Steel Frame 8' x 5' CNC - Advice Sought

**routercnc** · 28-05-2015

Hi Brenton,

As the table looks like its coming together I'll make a few comments on the gantry in post #1. If you simple space the Y axis rails further apart so that they mount nearer the corner you'll get a big stiffness increase. At the moment they are part way along the flexible face of the RHS. Also helps as bit with racking of the Y axis due to the wider spacing of the bearings.

When I've sketched out that type of layout I've aligned the upper & lower faces of the carraige with the top & bottom surfaces of the RHS, which puts the rails slightly inboard so that the fixing bolts don't hit the top or bottom surface.

You also talked about inserting an 'I' beam and chopping back to get clearance for a ballscrew between the rails. I can see where you are coming from but its alot of work. I remember some commercial machines welding a solid spacer under where the rails will sit (full length of gantry, wider than the rails, and say 6 - 8mm deep to suit) to give clearance to the ballscrew. Probably would need the seating face grinding true to recover any distortion from the welding prior to seating the rails. Although it's an offset from the gantry, some of the small loss of stiffness would be recovered by the spacers. Just another option that is relatively simple to implement.

The diagonal plate inside the RHS would be difficult to attach wouldn't it? Unless you are cutting periodic holes in the RHS to tack weld through.

**BrentonSpear** · 31-05-2015

Originally Posted by routercnc

Hi Brenton,

As the table looks like its coming together I'll make a few comments on the gantry in post #1. If you simple space the Y axis rails further apart so that they mount nearer the corner you'll get a big stiffness increase. At the moment they are part way along the flexible face of the RHS. Also helps as bit with racking of the Y axis due to the wider spacing of the bearings.

When I've sketched out that type of layout I've aligned the upper & lower faces of the carraige with the top & bottom surfaces of the RHS, which puts the rails slightly inboard so that the fixing bolts don't hit the top or bottom surface.

That is a good suggestion. I will incorporate that into my design.

Originally Posted by routercnc

You also talked about inserting an 'I' beam and chopping back to get clearance for a ballscrew between the rails. I can see where you are coming from but its alot of work. I remember some commercial machines welding a solid spacer under where the rails will sit (full length of gantry, wider than the rails, and say 6 - 8mm deep to suit) to give clearance to the ballscrew. Probably would need the seating face grinding true to recover any distortion from the welding prior to seating the rails. Although it's an offset from the gantry, some of the small loss of stiffness would be recovered by the spacers. Just another option that is relatively simple to implement.

Yes I have seen this done previously. I am going to adopt this approach.

I currently have 32mm clearance between the gantry face and the linear block face where the Z axis will attach.

I am assuming that a 1610 ballscrew and ends will need 60mm clearance between these two so I would have to make the spacer 15mm (after surface facing) and add another 15mm aluminium spacer onto each of the 4 linear blocks that attach to the Z axis to give me the clearance.

Will this thickness of spacer on the gantry and then also on the Z axis backing plate cause me issues?

Originally Posted by routercnc

The diagonal plate inside the RHS would be difficult to attach wouldn't it? Unless you are cutting periodic holes in the RHS to tack weld through.

I was thinking of welding this plate at two points either end.

**BrentonSpear** · 31-05-2015

Here is the revised gantry with the spacer.

I redid the measurements and a 15mm spacer from the Gantry face to the rails is enough to clear the ball screw.

**routercnc** · 31-05-2015

Yes, that was the basic idea on the spacer approach. I was looking into this a while ago (been thinking about a gantry upgrade) and you can reduce the spacer offset further with:
Custom ballscrew AC bearing housing +
Custom floating bearing housing +
Removing part of the flange on the ballscrew nut

You'll have to decide if it is worth the effort for a few more millimeters.

The diagonal plate down the centre is not worth it if you only weld at the ends. Better would be a shear panel set into each end to blank off the section. You could set the stepper motor with slotted adjustment holes into one of the plates (i.e. motor inside the section) to keep it tidy.

**BrentonSpear** · 01-06-2015

Originally Posted by routercnc

Yes, that was the basic idea on the spacer approach. I was looking into this a while ago (been thinking about a gantry upgrade) and you can reduce the spacer offset further with:
Custom ballscrew AC bearing housing +
Custom floating bearing housing +
Removing part of the flange on the ballscrew nut

You'll have to decide if it is worth the effort for a few more millimeters.

The diagonal plate down the centre is not worth it if you only weld at the ends. Better would be a shear panel set into each end to blank off the section. You could set the stepper motor with slotted adjustment holes into one of the plates (i.e. motor inside the section) to keep it tidy.

I am planning to add the end plates to stiffen things up on the gantry.