Holding the Workpiece

[Steven Wayne]

I recently modified the vacuum table of my cnc router. The mdf spoil board had been curling up and making it a hassle to get sucked down when I go to use the machine. When the company I bought it from was here setting up the machine I asked them what they do to hold the spoil board down to the table. They suggested wood screws into the vacuum table. Needless to say this project has been on the back of my mind for coming on three years.


First I had to draw the machine bed in cad. This went well considering I didn’t do any probing. I added brass threaded inserts to the vacuum table so I can bolt the spoil board down. While I was at it I switched to using strips so the t-tracks are exposed for clamping. Last thing was a X0 Y0 fence to make setups quicker.













This shows how this fence is used. Makes for quick setup most of the time.

New fence parts bolted down. Dry fit domino between X and Y in the corner.

Outside profile and surfacing done on the machine.

Fun project. Learned a lot. And, the spoil board no longer fights me when I start the vacuum pumps.

I really like having the X0Y0 fence. This makes the super glue/blue tape method a lot easier for me.

I recently bought a Hallmark ITTP touch probe so I can use probing cycles to set WCS offsets.

[bobby milam]

Steven sent you a PM on your machine. Looks good.

[Jim Becker]

Really nice setup there, Steven!

[Dennis Peacock]

Ok, back to an old topic.

How thick do I need to make my clamps to hold work pieces down?
I've made several from 3/4 corian that holds really well, but stick way high up on the work surface, preventing me from getting my dust boot low enough to do a good capture job on the dust. I do have a few pieces of 1/2" corian and I can also make some out of Red Oak, White Oak, Poplar, Cherry, Walnut, and etc if wood hold downs would work.
I tried drawing up a simple wooden clamp in VCarve Desktop and I wanted the end that would sit on top of the work piece to be stepped down so it would stick up so high but I ended up with something like 120 open vectors and not knowing how to "close" them.

I've seen several SVG files on the web but I'm still trying to learn my machine and just now finding out what my true limitations are. Clamping is one of my larger challenges right now as well as a Z movement that goes out of whack every once in a while.

[Jim Becker]

Dennis, the best clamps I've made so far are about .375" thick and made from scrap oak flooring that I ran through the planer. One end has the bottom relieved so there's a lip to go over the workpiece. The oak is strong but flexible ao these can be cranked down. They are much better than the ones I originally made from half inch plywood for sure.

That said....one must be adaptable when it comes to clamping some kinds of workpieces. You will use a variety of sizes, shapes and thicknesses over time and sometimes will have to fabricate one-time solutions, too. Also keep in mind that clamping is mostly about containing lateral movement. Vertical movement is less of a factor in workholding on the CNC, although with up-cut tooling there certainly is some vertical force involved. Sometimes I fasten strips of material to the spoilboard just to "box in" a workpiece for extra lateral support over any clamps or screws being used.

[Ronald Blue]

I made some similar to Jim's from some pine scraps that work pretty well. Not as strong as oak but seem to be sufficiently strong. I can see where something 3/4" or even 1/2" would be problematic. They could easily be in the way of anything being cut near the edge. I'm far from an expert but creating job specific hold downs could be necessary at times. Although some will be usable on a lot of projects.

[Ted Reischl]

...... Also keep in mind that clamping is mostly about containing lateral movement. Vertical movement is less of a factor in workholding on the CNC, although with up-cut tooling there certainly is some vertical force involved. Sometimes I fasten strips of material to the spoilboard just to "box in" a workpiece for extra lateral support over any clamps or screws being used.

+1 on that.

I use aluminum bars for clamps. Rockler sells a pair of table saw runners pretty cheap that work great as clamps since they already have a slot cut in them. Now that I have a mini mill I just make my own and am going to make a set out of 1/4 inch aluminum. It pays to put a little step on the end so it is easy to know how far into the work area the clamp is positioned.

[Grant Wilkinson]

I am brand new CNC user, so my opinions won't count for as much as others here. So far, I am using this system on my spoil board

https://www.amazon.ca/gp/product/B07...?ie=UTF8&psc=1

I like them because it means no aluminum tracks, which are pricey and bit-damaging. It was quick work to route the dovetail grooves in the spoil board and, so far, I've not broken any of them.

For hold downs, I am using CA and blue tape on thin pieces, screws from time to time and these for the rest of the time.

https://www.youtube.com/watch?v=wKiPfaA2oZY

So far, so good.

I plan to make a small vacuum table soon, as I have a Gast vacuum pump that I use for the vacuum chuck on my lathe.

[Jim Becker]

Grant, that system is fine, but isn't any different than using aluminum track in that you need the grooves in the machine bed for either the track or the dovetails and still need to groove your spoilboard to access them. The singular benefit would be no need for the track; the downside is that unlike the track, you need the angled inserts because using a common nut or bolt (in a pinch when extra clamping is needed) would potentially damage the dovetail as you tighten them. Even aluminum tracks wear from heavy clamping...trust me on that!

I have the Gast pump from my lathe, too, and it's only effective for very small, project-specific fixtures that have additional lateral support for the workpiece. A shop vacuum might be a better choice for a small vacuum table setup if you want to try that. I stopped trying to use the Gast pump and just went to tape and superglue when I don't want to or can't use clamps or screws.

[Grant Wilkinson]

Tks, Jim. You are right on all counts about the dovetails rather than track. I have my CNC mounted on 3/4" MDF, but then added another 1/2" piece to the top for the spoil board. I already had a bit to do the dovetail dadoes, but didn't have any spare aluminum track. Track here is very pricey. The dove tail fixtures were not cheap, but less expensive. Time will tell if I like my decision.

Tks for the info on using the Gast. I will likely only use it for small stuff, and since I have the pump already, I will likely try it for giggles. As far as using a shop vac, I've read that one issue is that, if the vac is not wet/dry, it relies on air flow to keep the motor cool. A vacuum hold down plate does not allow for any air flow, and this can lead to burning out the motor. I have no experience with this, but as my shop vac is not wet/dry, I think I'll not try to find out if this is true. :-)

[Keith Outten]

Dennis,

I make these from PVC. I like PVC because it is flexible and won't break over time, most wood species will eventually split. I quick cut these on my band saw and have a couple dozen of them in two sizes. They are basically free, made from project scraps. I have a major supply of 3/4" thick PVC sheet project drops that Norva Plastics gave me a few years ago. I can send you a flat rate box full to get you started if you want to give them a try.

[Keith Westfall]

I run a "T" track on my X & Y axis and I cut some 1" x .125" aluminum pieces about 4 inches long. Bent one end up and the other end down (slightly about .75 from the ends), drilled a hole in the middle, and then I can use T-bolts (of different lengths as required depending on the thickness of the material) and snug up the bolts and cut away. I also have an assortment of small wood blocks to match the thickness of the work-piece(s), that I place on the opposite end, so the clamp sits fairly level.

The slight down bend works well for on top of the piece, and the up bend sometimes allows me to "jam" it against a live edge to hold it in place. Especially those times I need to surface the whole board.

The aluminum seems to be soft enough to cut, in case it's not quite in the right position.

I have a small plastic container that I keep it all in and seems to work well.

I also have a strait edge on the log axis for reference, (trued with the machine for parallel), but a board across the end would limit doing long boards that need to overhang.

[Ted Reischl]

Aluminum is ok, but running into steel isn't.

Exactly, the value is in repeatable placement. I installed one on my mortiser for similar purpose.

Hitting aluminum with a router bit is not a problem. Good thing it isn't because I do quite a bit of aluminum cutting on the machine (on purpose).

So +1 on this comment Brian.

[Jim Becker]

Hitting aluminum with a router bit is not a problem. Good thing it isn't because I do quite a bit of aluminum cutting on the machine (on purpose).

'Depends on how hard you hit it, how deep and with what tool. DAMHIKT!!!! LOL Doesn't hurt the CNC machine much, but the tooling...well...

[Mark Bolton]

I recently modified the vacuum table of my cnc router. The mdf spoil board had been curling up and making it a hassle to get sucked down when I go to use the machine. When the company I bought it from was here setting up the machine I asked them what they do to hold the spoil board down to the table. They suggested wood screws into the vacuum table. Needless to say this project has been on the back of my mind for coming on three years.


First I had to draw the machine bed in cad. This went well considering I didn’t do any probing. I added brass threaded inserts to the vacuum table so I can bolt the spoil board down. While I was at it I switched to using strips so the t-tracks are exposed for clamping. Last thing was a X0 Y0 fence to make setups quicker.











This shows how this fence is used. Makes for quick setup most of the time.

New fence parts bolted down. Dry fit domino between X and Y in the corner.

Outside profile and surfacing done on the machine.

Fun project. Learned a lot. And, the spoil board no longer fights me when I start the vacuum pumps.

I really like having the X0Y0 fence. This makes the super glue/blue tape method a lot easier for me.

I recently bought a Hallmark ITTP touch probe so I can use probing cycles to set WCS offsets.

Its too bad they ran the T tracks end to end and didnt stop them short and give you some drop in pockets for the tracks. it would seem then you could easily pocket a new spoil board that was fully intact around the perimeter with the slots and drop-ins for the fasteners. may have helped your curling.

My spoil board will curl a little especially when it gets thin but I dont have any gaskets on the plenum and I never bother to seal the spoil board. I could definiely see the issues with the four individual pieces even with the gaskets. At least now youve got the file so running a new spoildboard is cake.

I have long thought about tracks for jobs that dont really require the vacuum all the time(so much more peaceful without it) but yet to get to a point where I have a solution Im willing to implement. Resurfacing/flipping the spoilboard daily is a major factor.