Replies to This Discussion

Permalink Reply by Michael T. on October 15, 2017 at 3:16pm

I think your the right guy to ask this question, when measuring on the plans. Are the measurements from the center of one line to the center of the other line?
I noticed measurements between drill holes is the center of one hole to the other.
But with parts are the dimensions from the outside of the lines edge or the center of the line?

Since I'm scratch building im big into measuring...lol

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Permalink Reply by Greg Harris on October 15, 2017 at 3:30pm

Again, I can't speak specifically to the CH640. If I understand your question correctly, they give you the dimensions of the overall part, including the thickness of the material, from outside edge to outside edge. I would highly suggest you don't measure anything on the blueprints themselves and then try to scale up. You will end up with errors that way. You are correct that rivet spacing and edge distance is measured from the center if the rivet hole. I'm not quite sure I understand your question, though.

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Permalink Reply by Michael T. on October 15, 2017 at 3:54pm

I've not seen the 750 plans, do they have scale drawings on them? on the 640 there are no scale drawings.

So the measurement of parts on the plans is to the outside edge. I was just wondering since the measurements from drill holes was from the center if of the lines...good to know

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Permalink Reply by Greg Harris on October 15, 2017 at 4:27pm

The way I understand it, Zenith blueprints are specifically NOT to scale, so that you don't try to measure the drawings and then scale up to make a part. I've built all of my parts using their printed dimensions, and with the 750, there is absolutely no reason to try to measure the drawing itself and then scale up. The 750 plans are definitely not to scale.

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Permalink Reply by Michael T. on October 15, 2017 at 5:42pm

I somewhat agree with You, but the question needs to be asked. Why is the ch640 not more popular?

I own a sonex plans set, that set is triple the size of the zenith set and loaded with scale drawings. I also own a bearhawk set and it's got of full scale parts..ready to photo copy and knock out.

The 640 plans set is not as friendly...still very good. Just not as cookie cuter as others

I have no complaints about it. Just observations

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Permalink Reply by Greg Harris on October 15, 2017 at 6:16pm

I have no idea why the 640 isn't more popular, either as a kit or scratch-built. It may be just because the homebuilder segment is obsessed with mostly 2-place airplanes, and you can find used Cherokees and 172s all day long that compete with the 640. I would guess that Zenair doesn't sell enough 640 kits or plans to justify the expense of significantly updating the blueprints...just speculation. It is my understanding that scale drawings lead to production errors. Different printing presses, copiers, printers, etc., produce different scale errors in the copying process. Therefore, if the drawings were perfectly to scale when printed by one vendor, they may be off by 1-2% when printed by another vendor, but there would be no way to know. And then there would be still more error when you photocopy them. Unless there is clearly missing dimensions and/or information (in such cases, a call to Zenith/Zenair is prudent), I can't see any reason to directly measure the drawings and then try to scale up. Some designs do a full-scale wing rib template for wood designs, but you still have to make sure the scale of the drawing perfectly matches your measuring tools, or you end up with errors.

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Permalink Reply by Michael T. on October 20, 2017 at 6:35am

I wanted to get your thoughts on metal working in cold weather. Is there any concerns about shaping metal during the winter months in an un-heated garage? I did a quick google search and didnt find much.

Forming ribs, bending brake usage...etc?

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Permalink Reply by Greg Harris on October 20, 2017 at 6:56am

That's a tough one. I formed a lot of my parts at different times of the year, sometimes when the metal was cold, sometimes warm. I heat my garage, but only when I'm working in it, so a lot of the time I'm working with metal that hasn't had a chance to warm up. I have not noticed any mis-matching between parts made in colder weather vs. warmer weather upon assembly. Where I suspect you may notice it, if at all, would be on the larger structures like the wings and fuselage, where you might get some very minor "oil canning" across some unsupported skin areas with changes in the season.The expansion and contraction is slight, though, in the temperature swings we're worried about. As long as you match-drill your parts when they are the same temperature, you really shouldn't have a problem. But I wouldn't worry too much about it unless your temperature ranges are really severe. And even then, it should be very a very minor issue.

There may be some other folks with different experiences on the forum here. I'm in mid-Michigan, where we have pretty moderate temps most of the time. One of the well-known concepts in airplane building, though, is that you don't need a controlled climate like with composite or wood construction. I really wouldn't worry too much about it.

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