Dual Drive on Long Axis?

[rweatherly]

Has anyone done a dual leadscrew on the long axis with a timing belt connecting them to the stepper motor?

[ViperTX]

Why would you want to tackle that problem??

[rweatherly]

The gantry wants to pivot around the current leadscrew. I have not been able to take it out with bearing/pipe adjustment. I figure two screws near each edge of the will correct that.

[triticale]

You might be trading one problem for another. Explore the causes of the problem first. Is the lead screw possibly a shosh off center? Could you increase resistance by increasing the bearing wheelbase?

If you mean that it is rotating around the axis of the screw, you need smoother-running nuts. Back off on the anti-backlash, and if that helps change the design. If you stay with draggy nuts, and ask the stepper to torque twice as many, that's another new source of grief.

[derekj308]

Hi rweatherly

Plenty of people have done exactly that for increased rigidity of an axis. When you are using el-cheapo home brew linear guides there isn't a great deal of resistance to 'twisting' of the table/gantry.

As a test of how your guides perform twist the table/gantry with your hands and see how much it moves. I'm tipping its a couple of millimeters and if so there are two ways to fix the problem. One is to buy proper linear guides to dramatically reduce the potential for twist (in homebrew terms it is negligible) or as you have discovered use two screws spread as far apart as is practical on your machine. As far as driving the two screws you need to compare costs of new guides vs. two screws and the stuff required to drive them. There is always the option of slaving two motors instead of making a synchonous belt system.

Just for interest sake take a look at the Mori Seiki japanese website (its ok there is an english version lol) and look at DCG theory of machine building.

http://www.moriseiki.co.jp/english/index.html

then click on the blue bit at the right of the screen which says
"Driven at the centre of gravity". There is a cool presentation on DCG.

Its way over the top for homebrew CNC but still interesting reading.

Cheers
derekj308

[jmytyk]

Dual screw design, I am working on building my machine right now- it will have dual 1/2-10 acme, belt driven. I have some mock-up pics of the drive layout in my build log. I do forsee some issue with the screw alignment, I have designed in enough adjustment so that i should be able to get it set up well.

Remember that the motor will be seeing about twice as much drag in the system as opposed to a single screw- spec the motor- i am using a 640oz/in @ 50v.

[joecnc2006]

you can use a system like this to keep the gantry from twisting.

[derekj308]

Cheap, practical, functional. I may just use that idea on my homebrew machine. Thanks joe2000che.

[mike hide]

Originally Posted by jmytyk Dual screw design, I am working on building my machine right now- it will have dual 1/2-10 acme, belt driven. I have some mock-up pics of the drive layout in my build log. I do forsee some issue with the screw alignment, I have designed in enough adjustment so that i should be able to get it set up well. Remember that the motor will be seeing about twice as much drag in the system as opposed to a single screw- spec the motor- i am using a 640oz/in @ 50v.

with that type of arrangement will you end up with a backlash problem or at least some "slop " in the system.

I am contemplating building this type of system .With the nut drive situated under the bed a pretty hefty moment results at the gantry, which is then transferred to the guide rails, essentially the cutting tool load times the distance from the cutter to the Xaxis drive nut.The smaller the moment arm between the nut and the cutter [coplaner]the smaller the moment. I am thinking of designing the machine with two Xaxis drives one either side of the bed and sitting just above it. this should minimize the moment eliminating the torque on the black pipe guide rails.

My main concern is how to drive the two acme screws from a single motor, belts and pullies or a second motor slaved to the first .!!!

[jmytyk]

wouldn't all moment calc's be based from the rails to the cutter head? the machine could operate in a mode without lead screws, to demonstrate what my opinion is... in addition the majority of machines us 2 rails for the x-axis, i think Joes2006 machine using 4 will lessen the importance of screw placement... all this is my observation- feel free to agree/disagree... _jon

[derekj308]

Hi Guys
Ideally the place which to apply the force to move the gantry/table is in the same plane as the linear guides. Plenty of people don't do it that way and have the screw in a less than ideal position and still achieve good speed and accuracy. In the case of Joes 2006 machine with the 4 pipes the ideal place to drive would be at a plane centrally located between the two levels of pipe. As Joe has pointed out in his diagram above you don't need to have the screw located centrally in the mechanism. By using cables to apply the force in the desired plane you can put the screw anywhere you like along the driven component of the gantry/table and it wont 'rack' since the forces on each side are the same because of the cables. The position it is in in the diagram above is ideal since it is as close as is practical to the cables line of action of force.

I would like to point out another consideration that can be made during a machine design is the position of the centre of gravity of the gantry/table being driven. During acceleration and deceleration a moment is generated about the linear guide due to the position of the centre of gravity (COG) relative to the linear guide. You can minimise this force by keeping the COG of the driven member as close as is practical to the linear guide. You could even add weight (he didn't say add did he, he's MAD!!!) to shift the centre of gravity to be in the same plane as the linear guides. If your system could cope with the additional torque and weight (deflection) it would allow much higher acceleration and deceleration of that axis. Of course the COG changes as other axis shift position but you can still design out the majority of the moment applied to the linear guides during accel and decel due to a COG not in the same plane with the linear guides.

Cheers
derekj308

[mike hide]

hi Derek

I have been piddling with some gantry loads as I see it . Again I have not fooled around with this stuff for a very long time . so feel free to correct me .

I have tried to minimize the gantry moment loads on the black pipe/skate bearing trucks and have included a hand sketch of how I think the loads are distributed .

I do not think it is a good idea notching the side rails so that the bed crossbeams carry through it virtually halves the beam depth and as all loads end up in the side rails it will severly reduce their bending strength [varies with the depth cubed if I recall. The loads from the gantry result in outward bending load on the gantry side arms which could be strengthened if necessary with and outer web .

The vertical loads on the truck should result in a differential loads on the front and rear bearings respectively ,the sidewall will experience a reactionary load to the gantry side load as an inward force on the sidewall. The black pipe will try and deflect downward at one end of the truck and upward the other trying to force into a shallow "s" form locally. This possibility could be reduced effectively in my opinion by nesting the pipe between top and bottom angle irons bolted into the sde rails along the length of the pipe and if required each tack welded at intervals along its length [belt and braces if you get my drift.

So in short I think if the moment is minimized by making the drive axis coplaner with the cutter axis, or as near as can be [obviously the cutter plane varies with the X axis position .The deflections in the bed and tracks I feel could be improved by the suggested mods .

I mean this in a constructive way ,and heavens knows I have been wrong many times ,this thread and Joes efforts have been a great inspiration to me and I do not in anyway want to his efforts .....mjh