# Help with creating dado for MDF



## JohnnyAbe (Apr 13, 2012)

Hi all,

Why when I create a 1/4" dado that my 1/4" MDF will not fit in it. I thought 1/4" MDF is truly 1/4".
I used a 1/4" spiral upcut bit in in my router table, created the dado then tried fitting the 1/4" MDF in the slot and it would not fit. I need to move the fence back a small amount and pass my board through again to get the MDF to fit.
I ran in to this same problem with using my dado blade on the table saw.

Can anyone tell me what I'm doing wrong?


----------



## jlord (Nov 16, 2009)

JohnnyAbe said:


> Hi all,
> 
> Why when I create a 1/4" dado that my 1/4" MDF will not fit in it. I thought 1/4" MDF is truly 1/4".
> I used a 1/4" spiral upcut bit in in my router table, created the dado then tried fitting the 1/4" MDF in the slot and it would not fit. I need to move the fence back a small amount and pass my board through again to get the MDF to fit.
> ...


It usually is John. There is MDF (heavy) & MDF light. MDF light is a shade lighter along with the lighter weight. I know for 3/4" MDF (heavy) is 3/4" & MDF light is about 11/16" in thickness. I have not run into material that was thicker than the stated size. Do you have a digital caliber? Maybe measure the thickness.


----------



## OPG3 (Jan 9, 2011)

I personally do not even measure material thickness that will fit into my dado joints, because of the dado-cutting methodology that I always use and has never failed me yet. Wood products are quite often variable from their stated thicknesses - this is just a fact of life.


----------



## JohnnyAbe (Apr 13, 2012)

Yes I do have a digital caliper. I'll check it tonight.
The project I'm working on is a drawer for my workbench. I cut a 1/4" x 1/4" dado 1/4" up from the bottom. Was planning on using MDF as the bottom. I did test the dado on 1/4" plywood and that fit really well. That's why I'm puzzled about the size of my dado. I would have thought that the spiral upcut bit I used would be the correct size for the MDF.
Maybe I'll just use plywood and be done with it.
I have been searching for plans for a dado jig that will let me use any size bit but I can't seem to find that. Hmm...invention time.


----------



## walowan (Jan 21, 2011)

Plywood will be MUCH better than mdf for a drawer bottom anyway, mdf is not very good at structural strength


----------



## DaninVan (Jan 1, 2012)

"Maybe I'll just use plywood and be done with it."
Excellent idea!


----------



## rickhester (Apr 9, 2012)

The Woodwhisperer has an exact width dado jig video at the following address: 139 – Exact-Width Dado Jig | The Wood Whisperer I've made a version of this and it works great.


----------



## The Warthog (Nov 29, 2010)

If the fit of the MDF is very close but not quite, you can get a very slightly wider dado by putting a strip of masking tape or two along your straightedge, or along your table fence, and cutting again. This will give you the few thousandths of an inch you sometimes need.


----------



## JohnnyAbe (Apr 13, 2012)

Thanks everyone.
I have seen the Woodwisperer's video. I'd have to come up with a way to make the jig work all they way down to 1/4". I haven't seen any that allow that.

I like the tape on the fence idea. I'll have to remember that for my next project.

I think for now I'll just use plywood. I'll also make sure that I run some scrap tests next time. Could add the shims to the dado blade to get the exact width


----------



## DaninVan (Jan 1, 2012)

_"Could add the shims to the dado blade to get the exact width"
_
This is a great example of where some tool applications favour one tool over another. Chris Curl was suggesting last week that he'd eventually be able to get rid of his tablesaw. The shimmed dado is a classic case of a perfect solution, and why a tablesaw is basic to any woodshop.
If I had to run 20 gables, dadoed on three sides, *3/4+"*, I'd guess that using a router would take at least three times as long. In this example the saw is safer and less fatiguing.


----------



## giltic (Sep 27, 2010)

Nice wideo you've found, rickhester !
This is an excelent example, that you don't need some fancy complicated stuff but just a simple home made jig to do the work.
I like it a lot and I'm going to build it.


----------



## RCclee (Mar 31, 2012)

Why are you using mdf? What is the upside? Never could figure that one out.


----------



## rickhester (Apr 9, 2012)

If you use the woodwhisperer's jig, it will work down to a 1/4" is you use a 1/4" straight bit.


----------



## JohnnyAbe (Apr 13, 2012)

I only selected 1/4" MDF because I was following some plans (that I modified to fit my workbench) and the material for the bottom of the drawers is (was) MDF. No other reason then that.
So I watched some of the video, need to finish. I'd be looking for a jig that will allow me to use any size bit at any given time. Needs to be flexible enough to cut dados from 1/4" to 1" and anything in between. Most of what I have seen don't are not this flexible.


----------



## jschaben (Jun 21, 2009)

JohnnyAbe said:


> I only selected 1/4" MDF because I was following some plans (that I modified to fit my workbench) and the material for the bottom of the drawers is (was) MDF. No other reason then that.
> So I watched some of the video, need to finish. I'd be looking for a jig that will allow me to use any size bit at any given time. Needs to be flexible enough to cut dados from 1/4" to 1" and anything in between. Most of what I have seen don't are not this flexible.


Hi Johnny - nothing wrong with mdf...Usually nice, straight and stable, especially once sealed.
The wood whisperer video was pretty good, I built my jig very similar. However, I don't quite understand why he said you MUST use a 1/2" bit and 5/8" bushing, just because that is what he made the initial cut at. Seems to me like you could use any bit/bushing combination that yield a 1/16" offset (bit and bushing diameters are different by 1/8"). You could make your 1/4" dado with 3/8" bushing and a 1/4" bit. If it were me though, in case the 1/4" were slightly undersized, I'd likely use a 5/16" bushing with a 3/16" bit.. Anything over 1/2" will work as built with the 5/8" bushing and 1/2" bit.... The setup will also work with the 3/8" bit and 1/2" bushing... Lots of options once you think about it


----------



## jw2170 (Jan 24, 2008)

jschaben said:


> Hi Johnny - nothing wrong with mdf...Usually nice, straight and stable, especially once sealed.
> The wood whisperer video was pretty good, I built my jig very similar. However, I don't quite understand why he said you MUST use a 1/2" bit and 5/8" bushing, just because that is what he made the initial cut at. Seems to me like you could use any bit/bushing combination that yield a 1/16" offset (bit and bushing diameters are different by 1/8"). You could make your 1/4" dado with 3/8" bushing and a 1/4" bit. If it were me though, in case the 1/4" were slightly undersized, I'd likely use a 5/16" bushing with a 3/16" bit.. Anything over 1/2" will work as built with the 5/8" bushing and 1/2" bit.... The setup will also work with the 3/8" bit and 1/2" bushing... Lots of options once you think about it


The reason the bit,bushing was quoted was to allow for the offset made by the initial cut. I guess Mark did not want to go into a great discussion on ways to achieve the same offset with different combinations.. ( I think?)

Many of the viewers would not understand the question of offset as easily as some of our members.

The only issue, which you have alluded too, is that with Mark's set up, you cannot cut a dado narrower than 1/2".


----------



## paduke (Mar 28, 2010)

With a 5/8 bushing any dado width can be cut under 1/2" with the proper size bit. 
Center line of dado will always be the same no matter the bit size.
I use both dado blade with shims (Freud Set) and "plywood size" dado bits

Attached is a sled designed to use 3/4' bushing and any size bit It was made to make a running dovetail but does a dado as well


----------



## bobj3 (Jan 17, 2006)

Hi

I use my jig all the time to put in 1/8" to 7/8" dado slots ,no big deal.. 

see jig below
http://www.routerforums.com/jigs-fixtures/2711-dado-jig-plunge-router.html

==


----------



## jschaben (Jun 21, 2009)

paduke said:


> With a 5/8 bushing any dado width can be cut under 1/2" with the proper size bit.
> Center line of dado will always be the same no matter the bit size.
> I use both dado blade with shims (Freud Set) and "plywood size" dado bits
> 
> Attached is a sled designed to use 3/4' bushing and any size bit It was made to make a running dovetail but does a dado as well


Hi Bill, you are absolutely correct. However, to do so, you need to do your computations from the centerline of the bit. One of the major points of the jig is that it is a zero clearance so you just line the fixed edge to the desired position and use the intended stock to set the moveable edge.. Little or no math involved. Personally, I think Marc missed a good opportunity to introduce the concept of offsets but James has a good point and he will likely introduce it in a later video.


----------



## jw2170 (Jan 24, 2008)

jschaben said:


> .... but James has a good point and he will likely introduce it in a later video.


ROTFLAO...

Shouda kept my mouth shut.......


----------

