MANUAL CNC CLAMPING:

CNC Fixturing with Screws, Glue & Toggle Clamps

When all you have on your table is a MDF sheet your clamping options are pretty limited. Having said that sometimes simple is the best... especially when you just start learning CNC.

Also, if your project is a "ONE-OF" then spending hours and dollars to create custom fixturing doesn't really make a lot of sense so you may want to resort to some pretty basic CNC hold-down techniques.

Screws:

Design your work with built-in scrap.. corners that are going to be cut off or inside cut-outs that will be eliminated.

Then in the G-code add "tabs" or places where the cut path has been interrupted by a bridge of wood that connects the good part to the scrap... at least as long as you need to keep it attached to the table... often the software automates this.

When the job is done, remove the screws and with a small saw, cut the tabs to release your work from the scrap.

Fast easy concept and CHEAP!

Glue:

There are numerous options out there from peal and stick adhesives to double-sided tape and kids' rubber cement. The success will be a function of how much holding power you need AND your accuracy requirements.

Some glues "creep". When the CNC tooling engages your wood, it puts a side load onto the cutter and the wood. Elasticized glues like say silicone or rubber cement can shift in response to this load and change the relative position of the wood, from its origin, affecting cut quality.

If on the other hand you're implementing a CNC based drilling pattern where all the pressure is down this may not be an issue, and glue clamping may offer the perfect cheap, clamping solution.

Clamps:

Toggle clamps are another cheap and easy method for CNC clamping. They're available in 100's of different sizes and configurations, for both vertical and horizontal clamping applications.

The challenge is to make sure that your CNC tooling doesn't run into the clamp. BEEN THERE DONE THAT! Sophisticated CNC software allows you to map out the location and specifications of your clamps and then creates the G-Code to avoid them, but most of the software you and I are likely to use doesn't have that functionality.

You also have to consider the substrate you are screwing your clamps into. If your CNC table is MDF, you may have a few challenges. Toggle clamps exert a surprising amount of clamping pressure. After repeated use, the clamp screws may pull right out of the MDF that forms the CNC table.

Try adding a bit of glue to the holes to reinforce the threads and gain them a bit of strength, but ultimately you're going to experience some frustration and may have to change the bed of your CNC table or convert to a different clamping method.

This is a great beginning, in learning CNC ... the next step might be a discussion of use of T-tracks as a means to avoid all those holes in your table.