Which Spindle To Choose? And How to Estimate Chatter (vibrations)
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- เผยแพร่เมื่อ 4 มิ.ย. 2024
- In this video, I will show how to choose a spindle based on specified cutting parameters. Furthermore, we will go through how to measure stiffness on a machine, and determine whether or not the CNC machine will be rigid enough to take the desired cut. Enjoy!
Calculate spindle speed, feed rate, RPM, spindle power, and the tangential cutting force here:
www.kennametal.com/us/en/reso...
As Mihai Dumitrescu reminded us, be aware of the fact that almost every CNC-machine has a different stiffness, depending on where the machine spindle has moved to. Routers will for example have the most deflection when the spindle is lowered fully, and located in the middle of the gantry. Another thing is that the machine's deflection also depends on which direction you're machining in.
Don't you think, it could be more stiff in router, if the Y gantry is horizontal and Z installed between the two Y guides that installed at the bottom side of the gantry instead of top side? In this case there is minimum torsion force.
How would I determine the stiffness of my particular CNC machine? And in what unit is measurement is such a characteristic measured? I m using a commercially made CNC, not a DIY build. I have the Onefinity Journeyman X50.
@@mickeymallette2663
You can read about sitffness here: en.wikipedia.org/wiki/Stiffness
k (stiffness) = F (force) / Displacement
Example: Take a robe around the spindle nose and put a dial indicator on the spindle to measure its displacement accruately. You pull it with a force of 15 N and measure a displacement of 2mm.
You stiffness will be:
15N/2mm = 7.5 N/mm
Here is a video of how it could be done: th-cam.com/video/OSkB5esVKkw/w-d-xo.html
I hope you continue this series!
Any updates on these amazingly informative videos?
Hi William; nice videos and also nice and good intentions to document the build. I would like to chip in with some thoughts, hoping they may be of some help. First of all, big manufacturers do FEA, then make mock-ups, many iterations, until theory and practice coincide. That calls for resources. We, hobbyists do not have the luxury to do that, therefore we make guesstimates and it is perfectly acceptable. In a gantry design, tool displacement depends on where on the X axis (transversal axis) the carriage is horizontally, and also where on the Z axis the carriage is vertically, at any moment in time. The influence on tool displacement of the horizontal milling force acting on the gantry (parallel to the ground) is rather small. The biggest influence is, as you may know, the rotation angle of the gantry beam caused by the mass of the Z axis acting at a distance X from the center of the mass of the beam. The cutting force has its share, but not that much unless the beam is very short and the motor is very strong. If you lower the carriage on the Z axis by, say 100 mm , you will have a certain tip of the tool displacement, but if you lower it 200 mm the displacement will be bigger. Also, 100 mm lowering the carriage in the middle of the beam will produce a bigger displacement than 100 mm lowering the carriage at the end of the beam. So, the beam should be designed to a specific Z travel, for instance: how should I design the beam so that to have a maximum displacement of no more than 25 N/micron at 300 mm Z axis travel (and obviously at the middle of the beam where it is got the weakest point). Let me give you an worked out example. Suppose a beam of rectangular carbon steel of 400 mm height , 200 mm width, 6 mm thickness; 800 mm unsupported length of the beam; horizontal distance between center beam and Z axis carriage center = 300 mm (out of which 100 mm is half the width of the beam); mass of the Z axis = 1500 N; cutting force = 200 N. Now, the parameter that we will vary is the vertical distance between center of the beam to the tip of the tool. If this distance is 300 mm (which means 100 mm effective Z travel under the beam, as 200 mm is half the height of the beam), the tool deflection will be 10 microns. If we lower the carriage down by 100 mm, (which means 200 mm effective Z axis travel) the tool deflection will be 14 microns. If we lower it further by 100 mm (300 mm effective Z axis travel), the tool deflection will be 18 microns. Lastly, lowering it down even further by 100 mm, the tool displacement will be 22 microns. So, the angle of twist is the ''hot'' parameter. Tool displacement is basically the amount of distance the tool deviates from perpendicularity, which is consistent with the angle of twist around the longitudinal axis of the beam. Chatter is partly caused by this displacement and I have noticed that in practice, when face milling for instance, the milling marks on the floor of the work piece are sometimes wrongly attributed to either the material the mill is made out of (steel, cast iron, etc), or to the lack of tramming the head of the Z axis carriage. These have their share too, but there is an amount of , let's just called them ''chatter marks'', that will be there forever by design, IF the angle of twist was not well conceived from the outset. In practical terms, a hobbyist should raise the work as much as possible to the Z carriage, as the accuracy is better the higher up the work is set to.
Hi Mihai, what a wonderful comment, you're absolutely right. For the record, I'm aware that the twist of the beam/gantry is one of the biggest causes of tool deflection, I do believe I have mentioned this in a couple of videos.
I'd like to add that also the z-axis itself has a very big influence. I feel like some hobbyists focus too much on building a rigid gantry, and still, end up with major tool deflection in the z-axis relative to the deflection caused by the gantry.
One advantage of casting the frame is that a shape optimization of a mockup CNC frame, very quickly tells you where to put the mass. Thus eliminating the need for 100's of FEA analysis with different components, welded or screwed together. Shape optimization is a very powerful tool when it can be utilized.
Have a great day!
Small correction to the video* "Tool Deflection Control: Critical to Your Success" from cnccookbook.com states that the deflection of the cutter should not exceed 0.001" (0.0254 mm). I however said in this video that the deflection should not exceed 0.02 mm (because we talk about the deflection at the tip of the spindle, and not the deflection of the end mill). Use this as a rule of thumb, actual chatter calculation will account for far more variables... This should however still provide us with a great estimate on what spindle to buy, without buying a spindle that can take heavier cuts than what the machine can handle. Have a great day!
It's great to finally find someone doing videos on diy cnc machine design based on desired performance. I've been looking for a long time and only seen a few other videos. Suggestion: it's not terrible but would help to lower the background music since your voice tends to blend into it (is this just a long cnc rap song? :)).
Thanks! I feel the same way, that's why a decision to share some knowledge. Great feedback I'll lower the track a bit. One day I might increase the music volume and rap about mechanical engineering in over the track :p
Hello. Good work! I'm designing my own machine now and really appreciate the work you put in to this very specific area of study. Any updates on the progress of this project?
Thank you, I'm glad to know that these videos are appreciated. I'm working on a new machine. There will be an update some time in the future🙂
excellent - thank you!
I think it's important to have mentioned the taper interface. Standard Steep taper holders like BT30/CAT40, etc are only taper contact and low in radial stiffness. Dual contact interfaces like HSK, Capto, and Big Plus are Face + Taper contact to the spindle and thus inherently have greater radial stiffness.
Your Chanel is absolutely awsome thank you for that ! Would ask you two questions though.
- wherebdonyoy study cnc machines university /country ?
- can you advice me a book or books so as to get good at cnc CNC Theory and calculations please ? i am starving for that.
Hi Bakir, thank you very much, your comment really encourages me to make more videos!
I'm studying at DTU (The Danish Technical University), which is located in Denmark. It's ranked the world's second-best university right after MIT, based on it's research. I think that's really cool!
I'm studying mechanical engineering, but at DTU it's only taught in danish. Some of the books we use for the courses are actually written by DTU, and some of those are written in danish. My all-around favorite book which we use at my university is called: Engineering Mechanics: Statics (
R.C. Hibbeler). If you understand everything in that book, you can make a career out of it lol. It's about all the fundamental calculations used to find the forces in static mechanical systems.
We also have a course called "Dimensioning and Strength", it's about the deflection of structures based on how the structure is mounted and what loads are acting on it. This is what all FEA engines are based on, these calculations will give you a deeper understanding of deflections through a given structure. Our book in this course is written by DTU in danish, so I encourage you to find one in English if this field interests you.
90% of what you have seen in my videos is self-taught knowledge, from watching absurd amounts of youtube videos, haha. It's much better to teach yourself to do some mechanical calculations.
Regards
William
Thank you very much.
How to decide distance between two lm rails.
Look up "2:1 ratio"
I am building a CNC router for machining steel etc - I have already purchased the spindle & steppers etc. The spindle is quite a high quality one with lower speed for steel cutting/with water cooling - 100mm in dia. weighing about 14 kg - I guess the fixed gantry design will be the best solution for that weight of spindle??
Hi Adam, I have to say that I might have been unnecessarily concerned about weight, but that's mostly because I want mass where it counts. I will cast my machine in a material similar to epoxy granite, and I will likely be building more than one machine, so I don't mind spending lots of time on the design and creating moulds for the machine.
You can actually get a lot of power from motors these days, so there's not much to worry about. However, you do mention that you already have bought some motors and drivers, maybe look at other people's machines that use a similar sized motor and sort of compare the weight of their machine to what the weight of your machine will be.
Have you bought servo or stepper motors? I would recommend servo motors for high load applications where some level of precision is required. If you take a look at the torque curves for stepper and servo motors, you will quickly find a reason to choose one over the other.
I have heard good things about the price/performance ratio for the Leadshine Servo Motors, but don't take my word for it ;-)
Hej, hvordan går projektet? 😊
Hello,
As I am a beginner I've never heard of the 0.02mm max deflection to avoid chatter rule.
Where does it come from ?
Thanks you !
Hi Geekoid, glad you're back! This article is a great read:
www.cnccookbook.com.s3-website-us-east-1.amazonaws.com/CCCNCMillFeedsSpeedsDeflect.htm#:~:text=If%20you%20do%20a%20little,that%20chatter%20can%20set%20in.
The deflection is a rule of thumb and in my video a talk about the stiffness of the spindle and not so much the stiffness of the actual cutter which the above article mentions can be up to 0.001". It's a great rule to determine which spindle you can use, without getting more power, than the rigidity of the machine can allow.
Still learning a lot from your videos, wish you well 👍
I kinda disagree with the 0.02mm threshold for chatter to occur. You can get chatter with even less deflection, or take monster cuts that deflect way more than that with no chatter (despite being prone to)
@Williams Garage Engineer, As this video is about 1 year old, i was hoping if there a way to communicate with you over a paid consultation ? an email even or something
Hello again I didn't receive your mail. I'll be deleting my previous comment within a few days.
@@williamsgarageengineering6543 wow I didn't get notified about this , will send you right away
i sent you an email since hours, hopefully it reached you ?
This guy is wrong about flute to RPM levels. The number of flutes effects maximuim/minimuim feed rates and HP requirement. It does not change optimal RPM. Optimal RPM is a function of the SFM rating and circumference of the tool. Has nothing to do with how many flutes you have. You can have like 20 flutes and still be able to run at the same RPM as a 2 flute endmill.
Hi Dave, as you're probably aware all motors and spindles have torque curves that show what the torque is at x RPM. Let's say I'm going to buy a 2.2 kW 24000 rpm spindle that has the following data points on a torque curve (these are hypothetical) :
0.5 Nm at 14000 RPM
0.8 Nm at 17000 RPM
0.5 Nm at 20000 RPM.
The ideal choice would be to run the spindle at 17000 RPM, as this is where we have the highest torque. This will likely allow us to mill faster, as the spindle might not have enough torque at 20000 RPM to take the required cut at the speed that matches 20000.
If you put a 20 flute endmill in a spindle that can only supply a torque of 0.8 Nm torque, then the chips that end mill would have to cut, will be soo small that the endmill is rubbing instead of cutting. If you want to cut a decent chip, then the end mill has to go slow, as it cuts 20 chips pr. revolution. So if this end mill requires 5 Nm torque at 3000 RPM, then that's not very ideal for a spindle that probably has 0.4 Nm torque at 3000 RPM. One could say that this end mill is not an optimal match for the given spindle.
Some spindles have close to constant torque curves... But in the end, the end mill has to cut at a given RPM at a specific torque, and if the spindle can't supply that, then you have wasted money on the wrong end mill.
chatter is caused by excess axis play the end mill bites and moves away from the cut back and forth, I do cnc service and when the ball screws and thrust bearings go you get chatter and sloppy parts, there is not much hope for a $200 engraving cnc router
people need to be realistic, a $200 cnc engraver is not a milling machine and not even a good cnc router, $200 cnc are designed for NO ! spindle side pressure, even for wood decient cnc routers start at $2000 plus and maybe can mill aluminum without to much chatter and wearing out the end mills , I am building a cnc router on a 1/2 aluminum plate with rect ball bearing linear rails with closed loop steppers with mist coolent around $1400
Hi Gary,
Good luck with the build, I'd love to see a video of your machine when it's finished.
The music adds nothing. It is distracting and it make it harder to hear what you are saying.
The music is great!