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double point arc
August 11, 2009, 09:46:43 PM
I want to construct an arc of a specified radius from one coordinate point to another coordinate point.  I am trying to use the double point arc tool, and I can't get it to work.  I can enter the first point, and then the second point, but then I hit nothing but snags.  This has to be simple.  HELP!

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August 12, 2009, 12:03:44 AM
I want to construct an arc of a specified radius from one coordinate point to another coordinate point.  I am trying to use the double point arc tool, and I can't get it to work.  I can enter the first point, and then the second point, but then I hit nothing but snags.  This has to be simple.  HELP!
The 2-Point Arc tool cannot be used that way - the 2 points define the diameter of the arc.
You have to do a little bit of construction to draw the arc you want.

In the attached diagram, assume you want an arc of specified radius from one end of the red line AB to the other.
First draw two complete circles of the required radius, but with centres at A and B as shown. I've used construction circles.
Then, using the Arc Center and Radius tool, choose  the intersection of the two circles (point C in the diagram - use the I SEKE) as the arc centre, and specify point B as the second point on the circle's circumference. Choose the same point B as the Start angle of the arc and point A as the End angle of the arc. Because the arc is always drawn anti-clockwise, choosing A as the start angle and B as the end, will define the arc compliment of the one shown.
You can then delete the construction circles and the job is done.

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Gary Wooding
Win10 64-bit,
TC21.2 x64 Plat, Bld59
TC16.2 Plat, Bld54.0
TCC 3.5

August 12, 2009, 05:46:30 AM
re: I want to construct an arc of a specified radius from one coordinate point to another coordinate point.

Use "Arc Tan to entities" with the "Through Point" feature enabled. You specify one point, then the second point. Now specify a radius/diameter and the start/end angles. You don't need to have any tangent entities available, just click wherever the points should be.

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John R.

V17—V21, 2015—2019
Designer, Deluxe, (Professional, Expert, Basic), Platinum
RedSDK enabled
Windows 10 Pro (1803), 64-bit

August 12, 2009, 11:38:13 PM
Use "Arc Tan to entities" with the "Through Point" feature enabled. You specify one point, then the second point. Now specify a radius/diameter and the start/end angles. You don't need to have any tangent entities available, just click wherever the points should be.
Thanks John,
I didn't know that one.
But I feel compelled to ask why such an undoubtedly useful feature was buried in a tool that deals with tangents rather than one that deals with points?

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Gary Wooding
Win10 64-bit,
TC21.2 x64 Plat, Bld59
TC16.2 Plat, Bld54.0
TCC 3.5

August 13, 2009, 02:38:36 PM
Thanks John,
I didn't know that one.
But I feel compelled to ask why such an undoubtedly useful feature was buried in a tool that deals with tangents rather than one that deals with points?

I don't know, but I would guess that the Tangent portion was thought of long before the Point feature when this tool was created in V3 or V4.

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John R.

V17—V21, 2015—2019
Designer, Deluxe, (Professional, Expert, Basic), Platinum
RedSDK enabled
Windows 10 Pro (1803), 64-bit

August 13, 2009, 06:28:41 PM
A curve approaching a point is tangential to it at any angle.  Good pick, John.

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August 13, 2009, 09:25:00 PM
I have tried the "Arc Tan to entities" with the "Through Point" feature enabled as suggested by John R.  I got the first and second points established via the status bar.  Then I specified and locked the radius in the inspector bar.  The program insisted on a third point so I clicked on the arc between my two points.  That is imprecise and I couldn't figure a way to do a local snap.  Adding in the third point changed the radius by the amount of error in clicking, even though the radius was locked.  Same thing with specifying beginning and ending radius angles, imprecise.

So one point would be how to enter this arc exactly.  Another would be, after it is established, how do you go back and change any of the parameters, by coordinates, not clicking.

This is the most frustrating program.  I guess spreadsheets are just plain easy.  Anyway, much thanks to all for the help and comments.  Establishing an arc of a certain radius between two set points is about day one-and-a-half or day two of high school geometry.  Why is it so difficult in this advanced program?

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August 13, 2009, 09:55:56 PM
I have tried the "Arc Tan to entities" with the "Through Point" feature enabled as suggested by John R.  I got the first and second points established via the status bar.  Then I specified and locked the radius in the inspector bar.  The program insisted on a third point so I clicked on the arc between my two points.  That is imprecise and I couldn't figure a way to do a local snap.  Adding in the third point changed the radius by the amount of error in clicking, even though the radius was locked.  Same thing with specifying beginning and ending radius angles, imprecise.

So one point would be how to enter this arc exactly.  Another would be, after it is established, how do you go back and change any of the parameters, by coordinates, not clicking.

This is the most frustrating program.  I guess spreadsheets are just plain easy.  Anyway, much thanks to all for the help and comments.  Establishing an arc of a certain radius between two set points is about day one-and-a-half or day two of high school geometry.  Why is it so difficult in this advanced program?

Here's John's basic method in a little more detail:

1) Go to Insert|Point|Cross, and place one Point at each of your set point locations. (If you know only the coordinates of these locations, press Shift+Tab to access the x field of the Status Bar and type the X coordinate of one point; TAB to the y field and type the Y coordinate; press Enter to plant that first Point object; repeat for the second Point object.)

2) Go to Insert|Arc|Tangent to Entities; highlight the Radius field of the Inspector Bar and type the desired radius, then click the "padlock" icon beside that field to lock the radius. Click the "Through Point" icon on the Inspector Bar (or right-click in the drawing and choose "Through Point" from the Local Menu).

3) Place the cursor on the first Point object and press V on your keyboard (an operation sometimes called V-snapping); place the cursor on the second Point and press V. (If the center of the phantom circle you see on the screen is on the wrong side of the two points, press Esc, V-snap to the second Point object and then V-snap to the first Point.)

4) Final step is to define the endpoints of the arc: V-snap to one of the Point objects and then to the other. Which Point you snap first will determine which part of the phantom circle remains as your finished arc. (The arc is defined in a counterclockwise direction by default.)

Henry H

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August 14, 2009, 08:41:49 AM
Another way:  Draw a rotated rectangle using your two coordinates as the first side.  Draw a circle tan to line against the first side with extended ortho snap, tab to the radius/dia. box to dictate your arc, use SEKE "m for middle" on the side of the rectangle closest to the circle centre to locate it.  Trim the circle with the rotated rectangle, then select the resultant arc inside the rectangle and relocate the selector point to either end and move the arc to the coordinate point, corner of the rectangle closest to the arc end selector point.  Select and delete the rectangle.

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August 17, 2009, 10:13:34 PM
Thank you Henry H, that worked just fine.  And I was not prompted for a third point.  One last question:  Can I go back and edit the arc to put a different radius on it?  I have tried using the select/edit tool and putting in a different radius in the inspector bar, but that leaves the center of the circle in place and moves the arc toward or away from it, ie off the two points initially established.  I would like to leave the arc on those two points and just change the curvature of it.  Can do/ no can do?

Thanks again for the help.

David Callow

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August 17, 2009, 11:43:41 PM
re: Can I go back and edit the arc to put a different radius on it?

As far as I can tell, it would probably be faster to create a new arc.

If the arc points are horizontal or vertical, you can scale the arc, but now you have an ellipse, not a true arc.

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John R.

V17—V21, 2015—2019
Designer, Deluxe, (Professional, Expert, Basic), Platinum
RedSDK enabled
Windows 10 Pro (1803), 64-bit

August 18, 2009, 01:36:27 AM
4) Final step is to define the endpoints of the arc: V-snap to one of the Point objects and then to the other.
Henry H
Interestingly, this doesn't work on my 11.2  Pro system.
When I V-click on the line, the point chosen for the arc end or start is the intersection of the the phantom circle and the radial line that passes through the place where I V-clicked.

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Gary Wooding
Win10 64-bit,
TC21.2 x64 Plat, Bld59
TC16.2 Plat, Bld54.0
TCC 3.5

August 18, 2009, 08:52:37 AM
4) Final step is to define the endpoints of the arc: V-snap to one of the Point objects and then to the other.
Henry H
Interestingly, this doesn't work on my 11.2  Pro system.
When I V-click on the line...

What line?

In any case, V-snap should not snap to an intersection. If it fails to snap to a vertex, then the probable reason is that the various entities are not all on the same Workplane. Select them and use Format|Place on Workplane.

Henry H

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August 18, 2009, 10:20:41 AM
What line?
I first drew a line and wanted to use its vertexes as the two end points of the arc.
I used V-snap on the line to define the phantom circle; that worked. When I use V-snap to define either end of the arc it behaves as I described.
It happens every time. Its in 2D and its all in the same work-plane.

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Gary Wooding
Win10 64-bit,
TC21.2 x64 Plat, Bld59
TC16.2 Plat, Bld54.0
TCC 3.5

August 18, 2009, 10:35:56 AM
I first drew a line and wanted to use its vertexes as the two end points of the arc.
I used V-snap on the line to define the phantom circle; that worked. When I use V-snap to define either end of the arc it behaves as I described.
It happens every time.

Yup. Happens here, too. Looks like a BUG.

EDIT: Definitely a BUG. Happens when V-snapping to Point objects, too. Makes the whole technique useless IMO, because the result is imprecise. I've submitted a Bug report.

Henry H
« Last Edit: August 18, 2009, 11:04:32 AM by Henry Hubich »

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August 18, 2009, 01:17:55 PM
Given the problem reported by lemel_man with Arc Tangent to Entities, I'll have to say that is NOT the preferred tool for this job. Here's another procedure, which does seem to work. I'll be grateful if anyone can either verify it or show that it's no good...

Place a "Cross" Point object at each of the two known points. Apply the "Fix Geometry" Constraint to each of them to assure that they won't move. Draw an "Arc Start/End/Included" arc that looks roughly correct, but do not snap it to the Point objects. Instead, activate the "Points Coincident" Constraint and use that to fasten each end of the arc to one of the Point objects. Then select the arc, open the Selection Info Palette, open the Metrics list, and change the Radius to whatever you wish.

Henry H
« Last Edit: August 18, 2009, 01:19:28 PM by Henry Hubich »

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August 19, 2009, 05:39:49 AM
Tangent to entities isn't too obstructive.  Snapping to/through the two points, tabbing in and setting radius, then entering twice gives a 360 arc (and the radius is precise), so node editing it can snap the beginning and end of the arc to the two vertices.  Henry, I tried your constraints method in 15, where it worked, but not in 10.5, which I still use a lot, because the "fix geometry" constraint wouldn't accept the point markers.  As a result, editing the radius moved arc and point markers relative to the centre of the arc.
« Last Edit: August 19, 2009, 05:50:34 AM by murray dickinson »

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