Up-Cut or Down-Cut Router Bits – Which to use?

 

It’s bad enough that there are literally tens of thousands of end mills and router bits that you can use on your CNC router, but then you have different variations of each as well.  In this post I will discuss the differences between Up-Cut and Down-Cut router bits and when you should use each.

Two Considerations With These Bits

First off, this article will take into consideration that most CNC routers will run with the spindle, or router, in the clockwise direction.  So when we talk about an up-cut or down-cut, we are addressing the way the flute and cut pattern is machined into the bit and not the direction that the bit spins.  I used to take for granted that this was a known fact, but I do get a lot of questions from people on this since the spiral directions are actually opposite from one another and they assume you have to reverse direction of the bit rotation.

Second, understand that there are differences between what we call a “router bit” and an “end mill”.  A lot of applications in a CNC router can use a standard router bit (which is designed for a handheld router), but there are some where this is not possible (or recommended), such as bits that have bearings connected to the end – these you would not normally use in a CNC router.  The main reason I mention this though, is because a lot of people use a big box store such as The Home Depot or Lowes as a source to get their bits for the CNC.  While these stores may be convenient, they do not offer the best selection of cutter types, especially for the CNC router.  Understanding this, I would consider shopping online, either from the place where you bought your CNC machine, or from a large end mill retailer.  You will find a much larger selection and for many size end mills, you will find both up-cut and down-cut versions.

The Up-Cut Bit

One of the biggest advantages of the up-cut bit, is the ability to rapidly evacuate chips out of the area that the bit is cutting. This is very important, especially when cutting hard materials, like aluminum, deeper than 1/8″ where you run the risk of recutting chips, which can ruin the finish, cause chatter, or break the bit.  No matter what material you cut, on most hobby-grade CNC machines, when you wish to cut deeper than 1/8″, you usually will take multiple shallow passes, and the up-cut design of the bit will help keep the channel clean from dust and chips.  This can be even more helpful when you are using a more rigid machine where you will take deeper cuts per pass.

When To Use?

Up-cut bits are often used when cutting hard materials like aluminum, brass and copper and are additionally the normal choice for cutting plastics and acrylics. Any time you need to ensure that the chips are removed from the cutting channel, it is best to use an up-cut bit.  Also for deep, narrow pocketing operations like mortises or for hold drilling, it is best to use an up-cut bit

When Not To Use?

I, personally, do not like using up-cut bits on laminates or plywood because the natural up-cut motion can leave frayed or chipped top surface edges, especially when cutting faster and more aggressively. I also do not prefer using up-cut bits on thin materials where I have no way of securing the center of the workpiece to the table, such as with thin plywood or plastics. If there is any bow in the material the natural up-cut force from the bit will cause the material to be pulled away from the machine bed as its cutting.

The Down-Cut Bit

The down-cut router bit is excellent for applications where you need a very clean, crisp cutting edge on the surface of the material, especially in hard and soft woods. The downward sheering action of the spiral cutters work a lot like a pair of scissors and sheer against the material that is being cut.  This sheering action is what leaves a clean cut edge.  Down-cut bits can still evacuate dust and chips but they do so much less efficiently than an up-cut.  A greater amount of chips will be left in the channel as the bit passes through.  Because of this, in many cases, shallower passes and slower feed rates might be required with a down-cut bit depending on your machine and the material you are cutting.

When To Use?

I tend to use a down-cut bit when I am working with any laminated materials, plywoods that are usually 1/2″ or thinner, or on thin materials where I don’t have a good way to fasten the center to the word table.  On plywoods greater than 1/2″, I typically will use a compression end mill, which gives me the best result on both the top and bottom finish of the material (more on Compression end mills in a future article).  On thin, bowed material, the down-cut bit works nicely because the natural action of the bit forces the material down towards the table as it is cutting. You will also find that when cutting small parts out, there is less change of a part being thrown from the bit engaging it as it is cut all the way through.

One unique use I have for a down-cut bit, is when I am working with MDF and I don’t want to use tabs to hold the parts in place once I cut all the way through. What I usually do is take passes no greater than the diameter of the cutting bit (so I will take 1/4″ pass depths with a 1/4″ diameter bit). This will force the dust back into the channel, which does not affect the cut quality, but as the dust packs into the channel it acts as a binder, which keeps the final part in place after the bit has made its last pass though the bottom of the material.  While this can work very well in place of tabs, you need to be mind full of your spindle (router) RPMs and your feed rate to ensure that you are not overheating the bit.  Your nose can be very helpful here – if you smell that wood is burning, then you are definitely overheating the bit, which will reduce its useful life and can even, in extreme cases, cause a fire.

When Not To Use?

I never use a down-cut bit on acrylics or any plastics greater than 1/16″ thick.  The added heat generated from recutting chips stuck in the cut channel can cause them to melt as the cutter passes through, leaving a very poor finish and requiring you to do some cleanup work just to get the part separated from the material when the job is done.

I also never use down-cut bits on metals.  I want to always ensure I am taking all precautions to keep chips clear from the cut channel, and just like with plastics, recutting metal chips cause the bit to overheat and in extreme cases, can get the chips hot enough where they weld themselves to the tip of the bit and can cause chatter, poor finish quality or even cause the bit to break.

Lastly, I will not use a down-cut bit in cases where I am looking for a nice clean bottom cut on the material.  The downward action of the spiral, works just like an up-cut bit does to the top of the material, except since it is pushing downwards, the bottom edge can not fall victim to chips and frayed edges.

Summary

The best scenario is to have both types of bits in your tool library so that you will never be caught without the proper bit for the material or finish quality that you are looking for. You can find up and down cut bits in many sizes, typically ranging from 1/16″ to 1/2″ in diameter.  My favorite brand of tooling is from Amana Tool.  I find their bits to be very accurate, leave an excellent finish and last a long time.