2D Diamond with rounded edges and exact dimensions

[liamgooch]

I'm having trouble modelling what should be a simple shape. It's a 2D diamond shape with rounded edges - but to an exact set of dimensions. The length needs to be 238mm and the width 70mm. The right end is also slightly smaller (narrow) than the left end.

Now in the file attached (Vent_Help v1)- I can get my edges rounded to the shape I am happy with, using the fillet tool on the extruded body. But the issue is that the dimensions are then less than required - as the fillet removes parts of the body to create the rounded edge.

I have tried experimenting with splines and tangents in the sketch (Vent v8), to try and get the rounded edges while keeping dimensions, however I'm either; unable to get a round edge, unable round the wider edges, or the tangent starts doing silly shapes.

Can someone please tell me where I am going wrong - and how I get the desired shape?

[Warmingup1953]

Why not start like this:

PS. I need to add another radius..Gimme 10

Edit: This is better I think:

[liamgooch]

Those tangents on the ends - every time I try to do that, I have this happen. How did you get it so round?

[Warmingup1953]

I created Construction lines perpendicular to the long axis....One at either end then a third horizontal at the top and a 4th to equalise the top 50mm radius

[liamgooch]

Okay this is definitely what I'm after - although I seem to be missing a step or two when trying to recreate it.

When adding the construction lines I can get the rounded shapes on the ends - but the curve then follows the circular shape too far and creates an indent before following the long straight line again. Additionally, my top curve is now too round - and I'm not sure I'm getting the same radius as you.

I'm obviously missing something - are you able to step me through where/how you are creating your tangents?

[Warmingup1953]

I think it may be a bit mouse control...which at times can be difficult to spot. In your case you appear to have more arcs and lines than needed. There only needs to be three arcs and two lines in this half profile. In my example, my 238mm Axis should also have been a construction line. 

Edit: I had to check how I do it...I Start here:

Then two tangent arcs approx the right arc length

Then I do the line an arc and a line in one smooth go

So from left to right: Arc...Line...Arc...line...arc...No more

[TheCADWhisperer]

@liamgooch 

Maybe starting with a Rectangle would be easier.

[liamgooch]

I'm sorry guys I'm pulling my hair out here - I think maybe I misunderstand how the 'tangent arc' tool works. I've attached a video showing what I am doing (wouldn't let me insert inline). 

I'm also unsure where you guys have clicked to create these arcs and lines. The file @TheCADWhisperer has uploaded has the correct geometry, but I'd like to cement my understanding of how this actually works.

Firstly, for the life of me I cannot get the tangent arc tool to work. Then randomly it does start working. But only when I have a second point to latch onto and a the middle line (going towards the top centre) already drawn - meaning the arc is not always 15mm as in your responses. I'm not 100% sure if that's important.

Then, I have no idea how you've managed to get the top arc to work. I can add two lines and make an arc between them. If it doesn't make a huge circle it will create the arc shape I need - but then because I've just had to guess where to place the two lines, I do not get the required width dimension of 35mm. I can start from the top centre and create 2 arcs of 25mm each, then create lines that attach the back to the outer arcs - but then I have to delete and recreate those arcs as they are not a smooth connection anymore.

[g-andresen]

Hi,

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@liamgooch  schrieb:

I'm sorry guys I'm pulling my hair out here - I think maybe I misunderstand how the 'tangent arc' tool works. I've attached a video showing what I am doing (wouldn't let me insert inline). 

Firstly, for the life of me I cannot get the tangent arc tool to work.

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1. The tangent arc tool in @Warmingup1953 ´ post refers to a vertical construction line.

However, this is not necessary if you use the center point arc tool, as I did in the screencast.

2. Here is a deliberately rustic way to create a sketch in less than 2 minutes.

günther