Neil,
I am new to cnc and this forum. I am in the process of building my own cnc machine. How does someone go about finding the velocity and acceleration for the stepper motors if they are not provided with a acceleration ramp? How did you go about setting your acceleration ramp?

 

Neil,
Thank you for the information and the pictures, much appreciated.

 

Dude, that is an incredible build. I love the magnetic dust brush, hollowed side braces, vacuum table, chain link wire guides, and heavy duty router. Would it be possible to make a couple separate builds for the dust brush and vacuum table with a little more info? Can the side plates be made on a routy?

 

Here's my attempt at a scientific answer... You will have 2 different speeds for your motors:

1. Rapids: This is the real max speed that the machine uses to move around while not cutting. As your stepper motor goes faster, it will have less torque(and consequently slower accel/decel). A heavier gantry requires more torque to accel/decel. The torque is non-linear for most motors but if you can get a copy of the torque curve for your motor you can probably calculate the acceleration curves and max velocity. T=rxF so 270oz/in=40mm x (mass of gantry)*(acceleration). So just solve for the acceleration and modify the units to be the same system and bingo. I'd subtract about 5% to compensate for rolling resistance. The max velocity usually is a result of the stepper driver board max steps/second. Mine maxes at 30,000 Hz so that's 30k steps/s. At 1.8 degrees/step that's 54000 degrees/s, or 150 RPM. At 40mm per revolution, that's 6000 mm/min or 100mm/s. That's 236.2 ipm.
   
2. Feed: This is the cutting speed and it's much slower than #1. It depends upon the material you are cutting, the bit you are using, the spindle power, and ultimately the torque of the motors

There are bunch of other speeds you can set as well... I set my manual jog speed very slow so I can manually position the spindle on the workpiece.

 

Thanks, that's a big help. I think I'll use plexiglass and horse-hair for the dust hood and 3D print the chainlinks. As for the vacuum table, many people say they use mdf and mill off the first layer so it's porous enough to let vacuum through. Does that work? Do you still need to cover all the unused parts of the table?

 

Mach3 controls the point of cutting based on the motor speeds. Automatically compensating for acceleration up and down in speeds to provide correct cuts. Accelerations main point is if you try to accelerate to fast you can stall the motors based on resistance of the machine to move. Remember stepper motors, like all electric motors, provide max torque at 0 RPM. If you attempt to accelerate too fast you will stall out.

Now if your machine is not stiff enough, acceleration can cause the machine to wobble around as the mass of the machine changes direction. Infact with the belts used on these machines it can happen if you have any sort of stretch in the belt.

As far as crisp corners you really want to run a router in Constant Velocity mode. That setting is in the center of the mach3 General Config screen. Also set your look ahead to 200 lines, that will provide for smoother operation also. The look ahead in combination with Constant velocity will give you crisp corners and smooth operation.

And yes the best way to get the acceleration and max speed for your machine is to just expriment. My big JoesCNC2006 can move at 200ipm rapid and acceleration is set to 20 units. These settings have so far allowed me to make rough cuts at 150ipm. But this is a heavy machine with acme screw drive not belts, weighs in at about 350lbs.

 

Hi Neil,
I have a quick question about running stepper motors. I have wired up my power supply, drivers, controller, and my stepper motors. I am using the usb cnc controller. I have set up my three axis. When I go to jog my motors they do not spin. They make a slight click when I change direction while jogging, but they do not rotate. Do you have any ideas on how I can fix this? What are you ideas on this issue, or do you know someone else on the forum that could help?

Zach

 

Neil,
Thank you for the ideas. I am using the pbx usb controller from probotix along with planet cnc software. The motors are 4 wire and they are wired according to the wiring diagram. I have the jog set at a slow speed and for a step i have it set for 1 inch. I am providing the motors with 36 volts. I am stumped.

 

I thought I would post that I solved my problem. Probotix incorrectly labeled their step and direction terminals, so I just swapped them and now the motors work great. Neil, do your motors make a click every few rotations? When I jog the motors they thump every few rotations. I assume its normal since all three of them do it, but I just wanted to make sure.

 

The help person from planet cnc said that the clicking is normal for the mk1 or pbx usb controllers while jogging. Neil, do you think that your cnc machine could handle a 2.5hp router weighing between 8 and 10 pounds?

 

Obviously PlanetCNC know best but that just does not sound right to me. The stepper's motion and and it's sound should be smooth when jogging.

Tweakie.

 

Neil,

What are the weak points of your cnc? I am in the process of building almost the same one, but my plates are made of mild 1020 steel and I am going with the 3mm belt. How come you didn't go with the aluminum v-slot wheel?

Zach

 

Neil,

Here are a few quick things that have crossed my mind about the deflection issue. I am wondering if the distance from the bottom set of wheels on your z-axis to the contact point of the bit into the material is to large causing to great of a moment around the y-axis. Have you tried cutting at a faster speed to see if you have less deflection at the tip? Also the way you have your base pieces arranged with the y-axis direction pieces under the x-axis direction pieces, I am wondering if by making those flush all the way around and adding legs would make it more stiff. Do the aluminum extrusions seem to be strong enough? I was wondering if they would be a little to small or weak for a larger application.

Zach

 

Neil,

Can you post more pictures of your cnc machine? I am wanting to look at how you built your x-axis.
Thanks

Zach

 

Hi Niel,

Thanks for all the information you have shared. I like the youtube video a lot.
Your build is very clean and tidy. Very professional. I am impressed.

I have been able to convert your solidworks files to parasolid format. Better than step format in my opinion.
You can place it on your parts list so others can get it. I have uploaded them on this post.

I am thinking of an identical machine. But I would like more Z, at least 180 mm.
I want to machine 200 mm high EPS (Polystyrene) and cut poplar plywood up to 12 mm thick.
Have you tried any foam?
Do you think that making the side plates higher will affect the rigidity of the system?

What cutters are you using?
You should get info on feedrate and spindle rpm from tool catalog, it is the tool manufacturers that know the ideal working conditions for their tools. You could be low on spindle RPM.Normaly wood tools are very specific. I have many clients using artcam on wood, special tools and normaly quite high rpm..

I believe the way the spindle or routers is fixed onto the machines is not ideal at all. I have been machining with routers and robots and the stiffer we attach the spindles the better.

With regards to dust on your V sections. You could attach a simple compressed air tube pointing on the V section where the rollers move, so that it removes any debris or dust. Sounds crazy, but it could work. From my experience, dust will go everywhere unless you have a VERY good vacum on the tool, even with all the dust boot apparatus . I have never seen a router with no dust on it. It sucks but it is the way it is.

At 2 min 22 seconds on your video, is that fastest speed you cut (when you press shift + arrow keys?
What feedrate can you go up to on this machine? or what feedrate are you using for the simulation?

One last question?
On your machine how do you set the length of the tool?
In case you use a different cutter for roughing , and finishing? That is something I am wondering. cause if you zero the part (part datum) with a tool, and use just the one to do the milling, no need to set correct length, as the machine will offset correctly. in other words it will know the height.

I am curious to know how this can be done. as this is important when doing a more complex milling job with different tools.

Thanks again and impressive build.
I have learnt a lot from your project.

Alex

 

This is the same for all machines. Some people have a conductive plate that the machine touches with the tool-tip and then automatically calculate the length. I don't have one yet so I loosen the collet and let the tool slide down to touch the part. Then move the machine down to where you want it on the tool, and then tighten it and tell the machine that z=0. This way you can change tools many times.

 

Hi Dude,

I understand the process.
I work with cnc, sell software for a living, so I am ok with all this.

My question was in the line of saving the tool length inside the software, MACH 3. If I have a toolchange device, where the lenght is constant, I do not have to measure everytime I change tool. Is this stored in MACH 3 or any software?

You do this if you have a collect, what about tool change? Meaning when you have a cone and you change the whole lot.

Dude, is there a post for your build?

Take care

 

Well I don't use Mach3 but in Linuxcnc there's a tool table where you set the tool length. I don't have any builds worth mentioning.