[Devastator]

I use dual leadscrews driven by one motor. No problems so far. If I did use two motors, I'd tie them together with a belt like JerryBurks did.

[yellow68gto]

I have a similar question regarding single vs dual screws. I am also using decomissioned cnc mill parts to build a very heavy duty fixed gantry router. My parts list include

2 identical 40mm x 6mm pitch ball screws from a haas vf5 with 30inches travel.

45mm thk profile rails the are 65" long

And a table from a vf3 that is 48x18x2.5 inches thick.

I was hoping to sell one ballscrew( only 1100 hours on it) and put that money towards my spindle.

I want to run the table lengthwise to help keep the machine more square. I am going for x50" y30" and z16" travels.

If I run the bearings under my table at 24 inches apart and a single ball screw down the middle, do you think the table is going to rack? I was hoping with the beef of the rails and bearings it would keep things stable. Also I could run 3 bearing per rail if that will help.

I am not looking at doing much steel but lots of aluminum.I am looking at the fire cnc bt30 spindle and 6krpm capabilities.

My parts I currently produce are approx 20x12x2.0 aluminum plate. Speed isn't as much as a concern as accuracy. I haven't even started looking at electronics yet. All I know is They will be big.

Any thoughts?

[JerryBurks]

.........Any thoughts?..........

If you have a massive moving table with such heavy duty bearings spaced apart 24" in x-direction I would think racking should not be an issue (but that is just my somewhat uneducated guess).
Racking is mostly a problem with moving gantries that can twist or bend and the closely spaced bearings (usually not more than 6-10") will move along x before the increasing gantry deformation under load puts up enough resistance. In that case the dual ballscrews will cure the racking.

[qbvbsite]

Hey,
I'm currently designing a 2 x axis motor design. Does anyone have a picture of the motors belted together? I plan on using 4 rails for the X (one set for each side) which each set having there own ball screw and motor (nema 34 1600oz). My gantry will be like the one in momus build, do you think if one motor fails i'll have gantry damage? Size of the machine will be 1500mm rails for the x and y and 400mm rail for the Z.

Thanks

--James

[hanermo]

I would not use slaved axis on anything important, expensive, rigid, or "industrial".

Where you use steppers and low power stuff, it wont matter, but any proper resolution/gearing/torque will break expensive stuff immediately.

I like the idea of slaved+timing belt .. and think its ok for less-rigid applications.

For my vertical z axis bridge, on a bridge mill/VMC, I will be using 2 vertical ballscrews, spaced 2 m apart.
I dont trust any of the slaving arrangements in corner cases, based on lots of experience.

Drive is servo, and force == 1500 kgf, = 15000 N.
More than approx 0.2 mm error and the bridge will rack, breaking some of the linear guides, maybe.
Even though the linears are very strong (50 metric tons rated) (6 rails, 12 trucks, 35 mm profile, 4000 kg rated load each).

Plan is to use large belts, and one servo.
Cost is not an issue - reliability is very much an issue.
Large belts, HTD8/30, cost approx 400€ for the whole arrangement, and this is ok, for me, for this use.
Approx 48 teeth pulleys (== 200 mm in diameter, several kg per pulley) at each end.
About 100 kg belt tension.

Bridge, going up/down, is == 800 kg in mass, fwiw.

For comparison, even tiny steppers (nema 23) do 500 kgf force, and 500 kgf up vs 500 kgf down = 1000 kgf differenential load.

This of it this way:
Can you put a full size family car on your bridge or gantry, at one end, without it bending/breaking, something expensive, permnanetly ?

(I can put a 3000 kg truck on end end. I´m unsure about putting on 2 trucks with racking stress at end end only..)

[wizard]

First thing first, I have no problem at all with a single motor driving dual leadscrews via a timing belt or other drive arrangement. It can work very reliably if properly engineered.

I have a similar question regarding single vs dual screws. I am also using decomissioned cnc mill parts to build a very heavy duty fixed gantry router. My parts list include

2 identical 40mm x 6mm pitch ball screws from a haas vf5 with 30inches travel.

45mm thk profile rails the are 65" long

And a table from a vf3 that is 48x18x2.5 inches thick.

This is sounding very nice even before we see pictures.

You should have no trouble pushing that table around with a single lead screw assuming suitable use of linear bearings. The question then becomes can you mount the bearings in a way that is suitable for the table to keep it free of racking and still have suitable travel. More info on where you are going with your machine design is needed.

I was hoping to sell one ballscrew( only 1100 hours on it) and put that money towards my spindle.

I want to run the table lengthwise to help keep the machine more square. I am going for x50" y30" and z16" travels.

That would be a good approach. However ultimately it depends upon the final arraignment of components and you may find that your rails are a bit short. Your linear bearings can't be more than 15" apart to get that travel, that in a machine that mounts the rails on the bed.

If I run the bearings under my table at 24 inches apart and a single ball screw down the middle, do you think the table is going to rack? I was hoping with the beef of the rails and bearings it would keep things stable. Also I could run 3 bearing per rail if that will help.

That would work but your rails are only 65" long. You wouldn't get your desired X travel. Racking is pretty easy to understand, the closer your two bearings for each rail are the harder it is for them to resist twisting. Your table is only 18" wide which means the lead screw at worst is only 9" from a rail. In your case I believe the real challenge is properly supporting the table in the long direction, bearings 24" apart imply lots of overhang or no support, which could be an issue depending upon the tables stiffness. My gut feeling considering this is suppose to be a router is that you will do OK.

I am not looking at doing much steel but lots of aluminum.I am looking at the fire cnc bt30 spindle and 6krpm capabilities.

My parts I currently produce are approx 20x12x2.0 aluminum plate. Speed isn't as much as a concern as accuracy. I haven't even started looking at electronics yet. All I know is They will be big.

Any thoughts?

Sounds like a big involved build, should be interesting to see what you come up with.

[yellow68gto]

Wizard thank you for the reply but I don't think I explained very well. My gantry X is going to have my 50 inch travel. Table Y is only going to have 30 inch travel.

Rotate the table 90 degrees in your thinking.my machine will be fairly square.

This is why I am asking about racking. I will be able to mill 25 inches from my Y ball screw center and 13 inches past each rail.

I am going to look into mounting the rails farther apart.I was just hoping to utilize the pick mounting already in my vf3 table.

Hope this helps

Ken

[wizard]

Wizard thank you for the reply but I don't think I explained very well. My gantry X is going to have my 50 inch travel. Table Y is only going to have 30 inch travel.

Must have gotten threads mixed up. I'm not certain I would go that route to be honest, ultimately it depends upon there results you expect. With such a wide aspect your will need dual lead screws.

Rotate the table 90 degrees in your thinking.my machine will be fairly square.

This is why I am asking about racking. I will be able to mill 25 inches from my Y ball screw center and 13 inches past each rail.

The problem with racking is that two things impact it. One is how far apart the rails are spread and the other is how far apart the bearing assemblies are on the rails I suspect you will have to work through the engineering calculations to determine how bad it will be with your layout.

I am going to look into mounting the rails farther apart.I was just hoping to utilize the pick mounting already in my vf3 table.

It might not be the rails that need to be farther apart but rather the bearings on the rails. It is really hard to tel until you walk through some simple deflection calculations.

Hope this helps

Ken

It would be far easier to turn things around and make the X the long axis. The narrow table Would make everything a bit easier to to set up mechanically.

[yellow68gto]

I understand but if I did the machine would take up the room of a car in my garage. I can probably mount the rails 44 inches apart and then the bearing would be mounted at the edge of the table which is 18 inches. I don't know any of the calculation or even where to start.

[hanermo]

Put the lead screw in the *center*.
This prevents racking, and the force is always alopng the axis centerline.

Use a couple of I beams(h beams) in steel to connect, above or below, so the screw is out of the way.
I would put it below the table (no crud goes into it.
About 200 mm tall I beam, crosswise, is about right.

[victorofga]

two screw driven with one motor that's not a bad idea..
properly sized belt could never fail..

also see very little chance for serious error between two screws..

wish I could get picture we built router around 96...

there were used 3 meter long screws

the 1250x2500 mm rectangle hasn't error 0.5 mm

you can measure only with tape measure.. but you would see a 0.5 mm difference

the timing pulleys were cut of steel with wireedm.....
holes were tapered and a tapered bushing was also cut..
bushing was threated and it worked without any keys..

setscrews never could hold so tight like these tapered bushings..

my friends has wireedm so it didn't cost nothing..