CNC Setup is a Balance of Many Factors.

I created a Free CNC calculator to help sort a delicate balance between Feed Rates, Chip Loads, Spindle Speed, and Power that go into finding a CNC setup the optimizes finish quality and minimizes the time required.

The concepts of setting CNC router speeds and feed rates are very confusing to hobbyists and even more seasoned enthusiasts. There is a delicate balance between Feed Rates, Chip Loads, Spindle Speed, and Power that go into finding a CNC setup the optimizes finish quality and minimizes the time required.

I searched for a good CNC calculator to help me understand how to set up mine What I found were bits of information scattered about but nothing that pulled the information together in a form I could understand better. And so I created my own.


Many many thanks for coming to my site and downloading copies to explore CNC settings. I received many compliments and comments on the tool and some suggestions for enhancements. It has inspired me to work on an upgraded version with additional features like compensation for Chip Thinning, better user controls, more warning flags. Adding a section allowing some custom materials.

I am happy to announce Explorer 1.0 has launched! Again that you for your interest in my tool.

Link to CNC Explorer 1.0 Introduction and Purchase. Also, downloads for Free “Limited” and “Lite” Versions.

CNC Explorer 1.0 Dashboard

OK let’s take a look at the calculator. This is a snippet of the top section of the full version. I am going to throw around a number of terms and if you are not used to seeing them I suspect it might be a little confusing. Probably another article down the road explaining the terms but for now, I want to show the sections of the tool. Hopefully, after showing a few examples it will become clearer.

The left side is where we configure our router/spindle limits. I have two sections for spindles so you can compare say the Makita RT071C against any other spindle you might install. I entered an 800W spindle which is what I was thinking of upgrading to. When I started developing this tool I had the idea it would maybe show if it made sense to go with a spindle. As I used the tool for different types of cuts I was surprised how it showed the strengths and weaknesses of both. One factor I didn’t consider before was the power of the spindle and how it can limit certain types of cuts. On the other hand there always seemed to be a solution that was a compromise of adjusting the feed rate or spindle speed. So yeah I put some defaults in this section. The Makita has an RPM range of 10K to 30K. If you wanted to use this router you would put in 0.932 KW which is the equivalent of the 1.25 HP of the motor. Since I choose an 800W spindle to test out that is what I left in as default. the third collum is for the CNC limits. I of course put in the Onefinty but had to guess on the feed rate limits. the Onefinity specification for X/Y rapid rate is 500 in/min. Most likely the mechanics i.e. stiffness of the machine will limit the actually cutting speeds. We will have to learn what the limits are. As a practical matter considering this is a “Hobbyist” level machine I would think a limit of 200 in/min. would be a good guess. The power of the spindle will in most cases be the limiting factor anyway. But you can adjust these limits at any time. The light blue boxes are free to edit. And that’s all the information needed to configure the calculator to match your setup. In the full version, all the settings for a particular cut use the interactive slider and toggle buttons. So we are free to play.

The top right section shows the results and conditions for our milling operation. The DOC (Depth of Cut), WOC (Width of cut) the Feed Rate, and a suggested Plung rate. The top row shows the SFR (Surface Feed Rate or how fast the bit cut edges are moving, MRR is the Material Removal Rate i.e. how much material is being removed per minute. This value is directly related to the amount of HP required and that is calculated and displayed. Finally, I thought it would be useful to convert into Wattage and that box will turn red if it exceeds the Wattage limit you entered in the spindle section. I was actually surprised how many times I tripped that flag when I was testing out different configurations.

To the left of the readout sections are all the controls to setup an operation. Starting with the blue slider you can adjust the router/spindle RPM. You can also manually type in a value in the dark blue box. Just to the right are a pair of dark green toggle buttons that will increment/decrement the desired Chip load by 100th in. Skip over to the yellow toggle buttons. Here we can select our bit diameter 1/8th, 1/4th, 3/8th and 1/2 inch bits. The light green toggle sets the number of cutting edges or Flutes of the bit. The dark brown toggle sets the DOC (depth of cut) in 0.01 increments. Next to it is the purple switch that picks the WOC (width of cut) as a percentage of the bit diameter. So for example if you want to make a slot cut then the full 100% of the bit is cutting. The last control, very light green picks a percentage to dial down the DOC in case we need to cut back on the power required to make a cut.

Chip Load is Central

Chip load is the amount each cutting edge or flute of a bit tool cuts off the material on each revolution. Dialing the amount is important because too much will wear your tool and possibly damage your equipment. Too little and the time required will be excessive.

I have seen this diagram all over so I don’t know who to give credit to but it makes my point. For best results stay in the sweet spot. You cannot adjust either the RPM or the feed rate independently. The two are locked in a solution and it depends on the desired chip load.

Chip Load is controlled by these factors:

  1. Feed Rate – How fast the tool is being driven by the CNC
  2. RPM of the tool – The rotational speed of the Trim Router or Spindle holding the tool bit.
  3. The Tool Bit Diameter. What is the cutting circumference of the tool?
  4. The Number of Cutting Edges or Flutes the Toll has.
  5. The design of the tool by the manufacturer

If you look on a tool manufacturing web site such as Amana Tools for each model you will find the chip load per tooth specified. The values are typically in the 1000th of an inch. You will also notice the values are dependent of the type of material being cut.

In my tool I built a graphical table of a sample set of different material and relate to the typical chip loads for 1/8″, 1/4″, 3/8″ and 1/2″ bits for each material. This list could be expanded as needed. Each tool diameter has a range of suggested chip loads.

Below the control buttons section is a table of different materials and the optimum ranges of chip load for each bit diameter chosen.

The table will highlight in yellow for each bit diameter selected. I collected these suggested chip load ranges from several sources and checked against one of the big bit vendors Amana Tool. All the sources are pretty much in agreement. Remember the dark green chip load switch will let us cycle through the range of chip loads and highlight in dark green. We can dial the selection to the proper range for our bit and material.

The caveat however is these suggested ranges are optimum ranges of chip loads. The case may be that our equipment is not capable of operating under the requirements. Limiting factors are going to be the speed ranges of the spindle, the type of bit used, what kind of cut are we making, does the spindle have enough power, is the CNC ridged enough to keep the bit on track. As you can see there is a vast degree of limiting factors. The best approach will always be to start a cut on the very conservative side. If all is going well start creeping up to the optimum settings.

Equipment Indicator Section

The last section of the tool is the equipment indicator section. This section keeps track of the operating ranges for our spindle and CNC. The valid range is highlighted in green and will slide up and down depending on the selected chip load target. You may notice for a given selection condition the RPM solution may be too low for the Makita or too high to the spindle. When we learn the limits of the Onefinity we might notice we are beyond the capability range. I find these range scale helpful in picking setting that gives some margin to back off conservatively for the initial cutting. the other thing I kept running in was the power limit of the 800W spindle and the indicators help back off some conditions to get the power back in line.

On to Some Examples

I tried to put into the tool useful things needed to set up a CNC for a milling operation based on the material stock, tool bit diameter being used the limits of the CNC machine itself, and what are the limits of the router or spindle on the CNC. It is interactive allowing adjustments to optimize the setup. The goal of the tool is to find a safe conservative initial setup to start a cutting operation. Sweet spots of both a Trim Router and a Spindle are indicated. I think this would help someone trying to decide to upgrade to a spindle including myself.

Cutting Aluminum

First, let’s pick a tool from Amana’s catalog. The HSS-1620 should work. It’s a single flute 1/4″ bit. The recommended Chip load cutting aluminum is in the range of 0.004″- 0.006″ and Max RPM is 18,000. All important numbers to plug into the calculator. a single flute 1/4″ bit. The recommended Chip load cutting aluminum is in the range of 0.004″- 0.006″ and Max RPM is 18,000. All important numbers to plug into the calculator.

So let’s say we are using the MakitaTrim Router. I changed the wattage limit to 0.932 for this example.

My calculator suggests a chip load of 0.006″-0.007″ just a tad higher than Amana recommends so we will drop 1000th picking 6-1000th for an initial target. The lowest speed the Mikta can go is 10K so we set the RPM to 10K. We want to make a finishing cut with a cut depth of 0.2 in and a step over 20% of our bit or 0.05 in. The calculated Feed Rate is 60 IPM with a fairly low power demand of 0.56KW. We are on the high end of the Chip Load recommendation so we don’t want to take more off the material. We are actually on the low end of the sweet spot and since we are going for a finish cut we should bump up the RPM. 18K is the maximum for this bit so let’s pick something in the middle, 14K

So now we would be cutting at 84 IPM and close to the center of the sweet spot. We can see the power requirement went up to 0.78KW. Both spindles could be used but the 800W unit would be running at just about it’s max and probably should not but driven at that level for anything but a short cut.

Cutting MDF

MDF is nasty material. But I just upgraded to a 1.5KW spindle and I am ready to rip through some cutouts in 3/4″ stock. The Chipload range for MDF with a 1/4″ bit is 14-17 thousand. I am going to pick 16. My bit has two Flutes My DOC is 3/4″ and this is a slot cut so my WOC is 100% of the bit. We are ready to hit RUN.

Well not quite. Somehow I got confused and thought I was on a $300K CNC with a 75 HP spindle. Seems like I need to back something down!

Well, the first thing is a Feed Rate of 320 IPM is crazy. And this is MDF after all so we are not going for an ultra-smooth finish. So let’s try backing the chip load down in half to 8 thousand. Hmm, the power is still crazy at 37 HP. The Feed Rate is down to 160 IPM still fast but better. OK well, lets cut down on the DOC, we shouldn’t be trying to cut this in one pass anyway. Try 0.25. Nope would have to back all the way down to 10 thousand per pass. That would take forever. OK, let us try backing down the RPM on my new fancy 1.5K spindle. Like all the way down to 1500 RPM and a Feed Rate of 25 IPM. Not great.

So actually I think the best solution is to drop down to an 1/8″ bit.

I think this is a pretty good solution. 1/8″ bit but only cutting 0.12″ per pass with 6 passes needed. Feed Rate is 60 IPM safe. RPM is low but OK at 6041. the MRR is much better not hogging out huge swaths. It’s going to take a while but it will be done safely. The moral of the story is cutting slots is hard work for a router.

It’s a Wrap

As I stated earlier twiddling the speed knobs for RPM and Feed rate you can set up your machine to cover a very wide range of jobs. I think the wide RPM range of a spindle is a big advantage in most cases to optimize cuts with larger diameter tools. Whereas a high-speed router is a plus with the smaller diameter tools. I think the calculator can help make a good decision on your upgrade.

As I always say play in simulation twice before making a cut once. So play with the tool on your own. Let me know if you have any questions or suggestions for enhancement.

I only play a knowledgeable CNC Operator on the Web.

If you use my tool please use all your common sense in setting up your machine.


References and Formulars

1 Comment

  1. Hi… I’m a CNC newbie. I have a Next Wave HD520 with the Shark 2hp water-cooled spindle and I’m running V-Carve Pro 11. I’m working exclusively in hardwood right now and am just beginning to learn about feeds and speeds. I’ve discovered your calculator but am not clear as to whether it will work for me as I (a) do not have Internet in my shop and (b) run my CNC using a USB Flash Drive and a pendant and not my laptop as my controller. Can I work with the calculator without having it connected to my machine and/or to the Internet? I need to be able to tweak the tool parameters offline at the shop and then plug the data into V-Carve Pro to save it to the thumb drive. Thanks for your help.

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