New CNC build gantry questions

[Capsuna]

Hello all,

I am beginning to plan my CNC router build and have a few questions about my gantry design.

First all, I came across a large lot of 60x60, 60x120, 30x90, and 30x60 extruded aluminum that I will be building the machine frame from. I also sourced a 1" 6061 T6 aluminum plate which I will be machining for supports, plates, etc. I plan on building a machine with a cutting area of 1850mm x 1250mm.

I did some math for the gantry, with the 60x120 extrusions, 68kg load in the center, and the span being about 1250mm, it will sag about .14mm. So I think that will be unacceptable and I want to stiffen it some without buying additional extrusions. So I was wondering how do I calculate the sag if I attach pieces to the tops and bottom of the 60x120 and make it a C shape (attached image)? So I will place a 30x90 on the top and bottom of the 60x120.

Also, are there cons of placing linear rails on the top and bottom faces of the gantry? Or should I stick with the side the z axis will be on? I am just thinking about how I will fasten these 30x90 extrusions to the 60x120s.

I look forward to this build and sharing my progress with everyone :)

Thanks for the feedback!


26809

[AndyUK]

routercnc shared a fantastic spreadsheet in the following thread; it can calculate the sag of some simple geometries, you might have some fun with the more complex profiles but take a look anyway.

http://www.mycncuk.com/threads/2023-Some-thoughts-on-cnc-machine-design-principles

Pros of placing the rails on the same face as Z are that the Z axis is a lot simpler to construct - you only have to have a single plate to connect the Y and Z carriages.

Con is that you increase the distance from your cutting bit to the gantry, which increases the rotational force around the gantry causing more deflection.

JoeHarris' build has a good description of how he joined the profiles in the way you're looking to do by using shims and then drilling and bolting through the extrusions.

[Capsuna]

Awesome, thank you Andy! I will take a look at the spreadsheet later tonight.

[JAZZCNC]

I've built in excess of 30 machines ranging from 2 x 2 to 10 x 5 using L shape gantry system and I can tell you 100% confidence that 2 x 120 x 60 arranged in L shape (not C) will easily handle 68Kg at this size.

Here's one using 120 x 80 at 1800mm length. with 28Kg Z-axis and doesn't even know it's there.

26813

[Kitwn]

Jazz,
There are lots of different profiles out there of differing cross section and strength at the same outer dimensions. Can you give us some details of the profile you use, especially the weight per metre? This would allow a reasonable comparison between manufacturers.

Kit

[JAZZCNC]

Jazz,
There are lots of different profiles out there of differing cross section and strength at the same outer dimensions. Can you give us some details of the profile you use, especially the weight per metre? This would allow a reasonable comparison between manufacturers.

Kit

Yes, no problem. I use 2 different types equivalent to ITEM and Bosch Rexroth profiles depending on machine size.
On the smaller machines,(up to 1000mm width) I use Bosch Rexroth equivalent 45x90 with a 10mm slot. 4.2Kg/M

The reason I use this type is that the slot spacing just happens to match the bolt spacing of BK12 bearings so it makes fastening easy.

On the wider machines, I use ITEM 120 x 80 (10.8kg/m) or 160 x 80 (13.5kg/m) with an 8mm slot. This does mean plates have to be made for mounting Bk bearings.
I prefer the 8mm slot because gives a more flat area for the linear rails to sit on when using 20mm rails The BR 10mm slot doesn't leave a lot of room spare.

[Kitwn]

Thanks Jazz, it's those little gems of wisdom that make all the difference.

I've thought for some time that I should gather up all these snippets of vital information for future use and have at last started a "CNC Instant Genius" Word document with the above as it's first entry.

kit

[AndyGuid]

...... I've thought for some time that I should gather up all these snippets of vital information for future use and have at last started a "CNC Instant Genius" Word document with the above as it's first entry.

Kit, Do yourself a favour and start using Microsoft OneNote instead of WORD. OneNote is designed for easy note-taking, and it automatically saves your edits and synchronises notes across your devices (such as laptop and mobile) if you like. It really is the Bee's Knees!! I wasted so much time not taking my son-in-law's advice to start using it earlier!!! And as much as I hate much about Windows 10, it's one positive is that OneNote is included for free with Windows 10.

Andy

[Kitwn]

Kit, Do yourself a favour and start using Microsoft OneNote instead of WORD. OneNote is designed for easy note-taking, and it automatically saves your edits and synchronises notes across your devices (such as laptop and mobile) if you like. It really is the Bee's Knees!! I wasted so much time not taking my son-in-law's advice to start using it earlier!!! And as much as I hate much about Windows 10, it's one positive is that OneNote is included for free with Windows 10.

Andy

Andy,
I've been an almost daily user of Word since the days when it came in a box of about 20 floppy disks so it tends to be the go-to ap for everything. I'll cetrainly have a look at OneNote when I have time.
It's hard to imagine we used to click on a particular feature we wanted to use and got the message 'Insert disk 14 in Brive B:' because even the HDD didn't have enough storage to hold the OS and all the applications at once.

You try tellin' that to t' kids of t'day and they'll not believe ya!

Kit