'proper' CAD drawing & inaccurate scanspeak datasheet?

[sorue]

hey guys, i plan to get my front baffle CNCed by a local shop. But because i don't know any CAD at all, i had to learn from scratch. The shop takes dxf files and this is the sort of template they need (not mine but another DIYer's files).
Required template:

http://www.mediafire.com/imageview.php?quickkey=ubi5mx7tvikk9ev&thumb=4

http://www.mediafire.com/?pt1as63h2a9ah0y

I'm not sure what kind of CNC router they're using, but if i understand the lady correctly, it seems like the dxf has all the measurements and dimensions already embedded inside?

Well, i didn't get the sample template at first and i didn't know what was needed so i ended up doing a 3d dxf, which the lady told me was utterly useless (yea i'm a total noob at this).

I'm using solidworks. I'm wondering if i can simply "Make drawing from part", and then the resulting 2D dxf will have all the required measurements already embedded, or are there more steps to be done, such as in this way?

The other problem i have is with regards to the alignment of the screw holes. The d3004/6600 datasheet lists the pitch diameter as 90mm. From the drawing, it seems like 90mm is the distance between the centre of the screw holes.

But I measured the unit i have and it seems more like the distance between the outer screw head edges. Am i measuring the wrong thing and what is the proper way of dialing in the distance between the center of two opposite screw heads?

The tweeter comes with an in-built gasket. The depth of the flange is 6mm, gasket included. If i have a 6mm depth routed out, after i screw in the tweeter, wouldn't it be too shallow? How much compensation is necessary to get a perfect flush fit?

As for the tweeter's outer diameter, if my unit measures 104mm, how much larger should the routed hole be? 104.5mm?

Finally, is it okay to use #8-32 screws with #8-32 hurricane/t-nuts for mounting both the tweeter and woofer? Or would it be better to get bigger screws for the woofer?

I hope you made it this far. Thanks for bearing with me! It's my first diy project so i'm trying to do everything to the best of my ability.

[Bill Schneider]

I can't answer the questions about CAD, so someone else will have to jump in for those.

I can help with the other topics though.

First, I don't trust all driver data sheets for complete accuracy. I've followed them before for cutout diameters, and found that the driver was sized too large to fit. Since that time, I have always measured my own and sized baffles cutouts from my own measurements.

To get perfect mounting hole locations, I use transfer punches from Lee Valley tools. They cost only $15 per set, and they provide peace of mind when marking driver mounting hole locations. Merely shim your driver to the center of the cutout, and tap through the mounting holes (see picture). This works if you drill your own holes - preferably in a drill press for good alignment - after the hard work is done on the CNC.



Regarding screw sizes - I try to use the largest screw that will fit in the driver recess. That often means mixing screw sizes on a baffle. That's my personal preference, but if a #8 works without mechanical problems, why not?

If you have gasket material on a driver, you must take into account gasket compression to have it flush mount nicely. I generally use the wider part of the blades on a pair of calipers and squeeze the gasket to simulate its compressed thickness. It takes a fair amount of pressure on calipers to squeeze gaskets like screws can, so put some ooomph in the squeezing. That's never failed me.

About clearances for flange diameters - a half millimeter is only 0.020 inch. That's only 0.010" per side (roughly the thickness of two sheets of paper). If you paint your baffles, that will be consumed quickly and you may remove/chip paint from the sides trying to fit the tweeter. If you double the clearance to 1mm, or maybe slightly less, you'll still have a very tight fit after painting.

[Bear]

My advice: talk to the shop and ask. They may have quoted you a price that assumed the file they would get would be CAM-ready with minimal editing needed. I'm not sure how much you are trying to have cut on a CNC, but if it is a lot of machining (e.g., recesses, dadoes, etc.), then it will get expensive if a technician has to re-draw the part on their end. If you are doing something simple like a driver cutout and rebate, then they may just need the dimensions specified using something as simple as a call-out so they have a firm grasp of sizes and reference points (many high-end machines have default circle cutting programs where you just enter a few coordinates and the machine does the rest; others will need a full CAD file to run through CAM software to compute tooling and tool paths).

[Wheels]

I can't answer all your questions, but here's what i know... according to the datasheet, the 90mm pitch is supposed to be the distance from center to center of opposite screw holes. i wouldn't think they would miss the measurment that far, but i have seen crazier things. what you could do is print a full scale picture of the front baffle and put the speaker on it to see how everything lines up.. you might have to cutout some paper and lay it one something so you wont damage the dome, but that is what I would try if you are at all skeptical. But if it were me, I would drill those by hand when you get the baffle back. You aren't going to mass produce them so the extra 10 mins it takes isn't going to cost you much. And you would know they are all going to line up.

On a side note before I forget... Please Please Please make sure your units are the same. It seems odd you are talking metric around a machine shop in North America. I know of horror stories where professionals in the business were getting bids on parts and one company came in at about a 10th of the price. they ordered 1000 parts from them not knowing they were building everything in milimeters, and they wanted inches. Just throwing that out. Also, most machine shops I've been around work in "mils" 1 mil is .001 inches (one thousandth of an inch) or .0254 mm. 30 mils = .03 inches = .762 mm. Just so your not confused if they throw that out there.

You need to know what the tolerance they can build parts to. Since they are using a CNC mill, it is not guaranteed that the hole will be X diameter. It will be X +/- some number. It should be less than 5 mils, (.005" or .127 mm) if they are at least a respectable machine shop. The number should be small enough to not matter, but it is worth checking to make sure. Also you should ask if it is a fixed tolerance or if it is dimension dependant. ie: where I work the number of decimal places given with the diminsion defines the tolerance. I know the measurements you are doing are defined by the .dxf file, which makes me assume it would be a fixed tolerance, but I don't know if it contains any diminsion information from the way you modeled the part. basically, 2 does not equal 2.0 and does not equal 2.000. I don't know if the .dxf will contain that detailed of information. I've just not been around it. I would probably ask someone at a solidworks forum that to make sure.

I would add at least .04" (1.01 mm) to the outter diameter. You can get away with less, but you aren't trying to make a compression fitting and you really don't need the tighter tolerancing. It will just make it more difficult to drop the tweeter in and you aren't going to notice a .5 mm gap on either side of the tweeter, and it give you more room for error in measuring it. If the gap bothers you, put a fine bead of caulking around it and paint right up to the tweeter. Then it will look like it grew in place.

For flush mounting it, the easiest way to make sure will be to mount the tweeter to a scrap board and measure how tall it is. We don't know what material the gasket is (I assume rubber, but I have seen foam before), or how thick it is. And even at that we don't know the density of the material or how much you are going to torque the screws, and even if we did, it would be faster and more accurate if you just measured it once it was mounted to something. If you have no way to mount it and measure it, I would guess (and I stress the word guess) you could compress the gasket half its height and get a good seal. So you would need the diminsion of the thickness of the speaker plus half the gasket height for the routing depth.

As for Solidworks to .DXF, I can't help you there. We don't use SolidWorks (I wish we did) and where I work we model the parts and the guy running the machine building it extracts what he needs for the CNC from the model file. I really think you would get better help on that from a Solidworks forum. Sorry, I wish I could be of more help there.

[Wheels]

Bill had a good point about paint. I would mask off the recessed area so no paint would mess up the tolerancing. Like he said, It will eat up the gap fast and possibly chip it when you put in the tweeter.

[NateTTU]

^^ Not bad for an electrical (wheels)...

[Wheels]

Nate, Don't forget I started on the mechanical side of life, then saw the light and went electrical

[lhwidget]

Hi,
The only problem you may run into generating dxf files from drawings done with Solid Works comes from how 3D packages generate drawings.

AutoCAD and most 2D drafting packages will scale the border to fit the image you are trying to plot, then scale output to the plotter to fit the paper. You can also plot an image at an exact 1:1 ratio (great for templates and screw ctr positions).

3D packages like Solid Works will scale the image to fit the paper. This scales the resulting dxf image also, and will result in a scaled final product.

The DXFs you posted are a great example. I opened the SampleFile.dxf and drew a 6 unit circle, then dimensioned the circle. This drawing is not scaled, it is full size. This DXF file would produce correct results if fed to a CNC cutter.

The draft3.DXF file will produce a CNC shape 1/8 the size you want. The output from the model was scaled to 1/8 its actual size to fit the A sheet template you used to generate the drawing. Next to your drawing, I drew a 6 unit line and 6 unit circle. Note the scale factor in the primary units dialog for this file, SolidWorks did that when it scaled to drawing to fit the A sheet border.

The confusion regarding metric or inch units is valid. The are two things to remember:
1) You make the drawing full scale (thus the resulting DXF file will be full scale also)
2) You then define what units the drawing was executed in.

The odd nomenclature I used above (6 unit line) is actually very accurate. You may be thinking in inches when you draw a 6 unit line, but your software won't know, nor will it care if you are drawing in mm, miles, parsecs or inches until you tell it so. Then it will assign the correct label to the units you used to draw the image and base the the necessary scaling calculations on the units you have chosen.

It's up to you to make sure the CNC programmer knows you drew the file in inches or millimeters. Either way, he should be able to handle it.

EDIT:
I forgot to mention it. The DXF file contains commands to: Go to this XY coordinate, place pen down (write, or start a line), then go to a new XY coordinate, then pen up. There may be a formula telling the plotter to travel in an arc, a circle, or a more complex 2D curve from the start to the finish points.

The CNC machine uses this data to position the cutter, then move it as the commands dictate.

A cad program uses the same data to draw lines on your screen.