Thread: Critique my 8020 design

I'd like to see the design but I run Linux which doesn't have an eDrawing viewer

Adding more pics

Just some of my personal thoughts.

* Beef up your main X axis rails 80x80 atleast or 60x120
* Ditch the 80x40 and use 40x40 for the legs, thread them and get some adjustable feet
* Use the square Linear Rails for your X axis instead of the round supported linear slides, I recon they'll handle the load better.
* Use 3 linear slides per side for the X axis (if you stick with the round supported linear rails)
* Beef up the gantry sides

Very nice design. I envy peoples 3D cad skills.

You asked for opinions and take this as that. My opinions.

Like Chris, I like square rails.

Spread the X axis bearings a little further apart. Even if this means increasing the machine overall length to maintain X travel extent.

Increase the gantry lower cross member width. Replace it with a plate as wide as the bearing width.

The weakness of a centrally driven gantry (racking) is improved this way.

Do all you can to keep the spindle as close as reasonable to the Y axis bearings.

And increase the rigidity of the gantry sides. Thicker plate or some kind of stiffening ribs.

"Keep it close, keep it rigid"

Greg

Ch4iS: Do you know where I can get 60x120?
I'm curious as to what advantage smaller feet would give? The 80x40 should allow me to further tie the corners firmly together.

I've already got supported round rails of set lengths (1100mm, 600mm, 300mm) so square rails or lengthening are not an option.
If I had my time again I'd make my purchases differently, but unless I wanted to spend a year reading non-stop the quickest way to learn (as always) was to do and find the areas that could be improved. If I was starting from scratch I'd probably be going for CRS rails + Ahren type carriages.

The X axis bearing blocks are currently 150mm outside to outside, I've been thinking of pushing that out to 200mm which will shorten my axis obviously.

I don't agree with replacing the lower gantry cross member with plate - having a thick member (giggle) improves the vertical rigidity. If need be I could double the width (use 2 pieces of extrusion).

Greolt: Solidworks is brilliant, amazingly intuitive and quick and I've only scratched the surface. If you can get a hold of a copy (student licences are often a cheap way if you are studying at all) then do it.

Originally Posted by pippin88

Ch4iS: Do you know where I can get 60x120?
I'm curious as to what advantage smaller feet would give? The 80x40 should allow me to further tie the corners firmly together.

Modular Components and Automation Australia - Item Aluminium

They are a reseller of the item brand of extrusion (German made) they are the only ones selling 60x120 as far as I know, but if you have SBR20 rails they bolt right too it, SBR16 I think has the same bolt pattern also.

Its not cheap last year it was $175 + GST per/m

The X axis bearing blocks are currently 150mm outside to outside, I've been thinking of pushing that out to 200mm which will shorten my axis obviously.
Worth doing with the height of your gantry sides


I don't agree with replacing the lower gantry cross member with plate - having a thick member (giggle) improves the vertical rigidity. If need be I could double the width (use 2 pieces of extrusion).
The X axis bearings support the vertical movement. It is racking that you need to be concerned about and a wide cross plate limits that. Racking is where cutting forces rotate (twist) the gantry around the X axis ballscrew.

Also as has been pointed out the gantry sides need beefing up and the centre of the spindle brought back inside your X axis bearings. You have a big lever there and you need to design balance and rigidity into it.

Also check out the dimensions of the y axis ballnut as it looks a bit tight to me.

Other than that you have a fairly standard design that works well.

I agree that making it a bit beefier would be good as you won't regret the extra stiffness nor mass when you start cutting stuff. Less chatter etc

Your just up the road from me so if you want to come and see my setup and get ideas / not repeat my mistakes then your more than welcome,, just PM me.

Cheers

Mark

Thanks for the feedback guys.
I'll move the X axis bearings further apart (I'm also going to add a 3rd truck in the middle of each side, they are pretty cheap).

I came across Lappy's build and found it quite interesting.
I was thinking that using 80mm flat bar aluminium in say 16 or 25mm thickness to replace my 80x40 extrusion sides (x axis long bit) could make some things easier.
Will check pricing tomorrow.

Mark, thanks for the offer, I might take you up on that sometime (time permitting - not enough hours in the day or days in the week!).

Glenn's build is a good example of what we were pointing out.

Gantry sides are 20mm by the look of it and the underplate across the gantry sides is wide to take limit racking.

You will get a shock at the price of thick ally plate as ally is priced per kg. That is why the extrusions are cheaper as there is a lot of air inside the material and that comes for free.

Couple of ideas

You could add alot of stiffness to your existing design by adding ribs at 90degrees to the side plates using the same thickness plate. Through bolted from the inside adding 3 ribs say 50mm wide would significantly stiffen the sides and be alot cheaper and more effective than just a thicker plate.
Doing something similar to the gantry ie keep the extrusion but add a back and bottom plate that extends say twice the extrusion thickness rearward away from the spindle side should help alot with racking and give you a nice shelf to run your cable management on.

In SW figure out where your center of mass is on the gantry and ensure that the bearing blocks are equally space either side.

Only other thing I would say is make it with the largest working envelope that you can afford (and do well) as increase in size is not proportional to the cost (drives electronics and motors being the same cost) I'd say 1/4 sheet then 1/2 then full sheet are the basic sizes to aim at and always have a bit of spare travel over and above that target.

Oh yeah.. if you get a spindle get one with a ER20 collet or larger you want to be able to use 1/2" routerbits not 1/4" as the difference in tool chatter is significant.

Cheers

Bar isn't too bad.
I can buy a 4m length of 80x25 flat bar for ~$240. That's $11.37 a kg vs $17 a kg for 40x80 extrusion.

I already own the ballscrews and rails, so changing size is not an option unfortunately. If I was starting again I would do things quite differently, but c'est la vie.

Thinking heavily of using flat bar (80x25mm) for the long side pieces
Advantages:
1. Easier - drill straight through and screw into extrusion holes
2. Cheaper - cost per metre almost the same as 80x40 despite weighing 66% more. Throw in the fact I won't need fancy fasteners ($2.50x10 each side) to join the bits, just normal bolts.

I'm also adding a 3rd truck to each X (and probably Y axis)




The angle / setback of the side plates and Y gantry is just a rough guestimate at the moment.
Will be adding fins to gantry side to stiffen.

That is much better but what we are seeing as week is the top of the gantry sides. The way you have the two edges converging to the bottom corners of the extrusion is your weak point.
I did a drawing of sorts and the width of the gantry sides is 10mm less than the thickness of an 80mm extrusion. Check it on your drawing by measuring at right angles off the back edge of the gantry side to the bottom front corner of the extrusion on y axis.