Feed Rates and Surface Quality

I have been getting some inconsistent results with my last few 3d carves. “Pencil Line” marks and poor surface finish. Something that used to happen a lot before I did my stiffening mod.
I think it is a combination of a dulling bit and too high a feed rate. Replacing the bit is easy. Figuring out the feed rates is not. :frowning:

I have been attempting to use some Feed And Speed calculators to come up with some good feed rates but have been getting wildly different results between the different calculators using the same data. (Spread sheet based on chip load data, FSWizard, GWizard)

I am finding the whole process confusing and frustrating. :cold_sweat: :rage:

I realize there is a tone of complex math involved with some of the more robust calculators but I am get results ranging from 300mm/min to 5000mm/min with different calculators using the same data.

I have come to the conclusion that I am going to have to run speed test for each bit and material to dial in a good feed range for my XC. So at this point I am just looking for a good starting point.

Playing with these calculators has helped me gain a better understanding of the math involved.
From what I understand?
Feed rate and RPM are a mater of Chip Load. Which is an interaction of the bit with the material?
Where Deflection of the bit (force causing things to flex) is mostly a factor of depth of cut?
I am a little vague about where With of Cut falls in on both?

I guess part of the problem is that I am using a router instead of a spindle so I have a minimum RPM as well as a max. (Something these calculators don’t seem to take into consideration)
Also even with a stiffing mod I still get some flexing in the gantry under high load. So I can’t take a high amount of deflection. But how much is too much? I have no clue, it is just a number, bigger is bad but what is the cut off? (I guess I will have to run some tests.) Also while many of these calculators have a wide range of materials wood is not one of them. And to make matters worse the hardness info I can find for most wood is in a deferent system then metals (with no easy conversion between)

While going over all of this I did discover that I had my plunge rates are way to high. I had forgotten that the max mm/min for the Z axis was significantly lower because it is a screw drive. 500mm/min.
I am not sure if GRBL is smart enough to limit an axis to its max rate of travel in spite of the feed rate it is given. But I when through my Tool Settings in VCarve and caped my plunge rate at 500mm/min on each tool.

I am going to keep plugging away at it and see if I can find more info on routers feed rate calculators made for woodworking.

If anyone has any calculators that they have found helpful or more info on the formulas involved that would be appreciated.

Adding “Wood Working” to my search has yielded a lot more helpful info. :smile:

Some Helpful Reference Reading:
Where to begin in a world with no standards
Woodworking Feed and Speeds

Bob Warfield over at the CNC Cookbook blog as an excellent post about finding the best feeds and speeds for lightweight CNC routers.

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Hi Aaron

I was in a similar position to you but I have found some answers. I have been doing some work, supported by Routercutter UK who are Whiteside stockists, and have been given a whole bunch of figures and parameters that should make life easier for the CNC crowd.

I have made a video that explains Chipload but I did not give any examples as I expect it to be followed closely by the video that I am now working on. Here is that link:

Before I get to the video showing Chipload in action with a particular set of cutters I have to finish and upload a video on fine tuning the X-Carve and testing its capabilities. I suspect that this video will help you more and I hope to get it uploaded by no later than Saturday.

Whiteside in the US have been very helpful and have provided the Chipload figures and a spreadsheet Chipload calculator to help me with my work. I am in the process of adjusting the spreadsheet to include metric calculations as well as the Imperial ones from Whiteside and I will provide this as soon as I can.



I have been playing with a demo of GWizard.
It has been useful in that the values it has come up with are producing better results.

Feed and Speed is still and issue as with the “Gas Gage” option any bit I set up has a feed and speed range of <200 mm/min to 5000 mm/min (max) and a RPM of “Too slow for your router” to “Too fast for your router” :confused:
I just pick the “Finish (2.0)” option and start from there.

Where is has been helping is with the deflection calculation and optimizing depth of cut.
Before I had been running
40% Width of cut and 50% Depth of cut for end mills and
10% Width of cut 50% depth of cut for ball nose.
By reducing the Depth of cut I have been reducing the deflection which is improving my results.

GWizard is doing some really strange things with the ball nose calculations though. (Changing the WoC values on it’s own. Or reducing it to 0 when I try to optimize cut depth)
I now know that ball nose bits have trickier math because it is a sphere shape making contact with the wood. So I am not sure if the problem is in the program or I just don’t understand how to use it with a ball nose. Unfortunately there is no easy to find help info or error messages on ball nose calculations o help me figure it out.