30 x 48 Questions

I’ve been considering upgrading from an older 24x48 CNC to an OpenBuilds model, but a friend recently purchased a LongMill, which led to some research on my part. I’m really impressed with Scienci in many ways, and am rethinking my strategy.

My application is bass guitar construction, so in order to support neck through construction down the line, I need an working area of about 48" in the long dimension. 30" in the other dimension is overkill, but there is not a downside unless I need to move to a smaller space.

One of my concerns about the OpenBuilds design is that the side rails are unsupported, although beefy. I’ve run into issues with unsupported rails on my existing CNC, and since I was going to build a “bowling alley” design with long side rails and a short gantry, that amplifies the worry.

It appears that the Longmill 30x48 is a short side rail, long gantry design. So this is my first question. My intuition and feedback I’ve gotten is that long gantry design would be prone to torsional flex in the gantry, leading to flex in the Y direction. Since this the “end grain” direction when cutting, it’s also the one that gets the most kickback from wood (hard maple in this case).

On the other hand, even though I’m not a mechanical engineer, the extrusions of the MK2 look like they would be better with regards to flex than the typical V-Slot design for the Openbuilds LEAD CNC.

I would be interested in other’s opinions. I’ve got two options. I could modify the Openbuilds design to use supported rails and a bowling alley configuration with a short gantry. This would be frankly a hassle, since I have to put together the whole parts list myself and make sure I don’t mess up anything. On the other hand, I could simply get a Sienci MK2 30x48 and be done with it. The only concern is Y axis flex of the long gantry.

I’d be very interested in comments about these two alternatives. Any help is appreciated.

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search for this video on youtube . He covers this. At 29 mins. he climbs up on the gantry during a carve.

I saw a different video from that guy about the Sienci, it’s one of the reasons I’m here asking about it. Having said that, I’m talking about torsional flex in the gantry. I know from experience that square tubing has very little torsional strength as compared to round tubing. Don’t ask, it’s a long story. But when I look at the Sienci gantry, I see a lot of things I like in terms of torsional flex. Diagonal planes. Radiusing. And I didn’t realize until I saw that video that the gantry is so much more beefy than the side rails. So I find it easy to believe that a longer Sienci gantry would equal or better a shorter standard V-slot gantry rail. And that’s why I’m asking about it.

But him standing on the gantry while cutting something doesn’t really have much to do with what I’m asking.

I would suggest reading the report here https://sienci.com/2022/09/09/stiffness-and-deflection-testing-on-the-longmill-mk2/

The short side rail as you mention was just to make it more practical in day-to-day use, since you can feed in full sheets through the front and back. It also give you easier access from the front for loading stuff. Doing the bowling alley would make things more rigid, but we just liked the convenience and a reasonable trade-off.

It’s so impressive that you are paying so much attention to the stiffness issue. What’s really interesting about those numbers is that the X axis deflection is not that different than the Y axis deflection, and the X rails are supported! (I’m assuming you measure Y axis deflection at the middle of the gantry?) That’s a tribute to how good the gantry design is. You’ve done a lot to lessen my worries, and of course the bowling alley setup is less convenient. I really doubt one could get that kind of performance with standard V-slot rails.

Keep up the great work!

One more question. I was taking a closer look at the MK2. I don’t use a dust collection boot, I use a dust enclosure, for various reasons. I was looking at the tops of the X rails. Do you ever run into problems with chips flying onto the the X rails and getting caught under the wheels? It looks like the running surface for the wheels is enough above the rest of the rail to prevent this issue, but I thought I’d ask.

But it is looking overall like I just need to figure out when I’ll have time to take my existing CNC offline and build a bench for a new 48x30 MK2!