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another newbie question

1026 Views 4 Replies 4 Participants Last post by  TimPa
as we prepare to buy a cnc, another concern is the table and clamping style/methods. eventually, we will be in a semi-production mode. have seen some very clever clamping methods that don't require a vacuum pump/table. I really don't want the extra noise or expense if possible.

I realize that what we are cutting plays a huge role in how it can be clamped. just curious what the experts have found over the years of use. any suggestions are greatly appreciated.

also, what is the method called when you almost cut the entire piece out but leave tabs that must be cut later to release the product???
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Hi Tim, and welcome. Where I work (University furniture design shop) every project is unique. Essentially the exact opposite from a production shop. We have a large CNC with a MDF bed an vacuum table which was $90k, isn't as noisy as you probably imagine, and is great for processing anything made from sheet goods. We also have 3 small CNCs (up to 36" x 48") with t-track beds and frames which handle just about anything made from hardwood including furniture parts and joinery and detail/inlay work. They could easily be jigged up for repeated production parts. They are often custom configured to cut challenging parts that can't be done using any other tool in our shop.

As you've stated it is what you will be cutting that will dictate how big and how solid of a machine you'll need. If what you'll be cutting might change over time then I'd recommend a machine with a bed that can be quickly re-configured. The CNC itself won't care if every file it cuts is different or if it cuts the same file thousands of times. For constant running though you'll want a true spindle rather than one using a router.

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I have seen a wide variety of methods being used. For smaller machine (Shapeko, Zen robotics, etc,.) I have seen people using double side tape.
Many people will screw down their pieces to the spoilboard. Most consider the spoilboard to be sacrificial and regularly cut into it, and then periodically resurface it.

I differ from most. I treat the spoilboard like the top of my workbench. It needs to be a flat reference to enable accurate work. Just like I do not purposely screw or cut into my workbench, I do not cut or screw into my spoilboard. Like my workbench, I have clamping provisions on my cnc (dog holes and t-tracks). Like my workbench, it is not a piece of furniture, and an occasional ding or two is no cause for concern, just make sure they are scraped to be below flush.

There are two main types of cuts using the cnc router. First is carving or engraving - there have been many posts here of signs or carvings. For the carvings to be uniform, the spoilboard needs to be flat, and parallel to the X Y plane of the cnc (I.e. Constant Z). That is why you will see people "surfacing" the spoilboard or milling it to ensure it is flat and at a constant Z (and also complaining about the huge amount of mdf dust this process generates). I chose to surface the under structure and attach a uniform sheet to the top of that. Less dust.

The second use of the cnc is to cut out parts (in software these are usually called "profile" or "contour" cuts). In these cuts the bit goes all the way through the workpiece. This is the type of cut that will chew up the top of the spoilboard and require periodic surfacing. To avoid this I simply add sacrificial spacers made from 1/8 hardboard under my hold down clamps. I can then safely cut through the stock by .03-.05 and never damage my spoilboard.

Many operations of course have both carving and profile cuts in the same piece, and are usually done in a single setup with only bit changes.

For either of these type of cuts, to secure the workpiece to the CNC you need to keep it from moving, and you keep it down flat against your reference. The common hold down clamps that engage into T-tracks do the hold down, but may still allow the stock to slide. On my CNC I have a series of dog holes along one end and one side. I use cheap plastic dogs in these holes (mine are from Rockler, but dowels and wood blocks would work as well). I then wedge the stock against these dogs to keep it from moving. I then have several have several clamps that hold the work down using T-slots machined into my spoilboard.

The cad/cam software (Vcarve, Aspire, ArtCAM or Fusion 360) you will use all have provisions for adding "tabs" to keep any cutouts in place - this serves the same goal of keeping the piece from moving and held down. Once the piece if done with cnc operations, you remove it from the cnc and either snap out the part, or cut it out with a knife or small saw, then sand or use a flush trim bit in router to remove the tabs.

My recommendation is to manually place the tabs where they are easy to trim, and not be stingy with them. As the profile cuts are usually done last, you don't want a workpiece to be damaged by prematurely breaking free!
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I use T-slot from 8020 and clamps. I tried the double sided tape at first and after watching a 1/2 finished 30" long piece of oak being slung round and round I went to the clamping method. If you screw your cutting piece into the bed it limits you if you have a precut piece that you don't have to profile cut when done.
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thank you all for the replies. I love this!

we are going to need an indexing rotary lathe for one project. some models have them table mounted while at least one has it off to the side. this decision may play a role in table style I guess...
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