Feed Rate vs Toolpath

[Steve Wonser]

I'm still struggling with determining feeds and speeds for new cutters that I purchase. In the past I've run into trouble by specifying a feed rate that my machine is capable of, but due to the short length or curves in the toolpath the tool feeds slower than I specified resulting in a change in chip load.

Is there an easy way to determine how a given toolpath will affect the ability of the tool to achieve the specified feed rate?

My machine is capable of 4" per second, how many inches of straight toolpath would be required to ramp up to that speed?

If the toolpath contains curves then how does that affect the actual feed rate?

I've read here that smaller diameter bits like to be run at faster RPMs, but that would require a faster feed rate for a given chip load. I just don't know how to guess at the actual feed rate my machine will use for a given toolpath.

Thanks,
Steve

[adrianm]

Listening to what the machine is doing is one of the most important factors in sorting out the speeds/rpms. If you're using spiral bits it's reasonably straightforward as they will make a really high pitched squeal if the feed rate is too low/rpms too high.

The other thing to look at is purely the quality of the cut and the feel of the bit temperature wise after a cut. It should only be warm to the touch not too hot to touch.

Whenever I have a new tool or material to use I start with the chipload tables to get me roughly in the ballpark no matter the size of the cuts and use the above to work from there. If you're doing a lot of smaller stuff it's worth looking into the ramp settings as well and making a custom setup file for those sort of jobs. It made a massive difference to the jobs I mainly do when I did that - http://www.shopbotblog.com/2008/03/a-ramping-the-vr-command-and-how-to-tune-your-tool-for-maximum-performance/