Question
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:
Max.X=500mm
Max.Y=500mm
Max.Z=200mm
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.
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!
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.
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.
Just for grins I checked a Vortex 3160 with CEL=1.75". The up-shear is ~20mm or more than .75".
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.
