Not all 1/4 " hardboard is 1/4"

I just found out that the drill press table I made last week to include a vac channel for sanding is almost useless.
Last weekend while making the table I routed a 1/4 recess for hardboard inserts... I thought I made a mistake when the hardboard sank lower than the top...I didn't understand how that happened. But I figured I had a brain fart. I went and bought a new board of hardboard 2 days ago and today I find out not all hardboard is 1/4". Of three different samples only 1 is actually 1/4" The measurements are: .196    =    5mm .225 .250 So even though they are all marked 1/4 they aren't. So to fix my sunken plate I glued formica inserts into the routed channel to raise it. Now I cut a true .225 plate and it is too high... Wish I known b4 I fixed a problem that wasn't really broken in the first place. Now I have to re-route the formica out.
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In many products, a nominal thickness is used. The actual product varies in thickness plus or minus an allowable amount. One that comes to mind is metal sheeting. The ranges for 29 gauge and 26 gauge overlap by a small margin on some of the charts I have seen.
I have made the same mistake as you. How many times can a person measure twice, cut once and still be wrong...... I am still finding out.
Jon
"tiredofspam" <nospam.nospam.com> wrote in message

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wrote:

In wood products, especially sheet wood products, there's no chance of it being plus anything, and also none that will remotely approach the nominal thickness. 7/16" "half inch" plywood is with us until they decide that we are dumb enough to buy 3/8" "half inch" plywood. (at which point the 7/16" will be labeled as "three quarters of an inch")
It's not like the government is going to force them to advertise the truth, or sell true dimensions. You'll notice that the class action suit regarding lies about the horsepower of air compressors has had no effect whatever on the lies about the horsepower of vacuum cleaners, tablesaws, circular saws, etc...
--
Cats, Coffee, Chocolate...vices to live by

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In Canada, poly vapour barriers used to be labelled as 2, 4, & 6mil. The thickness of each size hadn't been anywhere near the 2, 4, or 6 thousandths of an inch in 20 years. The government forced manufacturers to use the descriptions: Light, Medium, Heavy, and 6mil (which actually has to be 6 thou. and is required in residential construction and has a manufacturer's number printed on the poly along with the code approval numbers. As a side note, I had a building inspector reject some poly because it had what he thought was the wrong number on it. This was the manufacturer's number not the code approval number. I had to fax him the list of all manufacturer's numbers before he would allow it to be used. Seems like there should be an inspector for inspectors). Anyway, in anything lighter than 6mil, the manufacturers now make an even thinner product than they did before.
Jon
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Everything has a tolerance for manufacturing. Years ago, it was difficult to stay very close to the stated size. The poly extruders have gotten so refined that they can now hit it extremely accurate and now they stay on the minus side as the norm. Technology improved but the stated tolerances remain.
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Ecnerwal wrote:

also knew that hardboard wouldn't be exactly 1/4 but the .196 isn't in the ball park, and .250 being that much over was surprising. ... anyway I figured I could make replaceable inserts anytime but was wrong. I will make blanks when I fix my problem and be more careful next time.
Isn't measure once cut twice...three times... 8>)
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"tiredofspam" <nospam.nospam.com> wrote in message

.196 seems to be well under, but .250 is not over anything if the spec is 1/4"
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Edwin Pawlowski wrote:

Doh.... Thanks for keeping me straight.
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An unknown previous owner of my house made some washable filters for the furnace using galvanized sheet metal, plastic screening and some sort of filter medium. He made them exactly 20" X 20" and 10" X10" which is what is printed on the disposable filters. They aint as labeled, they're about 1/4" scant so these really well made filters sat up on top of the frame and let all sorts of dust through. Brian, in Cedar
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I'm so glad I ordered a dial caliper from Harbor Freight. With it, I've found that most of the stock I use varies from nominal dimensions...sometimes by a considerable margin. Now I make my cuts to fit the materials instead of relying on the stated size. For twenty bucks, it's a tool I highly recommend.
To combine this with another current thread...using the dial caliper, I was able to make a VERY accurate box joint jig on the second try (and tuned it for perfect usability on the third).
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Once you get that fixed, simply drill and counter sink a small hole in each corner of the routed out area, countersink far enough for the screw to sit flush. Then when you get hardboard on the thin side, adjust the screws up to level the insert. The Rockler router table I have uses the same idea for inserts of various thicknesses.
John Cunningham
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Thanks, I did that too my router table too. I think I will just use masking tape build up on the blanks if they differ. A little easier than trying to level the screws each time I replace a blank. Since the drums will have different size blanks it is a big deal. The drilling blanks I will go thru fast... I have holed the drilling blanks to suck up the shavings... works nice.
snowdog wrote:

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