Help please - stopped grooves in SketchUp?

[Eric The Viking]

This isn't a brick-bat - I probably chose the wrong material to demonstrate it.

It's supposed to be a stopped groove in a block of material. I'm trying to make ones like this with both ends stopped, but I've been bumping into SketchUp geometry problems, which are making it very hard.

Stopped grooves must get used in classical architecture a fair bit (fluted columns, etc.), so I was wondering if anyone had a clever (quick-ish) way to make them.

I've tried making spheres (and hemispheres), and using Follow Me to get the right shapes, but I keep falling foul of the design issue in SU that it approximates circles with polygons. So, I make a flat semicircle (in the 'ground' plane), but Follow Me on a quadrant coming vertically off the circle's edge (actually down, along and back up again) doesn't produce what I'd hoped. Here's the way I've been trying it:

The grey semicircle ought to "Follow Me" along the blue line, round the quadrant, along the straight and back up again, giving the shape outlined in black (the open semicircle end (right) shows where the lathe should finish).

Note: when doing this exercise, I deleted the black lines above, leaving only the grey semicircle (the source profile) and the blue Follow Me path. All the lines created come from the Follow Me tool.

I think the Follow Me tool sees the two quadrants on its lathe line as a group of straight lines (part of a polygon), and it lathes the circle along them, rather than round a true arc. So the last bit isn't a tangent to the circle - you get a face pointing the wrong way, which in turn means you can't extrude it. Here's what happens:

The two faces I've put texture on seem to be created by Follow Me. The one on the left _isn't_ the original semicircle, and they aren't coplanar, nor aligned with the axes (you can see the red axis cuts through on a slant.
If I turn on "show hidden geometry" and look closely at one end, you can see the weirdness clearly:

That was with setting the number of lines in each quadrant to 48. It looks like a barge boat if it's left at the default of 6. It's almost as if the start and end of each quadrant is wrongly calculated.

It's hair-tearing at the moment, but there must be a simple way...

Heeelp, please!

E.

 

[Brentingby]

Follow Me requires that the first segment in the path be perpendicular to the profile shape. If it isn't, the profile is first projected, not rotated, to perpendicular. The same thing happens at the other end. Look through the design click build blog at finewoodworking.com. Dave Richards has done a few different posts to show how to deal with it.

By the way, increasing the number of sides in the arcs does nothing to take care of the problem. It will reduce the angle between the profile and its projection.

 

[Brentingby]

There's a post he did back in 2009 about adding details that shows the principle. There are probably others.
http://www.finewoodworking.com/item/174 ... g-details/

 

[Graham Orm]

What about creating a ball of the right diameter and sitting it in the item then intersect faces then delete the lines that you don't want.

 

[Eric The Viking]

@Brentingby: Thanks - that exactly explains the problem! I'll go search for Dave's stuff.
@Graham: Yup - tried that first as being easiest. The problem is keeping the cylindrical surface, or the spherical one.

Sorry - being a bit curt, but couldn't resist looking to see if anyone had replied, but am now leaving a trail of plaster dust over the keyboard - back to the DIY temporarily...

 

[Brentingby]

Graham's ball idea could work, too.

I used the thing I tried to describe before of making sure the profile shape is oriented perpendicular to the path. I made a ball which I then cut on its equator. After selecting the right half of the hemisphere, I used the Move tool to stretch the hemisphere into a stopped groove.

 

[Eric The Viking]

Thanks Brentingby.

Following your prompt, I've just watched Dave Richards making a bin (drawer) pull, which is a flattened half dome.

Now you've enlightened me about the need for the path to be perpendicular where it intersects the template, it all makes sense. Dave's trick is to use a circle and rotate it ever so slightly about its centre, so that the intersection point is on the centre point of a side rather than a vertex of the polygon that substitutes for a circle. That gives you the perpendicular intersection automatically. when you've done that you can chop the circle up into quadrants and delete three of them.

I really appreciate the reply - I'd never have worked it out as there's nothing in the Follow Me help that I've come across so far (not saying it isn't there somewhere, mind).

E.

PS: Dave R has a nifty plugin for rounding over edges. I may very cautiously try getting the Ruby interpreter going later next week, when I'm feeling brave (running under Wine still).

 

[Brentingby]

I used his rotation method for my example, too. You can see that the faces around the top edge of the hemisphere and slot are narrower than the others.

 

[siggy_7]

You could make the object in your post then copy and reflect it - would work well for objects symmetrical about their centre.

 

[Eric The Viking]

@siggy: Yup, quite agree. The big issue was all the weird artefacts from the Follow Me operation.

Basically I got hung up on trying to model some hardwood trim strips for our new built-ins (with the chimney breast between them each has a tall projecting corner to make 'tidy' ). We've got some downstairs (lounge bookcases) that are simply decorated with three stopped grooves and a roundel at top & bottom. They look simple yet elegant and it would be nice to pick up the same theme.

I've learned a lot about "SketchUp foibles" in the last 36 hours! That includes Dave Richards' thing about working on large models and scaling at the end (if I'd done that in the first place, I might have twigged what was happening).

Also smoothing. On the basis of that, I'm going to have another go at the room model itself, too. Originally I skipped bothering to detail the skirtings, picture rail, windows, etc. as although I generated the long boards with Follow Me, there were too many construction lines, so at a distance the details went 'black'. It's overdue for a re-visit.

Cheers,

E.

PS: just had a go, using the tricks discussed above. Result:

This was done by...

1. creating a follow-me line thus:

• drawing a vertical "circle," 10" radius, using the standard 24 sided polygon.
• rotating it about its centre by 7.5deg. so that there's a face at the 12, 3, 6, and 9 o'clock positions.
• chopping it into a filled quadrant (9 o'clock round to 6 o'clock). Aligning 9 o'clock with the model origin, coz I'm lazy.
• scaling it very slightly, so that the polygon faces at each end of the quadrant fitted to the origin and to a 10" down, red-axis-parallel guide (probably could omit this step).
• making a component of it (needs more than one object, hence a filled quadrant); copying the component to the other end of the groove and flipping it; deleting unwanted geometry.
• exploding it, joining the ends together to make an elongated "U" shape, making a new component, and hiding it (temporarily)

2. Drawing a horizontal circle, 10" radius, on the red axis, touching the origin, rotating that by 7.5deg. and slicing into a semicircle.

3. Paste-in-place a copy of my Follow Me line, hide the original again and explode the copy (to get the right context).

4. Do the Follow Me operation! swap inside for outside faces & delete unwanted geometry (leaves a "D" at both ends and the Follow Me lathe line). remove top face. Select All & make another component of everything. Hide component.

5. Finally, draw the solid object to be 'grooved'. Copy+paste-in-place the groove (hide the original again!). explode the groove and immediately intersect it with the solid.

Job done, and it looks pretty clean, I think.

Thanks everyone!

 

[Brentingby]

Good work. I think you have a few unneeded steps in there, though. Still, you got it sorted.

 

[Eric The Viking]

Yup.

Some "insurance" moves in all the components, I guess. Will get more confidence with practice...

 

[Graham Orm]

Lost me....off to do some homework.

 

[Brentingby]

From what I've gleaned from Dave over the years:

1. Draw circle. Rotate so there are sides parallel to axes.
2. Draw and quarter the circle. Delete unneeded bits. Copy to opposite end of groove. Use Flip Along to mirror copy.
3. Delete radius edges. Connect remaining arcs with a line. This is the path for Follow Me.
4. Draw semi-circle for one end of the stopped groove. It does not need to touch the path edges but it needs to be lined up correctly.
5. Select path. Get Follow Me tool. Click on semi-circle. Leave back faces showing or reverse faces so they are showing.
6. Move groove into place on board. Delete faces over groove.

Make the initial groove any length you wish. You can change the length easily by turning on Hidden Geometry. Then select the edges at one end and use the Move tool to adjust the length.

 

[Shultzy]

I've just tried this and it seems to work quite well
Draw the "lozenge" shape - 2 half circles at the end of a rectangle
Draw a square underneath at right angles to, and equal to, half the width
Draw a quadrant in the square and delete spare lines
Select the perimeter of the "lozenge"
Select "FollowMe" and click on quadrant.
Delete spare lines and surfaces as necessary.

 

[Eric The Viking]

Shultzy - I think you're winning so far for easiness!

I always tend to be too reductionist - break a problem down too much then get hung up on details.

Of course I want a stopped groove, not to chop up spheres, etc., but you don't have to come at it from that direction, as you've just proved. Your method neatly side-steps the 'approximate geometry' issues, as they don't crop up if you start Follow Me in the middle of a straight line. I think you need to swap inside and outside faces at some point, but that's no big deal.

This all has turned out to be really interesting!

Really appreciative,

E.

PS: I'd been dreading later in the month, when my eval. copy of SU Pro expires and drops down to SU Make. I can't justify a full licence. All this is very confidence-boosting though, as the biggest missing piece would be the set of Solid Tools, but it looks like you can do pretty much everything without them, just thinking it through and taking a few more steps.

 

[Brentingby]

Good for you Shultzy. That's brilliant!

I can think of cases in which that method wouldn't work correctly so it's good to know more than one method.

 

[Graham Orm]

I've just tried this and it seems to work quite well
Draw the "lozenge" shape - 2 half circles at the end of a rectangle
Draw a square underneath at right angles to, and equal to, half the width
Draw a quadrant in the square and delete spare lines
Select the perimeter of the "lozenge"
Select "FollowMe" and click on quadrant.
Delete spare lines and surfaces as necessary.

BRILLIANT! Even I got that one to work! Well done Shultzy.

 

[Brentingby]

Graham, did you try the other way?

 

[Graham Orm]

Yes I kept getting a raised bit as if it was tilted back.