I want to cut a spiral/snail cam shape in 4mm acrylic similar to the image attached. For context, the cam will be roughly 100mm wide.
The main criteria is that when rotating the cam from the pivot - clockwise in this case - the distance between the pivot and a fixed point riding on the outer perimeter should increase in a linear fashion.
I can’t see any direct drawing tools in LB that can easily achieve this. I do have Illustrator which has a spiral tool, but that doesn’t work exactly as I’d like. There is also a method using arcs and the ‘golden ratio’ but that doesn’t look quite right either.
This doesn’t need to be perfect and I could probably get away with just tracing that image anyway.
I haven’t determined the exact dimensions at this stage. This is just a ‘proof of concept’ at the moment. As a rough guess for this cam I’m looking at a minimum radius of around 40mm to max of 80mm.
I will likely be making other sizes, but may only have to enlarge the artwork (except for the pivot hole).
This is a better example of what I’m talking about:
However, before anyone gets offended, I don’t really need someone to draw it for me because there may be other considerations that change the design. I sort of need a simple formula that always works, but maybe that might not be possible.
None taken Hence the question regarding whether this is a one-off. Initially I just wanted to make sure I was on the right path before committing too much time to a method only to find I’m going in completely the wrong direction. I’ve had a look to see whether there are any simple online cam design apps out there but I’ve not found anything really suitable. The example I posted above was created in Fusion 360 with an add-in I wrote some time ago (just for personal use so it’s not published). I’ll have a look at FreeCAD to see whether there’s a cam designer thingy in there.
Yeah, the one I posted previously (blue cross-hairs) was done in a CAD program.
What I find interesting is the the previous referenced version and the one you created have the same odd anomaly.
For example, if you rested the one you drew on a flat plane, the pivot point is always ahead of the perpendicular. Not sure if that is just the way it has to be - or even if that really matters.
The cam will be under slight spring tension, so I think it would want to rotate slightly in the anti-clockwise direction - which might cause an issue for what I’m using it for.
Or maybe I’m over-thinking it, as in creating a solution for a problem that doesn’t exist. It wouldn’t be the first time I’ve done that.
I think I know what you’re getting at. Where the cam is at its lowest point it immediately begins to rise. This looks odd but if you think about it it makes sense. If you need a short dwell section you’ll probably need to add a small circular arc section at the start then start the actual cam part a few degrees forward. Hopefully that makes sense.
Not sure if that makes sense and if we a meaning the same thing
To clarify what I mean: With the image you drew, if this was sitting as a solid object on a flat table then you applied weight to the pivot/axel, the cam would tend to want to move to the left - and likely continue to do so.
This is because the pivot is to the left of the point where it touches the table. If the pivot was directly aligned the the point it touches the table, then it would not move.
It may be of no consequence and maybe that’s just the way this type of cam has to be? In any case, this is about the level of my engineering know-how.
If I think about drawing this manually, I would use a turntable with paper on top, running at a constant speed with a pencil drawn straight across from toward the centre to the outer edge, making sure the time one revolution matches the time the pencil moves.
Ummm, I think I’ve gone mad thinking about it too much, and should just make the bloody thing!!
If you are interested, I’ll post a couple of images of what, where, how and why this cam will be used.
As you can see it has an identical shape to the one I first posted (yes I have laid one over the other to check ).
Ok, imagine the cam is not circular but instead it is a linear ramp. The ramp has a constant linear rise (or fall) along its entire length. Consequently if you placed a cylinder on that ramp it will always roll down it. All we’re doing is wrapping that ramp around a central point. Does this make it easier to see why its the shape it is? You’ll need something that adds a bit of friction such that the cam doesn’t rotate due to the load from whatever is acting as the cam follower. I hope this makes sense.
Once you’ve imported the spiral geometry into LB you can then draw a line with a length equivalent to your max radius then move it into position (see image below. The other end of that line will be your pivot point. If you want the cam subdivided into equal angular sections you can create a circular array of the line (and any labels) then do some trimming.
You’d need to move some of the stuff onto different layers and you’d probably want to position the text a bit better etc… but I’m just trying to cobble something together quickly to give you an idea.
Brilliant!! Thanks Marcus that’s pretty much what I’m talking about. Thanks for all your help. Now that I know what I need to do and when I know the exact dimensions, I’ll have a play with FreeCAD and do the rest in LB.
Cheers, Steve.
Thanks everyone for their input. Absolutely brilliant. I thought I may as well show how this “cam” is going to be used in context.
I think it may be interesting to others to see how LB has been used in more than one way to achieve my final (almost) goal. This is not some sponsorship by LB, but just an example how “I” have used LB with my co2 laser.
Apologies for the crappish images which I’ll try to explain.
The cam device will be used on a specialised router table to determine the distance required to route out the back of my hand-made picture frame moldings.
The picture frames will house some artwork that I have created using a masking technique. Parts of the mask were created in LB. The mask was cut using LB. The mask was applied to mirror-glass, weeded, sandblasted, painted and glue-chipped.
The following images are crude to some extent and posted as such - it’s late here and if anyone is interested I’ll explain in more detail at a later date.
The importance of the cam is to adjust the distance between the router cutting head and the fence as shown in the pathetic images I created. Oh yeah, that red coloured scribble is a spring. The said cam will be fixed on the rhs of the fence.
Not sure about that - there are 6 different spiral types to choose from, including plotted and Fibonacci, and options on all of them for the number of turns, steps, divisions, bias and inner radius. It can certainly draw a snail cam.
because there may be other considerations that change the design. I sort of need a simple formula that always works, but maybe that might not be possible.
Hence the question regarding whether this is a one-off. Initially I just wanted to make sure I was on the right path before committing too much time to a method only to find I’m going in completely the wrong direction. I’ve had a look to see whether there are any simple online cam design apps out there but I’ve not found anything really suitable. The example I posted above was created in Fusion 360 with an add-in I wrote some time ago (just for personal use so it’s not published). I’ll have a look at FreeCAD to see whether there’s a cam designer thingy in there.
