CNC Router Bits for Solid Wood

If the operation heats up the bits, high speed steel may last as long as carbide. December 2, 2011

I am new to the world of CNC routers. As part of a project that I hope will be my business in the future, I have bought a small CNC router to machine solid hardwood. The end products are small parts (average size not bigger than 50x50x20 mm). I have mastered the CAM software to the extent that I need, but am having difficulties deciding on the right type/material of router bits. I have tried several different types following suggestions of the dealers, but there are hundreds of types (just looking at the website of Amanatool is overwhelming), and I am in no position to judge which one is the right one.

Material to be machined: Solid beech exclusively

CNC router specifications:

Spindle: 1.4 Kw (1.85HP), 18.000 RPM max

Operations: Facing (on the top and sides to bring the work piece to exact dimensions), pocketing, contouring.

Forum Responses
(CNC Forum)
From contributor B:
A few words of economic guidance... Start with less expensive bits. It is highly likely that you will break a lot of bits over the first several months. HSS bits are 1/4 the price of carbide and actually hold a sharper edge. Better to break a $12 HSS bit than a $40 carbide bit.

The life of HSS bits vs. carbide will vary depending on your hold down system. If the solid wood parts are locked solidly in place, you will get more life out of carbide once you eventually make the change to the more expensive bits. However, if your parts cannot be locked down solid, as is the case with our curved moulding blanks, then you will get some vibration in the wood as you are cutting. This will force you to slower feed rates (typically less than 300 ipm) while retaining a high RPM (typically 18,000). The result of this type of cutting is that carbide will wear out sooner due to excessive heat. As such you will get a similar number of parts with a HSS and a carbide bit. That is why we do 80% of our cutting of hardwood parts with a 3/8" downcut spiral 2-flute HSS bit.

From the original questioner:
Many thanks. Before I was told that I can only use carbide bits, so never looked into HSS ones. This really relaxes my budget with trials!

Now there is the upward/downward issue. I understand that the differences are in chip removal and edge tearout. You mention that you do 80% of your cutting of hardwood parts with a 3/8" downcut spiral 2-flute HSS bit. The 2-flute choice is apparently a given in woodworking. Looking at the photographs on your website (nice pieces by the way) I observed that your work is mostly open, so the chips can easily find ways to go even if they are pushed downward. In my case however, let's consider a channel. Assume the end product will have a U-type cross section, and I cut the inner part with the router. I need clean edges on top and clean face on the bottom of the channel, since both will be visible. Let's say the groove width is 1/2", the tool diameter is 1/4" and the groove depth is 1/2". Which bit type do I choose? Do I now choose a compression up-down type assuming the up-down means no edge tearout and a clean face (what does compression here mean anyway)?

As a side note, the speed you consider slow (300ipm) is 4 times faster than the "faster than usual" speed on my router!

From contributor B:
Compression bits are rather pointless for cutting slots. They are a downcut spiral bit with the very bottom approximate 1/4" reversing into an upcut spiral. When you are cutting all the way through the material the bit goes about .1" below the bottom. The main downcut section is pressing the material downward and giving you a clean top surface while the small upcut section of the bit at the bottom is giving a clean cut to the bottom surface of the material.

You're going to need to use a standard downcut spiral bit designed to give a clean bottom surface. You can get by initially with a standard 2-flute bit but once you work things out you can spend a little more for bits designed for a cleaner flat bottom to the slot.

Given all this you are going to be compressing the chips into your slot. This will cause the bit to overheat and reduce its life expectancy. Since HSS stands up to excess heat better than carbide, you might find that in the long run you will stay with HSS. When we cut mouldings less than 2 1/2" wide on the CNC, we use this method since we would hit our hold down pods if we cut all the way through. We cut away the waste on the bandsaw and flush the edges (when necessary) on the shaper. One big downside to cutting slots this way, though, is that you are left with a slot full of tightly compressed wood chips. Removing them can be a pain - sometimes even 100 lbs. of air from of an air gun doesn't clear them out. You can solve this by running the bit through a second time after the slots are created. This will remove the bulk of the chips.

An alternative to fighting chip loading in the slot would be to do this in two passes. The first pass would be with a smaller upcut spiral bit that would run less than full depth. It would leave you with a mostly empty slot. Then you would follow up with a full width downcut spiral bit that would leave clean upper edges. Since the slot is already there the chips would have a place to go and not pack in tight like with the one pass scenario. All this is complicated, though, if all your slots are only 1/4" wide. You might pull it off with a first pass 3/16" bit but at 1/32" per side for cleaning you could end up with less than perfect edges.

By the way, 75 inches per minute is really crawling. If that is as fast as you can go without blowing the wood blanks off the table, then you should try to work out a more solid hold down system. You should be able to cut 1" hardwood parts at a min. of 150 ipm.

From contributor S:
The bit you described in the first paragraph is a mortise compression bit. Normal compression bits of other than very short cutting lengths have longer up-shear. Mortise compression bits have a 1/4" or shorter up-shear for routing pockets and slots or for very thin material. Check out the Vortex site.

From contributor B:
That's interesting. Every compression I've ever bought had about a 1/4" upcut. I didn't see standard upcut length on the Vortex site. What would that be... more like a 1/2"?

From contributor C:
Our [Southeast Tool] mortise compression bits are standard 3/16" on the up cut. This is so if you are cutting a 1/4" deep dado, the upcut does not come about the slot and tear it out.

From the original questioner:
Thanks for the clarification. I also find the two-pass machining idea very good. But why do you expect to have non-perfect edges?

On the speed side, I have to experiment more. I assume I have much smaller motors on the axes and as the spindle than the router you are using, but I probably get scared too early with increasing cutting noise.

From contributor S:
I doubt if there is a standard up - down cut ratio. It probably varies by manufacturer.

Just for grins I checked a Vortex 3160 with CEL=1.75". The up-shear is ~20mm or more than .75".

From contributor B:
I was thinking of the rough edges because your first pass would be with an upcut spiral. That will give you some minor tearing and chipping on the top edges. If you start with a 3/16" bit and finish with a 1/4" bit you only have 1/32" per side to clean up any tear out from the upcut pass. It will probably be okay, but it could be cutting it close.

From the original questioner:
Ah, I see. Thanks. What cutter brands do you prefer?

From contributor B:
I use Onsrud bits. I do this for several reasons:
1. The bits have done the job.
2. When I first started out I was breaking bits like crazy. Onsrud sent a technician out to help me solve the problem, no charge.
3. A number of years later after we had our systems down and were no longer breaking bits, I had a bunch from the same lot break. I sent them into Onsrud and they replaced them all.

From the original questioner:
Sadly they do not seem to have a metric collection. I emailed them anyway; they may have a European branch for that.

From contributor B:
Odd... I thought I had bought metric bits from them, but maybe not. I'd give them a call. They almost always have a tech available to speak with you.

From the original questioner:
There *are* metric ones, but they are buried in the lower levels of the site. Right now I am looking at the 52-411 (carbide though).

From contributor O:
I use carbide bits from Centurion Tools. They are inexpensive and last very well. I do all my designs and cutting in metric mm but all my bits are imperial (fractions of an inch) except a handful I asked them to make at 6mm for a specific job.

Using imperial sized bits and cutting metric isn't a problem at all except for specific actions like drilling holes where the diameter is critical. I just input the bits as fractions of a mm and the software takes care of it.

From the original questioner:
Many thanks. I will check them out immediately. Using imperial to cut and designing in metric is a very practical solution as you described.

From contributor M:
Just to clarify, we [Vortex Tool] offer compressions in both standard configuration (about equal amount of up and down cut lengths) and mortise compressions with shorter upcut lengths, either 1/4" or 3/16". Part numbers for the mortise bits in two flute 1/2" are 3187 for 3/16" up and 3189 for 1/4" upcut length.