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Lee Roberts
25-08-2010, 11:33 PM
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

GeorgeD
26-08-2010, 12:15 AM
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?

Ross77
26-08-2010, 12:42 AM
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 :smile:

George
Try here (http://www.roymech.co.uk/Useful_Tables/Manufacturing/Milling.html) or here (http://rogercortesi.com/portf/spindle/spindle.html) or hear (http://www.kennametal.com/calculator/end_milling_hp_calculations_in.jhtml;jsessionid=OF 5N3SUQ5E15BLAUCYOSFEVMCQFB0IV0) !

Ross77
26-08-2010, 12:56 AM
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......:smile::smile::smile:

motoxy
30-01-2012, 07:55 PM
While investigating feed rates I stumbled across this wee calculator. My be useful. Anyone know of any other calculators.....free

http://www.daycounter.com/Calculators/GCode/Feed-Rate-Calculator.phtml

Bruce

Jonathan
30-01-2012, 08:24 PM
Depends on your definition of free.

The one I use the most is:

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

motoxy
30-01-2012, 09:07 PM
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.

Swarfing
30-01-2012, 09:30 PM
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? :-)

Jonathan
30-01-2012, 09:55 PM
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.

motoxy
30-01-2012, 10:38 PM
oooo! this is going to be fun!!!!

Swarfing
30-01-2012, 10:46 PM
Which bit?

motoxy
31-01-2012, 11:07 AM
Which bit?

Your right, I am being very delusional!!

Swarfing
31-01-2012, 01:56 PM
lol......:lmao:

Fivetide
08-09-2016, 12:05 AM
Is it just me or does anyone else think Lees hot ? :yahoo:

JAZZCNC
08-09-2016, 12:48 AM
Is it just me or does anyone else think Lees hot ? :yahoo:

What you smoking.!!!!!!

Fivetide
08-09-2016, 02:11 AM
What you smoking.!!!!!!


Actually I'm vaping now , quit smoking the 40 a day death tabs 3 years ago at Christmas .. same time I quit drinking :) now I'm just an old boring bastard lol

Clive S
08-09-2016, 07:55 AM
now I'm just an old boring bastard lolGlad you joined the club:joker:

LeonBlack
01-11-2019, 03:45 AM
True, chip load is very important.

Chip load refers to the size of the chips produced during cutting. The main purpose is to get the maximum chip load possible, so you can increase productivity, reduce heats bits during work, and also prevent premature dulling. The chip load is to small means bits will become too hot and dull quicker; chip load is too high means cutters will cause a bad surface finish, and in worse situation, the bits break.

James Anwar
25-03-2020, 10:18 AM
Mr Lee: I am cutting composit door but every 2 or 3 door machine start chipping from top and bottom and some how tool broken advise me what type of bit do I use to get extremely fine cutting and bit last for a month at least as well

Lee Roberts
25-03-2020, 12:09 PM
Mr Lee: I am cutting composit door but every 2 or 3 door machine start chipping from top and bottom and some how tool broken advise me what type of bit do I use to get extremely fine cutting and bit last for a month at least as well

Hi,

I would try a compression bit/cutter, their flute's have upcut and downcut spirals all on one cutter, so down shear cutting on the top of your material and an up shear cut on the bottom of the material.

You can also get spiral up only and spiral down only, the choice of cutters is great now days, so always select the best cutter for the job/operation.

Of course you need to be using the right speeds and feeds or it won't matter what cutter you use.

Muzzer
25-03-2020, 04:09 PM
If you have a smartphone, there are speeds and feeds apps from all the big names there for free, such as Iscar, Mitsubishi, Sandvik etc. Check out the app store.

If you use Fusion, you can enter stuff like the surface speed and feed per tooth from the cutter manufacturer's site and it will do a lot of the calcs for you. You still need to experiment to see what your machine can do but unless you start with manufacturer's data you may not have a clue what the cutter can do or where it likes to work. Some of the modern cutters are pretty amazing when you use them with modern adaptive toolpaths.

I don't use CNC cock book myself. Bob Whorefields seems to be a bit of a knob and I've seen a fair bit of feedback suggesting that a lot of the numbers come out all wrong. If you have Fusion for free, there is no need anyway.

Admittedly this is for cutting metal and plastic, as I don't deal with The Brown Stuff myself.