QCAD.org Forum

Posted: **Sat Aug 30, 2025 10:09 am**

Hello,
I’m drawing patterns for leathercraft using QCAD.

In this picture, I’m curious about how to draw a line.

Starting from point A, I need to draw a line that measures 8 cm and ends exactly on line B.
How can I draw this so that it doesn’t go off line B and finishes precisely at the 8 cm point on line B?

I’d appreciate your help.

Thank you.

: 83aa268f-3822-4e33-a8de-496dc1b10f68.png (8.46 KiB) Viewed 18921 times

Posted: **Sat Aug 30, 2025 3:18 pm**

Hi,

I can't open the link you included.

One can attach screen captures and/or DXF/DWG files directly to a post.
See tutorial 'How to Attach a File to a Post'.

The LI line tool allows for restricting the length from the start point.
But that may or may not end on another line at 8 cm as that are 2 conditions.
Perhaps there is are solutions that complies with more than one condition.

Regards,
CVH

Posted: **Sun Aug 31, 2025 4:50 am**

Oh.. I'm so sorry.
I attached the image.

Thank you.

CVH wrote: ↑
Sat Aug 30, 2025 3:18 pm
Hi,

I can't open the link you included.

One can attach screen captures and/or DXF/DWG files directly to a post.
See tutorial 'How to Attach a File to a Post'.

The LI line tool allows for restricting the length from the start point.
But that may or may not end on another line at 8 cm as that are 2 conditions.
Perhaps there is are solutions that complies with more than one condition.

Regards,
CVH

Posted: **Sun Aug 31, 2025 7:01 am**

Hi,

Solved by compass and straight-edge.

First draw an auxiliary circle with an 8 cm (See lower) radius centered at point A:

Activate Circle tool 'Center Radius' (CR).
In the Option Toolbar: Radius = 8; Angle = 0 (Doesn't matter); Reference Point = Middle.
Place the Circle with its center on point A.
Let the cursor snap to the rightmost endpoint of the horizontal segment.
Followed by a right click = 'Cancel tool' when done.

The Circle is the locus of all points at a distance of 8 drawing units from point A.
This Circle may not cross Line B, touch Line B at a single point or intersect with Line B at 2 points.
With intersections, one of the intersections is the required endpoint of a line 8 drawing units long starting in A.

Then draw the required line segment:

Activate Line tool 'Line form 2 points' (LI).
In the Option Toolbar: Line Segment (Not an Infinite Line or a Ray); Length = cleared; Angle = cleared and both values don't matter.
Indicate the start point A.
Let the cursor snap to the rightmost endpoint of the horizontal segment.
Indicate an intersection of the auxiliary circle and Line B.
Let the cursor snap to the intersection.
Followed by a right click = 'Done adding segments' and a second right click = 'Cancel tool' when done.

Select and delete the auxiliary circle afterwards.

Remind that CAD values are typically unit-less.
8 cm is a radius of 8 drawing units long on a drawing in centimeters.
8 cm is a radius of 80 drawing units long on a drawing in millimeters.
And so on.

If you select only the newly created line segment, it should be 8, 80, ... drawing units long.

See Property Editor when selected.

Please include your QCAD version in newer topics.
The Option Toolbar for LI has changed over time.

Regards,
CVH

Posted: **Sun Aug 31, 2025 2:24 pm**

Oh.. Thank you so much..!!

It's a very simple method, but it's not easy to approach my thoughts. I solved it!

CVH wrote: ↑
Sun Aug 31, 2025 7:01 am
Hi,

Solved by compass and straight-edge.

First draw an auxiliary circle with an 8 cm (See lower) radius centered at point A:
Activate Circle tool 'Center Radius' (CR).
In the Option Toolbar: Radius = 8; Angle = 0 (Doesn't matter); Reference Point = Middle.
Place the Circle with its center on point A.
Let the cursor snap to the rightmost endpoint of the horizontal segment.
Followed by a right click = 'Cancel tool' when done.
The Circle is the locus of all points at a distance of 8 drawing units from point A.
This Circle may not cross Line B, touch Line B at a single point or intersect with Line B at 2 points.
With intersections, one of the intersections is the required endpoint of a line 8 drawing units long starting in A.

Then draw the required line segment:
Activate Line tool 'Line form 2 points' (LI).
In the Option Toolbar: Line Segment (Not an Infinite Line or a Ray); Length = cleared; Angle = cleared and both values don't matter.
Indicate the start point A.
Let the cursor snap to the rightmost endpoint of the horizontal segment.
Indicate an intersection of the auxiliary circle and Line B.
Let the cursor snap to the intersection.
Followed by a right click = 'Done adding segments' and a second right click = 'Cancel tool' when done.
Select and delete the auxiliary circle afterwards.

Remind that CAD values are typically unit-less.
8 cm is a radius of 8 drawing units long on a drawing in centimeters.
8 cm is a radius of 80 drawing units long on a drawing in millimeters.
And so on.

If you select only the newly created line segment, it should be 8, 80, ... drawing units long.
See Property Editor when selected.

Please include your QCAD version in newer topics.
The Option Toolbar for LI has changed over time.

Regards,
CVH

Posted: **Sun Aug 31, 2025 4:23 pm**

JayPark wrote: ↑
Sun Aug 31, 2025 2:24 pm
Oh.. Thank you so much..!!
It's a very simple method, but it's not easy to approach my thoughts.

It is in fact one of the mathematical solutions from solving a system of a quadratic function (Circle) and a linear function (Line).
Again a quadratic equation with: no, one or two real roots (Intersections) in X or Y.
Inserting the real root(s) back into one of the functions solves for the associated Y or X coordinate of an intersection.
But many planar mathematical problems can be solved by simply drawing them.

For the record:
You don't have to quote an entire post.
Before you hit the 'Quote [user abc]' button you can select a part of the post/reply by the user.
You can also edit this quoted text, in the above quote I removed the empty line and truncated the second line.

JayPark wrote: ↑
Sun Aug 31, 2025 2:24 pm
I solved it!

When considered as solved please add '[SOLVED]' to the title of the initial post by editing it.

Regards,
CVH

QCAD.org Forum

[SOLVED]In this case, how should I draw the line?

[SOLVED]In this case, how should I draw the line?

Re: In this case, how should I draw the line?

Re: In this case, how should I draw the line?

Re: In this case, how should I draw the line?

Re: In this case, how should I draw the line?

Re: In this case, how should I draw the line?