Z-axis design - help sizing materials please!

[routercnc]

Hi Jonathan,

The drawing helped, thanks. Wow, alot of overhang.

You could design a lower 'floating bearing'. This is where the lowest bearing block has two or more alternative mounting positions on the Z axis part which holds the router. For long reach work you bolt it where it is now, and accept the deflection. For shorter reach work you bolt it further down, spacing the bearings out so that the loads are taken towards the ends of the unsupported beam, rather than towards the centre. You need a moveable limit switch with this idea.

Another thought, although it doesn't really work with your requirements, is to support the rail at the centre, as well as the ends. The bearing blocks then run in the 'upper part' above this centre support, and the 'lower part' below this centre support. Your rail deflection is instantly halved, but there's a compromise with Z range.

It looks like you are using an 'I' beam for the router mounting plate. You're only considering the bending in one or possible two directions, which will give you good results on the deflection for these load cases. But since the router or spindle will be mounted off this, and the cutting forces will be offset, there will be a twisting force (Y direction cutting) and this section choice will give you poor results, particular with that overhang. Overall it might be worse than the original plate, all loads considered.

For such an overhang, you might want to choose RHS for the part which attaches to the router/spindle. This is good for all the load cases you are requiring. Ideally any bolting into this section should go all way through with spacers inside to get the loads into the section, rather than locally deflecting the wall. The calcs all assume good load transfer.

As Ross has already suggested, I suspect you don't need to mill down 400mm into things, but rather you want to be able to machine to a depth of perhaps 30mm into things which are up to 400mm deep. If this is the case you could limit your Z travel to say 100mm. You then block up the workpiece with spacers to suit. This would be fairly stiff for all cuts, but give you options in terms of workpiece sizes.

[Jonathan]

You could design a lower 'floating bearing'. This is where the lowest bearing block has two or more alternative mounting positions on the Z axis part which holds the router. For long reach work you bolt it where it is now, and accept the deflection. For shorter reach work you bolt it further down, spacing the bearings out so that the loads are taken towards the ends of the unsupported beam, rather than towards the centre. You need a moveable limit switch with this idea.

Yes! I was thinking that just hadn't got round to posting it...I'll allmost certainly do that. Maybe have 75mm or 400mm travel. Thinking about it I may as well have the lower bearing with several positions - it's only a few more holes after all.

With regards to supporting the rail at the centre I could do this when the Z range is limited, as above. It would make it take slightly longer to change between the two set-ups though.

It looks like you are using an 'I' beam for the router mounting plate. You're only considering the bending in one or possible two directions, which will give you good results on the deflection for these load cases. ...

For such an overhang, you might want to choose RHS for the part which attaches to the router/spindle... bolting into this section should go all way through with spacers inside to get the loads into the section, rather than locally deflecting the wall. The calcs all assume good load transfer.

Just to clarify I'm not using actual I beam, just 3 aluminium bits slotted and bolted together to make an I. I can't find RHS in the right dimensions so I guess I'd have to use a similar approach, just now with two 6mm aluminium plates instead of one. I'm concious that using RHS is going to further increase the horizontal distance from the router to the Y axis. Would it be ok to cut the front out of the RHS (to make a C shape) just where the spindle mounts. It's either that or get big enough RHS to fit the spindle inside?

As Ross has already suggested, I suspect you don't need to mill down 400mm into things, but rather you want to be able to machine to a depth of perhaps 30mm into things which are up to 400mm deep. If this is the case you could limit your Z travel to say 100mm. You then block up the workpiece with spacers to suit. This would be fairly stiff for all cuts, but give you options in terms of workpiece sizes.

Unfortunately that's not the case otherwise yes, I would just use blocks. One project I have in mind which needs a long Z axis is making wind turbine blades. That would, or course, be wood I should be OK.

Thanks for the help. I'll have another look for what steel RHS I can get hold of and modify the drawing accordingly.

[irving2008]

Unfortunately that's not the case otherwise yes, I would just use blocks. One project I have in mind which needs a long Z axis is making wind turbine blades. That would, or course, be wood I should be OK.

Forgive my stupidity, by why do you need all that z-movement for a wind turbine blade? If you're thinking of cutting them vertically 'on the hub' so to speak I think you run the risk of, as a minimum, the blade deflecting, or worse snapping off... anything that high a length to width ratio probably needs milling horizontally and adequately supported... or have i got the wrong end of the stick (or should that be blade?) :lol:

[Jonathan]

Forgive my stupidity, by why do you need all that z-movement for a wind turbine blade? If you're thinking of cutting them vertically 'on the hub' so to speak I think you run the risk of, as a minimum, the blade deflecting, or worse snapping off... anything that high a length to width ratio probably needs milling horizontally and adequately supported... or have i got the wrong end of the stick (or should that be blade?) :lol:

Yes you got the wrong end of the stick/blade there! :lol: I am intending to mill them horizontally. Near the hub, due to the twist on the blade, the Z distance is quite large. I'm intending to make a set of 3.2 meter (diameter) blades for a variable pitch wind turbine I've made. There are other things I want the big Z travel for, I just can't remember them at the moment!

I've found some 160x80mm steel box section. There's 5mm, 6.3 and 8mm wall thickness available. I think the 5mm should be ok. No idea how much it costs or if I can buy less than a 7.5 meter length....

http://www.steel-shop.co.uk/contentf...talogue(1).pdf
(page 7 - I live very near to them but I've never actually been)

[irving2008]

Yes you got the wrong end of the stick/blade there! :lol: I am intending to mill them horizontally. Near the hub, due to the twist on the blade, the Z distance is quite large. I'm intending to make a set of 3.2 meter (diameter) blades for a variable pitch wind turbine I've made. There are other things I want the big Z travel for, I just can't remember them at the moment!

I've found some 160x80mm steel box section. There's 5mm, 6.3 and 8mm wall thickness available. I think the 5mm should be ok. No idea how much it costs or if I can buy less than a 7.5 meter length....

http://www.steel-shop.co.uk/contentf...talogue(1).pdf
(page 7 - I live very near to them but I've never actually been)

Ah ok i see now... thats some serious machining you're planning! what material will these be made of (I know you said wood, but what type?). So each blade is 1.6m long approx by what at the root? the aspect ratio of a blade like that would be maybe 12:1 or more so the biggest cross-dimension is 135mm but I dont see that needs a 400mm travel. What am I missing?

[Jonathan]

Ah ok i see now... thats some serious machining you're planning! what material will these be made of (I know you said wood, but what type?). So each blade is 1.6m long approx by what at the root? the aspect ratio of a blade like that would be maybe 12:1 or more so the biggest cross-dimension is 135mm but I dont see that needs a 400mm travel. What am I missing?

Ash would be nice, maybe pine ... depends what I can get hold of really! Either way it's not a hard material, and anything I make which is a hard material won't be more than say 75mm tall so I think the making two of the bearings moveable and supporting in the middle is the way to go. Doing it that way would surely be sturdier than if I'd designed for 75mm in the first place - it will just take a bit longer to set up.

Here's one of my blade designs. This one was optimised for less Z axis travel but as you can see it's still quite a lot. The calculations show the root at an even steeper angle, and hence need a good Z travel. I agree this blade won't need 400mm, but I really don't want to be faffing trying to squeeze the blade into the available space!






The blade there is a 0.7m blade I made for a smaller with turbine. Plywood isn't so good...

Do you think the 160x80x5mm box section will be sufficient?