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 Post subject: Johnsinski's CNC router
PostPosted: April 2, 2020, 1:38 pm 
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At a request, I'm going to explain the CNC router I built a while back for cutting 1/10 scale RC body molds and other miscellany. It was originally intended to cut wood, plastic, epoxy, etc. But when I started my BEC I found it could do aluminum also, albeit slowly.

Specs:

Travel: X 42", Y 21", Z 10"
Ball screws: 5mm pitch, 16mm diam
Spindle: Brushless air cooled 1500W with VFD (variable frequency drive). ER16 collets
Steppers: 205in-oz Nema 34? HobbyCNC
Control Board: HobbyCNC Pro
Stepper Power: Big transformer rectified to 42VDC
Computer/Software: LinuxCNC
Frame: Horizontal Mill. Wood, Aluminum, Steel

The frame is sort of just a big thick wood box. It's 4x4s with laminated particle board (laminine?) facing. While it's nice to get it square, it's not necessary as there will be shimming...lots and lots of shimming. The CAD picture is not quite up to date, the only opening is the angled access area in front.

The quick jist of the build is get the main rails parallel and flat, then get the vertical gantry squared to that and finally the z axis squared up. I found it to be a bit of an iterative process.

I used a straight level (as a straight edge) to get the main rails straight. My rails simply have clearance holes for the bolts and I can tap the rail around and tighten those down. But, I also put shims under the whole rail assembly to get straightness in that direction (this would be where you want the frame to be as flat as possible).

Then you need to get them parallel, fairly easy if you have a caliper that big, or use a bar cut to length as a gauge.

Now the rails are straight and "parallel", but they may not be in the same plane. I used the "stretch thin piano wire across the corners" technique. You end up with 2 high tension wires going crosswise from the corners. Technically you need to "space up" one of the wires to compensate for the thickness. When the wires barely touch in the middle, you're pretty close to flat. There's a name for this whole process I can't think of at the moment.

Bonus question, what car is on the machine?

More next time. Add questions.

John


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master_07.jpg
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PostPosted: April 2, 2020, 1:55 pm 
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Quote:
Bonus question, what car is on the machine?


Porker 917?


Very cool build, if I had the space I'd be wanting to build one.


Rod


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PostPosted: April 2, 2020, 2:08 pm 
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917, yap.

I've got a seat of Proe that has NC capability. Not cheap, not for everyone. I've heard good things about Fusion 360.

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PostPosted: April 2, 2020, 2:26 pm 
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Johnsinski wrote:
917, yap.

I've got a seat of Proe that has NC capability. Not cheap, not for everyone. I've heard good things about Fusion 360.

I have used pretty much all of the professional CAD programs and have taught classes on some of them. Right now I am working for a company which has sort of a shoestring budget. They bought Fusion 360...it is quite inexpensive. Its OK for simple stuff, but heaven help you when you have complicated assemblies or parts. It is easy to learn and if you dont have to generate professional quality drawings, its decent. I think its hobby level stuff...again, good for many folks.
Me, I want my Solidworks back...

Nice job on the CNC router

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PostPosted: April 3, 2020, 2:47 am 
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I hear ya, gotta have good assembly power and mechanisms. I went for a job interview a while back, I don't remember the CAD system they had, but it was old and busted, not taking that job. I mean it was at a farm shop, but still...

I worked at a company in the early 2000s that was using DOS, I [PooPoo] you not. And get this, they had a 3D modeller (Euclid 3D (not parametric), bonus points if you've ever heard of that!), but you had to shut down DOS and reboot into Windows to use it. So it was just this cluster of 2D and 3D.

Aaannyway

CNC building pitfalls:

Ball screw bind: They bind when misaligned.

Ball screw wobble: This isn't huge but it can make a pattern in a fine 3D surface. Wobble can be from too high a speed (and too long of span) but mostly it seems to be from poorly aligned drives. I have direct solid connectors. Usually I see a rubber or spring type coupler of some sort, but I wanted solid. And that caused some problems, but I was able to move the steppers further away in the axial direction and make a much longer connector. I also had a stepper motor with a slightly bent shaft that was causing a similar wobble with the connector shaft.

Axis balance: in my case I really needed to counter the weight of the head/z axis, it's like 80lbs. I have roller chain going over a pulley to 50lbs of lead on a slide.

Stepper properties: I've had great luck with steppers as opposed to servos, but they have their limits, they dramatically lose torque at high speed. I've gotten the y-axis up to around 400 ipm with some futzing, but I have the limit set around 100 ipm for actual milling. With the 205-in-oz steppers I can barely prevent the table from moving at 100 ipm. Probably over 100 lbs of force.

Wood warps from season to season: If you build a wood CNC expect it to warp during the seasons. I did recently remake the y-axis gantry out of steel tubes. Normally if I'm doing something accurate I check squareness and flatness before and usually take a skim cut on the table for things that need to be flat.

The square you get from the hardware store may not be square: Get a square square.

Lost steps: That is the weak spot for steppers, but when properly set up it's not a problem. I had an issue with the HobbyCNC board where it would go into a "low power mode" and it would lose a few steps upon restarting motion or at unusually slow steps, but that has since been fixed. Normally I don't lose steps, even after 10,000,000 lines of 3D surfacing. I would go with bigger (slower more torque) steppers the next time around, but hey, it was my first CNC build.

Power system isolation: Something to do with grounds loops, that's why a transformer is used instead of a switching power supply. The computer connects to the control board via the Parallel port and in some case, wires melt if you try to use a switching power supply for the stepper power.

Don't use a router for the spindle: My first spindle was a small trim router, crappy bearings, worn out brushes, runout, slow, loud and just plain annoying for doing anything of significance.

One of the "lost pics" from the forum crash, an aluminum upright and my STAMP controlled servo squirter system. I have since put plastic sheets up anytime I use coolant. Oh, I also remade the head in aluminum since then too.

LinuxCNC is open source and available to install (obviously on Linux). Even if you don't have a CNC, you can play around with it.

This is one of my favorite homebuilt CNCs:

http://oneoceankayaks.com/madvac/madvac_index.htm


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PostPosted: April 3, 2020, 12:27 pm 
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Very cool.

Do you have any more details on the RC car bodies? Did you do your own vacuum forming?

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PostPosted: April 3, 2020, 2:38 pm 
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I am truly entertained by your automated spray bottle.

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PostPosted: April 3, 2020, 6:58 pm 
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I didn't fully engineer the squirt bottle and the wire pull rod eventually broke. :lol:

Yes, I vacuum form the bodies too, from design to final product all in my little shop. Here the porker 917 went 172 MPH, actual speed, not scale speed. Not my car, a customers car. That body was used on a car that set a world record a few years ago. I have a bunch of different bodies I make, just a small side business to help offset my RC addiction.


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PostPosted: April 4, 2020, 11:09 am 
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Was that gas or brushless?

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PostPosted: April 4, 2020, 9:23 pm 
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Most of the speed run guys are now electric/brushless. This one is electric/brushless.

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PostPosted: April 7, 2020, 2:09 pm 
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Hi John,

Thank you for taking the time to post your CNC router build!

Any reason why you chose to have the work piece vertical?

Mike


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PostPosted: April 10, 2020, 1:52 am 
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It's a bit odd, I know. In the big machines it would be called a horizontal machining center.

One of the advantages is chip evacuation. Gravity help this. On vertical mills you generally need strong flood coolant or high pressure air/mist to evacuate chips from pockets.

In my case it also simplified balancing the head weight. Well, the 50 lbs of lead I have on the back side of the y axis really balances the y-axis and head weight.

One more advantage in my case is that I only have one stepper wire that bends. The x and y axis wires don't need to bend or flex.

I generally don't have heavy things to machine so they are actually not bad to set up on the vertical. I usually have dowel pins to locate anyway. So normally I just lift the fixture up and slide it onto pins and it stays there while I bolt it down. It is actually a nice height to work on/set up/change tools.

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PostPosted: May 5, 2021, 3:20 am 
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I have a friend that works in a CNC shop that makes large tooling (molds, punch and die). I had him make a large aluminum "base" plate for my cnc router. I'm just trying to square it up.

I have "staggered" holes in it for 3/8" studs and 1/2" dowel pins.

It's really awesome, Thanks PS!


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PostPosted: May 5, 2021, 4:34 am 
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That will be a great addition to your machine.

I have started on the CNC conversion of my RF-30 clone milling machine.
Hoping to have it running before the end of the year.


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