plywood thickness

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David Werkheiser
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plywood thickness

Postby David Werkheiser » Fri, Aug 31 2007, 10:49AM

For those of you not using CNC to cut your parts, what thickness are you using for ply, 3/4\", 23/32\", or 11/16\". I hope this question hasn't been brought up recently.
Thanks, David Werkheiser

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Mike Bowers
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Postby Mike Bowers » Fri, Aug 31 2007, 11:00AM

I use 3/4 for thickness unless outsourcing.
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Postby Michael Yeargain » Fri, Aug 31 2007, 11:01AM

David,

To simply answer the question, it is a mater of actual thickness. Most import birch I have dealt with is 23/32\".

I use 23/32\" on my domestic birch as well and will continue using that unless a significant change occurs. I have not had any problems outside of human error that would warrant the use of any other measurement.

I would recommend using a micrometer and measuring about 10-15 sheets and make sure the sheet goods you are using is a constant.
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Postby mikesand » Fri, Aug 31 2007, 11:02AM

If I am cutting the job in house (no CNC) I use generic thicknesses (3/4, 1/2, 1/4, 5/8). I do so because when I print the job the part drawings dont show the deminsions in a readily usable way. All the measurements are made from one side and dont always reference the right edge. So I use the Job Print Out as a guide but I figure the dado, rabbit and hole locations on the fly. Because of this I dont need to build in the exact sizes.

If I am going to CNC then it has to be accurate.

Mike

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Postby Mike Seisser » Fri, Aug 31 2007, 4:35PM

I used to use actual dimensions and full dados before Mike Murray started cutting parts for me. After the first job (where I insisted on full dados against Mike's suggestion of blind dados) I quickly changed my mind.

In order to get a perfect fit on your dados, you need to know which sheet will be cut for which part. Mike explained to me that he measures the sheets to be cut and averages them for full dados. (Mentioned in a post above.) It made perfect sense to me as to why: if the dados are the size of the thickest sheet in the stack, then any parts cut from a thinner sheet will result in sloppy dados. If you take the smallest, then parts won't fit together. As it turned out, using the average method wasn't that great either (something Mike warned me about) because about 1/2 the parts didn't fit together, and I ended up either making dados bigger by 1/2 sawblade, or shimming where necessary.

After making the switch to blind dado construction, it's something I haven't had to worry about as the tenons/dados are all cut so precisely that differing thicknesses are no longer an issue.

And I'll say this: I wish I'd started using blind dados much, much sooner. Mike cut parts for the cab in the drawing below for me, and when I first unbundled them and dry fit them I panicked because I couldn't get them apart! (They fit together so well.) There's no confusion as to which way they go together, so even my guys can construct boxes without having to 'think too much'.

Sorry for the rambling about construction methods, but I felt the need to qualify my response to your original question.

Mike
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Postby Robert Glenn » Sat, Sep 01 2007, 9:54AM

David, I use 23/32 when not cutting on my CNC because it represents the closest resonable fraction to the actual size. When cutting on my NC I use an average actual measured size. I have gone to using \"classic\" core plywood for all of my CNC jobs because the thickness is very consistent. The plywood I'm refering to has the MDF outerply just under the veneer.

Mike, It was my understanding that the only difference between a blind dado and full dado was in the length of the dado. I use full dados when building FF cases and blind dados when building full access cases. Like everyone with a CNC we are faced with the \"material thickness\" issue when using dados, I would love to see more detail about what the joint actually looked like on your last project.

Thanks

Rob

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Postby Michael Yeargain » Sat, Sep 01 2007, 4:30PM

Blind dado cuts the actual thickness of the part (tenon) that goes into the dado so the parts would fit much more accurately.

While full dado cuts the dado but the actual thickness of the plywood (which varies) goes into the dado. Leaving a potentially loose fit.
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Postby Robert Glenn » Sun, Sep 02 2007, 1:20PM

Mike,

I think we may be talking apples and oranges here. What you are calling a \"blind dado\" I actually refer to as \"% dado or tenon thickness\". In the software you can have less than 100% thickness in the tenon and slot for either a blind or full dado option. Cutting the tenon to less than 100% thickness will not resolve the material thickness issue. Any movement in the Z axis is relative to the thickness of the material entered when generating the twd file and therefore the accuracy of the fit of the dado.

Rob

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Postby David Werkheiser » Sun, Sep 02 2007, 4:53PM

Thanks to all who replied! I am going to use 23/32\" thickness for 3/4 ply. I guess my question should of been, If you set 3/4\" as a thickness in sheet materials and your plywood arrives less in thickness, do you go into E-cabs and edit every cabinet in existing job?
When I build beaded face-frames, my stiles are 1 1/2\" wide, and I make up one large frame to cover multiple boxes. If ind. cabinets are off by +/- 1/32\", I am concerned stiles will not line up with box sides
Thank you again, David Werkheiser
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Postby Forrest Chapman » Sun, Sep 02 2007, 6:59PM

Hi All,

In responce to everyones comments,

1) A blind dado can be full thickness or any percent thickness but, is most commonly 50% of material and knotched front and back.
2) If you are machining parts you should use blind dados with any small variation of the defaults. Example (50%-.01fit-.02depth knotched 1\" front and 0\" back).
3) Material thickness should be a good average.
4) Plywood thickness is all over the board so don't obsess over it.
5) A blind dado will address the fit of the joint because no matter what the actual thickness the machine is referencing from the table up. It machines the tenon to what it thinks is half the thickness and the dado to fit. There may be some issues however. Example (where a stretcher meets the top edge of an end it may not be flush). This is usually not a problem except for wall bottoms that join an end with edge banded bottoms.

David, for your application you will have to use an average thickness and possibly shim you cabinets apart slightly or wait until you cabs are made to measure and make your frame.

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Postby Michael Yeargain » Mon, Sep 03 2007, 9:59AM

Robert,

I would think (not owning a router myself) that the 100% dado option would make for a great fit, as this would be relative to \"0\" Z depth. I assumed that the router would make a pass over the material to assure the actual 100% was true. Am I wrong?

As Forrest pointed out; the thickness variation would effect the tops (stretchers) and bottoms that are on base and wall cabinets respectively. That variable would be indexed from the inside of the cabinet making it protrude or recessed according to this variation. Correct?
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Postby Forrest Chapman » Mon, Sep 03 2007, 11:49AM

For 100% thickness dado since the software assumes the material is the specified thickness it will do nothing to the tenon only route the dado. This is why it causes problems when the thickness varies.

Mike, you are correct about where the variations show up.

Forrest

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Postby Robert Glenn » Mon, Sep 03 2007, 12:03PM

This forum is such a great resource!! What started out as a simple question turned into a string with some interesting nuggets of information and a potential jem for me. It was my understanding that the router refers to the material thickness and cuts from the top down to establish a less than 100% dada. If it truely refers to the table surface or \"Z = 0\" and comes up then that makes more sense and leads to many other ideas on how to manage the variable material thickness problem.

Great forum!!

Thanks Mike and Forrest for your comments!!

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Postby Rick Palechuk » Mon, Sep 03 2007, 12:04PM

Forrest, when you plow your dadoes do you try to set the width to match the bit size for speed at the machine, or do you look for strength of the joinery.

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Postby Michael Yeargain » Mon, Sep 03 2007, 12:13PM

can you do a 1% tenon width? Would that effect the fit negatively?
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