# Thread: How-To: Feed Rate Calculation

1. Today I received a question about feed rates regarding PCB’s on a DIY cnc router, so here we go.

In general when it comes to building a tool path; you will need to determine the correct feed rate. Different materials have different surface speed ratings for any given type of cutter.

As you can imagine the harder a material is, the slower we would set our speeds to. So, if we take the diameter of the cutter we would like to use and then also the surface speed of the material we would like to use, we can then calculate the correct RPM’s for our spindle.

Next we need to know the tooth load and also the number of teeth for the cutter, now we can determine the correct feed rate.

We can do this with the following calculations:

Spindle Speed = 3.82 * Material Surface Speed / Cutter Diameter
Feed Rate = Tooth Load * Number of Teeth * Spindle Speed

So using the above we could take a 6mm Cutter, a Surface Speed of 280 and this would give us a Spindle Speed of 178.3rpm. Now we can take our Tooth Load, times it by the Number of Teeth our Cutter has, and then times that by our Spindle Speed. This gives us a feed rate of 0.7, all units being in English of course!

Hope you found this useful and can use it to get that feed rate you have been after!

Lee

2. ## The Following 3 Users Say Thank You to Lee Roberts For This Useful Post:

3. What would be usefull,Lee...is a rundown of the majority of materials used for CNC and a surface speed graph?to simplify it more.

And sticky the thread in here?

4. Bastard beat me to it, I was just about to post some info on speeds and feeds, just the problem is quantifing the correct surface speed and when you consider wood, the chip rate seems to be the prefered option.....

Thanks anyway Lee

George
Try here or here or hear !

5. 6mm cutter on a PCB????

Lee, Isnt the main problem with small cutters the deflelection of the tool and not the the desired speed? For instant if you required 0.1mm precision for the cut and the tool deflected 0.5mm per newton then the limiting factor would be the tool? No

Or am I on cloud silly precision again......

6. While investigating feed rates I stumbled across this wee calculator. My be useful. Anyone know of any other calculators.....free

http://www.daycounter.com/Calculator...lculator.phtml

Bruce

7. ## The Following User Says Thank You to motoxy For This Useful Post:

8. Depends on your definition of free.

The one I use the most is:

http://www.cnccookbook.com/CCGWizardv40.html

(Unless you count doing some quick mental arithmetic and guessing!)

His is good because it calculates tool deflection, so you can get a rough idea of the surface finish and how close the tool is to breaking. I also use that when trying new materials by adjusting the depth of cut until the tool deflection is at the value I know is safe for that cutter in a known material. For instance I know on my machine that with a 6mm single flute cutter it will run all day with 1.6um tool deflection in aluminium but if I exceed that the surface finish is worse. So a few days ago I tried cutting mild steel with the same cutter on the router. Adjusted the depth of cut to get a calculated 1.6um deflection then did a test cut at that feedrate and depth of cut. It worked well...much less messing about to find the optimum.
Last edited by Jonathan; 30-01-2012 at 07:36 PM.

9. Thanks Jonathan.

£40 a year. I do not like subscriptions as I know that I will forget to cancel and just keep paying. But I see the lite version is a keeper even if you do not resubscribe. Limited to 1 hp.

10. I've had a chance to visit many a company that does machining. If a single one of them used there tools as it should against the maths i don't think any of them would get any work done? It's all well and good to get a starting point but different tools from different manufactures behave very differently. A manual machinist will always listen to the cutting and a feel for the handles. Depends on what coolant you are using if any at all etc etc. I have a favorite 10mm cutter whether blunt or not will cut any material i throw at it by just changing the speeds and looking at the chips. I have a set of 12mm cutters looks superior sharpness and cost a fortune that could not cut bullshit if it saw it.
In most cases just use the maths as a guide and learn how to machine (take it slowly and build up). Experience will come out better than the maths any time, running at 20 sqwillion miles an hour because the maths says so will just keep costing you.
Just for the record Jonathan (because i know you will defend the maths) before you jump back with an answer, I' am a performance engineer by trade and have seen many things that should not happen but do happen for any given reason work well or do not......Black hole surfing anyone? :-)

11. Originally Posted by 2e0poz
In most cases just use the maths as a guide and learn how to machine (take it slowly and build up). Experience will come out better than the maths any time
I agree entirely. I did a lot of manual machining on the lathes and Bridgeport milling machines at school before any CNC and I'm sure it's helped a lot as I know what sounds/feels/looks right when cutting metals, and what does not.

A bit of common sense applies here - start a bit below the calculated speed and work up. If the calculation comes up with something unexpected that looks like it wont work then research it further to find out why. The problem is you *can* be too careful as going too slow can break the cutter just as fast as going too fast. The formula's are guidelines, no more.

12. oooo! this is going to be fun!!!!

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