Working with 18mm Plywood

[icecactus]

When you work with 18mm baltic birch and all your equipment (besides festool) is imperial how do you accurately account for it?

18 mm = 0.708661
23/32 = 18.26mm or .26mm or 1/96" diffference
22/32 (11/16) = 17.46mm or .54mm difference

Do you just design based on it being close enough at 23/32? 1/96" is def close enough but do you find the error multiplies through out the project?

If I could switch to all metric I would in a heart beat since I am comfortable in both. Problem is, i have hundreds of dollars worth or incra/woodpecker rules in imperial and all my machinery is in imperial too.

 

[Sparktrician]

If I could get every one of my cuts within 1/96", I'd be in tall cotton.  Realistically, that's for other folks with more skills than I, so I don't worry about it.  With 18mm plywood, if I'm routing a dado, that fits as close to perfectly as I can imagine while giving a smidgen for expansion & contraction.  I call it good to go.

 

[sancho57]

I don't think you'll have a problem if you use a conversion Imperial-MM. Personally I don't find using 18mm ply all that difficult even when using all imperial.

 

[Acrobat]

Are you talking of a quarter of a millimetre.? Half I understand but ¼? Fit could be sloppy at ½ but really ¼ isn't a deal breaker unless your fitting something that has to be exact. I think just using whatever material you have and cutting to fit is fine.
You should ok for most jobs but try not mix 17 and 18 thicknesses together, that could cause a problem.

 

[Kev]

One way is to make your gross measurements in imperial and work with an 18mm spacer (measure out, then measure back)

Errors can have a habit of multiplying!

I actually feel it's easier to compensate for an imperial measurement when working in metric than the other way round!

 

[teocaf]

[member=23330]icecactus[/member]  you don't mention what design situations are giving you trouble, but you can use relational measurements instead of getting hung up on absolute measurements. 
for things like kerfs for any kind of plywood, etc, take a look at this little gadget from BCTW, the Kerfmaker, and get on the list for the next production run--you'll find it extremely useful in your shop:

 

[sancho57]

To the OP,

Re reading your post, I don't see where the issue is.  If ya cut a piece of ply 21 3/4 long by 47 1/4 wide, does it matter if it's 18 mm thk?

If ya a going to route a grove / dado, ya get a plywood set of bits. they are made to route 18mm grooves/ dados.

I honestly don't see a problem.

 

[icecactus]

I haven't run into an issue yet. I started to design some boxes in sketchup, and realized it's a pain to work in 32nds vs just using 3/4 like when I make something out of MDF. Once you start using 32nds in a project, it seems every measurement now is a 32nd.

It got me thinking what the correct imperial conversion would be or if folks just use 3/4". I would think it would be 23/32nds since its the closest and 3/4" is quite a bit off. But I figured is would ask since I am a newb.  [smile]

 

[ear3]

+1 on what [member=2205]teocaf[/member] said.  It doesn't matter if my shelf is 13 1/2" long, or 13 33/64ths, just as long as I'm able to reproduce this length down the line.  Whenever possible, try to do batch cutting and employ stops.  Keep a few setup blocks around to factor in +/-18mm or 2x18mm, or 2x18mm - 1/2" if you're routing dadoes in a shelving unit, etc.  Cut proud and trim flush.

My woodworking became a whole lot better when I gave up the idea of perfect measurements.  It was a bit uncomfortable at first, since I'd invested many years in becoming able to measure and cut down to at least the 1/64th.  I can still do it if I have to.  But I try not to obsess any more about pre-calculating everything if I don't have to, and instead, let the dimensions emerge from the piece as it's being built. 

   

I haven't run into an issue yet. I started to design some boxes in sketchup, and realized it's a pain to work in 32nds vs just using 3/4 like when I make something out of MDF. Once you start using 32nds in a project, it seems every measurement now is a 32nd.

It got me thinking what the correct imperial conversion would be or if folks just use 3/4". I would think it would be 23/32nds since its the closest and 3/4" is quite a bit off. But I figured is would ask since I am a newb.  [smile]