[jimluu]

Hi. I've been planning a 4x8 cnc router for quite some time now. It just dawned on me that I may not have enough space to have a horizontal table in my shop. Has anyone tried an upright design? I would appreciate if someone would point out some of the difficulties of a vertical design. Would save lots of space, and I may be able to convert the back side of the table into a panel saw.

I'm using a 2" ballscrew with a .2 pitch. Wondering how much torque needed to drive a 75lb gantry vertically with this screw.

[ger21]

Don't have any numbers for you, but just spinning a 2" screw requires quite a bit of torque. And your gantry will probably weigh quite a bit more than 75lbs. All the vertical routers I've seen have the gantry moving side to side, so you don't have to lift the gantry, just the Z axis.

[oopz]

you may look at this : .... ;-)

http://www.cnczone.com/forums/showthread.php?t=6840

its a wall mounted and off course DIY ...

[jimluu]

Thanks for the response! I've thought about a horizontal design, but if I can make a vertical design work, I'll save half the space. I'm planning to use a NEMA 34 motor with around 1200oz of torque or more. Do you think this is still not enough? I have not bought the motor yet and can always buy bigger ones. I'm wondering what happens if there is no power to the motor...will the gantry slide down?

[gth629e]

I'm wondering what happens if there is no power to the motor...will the gantry slide down?

It will depend on the drive system. If you use a ballscrew with high pitch or a multi-start ACME screw with high lead, then it would back drive with no power. A braking system would probably be in order.

Its an interesting concept though. Have you though about turning it on its side? That might save some space. Of course you would have to worry about the Z-axis assembly back driving lol.

You also have to think about how your will hold the piece you are working on. Gravity is not longer on your side.

Keep us informed!

Regards,

[jimluu]

If anyone out there knows how to calculate torque requirements...please help. My gantry will likely weight 75lbs, but lets use 100lbs to err on the safe side. My ballscrew is 2" in diameter and has a pitch of .2. I'll be mainly cutting plywood and MDF. I'm not familiar with any types of cnc physics and have a hard time interpreting some of the formulas posted else where in the forum. I would appreciate any help to see if it's at all feasible to go forward with this vertical design.

[gth629e]

Jim,

I would be glad to help. Below is the basic process most people use for claculating how much torque is required or how much linear force you can get from torque.

To find how much linear force you will get from your lead screw and motor assembly, use this formula.

1. Convert ball screw/lead screw turns per inch to pulley radius:
Example:
½-10 2 start = 5 turns per inch convert this 1 inch/5 turns per inch = 1/5= .2 = pitch

So pitch = .200

Then divide pitch by 2pi or 6.28
Example:
0.200 / (2pi) = 0.200 / 6.283 = 0.0318" (this is the equliv. radius pulley)

2. Convert motor torque into linear force: divide torque of your motor by the puller radius
example:

Torque = 200 Oz.-In.
200oz-in / .0318 in = 6289.3 oz. (linear force)
6289.3 / 16 = 393 lb (linear force)

3. Multiply result by the efficiency:

Ball screws have efficiency of .75-0.95 (I usually use .8)
Acme Lead screws have efficiency of about .45-.60 (I use .5 for rolled ACME screws and .4 for basic ACME threads with anti-backlash nuts)

393 * 0.6 (efficiency for ball screw) = 235.8 lb linear force.


You can see that in this example that at 200 oz in. you would have 235.8 pounds of linear force. Remember though, the rated motor torque for stepper motors is at holding torque. You would need to know about how much torque you would have at a certain RPM that would give you the desired feed rate.

Again, this is a basic calculation but it works well most of the time. The correct way is to break everything into inertial forces. The above calculations does not account for acceleration of the machine.

You case would be interesting as you would need more torque in on direction and very little in the other. I hope this helps.

I am making an excel spread sheet that will do a lot of this for you. When I’m done I’ll send it your way.

Regards

[jimluu]

Brian,

Thank you for your help. I would never have figured all that out by myself. Seems like the 1200oz motor that I have in mind should work ok. I'm going to hire out a local company for the frame then I'll post the results here.

[gth629e]

Jim,

I was just about to post and tell you that for startup you roughly need double the torque, and that would be even more true for your applications. However, with that much torque, you shouldn't have a problem lol. Thats a lot. I look forward to hearing an update. Keep us informed.

Regards,

[jimluu]

I wonder if having 75-100lbs of preload due to the vertical arrangement have signifcant impact on the wear of the ballscrew bearings and mounting bearings?

[gth629e]

I wonder if having 75-100lbs of preload due to the vertical arrangement have significant impact on the wear of the ballscrew bearings and mounting bearings?

It will have an effect but it should be manageable. For ball screws, you may develop raceways in the screw along one side faster than usual. For ACME screw, the nut would most likely wear more rapidly than usual.

For the mounting bearings, be sure and use a proper thrust bearing on each end in conjunction with a radial bearing. This will take the thrust off the radial bearings and the motor. If you do this, your end mount bearings should be fine.

If you need a diagram on how to use a thrust bearing in conjunction with radial bearings, just let me know.

Regards,

[jimluu]

Thanks again Brian. I would appreciate the diagram you'd mentioned. If you know a good source I'd appreciate that as well. Misumiusa seems least expensive so far, but $225 for bearings alone seems like a lot.

[Geof]

I just skimmed through the posts and had a thought. If you have the gantry vertical and do not counterweight it you may never have to worry about backlash.

[gth629e]

Thanks again Brian. I would appreciate the diagram you'd mentioned. If you know a good source I'd appreciate that as well. Misumiusa seems least expensive so far, but $225 for bearings alone seems like a lot.

Jim,

Check out this link as I do not have the thrust bearing diagram up on my site as of yet.

http://cncrouterparts.com/parts.html

Of course you do not have to use those specific parts but the general combination should be the same.

Thrust washer, thrust bearing, thrust washer, bearing plate, radial bearing, bearing block. Let me know if that is unclear. For cheaper bearings check out vxb.com. They have an assortment of radial bearings and some linear bearings. For thrust washers and thrust bearings, check out mcmaster.com. They have just about everything in the world and their thrust bearings should only be a few bucks.



Geof,

I have been thinking about the design and it is interesting. Although it will have its share of problems to overcome, there are advantages. As you said, there could be almost no backlash. It could save space. The longest axis (x-axis) support and bearings would not be due to flex due to the gantry weight. Much of the cutting dust would probably fall to the floor on its own etc. Like I said its an interesting idea.

Regards,

[jimluu]

Thanks guys. I'm a bit confused about the necessary bearings for my x-axis ballscrew. I made an error in my earlier post. My ballscrew is actually 1.5" with a .250 lead. The ends are machined already from the factory. I've included a PDF scan of the screw. I've looked at Brian's diagram and I'm a bit confused. I was going to buy a radial bearing/block and a fixed bearing/block. I didn't know about the washers etc. I guess I've spent enough money on this project that I'm not going to skimp on the bearings. Would you look at my PDF scan and spell out what's necessary? Misumi actually has a 30%off for first time buyers now.

Thanks. Jim.

[Oakley1975]

This is a good idea in many ways, as gth629e mentioned about dust falling off, others mentioning the space saving...

Something to consider would to place it at a slight angle (top back and bottom out) just so that the material could rest on pins or something a the bottom and it would lie back against the work surface until properly clamped... also the gantry and z axis would benifit from gravity slightly to fend off backlash.

We all like to think here,.. just a couple of ideas.

[joecnc2006]

I have thought about designing an alternate base for my 4x4 machine to make it vertical maybe at a 20 to 30 degree angle, with two motors on the axis it may be able to lift the gantry without a counter weight, but with cutting forces involved it may be needed will not know until it is tried.

Joe

[jimluu]

I've ordered all the major parts and am about to build the frame. Would square metal tubing be better than 8020?

[joecnc2006]

I used 80/20 for the side pieces and the gantry for the ease of bolting right to it, and ensuring a straight line of bolt paterns and parts.

joe

[martinw]

Constant force springs would help for the axes. No idea how much they cost though.

I would like to thank you for starting the thread. We are all short of workshop space.

Best wishes,

Martin