Question about racking on my x axis.

[planekrazie]

Ok, It was told to me sometime back about running 1 leadscrew down the center of my 48x72" table. I should have listened. Here is the problem;
when I push the left side of my bearing on the x axis to the rear of the table the right side moves about 1/8 to 1/4 of an inch to the front of the table. and vice versa. I posted pictures without my cutting boad on the machine so you can see and give me a few ideas on how to eliminate that problem. Here are some ideas that I came up with that might work. What do you guys think.
1. Using the single stepper that I already have, Take out the screw in the center and put on the left side of the cnc and buy another to put on the right side. Running a shaft with dual pullys and belts to both sides. Would this work? That would keep me for buying another stepper.
2. buying another stepper and screw and running 2 steppers for my X axis.
When I think about that one to me it looks like if one stepper skips a step then I am back in the same boat. The x moving with the left alittle ahead of the right.
3. Buying 2 more spb12 bearing blocks and spacing them out on both sides of the axis.

Please someone chime in on this problem. I would like to keep the cost of this one down.
The x axis lead nut is mounted temp until I figure out this problem. Also here is a pic of my 1st cut as it sits.
Here are the pics.
Thanks Steve

[ger21]

1)Something like this?
http://www.cnczone.com/forums/attach...6&d=1060103075
http://www.rmtucker.f2s.com/weardale...y/IMAG013A.JPG
http://www.rmtucker.f2s.com/weardale...o_Gallery.html

Should work fine, although it will be less efficient due to spinning 2 screws.

2)If your steppers are missing steps with 2 motors, why wouldn't it miss steps with a single motor? I see people say this all the time when talking about using 2 motors. You always here that if one motor loses steps it will ruin your parts. Guess what. If you have a single motor and it misses steps it will ruin your parts too. (It just won't twist your gantry apart) The bottom line with steppers, is that if you're missing steps, you have a problem with your machine that needs to be fixed.

3) You should have done this at the start. It would make a much stiffer gantry, but I don't think that this alone will fix your problem.

So either #1 or #2 would be the way to go.

[planekrazie]

Ill buy another stepper and lead screw. Also 2 more spb12. Hopefully ill get this thing done in the near future.
On a last note you said " You should have done this at the start. It would make a much stiffer gantry, but I don't think that this alone will fix your problem.
What other problem can I fix? Thanks Steve

Originally Posted by ger21 1)Something like this? http://www.cnczone.com/forums/attach...6&d=1060103075 http://www.rmtucker.f2s.com/weardale...y/IMAG013A.JPG http://www.rmtucker.f2s.com/weardale...o_Gallery.html Should work fine, although it will be less efficient due to spinning 2 screws. 2)If your steppers are missing steps with 2 motors, why wouldn't it miss steps with a single motor? I see people say this all the time when talking about using 2 motors. You always here that if one motor loses steps it will ruin your parts. Guess what. If you have a single motor and it misses steps it will ruin your parts too. (It just won't twist your gantry apart) The bottom line with steppers, is that if you're missing steps, you have a problem with your machine that needs to be fixed. 3) You should have done this at the start. It would make a much stiffer gantry, but I don't think that this alone will fix your problem. So either #1 or #2 would be the way to go.

[ger21]

If you'll only be using the laminate trimmer, you may not need to add the second bearing blocks.

[stevelang]

I don't know if I'm not understanding this but I think the problem is that the use of only one twin linear bearing per side is not enough, depending on which brand or type they are they area allowing a few degrees of movement, I would add two more linear bearings behind the existing ones, a few inches apart with some temp brackets and see what happens

[andy_ck87028]

In my simple way of analysing it, I think Gerry suggests viewing the problem from the perspective of levers.

If excessive force or excessive leverage caused by long length or short length turning forces is applied then presume you will hvae a problem.

Design around structural stability through solid triangles and the problem will not arise in the first place.

Hope this helps

Andy

[wcarrothers1]

Well I vote for #3..

BUT... I see your problem in flex of the 2 long sides of the gantry. by that I mean you only have 2 box section connecting the 2 sides with the metal rods. I'd say your flex is probably worst if the gantry is at the mid point between the front and the middle support.

I'm guessing that when your gantry is torqued the 2 rods are able to flex outward. With extra metal welded between that would not be able to happen.

Course adding another leaner bearing would spread out the torque on a longer section of bar as well.

I'm not a big fan of dual screws but that could solve it to although would probably cost more then adding strength to your table..

b.

[wcarrothers1]

You can see I have 5 pieces of steel going across my table perpendicular to the gantry X http://www.cnczone.com/forums/attach...7&d=1161702362

and in addition welded in a large X of steel to the underside of those to prevent one side of the gantry from moving with out the other. Course since I drive my gantry on both sides from a rack gear drive I'm pushing my gantry = on both sides rather then pulling from the middle. So I really don't get the torque spread issue from pulling from the middle while cutting on the out side.

But I think most of your problem is the table spreading appart while being torqued.. Either that or flex in your gantry alowing the toruqe turn. But your gantry looks well built although perhaps beef up the connections to the ends (ie triangle brackets or something extra)

Probably could set a section of steel in there (not weld it yet) and clamp it in and see if your movement does go down then weld away..

With your screw in the way it might be more chalenging to weldin an X but in any case more support for your MFD also could not hurt if you ask me.

b.

[Oldmanandhistoy]

Just a point on the dual screw dual stepper approach I go along with Ger21’s opinions. I am in the testing prototyping stages of a larger machine and am going the dual way.
I wont go into why here but will defiantly be using this approach on my X axis. Also just for interest if any; I have been driving two drivers from the same pins of my parallel port with no problems to date.

John

[bigz1]

I am about to install 2 motors on my X axis. I am using the X and A axis on my HobbyCNC board and slaving them through MACH3. Under no load I ran them several 1000" and the flags where still in the same place relative to each other. Perhaps some could enlighten me why they don't like this approach? I would have thought missed steps are bad whether their in a single setup or a dual one.

Liam

http://www.cnczone.com/forums/showthread.php?t=19603

[planekrazie]

Yea I can see the problem. I am gonna weld a few more cross members on the underside of my table shown here before I mounted the drive nut. I have 2 more bearings on the way. Both are sbp12 twn thompson.
Here is a pic before and Ill post a pic after. I just wanted to keep some of the weight down on the gantry. As far as gussets on the gantry. Ill make some more this weekend.
Thanks for all the help guys.
Steve

[wcarrothers1]

So since he is going to reinforce the table like I said do I win?

Really I think that has to be the easiest answer assuming your gantry is not flexing much.. To me the design looks great..

And painted the same color as mine

b.