Need Opinions on Z Axis Design

[RealManBearPig]

Hello!

Electrical engineer here playing as a mechanical. I am in the process of designing a moving gantry CNC router. Currently, I am working on finalizing the Z axis design so I can get an idea of where the gantry's center of rotation will be with everything attached.

Z axis travel will be 170mm max
2.2kw 80mm spindle
Aluminum plates are 1/2" thick with vertical 1/4"x5/8" pieces added for rigidity (may change to steel to get the spindle closer to the gantry)
HGH15ca linear rails with dual carriages
SFU1610 Ball Screw (May change to 1210 if I can find a nice ground one on ebay)

I am using fusion 360 and unfortunately the FEA simulation is no longer included in the maker version (May bite the bullet soon and subscribe to the full version). In the mean time, I'd like some opinions on which of the following would be the most rigid for the spindle plate


Option 1: Spindle mount does not extend past linear rail carriages and is mounted high up on the spindle
Option 2: Spindle mount is halfway on/off the rail carriages and mounts in the middle of the spindle.
Option 3: Spindle mount is completely below the linear rail carriages and mounted towards the bottom of the spindle.

All three options have vertical "stiffeners" attached to the back of the motor plate although without the FEA I'm unsure how much rigidity they're actually adding.


Similar Threads:

• opinions on vacuum fixture design
• Plasma Table Design Opinions
• Design opinions needed please
• Need opinions on machine design
• New Design -80/20 - Opinions Welcome

[ardenum]

All of them should be sufficient for light work, but in all honesty all of them are weak.

What kind of equipment have you got to work with?

Can you process steel?

Best and cheapest would be to buy a steel or an aluminum square pipe for the spindle box. Steel are readily available and aluminum ones go up to 200x100x5mm, 500m length costs here about 40EUR. 5mm ain't much so you'd need to strengthen one side with a plate. Steel would be enough on its own.

Need more data.

[RealManBearPig]

I have your basic woodworking tools (drill press, miter saw, band saw, table saw, grinder, etc). I currently have access to a vertical mill at work but I'll be changing jobs in the next couple weeks so that will go away. I've been keeping my eye out for a used bench-top mill. I haven't been able to convince my lady to let me bring home a big vertical mill yet but I'm still trying lol.

I can cut and drill steel but milling will be a challenge. Once I have the machine built and the accuracy determined, I plan to use it to upgrade itself.

I plan to use the CNC on all types of wood (MDF->Hardwood) and the occasional piece of aluminum. Given the deviation in C7 ball screws, I'd be really happy with an accuracy of 0.01" or better. I do have a concept in mind but I need to work out the general Z Axis design first.

By spindle box are you referring to the spindle mount? I didn't realize the factory mount was a weak point. Is this because of the width of the area that attaches to the spindle? Do you have any examples of a design using square tubing?

[ardenum]

By spindle box are you referring to the spindle mount? I didn't realize the factory mount was a weak point. Is this because of the width of the area that attaches to the spindle? Do you have any examples of a design using square tubing?

spindle box or headstock is the assembly that mounts to the X or Z carriage and houses the spindle, so in your case the spindle mount+plate.

The weak point is the plate. Usually people reinforce it with side plates but FEA tells a story that sufficiently thick tubing is stiffer and usually cheaper. If it's a steel tube than hands down much stiffer. If you got a tube that has enough space inside for the spindle and has a thickness of >8mm then you'd have to pay at least double in aluminum to get the same stiffness with a C shape.

There are no examples that I know of in diy community. But there's plenty of examples in industrial machines. Google images for "double column machining center".

[awerby]

If these are things you're considering building from scratch using woodworking equipment, I'd suggest taking a different path, and using an automation actuator to accomplish the same objective with a lot less work. They often have high-quality rails and ball screws, all mounted orthogonally in a rigid box with dust exclusion. Even if you can't use the motor it comes with, it will often be cheaper than buying all the components separately. Here's an example on Ebay (not that I'm suggesting you buy this particular one): https://www.ebay.com/itm/31255534524...AAAOSwyhBcezSJ

[RealManBearPig]

The weak point is the plate. Usually people reinforce it with side plates but FEA tells a story that sufficiently thick tubing is stiffer and usually cheaper. If it's a steel tube than hands down much stiffer. If you got a tube that has enough space inside for the spindle and has a thickness of >8mm then you'd have to pay at least double in aluminum to get the same stiffness with a C shape.

There are no examples that I know of in diy community. But there's plenty of examples in industrial machines. Google images for "double column machining center".

Thats an interesting idea. I'm not sure how I would go about mounting the spindle inside the square tube while also keeping it rigid.

If these are things you're considering building from scratch using woodworking equipment, I'd suggest taking a different path, and using an automation actuator to accomplish the same objective with a lot less work. They often have high-quality rails and ball screws, all mounted orthogonally in a rigid box with dust exclusion. Even if you can't use the motor it comes with, it will often be cheaper than buying all the components separately. Here's an example on Ebay (not that I'm suggesting you buy this particular one): https://www.ebay.com/itm/31255534524...AAAOSwyhBcezSJ

Thanks for the suggestion. However, buying one like that would still require a spindle mount and a plate... I'm not concerned with my ability to build this. Theres is a good chance that I will pick up a small mill before I get started with the build.
If after designing the Z axis the price works out to be cheaper to buy one on ebay then I'll go that route. That just takes all the fun out of it.

[luke peter]

Do you have a clear idea of what you plan to cut? The stiffness budget of your machine should drive your design decisions - the components should each be roughly similar, at around 10x the final at the tool stiffness target for the complete assembly.

Stiffness of bearing rail assembly would be improved by increasing the bearing spacing.

My machine is slowly coming together, but the maths suggests that my z axis is the weakest assembly. It is a box fabricated from lasercut plates, 10mm for the back, and "swiss cheesed" 8mm plates for the front and sides (stolen PeteEng's terminology on that one). See below.

Thats a fairly chonky boi - but will still benefit from beefing up the back plate to 20mm since I am getting calculated local deflection in the back plate itself.'

Its hard to tell what "stiff enough" is for your build without seeing the rest - I started with the desire to make good cuts in aluminium, and very slow/light cuts in steel as my design point, and targeted between 6ish and 10N/um of stiffness as a result.

I don't think you will have any issues under typical wood router loads with any of your intended designs (for what its worth).

[pippin88]

Don't forget tool length. Often people model the spindle and forget there is something actually sticking out from the spindle nose.

The rails on gantry carriage vs rails on Z plate debate usually forgets about the fact you actually have a tool with length.

I have rails on the gantry carriage, because this allows me to move the spindle up above the gantry.
This keeps the gantry as low as possible.

With rails on the Z plate it is likely stiffer with Z all the way up, if you only analyse the Z axis. However to get the same functional travel, the gantry must be higher which compromises stiffness of the whole gantry.

Z travel needs to be:
Longest tool bit you want to use plus clearance to get over the thickest part you want to cut.
E.g. 100mm long tool, wanting to cut full 100mm thick part. Z travel must be >200mm. Gantry clearnance must be >100mm.
Consider all the shortest tool you will want to use and whether you can reach the spoilboard / fixture (though it is often possible to use spacers to raise workpieces up if needed).