Need Advice on Spoil Board and Table Construction

[mattinglot]

I’m currently working on building the table for the lowrider, and could really use some advice on how the table and spoil board work so I can get it right the first time.

In case it affects the answer: I’ll be using my CNC mostly for cutting all the way through the material (such as pinball playfields).

Question 1: Is my understanding of spoil boards correct?

My understanding is that the lowrider rides on top of the table (NOT the spoil board), and then the spoil board is a board you mount to the table so that you don’t ruin your actual table top when the bit goes through.

It’s also my understanding that all the work holding happens on the spoil board. So for example if you want to use hold downs, you can drill a series of holes in the spoil board, and then stick t nuts in them.

The t nuts are recessed far enough that in theory, we shouldn’t have to worry about the end mill coming into contact with a t nut even though the end mill does come in contact with the board.

The spoil board also gets leveled by telling the CNC to cut a shallow box equal to the size of the board, ensuring that it’s level relative to the CNC.

Is there anything I’m understanding incorrectly or missing?

Question 2: Does this plan make sense?

Given the above, I plan to make a torsion box table out of MDF. The table top will be glued together, made as flat as possible, and the Lowrider will ride on top of it. The table top will have a bunch of tnuts in it to allow spoil boards to be attached to it.

I will then take another piece of MDF to use as the spoil board and attach it to the table top using the tnuts built into the table top and recessed bolts.

I will then tell the CNC to make a series of holes in the spoil board for tnuts for workholding purposes. The tnuts will also be recessed to avoid end mill hitting them.

Does this approach make sense? And is doing the leveling cut necessary/worth it if I take my time with squaring up the CNC and making the table flat?

Question 3: Is 3/4" MDF Sufficient for a Spoil Board?

It feels like 3/4" is not much material. Once the spoil board gets too beat up, do I have to toss it and get a new one, or is there enough material to refresh it a couple times by telling the CNC to shave off the top layer? I don’t want to risk an end mill hitting a tnut as I have to imagine that would be Bad.

 

Thanks so much.

[forgecnc (Aaron)]

For hobby CNC machines, usually the type of t but used is the one that’s hammered in, and that goes on the underside of the spoilboard, so you don’t really have to recess them.

 

3/4" is fine, you think it’s not a lot of material until you have to cut something large out of it…

 

Depending on how deep you cut into your spoilboard, you can refresh it a few times.

[clay]

Question 1: You’re exactly right on every point. Wish I had figured that much out before I had my machine built.

Question 2: Making a table out of MDF is fine. however, I’d make one modification if I was you. As far as I understand the Lowrider, opposite of the regular MPCNC, is more rigid the lower the cutter is. So adding a spoil board above the wheels will not only decrease your maximum Z height but also decrease your maximum amount of rigidity. I’d move in the opposite direction and have the spoil-board flush with the wheels or better yet, recessed a bit. Some people have even made height adjustable “drop tables” which are pretty neat.

There are a million ways to hold down material as well. T nuts, threaded inserts, vacuum table, double sided tape, super glue, the list goes on. One system that I quite like and want to try out next is this one. I went with half in threaded inserts on my build and inserted them at the bottom of my spoil-board so I have a quarter of an inch of material left over.

Question 3: If you measure the thickness of your material you shouldn’t have to cut much, if at all, into the spoil board. 3/4" is plenty.

 

[jeffeb3]

I don’t think surfacing your spoil board is necessary for through cuts. And you can even get pretty far with some tricks for things like engraving. If you build your table flat, then the Z error will be small enough and you can just add some depth to cover the error. If you get it too shallow, you’ve got to punch out those shapes and use a pattern bit to clean it up. Not a huge loss.

The great thing about v1 machines is you have the felxability to do it however you like. Here’s what I do

My wheels ride on my spoil board (which is 3/4" ply), I haven’t surfaced it. My build is 3’x4’ work area and I made the gantry go the long way (I was planning on building up to a full sheet, but haven’t needed to). I built a torsion box to make it pretty flat. I just use #6 1-1/4" wood screws through the material into the spoil board. At one point, I made some 2mm deep hole cuts in a 3x4 pattern in my spoil board. I measured the depth of each hole. They were all ±2mm with the farthest off being close to 5mm. I haven’t seen anyone else repeat it.

More detail on my hold down screws: I usually use the cnc to mark where to drill them. I used to make a separate gcode file with just the drill going 3mm into the workpiece at different places that are waste. Now, I just use the preview in cnc.js to move the bit to a clear spot and use the router to mark it and I go back and use a cordless drill to finish the hole and install screws. If it’s a small piece, I have some 12" or so long scraps. I put one end on the work and put a screw through the middle. If I’m using scraps, I need to be careful to not go too fast or it could loosen.

You need to be a little thoughtful on the height of the clamps. You don’t want your gantry higher than it needs to be or the router to be sticking out too much. If you have 1.5" of extra heught you need to clear because of your hold downs, that’s wasted rigidity. Screws can be mounted countersunk.

[mattinglot]

Thanks so much everyone, this is a tonne of good information. I didn’t realize the t nuts go under the spoil board, that makes a tonne of sense in retrospect. I will modify my design so that the CNC rides on the spoil board as well.