Help Design A CNC Unit

[Joe Wood]

Folks, could you help me design a CNC setup for cutting these various curved rafters only? The CNC shop I usually take these to has been so busy the last few years that it sometimes takes weeks to get them back.

The widest pieces I'm cutting is 2x12, the longest would be 12' so I’m thinking a 2’ wide x 12’ long table. Could the router travel up and down a single strongback above the table? I only work with clear wr cedar and Calif. redwood ~ 2” thick so don’t need a powerful motor. I'm hoping to just set the CNC table on one of my shop benches when I need to use it.

How do I get started with this design? Are there any good resources I could check out?

[Jim Becker]

It's unusual for a CNC machine to be more than 10' long, but you could certainly build something, particularly with AVID in a custom size. The machine doesn't care how far the spindle has to travel. It's just a matter of engineering the length of the various cables, etc., that have to be in the ladders so that full movement can happen.

As an alternative to buying/building a custom size CNC machine, you can consider making templates or having them made with a CNC and use those to create the rafters with a combination of a bandsaw close to the line and then pattern routing to final off the template. I have made more than one template that had to be bigger/longer than my 4x4 CNC machine can cut and it was just a matter of a simple flat dovetail to join the pieces together into the larger template.

[Bill George]

Folks, could you help me design a CNC setup for cutting these various curved rafters only? The CNC shop I usually take these to has been so busy the last few years that it sometimes takes weeks to get them back.

The widest pieces I'm cutting is 2x12, the longest would be 12' so I’m thinking a 2’ wide x 12’ long table. Could the router travel up and down a single strongback above the table? I only work with clear wr cedar and Calif. redwood ~ 2” thick so don’t need a powerful motor. I'm hoping to just set the CNC table on one of my shop benches when I need to use it.

How do I get started with this design? Are there any good resources I could check out?

There is far more to this than you think, are you prepared to spend say $10,000 or more on a custom table from AvidCNC? It could go as high as 15k and do you have the skills to create a file in gcode to run this machine? Granted it could be done in either VCarve Pro or Aspire. https://www.avidcnc.com/
https://www.vectric.com/products/vcarve-pro

[Michael Burnside]

Only thing I would add to posts above is that you could get by with a smaller machine if you employed tiling. That is, you mill one section and slide the piece through and mill the next, etc, etc.

The complexity of the piece you’re milling would dictate how difficult tiling is. If just a perimeter cut or simple pattern, tiling is trivial.

You could contact AVID as they may be able to do something custom, but as mentioned this endeavor is not cheap.

[Robert L Stewart]

A good local contact is a friend of mine. He is a CNC machine fabricator. He builds custom units.
John 760 803-0058 Ramona, CA

[Maurice Mcmurry]

Big CNC machines come up on Purple Wave Auction fairly often. Not local to California unfortunately. I am mostly clueless but see some go for far less than I expect.

Used Construction, Agricultural Equip., Trucks, Trailers & more (https://www.purplewave.com/search/CNC%20router?searchType=all&dateType=past&dateRanges=2023%2C2022&zipcodeRange=all&sortBy=current_bid-desc&perPage=50&grouped=true&viewtype=compressed&=undefined&searchTerm=CNC+router)

[Gary Campbell]

I only work with clear wr cedar and Calif. redwood ~ 2” thick so don’t need a powerful motor.

That is a totally incorrect assumption. Based on what I have observed over the last decade and a half your needs are exponentially higher than most casual CNC users and 2 -3 times most commercial user's requirements for spindle power and motor torque. Think 5hp and up spindle, and servos for drive power. Steppers will not keep up to a production schedule.

[Joe Wood]

Hey Gary Campbell, by Steppers, you mean multiple passes with lower depths each time? That's fine with me and how my other CNC guy does it.

[Jim Becker]

Hey Gary Campbell, by Steppers, you mean multiple passes with lower depths each time? That's fine with me and how my other CNC guy does it.
'In general', there are two types of motors used for movement in a CNC...steppers, which are functional and inexpensive -- and servos, which are heavy duty and generally more powerful and accurate because they move with constant fluidity rather than in small "steps". What Gary is saying is that for a production environment, steppers are unlikely going to have the power and longevity you want/need to make the investment worthwhile. Right tool for the job is the key.

[Michael Burnside]

An alternative is to have someone cut you a template out of phenolic or HDPE or even aluminum. Then, with said template, you rough cut the shape and clean it up with a flush trim bit using a router or shaper.

[Michael Burnside]

Hey Gary Campbell, by Steppers, you mean multiple passes with lower depths each time? That's fine with me and how my other CNC guy does it.

As Jim pointed out, Gary was talking about the CNC hardware, not the pass depth it takes. In general servo motors have a high, fairly flat torque curve and can be more accurate depending on how well the closed loop system is designed. I'm a EE and have designed several controllers using both types and they both work well (beware of cheap steppers), but not in the context of CNC. That said, if Gary thinks that's what you need, I'd listen. He has probably forgotten more about CNCs than any of us know.

[Joe Wood]

Where can I see some pics and some Info about these servos, the ones I'd be looking for any way.

[Gary Campbell]

Hey Gary Campbell, by Steppers, you mean multiple passes with lower depths each time? That's fine with me and how my other CNC guy does it.

Joe...
There are very few, if any cases where you could cut 2" thickness in 1 pass. I was referring to your not needed a powerful motor comment. My response was for both motion motors and cutting motor. Many of the low powered CNCs out there will cut 8/4 material just fine in multiple passes of .080 to .2 per pass at ~120ipm. Thats 10 to 25 trips around each part. If I was doing it to make a living I would have a machine frame and components that would do it in a max of 3. That frame will not be lightweight aluminum or extrusion, it will be steel, and the motion will be provided by servos to a 5-8hp spindle motor.

[John TenEyck]

How many of these do you need in a year. If it's a couple of hundred I'd go the template approach. If it's a couple of thousand I'd farm it out, be it a CNC shop or conventional one. Knowing little about CNC's and needing a custom one is a recipe for frustration, and a long payback period.

John

[Jim Becker]

Where can I see some pics and some Info about these servos, the ones I'd be looking for any way.
They really don't look any different than steppers, for the most part...black motors. :) It's how they are designed, built and how they operate that's the big difference. You'd not be typically looking for these as piece parts; rather, you'd be specifying them with the CNC manufacturer you chose to work with.

[Joe Wood]

I've never seen either, just trying to educate myself.

[Jim Becker]

https://i.ebayimg.com/images/g/o7QAAOSwtLVjyFm4/s-l500.jpg

[Alex Zeller]

I've never seen either, just trying to educate myself.

The main difference between a stepper and a servo is feed back. With a stepper motor the controller tells it how much to move and the motor moves that amount if everything it working the way it's suppose to. But there's no feedback so the controller just goes on to the next line of code.

With a servo the motor gets a signal from the controller to move. While the servo is moving it's also measuring how much it moved and it feeds that information back to the controller. The controller keeps sending a signal to the servo to move until the servo reports that it's moved the correct number of steps.

For example, if the code asks for a 10 step move the controller commands the servo to move in the correct direction. When the servo moves 1 step it tells the controller. Now the controller knows that it still needs 9 more steps. When the servo has moved 10 steps the controller then stops telling the servo to move and then goes on to the next line of code. More advanced controllers can keep track of how many steps are being lost by a servo and let the user know. With a stepper set up the controller doesn't know or care how much the stepper moved. It just moves on to the next line of code. So if the stepper doesn't move all 10 steps you've just lost 2 steps of accuracy. Obviously having feedback makes the machine more accurate.

Servos are more expensive because they have to have an accurate way of counting how many steps they move. Normally they will have an encoder to count how much it's moved. Not only does the encoder need to be able to count the number of steps but also which direction the servo is turning. Since they are more costly they often used in higher end applications. They also use things like rare earth magnets over regular magnets which also drives up the cost but does make them stronger.

[Joe Wood]

OK looks like I'll need some expert advice with this, can anyone do some paid consulting with me?

[Bill George]

OK looks like I'll need some expert advice with this, can anyone do some paid consulting with me?

Gary who replied here is in the business.