Improving Spoilboard Vacuum

Advice on ways to improve your spoilboard for better vacuum and better hold-down of parts. January 7, 2010

I need some advice on how to keep my dovetail drawer parts (1/2" Baltic birch) from moving while cutting. I have tried about everything I can think of - fresh spoilboard, onion skinning profile cuts, different feed rates and rmp, but am still having quite a bit of trouble. My vac is a 25 hp fpz twin turbo regen blower. Phenolic tabletop, gasket between spoilboard/tabletop and dividing zones. Any tips or tricks are appreciated.

Forum Responses
(CNC Forum)
From contributor J:
Is it possible to use tabs? I am sort of surprised that an onion skin of say .04 and a slow feed rate on the last pass won't work.

From contributor K:
You might try a lightweight MDF like Trupan or Tafisa for the spoilboard. It helped me quite a bit instead of regular MDF. Other than tabs it sounds like you have tried the normal stuff. A down spiral bit helps some to keep it down and keep dust packed in the cut.

From contributor M:
I cut loads of Baltic standard drawer parts on a regular MDF spoilboard with roughly the vacuum you use. As contributor J said, a .04 or so onion skin works very well for me. 3/8 Onsrud full compression for the first pass, full upshear for the second pass, relieved at the top for clearance, so the dust gets completely cleaned out of the groove. I run the last pass at 1600 IPM, though on drawer parts the reality is the machine never gets moving that quick.

It is an extreme rarity that I ever have a part move. Something else must be going on. Is the material very warpy, or is your vacuum leaking? How is the cutting strategy for the dovetails - are you forced all the way through on the first pass?

From contributor Z:
If you are failing with an onion skin and you are sure that the material is flat, make sure that your tool length is correct. Many times I have seen people fail with the onion skin concept only to find that the tool was measured wrong and was cutting through the part on the first pass when it was supposed to be leaving a skin. If your tool is off by a hair more than the programmed thickness of the onion skin, then you will cut through the part.

From the original questioner:
I am using Trupan Ultralight MDF 3/4" for the spoilboard. Typically only surface the top side. I am not sealing the edges, though. My drawer parts are produced with Routercad dovetail drawers program and toolpathed and coded with Enroute.

I have been cutting the profile cut in 2 passes with a Vortex 3/8" 3184 bit running 600 IPM @ 16000 RPM. My onion skin is a bit thicker than what y'all are using though. I have been cutting about 3/8" on first pass and 1/8" on final pass. I will try and reprogram with thinner skin.

From contributor R:
You mentioned you only surfaced the top side. We machine both sides of the spoil board. It gets rid of that waxed feeling, lets air through better. Also you said you didn't seal the edges? It is a must. With this LDF a lot of air gets sucked through the edges. I paint mine with primer and tape all around the table with duct tape.

From contributor E:
That vacuum pump is a poor choice for your application. High volume and low pressure and you are operating a 25 HP motor yielding poor results. Minor modifications to the spoilboard will not solve your problem. Was the pump the standard generic model that came with the machine? If not, why was it chosen? A high pressure pump would suit your application better.

From contributor O:
Or you could produce dovetail drawers with the proper equipment. The CNC router has become the Swiss Army knife of woodworking machinery.

From the original questioner:
The pump is what the CNC manufacturer uses (Shopsabre). I did upgrade to the twin turbo model for more holding. I like my "Swiss Army knife" machine!

From contributor E:
Unfortunately, this is what happens when a specific vacuum pump is supplied by a router OEM. Pity is this forum has many postings like this. I would suggest contacting Shop Sabre and exchanging this blower for a high pressure vacuum pump. Otherwise, the machine will remain unsafe (parts flying off the table), unproductive, and underutilized.

From contributor I:
You never confirmed you are machining both sides of the spoilboard on the initial setup. If you do not, you will never draw vacuum through the spoil board. This is needed to provide a flat and parallel surface as well as removing the sealer from the MDF.

From contributor E:
Stop adding insult to injury. Machining both sides of the spoil board is not going to do it. I can hook up a 10 year old 10 HP Sullair rotary screw vacuum pump with a sheet of MDF right off the shelf from Lowes and hold these kinds of parts down.

From the original questioner:
From what I have seen, many CNC manufacturers are using FPZ regen blowers for vac hold down. Are you saying this should not be done at all? And why?

From contributor E:
It is done primarily because of cost in order to keep machines in the under $50,000.00 price range. Vacuum pump selection should be determined by the application, not a price point. In the long run it costs the end user more in time, wasted material and the eventual upgrade to a style of pump that is effective.

From contributor W:
Some of it has to do with the speed of the spoilboard cutter. The way it was explained to me was the faster you move the cutter across the surface, the better it will be. Also try using regular MDF, not the ultra light, and surface both sides.

From the original questioner:
Success! After making some changes using several of the recommendations posted here, I have just gotten excellent results cutting out some of these tricky parts.

Here is what I did to get good results:
Sealed the edges of my spoilboard stock.
Surfaced both sides of the ultra light MDF.
Changed from top of material z zero to top of spoil board (gets a more accurate onion skin).
Make first pass profile cut on all parts before cutting through.
Onion skin is .04 instead of .125.

Thanks for the help!

From contributor B:
Glad to hear you got it working. Were you onion skinning all the parts before cutting any through when you were having parts move? I had a similar thing cutting pre-finished 1/2" ply drawer parts, and that is the primary change I made that solved it for me.

From the original questioner:
At first I was 2 pass cutting the parts individually, then figured out how to change the cutting order and cut the first deeper cut on all parts, then do the final pass of .04, but I was still having trouble with movement and added the other fixes which work very well. I am surfacing the spoilboard more frequently with drawer parts now as well.