Does SFM Matter?

Ever since I started milling aluminum with the Shapeoko and reading older forums (e.g. Practical Machinist), SFM is mentioned frequently and held in high regards as something to hold constant. But with videos from Winston Moy, John Saunders, and others either ignoring it or mentioning that it isn’t a big deal, I wanted to know if I should even care about it.

I know one should shoot for 800 SFM in aluminum with carbide but no hobby machinists seems to mention this. Does it really make a difference if I hold my SFM constant and adjust everything else? It seems like I can run fast or slow in aluminum with 1, 2, or 3 flutes, at high RPMs and low, and it doesn’t seem to really matter as long as I hold chipload constant, not SFM.


It’s an ideal guideline value for the folks who make tooling.

The problem is, our machines aren’t rigid enough to mechanistically calculate out feeds and speeds, so it loses its relevance.

I thought his guest blog articles were on point for this:

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In that same vien of older forums and PM posts;

the go to settings were slow rpm, heavy (thick) cuts, and what have you on the big solid cnc machines. the spindle would snow plow through the material because the machine had brute rigid force. go back further and adaptive clearing wasn’t a thing, HSS was king, etc.

for these little nonrigid machines with router spindles/speeds and small diameter tooling it’s a different ball game. now you got this lil gokart with a shovel on the front for snow plow of a spindle. fast light cuts at the material, not too much - not too little. datron practices are a good idea to keep in mind as opposed to those of old haas/fadals.

SFM will put you in the ball park, but then you have to keep in mind things such as chip thinning and lack of rigidity. it’s all intertwined and the machine capabilities must be accounted for first and foremost.


Sounds good. Thanks to you both for your knowledge and insight. Just trying to figure out what matters and what doesn’t in this.

no worries. it’s a lot to take in and the information out there is so broad and can be either dated or not apply to these small machines.

the thing is the big machines had a larger range to work with. all settings will ‘work’ with the risk being mostly broken tool or bad cuts/finish. they typically have rigidity and power to back the feeds/speeds - even when they’re off a bit. on our little machines we have a narrower range as the spindle can stall, motors or belts can skip,etc on top of broken tools and bad cuts/finish.


I’m pretty new at it, but from what I can tell, when cutting aluminium chipload per tooth seems to be a better indicator for less rigid machines like the Shapeoko.


Chipload is the actual work being done. I don’t worry about SFM unless cutting hard materials like steel or titanium where your temperature sweet spot is much much smaller.

I like to limit myself to 3000sfm for aluminum. Like Paul said, we are much more suited to a Datron style of machining. To me that means as high rpm you can run and also take your required chipload at without any signs of chatter or deflection.

Cut temperatures can decrease and cutting forces lower as rpm rises.

I faced Ti the other day with a 0.750 insert facemill at 10krpm. The book said I was 400% over maximum recommended SFM but when CAM’d to chipload and chip temperature the cut was possible.

This machining thing is the closest thing to the Matrix I can think of. You can totally bend the rules


I think I’ll cut an inspirational plaque with this motto ! :slight_smile:


Right, that’s what I understood from your knowledge and experience on it, I just wasn’t sure how it squared with a lot of old-school style machining knowledge. It almost seems like we need entirely separate parameters with what we are doing. Thanks for your thoughts.


Nothing to add @The_real_janderson, except thanks for asking the question. I recently learned of this parameter and decided it wasn’t relevant to what we do with our machines. But I still wondered…

Now I know.


Yep, just wanted to clear things up. Glad you learned something from it.

I used to worry with all this until I got G-Wizard. Once you set it for a Shapeoko with a 611, all you have to know aside from the obvious tool parameters and material type is the RPM you will be using. There is a chart on the wiki that has suggested RPM settings for various materials. Once I have everything plugged in, I use the slow/aggressive slider to get my chip load to the specs of the tool and get tool deflection acceptable. Then just plug everything into Fusion360’s CAM. I have not broken a tool since using G-Wizard and there’s processing power left in my brain to not screw up my offsets. In general, I have found that 50% on the aggressive/slow slider is conservative and well below the machine stall or tool breakage points. A good thing to remember is that on a low power machine like ours (under 1HP I think), G-Wizard requires no annual renewal fee after the initial purchase. I have easily saved enough money in tool life to pay for it.


That totally makes sense. HSMAdvisor and GWizard are both on my compare list to think about buying. It’s just a fairly expensive purchase for me now but I hear good things about them.

The pricing on G-Wizard has a bit of subtlety to it — the on-going payment gets one unlimited horsepower in the application, but each year of on-going payment gets on 1HP in perpetuity. Since the trim routers and spindles suited for use on these machines average 1.25HP, just a year or two’s payment gets one a usable power rating:

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Yep, absolutely, but that’s still at least $80 which for me is a large percentage of discretional money every month that I’d rather spend on project materials right now.

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It goes on sale every so often — esp. look out for sales which bundle MeshCAM.

If you have an Android or IOS phone FSWizard is free. There’s an upgraded version with more materials for $17.99. I’ve downloaded the free version and played with it a little, but have not had a chance to try cutting anything yet. Might be worth checking out if you’re on a budget?



Hey, I’ll check that out. I have a bunch of Google Play credit I can use.

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