Fixed gantry or column for mill rigidity?

[NIC 77]

If all else fails and I can't find a local place to machine the columns flat, I may end up building a rig directly on my table (which *is* flat) and use a guided grinder for the job. And if a pig flies and somehow that works, I could also use it to make the Bridgeport a mill again.

I experimented a bit with trying to do some kind of guided grinding in the past. Yeah, it didn't work. You really need a proper surface grinder. That pig's not going to fly without throwing lots of money and time at it.

In my case, the pieces were 48" long, so there was only one place in town that could do it, and they wanted a huge amount of money.

You can do it by hand though. As long as you're "handy" with an angle grinder and high grit sandpaper flap disks. Lol. Get it, by hand, and handy, I crack myself up.

I'd actually like to get myself some good hand scraping tools to complement this technique. Don't laugh, it actually works, but requires oh, so much patience.

Here's the vertical sketch I've been working through:

Is this your latest design?

If you stich weld some steel flat bar onto the columns, the rails can mount to the flatbar, and you only have to worry about making the flatbar flat. I used 1.5 x 0.5 flatbar for 25mm rails.

I'm guessing that you will be adding feet to the bases of the columns to bolt them to the base and that the columns will be welded together.

That's how this thread started. I want to build a machine to replace the BP. Maybe I should just reuse the base, knee, ram, etc and build a CNC around that rather than trying to fix or convert it. The column would make a sturdy support for sure.

We certainly do put alot of effort into our projects. I know I'm certainly not very efficient at this, or in making decisions when it comes to my own personal projects.

I came across this the other day:

https://www.kijiji.ca/v-other-busine...ect/1483778127

but I have no space for it!

[JRoque]

Hey NIC,

That is more or less the latest design, though I added feet to the two columns. I will also be closing the gap between the columns once I know where the will be, etc. It will be quite the challenge to make those 3 individual pieces coplanar to each other.

You are correct, only area under the 45mm rails on the two vertical columns will need to be square to the base table. The table itself is ground flat so that will be my datum for everything else.

I'm right handed but I'm pretty sure I was born with two left hands. My arms could've been tree trunks as far as my control and artistic ability go. I'm going to need some sort of machine to help out with this.

The idea behind grinding the columns was to use two rails mounted to the table. A 'cart' bridge would be built with a rail on each side of the column while it's sitting flat on the table. I have a grinder mount for the lathe (somewhere) and a Precise grinder/spindle I thought I could use. Where the flying pig comes in is that the contraption will probably take longer to design, build and proof than the rest of the machine.

I did try to find a nearby CNC shop years ago with a large enough, proper surface grinder to fix the Bridgeport but no one I contacted had it. Perhaps I should try that again.

That looks like a good machine in your link. I also found a nearby "compact" VMC that looked great and had a reasonable price. The damn thing is 10,000 pounds (4500 kg) and would barely fit through my garage door. Plus, by the time these machines make it to us garage warriors they are beaten to near death, need a bunch of parts that in many cases are no longer available, require 3-phase power or converter and an industrial size air compressor that's equally large as it is loud.

Regards,
JR

[pippin88]

I see a lot of flat plates - bad.

If the table does not travel between the columns there is no point having two separate columns. It will just make alignment harder.

[JRoque]

Hey Pippin, thanks for the feedback.

I agree it will be much harder to align those 2 surfaces, perhaps even beyond what I can do. The reason for that is to have a wider column for better support. A single column (structural steel tube) is 12" (304mm) wide x 6" (152mm) deep - not nearly deep enough. A single column would have to be cut at the base to allow the screw/nut through, further weakening the structure.

What would you suggest would be best given the requirements and materials for the build?

Thanks,
JR

[JRoque]

Hi Pippin,

Something like this?:


X/Y table and base are still draft. Not sure if I'll do a whole plate or some sort of saddle. On X, I think it'd be best to have the trucks bolted down directly under the spindle and have the rails move instead, but I'm looking for feedback on that as well.

Thanks,
JR

[JRoque]

Hi all,

So what is your take on using a single column like above vs two? It would certainly simplify my build quite a bit. Maybe I can insert a brace/spine through the center of the square tubing and hold it with bolts front and back to add rigidity.

Peter, since you're the epoxy granite guru, if I fill the square tube with it, can I forgot the internal bracing I mention above? I have no experience with that stuff and see it as not much more than a bit of play sand and glue I am planning on using it but it was more for vibrations than relying on it for rigidity.




BTW, I received the spindle motor and controller from Berderga factory in China. Thing of beauty, that package. Threw it up on the bench for some basic testing, all nice. Relatively lightweight for 2.2kW machine (50 lbs/22 kg), 240V, 8,000 RPM, RS485, analog and digital I/O (some optically isolated), including step and direction inputs. It has encoder output that can be fed back into the NC board for very accurate positioning for tapping, automatic tool changing, etc. Software to review, change, save/set/backup settings, tune and debug. Can't wait to put it to work.





Regards,
JR

[peteeng]

Hi JR - From the work I've done on this so far:
1) Thin tubes lozenge ie they change sectional shape under load. Filling with something helps retain the section shape. EG does this so does expanding concrete which is easier to use. See below link
2) To stop lozenging or minimise it you need to use a very thick walled section. This helps with mass and stiffness. PU expanding foam of high density can help with resonance at minimal weight gain if weight is an issue. Use 300kg/m3 foam.
3) I think the expanding concrete is the go if weight is not a concern eg its a static component.
cheers Peter

https://www.cnczone.com/forums/mecha...rum-posts.html

https://theconstructor.org/concrete/...-cement/23780/

[JRoque]

Hi Peter,

Excellent! I thought of expanding concrete since I've used that in other applications (fixing the pool) with success but thought it would be too brittle to withstand deflection and vibration. Weight is not much of a concern so concrete is viable.

Obviously, having the wider support of 2 columns versus a single one is better but, would a single, 12"x6"/300x150mm, 0.375"/10mm wall square tube be enough? Fusion says it would be unless I apply >500lbs/226kg of horizontal force at the very top of the column.

Thanks,
JR

[NIC 77]

So what is your take on using a single column like above vs two?

Dual column if you want a larger work area and have your table pass between the columns.

If not, still a dual column, but with a rectangle welded between the two columns so it's essentially one piece. People sometimes get these connecting pieces plasma or laser cut then weld it together like a puzzle.

You need to start drawing everything out, motors, motor mounts, ballscrews, ballnut, etc. Then you will see that having a nice recess in there will help to make your motion coplanar when you consider your ballscrews and your linear rails working together.

Unless you want to slot the column and run the ballscrew inside of it. But you're already talking about filling that with EG or expanding concrete.

Obviously, having the wider support of 2 columns versus a single one is better but, would a single, 12"x6"/300x150mm, 0.375"/10mm wall square tube be enough? Fusion says it would be unless I apply >500lbs/226kg of horizontal force at the very top of the column.

The way it's drawn, the plate at the bottom might cause more deflection than the columns. Plates bend easily compared with square tube.

Are you doing an FEA analysis in fusion? Did you include the whole design including the bottom plate? What are you using as an acceptable amount of deflection?

[peteeng]

Hi JR - With machine design there are two design factors that need to be specified:
1) Static deflection - what you are measuring in FE
2) Dynamic deflection - this is a bit hard for the DIYer

For a commercial VMC they run at 150N/0.001mm or stiffer (um) in x,y and Z. For small cheap benchtop machines they run at 5N/um. When you do FE in fusion it "bonds" connections together. You can make connection stiffness adjustments. But a welded connection is 100% efficient and a bolted one is about 50% for rough work. So pick say 50N/um or calculate what you have at the moment and see what range your in. The load is applied at the nose of the spindle or at a nominal tool center just be consistent. I use 1000N as a nominal design load for a mill this is more than the tool load expected but makes the maths easier. So if you aim at 100N/um means you are looking for less then 0.01mm deflection. All other structural issues are logical, construction or functional issues; you are in charge of those. Peter

In plunge drilling you can 200kgf say 2000N but the Z dirn is usually really stiff. But best to check it out...

[JRoque]

Hey NIC, thanks much for your input.

....still a dual column, but with a rectangle welded between the two columns so it's essentially one piece.

Yep, in the 2 column version, there's a 6" (~150mm) gap where a 6" square tube would be welded. But a previous comment is making me rethink that. If I go with a two-column design, and tie those up with a middle square tube, now we're talk about 3 different tubes/surfaces to align up and account for weld warping. It would also make it hard to machine at that width (and heft).

Unless you want to slot the column and run the ballscrew inside of it. But you're already talking about filling that with EG or expanding concrete.

I was initially concerned with drilling/cutting through the column as that would weaken the base. Added to that is that I would be filling the column with some deadening/stiffening material. The solution: weld a pipe in the drilled column hole to route the ballscrew through.

The way it's drawn, the plate at the bottom might cause more deflection than the columns. Plates bend easily compared with square tube.

I really don't see that. The plate will be welded to the square column, it's 1/2" (~13mm) thick, and it's bolted down to a 1" (25mm) steel plate table. The bolts are 3/8" (~10mm). The 'lip' of the steel base plate is 1" (25mm) wider than the column which gives it very little chance of deflecting. If necessary, I would weld the plate to the base as well.

Are you doing an FEA analysis in fusion? Did you include the whole design including the bottom plate? What are you using as an acceptable amount of deflection?

Yes, Fusion 360 (a pain in my Rhino/AutoCAD behind). I did not include the bottom plate, yet - but I will. For deflection, I did not know what to expect. I applied 500 pounds of force in the Y direction, at the top of the ~42" column, and it deflected 0.001", per Fusion analysis. I thought that was pretty good because my spindle will be down on the column, much closer to the table, when it's actually cutting.

I was trying to find out if vertical columns would be acceptable vs a fixed bridge. While the results were good, I did not yet include the Z rails, plates, spindle housing, etc. That will be next.

Thanks,
JR

[JRoque]

Hi Peter, thanks for your insight.

I'm going to load the design in Fusion and try this out again from table/base to tip of the spindle. I did use higher forces previously but only on the column, and the base was assumed to be fixed. Once I have something I can import into Fusion again, I will check x, y and z for deflection. I think I a single column will twist to X forces before Y/Z ever notices, we'll see.


Hey, thanks once again for everyone's input here. Sorry for the million questions and ambiguity with the design, but I'd be missing an opportunity if I didn't pick your brains. Much appreciate your comments and help.

Cheers,
JR

[peteeng]

Hi JR - When evaluating a machine configuration usually a "trade study" is done. Short for trade off study. Simple configs are made and compared so you can get a direction on what works best. people tend to dive in and make detailed models before they settle on a config. Up front keep it simple, get an understanding of what's happening and have a spec to aim at. That also includes what your trying to achieve. Its a long haul to design something to a detail level then decide its not what you wanted! But then its better to do that in CAD than in real world metal...My hard drive is littered with "failed or parked" designs for various reasons. I tend to take a philosophical stance on the machine and then use that to see where it goes...Peter

eg only use laser cut and bolted parts or only use parts that can be built in my garage or only use plywood, etc etc

[JRoque]

Hi Peter,

Right on. I strip the machine down and leave only basic structural elements to run the simulations. Having everything would give me a closer to reality result but I'm basically looking to meet a minimum requirement. Adding more stuff to it will (typically) improve stiffness. This is all an approximation and I'm fine with that.

Fusion 360 doesn't like me, and frankly, I don't like it either. It will take me some time to set all the parameters, figure out how to "stitch" parts, tell it I'm using bolts in others, etc, to then run the stress tests. In the meantime, I have a more pressing issue with my spindle drawbar. I will post that next here just in case you knowledgeable bunch also know about my type of drawbar.

Regards,
JR

[JRoque]

Hi all,

This is the drawbar to my CAT40, Shizuoka B-5V spindle. It looks in good condition but I still want to take it apart, finish cleaning the caked-on gunk, grease it and put back together within specs.



Here's a scratch sketch with some dims:



If you look at the pic, you might see some tool marks from when the previous owner presumably took it apart. Unfortunately, everything seems to be Loctite down. I tried to undo the 43mm part at the top, threaded to the 13mm shaft, but it won't budge without further marking it.

In the picture above, you can see a nose section where the 6mm ball bearings (not shown) go. I thought of putting a rod through there to twist it off from that side but don't want to mess with the precise bearing holes as that is critical.

The 84 Belleville washers have no tension right now - not sure if that's right or if they need to be preloaded. I don't have a drawbar cylinder at the moment so have been using a puller to push it open while testing. My standard CAT40 toolholders seem fine. They grab and tighten up early as I'm closing the drawbar down.

I was going to call it a done deal when a voice in my head started questioning how much pull was really going on. Didn't want to spend more on a pull gauge than the spindle so I started looking and found a BT40 gauge that seemed good. However, the BT40 drawbar tension gauge (with a BT40->CAT40, 45 deg, Haas-type pull stud) barely holds on, tightens up much later than other tooling, and it peaks at 200 lbs (90kg) of pull - not good.

Which brings me to that nut you see in the picture with the drawbar. That threads to the spindle housing and keeps the drawbar in by pushing down on the 43mm nut. I can push in the 13mm rod, compress the washers and then move the nut further down. I can do that until the drawbar is no longer able to close back up as it's pushed in all the way. That doesn't seem like what I need to do to tension the washers. It does keep the drawbar in and could be used to adjust the tension some but I think the washers need to be tensioned differently.

So, am I right to think the washers need to be preloaded, even while the drawbar is outside the spindle? Hopefully it's not the full 1500 pounds (680 kg) but something more than the sloppy state they're in when outside the spindle.

Any and all clues, including suggestions on how to organize my thoughts so my posts are shorter than Magna Carta, are welcome ;-)

Cheers,
JR

[SUBMICRON]

I see a lot of flat plates - bad.

If the table does not travel between the columns there is no point having two separate columns. It will just make alignment harder.

100% Right Y shaped column is the solution, not 2 seperste columns

[ranaalex]

A fixed gantry (or "bridge") design like you've drawn is more rigid than a moving gantry, and it can be made very massive, since it doesn't have to move. The "column" type mills are typically small machines; larger ones are usually massive castings in a "C" configuration. But mills like the one you've drawn are becoming more common in industry. They are easier to build if you don't have an iron foundry, and work well if attention is given to rigidity in all parts and assemblies.

You say this is a "CNC/manual" mill; I'd suggest you choose one or the other. Ball screws are used for CNC, they're not used for manual mills because they tend to back-drive - you'll need a way to lock down any axes you aren't using for a particular move. Is this machine supposed to have dual spindles and another axis parallel to Z? If so, you'll need a way to drive the two separately, this gets complicated. Also, you'll need longer rails for X, and a footprint about twice as long as you've indicated if the tool is supposed to reach the whole table area. Read more here https://inspecthome.com.au/