Wanting to build a CNC

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01 Nov 2010 21:04 #5012 by tlhiv
Wanting to build a CNC was created by tlhiv
First, I would like to apologize because I am very new to CNC's. I know what CNC's do, and I have a basic understanding about how they operate, but I am trying to obtain some knowledge from the experts on this forum for building my first CNC.

My brother-in-law is a professional welder (and a very good fabricator), and I have a degree in Electrical Engineering. I am also VERY familiar with Linux (as I have been using Linux exclusively for over 10 years now). He and I would like to build a plasma cutter CNC. He has told me that building the table should be absolutely no problem, and it is my understanding that all of our cutting will be done via EMC controlling several stepper motors. We would like for our CNC be 5 feet wide by 10 feet long (large enough to accommodate a 5'x10' sheet of plate). We realize that 4'x8' are pretty standard, but he assures me that 5'x10' is out there as well.

Our plan is to build the entire table out of carbon steel and the "CNC stuff " (i.e., rails, tracks, gears, etc.) out of aluminum. We are anticipating that a stepper motor will need to reside on each side of the table in order to run one axis the entire length of the table. These motors should always be turning at the same speeds and should start and stop at the same time. There should also be a stepper motor that moves the head across the table as well. We do not (at this time) see the need for a vertical direction since the cutter will either be on or off (unless I am misunderstanding). However, when we purchase our parts for building the CNC, we would like to go ahead and have vertical support in our driver board (even if we don't use it initially).

I am pretty comfortable with CAD programs, and I see no problems converting CAD output from DXF to G-CODE. Perhaps, EMC can do this for me (I am just starting to learn all of this, so I am not sure of what EMC can do and what it can't do). I have no problems assembling a driver board from scratch if need be, but I would like to know what motors are preferred (perhaps sizing is an issue here) as well as driver boards (either already assembled or design plans) are preferred for EMC.

Thanks for the attention, and I look forward to hearing from your expertise.

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01 Nov 2010 23:41 - 01 Nov 2010 23:43 #5013 by BigJohnT
Replied by BigJohnT on topic Wanting to build a CNC
I have built a CNC plasma cutter from scratch, while not a big as your planning I can handle 4 x 8 sheets in 36" sections. I used belt drives and a drive shaft to each side of the gantry. The water table is real important to keep the dust down... I used steppers with Gecko 203v drives and a 64v power supply. You need Z for sure as you have to move the torch up and down to follow the material as well as accommodate different thickness material. I designed a floating head with a micro switch to "touch off" on the material then move to pierce height.



EMC2 is light years ahead of other controls when it comes to high speed cutting with lots of little arcs and lines.

When you design your machine make sure the X and Y axis move real smooth so your cut is clean. If I was to do it again I would use linear bearings for X and Y. Think about making provisions for a spotting drill if your making parts for other people that might have the need for holes.

Any step and direction drivers will work with EMC.

John
Last edit: 01 Nov 2010 23:43 by BigJohnT.

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01 Nov 2010 23:52 #5014 by tlhiv
Replied by tlhiv on topic Wanting to build a CNC
John,

Thank you for your reply. I would be very interested in knowing how you built your tracks or if you bought pre-built ones. I would also like to know what size steppers you have and what drivers you are using. Also, if you could give me an estimate of how much said motors and drivers cost, that would be great. When you say that you can handle 4'x8' sheets in 36" sections, do you mean that you can cut 36" at a time but you simply have to slide your plate down? If so, perhaps that's all that is needed. This is where experience can really be helpful to us because we just don't know what to expect.

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02 Nov 2010 10:59 #5017 by BigJohnT
Replied by BigJohnT on topic Wanting to build a CNC
I built the rails from Frameworld and 80-20 extruded aluminum. I used their slides and build everything else up from scratch. I liked the Frameworld slides better.

Some photos here.

s47.photobucket.com/albums/f163/johnplctech/Plasma%20Cutter/

The rails sit on top of a steel tube frame and only touch at the corners. The corner blocks have a hole in the bottom that sits over a socket head cap screw so I can lift it off easy if I want to. This allows me to slide a 4 x 8 sheet between the aluminum rails and the steel rails.

Things I would do different and or might change one day...
Have the slats inside of the water table so I can bring the water up to the bottom of the material.
Use a linear rail for the Y axis.
Use the next bigger size belt for the X axis.
Have a way to quickly square up the sheet to the table and hold it in position.

I have a Hypertherm 1250 with a machine torch. I used Gecko 203v drives and Automation Direct steppers. I'll have to make up a list of parts I used...

John

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02 Nov 2010 11:42 #5018 by tlhiv
Replied by tlhiv on topic Wanting to build a CNC
John,

It sounds like you've put quit a bit of cash into your parts. We're not against doing this, but this is our first CNC build and at first, we're not sure how much business we're going to be able to generate with it. So, we're probably going to want to go with some kind of entry-level system at first, and then as we generate business we could build a more sophisticated system.

Not that we're looking for the cheapest parts out there, but are there any recommendations for less expensive drivers, stepper motors, transformers, and rails? Again, if that means that we have to build the drivers and rails from scratch, we don't mind doing this.

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02 Nov 2010 12:54 #5021 by BigJohnT
Replied by BigJohnT on topic Wanting to build a CNC
You could do some googling and find some cheaper ways to spend your money and depending on what kind of quality you want out of the final product you might get away with building everything from scratch but I don't know personally of any that work well. We did do our first few cuts with the hand torch and my other brother John did have to hold the trigger while the machine ran... but that got old in a few minutes. And there is a workaround to use a hand torch and have the CNC control turn it on and off... Without a mill handy to make the parts needed to "connect" the frameworld rails together and make the gear boxes, what I did might not be an option. In the end trying to build a CNC machine for little or nothing is usually a time sink. Keep in mind the speeds you need to have on a plasma on thin material can be as fast as 415 IPM so you have to design your machine to achieve those speeds if your cutting thin materials. For 14 gauge from memory it is in the neighborhood of 135 IPM with the fine cut tip and 220 IPM with the 40amp nozzle. You can download a Hypertherm user manual and see the cutting speeds for different materials and tip combinations. Also your plasma needs to be CNC ready ie you can turn it on and off with the CNC control. High Freq start plasmas like Harbor Freight ones and CNC controls usually don't get along well with each other.

I spent many hours cruising the plasma forum over on the zone before making the decision on what plasma cutter I wanted to purchase for an eventual CNC machine. Then spent many hours researching and planning before I made the first piece of my plasma cutter. In the end if your trying to make some money with your plasma cutter you need one you can make the parts fast to be competitive. What I mean by that is the time you spend between getting a drawing and making sparks and having a finished product needs to be very short.

If you can build a driver as good as the Gecko 203v for less than $147 and have no questions replacement your real good! You drive needs to be able to morph from 10 micro steps to single steps as the speed changes for smooth operation of your steppers and handle 80 volts to have enough power for the acceleration needed to keep the speed up during direction changes. I ended up using a 64 volt power supply after doing all the calculations. So a cheap 12 volt or 24 volt driver won't cut it in a plasma cutter.

The steppers from Automation Direct was the lowest cost item of the build at about $30 each. Looks like they are $39 now for a Nema 23 triple stack 276 oz-in holding torque stepper. At the bare minimum you need an X and Y axis. I have used a drag tip on mine once or twice on the hand torch. What you see today on my Z axis is not what I made my first cut with, but evolved to that after some experience.

You might consider an all steel design if you don't have a mill handy to make the aluminum parts.

John

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02 Nov 2010 13:12 #5022 by andypugh
Replied by andypugh on topic Wanting to build a CNC
If you are comfortable with steel fabrication, then something similar to a Mechmate might suit you:
www.mechmate.com/

Dallur has built a monster plasma cutter for making boat parts. He is active on the EMC2 mailing list and IRC, but I am not sure if I have seen him here.
www.dallur.com/index.php?id=130

The Mechmate plans are a bit expensive at $100 so I probably wouldn't suggest buying them unless you intend to make exactly that machine. (It is a router, so not quite the same sort of thing as you want anyway). A router needs to be a lot less vibration resistant than a plasma cutter, I think you could make a very adequate plasma simply using roller bearings arranged around square or round section tubes. There is mainly-MDF router that uses this approach:
If I was building such a machine I think I would look very hard at using bicycle chain as the drive system (anchored at both ends, tensioned and running through a system of sprockets). EMC has leadscrew compensation capability, so this could be made as accurate as you need.

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02 Nov 2010 13:39 #5029 by tlhiv
Replied by tlhiv on topic Wanting to build a CNC
We don't anticipate that this machine is going to constantly be making us money. In fact, we don't even anticipate that we will have much business at all in the beginning. My brother-in-law lives in a rural town, but this is as much of a "project" to us as it is something to make money for fabrication. In fact, we don't even have a plasma cutter yet. We have considered (at least for the beginning until we have our proof of concept satisfied) using an Oxygen/Acetylene torch instead of a plasma cutter until we can justify buying a plasma cutter.

I see no problem purchasing 2 of the drivers and 3 motors (I'm thinking 2 of the 3 motors will be running in parallel to one another on each side of the table as hopefully can be controlled by the same driver. I'm not sure if that's doable or not, but I can't see why it wouldn't be. Also, it would seem that a solenoid could be used to "squeeze the torch trigger" and release it for cutting. So, recapping it would seem that we might be able to get away with 2 drivers, 3 steppers, a power supply, and a solenoid + rails.

Thoughts?

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02 Nov 2010 14:06 #5030 by BigJohnT
Replied by BigJohnT on topic Wanting to build a CNC
I use one motor and a drive shaft between the two rails to drive both sides from one motor. If you put a motor on each side you need a drive for each one. The main thing up front is to get a good X Y table then add on as needed.

Flame cutting actually gives you the ability to cut thicker materials. You would have to fool around with the dwell (preheat) times and speeds I assume but a very workable setup to get going. What kind of parts are you thinking of cutting?

Spend a lot of time in the plasma forum over on the zone and see what others have built, some are very clever and use common materials. The weight of the gantry needs to be kept as low as possible and stiff enough not to flex with the loads from the torch and direction change. The heavier and harder to move the more power you need. I've seen skate wheel rails, tube rails with bearings and all kinds of clever things over on the zone and elsewhere in my search for what would work for me.

John

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02 Nov 2010 14:08 #5031 by BigJohnT
Replied by BigJohnT on topic Wanting to build a CNC
This is a link to plasma machine builds on the zone

www.cnczone.com/forums/plasma_edm_other_...machine_project_log/

John

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