[jonnom]

Hello everybody,

I'm currently planning a new CNC machine for my shop (strictly hobby), i'm looking to build a versatile machine for wood/plastic/aluminium and would love some input on the design and parts, please note i'm no engineer so everything here was put together by looking at other peoples experiences from this awesome community.
so the plan is as such:

• frame from 60x60 extrusions + gussets and 30x60 cross beams (from paletti)
• gantry risers are 20mm machined aluminum (or double 15mm?)
• gantry is 40x160 extrusion
• Linear guides are 20mm HGR rails from HIWIN + wide blocks (normal blocks on Z)
• Ballscrews + FK/FF style supports at both ends are 1605 C7 (unless someone can convince me otherwise)
• Direct drive Nema 23 oz 400 3A motors + Gecko drive
• Spindle will probably be Air cooled 2.2Kw VFD

my questions to the gods of CNC bilidng are:

• direct drive or belt driven? what kind of belt/width?
• will 60X60 extrusions do the job or should I opt for 80x80?
• will the 40x160 extrusion gantry be solid enough or should I go for 80X160?
• Is there a certain direction for the FK/FF supports? I can't see this referenced anywhere
• I cannot seem to decide if I should go for a 10 pitch or a 5pitch ballscrew (or if i need 20 vs 16 mm screws in the first place)
• air vs water cooled spindle? cant decide on this one either, prefer to keep it simple, but also quiet

would love input on these questions/the design in general,
Jon

[pippin88]

10mm pitch screws. For wood you do not want 5mm pitch screws.

[wizard]

Hello everybody,

Welcome!

I'm currently planning a new CNC machine for my shop (strictly hobby), i'm looking to build a versatile machine for wood/plastic/aluminium and would love some input on the design and parts, please note i'm no engineer so everything here was put together by looking at other peoples experiences from this awesome community.
so the plan is as such:

• frame from 60x60 extrusions + gussets and 30x60 cross beams (from paletti)
• gantry risers are 20mm machined aluminum (or double 15mm?)
• gantry is 40x160 extrusion
• Linear guides are 20mm HGR rails from HIWIN + wide blocks (normal blocks on Z)
• Ballscrews + FK/FF style supports at both ends are 1605 C7 (unless someone can convince me otherwise)
• Direct drive Nema 23 oz 400 3A motors + Gecko drive
• Spindle will probably be Air cooled 2.2Kw VFD

As sized this would be a light duty machine. It is important to understand what you Z axis height is and the travel there. the height impacts torque applied by the gantry to the X axis bearings and any twist that will be imposed on the gantry beam. It is an important dimension that you at once want to minimize yet not be too short that accomplishing your goals becomes a problem. In any event your drawing seems to indicate that the vertical supports have the X axis bearings to close together.

The problem with spreading out the X axis bearings of course is that you have to make the machine longer to achieve the same cutting area. I would look at this area closely and also make sure that the vertical supports for the gantry beam are stiff enough. Spacing the bearings as far apart as the gantry is high is a good rule of thumb.

my questions to the gods of CNC bilidng are:

• direct drive or belt driven? what kind of belt/width?
• will 60X60 extrusions do the job or should I opt for 80x80?
• will the 40x160 extrusion gantry be solid enough or should I go for 80X160?
• Is there a certain direction for the FK/FF supports? I can't see this referenced anywhere
• I cannot seem to decide if I should go for a 10 pitch or a 5pitch ballscrew (or if i need 20 vs 16 mm screws in the first place)
• air vs water cooled spindle? cant decide on this one either, prefer to keep it simple, but also quiet

1. It probably doesn't matter. Personally I prefer belt drives in most cases. The reason is that it allows more favorable motor positioning, eliminates the need for zero backlash couplings in favor of belts and pulleys and lastly allows you to select ratios other than 1:1. On the other hand direct drive stepper driven solutions work perfectly good in wood routers.

2. If money is no problem go larger. This is a fairly large machine but there are a number of factors to consider. For example will it have its own frame or sit on a bench?

3. Honestly I'd look for a steel beam for the gantry. Something around six to eight inches square. Yes it is a bit more work but that is offset by lower costs for the raw materials which by the way are better materials for a gantry beam. Remember you gantry beam is totally unsupported beyond the two uprights, it needs to be strong.

4. Ball screw supports by design have to support thrust in both directions so that isn't an issue. When designing your machine though it is always a good idea to consider how you will service the machine after it has been built. Sometimes designs that look good on paper are a real pain in assembly and service.

5. Well you make you decisions based on the best engineering solution that fits the budget. Almost all machines are compromises in design to hit a budget or other limitations. In your case you need to consider that max speed of the steppers (at a reasonable torque level) and the max feed rates and rapids you want to achieve. most likely you will want to the faster ball screws to get good feed rates for working in wood. This could compromise operations in aluminum though. However in this case, a lightweight machine, you will be limited in what you can do in aluminum anyways.

6. Water cooled is noise is an issue for you. This does however require additional hardware.

would love input on these questions/the design in general,
Jon

Further comments:
1 add more cross beams, probably 250mm centers
2 the vertical supports should be thicker, cast structures or built up tube type structures. You can of course go with really thick risers but that generally gets to be expensive. In any event don't try to implement something as thin as 15 mm.
3. don't go with extrusions believing that precise machining isn't required. Extrusions are't always flat enough to mount linear rials on and can be a huge pain when it comes to getting rails aligned properly. It isn't uncommon for people to punt and mount the rails to sheet material that are then mounted on the extrusions. Remember the better the rails you buy the more important it is to have them parallel and in the same plane as the case may be. Some orientations of the rails will make this easier, in your case you might want the linear rails on the X axis on top of the beams.
4. If you are going to use belts on any axis it sometimes makes for good practice to add idlers. The interesting thing with belts is that you can tune them by adjusting the tension. if there are long unsupported sections then you can have issues with the way the belts behave. You probably can't see this in wood but it can be noticeable in high machinery where there are high expectations of cut quality.
5. Your general arrangement of the machine isn't bad at all. Don't be afraid to experiment with the machine before you start cutting metal to build it, it is far better to explore in CAD then in metal (well to a point). My biggest concern is in the risers and the gantry beam of your current design.
6. Make sure you understand what your expectations are before you go too far with this design. It is a perfectly good machine for wood (once minor problems are addressed) and most plastics but it is going to be an extremely light duty aluminum machining router.

[jonnom]

Thanks for the great info Wizard!

I'm going to stay with Alu profiles for now as I prefer to have something modular that can grow and change if needed (might want to add some Z clearance at some point for a 4th axis). Also i re-thought the dimensions and decided it would make more sense to have it wide enough for full sheet, length can grow later if needed. The total machine size is now 1500 x 1350, gantry beam length is 1300, workable area is about 1250 x 1050.

I really would like to be able to also machine aluminium (obviously not expecting the same speed/quality as a proper vertical mill, that's the next project, but i do want to be able to machine 20mm sheet aluminium at reasonable quality, speed is not a huge issue as it is a hobby machine)
Ive reduced Z clearance to 150mm to help with regidity.
motors stay nema23 400oz stpeppers (left over from another project), 2.2Kw VFD water cooled spindle, 20mm Hiwin linear guides + wide bearing blocks, 1610 ballscrews.

The frame is now 80x120 + 40x80 cross sections, bearings have been set further apart, gantry beam is now 80x120 bolted (almost) directly to the bearings on the x-axis profiles to add rigidity (i hope), according to manufacturer with 500N force on the middle I should only see 0.02mm of deflection, I think this acceptable for my uses. Iv'e also added belt drives all round + idlers (is there a certain rule to idler placement in regards of the belt?) hoping to tuck the motors away as you suggested and to add some torque/speed adjusting room due to the 10 pitch ballscrew.

I was thinking of changing all the machined aluminium parts from 15mm to 6mm steel (iv'e modeled this on motor brackets for now) - this should reduce price a little and as they only serve the purpose of holding everything together it shouldnt change much, right?
The base plate of the machine will be a heavy 20mm thick quartz counter top surface to add weight to the base (it's free) - should add another 100kg to the base and a nice flat work area to work on, ill probably epoxyy in some threaded inserts somehow. haven't figured that out yet, might just bolt some ply on top or a proper alu t-slot machine base.

What if i fill the main cavities of my X + Y beams with epoxy granite and some steel rebar across? could this help increase rigidity/dampening or will it just make my gantry beam extra heavy with no real effect? it would add an extra 12 kg of EG to each of the 3 80x120 beams, i'm attaching an image of the profile.
Total wieght of the machine would probably be in the vicinity of 200kg - 250 kg.

would love additional feedback!

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