Help with resizing?

Not precisely a resizing question and I hope this is not an inappropriate use of the forum. I only use Google Sketchup for making project plans and for sharing ideas with others online. Rarely, I have a need that is very basic, but beyond the abilities of sketchup.

I wonder if someone would be so kind as to make an adjustment to the attached dxf file? I don't know, maybe a beginner wouldn't mind doing it for practice?

The file is a 2d image that describes the contour of a concavity at its widest cross section. So with the image on the desktop as it is opened, rotated 360 degrees on its vertical axis a 3d concavity would be described.

Now with the 2d profile intersecting it's 3d concavity across that central axis, keeping the 3d section fixed, bring the 2d plane forward exactly 11/32 of an inch.

If I've described this appropriately, the resulting 2d curvature will be slightly reduced in scale as it intersects a narrower cross section of the concavity.

This is the resized dimension that I need. I can't seem to perform it on Sketchup. If someone could accomplish this and repost the corrected dxf file I'd be very very appreciative.

If this is possible to do in non pro sketchup, perhaps someone could give me some tips? I couldn't even get beyond importing the dxf file. Or if this is an improper help request for this forum then delete this post with my complete understanding.

Thanks so much,

Brad.

Waveguide-TMM-profile.dxf

I'm looking at the profile, and your wording has me concerned. I didn't want to waste time giving you the wrong profile, but I interpreted your post with a possibility of 4 different outcomes.

There's actually a fifth.. more disturbing option, but let's get the easy ones out of the way in case it's all you need.

The attached DXF shows 4 options (with an indication of how it was changed).

Option #1- I moved the lower line upwards 1/32" without changing the sides at all. Then extended that lower line so that it touched the sides again (as it would need to become longer) then trimmed away the side lines that projected past it.

Option #2- I "warped" the sides as I moved that lower line upwards, automatically changing the sides to accomodate the new position of the line. (They are Splines.. accelerated radius bezier curves, there's not much more I can do with them with no data points to calculate them with.)

Option #3- I moved the upper line downwards without affecting the sides, and then trimmed the lines accordingly (like option #1)

Option #4- I moved the upper line downwards and warped the sides as I did it. (like option #2)

I can only assume you didn't want it made into a 3d object in AutoCAD? Your description of the object in 3d seemed only to make us understand what the item was and a description of how it was supposed to change.

Problem was.. it's not all the same language..

ie- "2nd plane" on a concave depression could refer to the plane at the bottom of the hole, or the plane at the surface of the hole. Moving a plane "forwards" would depend upon what angle you were veiwing it from. There was mention of a shallower hole, so I assumed the top or bottom of the hole was supposed to move towards the other.

The other.. "disturbing" Option #5 was to create the 3d shape and then slide either the bottom or top plane sideways (or forwards in 3d.. same thing) so that a round hole would only be symmetrical in two directions. I'm really hoping that's not what you wanted. It's possible, but not something I'm going to mess with.

You mentioned the "resulting 2d" which suggested that you just wanted the 2d, and the rest of the description was stuff that didn't pertain to the actual lines/arcs themselves, except to make a revelation regarding what the item was.

On working with 2d, it seems better to describe the 2d aspects simply, and directly, in laymen terms..

ie- "This 2d profile was drawn 'too thick'; could someone move the lower edge 11/32 of an inch upwards but maintain the existing radius on the sides?"

Waveguide-TMM-profile[mod].dwg

Hopefully a description of the application may help. I'm hoping that the dxf file, if opened in a proper CAD program, will be properly scaled. I attached a pdf with dimensions below just in case.

That dxf file describes the curvature of a 'waveguide' recess on a speaker baffle so that a tweeter (hi freq dome driver in this case) can be acoustically assisted. Here's a pic

The waveguide above is a plastic molded version and the builder/designer also supplied that dxf file in case anyone decided to have a go at turning one on a lathe.

I decided that it might actually be easier and more accurate to make a cutting bit to do the job on a drill press and the results were very promising.

The bit above was made out of scrap steel in only about an hour and

worked very well in MDF.

Bear in mind that this is a drill press running at 650rpm. Of course this is not intended for high speed router use and with a 7/16 shaft, it would be virtually impossible for anyone to fit it into any commonly used router collets anyway.

I'd like to share this idea with anyone else building this design, and a couple of other DIY designs which utilize that plastic waveguide, or even produce other profiles with some ease, but I recognized a minor catch. I am determined to fab something that anyone can construct off the shelf at home depot and without precision tools, mills, lathes, etc. The blade is fashioned precisely on the template of that dxf file I attached. The use of angle iron and a common 7/16 bolt (nominally 1/2 inch) requires the cutting edge to be offset from the axis of rotation by half the diameter of the bolt in addition to the thinkness of the iron. If my math is right that's 7/32 plus 1/8 which is 11/32. That alters the profile of the finished cut. In order to correct the template an intersected plane of the cutout exactly 11/32 forward of the center axis is necessary to draw in order to correct the blade profile for the next version of the bit. CAD seems perfect for this. But it's beyond my skills.

Waveguide100 98.pdf

I don't know anything about the application (in terms of shortening an accoustic cone). I DO know about creating cutting profiles though. I've never seen an offset like that before but I can certainly see the usefulness, and what problems it creates.

I'm still trying to fathom the angle of the cutting edge of the blade and how you're doing any cutting as opposed to merely rubbing the wood into submission with a thin edge of metal.. but that's an issue for your end. If you hadn't provided pictures, I never would have believed it possible.. though I guess the offset of the blade accomodates that.

I think we can kinda' start over a little here and I'll see if I can help you.

You've calculated the distance from the center of the rotational shaft, to the outer most point of the lower flat portion of the blade and and have determined the width of that point needs to decrease, or increase, due to a new shaft size?

Changing the shaft diameter doesn't change anything.. it's spinning and not cutting, only the distance between the blade edge and the centerline of the shaft has any affect on the hole that's created.

The further away the blade is from the shaft centerline, the larger it's path through the wood will be, which means a wider hole.

The same size blade, moved closer to the centerline of the shaft, will result in a smaller hole, even if the blade profile hasn't changed at all.

If you're trying to match numbers at various elevations, you'll find that the distance to the shaft centerline from those points is one of the two critical dimensions to maintain (the other being the height, but that's not affected by your offset rotation).

If you give me the specs you're trying to reach with your hole, I can construct a profile that will hit those marks if you can provide me with the distance from the centerline of the shaft to the backside of the blade when it's mounted. (It won't be perfect, but then we can go back and forth with blade pitches along the blade afterwards to fine-tune your profile.

If that was the blade profile you attached, it doesn't match the pictures you provided. In the picture, one side of the blade (at the narrow point at the tip) is cut back sideways and seems to align with the drill shaft.. yet the DXF shows a symmetrical shape. If your shaft isn't centered end-to-end on the blade, one entire edge is just shaped for looks (and to avoid the edges of the cut hole).

I don't mind doing the calculations and making the profile if you look at my attached PDF and get me what I've requested to make the blade shape.

I need:

#1- The distance to back side of blade from shaft centerline.

#2- The distance from shaft centerline to cutting edge of blade at the closest point (which either matches the diameter of the hole at the smallest point, or I'm lost and perhaps someone else can assist you.)

#3- the resulting hole specs you're trying to hit.

#4- the overall height of the Blade itself (the height of the flange.. I assume you aren't using the full height, or you'll have a resultant ledge.. or maybe you want that.)

If I'm still not getting it, I'll leave you alone in the hopes you can find help from someone else.

Waveguide-TMM-profile[1].pdf

Mike, thanks so much for your assistance. This is essentially a larger version of a common flat blade wood boring bit, and the blade edge angle to the work is the same. Not the most efficient design, but a practical compromise for occasional use. The finish it leaves in MDF is less pretty than what we get from high speed carbide tooling, but a little sealing and sanding with primer cleans that right up.

I'm still not able to describe this sufficiently.

Imagine slicing a whole apple pie in half right down the middle.

looking at its cross section you'd observe the profile of the bottom crust along the bottom and rising to the edge.

Take another identical pie and slice it likewise but one inch to either side of the exact center.

Now looking at this cross section the profle of the bottom crust is scaled somewhat smaller than the first.

I'm suggesting doing the same thing to the "pie" that is described by the internal volume of that waveguide.

Slicing it right down the center reveals the exact contour of that dxf file.

Slicing it 11/32 away from that center, which is the exact distance from that center to the cutting edge face of my fabricated bit, reveals that somewhat smaller profile I'm looking for.

It ought to be a pretty straightforward exercise in CAD for anyone just as soon as I'm able to get the language straight. 11/32 is the only number needed.

Ok, well, like I said.. if my PDF wasn't addressing the issue, then it's best if someone else helps you. I thought it did (and still do) but there's apparently a language thing here. Sorry.

Good luck!

BTW- if you find someone, I recommend letting them tell you how to do it, and simply providing what they ask for, because they'll better understand the limitations of their program, and the benefits of methods you aren't aware of. You can draw anything, but how you get to that point will differ between program abililities, geometry knowledge, and knowledge of the artist. If it's not what you want the first time, determine the problem and try again.

Also, you do not have a blade profile, you have a hole profile which you will attempt to cut by an offset blade. Introducing that earlier into the conversation might help, as people doing free favors aren't likely to want to spend TOO much time trying to decipher geometric descriptions, provide drawing feedback, when the problem/solution is a lot more obvious than that.

Here's a final tip. Ask an AutoCAD user to make the hole in 3d and then give you a 2d section profile (or slice) of the hole that is 11/32 away from the centerline. It'll take a 2010 user less than a minute to do so. I'm on 2004, so it'd take considerably longer, and the questions I asked would still need answering, but most of them are in the hole profile. You're saying I still don't understand, so apparently it'll take even more than that.

It's what you're asking for, and it's what I was stumbling for in my PDF to you, showing how that "blade profile" you provided, would cut as an offset blade, assuming the position of the shaft, labouring under the impression that... well, you get the picture I think (in PDF form, no less).

Again, Good luck!

Here is what I was eventually in Google Sketchup this morning.

The red plane is the new blade profile. It does turn out to be very simple CAD fix. The new blade profile corresponds to an 11/32 shift from the center axis of rotation.

corrected DXF.zip

The terminology did it.. like I said.. it was a language thing, because the drawing you have here, is the same thing I showed you a top view of in my PDF.

I didn't get it RIGHT because you had just told me that your original DXF was a blade profile..

The blade is fashioned precisely on the template of that dxf file I attached.

Your picture directly above it, showed a similarly shaped blade, so I saw no reason to doubt you.

I asked the questions that would have identified and remedied the situation, but you felt it important not to provide that information, apparently, which seemed insulting in the brisk dismissal.

Honestly? At that point I thought your project was doomed if you couldn't see that the distance between your shaft centerline and the knife edge was the whole point. I tried explaining that in the PDF, but when my request for info was denied, I figured it was time to step back and leave.

I'd already said in my previous post, that if I still didn't get it I'd back out to let you work it out with someone else that might be able to help.

I wasn't going to go futher, but then when I got home, I suddenly noticed that in your response to my PDF, you called your DXF the hole profile, and I realized I'd been wasting my time thinking that the alteration of the shaft size to a new diameter, and corresponding change to the blade, was the issue.

The information I had been asking for was almost completely answered by the mere fact that it was the hole drawing, not the blade drawing. The rest was answered by your 11/32 measurement.

I came back and edited my post to explain the "real question" and couldn't help but share a little prior frustration with you at that point.

Anyways.., it was just the frustration of miscommunication on our parts, luckily you're all set now. I apologize for the tone I used.

Mike I'm really sorry. Anyone who takes time out of the day to give help to others ought to be congratulated and cut more slack than I've given. I was just very frustrated by this. I have a short fuse sometimes.

Thank you for the suggestions. I'll calm down.

Mike I'm really sorry. Anyone who takes time out of the day to give help to others ought to be congratulated and cut more slack than I've given. I was just very frustrated by this. I have a short fuse sometimes.

 

Thank you for the suggestions. I'll calm down.

What are you apologizing for? Long winded miss-information!? It's not your fault.

KC

Misunderstandings happen.. I think he was being polite, KC, which I appreciate, but yeah, that was my bad not his.