1st Build, Second machine

[JAZZCNC]

Put the rails on the front plate and the bearings on the back plate. This way the rails help stiffen the front plate.
The carriages then go low on the backplate and it makes for a very stiff z axis with max support where it's needed.! close as possible to the cutting end.!

Edit: Also if you pocket the bearings into the back plate slightly you regain some of the lost stand off from gantry. Also you can use one of the pocket edges as a datum ref point for alignment of the bearings. If you also do the same with the rails into the rear of the front plate, (but not quit as deep) this as the same affect and also another datum ref edge.
The net affect of both brings the stand off from the Y axis down slightly and also creates accurate ref edges to work from which are essential for setting up profiled linear rails correctly.

Hope this makes sense.! If not just ask and i'll try to explain better.

[Jonathan]

Put the rails on the front plate and the bearings on the back plate. This way the rails help stiffen the front plate.
The carriages then go low on the backplate and it makes for a very stiff z axis with max support where it's needed.! close as possible to the cutting end.!

If the rails are contributing significantly to the stiffness of the material you are using for the Z-axis then that material should be made thicker/stronger. Still definitely put the rails on the front plate, but primarily for the second reason - the supporting carriages are closer to the bed.

[JAZZCNC]

If the rails are contributing significantly to the stiffness of the material you are using for the Z-axis then that material should be made thicker/stronger. Still definitely put the rails on the front plate, but primarily for the second reason - the supporting carriages are closer to the bed.

Not sure why you felt the need to post that.? . . . My point was the rails on front plate is much stronger way and no where did I suggest useing it prop up a flimsy front plate.!! . . . Thou if we are splitting hairs then a correct design should factor in all materials plate rails etc to give optimum strength without over engineering which wastes money and effiecency.

Bottom line rails on front is stronger.!!

[Jonathan]

no where did I suggest useing it prop up a flimsy front plate.!!

This, in my opinion, suggests that:

Put the rails on the front plate and the bearings on the back plate. This way the rails help stiffen the front plate.

Either way obviously I agree that rails on the 'front' is strongest. It's one of a number of things virtually everyone does wrong to start with, myself included. It also reduces the length of material required for the Z-axis. The only disadvantage I can think of is that you're also having to move the stepper motor and screw against gravity ... but that's almost certainly negligible.

Ideally you should also machine a slot in the plate for the rails so that one of them can be clamped in place. The same is true for the bearing blocks. If the machine crashes it helps stop them move out of alignment. I don't know if you have a milling machine though?

Not sure why you felt the need to post that.?

Perhaps there wasn't much point. I thought I was reinforcing/clarifying it.

[JAZZCNC]

This, in my opinion, suggests that:

Ok obviously miss understanding but either way flimsy or strong it's fact the rails would help support.!

First let me say dont be fooled by the lack of post's, I've been building diy cnc machines both for my self and others for meny years so I know very well all the newbie mistakes and pitfalls I just havent posted on here.

Regards the down side and your thinking you have to move the steppers with the front plate. It makes no differnce rails on rear or front both you can still have the motor static with the right design. Your right thou the difference is negliable.
I have 2 machines a large and small the large machine use's the moving screw,stepper arrangement with nema34 stepper and the smaller machine use's the static stepper,screw with nema23 stepper both work equally well. The only difference or advantage to the moving stepper arrangment is that it use's slightly less material because you dont need the high backplate but again the saving is negliable.

Eh eh now here's my chance to point out or clarify something you said.!!! . . . When using Profiled linear rails it's important you have a datum referance edge but not for helping stopping them move when crashed, the main importantance is for the alignment when installing. The point being that you fasten one rail tightly up against the referance edge using the correct edge of the bearing or rail depending in which orientation you have them ( they do have a right and wrong way).
Then you tighten the opposing or what I call floating rail or bearing losely then slowly move the gantry or table along the rail nipping the bearing or rail as you go along feeling for any tight spots and losening adjusting the floating rail or bearing to eliminate tight spots. . . . Simplezzs.!

[Jonathan]

Ok obviously miss understanding but either way flimsy or strong it's fact the rails would help support.!

Of course, but if it's helping significantly you're probably doing something wrong.

First let me say dont be fooled by the lack of post's, I've been building diy cnc machines both for my self and others for meny years so I know very well all the newbie mistakes and pitfalls I just havent posted on here.

I've read plenty of your posts on CNCzone. Well, I'm pretty sure it's you. It is obvious from the few posts that you have made here that you know what you're talking about! I have not posted on CNCzone since 5 years ago as I figured I would never have time to do any machining if I did it too much.

Regards the down side and your thinking you have to move the steppers with the front plate. It makes no differnce rails on rear or front both you can still have the motor static with the right design. Your right thou the difference is negliable.

Fair enough, me not thinking...

When using Profiled linear rails it's important you have a datum referance edge but not for helping stopping them move when crashed, the main importantance is for the alignment when installing.

Agreed, the more important factor is to attain good alignment of the rails. It also explains why the sides of the bearing blocks are ground. The reason I said about stopping movement after a collision is that I remembered reading it here, page 13 where it mentions that 'it is possible that the rails and the blocks will be displaced when the machine is subjected to vibrations and impacts':

http://www.hiwin.nl/DownloadCenter/f...9TE14-1006.pdf

Then you tighten the opposing or what I call floating rail or bearing losely then slowly move the gantry or table along the rail nipping the bearing or rail as you go along feeling for any tight spots and losening adjusting the floating rail or bearing to eliminate tight spots

That's what I did and subsequently checked it with a dial indicator.

I won't pretend to be any sort of expert on profile rails - I've only just researched them and bought mine. I still may get some for my X-axis (2000mm rails). I gather you know of a good source?

[JAZZCNC]

Agreed, the more important factor is to attain good alignment of the rails. It also explains why the sides of the bearing blocks are ground.

Also Jonathan If you look carefully the rails are ground or have ref edge not just the blocks. . . Very easy to miss.!

[JAZZCNC]

Why do my posts keep getting sent to the moderator for approval.???? . . . There's nothing wrong in any of them. . . Arghh My pet hate is sensorship.!

[Jonathan]

Why do my posts keep getting sent to the moderator for approval.???? . . . There's nothing wrong in any of them. . . Arghh My pet hate is sensorship.!

This censorship is applied to all new users now by the moderators. Most likely to reduce people signing up purely for advertising or that sort of thing. Must be annoying, unfortunately I can't help.