# When Is 1/4" Not 1/4"?



## patlaw (Jan 4, 2010)

Please explain why 1/4" plywood does not fit into a dado cut with a 1/4" router bit. I had to move the fence and rout twice to get the bottom to fit into the dado. Obviously the plywood is to thick, or the 1/4" dado is too narrow. The difference is too small for me to see. Which is more likely? 

It's time consuming to have to cut the dado three times (shallow cut, full-depth cut, widening cut). The plywood came from Lowe's. There's nothing special about it.


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## Brendoniuss (Feb 3, 2016)

I think it has to do with different tolerances between sheet goods manufacturers and the finishing they go through? Sanded vs unsanded. Just a guess..

A set of calipers or a micrometre would allow you to determine the exact thickness and choose an appropriate size but I've only ever made multiple passes. More work but a custom fit each time. Good luck! 

Sent from my SM-N910W8 using Tapatalk


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## DaninVan (Jan 1, 2012)

What kind of plywood is it, Mike?
Sanded one side Fir sheathing?
Not being critical, just looking for more info. I have no problem using sanded sheathing for shop stuff myself.
Can't get decent Hardwood plywood up here on the Sunshine Coast, so if it isn't critical to me...


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## patlaw (Jan 4, 2010)

DaninVan said:


> What kind of plywood is it, Mike?
> Sanded one side Fir sheathing?
> Not being critical, just looking for more info. I have no problem using sanded sheathing for shop stuff myself.
> Can't get decent Hardwood plywood up here on the Sunshine Coast, so if it isn't critical to me...


Home Depot Special


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## Stick486 (Jan 4, 2013)

patlaw said:


> Please explain why 1/4" plywood does not fit into a dado cut with a 1/4" router bit. I had to move the fence and rout twice to get the bottom to fit into the dado. Obviously the plywood is to thick, or the 1/4" dado is too narrow. The difference is too small for me to see. Which is more likely?
> 
> It's time consuming to have to cut the dado three times (shallow cut, full-depth cut, widening cut). The plywood came from Lowe's. There's nothing special about it.


WTB one (bit or ply) is metric and the other (ply or bit) is Imperial...
save work...
cut the dado on the TS...


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## PhilBa (Sep 25, 2014)

I'm a little confused, you are saying the dado you cut with a 1/4" bit is too *small* for 1/4" big box plywood? That is completely backwards from what I've seen. Usually the plywood less than 1/4" thick and the dadoes are loose. maybe you had 3/8" which is about 1/16" over 1/4".

Calipers would be a good idea anyway.


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## Ratbob (Apr 6, 2015)

Calipers will tell the tail, but in the link Mike posted the spec sez the 1/4" ply 'Actual Product Thickness' is 0.47" (just over 15/32).


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## Herb Stoops (Aug 28, 2012)

Ratbob said:


> Calipers will tell the tail, but in the link Mike posted the spec sez the 1/4" ply 'Actual Product Thickness' is 0.47" (just over 15/32).


I saw that too, and thinking closer to 1/2". They should have said "At least /4" thick"

Herb


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## Ratbob (Apr 6, 2015)

I guess great minds think a like Herb, not sure how I got included in that though.:no: I guess even a broken watch is right twice a day.


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## cocobolo1 (Dec 31, 2015)

Herb Stoops said:


> I saw that too, and thinking closer to 1/2". They should have said "At least /4" thick"
> 
> Herb


Sounds like little more than an advertising error. They meant 1/2" it seems.


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## Cherryville Chuck (Sep 28, 2010)

Usually finish ply is smaller because of the sanding not bigger. I know metric comes in 3mm for drawer bottoms and backs but I thought much thicker came in even mm such as 6 or 8. 8mm would be quite a bit thicker, about 5/16.


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## fire65 (Oct 29, 2008)

The same reason your toilet paper is much smaller than the holder, a candy bar is 2 inches, etc. MONEY, they keep making it a little smaller, then a little smaller thinking we do not notice. WELL YEAH we do, especially in wood working. I was noticing today the under size 1/4" router bit I use for my drawers is not working. I am getting a gap with the oak plywood now. 
Before long we will have to use 1/2 to fill a 1/4 in dado.


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## PhilBa (Sep 25, 2014)

homedepot in the US doesn't sell metric. 1/4" will be be something 7/32". If it was thicker than 1/4", they'd be selling it for thicker (and more $$$).

Maybe it was mislabeled product but 1/4" nominal is never thicker than 1/4".


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## DaninVan (Jan 1, 2012)

Let's not forget the *5/16" ply *that's available out there. Easily confused for 1/4".
Home Hardware - 4' x 8' x 7.5mm(5/16") D-Grade Spruce Plywood

Apparently this is a not uncommon issue with HD.


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## Herb Stoops (Aug 28, 2012)

DaninVan said:


> Let's not forget the *5/16" ply *that's available out there. Easily confused for 1/4".
> Home Hardware - 4' x 8' x 7.5mm(5/16") D-Grade Spruce Plywood
> 
> Apparently this is a not uncommon issue with HD.


HD here would be sellin that for 3/8", never seen 5/16". 

What is the metric 2X4 size?

Herb


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## Cherryville Chuck (Sep 28, 2010)

Herb Stoops said:


> HD here would be sellin that for 3/8", never seen 5/16".
> 
> What is the metric 2X4 size?
> 
> Herb


38mm x 89mm. That`s how it reads in the Canada Building Code. Because metric is the law up here the Code is written in metric. It`s not supposed to be nominal in metric, it`s supposed to be actual. 

If that ply came from offshore, especially China, the chances are it`s metric. Just about everything I`ve seen out of China is a metric equivalent of the Imperial version.


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## Herb Stoops (Aug 28, 2012)

Cherryville Chuck said:


> 38mm x 89mm. That`s how it reads in the Canada Building Code. Because metric is the law up here the Code is written in metric. It`s not supposed to be nominal in metric, it`s supposed to be actual.
> 
> If that ply came from offshore, especially China, the chances are it`s metric. Just about everything I`ve seen out of China is a metric equivalent of the Imperial version.


So what is 38mmX89mm equivalent in inches (actual)


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## cocobolo1 (Dec 31, 2015)

Herb, there is approximately 25.4 mm to one inch. Or 2.54 centimetres. Same thing. The 38 x 89 is 1 1/2" x 3 1/2", a normal 2 x 4.


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## sunnybob (Apr 3, 2015)

It might be a metric thing.
If you have got hold of a 6mm bit, thats fractionally under 1/4".
some charts give you a straight conversion between 6mm and 1/4" but they have rounded things up and down. a true mathematical conversion of a 1/4" is 6.35mm. so if it is a 6mm bit, the board wont fit by 0.35mm which is just over 1/32"


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## DaninVan (Jan 1, 2012)

5/16" is 8mm.


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## cocobolo1 (Dec 31, 2015)

DaninVan said:


> 5/16" is 8mm.


Well, 7.9375mm if you want to split hairs, but who's counting? >


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## DaninVan (Jan 1, 2012)

*A BC Product Then!*



patlaw said:


> Home Depot Special



So, it _is [/I ]sanded sheathing grade plywood.
They call it pine ...
https://www.google.ca/search?q=Pine...&oe=utf-8&gws_rd=cr&ei=iVW0VrXGM9XKjwOM357wBA_


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## LazurusLong (Jan 20, 2015)

DaninVan said:


> Let's not forget the *5/16" ply *that's available out there. Easily confused for 1/4".
> Home Hardware - 4' x 8' x 7.5mm(5/16") D-Grade Spruce Plywood
> 
> Apparently this is a not uncommon issue with HD.


Product in the wrong bin or mislabeled??? Never!!!:no::no::no: :grin:


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## Garyk (Dec 29, 2011)

After running into the same problem a few times I invested in a set of Whiteside plywood router bits. They are sized for the (mostly imported) plywood. Stick has the answer in the imperial & metric comment. Sorta like metric vs SAE wrenches & sockets. Like Mike I always use my digital calipers first to find the true thickness. The Sandiply sold in Lowes & HD is notorious for never being a consistent size. Like the old saying says " measure twice & cut once"


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## patlaw (Jan 4, 2010)

The plywood and bit are both labeled as being 1/4". Whether either is mislabeled is yet to be determined. If I find the answer, I'll let you know. Thanks for all of the thoughts so far.


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## N'awlins77 (Feb 25, 2011)

Plywood is always thinner, then labeled. Sanding.
You need a set of these:
Undersized Plywood Router Bit Set | Rockler Woodworking & Hardware


Rockler Undersized Plywood Router Bit Set
(1) 23/32" Straight Bit for 3/4" plywood
(1) 31/64" Straight Bit for 1/2" plywood
(1) 15/64" Straight Bit for 1/4" plywood


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## patlaw (Jan 4, 2010)

N'awlins77 said:


> Plywood is always thinner, then labeled. Sanding.
> You need a set of these


I'm confused. If the plywood is a full 1/4" or even thicker, how will undersized bits help?


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## PhilBa (Sep 25, 2014)

patlaw said:


> I'm confused. If the plywood is a full 1/4" or even thicker, how will undersized bits help?


It won't. I think people have misread your post. The only thing I can think of is mislabeled wood. I guess a mislabeled bit is possible but that seems a lot less likely. You should be be able to put a tape measure or other ruler up to the wood. I would measure the slot cut by the bit rather than the bit itself. While not super accurate, it would probably tell enough of the tale.

by the way, another approach to getting overwide wood to fit is to sand or plane down the edges of the bottom piece (on the bottom side) with a fairly shallow bevel. That's how it was done in the old days,


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## Cherryville Chuck (Sep 28, 2010)

DaninVan said:


> So, it _is [/I ]sanded sheathing grade plywood.
> They call it pine ...
> https://www.google.ca/search?q=Pine...&oe=utf-8&gws_rd=cr&ei=iVW0VrXGM9XKjwOM357wBA_


_

Yep, says right in the particulars that it is 0.47" and 1/4". Obviously there are no math scholars working for them. Definitely made as sheathing, the width is just enough below 4' wide to allow for expansion and an actual 1/4" thickness wouldn't pass code for sheathing._


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## patlaw (Jan 4, 2010)

According to General No. 147 digital callipers, the plywood is exactly 1/4". Therefore, the router bit must be undersized slightly. The dado is (now) 17/64". I guess I have a perfect 1/4" thick piece of plywood with a perfect 1/4" router bit in a router with zero runout.


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## DaninVan (Jan 1, 2012)

"HD here would be selling that for 3/8", never seen 5/16". 
-Herb
They sell it up here for flooring underlayment.
I called it sheathing for want of a better term, But Charles is correct, the minimum thickness for actual sheathing applications is 3/8".
http://www.canply.org/pdf/main/plywood_handbookcanada.pdf


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## Herb Stoops (Aug 28, 2012)

DaninVan said:


> "HD here would be selling that for 3/8", never seen 5/16".
> -Herb
> They sell it up here for flooring underlayment.
> I called it sheathing for want of a better term, But Charles is correct, the minimum thickness for actual sheathing applications is 3/8".
> http://www.canply.org/pdf/main/plywood_handbookcanada.pdf


Underlayment grade here used to be ,"no voids"

Herb


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## DaninVan (Jan 1, 2012)

That was my understanding up here as well, Herb. Disastrous if a spike heel punches through a lino installation, due to the top veneer collapsing. 
Could end up being a couple thousand $$$ to repair, assuming a big floor, and still under warranty. Ouch!
My house damaged by estate agent. - MoneySavingExpert.com Forums


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## Nickp (Dec 4, 2012)

DaninVan said:


> That was my understanding up here as well, Herb. Disastrous if a spike heel punches through a lino installation, due to the top veneer collapsing.
> Could end up being a couple thousand $$$ to repair, assuming a big floor, and still under warranty. Ouch!
> My house damaged by estate agent. - MoneySavingExpert.com Forums


I guess I should put my pumps away...


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## Stick486 (Jan 4, 2013)

Nickp said:


> I guess I should put my pumps away...


walk on your hands...


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## N'awlins77 (Feb 25, 2011)

I agree with Phil, it must have been a misprint. All ply, that I dealt with, is always thinner!


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## DaninVan (Jan 1, 2012)

*Not Mislabeled.*



N'awlins77 said:


> I agree with Phil, it must have been a misprint. All ply, that I dealt with, is always thinner!


Mike checked his material with calipers...it's _exactly_ 1/4".
Router Forums - View Single Post - When Is 1/4" Not 1/4"?


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## Garyk (Dec 29, 2011)

Interesting : if both the ply & the bit is measured to be 1/4" with calipers why would it not fit ? The only other element would seem to be heat from the bit speed, but heat would cause expansion, wouldn't it ? Any engineers on the board who could come up with an answer ?


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## DaninVan (Jan 1, 2012)

*Not Skinny Enough*



Garyk said:


> Interesting : if both the ply & the bit is measured to be 1/4" with calipers why would it not fit ? The only other element would seem to be heat from the bit speed, but heat would cause expansion, wouldn't it ? Any engineers on the board who could come up with an answer ?


Think of it in decimal form, Gary.
The channel = .25"
The plywood =.25"
Maybe if the plywood was .23"
There _has_ to be an allowance _somewhere_ in the concept.


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## Herb Stoops (Aug 28, 2012)

get a bigger hammer.
Herb


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## DaninVan (Jan 1, 2012)

So what _are_ the _practical_ allowances one should aim for?
There's context to be considered of course. If you're rebating the back dust panel on a kitchen cabinet you're building, obviously the fit isn't anywhere near as critical as the parts of a jewelry box.


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## Cherryville Chuck (Sep 28, 2010)

Type of glue used could be a factor too. A thin glue like polyurethane would be better with a tight fit than T2 with a high solids content.


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## PhilBa (Sep 25, 2014)

That's a good point about glue - water based glue will swell the wood a bit making the fit tighter. 

The reason that a 0.250 board would not fit in a 0.250 dado is that there is a fair amount of stuff sticking up from the cut surfaces. It makes it very hard to slide the board in. You could probably hammer it in, though. Look at them under a microscope to understand - pretty rough looking. I've done some experimenting in my box making and there needs to be at least a couple of mils of clearance to have a "tight" fit. Especially with glue on.

I think this was alluded to earlier but, a 1/4" bit may measure 1/4" but when chucked in any router, there is some runout so you get something wider than the bit. I was having problems with a box joint being loose and found that the bit and router collet were adding about 10 mils (.010") to the cut width. This resulted in a .020" gap (more than 1/64") which made the joint rattle when dry fitting. Turning the bit 90 degrees took it down to about 4 mils and the fingers fit nice and tight.


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## DaninVan (Jan 1, 2012)

So, Phil; your vote would be for a 4 thou allowance, minimum?


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## PhilBa (Sep 25, 2014)

That was my experience with box joints in walnut and maple. Not sure how softwoods would fare. Note that for the box joints, the error is doubled. I think in practice the tightest fits are going to be when you need to slightly sand down the "tenon" piece. Not a good idea for box joints but OK for a drawer bottom (plus looser isn't so bad there anyway).

This discussion makes me wonder about exact width dado jigs and why they work. I think it's because of bit runout which adds just a little width to the dado. 

Fun topic, makes me think.


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## DaninVan (Jan 1, 2012)

I'm just heading out for a hike around the neighborhood with my retired machinist buddy. I'll ask him for his take on this problem in physics.
I know that they worked to incredibly tight tolerances (aerospace), so much so that components had to be kept at controlled temperatures or they wouldn't fit.


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