CNC Gantry Build. The $1 mistake that started it all.

[ChipsBFlyin]

I've been lurking around this site for some time and have always wanted to build my own machine. I shied away from starting a build because I questioned the longevity of an epoxy granite frame, though some of the machines I've seen built have been impressive, and because I had no interest in making my own steel frame because of the machining required after. My main interest at the time was building a VMC, until I saw a Datron in action. I then started researching about making CNC gantry machines with a granite base, and that's when I stumbled upon @fox3d 's granite gantry machine, can be seen here:

https://www.cnczone.com/forums/epoxy...54358-cnc.html

I was surprised at how well the machine cut with his spindle set up. Shortly after, I found myself looking into large surface plates, needless to say I was blown away by cost of some of these plates. Once they go above 18"x24" the price sky rockets. I then started looking at some featured at auctions, and out of curiosity I wondered how much these large surface would plates sell for. I placed a $1 bid and about a week later, I received an email notification that I had won a 36"x48"x8" surface plate and that had a few days to pick it up. I was completely taken by surprise and now had to figure out how I was going to move this 1,000lbs+ monster piece of granite.

Once I got the surface plate home, I began taking measurements and started modeling it in SW.

This is what is what I have so far:

25mm Hiwin rails on X, Y and Z.

Ball screws: TBD

Control: Centroid Acorn? I would like to rigid tap, and be able to run HSM tool paths without the control slowing down.

Spindle: Jianken is up there on my list, but still researching, so I am open to suggestions. I'm thinking 15k-20k spindle with ATC. I mainly plan on using a 1/4-3/8" end mills, maybe 1/2" once in a while. I would like to be able to drill 1/2"-9/16" max, with 0.159-.3125" being common.

Gantry: 6" x 12" x 63" long, 0.250" wall tubing.

I plan on only cutting aluminum on this machine, so the machine will be fully enclosed to keep chips and coolant inside.

Machine travels: X - 48", Y - 30" and Z - 12"?, these specs may change as the design progresses.

Any recommendations for sources for ground ballscrews, Hiwin rails and servo motors are welcomed, and what places/brands to avoid is also appreciated. Any feedback and input on the machine design is also welcomed. If you think I am making a mistake, please point it out.

Here is my machine in its current design stage:

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[peteeng]

Hi CBF - Looking good. 25mm cars is a good size get medium or high preload cars not nominal cars. The top of the columns can be thinner. The max moment is at the cars so having such a wide top does not really help much. But it probably does not interfere with anything. Are you going to have your steel fabrications stress relieved? At the level you want to build its a really good idea. The moving Z tool plate has 4 bearings on it, The lower bearings are the ones that do most of the hard work. especially bumps and crashes. So if you can get these lower bearings down further and reduce the moment on them its worthwhile. These two bearings are the ones that wear most as they are closest to the applied load. Either make the saddle wider and bring the cars down or stagger them the other way so they are below the saddle bearings..... Peter

[mactec54]

The Gantry support arms are similar to the Datron design your Linear Bearing mounts look better than what they have for their machine, looks good, may want to spread the base out.

DMM is a good place to start for Servos 20Bit Encoders, TBI for Ballscrews

https://dmm-tech.com/

https://tbimotion.en.ec21.com/TBI_Ba...--9941262.html

[ChipsBFlyin]

Hi Pete,

Thanks for the tips on the preload. By a wide top, are you referring to the gantry? I measured several commercially available cnc routers and 6"x12" was the average I came up with for their gantry. My friend who owns a Datron was kind enough to provide me with a rough gantry measurement, he said it was approximately 7"x10"x56"L.

I do plan on sending out all welded structures for stress relief before any machining. I am curious about this though, I've searched and have not been able to come up with a number of how much a non SR weldment may move over a distance. I am sure this is because there are many factors involved, but it would be nice to to have at least a rough idea of how much could be expected.

Attached you will find an updated Z axis plate. I originally had the rails spaced apart 7", changed them to 8.5". I don't think I can lower the Z axis bearing blocks any lower as that would affect my Z retract clearance, is there any way around it? I've seen several of the big boys, like DMG Mori, Haas and another that I can't recall, mount their bearing blocks in a similar fashion. I do plan to box the Z axis plate to increase rigidity, can be seen in the attached thumbnail. I will post more updates as the design progresses.

@mactec54, thanks for the links. The base that I have is the steel base that come with the surface plate. I may update it at some point, most of the cutting occurs on top of the stand, the rear is only for the tool changes.

Do you guys know how I can insert large images, instead of just thumbnails?

[peteeng]

Hi CBF - The deflection due to stress relief is hard to predict. Lets say that it will deflect enough that the linear rails will not function they will jam if you machine them first. I had 2 saddles made last year with very clear instructions on how to weld (preheat small welds and placement etc) when I got the parts and assembled them the rails jammed. They were cupped about 1mm. The aim was to not machine them but that hope failed. So I had to have them corrected.... SR is the best path. Be a little generous with your machining allowance especially over long distances.

I meant that the top of the columns can be narrower as the max moment is at the bearing not at the elbow. The gantry size is fine.

re: residual stress - steel shrinks 6% when it goes from liquid to solid, then there's also the local expansion/contraction of the red hot material to over 1000C with a CoE of 12e-6m/m/c there's enough stress to tear the welds in over constrained fixtures.... Peter

[ChipsBFlyin]

Pete,

Thanks for the sketch and sharing your story. I narrowed the top of the columns by 1.5", I'm guessing I can take off more material and still be ok, but I kinda like the way they look, and if it doesn't hurt anything then I would like to keep them that way. I will be SR all components that are going to be machined.

This brings me to my next question, can I get away with the stand not being heat threated? Its going to have leveling screws that thread into the inserts on the bottom of the surface plate and support the end of the Y axis rails mounts. I tried to make my stand design similar to the one found on a M8Cube, but I'm not sure that the design I have so far is sound.

Please note that those pins sticking out on top of the stand are meant to be the top leveling/support screws. I just haven't done a detailed model of the stand yet.

[mactec54]

Pete,

Thanks for the sketch and sharing your story. I narrowed the top of the columns by 1.5", I'm guessing I can take off more material and still be ok, but I kinda like the way they look, and if it doesn't hurt anything then I would like to keep them that way. I will be SR all components that are going to be machined.

This brings me to my next question, can I get away with the stand not being heat threated? Its going to have leveling screws that thread into the inserts on the bottom of the surface plate and support the end of the Y axis rails mounts. I tried to make my stand design similar to the one found on a M8Cube, but I'm not sure that the design I have so far is sound.

Please note that those pins sticking out on top of the stand are meant to be the top leveling/support screws. I just haven't done a detailed model of the stand yet.

The M8Cube has and outer structure that is supporting the frame like what you have done you may have to add some cross members and sit the trash bin on top instead of on the floor
The base frame is also attached to the Main Base of the machine not just sitting on leveling screws this would not be very good to do it like this with leveling screws

Yes the stand should be fine as long as there is no stress where it mounts to the machine frame

[peteeng]

Hi GBF - stand non treated OK. Will need a few more cross braces I think. Cantilevers are like tuning forks and can vibrate easily. Peter

[ChipsBFlyin]

Thanks for the input. Will revise the stand. The tubing pictured in the stand above is 1/4" wall 4"x4" square tube. Does the stand benefit from heavy wall tubing or can I use same tube size but 1/8" wall? I don't have a forklift or crane to move around a heavy wall stand like that, as I fabricate it.

[peteeng]

HI CBF _ 1/8" OK. Does your CAD have FE? If so do a modal analysis and this will show you how the frame will vibrate and at what freqs. If you publish the stand step file I can run the modal if you like. Peter

[ChipsBFlyin]

Pete,

Thank you for the offer. I think I will take you up on that. Speaking of cantilevers, am I setting myself up for failure or other issues with my Y axis rail mounts offset from the sides of the surface plate like this? As you can see from the picture, they are not supported by the stand. My reasoning for designing the Y axis mounts like this was to isolate the rail mounts from the stand, to avoid potentially tweaking them. Also to get more X axis travel and space for the chips/coolant to drain to the enclosure (have not designed that yet).

[peteeng]

Hi CBF - all those bits have to be lined up and the welding will be a variable. I'd try to make the "side legs" as a single piece vs so may bits. But you are SR and final machining so that would solve the issue. But maybe a sheet or plate construction vs all those bits? swarf clearance is a valid thought, there will be lots. Up to you. Simple is best and machine stiffness/simplicity trumps swarf... Peter

just a thought with heavy thick slabs of stuff they deflect down due to gravity. If you support it at 60-55% its width this is the spot that it stays flat but look that up. Plus if you support at edges it is supported at its first vibration node so its easy to excite. Holding it at 55% is something like its third harmonic.... which is unlikely to get excited....

[mactec54]

Thanks for the input. Will revise the stand. The tubing pictured in the stand above is 1/4" wall 4"x4" square tube. Does the stand benefit from heavy wall tubing or can I use same tube size but 1/8" wall? I don't have a forklift or crane to move around a heavy wall stand like that, as I fabricate it.

You make the stand so you can fill it with sand or a mix of epoxy and sand, this will dampen the mass hollow steel tubes, how they vibrate 1/8" is ok, with the weight you have on it 3/16" or 1/4" will work also, I use a lot of 3x3x3/16" wall for my base frames which are similar to what you are doing.

[ChipsBFlyin]

This is what I wanted to do originally. The problems with this design are, finding someone with an oven that can fit a weldment that's approximately 6' x 6' x3', and finding someone to machine the top 2 Y axis rail surfaces. I'm sure it would probably be quite expensive to have a shop machine it, even though its just a single op and tool.

[peteeng]

Hi CBF - That design has a few issues. As you say you need a large heat treaters to SR the frame but also the load path from the tool to the job is thru the frame and thru many elements of the frame. The loadpath needs to be as short as possible and as simple as possible to produce a stiff machine. No point in having that huge base and a wimpy frame connected to it... Peter

[mactec54]

This is what I wanted to do originally. The problems with this design are, finding someone with an oven that can fit a weldment that's approximately 6' x 6' x3', and finding someone to machine the top 2 Y axis rail surfaces. I'm sure it would probably be quite expensive to have a shop machine it, even though its just a single op and tool.

Where are you located there are some that can do the whole job stress relieve sand blast and machine and it is not that expensive, the Heat treating will be in the $100 range machining $300 to $400 range depending on ease of setup.

The company I use can Grind more than your 6' x 3'

[mactec54]

Hi CBF - That design has a few issues. As you say you need a large heat treaters to SR the frame but also the load path from the tool to the job is thru the frame and thru many elements of the frame. The loadpath needs to be as short as possible and as simple as possible to produce a stiff machine. No point in having that huge base and a wimpy frame connected to it... Peter

They design like this to have a chip tray under the table, which can be pulled out from the front, they work well the Datron is designed like this.

[ChipsBFlyin]

Where are you located there are some that can do the whole job stress relieve sand blast and machine and it is not that expensive, the Heat treating will be in the $100 range machining $300 to $400 range depending on ease of setup.

The company I use can Grind more than your 6' x 3'

I'm in Brooklyn, NY. I don't think anyone near me is touching this for less than a couple few grand for machining. Everything is so expensive around me, a 1/4in wall 2"x2"x24' long tube is $279.

[LorenBMS]

Some material sources outside of Gotham. All will UPS ship.

https://www.onlinemetals.com/en/buy/...sort=price-asc

https://www.midweststeelsupply.com/s...eelsquaretube#

https://www.mcmaster.com/steel/shape...thickness~1-4/

[mactec54]

I'm in Brooklyn, NY. I don't think anyone near me is touching this for less than a couple few grand for machining. Everything is so expensive around me, a 1/4in wall 2"x2"x24' long tube is $279.

This company is a 1000 miles from you, they do everything https://pgisteel.com/steel-plate-ser...nealing-steel/