[ty1295]

Working on my plasma table again. Frame work done, X axis done via ballscrew.

Not sure how I want to do the other axis.

I have a rack/pinion mounted on both sides, with the teeth facing down (idea is so dirt doesn't fall into teeth).

On the gantry I have a driveline that runs to both sides, servo mounted on one side driving that shaft. Output of that shaft obvioulsy needs to transmit power to rack (well pinion). Driveline Shaft is about 5-6" above the rack.

At first I had a swing arm setup with a spring pulling up on it to force pinion on to the rack. It was rather sloopy and I just didn't like it.

Need some ideas, either spring loaded or not. Pictures are even better. I may try to get some pics myself tonight.

Ready to move forward on this but after a few days not sure how.

Found one picture close to how mine is setup, but since I have the driveline to prevent racking of gantry I can't mount motor directly onto rack as it is here.

[ViperTX]

don't use a spring use a rigid setup.

[freak_brain]

but doesn't a rigid setup get tight and loose as it goes across the rack. Mine sure seems to.

[eman5oh]

Have you looked into anti-backlash pinion gears. The gear is spit and and has a preload to push the two pieces in opposite directions. Here is a link. http://www.wmberg.com/catalog/pdf/b00b117a.pdf and a picture.

[ViperTX]

so, is the looseness due to variation in the rack....its mounting...on in variation in the distance the pinion is from the rack....which is most likely due to the flatness/straightness of table guide assembly.

[Apples]

Hey ty1295,

That's my plasma machine. The spring setup works brilliant. I have to make sure that the acceleration is not set to fast in Mach3, otherwise it will jump gears and the rack. But other than that is is great.

Have you seen the video?

Peter

[jeffery71]

I also have a spring setup.. It works great for me and I dont' notice any backlash. One thing to keep in mind is its best to keep the pivot point of the motor mount/spring assembly in the same plane as where the rack and the pinoin mesh together. (there has to be a better way of explaining that..lol) By doing this it is much less likely for the pinion to jump a tooth on the rack.


Jeff T.

[Geof]

.....The spring setup works brilliant. I have to make sure that the acceleration is not set to fast in Mach3, otherwise it will jump gears and the rack.....

Rig an adjustable stop behind the arm that holds the pinion and adjust this so the pnion can move the distance needed to accommodate variation in the rack position but cannot move far enough to jump a tooth.

[Apples]

Geof,

That sounds like a good idea. Might have to do that. I am having problems at the moment with the gantry wobbling at one end(opposite to motor end).

Looks like I am going to have to buy another Gecko and motor. Hopefully this should fix it up really good.


That end probably moves about 4-5mm. ie. I do a fast jog at high speed then stop and the end of the gantry wobbles.

Peter

[sol]

Here is how I did it, http://solsylva.com/cnc/4cnc54.html (scroll down) works well; once adjusted it has never jumped a tooth. Went with this system because I could not come up with an inexpensive swivel that would be backlash free. The adjustable gibs dealt with that problem easily enough.
The arrangement of the set-up also tended to the problem Jeff T mentioned in post #7.

[Geof]

Peter;

I looked at the picture someone posted that you say is yours. Your gantry wobble may be coming from flexing in the frame you have tieing the two sides of the gantry together. From the picture it is possible to see you did a very sturdy job of mounting your guide rails but it appears you used fairly light angle to tie across from one side to the other; lighter than your rail supports anyway. If it is possible see if you can weld on extra angle to turn them into square tubes; the stiffness will increase considerably.

[energyforce]

Geof,

That sounds like a good idea. Might have to do that. I am having problems at the moment with the gantry wobbling at one end(opposite to motor end).

Looks like I am going to have to buy another Gecko and motor. Hopefully this should fix it up really good.


That end probably moves about 4-5mm. ie. I do a fast jog at high speed then stop and the end of the gantry wobbles.

Peter

My cnc is going to have hardened shafts at each side for the x axis with 2 pillow block bearings placed 12 inches apart on each shaft. The shafts are predrilled and mounted on shaft rails. There is only one motor on one side powering the x axis by use of a rack and pinion. The table is very very sturdy and heavy duty and stiff in all directions. I cant see any wobbling happening here from this setup. What do you think? (asking for your input).

[freak_brain]

energyforce,

what kind of width are we talking about? How wide is the gantry you will be driving from one side?

-Allen

[Geof]

My cnc is going to have hardened shafts at each side for the x axis with 2 pillow block bearings placed 12 inches apart on each shaft. The shafts are predrilled and mounted on shaft rails. There is only one motor on one side powering the x axis by use of a rack and pinion. The table is very very sturdy and heavy duty and stiff in all directions. I cant see any wobbling happening here from this setup. What do you think? (asking for your input).

Peter;
It is a bit difficult to say anything definite just from a description, can you post pictures? I am having difficulty visualizing how your pillow block bearings are attached to or running on your shafts.

[energyforce]

Shafts are approximately 6 feet apart running parallel to eachother. Pillow blocks are approximately 10 inches apart center to center. I attached a 3D model. The y axis gantry is mounted to the pillow blocks and is very sturdy (wide and thick) and is made of aluminum (for weight reduction) to reduce inertial forces from acceleration. As for the weight of the gantry, I can only guess. To be conservative id say about 200 lbs. We would probably try for 150lbs as an end result.

[Geof]

Okay so your bearings form a rectangle 6 feet across the width of your table and 10" along the shaft on each side and the bearings are on "legs" either side of the gantry that reach down the distance of your gantry clearance above the table plus the distance your shafts are below the table.

You might get noticeable twisting. Consider if you locked one side firm and applied a force on the other side trying to slide it along the shaft. Any looseness between the bearings and shafts or sideways bending of the "legs" is multiplied by 7.2 (72"/10") in the longitudinal direction. Similarly your longitudinal force is multiplied for the twisting force. If you apply a force of 50 lbs then each bearing on the shafts is carrying a sideways force of around 90 lbs.

You will have to concentrate on making the gantry "legs" stiff sideways. For the best combination of stiffness and weight reduction these should be made as a box structure. Of course if you can tweak a bit more distance between the bearings that will help.

[Apples]

Geof,

About my wobble in my gantry. Yes I have look at the wobble and noticed that the angle iron is too small. I have welded so far so small strips about 2-3inches long and about 1/2inch wide across the top of the angle iron. I have also ordered another motor for the other side.

I will weld another bit of angle to the trucks with the small angle to make it stronger. I could actualy see the light gauge angle bending when I pushed from the other end of the gantry.

So hopefull after this with another motor and welding more angle on everything should be good.

If this is still wobbling after this I am going to rip everything out and put in the Bishop Wisecarver bearings.

I am hoping to cut all day with this.

Peter

[BIGCNC]

On machines i build i use air cylinders for primary engagement and a small spring to provide tenision when the air is off, so position is not lost. I use a small regulator to set pressure on the rack, max pressure on the rack is 38lbs .

[Apples]

Geof,

I have done some shopping!

I am now using the Bishop Wisecarver bearings (number 2) and also the matching track. I will have the track running on the top and the bottom of a piece of box steel. So I will have 2 bearings on top and 2 bearings underneath. The bearings underneath will serve to hold the gantry down when there is a fast change in acceleration and deceleration.

Althought the Hiwin type rails look flash, and even though I could get them for about an extra 2-300 dollars all up for the whole table. In my opinion from what I have heard people say I agree that the Bishop Wisecarver vee bearings are the superior choice for a plasma table. Mainly due to the fact they will be more tolerant to the plasma dust.

With the gantry I am now making it out of 3"x3"x1/8" box, 75mmx75mmx3mm.

Hopefully with the strong box as the gantry and the vee bearings and a dual drive setup It should all be good, and ther should be no racking of wobble.

Peter
Australia

[Geof]

Talk about B.S.H. engineering; not familiar with the acronym? Built like a "Brick S**t House".

That should be stiffer. One thing I suggest is don't weld things up after aligning as you describe in one of your pictures. If you introduce distortion during the welding you are sunk.