Cutting tolerances

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Cutting tolerances

Postby MickinOz » Fri Jan 03, 2020 7:18 pm

When I was a pup, I wanted to build a boat. It never came to fruition, in the 70's fibreglass boats had become very plentiful and cheap so marine plywood was not as commonly available as it had been, and the revolutionary epoxy resins weren't yet common either.
But I did a lot of studying.
One of the common threads in all the magazines and plans I read was that <1/8" (3.2 mm) was acceptable tolerance and a hull built to this would still look "fair".
Which brings me to my question.

I've cut my two side walls.
Laying one on top the other (not easy to align accurately) I think there is a difference of maybe 1/12 " (2mm) side to side.
Is this good enough?
I foresee lots of potential catastrophes should I try to machine them closer, and when you think they will be 61" apart, the difference should be impossible to detect.

What does everyone else do? Honest answers only now. Anyone who says theirs are absolutely perfectly matched may get called a skite, unless they have a CNC machine. :lol:
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Re: Cutting tolerances

Postby tony.latham » Fri Jan 03, 2020 7:36 pm

I don’t know what a skite is but why can’t you make them identical with a router and a pattern or trim bit?

When I make a teardrop, the first thing I do is make a full size pattern so each wall is a mirror image.

T


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Re: Cutting tolerances

Postby MickinOz » Fri Jan 03, 2020 7:48 pm

tony.latham wrote:I don’t know what a skite is but why can’t you make them identical with a router and a pattern or trim bit?

When I make a teardrop, the first thing I do is make a full size pattern so each wall is a mirror image.

T


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To call someone a skite is to say, in a jocular fashion, they may be just stretching the truth somewhat.

The truth is, the weather is all over the place and my plywood is swelling and bending and carrying on something fierce.
It was 44C at 5% humidity yesterday when I cut the first sheet, and 21C @ 68% humidity when I cut the second one last night.
So this morning the sheets are a bit buckled, hard to line up, etc.
Hence what's the level of tolerance that is considered acceptable.
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Re: Cutting tolerances

Postby tony.latham » Fri Jan 03, 2020 7:52 pm

It was 44C at 5% humidity yesterday when I cut the first sheet, and 21C @ 68% humidity when I cut the second one last night.


I get the same swings here in Idaho or at least something from 100% to 10%.

Right now, or whenever you read this, both sheats will have the same moisture content. Screw them together and true them up with the router. Or live with the 2mm.

Here's a shot of cutting off the headliner shelf on a stack of two wall skeletons.

Image

They're identical.

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Re: Cutting tolerances

Postby edgeau » Fri Jan 03, 2020 8:30 pm

I did the same thing, use one as a template for the other and a trim bit on a router. That said, there are plenty of other bits that are not quite true or square that I had to live with or work around. If you don't have a trim bit and you are in a remote region I reckon you could probably work with what you have. That expanding polyurethane glue will help fill any resulting gaps when the roof goes on.

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Re: Cutting tolerances

Postby MickinOz » Fri Jan 03, 2020 8:51 pm

Thanks fellas. I do have a trim bit, just consider it somewhat risky, when I have it pretty close, to go fiddling with it.
It's the price of running with what materials you can get.
Structural ply is just not as stable and dimensionally tight as marine ply.
A couple of guys commented on my build thread that I was being a "bit precious" and carrying an "attitude" when I was bemoaning the dearth of good quality materials within travel distance, but this why one should get the best possible materials to start with.
Might do a trim yet.
Going out for another measure up.
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Re: Cutting tolerances

Postby MickinOz » Sat Jan 04, 2020 3:57 am

tony.latham wrote: They're identical.


BTW - Skite! :lol: :whistle:
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Re: Cutting tolerances

Postby swoody126 » Sat Jan 04, 2020 7:12 am

w/ horse shoes, hand grenades and atom bombs...

close counts

and at 50yds from a galloping horse it'll look just fine

come to think about it that's how most folks will ever see it

2mm is tantamount to an RCH

cut to fit and trim it out

you can't see both sides at the same time anyway ;-)

sw
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Re: Cutting tolerances

Postby tony.latham » Sat Jan 04, 2020 11:49 am

BTW - Skite!


I have written a couple of middle-of-the-night novels, so perhaps it's true.

:frightened:

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Re: Cutting tolerances

Postby Squigie » Sat Jan 04, 2020 12:42 pm

This is one of those cases where you should do what you feel is adequate.

But, I have to be the guy that falls back to the old cliche: Do it right, right now, so you don't regret it later.

I know that not everyone has a router, but it should be easy to clamp the two panels together and trim them to match ... with a router.
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Re: Cutting tolerances

Postby MickinOz » Sat Jan 04, 2020 3:27 pm

swoody126 wrote:
and at 50yds from a galloping horse it'll look just fine


sw

Me dear ol' ma would say " No one's going to get off a galloping horse to look at it."
What does RCH stand for?

Edit: never mind, the Urban Dictionary set me straight. :lol:

Squigie wrote:This is one of those cases where you should do what you feel is adequate.

But, I have to be the guy that falls back to the old cliche: Do it right, right now, so you don't regret it later.

I know that not everyone has a router, but it should be easy to clamp the two panels together and trim them to match ... with a router.


I've actually got two of the them.
Yeah, as I lie here in bed on Sunday morning, air conditioners of, listening the rain on the roof, and marvelling that two days ago it was 115F and 5% humidity, I'm leaning toward a little trim.
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Re: Cutting tolerances

Postby MickinOz » Sat Jan 04, 2020 5:05 pm

tony.latham wrote:
BTW - Skite!


I have written a couple of middle-of-the-night novels, so perhaps it's true.

:frightened:

Tony

Cheers mate.
BTW calling someone a skite, even a bloody skite, is by way of a friendly tease, never used to be nasty, just so you know. :beer:
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Re: Cutting tolerances

Postby tony.latham » Sat Jan 04, 2020 5:34 pm

never used to be nasty...


Nor was it taken that way. :beer:

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Re: Cutting tolerances

Postby MickinOz » Sat Jan 04, 2020 9:26 pm

Matchup 5.1.2019.JPG
Matchup 5.1.2019.JPG (84.12 KiB) Viewed 1273 times

I'm glad I put the question here.
The missus said no-one would ever notice, I thought no one would ever notice, but in the end I clamped it all together and trued it all up with the trimmer bit in the router, and touched up a few spots with the palm sander.
They are now identical.
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Re: Cutting tolerances

Postby edgeau » Sun Jan 05, 2020 1:47 am

MickinOz wrote:in the end I clamped it all together and trued it all up with the trimmer bit in the router, and touched up a few spots with the palm sander.
They are now identical.


Skite

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