Re: Lead screw/Guide rail install/pre-load (machine design thoughts)

--- In CAD_CAM_EDM_DRO@egroups.com, ballendo@y... wrote:

> Ballendo (may I call you Balls? :),
>
> snips/thoughts/answers interspersed below:
>
> >I'm doing what is essentially a 3 axis router: 48" x 36" x 18" of
> >cutter travel. It'll have the z-y plane fixed in x, with a
> >travelling x table moving below the y-z plane. So it's a little
> >goofy in that the part moves only in x, while the router moves in

y-

> >z.
>
> This is a very common "bridge" design. Not goofy at all! It IS a

good

> choice since you want 18" of Z axis. With this design the Y and Z
> axis is stiffer than a gantry style.
>
> The disadvantage is that you need TWICE the space to operate(2

times

> the X axis travel length), and the material costs will be higher.
> Also the mass of the "moving table" load will change, depending on
> what work is loaded on the machine. This means your drive system

will

> need to be set up to handle the expected material load. And

supports

> for long work will need to accomodate this "movement".

I will actually be moving a 10ft pipe beneath the cutter, but will
get by only moving it about 48 inches. I'll use the machine in a big
building, so the size will be tollerable.

>
> >Because I want to maintain some decent accuracy, I'll be using a
> >linear strip encoder to close the feedback loop for the long axis.
> >This should allow me to simply drive the table to the correct
> >position and not be concerned with the accuracy of the lead screws.
>
> This is fine, but way overkill for your "stated" application.

Rolled

> thread leadscrews(ball and acme) typically have an .003/foot
> accuracy. And it is generally non-cumulative. Your MATERIAL(pvc) is
> going to "move" much more than your machine accuracy!
>
> Unless you already have 'em(the linear encoders), think about none

or

> rotary style,IMO. EMC now has leadscrew compensation (just like the
> big boys) so you would be able to "dial-in" YOUR screws. If you use
> EMC.

I acknowledge the linear encoder is not really required for the pipe
work, as +/- .032 is acceptable. I was flirting with the idea of
using very inexpensive drive components and essentially making up for
it with the encoder. In addition, I have no real feel for how much
of a problem loosing steps or position with a stepper is. I thought
the linear encoder could, in theory, negate the effects of any slop
in the design, and allow me to accurately drive the table. I was also
unaware that the lead error was non-cumulative. The acme leads I was
looking at were +/- .006, so I thought I'd potentially be out .036
from the screw alone. Then for small parts, I already have
nearly .003-.004 just from the lead screw. I would like the setup to
be as accurate as I can reasonably get, since I plan on trying to
swap out the router and mill small parts for future manchine work.

>
> You won't need the RIGID mounts we have been talking about for this
> machine. You didn't mention travel speed, but the industry std. 5/8-
> .200 ballscrews will be plenty rigid for these travels at nearly

any

> speed you might realistically want to attain. I'd suggest a FIXED-
> SIMPLE for all axes.(either angular-contact or my "cheap" radial
> bearing version)

Sounds good. I have virtually no speed requirements :)

>
> Be sure to use a GOOD Y and Z linear bearing system. Y, because

it's

> the mounting surface for the Z; and Z because you are dealing with

a

> LOT of leverage when you extend out 18 inches in Z!!!
>
> Ron, Let me ask you a question. Do you REALLY NEED 18 inches of Z
> travel???
>
> Very often, the principle I spoke of (earlier post) comes into play
> while we are deciding what size machine to make. "I'm not sure, so
> I'll make it big enough for anything!..." This sort of thinking

can

> lead to failure as the project "snowballs" into problems. Or
> paralysis by analysis.
>
> If you are using 10 inch PVC tubing, you NEED 10 inches of travel,

at

> least, right? So why not 18??
>
> On the other hand: Maybe you need at least 5 inches of Z travel
> (probably 6 or 8 inches), and a BRIDGE that adjusts up and down to
> accomodate the different material thicknesses!

That is a very good idea. I do understand the thought process you are
describing, and yes, it's familiar :} I expect to be able to reduce
the z requirements.

>
> The resulting machine will be MUCH stiffer, cost MUCH less, hold
> better accuracy, and still do everything you expect.
>
> Keep in mind that most of the tooling you will probably use is

about

> four inches long (or less, router bits for plastic max out at about
> three inches, unless a specialty product) and having 18 inches of
> travel will not really let you cut any deeper as the bit/machine
> spindle interference limits the cutting operation.
>
> Now I realize I don't have ALL the info. And I may be wrong. But...
>
> I've watched this process enough times to know that someone on this
> list will benefit from what I've written.
>

Exactly ! I agree, and thanks for taking the time to steer me in the
right direction. I appreciate your input.

Best regards,

Ron