Z AXIS - Moving bearings or moving rails ?

[TrickyCNC]

I'm going to build my Z axis later.
I've been looking at various machine builds and getting some ideas.

There seems to be two ways to approach it.

1. Fix the bearings to the gantry carriage, and have the rails move up and down with the router.

2. fix the rails to the gantry carriage, and have the bearings ride the rails , up and down with the router.

as far as I can see. with the bearings fixed, and the rails moving, it makes for a more compact machine, only being tall when the router is fully up.


Are there any pros and cons to either method ?
Rigidity ?

Unless I hear otherwise, I'm moving towards the fixed bearings design.

TIA for any suggestions

Rich

[microcarve]

It depends on what kind of machine you're building and what
it's intended purpose is.

Mine is right for the design of machines I build. Reason is,
the further the bit is extended, the more the bit can act as
a lever. Leverage is what causes flexing or bit breaking in
many machines.

And that's why I always suggest use of layers...like MDF...to bring
the work closer to the bottom of the Z axis.

It can work like this. If you write your name with a pencil and hold
the pencil normally, it's easy. But the further you hold the pencil
from the writing end, the harder it gets to write clearly.

It can also depend on lots of other things. Like your choice of rails.
The uprights. How the X rails are oriented. The backing for your
Y rails.

If you want the bearings to carry the weight of the router, it's mount,
whatever fixtures it has, and reach down a long way, then
precision linear rails like Hiwins will handle it all better.

With something like round shafts...that can be the cause of binding
with the extra weight. Again, depending on how those bearings are
oriented....and/or what kind they are.

The further a Z needs to reach down, the more rigid and stable the
design needs to be.

I've built them both ways many times. For the small machines I make
now, the way I do it is the best way for that design....and what it's
supposed to do.


John

[ger21]

My Z axis has the bearings moving. My carriages are only 200mm tall, but the rails are 380mm long. So if I had the rails moving, I'd need to double the height of my carriages.
Also, if the rails move, when the Z axis is lowered all the way, a portion of your rails equal to the length of travel will be hanging down unsupported. When my Z axis is fully extended, it's just as supported as when it's all the way up.

Jmo, of course.

[TrickyCNC]

My Z axis has the bearings moving. My carriages are only 8" tall, but the rails are 15" long. So if I had the rails moving, I'd need to double the height of my carriages.
Also, if the rails move, when the Z axis is lowered all the way, a portion of your rails equal to the length of travel will be hanging down unsupported. When my Z axis is fully extended, it's just as supported as when it's all the way up.

Jmo, of course.

let me just translate that to English, so I can picture it better in my head :P

(yes, I used a calculator for the 15" bit )

[ger21]

I specifically swapped it to metric, thinking it was better for you. Yes, it's definitely 8" and 15". (The rails are 380mm, though.)

[TrickyCNC]

John

You don't give any explanation as to which way would be more rigid or why.
Or maybe I missed it ?

I know about leverage and long Z travel. I still need to decide how much travel I need/want

I have 6 1/2" clear under the gantry, with the thinking that I could carve 6 x 6 posts or corbels if I felt the need.

Yes, I am thinking of making different sized raised beds, to bring the sheet stock up to the gantry.

I'm not sure what I will use the machine for, as I'm a newbie, and the possibilities just seem endless !

I have approx 25" x 25" X and Y, with the 6 1/2" clearance under the gantry

Thanks

Gerry

Looking at your router mount, there is a lot of distance (leverage) from the bit, to the bearings. Does this show up in use ?

also, as I see it, the bearing support would be the same (amount ) which ever way round you fixed them ?

If your router mount was tall - 15", with rails,
and the bearings were 8" tall on the carriage,
that should work ? no ?

that is what I am thinking of doing.

I think Louie's is set up this way... but I've seen that many Z's in the last couple of days I might be getting confused !

Thanks

Rich

[TrickyCNC]

I specifically swapped it to metric, thinking it was better for you. Yes, it's definitely 8" and 15". (The rails are 300mm, though.)

ha ha ... I thought that's what you had done.

I don't get why the Europeans are trying to force metric on us... I still use Imperial / English.

I mean, in the UK we all still measure our height in feet and inches
our weight in Stones and pounds

We drive in MPH although we now have to buy liters of petrol
We work out fuel usage in MPG

We go to the pub for a Pint !
We eat 1/4 pound burgers !

etc. etc. ... rant rant ... LOL I could go on forever

[ger21]

Looking at your router mount, there is a lot of distance (leverage) from the bit, to the bearings. Does this show up in use ?

I haven't finished building it yet.
Lowering the bearings any further would result in a reduction of Z clearance.
If tried to design it so the carriage / router mount will have next to no flex, with the bearings mounted to the carriage.

If you take the image I posted above and swap the rails and bearings, you'll need to slide the bearings up 7" in order for the Z to go up. Which would result in an additional 7" unsupported.

Designing a Z axis can be very complex, especially as travel gets longer. The longer the travel, the harder it is to make a rigid Z.

[TrickyCNC]

Designing a Z axis can be very complex, especially as travel gets longer. The longer the travel, the harder it is to make a rigid Z.

hmmm.... I though it should be easy ... every machine has one !

I'm sooo impatient now !

I'll rig something up with a couple of inches travel, just to get me working, then I'll think on it some more.
I'm thinking what I have laying about in the garage / workshop that will do as rails, I may end up with copper water pipe for my 1st Z rails, if I cant find any steel.

I just want to cut something now

I'll make it so I can easily swap Z's to the carriage, then I can experiment a bit.

Thanks both

Rich

[louieatienza]

I would say,the longer the z travel you need, the better to put the rails on the carriage. The shorter the x travel, either way would work but might be better to put the bearing on the carriage for ease of fabrication. Reason being is that with moving bearings, you're limited by the length of travel by how far your router hangs below the bearing. This would normally not be an issue unless you had needs that require a longer z travel (tall fixtures like mill clamps, tall workpieces, rotary axis, 5-axis head...)

Either way has advantages and drawbacks. I prefer to have moving rails, since the lower bearing will be closer to the work at all times, and that is the bearing that will take the grunt of forces. But then the gantry legs have to be taller. If the bearings move, then the spindle is always "hanging down" from the lower bearing, and it has to hang down the max z travel. But the gantry can be made lower. I wouldn't rely on rails to help support the carriage on a moving rail system

Also a moving bearing system is less complicated to build. Mounting bearings on the carriage saddle may require a separate plate to do so, unless you had flanged bearings...

[microcarve]

John

You don't give any explanation as to which way would be more rigid or why.
Or maybe I missed it ?

I know about leverage and long Z travel. I still need to decide how much travel I need/want

I have 6 1/2" clear under the gantry, with the thinking that I could carve 6 x 6 posts or corbels if I felt the need.

Yes, I am thinking of making different sized raised beds, to bring the sheet stock up to the gantry.

I'm not sure what I will use the machine for, as I'm a newbie, and the possibilities just seem endless !

I have approx 25" x 25" X and Y, with the 6 1/2" clearance under the gantry

Thanks

Rich

It's probably a safe bet to say that as a newbie DIYer, it'll be better
to plan for the rails to be fixed and the bearings moving.

Your X rails are fixed...Your Y rails are fixed. The Z is basically a smaller
X or Y. Imagine the complications of trying to use your bearings as fixed
and the rails moving on X and Y.

That's a good sized machine you're building and it'll highly depend on
how it's made....what it's for....materials and construction...etc...etc...

The Good...and Bad...thing about DIYing a machine is...there are no rules.
Good because you can break whatever rules someone tells you exist.
Bad because it depends on how good you are at breaking the rules with
mechanical stuff.

Some drawer slide machines can machine metals. Some welded steel
machines can barely carve wood. IMO...it can be a good idea to make
anything at all that works to get the feel of the mechanics involved.
If you see it working...or not...that's valuable experience. Verrrrrrry
few people are going to design a winner first time out. The lowly MDF
machine has it's place...


John

[PaulRowntree]

I'll rig something up with a couple of inches travel, just to get me working, then I'll think on it some more.
I'm thinking what I have laying about in the garage / workshop that will do as rails, I may end up with copper water pipe for my 1st Z rails, if I cant find any steel.

I just want to cut something now
Rich

In my line of work we have an expression : Two weeks in the lab can save you 5 minutes work in the library.

The Z is critical, and you should (IMHO) spend the time to think about it rather than just do something to get something quickly made. Copper pipe is just a bad idea as it will flex and worse, permanently deform quite easily. I would suggest that you make a design that makes the most of your carriage clearance, up to a max of 6" in Z. Try the stationary rails approach, and then build to the best of your ability. If you want to change it, your machine will then be good enough to help you out. If you build quick and dirty, it may well be inadequate to make parts for Mk II.