stepper motor size - again !

[TrickyCNC]

Yes, I am new.

Yes , I have been searching the forum.

No, I cant find the answer yet !

The question gets asked a lot, but no real answer is given. Usually just 'get the biggest you can afford' type answer .

I want to build a machine for routing wood.

I understand the gantry weight calculations etc, but I cant find any information of the force required for router cutter size with depth of cut and feed rate, with regard to stepper motor size/torque ! ???

Even just a point in the right direction would help :-)

TIA

Tricky

[ger21]

Originally Posted by Tricky! The question gets asked a lot, but no real answer is given. Usually just 'get the biggest you can afford' type answer .

Which happens to be 100% the wrong answer.

For a homebuilt router, you can probably assume that cutting forces won't exceed 30-50 lbs.

The reason the question never really gets answered, is that the person asking never provides all the needed info to answer it. In most cases, they don't even know what that info it.

The most important info you need to know, is how fast you want the machine to go, and how fast do you want to accelerate to that speed.

Acceleration will typically take more force than cutting, so cutting forces aren't really that important (imo).

Say you want to accelerate a 100lb gantry to 500 ipm at 0.25G.
You'll need to provide 25 lbs of force.

So you figure out how much torque you need to get 25lbs of force.

But, with steppers, it's not that easy. A stepper's rated torque is holding torque, when it's not spinning. The faster it spins, the less torque it has.

So, you'll need to find out what rpm it'll be spinning at when you get to 500ipm, and make sure it has enough torque at that rpm to get you your 25lbs of force.

Then find a motor that fits the bill. Keep in mind, that you can pick two motors with the same holding torque, that will provide vary different performance.

You can get a 400 oz motor with very high inductance, that will probably only have 100 oz of torque over 300rpm. Or you can get a very low inductance 400 oz motor that may still have 200 oz of torque at 1000rpm. This motor will require a more expensive drive to attain that performance, btw.



Or, you can take the easy way out, and buy a Gecko G540 with 3.5a, 380oz motors. This setup works well for 90% of the machines you see built here.

[TrickyCNC]

Originally Posted by ger21 Or, you can take the easy way out, and buy a Gecko G540 with 3.5a, 380oz motors. This setup works well for 90% of the machines you see built here.

Ha ! I've seen that a few times too
I'll see if I can find a UK supplier.

I may look into building my own driver board too, for the same reasons as I am building my own CNC machine. To learn, and to save money.

I just had it in my head that the effort needed to cut the wood , say 12mm deep 8mm cutter at a feed rate optimum for the cutter would take more force than just getting the gantry etc up to speed. I know that hand held routing take a bit of force for a decent feed rate. the cutting action wants to take the router off in all directions !

Thanks for the reply anyway

[Drools]

Gerry, where can I get a good 3.5a@380oz motor to match up with my G540?
Thanks

[ssutton]

"you can take the easy way out, and buy a Gecko G540 with 3.5a, 380oz motors"

Go with this advice, its sound.

[ger21]

Originally Posted by Drools Gerry, where can I get a good 3.5a@380oz motor to match up with my G540? Thanks

Stepper Motor

[TrickyCNC]

Originally Posted by ssutton "you can take the easy way out, and buy a Gecko G540 with 3.5a, 380oz motors" Go with this advice, its sound.

It might work, but doesn't tech me much !

or answer the question for others that follow this path .

I found this type os answer whilst searching, but I like to know "Why" ! :-)
Especially as I just found the UK price is £300 too (just for the driver board) . A bit pricey !

I can get 3 motors and a 3axis driver for half that. so you can see why people go the cheap route, even if it is miss guided .

[ger21]

Originally Posted by Tricky! It might work, but doesn't tech me much ! or answer the question for others that follow this path . I found this type os answer whilst searching, but I like to know "Why" ! :-)

Because it has features that no other drives in it's price range or cheaper have, and it can drive bigger motors faster than cheaper drives can.

Most cheaper drives are only rated for 24-36V. The G540 can handle 50V safely. Speed is proportional to voltage, so this gives it a large advantage.
It's also good for 3.5 amp motors, while most others are rated for 2.5amps.
These features allow the G540 to drive 380oz motors faster, with more power, than other similar drives running 280 oz motors.

You might look into getting a package from Stepper Motor, Stepper Motor Driver,CNC Router, Stepper Motor Power Supply, Stepper Motor Kit, DC Servo Motor, Stepper Motor, Stepper Motor Driver, CNC Router, Stepper Motor Power Supply, Stepper Motor Kit, DC Servo Motor, Keling Technology Inc, USA
A package with 3 motors and power supply is about $460.

[stevespo]

Gerry laid it out pretty darn well. What else exactly are you trying to answer?

If you want the underlying formulas involved, they're described in any Physics text. There are detailed articles all over the place, including the Techno-Isel catalogs, Excel spreadsheets, and many websites.

What size stepper motor do I-need..

One issue I struggled with is that it's very difficult to come up with reliable data for a given motor, PSU and driver combination. Unless you have a verified source, you just don't know what exact performance you'll have at any given RPM. So, you look at comparable setups, you generalize and play guessing games with ranges and you still come up with a lot of gray area when it comes to sizing. It's not an exact science with DIY components and without the RPM/Torque data for the specific motor, PSU and driver you're interested in using.

Beginning builders reliably and predictably buy high TPI screws, poorly sized motors and inexpensive drivers, believing that speed is not important and that they are saving money. Soon after, they are frustrated with long waits and poor performance. The only option is to upgrade screws, motors, power supplies, drivers, etc. You end up spending much more over time.

This seems like a developmental phase that every builder has to learn about the hard way, myself included. I've gone through multiple drivers, PSUs, motors, etc. At least I built a very solid mechanical foundation and that has been very solid and accurate for the past 4+ years.

If I had followed similar advice, I could have saved myself MANY hundreds of dollars and had even more productive time with my machine. It's true, I learned a lot more, but you always pay a price.

Steve

[TrickyCNC]

Thanks Gerry and Steve.

The electronics and principles of power I understand.

What I did expect to find on my searches, was some kind of formula telling me about the force needed to cut wood/mdf/aluminium/plastic etc etc.

If I have certain size cutter, spinning at a set speed, cutting a set depth, and moving at a set rate.

Obviously a bigger cutter, cutting deeper and feeding faster will need more X,Y drive power than a smaller cutter taking more passes and feeding slower.

Too fast and you break the cutting bit ... but how fast is too fast ???

There must be some info out there that will let me work this out !

I have seen mentioned once somewhere that it is optimum to have a fast enough speed/feed rate so you are making chips and not dust. There must be a formula for that too. But I cant find it.

[CarveOne]

This is one of those things in life, like voodoo, that just can't be quantified reliably with a mathematical formula.

Different cutter bit designs of the same diameter have different shear strengths. The same cutter design cutting in different materials act differently. The same material in different production runs may behave a little differently and reach shear strength at different cutting speeds and/or depth of cut. When it runs into a knot, the cutting conditions change and the bit breaks. How do you write a formula for calculating "Stuff happens"?

The only thing that can be counted on is that you WILL break bits occasionally. Keep spares on hand.

CarveOne

[stevespo]

Ditto to CarveOne's comments above..

Tricky, there are formulas for calculating cutting forces for a given feed rate based on RPM, material, chip load, DOC and a specific cutter - but they still don't account for the design of your machine and capability of your spindle. If you can find published chip loads, then that is a pretty good start. A small amount of deflection or vibration and all bets are off.

https://www.onsrud.com/xdoc/How-the-...Affects-a-Tool
Milling Speed and Feed Calculator

If you have a 2 ton machining center, then mass, rigidity, HP, etc are fairly well defined. If this is a homemade MDF router with a laminate trimmer, then the standard published variables might not work at all. You might have to reduce the feedrate by 1/3, 1/2, or more to get halfway decent results. The RPM or DOC may also have to change drastically, and maybe you'll never get the results you want, period.

These are the variables you need to deduce through trial and error, using the specific machine in question. There just isn't any other way. Even with the big, powerful machining centers, you can bet that the machinist/operator/programmer is making adjustments based on the particular tool, the specific material and the desired finish on the final product.

A standard carbide woodworking router bit will have very different characteristics than a HSS endmill or a high performance tool designed specifically for the application. Just because they look the same doesn't mean they will perform the same on your router.

As far as the force required to move that particular tool, at a given feed rate, RPM, DOC, in a variety of materials you need to start with a reasonable estimate and just move on from there. How much force do you need to move a hand held router taking a 3/4" deep cut with a 1/2" bit in a piece of maple? Is the tool sharp, are you using dust collection, do you have adequate power, etc, etc.

Figure out a best case, and a worst case and go from there.

Steve