Had Another Go

[lee websterParticipant @leewebster72680]

Hi Nigel.

I haven’t posted a comment for a while, but this thread has interested me. Learning to use the sweep tool is a good thing. But, for someting like the simple frame you are trying to draw, why not just creat a sketch on the XY plane and then extrude it into a solid? Sweep takes two sketches on two planes, whereas drawing the frame on the XY plane is one sketch on one plane. Plus of course you would need to use the shell tool if Alibre has one to create the U shape.

If you draw the frame as above can you mirror the sketch to create the othe frame rail? I don’t use Alibre Atom because it requires internet connection.

[David JuppParticipant @davidjupp51506]

Lee,

Atom3D only requires internet connection to validate the licence every 30 days.  Internet is not required during operation.

Yes Atom3D does have a Shell tool.  Nigel has a chassis rail example done using shell.

[Nigel Graham 2Participant @nigelgraham2]

Until someone suggested using “Offset” to reduce the number of steps a bit, I just used rectangles, lines and trimming to form the basic ‘C’. Not the Shell tool – but doesn’t that leave the far end of the profile closed, like the bottom of a box?

I’m a bit puzzled by what Lee means though. The chassis is made up of channel sections not solid bars, and is not a simple rectangle in plan, so I can’t see how it can be extruded in one go either lengthways or top-down as an extrusion.

[Nigel Graham 2]

On Nigel Graham 2 Said:

the Shell tool – but doesn’t that leave the far end of the profile closed, like the bottom of a box?

Depends which face(s) you add to the Remove Faces list.  By default Shell doesn’t remove any faces from the model, it just hollows out the inside.

With some thought about faces to remove, and perhaps thickness overrides, the Shell tool can be a shortcut to modelling some shapes.

It’s very rare for there to be only one single way to model a particular part.  Different users may find very different approaches to make most sense.

[Nigel Graham 2Participant @nigelgraham2]

I was trying to think what I’d used Shell for, but eventually remembered it was for the wagon’s coal bunkers, open-topped boxes fabricated from steel sheet. Not its chassis.

[David JuppParticipant @davidjupp51506]

Error correction time:

I stated previously that in a Sweep, the position of the Path sketch doesn’t matter.  Jason did some tests which showed I was wrong.

I had always believed that the path is followed even if it is offset from the profile.  Turns out the finished sweep actually sits parallel to the path sketch, so the relative lengths of portions of the result can be affected by the position of the path in some cases.

So not only for understandability, but also for accuracy the path should coincide with the profile – you’ll have to decide if touching the profile is OK, or if the path should run through the centreline of the profile.

Apologies for any confusion caused.  I’m checking with Alibre QA if this is working as intended, and if the Help file needs a little more adding to cover this.

[Nigel Graham 2Participant @nigelgraham2]

I had thought the sweep was parallel to the path, so the same layout and length, but displaced.

I’d read the manual as saying the sketch does not need be on the path, which seemed strange but I took it to mean the calculations would place it on the path. Only how would it be centred? On the sketches’ axis of symmetry, on one corner if a polygon, on some arbitrary point on the circumference of a circle?

Tht may not matter if the object is floating about as a Part, I suppose.

[David JuppParticipant @davidjupp51506]

Nigel,

The sweep feature always goes through the profile sketch.

The path that the sweep feature follows is parallel to the path sketch, and anchored to the profile sketch.

If the path sketch is offset from the profile sketch, the parallel application of the path sketch can result in some lengths changing from those in the path sketch, when the feature is generated.  It’s generally not a big effect, but can make a difference.

For your chassis rail, it can result in the angled portion getting moved along the rail slightly.

[JasonBModerator @jasonb]

These are the couple of sweeps that I sent David with my Query.

In both cases the lower square has the path against it’s edge. However the upper square has the path sketch offset. Looking from above shows that the upper square remains a coonstant distance from the path so in teh case of teh angled one the main leg of teh chassis would be longer and the front section shorter. In the case of teh bent one then the vbend radii change as they are equally spaced from a springing point further away so the radius alters.

 

 

There are times when you want it parallel to the path such as this bead running around the opening but set 0.5mm from the edge which was the path.

[Nigel Graham 2Participant @nigelgraham2]

Oh Lor’! Gets worse.

I thought I was beginning to understand how to use that Sweep tool but you have just warned me it holds a confusing trap hard to avoid, making it more difficult still.

I’ll have to plan any models either to use a work-round in the CAD models (simplifying or omitting such awkward shapes), or by designing the project itself without them physically.

.

Having investigated the possibility, I am tempted to overcome all this palaver by scrapping the chassis I have made and replacing it with one with fully-parallel sides. The narrowing for the steering clearance is then by making each side rail as two straight lengths bolted together, overlapped, with a plate spacer. The displacement each side is 1-1/4″, so the spacer is only 1/4″ thick . No awkward angles, easier machinery mounting, more room for it. The problem apart from material cost and work time, is losing space needed for the steering-gear.

Ironically this CAD model is supposed to help me work out how to install the engine and road-gears in a rather cramped space.

 

I’ve also another design task. I have lost the long gib-head key between the rear wheel and axle, a rather awkward one because the axle and hub keyway widths differ slightly, probably <0.01″ total error. So back to the books to determine the correct dimensions then make a new one, 1-in-100 taper and all, but of dual width.

[JasonBModerator @jasonb]

There is no trap. If done how I first suggested then it will work fine and a bit of forethought will avoid any problems.

The only thing I would say is that I drew the path for the inside edge of the chassis to follow, if your dimensions were taken from the outside edge then that is where the path and C section sketches should meet. To me it made sense to use the inside edge as that is what all the other main cross rails fit to.

Don’t give up on sweep as it is a feature that can be used a lot and giving up will restrict you.

[Nick WheelerParticipant @nickwheeler]

It’s only a trap if you blunder into it unprepared. Being prepared enables you to swerve around and carry on unaffected.

 

To be prepared requires more practice by repeating exercises different to the couple you’ve recently accomplished. Doing this is a long established learning and teaching method. Moving on before you’ve mastered, or at least understood the fundamentals, is deliberately setting yourself up for disappointment.

Jason’s new examples demonstrate how more sophisticated techniques can make complex parts easier.

I designed my hotrod chassis rails using the external dimensions as they need to fit inside the body that I already have. When I actually build the thing, I’ll pull the internal dimensions as they’ll be easier to measure on the build table. And when I have the flat parts laser cut, I’ll have some templates cut for the angles as the chassis taper turns out to be a hard to measure 7.9°

[Nigel Graham 2Participant @nigelgraham2]

Well, that “blunder into it” is the point. I’d not even known this can happen until Jason described it, and it seems very easy to forget these subtle points, be confused and make a mess of things as a result.

I have tried setting up that frame in 3D many times by now by different ways and it either just goes wrong or sort of “works” but in a very clumsy and inaccurate way.

I was using the inside dimensions all along, but because the frame tapers, I found I could not constrain those two cross-members accurately to both sides in their right places.

…….

Several years ago I fabricated two hexagonal frames from black (well, very rust-brown) steel bar, about three feet diameter. They didn’t need very close accuracy, but I still wanted reasonably good hexagons that would screw to other components correctly.

The very “pre-loved” steel was too rough and the bars too long for reliable setting by combination-square.

So I made each as three pairs, aligned to pencil lines set out on a sheet of 12mm plywood by standard geometrical construction methods and long beam-compass.

 

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