[suspension]

hi everyone,

I an creating this thread to get some advice from experts in this forum for the selection of various parts for the new CNC build that I am planning to start for pro-hobby and basic professional work. The design is heavily based on following work:


I've started the design of base using Fusion 360. See the attachment.


In order to finish the cad model for the base, I need to select servos, ball screw and related components like end supports, couplings.
Since this is mostly for hobby type work, the amount I can spend is limited, but it does not need to be super cheap. (I already have a 6040 CNC and the reason I am making this machine is to have better stability and accuracy).

Some questions that I currently have regarding above selections are:

1. Which servos are more suited to this? AC or DC servos? The total weight of the machine would be around 800KG at the end.
2. In this thread, JMC brand is recommended for servos for this build. I think they are reasonably priced and also seem to have good quality. I also checked DMM and their pricing is too high. Also checked Delta servos and those are also too expensive. Apart from JMC, I would like to see if there are any other Chineese good quality brands with reasonable prices. Also what are your thoughts on various servo motor options listed in AliExpress? Such as this one.
3. Which accuracy level is required for ballscrews of this machine? C3? C5? C7? And which manufacturing process is more suited (ground screws, rolled screws, etc)? Which nut (internal circulation, etc)? Preloaded or not? If preloaded which type? any specific recommendations for fixed and floating end supports?
4. Based on above what are some of my options for brands and suppliers for ball screws? I have already looked at brands like tbimotion and terry-machinery. Hiwin seem to be too expensive. (However I will be using Hiwin for linear rails/guides). What about various suppliers/brands from Aliexpress? For example this one?

I will be having more questions on base later in this same thread (for example on how to machine various critical surfaces, how to make the mould, etc), but above are most important at the moment.

Any help is really appreciated.

Thank you
S

[ardenum2]

I've read a few things about those drive integrated jmc's and people were complaining about lack of proper manual and tuning issues.

[suspension]

I've read a few things about those drive integrated jmc's and people were complaining about lack of proper manual and tuning issues.

I think the quotation they gave me refers to this motor instead which does not have an integrated driver.
Not sure if tuning issues are same with this motor and compatible driver (JASD4002-20B).

[suspension]

I have changed the base a bit.

[ardenum2]

there's not much difference between iso30 and bt30, bt30 can also be used without the 2 dongs. Here's a few motorized spindles:

iso20 toolholder https://www.aliexpress.com/item/1005004798512348.html

bt30 toolholder https://www.aliexpress.com/item/1005004206523750.html

here's a real machining center spindle it costs $4500 without tax and shipping, it might be too big for the machine though, refer to the cad file.

https://ctbservo.com/product/motoriz...000rpm-140n-m/

[catahoula]

I went with c5 ground ballscrews, preloaded double nut TBI brand from BST Motion (aliexpress). C3 are quite expensive. C7 (rolled) aren't really precision enough for preload. C5 seems appropriate to me with ballscrew mapping. For preload, the seller made a recommendation that i followed (i think it was a lighter preload than I would've ordered otherwise) and I'm glad I did- they're plenty tight and being tighter might create a lot of heat etc. of course, the tighter preload might last longer, maybe i'll find that out in a couple years.

[ardenum2]

the tighter preload might last longer, maybe i'll find that out in a couple years.

really? I was under the impression the higher the preload the less life the ballscrew/ball bearings will have.

[catahoula]

yeah i would generally agree with you, but i'm also considering the possibility that since the preload will loosen over time on all ballscrews, maybe you get more time at zero backlash with a tighter preload, even if it means the overall life is less. just hypothesizing here

spent years as a bicycle mechanic and saw many an over-tightened hub that survived a very long time, fwiw

[joeavaerage]

HI,

I would like to see if there are any other Chineese good quality brands with reasonable prices.

There are plenty of cheap Chinese servos, and most of them to be fair are 'fair to even good quality'. Its not that I have an issue with cheap Chinese servos but all of them
miss the setup and tuning software. That's the issue. There are hundreds of parameters that need to be set in an AC servo, while many can be left at default values
there are many that cannot. Do you know what those parameters are? Will you be able to decipher the Chinglish manuals?

I recommended Delta and DMM not because they are the cheapest but because they are the 'cheapest of the quality Chinese made brands that have setup and tuning software.
If you want expensive try Yaskawa or Seimens....they'll make you weep!.

The machine in the video link you posted uses 750W (80mm frame size) Delta servos, the identical units that I use, including the electromagnetic braked servo for the Z axis.

The company whom supplied these servos to me (China) have their own brand called ToAuto. I have some history with this company, all positive. I do not have any of the ToAuto servos
but would be happy to buy them on the basis that I feel I have the support of a good supplier. Given also that I have had some exposure to AC servos including Delta, Allen Bradley, Omron,
( a Yaskawa clone), Mitsubishi and Lexium (Schneider Electric) I would be happy enough to program a ToAuto but I still pay an extra $196 to get genuine Delta servos. I vote with my wallet
and pay the extra to get the quality I want and the setup and tuning software to match.

https://www.fasttobuy.com/nema-32-cn...he_p35883.html

Craig

[suspension]

there's not much difference between iso30 and bt30, bt30 can also be used without the 2 dongs. Here's a few motorized spindles:

iso20 toolholder https://www.aliexpress.com/item/1005004798512348.html

bt30 toolholder https://www.aliexpress.com/item/1005004206523750.html

here's a real machining center spindle it costs $4500 without tax and shipping, it might be too big for the machine though, refer to the cad file.

https://ctbservo.com/product/motoriz...000rpm-140n-m/

Using BT30 without dongs may not be appropriate if I were to do some steel milling?

[suspension]

Given also that I have had some exposure to AC servos including Delta, Allen Bradley, Omron,
( a Yaskawa clone), Mitsubishi and Lexium (Schneider Electric) I would be happy enough to program a ToAuto but I still pay an extra $196 to get genuine Delta servos.

Craig

May I know how you arrived at $196? Because I am also prepared to pay that much extra, but a brief glance across prices shows me that ToAuto 750W motor + driver + cables are priced at 242 while for delta its 648 (B2 series).
Th JMC on the other hand quoted 152 for 400W version.

Also, could I please know which type of servo matches CNC machines better? AC or DC?

I also see they offer software for tuning:
https://www.jmc-motor.com/oftware.html

[joeavaerage]

Hi,
the price for a 750W Delta B2 series servo kit from Fast-to-Buy is $438USD, while the ToAuto 750W is $242USD, difference $196USD

https://www.fasttobuy.com/flange-80m...er_p28084.html

I have bought three of these servos in the last two months, two for me (fourth and fifth axes) and one for a friend. All have been $438USD. Shipping to New Zealand has varied from $150USD (3 day Fedex)
to $156USD (3 day Fedex). The record time is 2 days 19 hours and 47 minutes from pick-up ( Hong Kong) to delivery (Christchurch NZ), and the slowest time was four days 5 hours 20 minutes.

Also, could I please know which type of servo matches CNC machines better? AC or DC?

DC servos are going the way of the dinosaur. They are still quite capable of good performance but modern AC servos are just that much better and way WAY more flexible/programmable.

Its your money and therefore your choice. I too made my choice and it was Delta B2 series.

Craig

[ardenum2]

Using BT30 without dongs may not be appropriate if I were to do some steel milling?

not a problem, steel depends on the spindle and stiffness of the frame, bt30 without dongs will work just fine as long as there's enough clamping force in the drawbar, there's a chance you could swap the spring discs inside for more clamping force.

You said your machine is smaller than stefs, a c frame at this size might not be stiff enough, but it all comes down to the spindle. If you want to use that proper spindle for $4500 than you probably need to buff up the frame or use a slightly different one. Have to do some structural to make sure how much your frame deflects when under load during steel cutting.

with a router spindle like the first two links, you could be cutting steel but in a different way, check dental mill videos for how it looks like.

Are you sure about 400W and not 600W or 750W?

[suspension]

Would getting rid if the C frame be enough to host a proper BTR30 spindle with a servo?
I will check the dental mills.
If I were to use BT30 driven by a servo instead of a router ATC that use ISO30, what additional benefits do I get? Such as tapering threads?

Regarding power of the servo, I see that there is no much of a difference in price, so for 3 axis I guess 600W or 750W is also ok. But if the price gap is substantial, I would like to stick with 400W if that's possible.

[ardenum2]

Would getting rid if the C frame be enough to host a proper BTR30 spindle with a servo?
I will check the dental mills.
If I were to use BT30 driven by a servo instead of a router ATC that use ISO30, what additional benefits do I get? Such as tapering threads?

Regarding power of the servo, I see that there is no much of a difference in price, so for 3 axis I guess 600W or 750W is also ok. But if the price gap is substantial, I would like to stick with 400W if that's possible.

for a bt30+servo a c frame is probably the best but needs to be beefy to take advantage of it. If you go down this route high speed machining is probably off the table, these bt30 cartridges usually run at max 12000rpm, most cap out at 6000rpm due to the bearings, which is why you need more stiffness to support heavier cuts.

vulcanmachineco(attached) makes a belt driven bt30 but not an a c frame, its a double column raising gantry, the stiffest we in the diy can hope to ever get, but to replicate it in EG could be tricky, you'd probably have to redesign the saddle instead of resizing it and figure out how to fit bt30+servo onto it.

[joeavaerage]

Hi,

Regarding power of the servo, I see that there is no much of a difference in price, so for 3 axis I guess 600W or 750W is also ok. But if the price gap is substantial, I would like to stick with 400W if that's possible.

When I was initially designing my machine I had intended to stick with 400W servos also...but I must confess without any epsecially good reason, size, convenience, cost all seemed to indicate 400W.
When I came to buy them however I found that a 400W Delta B2 was $398 and a 750W B2 was $438....a mere $40 more...so I got the bigger ones.....why would you not?

My first calculations did not include rotational inertia...and that lead me to the conclusion that even 400W would have been an overkill. Once I realised that my calculations were deficient and I re-did them
but now including rotational inertia I realised that 750W was good enough....but I would have been severely disappointed with 400W.

To be honest you are a long way from deciding on the power of the servos. The actual parameter you need to decide is the rated torque of the servo, and that in turn will be ditctated by the pitch
but most importantly by the diameter of the ballscrews. Rotational inertia of a ballscrew is proportional to the fourth power of diameter, ie very VERY sensitive indeed to ballscrew diameter.

A 400W servo (1.27Nm rated) might be fine for a 20mm or even a 25mm ballscrew but would be hoplessly underdone on a 32mm ballscrew.

Once you firm up your design with the diameter and length of the ballscrews THEN do the calculation and determine the required torque to meet your performance goal. That is what determines the power
of the servo.....not some heuristic guess.

Craig

[suspension]

Thanks for the calculation Craig, really helpful. I wonder if there is any online tool or software that lets me do these calculations by myself.
As per the ball-screw, based on Stef's mill which is much bigger than this, I suspect I will ever need 32mm. In fact I was thinking about using 16 mm or 20 mm. I think I need some advice there as well.
In addition to the ball screw diameter, would the pre-loading has a significant effect on the torque?

[joeavaerage]

Hi,
the minimum size of the ballscrew is that size required to preclude 'whipping' where the ballscrew spins that fast that it start getting out of line. There are some calculations floating around
that attempt to quantify it, but generally speaking the longer the ballscrew the bigger its diameter must be in order to avoid whipping. A related matter is the pitch of the screw. A fine pitch screw
will have to spin fast to meet your max velocity requirement and be more prone to whipping than a coarse pitch screw. The downside is that a coarse pitch screw requires more torque to achieve
the same acceleration and cutting force.

The maximum size ballscrew is that you can afford, and afford to buy a big enough servo to accelerate it. I've seen some guys thinking 'hey I'll get 36mm or 40mm ballscrews because they're so stiff,
only to find they need a 3 kW servo to spin them.

I have taken the approach that a fine pitch screw maybe a little slower but have greater cutting force authority......and so I have gone for 32mm diameter 5mm pitch screws. They are 648mm long
and will not whip at any realistic speed. In fact I spin them up to 5000rpm which give me a rapid traverse of 25m/min. The 32mm diameter means that they are stiff and accurate but require
decent servos to spin them, again I selected 750W.

Others favor course pitched screws, say 10mm or even 20mm. They argue that the screw need spin very much slower to get the required speed, and gearing could be used at need to increase
the torque of the servo to offset the greater torque demand for cutting forces and acceleration.

It is somewhat a matter of style.

The essential point is that the servo, any gearing, and the ballscrew must match the machine. If you think servos are expensive then you have not checked out the ground C5 and C3 grade screws.
One of the other contributors to this thread is of the opinion that some of the cheap Chinese made screws advertised as C3 and C5 grade while being less than a tenth of the price of the industry leaders like
THK, NSK, Bosch Rexroth etc. It is very seductive to believe that such high quality screws can be had at such bargain prices....but I disbelieve, I think is a case of the Chinese lying yet again.

Bear in mind that quality ballscrews are the absolute heart of any linear motion control system like a CNC. The better the ballscrews the better the result, and C5 and C3 screws (genuine) are worth way WAY more
than the servo that drives them. May I suggest you start doing your research on ballscrews now....they are commonly the most expensive piece of the whole puzzle.

C7 screws are rolled and have a linear accuracy of 50um/300mm. That sounds good but they can and do have cyclic error, and is uncontrolled but can easily exceed 35um/rev. If you are targeting 0.1mm
accuracy then these are ok. If you want 0.01mm accuracy they will not be up to it.

C5 and C3 are, and traditionally have been, made by grinding, and are mega expensive. A C5 screw is about 18um/300mm, which is better, but the real kicker is the cyclic which is 8um/rev.
C3's are better again, about 12um/300mm and 6um/rev. So both C5's and C3's are desired and required if you want accuracies of 0.01mm.

If a C7 screw, say 25mm diameter and 1000mm long is $100, a C5 25mm diameter and 1000mm long from THK or NSK will be over $1000, and C3's even more again.
If we as hobbyists could find C5 and C3 screws we could make much better machines, and I would advise caution before believing the advertising hype. Quality costs. BS is cheap.

I got my screws (ground C5's double nut, called BNFN by THK, 32mm diameter, 648mm long, 5mm pitch with two FK25 support bearings) secondhand from Korea. When I got them they still had the
original test certs....like they'd never been used, although they were advertised as second hand. I got three sets (screws/nuts/supports) for $1000USD including shipping to New Zealand.
Had I bought new they would have cost in excess of $2500 each. In short I could not have afforded to build my machine WITHOUT being able to get these absolutely superb screws at such a price.
I got new old stock THK linear rails and cars from the same company.

Once I secured these components (ballscrews and linear rails) I then designed my machine. I designed it to take advantage of what I could get second hand......I did not design my machine and then
try to find the really expensive and hard to find bits, I did the reverse.

https://www.ebay.com/str/industrialp....m47492.l74602

This place 'would put a horn on a jelly fish'.

Craig

[suspension]

Hi Craig,

Thanks again for very valuable and clear information.
Based on stef's selection (20mm for X/Y and 25mm for Z) and considering the smaller size of my machine, I am thinking of 15/16 mm for X/Y and 20 for Z. This is not backed by any calculation.
For pitch, I think I am ok with 5 or 10. I'd try to use the one which gives me a cost benefit.

As per vendors, I checked the ebay seller. I think he stocks a very good collection of items and his prices are very fair assuming the accuracy/backslash are accurately specified. Only for certain items he specify the accuracy class however.
In addition, I was researching TBI motion (I think they are Taiwanese) and gives pretty good accuracy of 18/3000 for Rolled C5 at 46USD per meter. (https://www.tuli-shop.com/tbi-motion...-scr-1604-f-c5).
C3 is much more expensive at around 193 per meter. (https://www.tuli-shop.com/tbi-motion...-scr-0801-g-c3).
I have seen elsewhere in the forum someone recommending this brand. The total cost will be higher when end machining, bearings, ball-nut are added however.

Based on above two, I certainly prefer the e-bay seller for used ball screws as they are of good known brands, seller has 100% rating and will have similar cost to TBI for example when bearings/nut/end machining included. The way I understand, the linear accuracy does not change with use? I will need to make sure that backslash is low (perhaps ask the seller?), there are no major cosmetic issues before buying any used one. Any other things that I need to be concerned about?

Thanks
Sus

[peteeng]

Hi Sus - Hiwin publish the equations for screw vibration limits in their resources section. I've attached my motor design spreadsheet for you. TBI is a good medium/high value brand. Consider a preloaded screw as well.

You have not stated the size of your machine but a mill usually is small enough not to have vibration issues. But it can have screw buckling issues due to the smaller pitches used hence larger compression forces. Look at Hiwins calcs they cover buckling, vibration and forces generated...Peter

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