Stepper porn?

[sansbury]

I was just reading a message where a guy said that 150oz motors were "absolute minimum" for a Sherline and that up to 250oz were still in the "sweet spot." I am genuinely curious about why people seem to over-spec steppers so heavily, maybe I am missing something?

I've been building CNC machines for about a year now and am on #3, an X1 conversion, after building two router tables, one for a Dremel, and one a Porter Cable. All three have been run using the same 125oz unipolar motors and a 24V Hobby CNC setup. The larger router used 1/2-10 Acme and could reliably jog at 90IPM, and generates enough sideforce to stall the router. The X1 will move the X/Y axes at 14ipm at least, which is quite tolerable on a machine this size.

I have seen people putting 425oz motors on these mills which has me completely flabbergasted. I suppose you could put brass gibs in and tighten them up like the heads on your car and gain something that way, but more oft I suspect people over-spec steppers because they think that if 125oz will work, then 425oz will be four times better.

Granted, the price difference if buying new steppers is pretty small and you can go from 125 to 300 for $10 or so per motor. So, why not?

Well, one thing, which some may think is silly, is that on my X1, those puny motors are just strong enough to cut about as hard as the machine should. Which means that, when (not if) the operator (that's me!) crashes it, the first thing to give will be the motors. While I don't count on this as a safety mechanism, it has prevented me from so much as stripping the infamous "cheese gears" such as when I mistakenly sent a .250" endmill straight into the side of a .750" block of Al at 12ipm. Oops!

At some point things start to add up. Heavier motors require more work to support, and perhaps force you to use more costly drivers to operate. But the real cost I think comes from the lazy thinking that leads to over-specing. There are times and places for servos, ball screws, and many more costly things, and if your budget is open-ended, then by all means do your part to keep the American industrial worker employed.

Anyway, I looked on Sherline's site, and it seems like they sell their mills with 136oz steppers. Are they under-powering their machines?

/rant

[Stepper Monkey]

It's a complex issue, but I would agree with you that overspecing motors is a serious problem. It is something discussed on several threads in the past.

With proper drivers and voltages and a well-matched motor 100/oz can be easily all that is needed for a machine that size, I run at that end of the range myself, but that is the exception for people running them now. Many setups are inefficient enough they couldn't get their machine to even move reliably with one that size. Hopefully as our hobby gets bigger we will find better and more sophisticated drivers will become more available and cheaper, and knowledge about them little more common. Frankly, the complexity of the subject intimidates most people, and even for those that it doesn't budget and availability limit the choices.

That being said, I'm also the guy who suggested 125-250/oz motors for the Sherline, too. I was oversimplifying for a newbie, but it mainly still comes down to the drivers, and with the types of relatively unsophisticated drivers and lower voltages commonly used by hobbyists on small machines that whole range generically works. In fact, I mentioned that range to hopefully stop people from putting BIGGER ones on which is unfortunately the trend. I'd just be happy if I never saw another 496/oz motor-equipped mill driven by a Xylotex and a 12v computer supply, with the owner convinced that the weak performance of the motors means building adapters to go up to stronger Nema 34's!

To get where you want to go, it would take folks being willing to understand the complexities of what makes the right motor for a machine/intended job/driver/supply/motor combination, which is not trivial or a small number of equations. They also have to fight the Tim Allen effect, and a lot of FUD and misinformation from salespeople first. Then they would have to be able to afford the solution. We have a ways to go yet, but it's getting there.

[sansbury]

Originally Posted by Stepper Monkey That being said, I'm also the guy who suggested 125-250/oz motors for the Sherline, too.... In fact, I mentioned that range to hopefully stop people from putting BIGGER ones on which is unfortunately the trend.

LOL! That actually does make sense in a perverted, upside-down sort of way.

The most irritating part of it is that it doesn't even do you any good. Once you have sufficient torque to move the table at the desired rate, plus a little headroom for the leprechauns, adding more produces no measurable gains.

There is a whole other issue your post raises which is the unreliability of some specs. As new as this hobby still is, you have vendors who don't know what they're selling marketing to customers who don't know what they're buying.

Come to think of it, perhaps you and I should get together and sell NEMA34 motor adapters for Sherline and Taig mills? We could probably corner the market. I got into this hobby by way of DIY audio equipment (long story) and we could use vaguely-understood terms like "superior dampening" or "improved regulation" to describe the advantages.

Of course we'd also have to come up with a good conspiracy theory that explains why the manufacturers don't sell those size motors themselves. Perhaps something about how the superior dampening provided by the larger coil size in the motors reveals resonance issues in the mill due to design flaws which can only be resolved by adding some other doohickey we also sell. To cap it off we could have a youtube video showing the axes being jogged at 300ipm. Don't you wish yours could do this too? Call now!

[The Blight]

I only have one mill (X2), and I'm using 495oz-in motors on it. With the gibs as tight as I have them, I would have to say that these motors are perfect. Any tighter, and the motors will stall. For most people this is a bit of an overkill, but I wanted the machine to be as rigid as I could get it, so I went with the biggest I could find in the nema23 size. They only cost a couple $ extra, so I didn't really care. I also planned on using the same motors on a bigger machine later on.

I also have a scratch built CNC Lathe which is using 250oz-in motors, because I'm using linear rails on all axis. It has plenty of power to run the machine, and I could probably have lived with smaller motors, but at the time, these were the only ones they had in stock.

For my next project (PCB router), I'm using 80oz-in motors. These have plenty of power. I was looking at using some tiny 14oz-in linear stepper motors, but because of the lack of speed I ended up using some bigger ones.

One factor which have a lot to say when it comes to selecting power is the pitch of the screws you are using and the type.

Maybe we should create a sticky with recommended stepper motor sizes and power supplies for different types of machines. This question pops up from time to time, and most times they never get a reply. The only problem with this is if someone takes our advice and ends up with a motor too small for their application. Might be better to go with the overkill and suggest 300..

[Stepper Monkey]

I kind of like the idea of sticky - you could do a grid with common machine/driver combos and one or two recommended motor types or ranges of inductance for different common uses we put them to. It would still be pretty oversimplified but it would be a lot better than nothing!
For those that want to take it further, maybe add some basic rules of thumb for how sizing is determined, as there are almost an infinite variety of motor types with a lot of different performance curves that people come across surplus they might like to know how to use.
I'd be perfectly willing to help out making one, and could probably even do it passably well if I have to, but I know there are minds here far more practiced and familiar with the subtleties of the subject that would be much better at it. Any takers for making a sticky? At least to help lay out the initial equations?

[blades]

My motors are in the 100 oz range, and are just a touch underpowered, IMHO. I can't ride the brake without the motor losing steps. Lost steps can = scrap, so I can see the merit in larger motors, especially ones that provide enough torque to allow some drag on the gibbs. Mine is a very simple and budget oriented conversion, so I have no immediate plans for larger motors........ yet.

[Riceburner98]

I can admit to being guilty of this... Basically being a newbie, and having my newly-bought Taig continuously stall until I loosened things up so much that I had really bad backlash / slop in the axis. Started with whatever 125oz motors came with it, then bought the 269oz ones Xylotex sells, and that let me tighten things up some. The best (and correct) way to fix it probably would have been to sit down and spend the time lapping all the ways and getting it perfectly adjusted, but at the time I didn't know any better and just figured "bigger is better". At least I didn't get the 400+oz ones. The next machine in the works is going to use linear slides w/ 5tpi screws, and I'm likely going to swap the 269oz motors in there unless there's a good reason to use the 125oz ones? Guess I could try both and see what happens..

[sergizmo]

As an owner of a CNC Sherline, I agree completely with the OP. I got it direct from Sherline with the 136 oz-in motors. Rapids are set at 24, which is plenty for a machine this size. The whole mill weighs 45 pounds (well, 69 since I bolted it to a 12 by 9 granite plate, but that doesn't really count), has 1/4-20 leadscres on XY and 3/8-20 on Z, and is made almost entirely of aluminum except for the column which is steel. Although it is steel, it isn't all that beefy and heavy cuts will flex it I'm sure. There is no way the steppers are the limiting factor or weakest link. The machine is simply not that strong, and bigger steppers could only cause further damage if a crash occurs or the limits are hit.

What is even more astounding is people who put servo setups on a Sherline (I've seen a couple videos). That's like a Formula 1 engine in a Civic. Do they want to fold it in half?!?!?!?

The Taig is a little beefier and has 1/2-20 leadscrews. It could probably use something in the 150-170 range, but much above that would be overkill as well.

Serge

[Horsedorf]

Sure seems to be a lot of knowlege on this thread It'd be nice to have a little vb program where you could click on what features you intend to have/use and what kind of cuts and rapids you are going for and having it vomit up an oz-in rating for a stepper... Eh, maybe some day.. though I do wonder just what information would be needed for a program like that.. like..

acme vs ballscrews.
ground ways vs milled ways vs Linear slides/rails..
Media being cut?
max depth of cut machine can (should be able to do)?
Would max rapids (desired/obtainable) be an input? or should it be an output to such a program?
Does it even make sense to have a program like that? It SEEMS like it since I see the use of the phrase "rule of thumb" used a lot.

[The Blight]

There are a lot of variables to take into consideration. I don't know much about VB, but I can make a spreadsheet in excel. I just don't know what to include and everything, so my suggestion will have to be this: Create a thread where everyone can reply with their setup (steppers, drivers, screws, ways, and type of machine), and the experience they have had with it. Out from that we can start to filter out what is a good setup. I guess we should back it up with some data and calculations.

It all also depends on weather or not you have modified or enhanced your machine in any way. My mill has gone through several small modifications (nothing really serious), and I would have to say that it's above specs when it comes to this type of mill.

A reply form for this thread could be:

Name of machine/type: X1, X2, X3, sherline, taig..etc
Stepper motors: X:125oz Y:150oz...etc
Drivers: G201, G203..etc
Power supply: VA, voltage.. etc
Screws: Ball screw, roller screw, acme...etc
Ways: Dovetail, linear rails, lapped, scraped...etc
Spindle motor: Watt, RPM, HP...etc
Rapids: IPM / MM/M
Depth of cut: Inch / MM @ cutter width
Experience/other: Repeatability, backlash, slop, rigidity, modifications (mods done to the parts that have not been covered).

Do we need more?

[RickOmatic]

The great thing about cnczone is the wealth of actual user knowledge of machines that are out there! It is always hard to figure out what hardware may be nessisary for every application. With so many variables..Ballscrews or acme....cast iron dovetails or linear bearings....cutting foam or titanium....
My 2 cents worth, as I tell people when I do my "Cheap & Free" talks is to just build something! You can always upgrade, sell your first unit on e-bay or Craigs list, and build or buy something bigger and better!!
My RickOmatic (in the photo section) has seen Many, many years of faithfull service. It still has the first version of Hobbycnc's driver board and 60 oz steppers! Granted it has a 3-1 belt reduction, but it still does everthing I have ever needed it to do! So, follow the forums, and get started! Rick