a Steel Box Section Build with SBR20 & Ballscrews Plus a few questions

I'm using two 3Nm motors to drive my X axis. 68V PSU.Steel gantry, with spindle, etc, probably weighs around 50kg. 2005 ballscrews, driven 1-1 through (from memory) 20T pulleys. Gives me about 5m/min rapids. That corresponds to about critical speed of ballscrews and corner speed of motors. 2010 might give me a bit more speed but probably at the cost of acceleration which is more useful if you are doing fiddly detail cutting.

So I still haven't finished my Frame still after a long long delay in making the CNC after house moves work and another daughter it's time to get this thing back on track again, when I started this I never in a million years thought I would still be making it.

Although I haven't been making any progress on my machine I read through the amazing posts on here most days still.

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So I have been redesigning my original again and I have found that Item does a 45x90x90 10mm Profile

https://www.aluminium-profile.co.uk/...file-kjn992387

The plan would be to put the two together with bolting through one and then into 'T nuts on the second profile.

It will then be wider than the width of the frame to allow the full width of the cut (Which I didn't have previously)

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I need to change the bracket so that it doesn't get in the way of the Z

It will have a movable shelf which I haven't added to the design in fusion 360 as yet so the spindle won't need to go right down to the bottom shelf.

It is roughly the same pricing as a 200x100 cold-rolled box section but I think it will be easier for me to complete the gantry with this instead of the steel and not have the resin to be put on to get the accuracy on the rails as time in the workshop is still limited these days.

Looking at the profile it seems pretty sturdy and it would have 5mm on each side of the rail to attach on the slots. But before I get too carried away with the design I thought I would check if this is a good possible profile to use.

Any thoughts would be most welcome.

Thank you

Have a look at John Ward's site.
https://www.youtube.com/watch?v=Xbiipb6nnZg
You will achieve greater accuracy using flat plate fitted on top of your frame and shim at mounting positions.

Pointless using such a heavily built gantry on such a weak flimsy frame.! You seriously need to brace that frame-up.

Good Morning

Thank you for the replies, Thanks Colin I will look through the link very appreciated, Thank you Jazz yes the actual frame that I have pretty much built most up and has more bracing of each corner and I'm adding diagonal pieces on the Horizontals of the bed and then the second top shelf (not shown) will also be bolted to the mainframe to add more rigidity.

Because I had that part built its more of the Gantry design I'm changing from the 50x50 box Gantry I originally had as my design to the Aluminium Gantry, but I wasn't sure if the Gantry would be as good and as strong as the 50x50 Box design I had previously,


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The profile I'm thinking I using is this but I couldn't find too much about the strength of it in terms that I understood (Me not clever enough not Bosch's information)

Attachment 29547

I will use the Profile to create a stable base to connect it to the Plate. the Screw and mounts will be put on ali plates because of the room available on the 45mm and I need 60mm width for the screen bk12 fittings.


But I like the words from Jazzcnc used Heavily then I will continue along that route with my design I haven't ordered the profile as yet and I'm trying to get the rails on order soon.


I see Aliexpress and ebay don't seem to all say Hiwin rails now although the part numbers they use are still the same.

Fred shows that he sells original Hiwin still and also a 3rd party range which seems to be a lower cost but has anyone used these instead of Hiwin or is it just false economy and just go for the Hiwin rails?


Thank you

Rob

You can also skin the frame with sheet material (rather than diagonal beams) for bracing

Quote:

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Originally Posted by pippin88

You can also skin the frame with sheet material (rather than diagonal beams) for bracing

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Sheet material will do very little for it other than turn it into a Big Drum. It needs proper and substantial bracing to handle the forces a heavy gantry will put on it. If done properly it will also reduce resonance/vibrations.

Thank you very much for the replies, I have been adding to the design of how I have made the frame, although I still have the horizontal cross members to put together.

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Also wondering if anyone has used Fred from BST Automations own Linear rails ? or is Hiwin the way to go,

Thank you

Quote:

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Originally Posted by grain_r

Also wondering if anyone has used Fred from BST Automations own Linear rails ? or is Hiwin the way to go,

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Yes, I use lots of them and they are good for the money, but they are not as good as Hi-win so don't expect Hi-win quality. That said they more than good enough for a serious Hobby level Router and if maintained properly will last for years and they are nearly half the price of Hi-wins so no one should expect them to match Hi-wins really.

If you're wanting the best Stiffness and working the machine hard in a production environment 10-12hrs per day 6 days a week then you should go with the Hi-wins.

Re the frames It's still a little weak on the bracing IMO, because the sides are so tall the short end diagonals don't extend far enough across the beam, same down the sides, this will allow the cross beams to flex as the sides apply a bending motion to them. You would be better with another lower cross beam with diagonal braces between them.
My first machine was built similar to this design but with short sides and it flexed at this point, I later braced it up as mentioned above and it was much stiffer.

If you don't need to pass material thru the machine I would also fit triangle plates to the ends to brace the sides. Those tall sides with spindly tubes are a massive weak link in this design, why such tall sides with such a short Z-axis.?

Hey dean great thank you very much for the information on the Rails I have been looking and will be most likely going for those OEM rails, It will be purely a home Cnc just for hobby work so sounds good enough for me.

I will add some more bracing to the design higher up too.

The original plan was to make foam moulds for fibreglass parts for a car I was building so I could then keep layering up the foam and moving the shelf down to create the arches and other large parts, But that has changed now :(maybe one day) but its going to have a movable shelf for sheet woodwork so once the shelf is in that should also add to the strength too.

I originally left the ends open for end-grain cutting but I will add the end bracing too and I can always take them off when I'm doing normal sheet work.

thank you