Help me pass my C&G CAD Exam

Hi I could use a little help with this, I am currently doing the C&G level 2, 2D CAD Exam. Bizarrely this part of the exam lasts 2 hours, with 1.5 hours one week and 0.5 the next week, this gives a chance to find answers to any problems and I have a problem.

We had to draw a screwdriver and a wood screw, fine nice and easy, but I cannot work out how to finish the handle off.

I am 42 years old and this is a career change for me due to ill health, used to be a builder but now I am doing the drawings instead. If I can work this out I have a fighting chance of getting a distinction from C&G.

As you can see from the picture, the curved section of the handle needs to be inserted and trimmed to suit, I assume eyeing it in is not good enough. But I have no idea how to go about this and despite looking on the interweb, I'm no closer to a solution.

Any help or pointers would be appreciated.

Screwdriver for C&G.dwg

I do not see where you would put the circle. the lower curves on the handle curve inward not out. So assuming that's what you are trying to accomplish using the circle and just offsetting it from center to get the curves you need based on your example image.

The first thing I would do, assuming the curvature of the circle is correct to achieving the proper curve needed to complete the assignment, is to find the middle of the open space between the top half of the handle and the bottom of the screw driver.

After finding that midpoint I would move the circle from its center to the mid point you just located.

I hope this helps you to keep moving forward in your assignment.

Well.. doing the geometry, you can see two things:

#1 you know how far the centerpoint of the new circle is away from the centerpoint of the old circle, since they'll only touch at one point. Circles are funny things.. if they touch each other at a single point on their radiused edge, they ALWAYS describe a smooth curve.

#2 - to ensure a smooth transition to the lines making up the corner at the lower portion of the circle, you have to position the circle the same way (only one point touching). This means you know the distance from that corner to the centerpoint of that circle too. Again, this insures a smooth transition between arc and line.

This means you have a distance to the centerpoint of the circle from two positions, but you don't know the angle from these points.

So.. what shapes represents a constant distance from a single point? Or better yet.. what shapes have a specific distance radiating from a centerpoint?

Use two of those.. one above, one below, (sized to the distances discussed in #1 and #2) and where they intersect, it shows the only possible position for the new circle of that specific size, to encounter the two positions you want it to reach..

I hope that does it for you. Good luck!

Thank you for your reply's, it's good of you to help, but I don't understand.

Maybe I have started off wrong. The location of the circle on the right is irrelevant, in fact the circle itself is irrelevant, it is the curve with a radius of 12.5 that is needed to make up the bottom section of the handle. My assumption was to use a circle and trim out, my assumption could well be wrong.

My simple way from what you done already is to just OSNAP the lines you have to the main handle and shaft looks ok to me, might not be a techical way of doing it but it works

Thank you for your reply's, it's good of you to help, but I don't understand.

 

Maybe I have started off wrong. The location of the circle on the right is irrelevant, in fact the circle itself is irrelevant, it is the curve with a radius of 12.5 that is needed to make up the bottom section of the handle. My assumption was to use a circle and trim out, my assumption could well be wrong.

Hi.. the location of the circle is critical because it positions the curve. You can't position a single radius curve without having the centerpoint in a specific position, even if it's not what you're intending to use. The centerpoint of a circle IS the centerpoint of any curve of the same radius.

Otherwise, what's the point of having a radius to begin with (points radiating from a single point at a specific distance)?

Trimming it out is exactly the way to do it.

Tell ya' what, try placing it as I explained first, and THEN come back and tell me I'm wrong, or you know, say "wow! Works like a charm! The technology used on that seemed extremely advanced, was that shared with you by aliens visiting from the future?"

And I'll be like.. "Holy crap that dude is smokin' CRACK!"

Seriously.. it works.. I do that kinda' thing a lot.

If the method escapes you, google the actual definition of the term "tangent" and consider how far the centerpoint of an arc/circle is away from a line/arc/whatever that intersects it.

The curve and the "hard corner" are all touched by the new radius, tangentally.

Armed with that info, google the term "swing an arc" for the basic concept of how to use that info to locate the centerpoint of the circle/arc.

Then trim down your circle, mirror it, and your done.

Hmmm, Is this the complete question? what tool or technique are they testing you on? There are a lot of ways to achieve this, but presumably they are loking for a specific result. Without knowing what they are looking for it's difficult to come down on the best method of getting there...

In this example, I've used a 2P circle and TTR circle to make a curve that is tangent to the existing handle geometry...

Perhaps this diagram will help.

Offset the upper arc by 12.5 (red line) , because you know that the upper arc and the infill are to be tangential.

Draw a circle of radius 12.5 (blue circle) at the shoulder point, because you know that the infill touches this point.

Where the red and blue arc intersect is the centre of the infill arc, so move your circle so that its centre point is at the intersection, and trim.

I like it!

And, it's more acurate than my way...

Sorry Pablo but your method left a gap where the curve touched (or didn't touch) the top corner of the shaft.

Mike, erm, sorry mate, no idea what you mean.

eldon, your the Daddy, thank you

Me can feel a distinction coming on!!

Thank you to everyone for your help.

I do forums, I'm a long time member of loads of 'em and believe in giving back what you take. Now I can't help you fellas with CAD, but I make a mean website, have a good understanding of SEO and have been a builder for over 20 years. If you need anything, shout.

All I did was describe what Eldon has now illustrated. Had you actually sat down and tried it, rather than just read it, you'd have learned how to do it with my first post.

The radius of the upper circle, plus the radius of the new circle (with the specified radius) equals distance #1 (which is the distance from centerpoint to centerpoint)

The radius of the new circle equals distance #2 (which is the distance from the corner to the centerpoint).

"swing and arc" at those distances (meaning draw a circle or arc with those distances). One from the centerpoint of the upper circle, and one from the corner.

Where those intersect, you have your centerpoint for your circle.

I was trying to permit you to do it yourself and learn it.

All I did was describe what Eldon has now illustrated. Had you actually sat down and tried it, rather than just read it, you'd have learned how to do it with my first post.

 

The radius of the upper circle, plus the radius of the new circle (with the specified radius) equals distance #1 (which is the distance from centerpoint to centerpoint)

 

The radius of the new circle equals distance #2 (which is the distance from the corner to the centerpoint).

 

"swing and arc" at those distances (meaning draw a circle or arc with those distances). One from the centerpoint of the upper circle, and one from the corner.

 

Where those intersect, you have your centerpoint for your circle.

 

I was trying to permit you to do it yourself and learn it.

I know mate, I'm not knocking you and I appreciated your help and yes I wanna learn it and understand it, I ain't gonna learn nofink by just copying. I simply did not understand your explanation, but that is in no way a reflection on your ability to explain yourself or use CAD.

So are we friends???

Hug??

:D

All I did was describe what Eldon has now illustrated. Had you actually sat down and tried it, rather than just read it, you'd have learned how to do it with my first post.

Mike, sometimes a picture is worth a thousand words. :wink:

Hi footprint; good luck with the C&G.

I would suggest that as well as cad tutorials you should be looking at basic geometry texts.

A good one of these will show you how to use triangles and radii to layout most of the shapes you need draw in engineering. Be careful though because some can be confusing to follow unless you are also doing an algebra course.

I know that you don't fit into this age group, and hope not to cause offence, but I think that for an adult who is not used to draughting constructions then the stuff written for 12-15 year olds is a good place to start.

heh.. No bad blood here..

I just got all excited to have an answer for you, especially in the situation you described, plus following an incorrect answer. It's really cool to be able to offer an insight that was born of experience, rather than actively learned anywhere.

It's a tad frustrating though when you feel like the person didn't really give the info a serious go, and was simply waiting for an answer to be handed to them instead.

That's not the case here, I'm just relating the initial impression I had. I guess it's worse though when the "correct" answer was produced later and apparently unbeknownst to all, it matched what I described.

It's all good though. CAD 64 is right too, a picture is worth a 1000 words. I usually can't download the files into my AntiqueCAD, so I don't usually think in terms of redrawing stuff and taking pics of it.

I gotta' try and remember that not everyone's MindCAD looks like mine.. I described "new circles", "upper circles" etc.. and that's kinda stupid to think someone's gonna' be able to follow that.., lol

When you think about it.. that's the whole point of having AutoCAD in the first place. So I guess it's like walking over to a friend's house to have them come by to see your new car.

Sorry if I got too weird, on you.. what, with alien technology and all that.. lol

.... plus following an incorrect answer. ....

How was my answer incorrect?

I am sorry to cause a commotion, but even my head was reeling from MikeScott's verbiage, and I thought a picture was easier to take in.

How was my answer incorrect?

There is a incorrect assumption, shown in your second diagram, that the infill circle lies on the midpoint of the vertical line that you have drawn.

The attached diagram shows both constructions, and your lines do not pass through the true centre.

Seath, Eldon's right.. The problem lies in the fact that that vertical line you added shouldn't automatically be at 90/270 degrees. It might work by luck in some instances, but it's not something that takes the upper shape into account.

The tangent point (where it hits the upper circle) is a little to the right of the top of your line.. which would put the centerpoint of the required arc a little "off".

That vertical line would have to be calulated, because it has to be on that upper tangent point, wherever it is, or it's in the wrong location... resulting in a bad curve transition.

I have a question though Seath.. once you had that line.. how did you manage to ensure it touched the two points of the circle (in blue on your diagram)? Did you just keep moving it until it hit? I do that kinda' thing sometimes and hate the fact I can't always nail it, and have to approach it from a different route.

It can be all to easy to jump in thinking about what Autocad command you should use, when you should really be thinking about the geometry!

To Foot Print - The construction industry in the UK is short of draughtsmen with trade experience. I wish you all the best!

It's funny how these things come along:

http://cadtips.cadalyst.com/curved-objects/draw-tangent-lines-and-arcs