BuildingAfloat

[Robin Hewitt]

No need to break your brain if you use sensible units such as the British standard fat bloke

The standard fat bloke weighs 16 stone which translates conveniently to a downward force of 1000 Newtons.

Direct drive on to a 5mm pitch screw requires 0.8 Nm of torque to lift one fat bloke off the floor.

Look at your cutter in it's collet and ask yourself, how many fat blokes could stand on the sharp end of this before it snaps?

Multiply by 0.8, call it Newton meters and anything above that is superfluous.

[Jonathan]

I'd be more worried about accelerating the fat bloke, although it's still not going to change Robin's conclusion that a smaller motor is fine for your Z-axis.

The question really is do you envisage using the motors on a larger/heavier CNC Router, or a milling machine in the future? If so then it's sensible to get the 3Nm motors as they a very likely to be well matched to the future machine. If not then there are motors with a lower rated torque, but would perform better on your machine due to their lower inductance enabling them to reach a higher speed than the 3Nm motors. For instance play around with Irving's spreadsheet to compare this 1.85Nm 1.6mH one to the 3Nm motor for your size machine. The annoying thing then is you're spending more money on a smaller motor, so if they do turn out to be better in theory you've got to be sure you wont regret not having the bigger motors in the future for something else. Additionally the 3Nm motors are fit for purpose, so if they're cheaper than the better alternative then why not?

One point about the X-axis slaving - I wouldn't compare two motors and no belts to one motor and belts, since the belts have so many advantages anyway that you should use them with either system. Given that, out of your list of reasons, the only one that applies is you pretty much eliminate the chance of racking the gantry if the screws are linked. What are the chances of one motor stalling if you have two? Jazz said he's never had his stall since setting up with one motor and I've not had problems with two motors after tuning.

With both the motor selection and drive mechanism, in the end either system is fit for purpose so there's no point in me suggesting that you should use one or the other. What is worth suggesting though is if you do stick with trapezoidal screws, then consider substituting the nuts for the 'wonky bearing drive nut' method I started out using:






Very low backlash, low friction and I bet you could make it with hand tools. There's certainly room for improvement on my design, some of which is discussed in my build log.

Hopefully I'm wrong, but it seems from your previous posts that you're considering not machining at least one end of the leadscrews presumably to save cost? If so forget it since the accuracy will be so poor.

[BikerAfloat]

Robin,

That is excellent, when I stop laughing and the tears have cleared, I'll see if I can find how many fat blokes can stand on the average cutter, should be an interesting Google!

Now looking at Irving's stepper motor tutorial, will then see if I can estimate the mass of my machine components and see where I go from there. Since I'm quite a bit heavier than a BS Fatbloke I keep thinking I could snap (or at least bend) most things that will fit in a Kress collet!

Do we have shear force tables for typical cutters anywhere? Is it a common figure maufacturers supply? or do we just go by Dia of the cutter and its material spec?

I'm guessing snapping during plunging is much less of an issue as the material is in compression. So am I right in thinking that as long as the shear force for a cutter is not exceeded things should be OK?

Cheers,
Geoff.

[Jonathan]

Do we have shear force tables for typical cutters anywhere? Is it a common figure maufacturers supply? or do we just go by Dia of the cutter and its material spec?

Unfortunately it's not trivial to get an accurate idea of the forces on a milling cutter since there are so many variables, some of which are hard to quantify. But if you say 20N for cutting aluminium and 50N for steel with your average size cutter (very vague statement there on purpose) you wont be far off...

I'm guessing snapping during plunging is much less of an issue as the material is in compression. So am I right in thinking that as long as the shear force for a cutter is not exceeded things should be OK?

Quite the opposite actually as the geometry of the cutter is not favourable. Plunging with a milling cutter puts a very high axial force on the spindle, so it's generally something to be avoided by instead moving the cutter in Z and X or Y at the same time to enter the material - i.e. ramping.

[JAZZCNC]

Ok lets bring this back to real world use and implementation.?

While the motor calcs like Irvings are great they are really only good for giving a ballpark figure IMO.!
Unless every aspect of the machines friction and resistance to movement etc are known, which can vary wildly dependent on design, build quality and component quality then it's very difficult to accurately choose the perfect and ideal motor.
Also like Irving pointed out it's very difficult to get real useful motor torque curves graphs from manufactures.? When dealing with cheap motors this is no accident either because they would show that they are really quite below what people might expect.? Only the better motor manufacturers tend to give them and these cost far more money than the average DIY user would want to spend.!

In real DIY built in the shed, garage, basement or coal house and soon to be back of a boat.!! . . things don't often work out like the Calcs or graphs would have you believe so some (plenty) sticky wiggle room needs to be factored in.
Then there's electronics side and in-balance that using different sized motors brings to the control box and it's power requirements.?
Lets take this machine or system for instance.!
If ball-screws are used then it may be possible to use 1.85Nm motors which will run on 40-45V using 50V drives for 2 axis but the 3rd twin screw setup will need at least larger 3Nm motors and 65-70V and 75V drives. This means 2 different PSU's costing extra money and space.
The only advantages to the smaller motors will be they spin slightly faster which really in all probability won't be required, they will accelerate slightly faster but again the little extra will be negligible in real use for this machine, the total cost will be slightly less.
The while on paper the larger motors will accelerate slower and spin slower in real world use the difference won't make much if any difference to this machine other than cost. This is offset by the extra PSU required and not so massive.
The larger motors on the other hand will give great over build "wiggle room" while not having any real detrimental affects to machine performance.
Yes While it's very true Bigger is not always better in this case the difference won't be so much it affects performance to be worth bothering about.!

Then there's the longevity factor.? An over spec'd machine tends not to be so stressed and can run well within it's limits giving long life to both motors and drives.

That's my view on the motor choice obviously others have there's.!


Regards the twin screw and 3.1Nm then yes if you use ballscrews with 10mm pitch and gear either 2:1 or 1.5:1. This will increase torque and resolution while still keeping the motors in a nice RPM range well below the corner speed while cutting. The sacrifice will be rapid speed which to quote John S only any good for "dick slapping".

Like Jonathan I urge you to use ballscrews and the difference in cost if bought from china won't be very much and really lead screws can't be compared with even cheap ballscrews unless high quality lead screws which cost more than ballscrews.!

Use 16mm as well not 12mm.

Also if you use the 90x45 profile from KJN then the BK/BF blocks fit the slots and can be bolted straight on. Also the BK/BF blocks from china are ridiculously cheap and I see no point in making your own.?

[Jonathan]

The sacrifice will be rapid speed which to quote John S only any good for "dick slapping".

That and bed surfacing. It's much more useful to have high acceleration, since this dictates how fast the machine can go round corners which is important for toolpaths with fine detail.

Like Jonathan I urge you to use ballscrews and the difference in cost if bought from china won't be very much and really lead screws can't be compared with even cheap ballscrews unless high quality lead screws which cost more than ballscrews.!

Use 16mm as well not 12mm.

Yes and yes...

though I have seen 8'x4' machines with 12mm

Seeing them doesn't mean they're any good! Unless you need a rotating nut on a 16mm screw (which is pushing it for 8') you'll need at least 20-25mm ballscrews on an 8' axis to get sufficiently high critical speed.

Also the BK/BF blocks from china are ridiculously cheap and I see no point in making your own.?

I make them myself since then I know the quality of the block and bearings is good and I don't want to limit my design by having to fit standard size blocks in. But for your average machine and particularly without the tools to make them yourself the blocks from China are excellent value.