Fastest cut speed

CanisLupus
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Fastest cut speed

Post by CanisLupus »

What is the fastest cutting travel speed you have personally used with your CNC machine? That is, how fast was the spindle traveling while cutting solid wood or baltic birch plywood? Was the travel speed maxed out? if not, would you have increased the speed? Why wasn't it maxed out (reached machine capacity, cut quality, etc.)?

For this exercise, consider 2' x 4' and 4' x 4' bed sizes on hobbyist / entry-level pro machines. Assume toolpaths with minimal change in direction to allow for maximum acceleration and sustained cut speeds. What is the maximum speed in your ideal but realistic cutting world?

A friend and I are discussing machine capacities versus practicality versus operator desire for smooth cuts and reduced manual finishing.

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Re: Fastest cut speed

Post by TReischl »

The fastest I have gone is 800 IPM. Just to see if it were possible on my machine. That is where it stopped, I have no idea of the top end but it seems to me that CNC Router Parts had a video of the machine running at a feedrate of 1100 IPM.

The machine is basically an Avid (formerly CNC Router Parts) design and parts. The X and Y are rack and pinion drives, the Z is ballscrew.

Here is a link to the video: https://www.youtube.com/watch?v=Pt8n43_YRjI&t=85s

I typically will rough pieces at 350 IPM. Finish feedrates depend on the material.

Things happen fast at 800 IPM and frankly on a machine my size (3 X 4) it is a bit unnerving. Another problem is chip control. At that feedrate the chips are projectiles and the dust collector shoe has a hard time keeping them from being flung all over the place.

Here is another video running at 350IPM: https://www.youtube.com/watch?v=tFA_1PJS32U
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Re: Fastest cut speed

Post by Leo »

I can cut at 500-600 and possibly faster. Heck, I can most likely cut at 800 if I really wanted to.

---- BUT ---- I do not.

Just because I can does not mean I am comfortable with it.

Also it depends.

It depends on a LOT of things.

The list of - it depends - is a long list and I will not even attempt the variables.

Some machines can cut at over 1000 on entry level / PRO machines. Some will MAX out of 50-75 IPM in the hobby level.

What is the fastest speed can an average car go?
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TReischl
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Re: Fastest cut speed

Post by TReischl »

Leo wrote:
Wed Mar 11, 2020 8:27 pm
I can cut at 500-600 and possibly faster. Heck, I can most likely cut at 800 if I really wanted to.
From what I have seen of your machine I would certainly think so.

I might be the only person on the planet that does this, but. . . . I have my post set up so that it does not output G0 moves, instead it outputs a G1 with a feedrate controlled by a variable defined at the top of the program. Since I do it that way my rapids when moving the machine manually are still fast but I can slow them down during a program if I want to observe better. I also do a couple of other weird things, like move the machine to program zero and then do an optional stop at the beginning of every program. That way I can make sure that the program is running from where I planned to run it. It has saved me a couple of times now. Machine rapids to wrong spot, ooops, Ted, you forgot to reset your zero for this part! If I am doing multiples I just comment that line out when I know it is in the right place.
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Re: Fastest cut speed

Post by CanisLupus »

Leo wrote:
Wed Mar 11, 2020 8:27 pm
I can cut at 500-600 and possibly faster. Heck, I can most likely cut at 800 if I really wanted to.

---- BUT ---- I do not.

Just because I can does not mean I am comfortable with it.
<<< snip >>>

What is the fastest speed can an average car go?
That's a good analogy and illustrates my point. My vehicle could easily reach speeds 130+ mph. I've never driven it pushing the pedal through the floor. However, I typically drive 55 - 70 mph and occasionally 80 or so if I'm driving through certain western states. Certain factors affect those routine speeds, like weather.

Back to the CNC example, even if a machine can cut at 1000 IPM, for example, is it likely you'll spend your time cutting at 300-400 IPM? That's what I'm really looking at.

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Re: Fastest cut speed

Post by Adrian »

Using the car analogy one important thing to bear in mind is where the car is running. On a mile long straight the car can go 0 to 130 and back to 0 again. Try doing that in a 200yd stretch.

I've seen lots of posts (admittedly not often on this forum where people seem to have a better understanding) from people saying they cut at all sorts of speeds that mathematically just aren't possible with the size machines they have. There's no way they can accelerate and decelerate in the bed size let alone on the size of jobs they're cutting. They might put that feed rate into the database but it doesn't mean it's running at that speed.

I generally run nowhere near the maximum recommended cutting speed of my machine (600ipm) as the size of jobs I cut and the amount of curves they have means I would never get near it. The majority of cutting I do I use a feed rate of 360ipm but I doubt more than 10% of the sheet runs at that actual speed.

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Re: Fastest cut speed

Post by TReischl »

This subject has interested me enough in the past to cause me to create a spreadsheet to figure some things about acceleration, time and distance to reach a certain speed.
acc.JPG
This shows that at an accel of 25 inches/sec 2 it takes the machine 2 inches to reach 600 IPM, it will also take 2 inches to slow down if it is making a sharp turn at the end of the move.

The chart on the right shows the same info to reach 300 IPM. Because acceleration is based on time squared these calcs are not linear. The one on the right shows that it takes 1/4 the distance to reach one half the velocity of the one on the left.

I was sort of surprised by what this stuff looks like, I was thinking that it would take much further to reach a speed of 300 IPM. The second video I posted above is an example of why a person might want to program for the fastest possible feedrate that does the job. During that roughing there are long runs where the machine runs at 350 IPM. Later in the video when it is finishing shows what happens when the moves are much shorter and involve rapid changes in direction. The machine slows down and does not reach programmed feed rate. But it is running as fast as it can.

BTW, 25 in/sec2 is not a really fast acceleration for a machine. I have had mine as high as 37 but I was not comfortable with the way the machine was reacting as a whole. The machine is not heavy enough to resist the forces those accels/decels create. Here is what happens when the accel is increased by 20% in the first calc on the left:
c2.JPG
So not only is speed important but so is acceleration. There are folks who aim to have their machines achieve an acceleration of 50 in/sec 2. At that accel it would take a machine 1 inch to reach 600 IPM velocity. That occurs in .2 sec.

Edit: One thing I have not figured out is how the feedrate reacts to curves. I know it will slow down for tighter curves, not as much for larger ones. But since I do not have access to the algorithm used in the control I cannot compute finite values for say a circle of 5 inch radius being programmed for 300 IPM.
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Re: Fastest cut speed

Post by TReischl »

OK, I went crazy again:
c3.JPG
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Re: Fastest cut speed

Post by Adrian »

Surely a lot depends on the mass of the gantry, spindle assembly though? A lightweight router on an aluminium gantry is going to have a lot less mass than a 6hp spindle on a steel welded one for example.

All my experience with acceleration, deceleration is with the design of racing cars rather than CNC admittedly but my gut instinct says mass will play a major part and I expect the ability of the steppers/servos to handle that deceleration without skipping as well.

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Re: Fastest cut speed

Post by TReischl »

Adrian wrote:
Thu Mar 12, 2020 9:20 am
Surely a lot depends on the mass of the gantry, spindle assembly though? A lightweight router on an aluminium gantry is going to have a lot less mass than a 6hp spindle on a steel welded one for example.

All my experience with acceleration, deceleration is with the design of racing cars rather than CNC admittedly but my gut instinct says mass will play a major part and I expect the ability of the steppers/servos to handle that deceleration without skipping as well.
You are exactly correct Adrian. That is why I made the comment about how my machine started to react at 37 in/sec2. The mass of the gantry causes a problem at that accel/decel. At that point the motors are starting to move the machine itself, not just the driven elements. That whole (for every action there is an equal but opposite reaction) starts to make itself known. Trying to accelerate the mass of the gantry too fast causes a negative reaction in the base of the machine. And then there is the thing you touched on, the motors may not have the ability to accelerate the gantry without losing steps.
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Re: Fastest cut speed

Post by rtibbs »

You guys have way too much time on your hands :D

It does keep ones mind active though

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Re: Fastest cut speed

Post by ger21 »

For a given RPM, the faster the feedrate, the longer the tool life will be. (in the vast majority of cases)
In a commercial setting, we always cut as fast as possible, with the limit being the quality of the cut, and the ability to hold your parts in place.

The only reason to cut slower is to get a better finish.

On a lighter weight machine, the rigidity of the machine can affect cut quality, so this can also limit your speeds.
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Re: Fastest cut speed

Post by IslaWW »

Ability to hold the material, horsepower of the spindle, power of the driving motors (steppers or servos) along with rigidity of the frame are the factors that affect maximum cutting speeds on sub $100k machines. In many cases on lesser priced machines will not even come close the mfgr's recommended chipload.

With all of the above being true, it is the operators job to run at the fastest feedrate and lowest RPM that returns acceptable edge quality and part sizing. This is how CNC machines are placed into "price classes". The ability to hold a given tolerance at a given feedrate in a given material.

An easy test is to cut a 4" circle, both climb and conventional at different chiploads. Take a 1/4" 2FL upcut bit, .25 pass depth, 15krpm and cut it at 50, 100, 150, 200 and 250 ipm (if you can) and measure the difference in size with calipers between the 2 circles cut at those speeds. You will find the optimum chipload for your machine.
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Re: Fastest cut speed

Post by TReischl »

"With all of the above being true, it is the operators job to run at the fastest feedrate and lowest RPM that returns acceptable edge quality and part sizing. This is how CNC machines are placed into "price classes". The ability to hold a given tolerance at a given feedrate in a given material."

Gary, where do these "price classes" exist? I have been around a long time and have never heard the term as you stated it. Sure, I have heard something like " Such and such is a $100K machine" but that is not a class, just a statement of the cost of the machine. I dunno, I think more in terms of "price ranges" rather than "classes".

I have never heard of anyone placing a machine into a "price class" based on the feedrate, rpm, and part sizing. In my experience machine prices are determined in a much different manner. A spec is created for a machine to cut at X ipm, at Y rpm, work area of AxBxC, tolerance of .xxxx, etc. Then preliminary design work is done to get an overall idea of what will be required to build such a machine. Then work is done to estimate how much that machine will cost to build. After that margins are added for things like profit.

Nowhere that I have found says something like "if the machine can cut at 1000 IPM, and hold a .0002 tolerance then it is a $XXX machine.

IMHO machine cost is controlled by physics. That is what a machine designer does, figure out what materials, motors, etc must be used to operate within the laws of physics while achieving the desired result.
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Re: Fastest cut speed

Post by Leo »

When I was buying industrial machines for the workplace, what we were most concerned with was repeatability and reliability. Price found its own place.

On my last job, the last machine I bought was a Studer S33 cylindrical grinding machine. I needed to hold something in the neighborhood of 20 millionths on roundness. The machine actually did perform. It was also in the $400,000 range.

Its all relative.

In my first post I believe I said ---- It Depends. It depends on a LOT of stuff. All of what is mentioned in all the posts posted here is only SOME of that stuff.
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