[forgetcolor]

I'm building a JGRO router and relatively new to routers in general. My question is what diameter bit do people generally use for CNC? I'll be mostly cutting softwoods, plywoods, and MDF. Also, how much longer does the bit need to be than the depth of cut (if at all)?

thanks

[Woodworker24]

Hi Forgetcolor,
I finished my router about 5 months ago and have found that 1/4" is the most common, depending on how fast you're cutting you might want to go to a 5/16". If you are planning on doing plunge cuts make sure you buy the up-cut bits unless you can ramp into the material, hope this helps

[thaswell]

You can use any number of different bits. As for the length it is best to not go any longer than necessary. This is because the closer you stay to the bearings the easier it is on the router. In cutting wood this is not a huge factor but is always a good thing to keep in mind. I use bits from a pointed bit for engraving to a 3/8 up cut for roughing. You can use different shaped bits to cut decorative edges such a cove or special bits for cutting light aluminum flat stock. Your choice of a 1/4" up cut is a good choice as long as it does not splinter the top surface of the plywood. A good tool supply store can help you pick the right tool for the job

Good luck and have fun. That is what it's is all about!

TH

[Charlm]

The shank size should be an indicator to how big the cutter should be. If your machine only takes a 1/4 inch shank then you should not really go any bigger than this until you understand what your machine can do.

Straight double fluted tungsten bits are a starting point for a nubie and you will have varying results. When you change feed rates, cutting speed and depth of cut this will also change how much you can cut properly. Over time I have been able to work out how deep I can cut at a standard speed. I set this quite slow about 100 mm per minute and then i scale this up when i am running my machine. Material type and bit speed will change what is the best speed for your job.

My suggestion is to make a simple test code that takes up a small area of your material and see just what is best for that material before you try to cut your real job. I have a simple test code that makes a recess that is the right size for a 808 bearing (i had a few of these left when i finished my machine). I muck around with feed and cutting speed until i am happy with my result (I am interested in finish and I need to be able to firmly fit the 808 bearing into the recess without slop or it being too small for the bearing to fit into).

Hope this helps....

[CNCRouterm]

Hello,
Tool diameter is largely a preference, within the constraints of machine capabilities. For me, 1/2" tool diameter is preferred for wood products, 3/8" for plastic or aluminum. The 1/2" choice for wood products is because that size is a commodity size, it is only about 12% more money than 3/8", and allows for more chip load. The 1/2" tool is also much stronger, so it is far less likely to break than the 3/8", the finish is also better at the elevated feedrates I typically run at as well. For your product mix, I would use a 1/2" compression tool most of the time, unless product details demanded a different size, such as 1/8" max inside radius or similar specs on the customer prints.

The first CNC I put together used Porter Cable 3-1/2 hp routers for spindles, they had a both 1/4" and 1/2" collets, which still is pretty limiting, but workable. That type of spindle is going to be the limiting factor in tool selection, as a 1/2" compression tool with 1-1/4" cut length is quite capable of stalling the router long before you get to the designed tool limits. My current machine is not a DIY effort, so I have to dig through the cobwebs in my brain to get in your shoes.

In short, use the largest practical diameter tool, use the shortest cut length. The tool will have more flute than cut length, so for most applications, a 1" cut length tool (which will likely have 1.02 to 1.032" cutting edge) is valid for 1" material, provided you can hold Z reasonably well. If your machine deflects easily in the cut, a bigger diameter will impart more deflection, but be more resistant to breakage, you have to decide on the trade offs.

What feed rates is your machine capable of? I usually program feeds and speeds based on material and chipload. Different tools have different chiploads, as do different materials. Bigger tools can carry bigger chiploads as a rule. For a 1/4" compression, in 1/2" mdf, I used to run about 18K rpms at 16500mm/min or 650 ipm feedrate, which is about 0.018" chipload (feed per flute per revolution). Bear in mind that my spindles are modestly beefy router spindles with hsk63 tool holders. Back in the old days, the old Digital could barely make 400 ipm on a good day, and the spindle mounting was not all that rigid. When they upgraded to a Perske with variable frequency drive, that help alot, but still, the machine was not all that rigid or fast.

[dcorbett]

There are speed (rpm) & feed (ipm) calculators online you can use to find the best cut.
You need to know the shaft rpm, the tool material limitations (designed chipload), and the tool dimensions when setting up.
Soft woods, like pine are easy, and you can get by with a high-speed steel.
MDF will most certainly require something harder to last any time at all, and you will need to be careful to set up the speed correctly - when working with carbide, you need to bite off a lot of material. I found out the hard way. I tried setting up using HSS settings, and the carbide bits kept breaking...slowing it down only made it worse. Turned out that the carbide was flexing instead of cutting. Sometimes it is counterintuitive.
Your best bet is making friends with the tool guy in your local woodshop.