Some operators like this bit as it packs the sawdust in the groove being cut

Some operators like this bit as it packs the sawdust

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and helps hold the material in place on the table surface. Some operators like this bit as it packs the sawdust in the groove being cut, this helps hold parts in position and helps maintain vacuum when that type of hold down system is used. Disadvantages are the packed sawdust is not removed by the dust collector and must be scraped or brushed off manually. Also this bit is not appropriate for thermoplastics as the "packing" of the dust re-welds the parts together. When making "through cuts" the down cutting force can splinter the bottom face of veneered or other susceptible materials
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Compression – The third type of spiral bit is a hybrid design of the up and down cut bit. This bit has a up cut portion on the lower part of the bit and down cut on the remainder of its CEL. Material being cut is augured downward from the top of the material and upward from the bottom. This compression of cutting forces results in a clean, splinter free top and bottom face. A special type of compression bit is the Mortise compression. This style has a much shorter up-cut section than the standard compression bit and is used for grooves and dados as shallow as ¼" while still having a splinter free top surface. There are other variations of these three types, one involves the angle of the helix, fine tuning the bit for particular materials. Bit Ends Some router bits are designed for plunge cutting and some are not. Standard End – bits will plunge cut nicely, these bits can be identified by the swallow tail style profile of the end of the bit. This bit is the right choice when cutting profiles. The disadvantage of this style bit is it leaves marks or swirls when pocket cutting. FEM (flat end mill) this style of end geometry is designed to cut a smooth bottom surface when pocketing. The disadvantage of this style is it need to be ramped into its cut. Forgetting to do this will result in a burned bit. The end profile of this bit is flat. FEM end bit Standard plunge end bit Compression Bit
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