Another router question...

[knobcreekman]

... probably a dumb one

Does a 3/4" sraight bit make precisely a 3/4" groove that would perfectly fit a 3/4" piece of MDF or ply?


thanks for any help and feel free to laugh if it's as simple as it sounds

[Black wallnut]

Some do and some do not, and some MDF will fit and almost no plywood will fit.

[JimD]

Just to be clear, the biggest problem is that 3/4 plywood is not really 3/4 inch thick. 3/4 MDF and particle board are typically 3/4 thick. The bit is 3/4.

There are also router bits about 1/32 undersize to better fit 3/4 plywood.

Jim

[Mr__Bill]

Perhaps.

Moved in precisely in a straight line a 3/4" bit will make a 3/4" groove. Any deviation from the line and while the groove is still 3/4" at any given point, taken as a whole with rigid 3/4" stock it's no longer 3/4" wide in a straight line. If you follow my reasoning.

The router bit is milled to more precise specifications than the MDF is manufactured to. Add some humidity to the air and the surface gets a bit fuzzy adding to the perceived thickness of the product.

In practical application some sanding somewhere is always needed. That at least has been my experience. That said, a 3/4" bit for 3/4" stock is a good starting point, or you can make two passes with say, a 1/2" bit and fine tune the groove to fit the actual measurement of the 3/4" MDF

edit: I forgot to say that plywood is never exactly the size it's rated as. Most is undersized, 3/4 is now 17 mm or 23/32 but not really it's just a bit smaller than that. Hardwood plywood can sometimes be a hair over size and is closer to being the size it's stated as being.

Bill
in my other life I sometimes have to write weasel word reports, can ya tell?

[LCHIEN]

yes and no.

A 3/4" straight bit will plow an exactly 0.750 inch wide groove.
However, 3/4" plywood is not ever 3/4" and although not completely standardized, will be approximately 1/32 less, more or less.
MDF is usually close to the correct dimension for thickness (I think).

They sell plywood bits, for which the 3/4" size is supposed to be undersized to fit plywood, and usually 23/32", but as I mentioned before, the ply wood will still vary from this thickness so it may or may not fit well. THe best way to get a perfect fit is to use a 1/2" bit and make two passes, one to set one edge and the second a precision space away to set the other edge. YOu may find some jigs around the internet that will allow you to clamp the piece of plywood between a couble of guides and the use the guides to cut the two passes i mentioned.

[knobcreekman]

hey guys thanks for the info! i'm just new to all of this and want to save myself any headaches i can by asking questions BEFORE i begin

[LCHIEN]

hey guys thanks for the info! i'm just new to all of this and want to save myself any headaches i can by asking questions BEFORE i begin

it was not a dumb question, as you can see the answers are not obvious and experience based.

We've all had the headaches. Wish i had someone to have asked.

[cabinetman]

... probably a dumb one

Does a 3/4" sraight bit make precisely a 3/4" groove that would perfectly fit a 3/4" piece of MDF or ply?

A 3/4" straight bit cuts a 3/4" groove. A piece of 3/4" ply or 3/4" MDF will fit a 3/4" groove. If the groove is not straight, like if it drifts off a straight edge it will still cut a 3/4" groove, but not straight. The substrate selected for the fit should be checked for thickness before machining the groove. It would be wise to run a sample groove in a scrap to check the fit.
.

[cabinetman]

yes and no.

A 3/4" straight bit will plow an exactly 0.750 inch wide groove.
However, 3/4" plywood is not ever 3/4" and although not completely standardized, will be approximately 1/32 less, more or less.
MDF is usually close to the correct dimension for thickness (I think).

I don't agree with the statement (boldface). To say 3/4" plywood is not ever 3/4, is simply not true as 3/4" plywood is 3/4". I can still buy 3/4" hardwood plywood that is a full 3/4" thick. It depends on where it's bought and what you want to spend. You can also get face veneers .0625 thick.
.

[Black wallnut]

In an ideal world a 3/4" bit will cut a 3/4" grove however if your router has the slightest bit of runout, or the bit has the slightest bit of runout, or the bit is not perfectly milled (or made) then it may not cut a 3/4" grove.

[JimD]

I rarely cut dados with a router for several reasons. First, my Freud stack dado blade on my table saw is easy to adjust for width to match the material - router bits are not adjustable. Dado blades have many more cutting edges so they stay sharp longer (but cost more to sharpen). I also do not enjoy the screaming noise from routers. And it is difficult to guide so that the dado has a constant width.

When I use a router to make dados, I usually use a jig that works with a 1/2 router bit and 5/8 inch template guide. It guides both sides of the cut so you CAN make a straight dado. And it is easily adjusted for the width of the stock.

Jim

[LCHIEN]

I don't agree with the statement (boldface). To say 3/4" plywood is not ever 3/4, is simply not true as 3/4" plywood is 3/4". I can still buy 3/4" hardwood plywood that is a full 3/4" thick. It depends on where it's bought and what you want to spend. You can also get face veneers .0625 thick.
.

OK, I should not have said "Never". But probably 95%+ of what you can buy that's labelled "3/4" " will be undersized of 3/4"

[phi1l]

I expect that is starts out 3/4 before they sand it. So I would expect sanded ply to be a bit under the nominal thickness...

[cabinetman]

OK, I should not have said "Never". But probably 95%+ of what you can buy that's labelled "3/4" " will be undersized of 3/4"

Like I said, it depends where you buy it and what you're willing to spend.
.

[LarryG]

KCM, here is one version of the type of jig that Loring mentions. It's about as simple to make and use as anything I've tried or seen. Mine is sized to handle workpieces up to about 12" in width, which covers most bookcases, wall-hung cabinets, and the like. A longer version could be made if needed, but if so it would be a good idea to also clamp the far edge of the jig halves to keep them from deflecting laterally.

Secondly, if you're new to routers I recommend picking up a copy of Bill Hylton's most excellent book, Woodworking With The Router. No one here minds answering any questions you might have, but Hylton will answer questions you didn't realize you should ask. This title is THE book to own about routers and all things related.