loft lines radiate out from the sides of the surface object the result is unusual.


Fig 12


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If the surface objects are angled the loft takes on a more unusual shape (below left), note lofting with the angles greater then +/- 45 degrees probably won’t work and create an error. Lofting can also be carried out between an Acis solid and an Acis surface created by ‘surface from profile’. As can be seen below right TC will loft from the solid object as expected and from the sides of the surface object.


Fig 13


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Post editing a face to face loft.


Face to face lofting does not have a compound profile option, and lofting face to face will normally break an extrusion link when the extrusion is created using and compound profile, EXCEPT, if Part tree - Acis / allow editable history are turned on, in which case one can complete some post loft editing.

In the example below a circle and filleted polygon are used for the profile, these are extruded with the compound profile option turned on, they are then angled using 1 degree and –1 degree ‘Y’ direction as previously, and face to face lofted.


Fig 14


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Once lofted, the polygon profile was selected and scaled times 2 in the Y direction, the loft will update to the new shape, the 2d profile is included to shown the resultant loft mimics the change in profile shape, the ‘ribs’ in the 2D profile are simply due to the way the profile was originally filleted.


Fig 15


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One can also use node edit to change the profile shape, in the example below the centre ’rib’ is node edited to increase its size, as with any alteration one must exercise a little care when node editing.


Fig 16


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Post editing can also be carried out if an intermediate object is used, the best way in my opinion, is to slice out a central section, as discussed earlier, then Trace around the piece using a polyline incorporating lines and arc’s to form a closed profile, Note - turning on degenerative faceting may be needed to snap the polyline in place, this profile is then extruded with compound profile to produce a thin section for the intermittent faces.

The resultant loft can then be edited in node edit mode, (be careful not to over-do the changes).


Fig 17


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The final pictures show why I would prefer to use a polyline to trace the centre section, No 1. is a spline traced and adjusted in height, No 2. is traced with arcs and lines and adjusted, (NOTE turning degenerative faceting may allow the difference to be seen more easily), as can seen a polyline more accurately reproduces the lofting lines to give a loft less prone to errors or kinks.


Fig 18 and 19


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Estimating a face to face curve

This last page is regarding the estimation of the face to face loft curve.




This page has been written as an example, It is highly unlikely you would need to estimate the curve unless one was drawing 2D which will be turned into 3D at a later date.




In this first example we will draw from profiles at right angles, figure 20 below the 2d profile and 21 shows how cylinders would be set up for the loft.


Fig 20 and 21


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To start, we draw construction lines tangent to the profiles, and because this loft will have 2 curves (top and bottom) we need four construction lines as shown in Fig 22


Fig 22


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Whilst talking of the construction lines, in straight objects this tangent is effectively perpendicular to the face, however going from a cone to a cone the construction lines would not be perpendicular to the face,
which needs to be remembered for an accurate estimate of the loft (see Fig 22b).


Fig 22b


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The next step is to draw diagonal lines linking the profiles where the loft will start and finish as in Fig 23, we can read the length of the lines in the selection palette.


Fig 23


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For the next step we need points which are one third of the line length, one could use the edit tool to divide the lines into three and use the vertices as snap points, in this example we will use arcs and the type the figures into the inspection bar fields. (there are many others ways to obtain the desired results)




Using the arc tool snap the a line vertex, in the inspection bar, type in the radius and lock field to prevent accidental alteration, draw the are so that it intersects its relevant construction line, repeat at the opposite end of the line, and then repeat with the second line, the result is shown in Fig 24


Fig 24


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Activate the spline by control points tool, right click and select properties, under the curve section tick 'show frame', this is simply so one can instantly see if the spline is snapped correctly.


Fig 25


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Snap the spline into place, using vertex, first arc/construction intersection, second arc spline / construction intersection, end vertex, repeat with second line, DO NOT be tempted to add more nodes, the spline should end up with three frame segments only, Fig 26 shows the finished drawing, the spline (shown green) is how a face to face loft will connect the two profiles.


Fig 26


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And just to prove it :-) Fig 27


Fig 27


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This second example uses two profiles which are on different workplanes, as can be seen in Fig 28 we will project the position onto a single workplane.


Fig 28


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Because we are now on a single workplane the setup is the same as previously described, except in the case we only need one curve, thus only two construction lines, again the line length is divided by three and two arcs drawn Fig 29


Fig 29


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The spline by control points is again snapped to the vertices and arc/construction intersections as in Fig 30


Fig 30


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When checked against an actual face to face loft, Fig 31, it is very close, the discrepancy is probably due to our drawing being 2D whereas the loft is curved in multiple directions.


Fig 31


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In this next example we will attempt to a more accurate curve by using a 3D spline. This involves setting up two different workplanes, one for each of the object faces, as can be seen in Fig 32 a construction line is drawn on each workplane


Fig 32


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As can be seen in Fig 33 which shows a rotated view the construction lines are tangent to each object and thus different workplanes.


Fig 33


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For ease, a 3D polyline is drawn as the diagonal (this simply saves having to create a third workplane), its length is read from the selection palette and divided by three, when drawing the arcs ensure they are on the correct workplane for their intersection with the respective construction line. Fig 34


Fig 34


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Using the 3D spline by control points snap to the vertices and arc/construction intersection as previously, and as can be seen from Fig 35 and 36 this gives a reasonably accurate representation of any resultant face to face loft.


Fig 35


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Being honest, normally there is no reason to carry out the drawings shown above, it is quicker to just loft and see what results, the above was typed simply to give an understanding on the loft shape can be simulated in 2D.







There is one last example, generally one cannot influence the shape of the face to face loft, one cannot move profiles or add guidelines, however, one way drawing in 2D could be used, is the check a loft using different shaped profiles,




I'll not go through the procedure again, but using same example as previously the right profile shape has been altered, Looking at Fig 37 below, this change in shape alters the line length (new line shown red), and thus when divided by three alters the arc / construction line intersection, (shown orange), the resultant spline (orange) takes on a different shape when snapped into position,


Fig 37


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As can be seen from this last image Fig 38, the resultant loft takes this new shape, this is simply due to changing the shape of one profile.


Fig 38


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There may be occasions when the above is not applicable and the loft is not as expected, but the above was written in the hope of providing a better understanding of how TC attempts to creates a face to face loft, this should be viewed with the other lofting tools, for example normal lofting, loft with guidelines etc in deciding the best method for any given situation.




The End



Contributors to this page: AndyUK .
Page last modified on Monday 23 of May, 2016 02:08:48 PDT by AndyUK.