View Full Version : Sacrificial wood on base frame?

Mon 04 July 2011, 08:56

My router table needs a good wood surface to work from but it's cutting area is less than the actual area available so I was gonna use clamps in the excess for work holding. Current thoughts are 18mm MDF not leveled over the entire machine surface, then a sacrificial 8x4' -15mm plywood sheet in the centre, leveled.

I can replace the sacrificial sheet at anytime with stock from my local DIY store but they don't stock the larger 3mx1.5m sheets :mad:, I have to special order from a different wood supplier at some expense. Given I can't machine over the entire MDF sheet area, it will be used for clamps and supporting surface only.

Is this a normal approach or should I just stick with one large, expensive board that has a routed, level cutting area (rectangle pocket) and replace this when cut to bits? Any alternatives? Is MDF/plywood combination ok or should I coff up more and go plywood for both?



Mon 04 July 2011, 14:10
I'd go with MDF for both, and the only thing that matters is that you get a good glue joint down to the base piece. It doesn't really matter how you piece together the
sacrificial board, do whatever is inexpensive. MDF will behave in a more stable fashion as you skim it to resurface it, until it's gone. Of course, the dust is nasty, so control it.

Mon 04 July 2011, 15:43
Thanks. I will go for MDF for both boards, but hadn't considered glue, was gonna just bolt right through the two boards. I looked in cad and I can bolt through both boards and the metal frame holes if I wanted.

What glue would you use? I saw the mechmate drawings suggesting bolts, capped off with epoxy.


Mon 04 July 2011, 16:07
I saw the mechmate drawings suggesting bolts, capped off with epoxy.
Yes, this is the main support board. You DO NOT cut into it. On top of that spread white/yellow carpenter glue with a squeege or a fine tooth trowel, before it sets up apply the 'spoil board' and clamp edges and weight down the field area till dry. You will surface the top of the spoil board with a surface cutter when your machine is calibrated properly by reading about the process on this forum. After the surface of the 'spoil board' becomes pitted with 'cut lines' over time from cutting through your work, you will surface the 'spoil board' again and repeat until your cutter starts to make contact with the glue line. At this point you will resurface leaving a little glue line showing, but will not cut into the support board. A new 'spoil board' will then be applied as before and start the process over again.

Tue 05 July 2011, 03:13
ok. I figured I was going to resurface but not until I was down to the base board, can see why using through bolts in both boards would be a problem.

Will look into glue brands this side of the pond.



Tue 05 July 2011, 05:59
giggler, I personally don't care about the brand of carpenter glue... As a matter of fact, any glue which can bond MDF will do the job... Even starch glue...
The glue won't have any affect on the machine performance of the audio quality of the NOISE...
If I were in your shoes, I'll spend more time how to have evenly spreading the glue & even clamping pressure....

Tue 05 July 2011, 06:29

thanks. I can get contact adhesive from my local wood yard so probably will brush that on both sheets.

my base boards are now 10'x4' sheets, 2off cut down to my bed size and on top will be an 8'x4' sheet of same MDF, standard density 18mm that I can cut to bits over time.

i'll have to think about clamping, perhaps lots of car batterys spread over the board as i've a few around. I was gonna mark the corners of the base board with the machine tool so I get good square alignment of the sacrificial board on top relative to my cutting tool.


Tue 05 July 2011, 17:44
Contact adhesive will grab immediately after the set time has passed. You will need to make sure your top board is positioned correctly before contacting the other surface. There will be no adjustment after it grabs. If this is the way your going to do it you will not need clamps or weights. You will need to pressure roll any air between the boards and use a mallet to apply enough force to the surface to make a good bondable set.

Tue 05 July 2011, 18:11
I personally have done a little indicator to my spoil board glue ups....

I use a mixture of :
1/2 white glue
1/2 wood glue
..a little bit of water
blue or red food coloring

Mix all together and then spread with a thick nap roller, then use notch trowel to get the proper 50% coverage, then put the MDF on top. Add sandbags....wait a day.

The food coloring gives you a quick indicator when you have surfaced to far!

....my little shop tip for the day.

Wed 06 July 2011, 05:11
The bondit contact adhesive I was gonna use mentioned the following sales blurb:

premium grade, solvent-based, neoprene adhesive with high grab and long open time to enable re-positioning

I do have some wood PVA glue that a die cutter gave me, its over a year old now and been sitting in my garage unopened, perhaps this would be better. Thing is I don't have anything to clamp as yet, I might be able to put weight on the glued board? Theres about 130mm of border around the sacrifical board against the base boards so my 4" G clamp set won't fit.

Why do I have to make sure air bubbles are removed since I am gonna level it with a router tool, does it really matter if some remain?


Wed 06 July 2011, 05:55
Hi Giggler

Seans tip is about the colour is a good one and I have put it in the brain bank (Thanks Sean).

As a general rule if you are unsure about something do a test piece.
Most people are quite astounded when they try to pull apart a properly glued joint esp two flat surfaces bonded together.
So get a few bits of what ever you intend to use and glue away... then destroy them !

I love laminating wood together for both effect and strength.
My biggest tip is to use plenty of glue, maybe spread with a cheap body filler plastic squeegee THEN wiggle the two surfaces together in a circular motion this really gets the two surfaces mating together nicely.
You will feel the top sheet just start to float when it is done properly.
Clamps, weights, cement bags or all of the above will do the rest.
Finally clean up any excess glue after the squeeze has been applied for a clean job.


Wed 06 July 2011, 05:56
PS - Work Fast.


Wed 06 July 2011, 06:25
PVA glue is my preferred glue for bonding any wood based product, that includes MDF. I'm not aware of shelf life for these stuff... The only thing I have to do is add some water if the viscosity is too much for easy application.

Wed 06 July 2011, 06:48
Minor air bubbles in the glue are not a problem in this application. What you want to avoid is a large area that is either not bonded, such that it could break free when you happen to cut around it, or is bowed up, such that it flexes up and down and causes unwanted variation in your work piece.

Wed 06 July 2011, 10:37
The following link: http://platchem.bpweb.net/images/a40tech.pdf
will explain how to use the product. Unlike wet set glues a contact glue is allowed to dry to the touch before setting the boards. There will be no ability to 'reposition' a panel after it makes contact. Any air trapped between the boards, as explained above' will cause the spoil board to seperate from the support board as the spoil board becomes worn down.

PS. If someone were to set two panels together while a contact adhesive was still 'wet' the adhesive would never bond the panels together.

Wed 06 July 2011, 10:42
OH! Sean's idea with the food coloring trick is great:)

Wed 06 July 2011, 16:12
I have alsogone over to the paint department at my old company and used a couple spoonfuls of universal tint for the same use.

Bottom line - you can see (even on the edges of the spoilboard) how much material you have left.

Wed 06 July 2011, 22:32
Minor air bubbles in the glue are not a problem in this application. What you want to avoid is a large area that is either not bonded, such that it could break free when you happen to cut around it, or is bowed up, such that it flexes up and down and causes unwanted variation in your work piece.

nice insight

normand blais
Thu 07 July 2011, 09:34
I use to put weight on the new board to glu it . Now I use screws with washer ,starting in the center and spiraling toward the edges to avoid air bubble .
Last time I broke a screw flush with the table ,I dig a bit around it and grab the stub with the drill chuck ,and then reverse .I find it better than visegrip