This article is authored by Carbitool, a leading woodworking tool manufacturer in Australia. In this article, Carbitool discusses the benefits and drawback of an upcut and downcut router bit, as well as when and how to use them.

When there are so many options available to woodworkers it can be difficult to pick the right tool for the job.

A question we often get asked here at Carbitool regarding spiral flute cutters is whether to use an up cut or down cut router bit. And the answer is that it all depends on your machine, materials and your desired finish. When it comes to picking the most suitable bit — it will be determined by your job’s particular demands.

That’s why we have compiled this handy blog to ensure that you get the perfect finish — every time.

The Differences Between Up Cut and Down Cut 

Spiral flute routers are available in both up cut and down cut in a wide range of diameters which means that you have plenty of choices when it comes to selecting the right tool.

Up Cut Spirals 

Up cut spirals work similarly to a HSS twist drill. Under right hand rotation, the spiral pulls upwards — bringing the swarf up and out of the workpiece. This can be a huge advantage for mortising as it can remove swarf from a narrow, deep mortice.

Importantly, it also exerts an upward force on the workpiece. An up cut spiral might not work as well for edge finish on the top surface because the upward cutting action can lift the timber edge and result in splintering but it will reduce splintering on the bottom edge. Set your cutter height so the bit protrudes through the bottom of the timber and you will have an up cut spiral action pulling up into the bottom edge. This will produce a chip free, clean finish on the bottom edge.

A potential disadvantage of up cuts is that when cutting thin, light materials such as Acrylic sheets the up cut may lift the workpiece and distort the cut or cause vibration.

Down Cut Spirals 

The opposite is true of down cut spirals. 

While they still operate in a right hand rotation, the spiral runs downward applying a down shear pressure to the workpiece’s top edge. This will stabilise the cutting process on the top edge but might leave the bottom edge prone to breakout. Down cut spirals are not generally recommended for mortising as the swarf will be directed down into the bottom of the slot making it difficult to clear sawdust from the cutting operation. Finishing passes with a down cut can be effective in this case as less swarf is generated when performing finishing passes and top edge tear out is eliminated by the downward force applied.

The downward cutting action will tend to push thin, light materials down against the bench reducing vibration during machining.

The benefits and disadvantages of up cut/down cut spirals are reversed if your machine is bench mounted or set up in a router table in an inverted position.

Working With Different Materials

Often wood machinists with the most demanding requirements, working to the tightest tolerances such as Boxmakers and Timber instrument makers will be using the most challenging timbers. 

Tonewoods such as Ebony, Rosewood, Maple, Alder, Blackwood etc offer great tonal properties & stability but achieving the perfect finish can be easier said than done. Some timbers are particularly hard to deal with and require more advanced tool geometry. Light timbers such as Cedar, Spruce and Pine can easily splinter. It can also be difficult to eliminate a furry finish when machining these materials. The upward or downward force exerted on the workpiece by spirals is usually the answer.

Composite materials are comprised of layers and/or particles or fibres held together with epoxy, polyester or acrylic resin adhesives and can be susceptible to delamination when machined. Whether it’s an advanced composite such as those used in aerospace or an everyday composite like MDF, particle board, plywood or solid surface benchtop material, a “one way spiral” may cause chipping or delamination on either the top or bottom edge.

Right Bit For The Job 

Spiral bits offer another advantage. 

In recent years, spirals have taken the next step and have been developed into 2 way spirals with a section of down cut at the shank end which intersects with a section of up cut that extends to the cutter end. As both spirals push towards the centre they compress the board as they cut — which is why they are called ‘compression’ bits. Compression bits will press the composite’s layers together during cutting preventing delamination and ensuring a chip free finish on the top & bottom edge simultaneously. The extremely sharp cutting edges of compression bits help to limit the heat generated when cutting resinous materials. High relief angles assist when cutting softer board product which can undergo “springback” once the chip has separated from the board. Check out our blog “6 Ways to Master Compression Cutters” to learn more.

Original Source: https://www.carbitool.com.au/2021/11/up-cut-or-down-cut/