Feed rate

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Feed rate

Postby cusoak » Tue Jan 16, 2018 1:39 pm

I was watching a youtube video and the person was talking about feed rates and was talking in percentage's and not inches per min.
Is that because his cnc has a limit to how fast it can move. He was using mm also
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Re: Feed rate

Postby Adrian » Tue Jan 16, 2018 2:01 pm

Rather than creating lots of different tools for different materials some people create the tool once at a set feed rate and then cut at a percentage of that using their control software.

I have multiple definitions of each tool set for different materials personally. I also run in inches per second even though I do everything else in metric.

There may be other reasons for working a feed rate as a percentage but that's the only one I've come across. The other thing it might be is that he was talking about stepover rather than feed rate. That is usually expressed as a percentage.
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Re: Feed rate

Postby IslaWW » Tue Jan 16, 2018 9:25 pm

Jeff...
Feedrate is feedrate. Like Adrian says many use a fixed then adjust using "Feedrate Override" which is a feature of most control software's. FRO is shown on most controls as a percentage. But an FRO percentage is useless unless the base feedrate is known.

On a given feedrate, most modern controllers use a keyboard input and/or G/L/M command to change feedrate from 2% to 200%
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Re: Feed rate

Postby Leo » Tue Jan 16, 2018 11:59 pm

I find naming feedrate as a percentage to be quite odd. Percentage of what?

Feedrate is going to be dependant of the ability of a given machine, but there IS some scientific baseline to work from. It does take a little math to figure it out. It's not hard or complicated but it is math and not just a guess. As a person gets more experienced, a tongue in cheek, is really not so, but an educated assessment of feetrate.

The rule of thumb I like is -- you want to make chips, not dust. Dust means feedrate is too low.

Another is - if the cutter is smoking, getting discolored or burning --- the RPM is too high.

Feedrate is calculated by getting a chipload rating from a tool manufacturer, then multiple by number of cutting edges (flutes).

https://www.guhdo.com/chipload-calculator

There are LOTS of calculators out there - this one is nice because you can change RPM or feedrate until you get a decent CHIPLOAD.

Soooo - FEEDRATE - is NOT the only concern. You need to figure out CHIPLOAD, RPM, number of flutes, to get the proper "feedrate". AND - you need to match all that to your particular machine.

You CAN reduce the chipload by varying the RPM and feedrate to adjust your chipload. If your machine is complaining, you can adjust further to reduce the chipload.

A tiny bit of algebra A+B=C You could say 5+2=7 OR 4+3=7 OR 6+1=7 The combination of numbers really need to add up to something. That is how feedrate works, but it works with RPM and number of flutes and comes out to chipload.

Keep in mind - in wood - you do not want 4 flute cutters, and single flute cutters work great. It's all about allowing the chips to evacuate. It's also to allow the chipload calculation to work to your favor in case you need to reduce the number of flutes.

I do hope this helps a little.
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