recommendations for a quality 6040 CNC machine

[glenenglish]

I've got a 3040, and a 3020  from 13-15 years ago ,
Built really solid, straight, square ! no backlash, excellent quality, I run with a 18k RPM water motor and Planet CNC controllers
It is mainly large aluminium block and bar and steel with std extrusion deck.  heavy.
But
In the past 10 years I bought  2x  6040 machine chassis.
Both were built of types of extrusions, rubbish ball screws and carriages, rubbish bearings. Junk.
They weigh about the same as the old heavy 3040.

Has anyone got suggestions of a source for a higher quality end of the scale 6040 chassis ?
Happy to spend money...
Does not matter if it comes with some chinese copy controller or not, I'll probably just toss it.
I need a large deck surface area for light duty sheet ally.

thanks
glen

[Doctorandus_P]

If you're happy to spend the money, then buy a Datron. They don't list prices, and I guess you it will make you really happy to get rid of your money in this way. 

Sorotec (from Germany) also makes some nice routers, although I would avoid their cheapest offerings, and go with at least two linear bearings per rails. You can expect to pay around EUR3000 or 4000 for a Sorotec, but that's exclusive motors and electronics.

FS3MG is also a nice machine, and sold via Ebay. Gotteswinter has one (or an FS4MG) as an upgrade from his Sorotec.

Another machine worth mentioning is the RatRig Stronghold Pro. It similar to the "improved" Shapeoko and X-Carve machines, but the "Pro" version has ball spindles.

For the Chinese machines. There is a lot of garbage, but there are also some decent ones. The absolute minimum I would consider for myself will be with rectangular rails which are supported over their full length. There are some with blue anodized parts that conform to this rule. And you can buy such a machine (a small one) for less then EUR2000. But in general, you get what you pay for.

And DIY is also an option, especially if you already got such a router. One model I particularly like is in the attached picture. I found it on instructables and it claims to cost EUR500, (but that is probably for the material only, and not for time, toolwear, etc). This model is mostly a clone from a Sorotec. I suspect that even made from wood it will perform decently if you stiffen up the frame by adding some steel beams in important locations. (The Sorotec version has a thick aluminium plate on the back side of the gantry to stiffen it up.

Also, flat ground aluminum tooling plate is easily available (in the EU, I guess GB too) And it costs around EUR10 to EUR15 per kg. Designing and building one yourself is a valid option, but it depends a lot on:

* You skills.
* The value of your time.
* Whether you consider it fun or a nuisance.
* Whether you manage to buy something in your price & quality range.
* Moon phases and planet alignment.

With DIY you also have a lot of options. Do you want "cheap chines rails" or better quality (Hiwin, SKF, INA, etc) There are also huge differences in prices and qualities of ball spindles (even from china). A rolled 1605 spindle can cost around EUR50 while a ground C3 tolerance ball spindle of the same size can easily be EUR300 or more.

[glenenglish]

thanks for the comments

[Doctorandus_P]

That is a bit of a short answer.
I gave some examples with different price / quality / DIY / precision tradeoffs. If you want better advice, you have to narrow down this range of compromises first. There also appear local manufacturers to be sprinkled around the world. but they may be a bit hard to find. Is there anything near you (and is that important to you?)

[glenenglish]

no other important items.  Certainly rails rather than rods for bearings, specially for large machines.
This company has received good reviews from like minded critical hobbyists :

https://www.omiocnc.com/products/x6-2200l-usb.html

they have a range of platforms. Sorry I wont consider anything inside western Europe, all prices out of this world from any other continental point of view. I think Europe can only compete with Europe...

[Doctorandus_P]

Don't blame the Europeans. GB let itself being mislead into Brexit by a handful of shrewd weazels. If you as me half the world is going to hell because of a small number of manipulative bastards who manage to "convince" a majority of idiot voters, and sensible people are turned into a minority, but that's enough (too much already) of politics.

----------------
The machine you mentioned is a quite common class. It is around the minimum that I would find decent enough to actually consider buying. And there are many similar machines from China. Look at this one is for example:

https://www.aliexpress.com/item/1005006477030793.htm

(It's also similar to the version with the "blue anodized frame" I mentioned earlier). And they are one or two classes "lighter weight" then the Sorotec or FS3MG.

If you go for such a machine, then consider a few details. First, I suspect the ball spindle has regular deep groove bearings. Replacing them with angular contact bearings may be advantageous. A second thing is that I don't understand the Y-axis rails. They are always "facing upwards", while the machine would be better if the rails itself was mounted to  the table. This would stiffen up the table, and the whole bride would be connected directly to the table and this reduces relative Z-movement between them. Rotating those rails 90 degrees such as in the "wooden router" (Sorotec clone) is probably even better. It makes the table itself quite thick, and thus easier to make it stiff. And of course stiffen up the back side of the gantry.

But before modifications, start by using it "as is". It would be nice to know how much modifications are actual improvements

[Infraviolet]

"Certainly rails rather than rods for bearings"
You mean the HGR* style "square" rails with carriages rather than supported SBR* linear guides?
Is there a reason behind this, I'm interested to understand it. I know unsupported rods are vastly inferior, as the rod can flex in the middle a bit and thereby ruin the rigidity. But for a given size aren't SBR* rails usually somewhat beefier and less flexing prone than the rather slender looking HGR* square rails? And SBR* , with the C shaped cross-section carriages, less likely to bind up if there's ever so slight a misalignment?

[glenenglish]

So, FWIW  my old heavy 3040 has 20mm round bearing rods, and also the newer 6040 machines I have also have 20mm round bearing rods, not sure of the ID of these.
longest axis carriage lengths are the same on both 6040 and 3040 (110mm) 

[Doctorandus_P]

Th square rails have grooves for the balls ground into the rails, and with that you get convex balls in concave grooves and this generates a line contact. With the round rails you get convex balls on a convex rod and this is a point contact. As a result the round rails have a  lower load capacity for the same size.

I tried to get some numbers, as I've collected some tables, SBR20UU (Unknown brand) has a Dynamic / Static load bearing of 882N / 1370N and even an MGN9C (Hiwin) has 1860N / 2550N Dynamic / Static load bearing. (Note that MGN series only has two rows of balls. If you go to HGH20CA the load bearing becomes: 17.7kN / 37.883kN

Other advantages of the rectangular rails is that they are more compact (Which gives more choice in mechanical construction) and the mounting holes are closer together, which allows for better mounting of the rails and a stiffer overall construction.

I have once seen SBR12 rails, and those are barely usable at all. There is hardly enough space for the bottom rows of balls to wrap around the axle (for the supported axle version), and they almost fall of by themselves. I have bought SBR16 rails myself (quite some years ago) and they are OK-ish but I would never buy them again. There was also a height difference in the aluminium extrusions which caused problems with assembly. About the only advantage of SBR rails is better wipers because of the simple smooth construction. Lot's of wood working machines (even with square rails) have their rails exposed and apparently it is not very much of an issue. The rails do need maintenance though. You should put oil into the bearing blocks on a regular basis. Not only to prevent them from running dry, but also to flush out and remove contaminants.

Also if you look at the performance to price ratio, then MGN12H is about the same price as SBR20, but much stronger. Even if you do not need the strength itself, it still translates in more stiffness and longer life time (because of lighter relative load). Also beware of the difference between MGN12C and MGN12H. The "H" carriages are a bit longer (They also have a better performance to price ratio). I would only buy the short carriages if there really is no room for the longer ones. (Price difference is very small). Also note it is bad practice to mount the carriages right next to each other. You should have at least about half the carriage length between the carriages. The reason is to have some adjustment for production tolerances. There is always some (micro meter level) difference in height and this can (and will) be a cause for jamming if the rest of the construction is stiff (If you make it of wood, then the wood will just bend a bit).

If you are into DIY, then the complete sets are very attractive. You can get a complete set for 3 axis (400 / 700 / 1000mm) for around EUR350. Such a set has:
* 2x HG rails of each of those lengths.
* 12 HGH20CA Carriages.
* 3 ball spindles size 1605.
* BK12 and SK12 bearing blocks for the ball spindles. (Probably the cheap versions, consider to put angular contact bearings in them).

There are also different sets. I've also seen them with 400 / 1000 / 1500mm rails length and I've seen them with 25mm rails.

This is (of course) all relatively cheap Chinese stuff. It's nice to get started with, and for a simple machine it's "good enough". If you're planning to build a really nice CNC machine (such as the VMC from Stef van Itterzon) Then Genuine Hiwin or better makes more sense (Such a machine will be quite expensive to build anyway). But the rails are interchangeable. For DIY it's perfectly all right to start at a low cost, and when your machine turns out to be used a lot and the rails wear out prematurely, you can interchange them for better quality rails. When you use your machine a lot, the higher cost of better quality parts is also not such an issue as for a curious starter.

[Doctorandus_P]

After reading your post again:

I know unsupported rods are vastly inferior, as the rod can flex in the middle a bit and thereby ruin the rigidity. But for a given size aren't SBR* rails usually somewhat beefier and less flexing prone than the rather slender looking HGR* square rails?

No. The HGR rails themselves are indeed "thin and flexible", but they are just the rails. They have a lot of mounting holes and they should always be mounted on a strong and stiff surface that gives strength to the total construction.

And SBR* , with the C shaped cross-section carriages, less likely to bind up if there's ever so slight a misalignment?

No. Both versions are very sensitive for misalignment. The SBR can rotate a bit around around the round axle, but they are (at least) just as sensitive to misalignment  in other directions. Also, if your rails have a high load capacitance, then they may overcome misalignment by just bending the construction a bit. If the load capacitance is very low, then it is very easy to overload and damage the bearings due to overload caused by misalignment. I have heard several complaints of "cheap chinese stuff" breaking and wearing out quickly. I suspect that most of those are just mounting and alignment errors. In general, Aluminium extrusion profiles are not straight enough to directly mount such rails on. Often it works, but you have to be careful. Your rails may need shimming to make it possible to mount them properly.

And another note:
You should always take apart the chinese carriages, wash them with a solvent and re grease them. They (nearly) always have grinding grit and shavings in them that makes them run more rough and cause extra wear.

[glenenglish]

"The HGR rails themselves are indeed "thin and flexible", but they are just the rails. "

But they are never run without being screwed down to a supporting beam.