Welcome to todays world, Bryan.

Ply is getting thinner each year as most is now a metric thickness.

There are special straight cutters that you can buy just for plywood.....

 

I've heard that plywood starts out at the right thickness and then it gets sanded. The easiest way to get a perfect fit is a jig that James (jw2170) has posted before and cut with a smaller bit to both edges. Otis (OPG3) has also posted one that works the same way. It is the only sure fire way to get a perfect fit with today's plywood. Check out James' and Otis' uploads to have a look at them.

 

Check this video out.
139 – Exact-Width Dado Jig | The Wood Whisperer

 

Grizzly has a set of 3 bits, that work, sized for the most popular plywood.

Baker

 

Hello Bryan,
I have worked in both plywood and waferboard mills for many years and was surprised at your comment that 3/4" plywood is actually 23/32". We always strived to make 3/4" an actual 3/4" but in reality there are many factors that can contribute to tolerances that can be quite varied such as accuracy of the veneer thickness being peeled, along with the mix of species glued up for the inner core. We used poplar and spruce either together or separetly which requires different pressing pressures and heat. There are many more variables which can also vary from working shift to a different working shift.
I would suggest measuring the thickness of each board before using. Good luck.

 

Bryan, I know this is a router forum but that occasionally leads us to other areas. If you have a stacking dado blade this might help with tweaking. When using mine to cut dados or grooves I lay the stock on the bed of my table saw (cast iron) and stack the blades and chippers beside the stock to get the width to match. Stacking dados usually have shims which can be used to tweak the final dimension. Once I know what I need I mount the pieces in the saw and do a test cut on scrap stock. If I need to add or remove a shim it's fairly easy to do.

Peter

 

Plywood is not true 3/4", neither 23/32", it is only an aprox measurement. If you want a perfect fit using your router, make the first cut with the 3/4 ply bit and then measure with a caliper, if it is still too tight then make a second pass removing just a hairline.

Easy enough with a straight guide and shims.

 

It seem like no two pieces of plywood are the same so here is a jig.( one of many different design but you get the idea)
http://m.youtube.com/watch?autoplay=1&v=cn_66sOqVUs

 

Dado Jig for Any Thickness of Material

Here is my version of a dado jig. All the clamps are built in, moving it only requires turning one of them (after initial set up) and it relies on the material being worked for set ups, so different manufacturing thickness is not an issue with it either:

http://www.routerforums.com/jigs-fixtures/44539-kellys-new-router-jig.html

To use it, I use, for example, a half inch cutter head to make a 3/4" cut, a 1/4" for a 3/8" cut and so forth.

 

Byran

I feel your pain and we have all been there. I have purchased from CMT the plywood set that has the router bits sized to fit todays plywood. That being said not all plywood is the same size.

I suggest two options for you to get good fitting dados: 1) make a jig as posted to use a router. 2) I prefer personally to use the tablesaw because I have not gotten around to making to jig. When using the tablesaw I use a dado stack set from Dimar. The set comes with shims which are color coded. I use a caliper and measure the plywood first and write down the measurement. Next I set up the dado stack and refer to a cheat note for the size based on (2) blades c/w chippers. I intentionally cut my first dado larger and insert the plywood into the groove. With the plywood inserted in the groove (the joint will be loose) I then grab the shims and insert the shims into the groove until I have a tight fit. This tells me how many shims I need to remove to achieve a tight fit. You can also use the caliper and do a bit of math to calculate within a few thousands to get the same results.

Play around with it and you will find out what works best for you.

 

Byran

2) I prefer personally to use the tablesaw because I have not gotten around to making to jig. When using the tablesaw I use a dado stack set from Dimar. The set comes with shims which are color coded. I use a caliper and measure the plywood first and write down the measurement. Next I set up the dado stack and refer to a cheat note for the size based on (2) blades c/w chippers. I intentionally cut my first dado larger and insert the plywood into the groove. With the plywood inserted in the groove (the joint will be loose) I then grab the shims and insert the shims into the groove until I have a tight fit. This tells me how many shims I need to remove to achieve a tight fit. You can also use the caliper and do a bit of math to calculate within a few thousands to get the same results.

Play around with it and you will find out what works best for you.

I like that idea--even though i keep meaning to build the router jig too!!

earl

 

Bryan I have said before that I don't run a dado type groove to be the exact thickness of the inserted panel, I run the groove at only 60% of the thickness or whatever cutter is closest and then I rebate the underside of the insert, what this does it that it gives a perfect fit and it also give you a clean shoulder the set the width from, you can set the width by having an exact depth to the dado and an exact length to the insert, however I always find that glue in the dado does play a part in making this harder so with the rebated shoulder then there is room under the insert for glue to settle in, I don't suggest a big space, if I go on say 6mm then I run the dado at 1/4 or 6.5mm and I cut the shoulder at 6mm and this leave a .5 space, just a little bit under the insert, once you change to doing this then you will do it all the time as it make the insert fit perfectly, every time. N

 

That's an interesting approach, Neville. Thanks very much.

 

Hardwood plywood is usually the correct dimensions and my Whiteside bits usually work. However as Fred alluded to it depends on the mill producing the product. The imported stuff is all over the dimension spectrum. I have a few test blocks made up and can easily check the size before I buy the plywood. If it is too far out I look elsewhere. If the project is important I stay away from the big box stores and buy from a local lumber retailer. (in TN) cost a little more but I'd rather support the the mom & pop businesses.

 

Thanks Peter. I'm looking for an excuse to get a nice stacking dado - this might lead me there. But for this project, I'm cutting dadoes in a carcase that's 20 inches by 70 inches - pushing something that wide through my table saw might be a hairy experience.

 

Thanks Mike and John for the pointers to the dado jigs. It's too late for this project, but next time ...