Jump to content

Accuracy not spot on.


Recommended Posts

I have a question about accuracy.

 

I am new to CAD.

 

I have established six construction lines at 30 degree intervals all passing through a single point (20,10). These lines were to be the major axes of a 12 sided polygon (dodecagon).

 

However, when constructing the polygon with a centre of 20,10 and a radius of 5 some of the apexes did not lie on the construction lines.

 

Admittedly the error was quite small amounting to only about 2 or 3 mm in real world terms but would you expect AutoCAD to be absolutely spot on in such a construction?

 

Frankly I can’t decide if it’s the construction lines OR the polygon that is in error or a bit of each.

 

Perhaps it would be better to draw a circle of 5 radius then draw the polygon as a series of lines joining the point of intersection of the circle with the construction lines.

Any thoughts?

Link to post
Share on other sites

Perhaps you are not making best use of the Osnaps. It works perfectly for me. Perhaps for further help, you could post a screen shot/ drawing file, and maybe someone can puzzle out what is going on with your working :)

Link to post
Share on other sites

also check that the screen has had a REGEN. Sometimes AutoCAD lags behind and displays a low res image whist retaining all its geometry accurately.

Link to post
Share on other sites

Now here’s a funny thing,

 

I have redone the exercise and – guess what – the polygon was spot on!!!

 

I haven’t a clue what went wrong previously.

 

BUT-

 

When I completed the polygon it insisted on placing itself with the apexes half way between the 30 degree lines, so the mid point of the flats were on the 30° guides. The only way I could get it to sit with the apexes on the 30° guides was to rotate it by 15°

 

Have I missed some element in the construction of the polygon?

 

Thanks for your help and watch out for me again – I am only on the foothills of the learning curve.

 

Thanks again for your help.

 

Jim

Link to post
Share on other sites
Lee Mac

I think this would depend upon whether you have it inscribed inside the circle or otherwise?

 

EDIT:

 

upon experimenting, you can rotate the polygon to whatever angle you like, upon drawing it.

Link to post
Share on other sites

I tried it both ways Lee Mac.

 

It made no difference - but it was more difficult to get the radius right when NOT inscribing!

 

Thanks anyway.

 

Edit: I ended up rotating it! - thanks

Link to post
Share on other sites

Rotate the polygon as Lee stated. Whatever angle you require.

 

Get going, not that difficult in any version of ACAD.

Link to post
Share on other sites
MisterJingles

If I can hijack this topic since James' question was answered.

On accuracy, if I am dragging an object around and I want to drop it but it must sit flush against another line, what function should I be using? Im guessing it has something to do with the snap function. Or does it?

What I find myself doing is moving the object and zooming as far as possible to get it as close to the line as I can but this cant be exact.

 

The same goes for drawing a line which I want to snap perpendicular to another line. Currently I just extend it past and then trim it but that seems to be the long way around.

Link to post
Share on other sites

At the bottom of your screen there are a couple of buttons (SNAP; GRID; ORTHO; POLAR etc) - right click on the one that says OSNAP and select Settings. There you can choose what OSNAPS you want always running (I recommend at least Endpoint, Midpint, Intersection as a start - you'll find your own setup in time) and make suer the Object Snap On box is ticked in the upper left corner)

 

Now when you start a commane (like Polyline) and get close to another line - you'll see yellow-ish sqauers popping up, these are your OSNAPS, when one of them is lit up and you click to end the Polyline command, the line will snap to the OSNAP that was lit up.

 

And in the future, please don't hi-jack threads - if your comment doesn't directly relate to the OPs question, start a new thread.

Link to post
Share on other sites
MisterJingles

Ok I apologise, I thought it was preferable to starting a whole new thread. Wont happen again.

Will fiddle with the osnap settings as suggested.

Thanks

Link to post
Share on other sites
I have a question about accuracy.

I have established six construction lines

Any thoughts?

 

Instead of drawing construction lines, draw one of the major axes. Then using the polygon command, you can draw your 12 sided polygon directly based on the mid point of the axis being the centre of the inscribed circle, and the end point of the line being the radius. Your polygon is now perfectly aligned without need for rotation :D

Polygon.jpg

Link to post
Share on other sites
  • 1 month later...

It's some time since I checked out this thread - I had not realised that there was additional posts since my last one.

 

Sorry that I have not thanked contributors sooner.

 

So Belatedly - Thanks for your help.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...