New design, need critiqued

[CtrlZ]

Hi all, I'm new here, and this will be my first CNC router build, so please bear with my ignorance.

Recently the company I work for has realized the need for a new CNC router table. We've decided to build our own for a few reasons. One reason is, we are on a budget and have some of the components on hand. Also, we can design a custom machine to suit our needs. Not to mention, it will be a lot of fun!

I am designing this machine, but I would appreciate some feedback before I get started. The design is a 48" X 96" 3 axis router table with approximately 18 inches of clearance under the gantry. We will be using it mostly for milling wood parts. The spindle and servo motors are just simulations, and I'm not showing the bearing blocks for the ball screws, but I am planning on using some large AC servos in a direct drive system with spider couplings. I'd like to know everyone's thoughts on the design. Is it rigid enough for our application? What size/type ball screws? Etc.Please critique away.

My goal is to make as sturdy of a machine as possible without blowing the budget on material and man hours.Thanks in advance for your comments.

[Herbertkabi]

My opinions:
1.) Your Z axis! You have constant max long arm from top of endmill to z-wagons - why? Rails I like to see on the moving part of Z, rails making moving part of Z-axis more rigid (then wagons will slides on the bridge and this part will be as short as bridge widht). Then additional plate between bridge wagons and Z-wagons ... or bridge rails and wagons to the other side of bridge.
2.) Dont try to lighten your machine. Why to cut windows on the bridge?
Very modern ? ?
This is milling machine, not any rc-airplane. As massive as better. Later you will want to make it more massive anyway and you will close all windons to keep your machine as clean as possible. Also you will think: oh dear ... why I used so thin materials... why I did not use at least twice thicker ...
Added photo of my machine I built many years ago. All axles have 4 wagons, X-rails are under the bottom, y-rails on the back of bridge and z-rails on the moving part of Z.
cheers,
herbert

[ger21]

Do you really need that much Z axis clearance/travel? Definitely a weak point. I'd also make the gantry beam taller, and spread the rails apart a bit more.

[CtrlZ]

Thanks for the replies! Not what I was hoping to hear, but just what I needed. Sounds like I need to work on the design some more.

Should I even bother with the extruded aluminum for this size table?

[Phife]

While is looks pretty, I would have to agree with the other guys, 18" Z and the way its designed is a huge weak point. I find my 8" Z travel is plenty for stock up to 3.5-4" max. If you are planning on mainly cutting sheets you dont need 18".

The X-Y uprights look thin, make those shorter and beefier. Space out your bearing blocks more.

How are you going to make those blue pieces on the bed/frame of the machine? are they metal? sheet metal? I'd avoid the fancy shapes and sheet metal and go with solid aluminum/steel.

All the cutouts and fancy shapes will end of costing more money in the end.

Use the aluminum extrusion if you dont have access to welding equipment or someone who can weld for you. Otherwise go for square and rectangle steel tube.

Oh and if you are planning on cutting sheets I'd go for a working area larger than the sheets, say 49x97" so you can cut around a full sheet.

Just my 2cents

[florins]

Hi CtrlZ,

I believe that you have a great start, you should just work a lot in order to get the things squared first on paper, and just later in reality - much cheaper this way . Not too much to add to what the other guys are saying (and I agree).

Firstly, you would probably like to extend the distance between the rails as much as possible, in order to increase the momentum of inertia and reduce vibration and flexibility. Looking to the Y rails, I can see that you placed them on the lower slot of the top beam and on the top slot on the lower beam. It should rather be the other way round, use the top slot on the top beam and the lower slot on the bottom beam, to increase the distance and momentum.

The beam connecting the two Y columns should be much stronger compared to what the drawing is showing - probably an extruded profile can help there.

Then the Y-Z carousel just seems way to high compared to its depth. I know that all the other guys said the same, but it's very important that the height should be twice (worst three times) the depth - this is based on practical experience, but can be calculated as well depending on the materials used. If you wish to keep the height for whatever reason, then consider to increase the depth of the carousel (and adjust accordingly the X axis).

Last, if possible, for stability reasons, try to keep the spindle in the centre of the carousel base.

Hope this helps, good luck with the design and work!

Florin

[CtrlZ]

Wow, thanks again for all the replies! I agree with all of you and some of the things you brought up I already had concerns about, and also some things I didn't think of. This is a big help and I will definitely use your advice.

The blue parts are all just 3/4" steel and most everything else is just 3/4" aluminum or extruded aluminum. We do have welding capabilities but I was trying to make everything in components to save on man hours but maybe I'll rethink that now. We probably don't need 18" but we do need 12" minimum. We will need to mill some wood columns that are 12" X 12" and I can see were we could possibly need a little more for custom jobs.That's one of the reasons for the custom design, but I don't think I'll have a problem beefing up the design with all your advice.

Any thoughts on the drive screws? I was planning on 1" for the X and 5/8" for the Y and Z. Is that large enough. Or do I need to wait until the table design is complete to decide on the screws? Also, rolled or ground and what tolerance for milling wood? I know what the best is but there is a big difference in price and the lower the material cost ends up, the better chance I have of getting a green light from the boss. I don't want to compromise though if it's not wise.

[stevespo]

Ditto. For sheet goods and even fairly deep relief carvings 6-8" of clearance and travel on the Z is typical.

I would go for 1" screws with 1TPI on the X, and something similar on the Y. You can go smaller diameter on the Y, but make sure you calculate the point at which both screws will start to whip before settling on the TPI. On the Z, a 5/8" 5TPI will give you decent performance, but you could also use 2.5TPI or similar.

Steve

[Phife]

what sort of milling do you need to do on the 12x12 columns? are you planning a rotary axis?

[CtrlZ]

@Steve, thanks for the advice. I'll upgrade the Y axis for sure.

@Phife, we are only doing shallow v groove cuts and flutes on the 12" columns. We may add a rotary axis later, but not for the large parts. We have CNC lathes with milling capabilities, but they have limitations. Some of the limitations can be overcome with software changes, but if the need arises we may decide we need a fourth axis on the router later. Maybe not, but I did have it in mind to keep the option open for the design.

[Phife]

Well if you can make some sort of removable bed so you can drop the large column into the bed you would not need such a large Z axis.

Your long Z axis will come back and bite you in the ass. Its such a large cantilever point that when you try cutting out sheets and hardwoods at any reasonable speed you will have trouble keeping the Z from deflecting.

If you need such a long Z then you need to beef up the entire machine.

Also I think you will be better off with R&P drive for your X, a 1" x 8ft ballscrew is going to whip and limit your cutting speed.

[CtrlZ]

Hey thanks phife, great idea! I had thought about an adjustable table when I first got started but it was somewhat of a convoluted idea, but I never thought of just a removable table. I think I can make that work.

What about rack and pinion for the X axis?