To resurface or not to resurface…?

Please settle a disagreement. For a CNC machine to provide desirable results, is resurfacing/flattening a spoilboard something that should/must be done at least once, or is it something that should never be necessary? Why/why not?

I’m not going to say which side I’m on, just want to get some informed answers, as we’re both fairly new to this. Thanks!

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Wow, highly subjective & presumptuous.

First, are both parties willing to accept the conclusions of this discussion as the ‘correct’ answer?

“desirable results” is the subjective bit. If you are making large decorative items that do not require a lot of precision, I would think the original equipment as delivered by the manufacturer is sufficient. You can still surface the part you are machining to get the top flat. Small discrepancies in the thickness are arbitrary.

If you are making parts that require high precision, like I am, then you want the table as perfectly flat & parallel to the MCS (Machine Coordinate System) as you can get it.

If you can achieve “desirable results” without surfacing, don’t bother. :wink:

Precision has to start somewhere, and things being level, plumb, and at right angles is a chain which continues through every aspect of the machine, either in a continuous linking where things “just line up and work”, or a disconnect where something not being on the same plan as another part, or not plumb at 90 degrees to some other part will cause either a problem, or a cut to not be made as well as one wants.

Can one slap together a machine, through a piece of stock on an untrammed machine, and make a cut w/o an overt problem? Of course — we have customers do this every day.

Is that how one gets the best possible part w/ clean cuts which is dimensionally accurate? No.


@Tod1d and @WillAdams nailed it, but if you need evidence about why sometimes it’s required, here a usecase for you:

  • suppose you have a (large) piece of stock that we will assume is perfectly flat and has a constant thickness all around.
  • attach it onto an unsurfaced wasteboard, zero Z somewhere on top of the stock, then run a v-carve job that e.g. carves letter with a shallow depth of cut over a wide area/widht/length

Chances are, in some places the letter will be carveed slightly too deep, and in other places sllightly too shallow, or even not carved at all and the vbit will hover above the surface. We’ve all been there, it’s a good way to ruin a vcarve job.

Now surface your wasteboard and do the same: chances are the vcarve job will have even thickness across the piece.

And now a counterpoint: suppose you have a large piece of stock and you want to vcarve some text on top surface, and it’s ok to run surface the top of the stock first. Even with an unsurfaced wasteboard that may tilt the stock at a slight angle compared to machine axes, the surfacing pass on top of the stock will leave you with a surface that is guaranteed to be parallel to the machine axes. And if you run the same vcarve as before, it will likely come out perfect. This is one example where wasteboard surfacing does not matter.

All that said, I don’t see any drawback to surfacing your wasteboard, and there are definitely cases where it’s required, so it should be a fairly short discussion :slight_smile:


@Tod1d : I don’t know, but currently both parties are getting their information from YouTube, and this forum is frequented by people in the industry.

Sorry, “desirable results” IS relative. We’re not machining medical devices or anything that requires very high tolerance, but I’d say that when you make a 10” round wood plaque with names along the edges and some are as expected and some are barely visible, I consider that undesirable.

@WillAdams : Agreed.

@Julien : I’ve heard of people surfacing their material, but if they’re doing that every carve, that sounds like a poor use of time (and material), especially given we currently run our material through a jointer and planer before carving.

The main issues we are struggling with is the increased consistency of the products generated vs the cycle of continually (hopefully slowly) going through and paying for spoilboard that is subject to environmental changes.

I appreciate your insights!


You could consider using HDPE for the wasteboard, then humidity changes would not a concern anymore.
I used one for a long while and liked it.


Personally, I’d just surface the material. If you’re worried about wasted material, don’t use the planer at all, or just stop 0.1" short of the thickness that you want and let the CNC finish the surfacing. It takes a little time (CC estimates a 12"x12" square doing 0.05" DOC with a 1" surfacing bit at 5 minutes of time) but if it gives you a consistent result with very little effort, that sounds like a good thing to me.


Another material option for a spoilboard/slats is cellular PVC — I use that on my SO3 XL w/ HDZ:


Some of us don’t have jointers and planers so we surface the spoilboard and the material. Yes it is time consuming but not enough that I can justify the extra expense of the planer yet. It is on the list for my shop but other tools have come first.


Surface and tram, yes!

I deal with .001 to .002 specs, it is important to me to have it accurate. With the humidity here in the Seattle area the mdf swells in various ways. Yes surface and tram!

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The Shapeoko machines are great but if you have an XXL you have a lot of real estate to cover and materials like MDF can swell and shrink. So an initial surfacing can reveal you need to tram your router. IF you tram you need to surface your spoilboard a second time to prove your router and spoilboard are square and plumb to each other. In use the spoilboad tends to get torn up. As long as the random patterns are not too big then you do not need to surface but if you want a nice clean surface then you can surface the spoilboard several times before you need to replace it.

Traming is setting your router/router mount to be square and plumb with your spoilboard. Surfacing is simply removing the top layer to clean up the surface from the inevitable scarring you will experience by cutting through projects. You can use the bottom (spoilboard) of your project for z zero to minimize scaring.

They call it a spoilboard for a reason because it gets spoiled. The newer SO4, Pro and HDM are using the hybrid tables and the t-tracks are close to the surface so using the spoilboard as the z zero would make more sense than on the SO3 or on the newer ones that have a supplemental mdf spoilboard sitting on top of the hybrid table. You dont want to cut through your t-track on the newer machines.

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@b-radg916 In addition to all the many good reasons cited about why one would do it at least once (and then periodically because of environmental changes and cuts in the surface of the spoil board. Consider a surface that is on average parallel to the x-y plane of the x-y rails, but locally wavey. This lack of local flatness can create issues with the painter’s tape and CA glue workholding. While the waves may not be so bad to create this gaping under the work piece, my cut-thrus into the spoilboard in the most heavily used left-front corner eventually make this unreliable for tape-CA, and I re-surface…it’s not a once and done.

does it need to be flattened once? Nope. Also do not need to square the x-y rails, tram z, calibrate x/y/z travel distances, calculate feeds n speeds, or any of a host of things. If your friends are getting the results they want, no need to change anything. These, and many more of the standards of practice, increase accuracy and precision as well as potentially reducing process errors.

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It is $25 for a tolerable surfacer, takes less than 5 minutes to program and less than 15 minutes to run, and most importantly makes the bed parallel to the plane of motion. Just DO IT.
Not even sure why it is a question.
After surfacing, use a plate of glass on the spoilboard, and an indicator on an arm to tram spindle to vertical (I mean perpendicular to the table).


I bought a cheap set of 1-2-3 Setup Blocks on Amazon. They are cheap and get your router trammed quickly.


Hi Bradley- Depends on what your cutting. Of course a good surfacing makes sense. But I run several cncs at my shop and unless we are cutting wood planks or signs, surfacing doesnt matter. Most of the time we are cutting 2" HDU foam that will become molded and cast in fiberglass, so that level of precision will become lost anyway. Here is another unique situation. I’m about to cut wooden signs for a theme park and they specifically want the signs to look hand carved and not perfect or polished, so not having a resurface will actually be a benefit in this situation. But in general you should resurface at least every quarter.


I didn’t, surface my waste board at first. But, I soon saw letters on one end that were deeper than I’d programmed, and letters on the other end that were shallower to not carved! It’s a simple process and the tool paths are available for free! I’d do it!


I have found that it was necessary because I used stop block and hold downs by screwing not the MDF spoil board. Often when removing screws it would raise the MDF like a small ant hill. This, creating an uneven surface for the future. I have been using my SOP xxl for over a year and resurfaced 3x. I am ready to change out the MDF for a fresh spoil board. FLAT makes a big difference. Both spoil board and stock material. I make the mistake often early on my journey of CNC. Beat of luck in the woodshop! Tom​:+1:t2::four_leaf_clover:

Where would there be a cut file for a 4XXLPRO 33x33

A PDF of the wasteboard itself is at:

for a surfacing file, just work out the working area and cut that as a pocket.

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At its core, CNC is about tracking your numbers. Go Up, Down, Left, Right, Fast, Slow… all to a given set of increments.

All of these number sequences are built off of where you set your start point. Your X, Y, and Z (0, 0, 0). If you do not have a square and level set up to ensure to your basic start point of 0,0,0 is correct then you cannot accurately tell your machine what you want it to do.

With all of that being said, everything we work with has tolerances in the build specs so there is no such thing as perfect.

My two cents, best of luck.