Dynamic blocks - stretching a block (triangle) whilst keeping an angle constant.

[c.mitchellQ7AMQ]

I am attempting to create a dynamic block in AutoCAD LT, whereby a simple triangle has a linear stretch along the X axis and in turn it stretches along the Y axis, keeping the Hypotenuse-Adjacent angle as a constant.

I have got as far as applying a linear stretch, and then adding a second stretch to that parameter and setting the angle offset to 270, however it's the final part I'm struggling with. Being LT, I cannot use geometric/dimensional constraints in the block editor. I have seen a block somebody else has done, where they have the distance multiplier set to a 1.73 number, and seems to work for them. However that keeps it at whatever angle they initially wanted it kept at.

My question is how do I make sure my triangle retains its 17.5/20/25 degree angle (I'm going to create multiple triangles at different angles) while using the linear stretch actions? If it is a case of mathematics, and working out the relevant distance multiplier, I'm struggling to understand what it is I'm calculating to therefore type in the distance multiplier.

Any help is appreciated.

[h_s_walker]

Why not just use a scale action on the linear parameter instead of the stretch. That will keep the angles constant

[c.mitchellQ7AMQ]

Ah sorry, I should have mentioned that - I have objects all around/over the triangle which will just move as the triangle increases. Basically the triangle itself is a construction guide, with various roof/ceiling details around it. The ridge will stretch upwards with that point, and the eaves/ceiling will stretch horizontally as the angle that I require being constant is stretched. Applying the scale action will scale those objects up too.

[h_s_walker]

You can have multiple actions attached to one parameter. Use a scale action on just the triangle and use a stretch / move action on the items which need to stay the same size. See the attached drawing

[c.mitchellQ7AMQ]

Thanks for that. Unfortunately though, I require all angles to be maintained as it is stretched. i.e. in the case of my first triangle, the bottom right angle at 90, the bottom left at 17.5, and top angle at 72.5 - which is where I think I must need the distance multiplier to be a certain value for the Y stretch.

I've attached a basic version of my block. As you'll see, the base point is the bottom of the triangle in the middle of the block, and stretches both ways based on the one horizontal stretch. Ideally what I'm needing is that angle (17.5 in this case) kept the same when it is stretched out, so that it automatically stretches the Y stretch - as currently I'm having to manually stretch that and pre-draw the correct triangle beneath the block to then fit the points to. 

[h_s_walker · Accepted solution]

A quick bit of googling and fiddling found the answer

Delete the vertical linear stretch parameter and it's associated action.

Add a new stretch action to the horizontal linear stretch parameter and change the parameters according to the image below. I would suggest you use the calculator as I have shown. The first part of the equation before the divide works out how high the vertical of the right angle triangle is and then you divide that by 600 to work out the vertical distance multiplier

[c.mitchellQ7AMQ]

Spot on! That's exactly it! I knew it was either the CAD parameters I wasn't getting right, or the mathematical equation part itself. I know this therefore doesn't render the answer CAD related haha, but thanks very much!

[h_s_walker]

I'm an idiot. You only needed to use TAN 17.5 in the calculator. the *600 and /600 cancel each other out. It's too long since I did any maths.

[c.mitchellQ7AMQ]

Bloody hell. Yeah. I never realised it would be that simple of a solution. Thanks anyway, Id have struggled for ages on that one!