[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]

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]

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 :cheers:

[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]

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

Stepper Motor

[TrickyCNC]

"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]

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

[TrickyCNC]

Thanks again then.

So ... around 400 Oz/Inch and go from there.

Choose my driver carefully ...
Anyone make their own ? I'm reasonably OK with electronics. Although Audio amplification is more my thing !

I'll have a read through the driver section

[CarveOne]

The current "standard" for DIY machines are the 380 oz steppers, 48vdc PSU, and the Gecko G540 four axis driver kits from various resellers. This will drive most any size machine up to a 4' x 8' with reasonable power for most things we do. It's a very good off-the-shelf "buy it and install it" solution for a beginner. There is no real need to do all the complicated math to insure that you know what is happening and why. It's fine if that is your thing. The guys here have done all that and share their trials, successes, and failures readily. The bottom line is that the best designed machine in the world won't work well if you don't have the skills to build it properly.

You can buy lower cost motor and drive kits and get less performance for less money. The Gecko brand drives have features not found in low cost drivers and are worth the extra money in my opinion.

If you want to design, etch, and build your driver boards yourself go right ahead. Many people do just that. They spend time and save money if they have the skills to do so.

If you want a commercial quality machine on a DIY budget you can do that. Learn all you can here and dazzle us with your abilities. It's up to you. We'll watch the show. Oh yeah - post plenty of pictures and videos please.

CarveOne

[TrickyCNC]

hmmm.... lots to think about.

Gecko doesn't seem to be about in the UK. I can find an outlet, but they aren't really advertising anywhere.

I really want to understand how these things work, not just follow like a lamb. I am the person that takes things apart to see how they work, rather than just use it because it does !

I would like to know what I could do with a 270 Oz motor for instance, if only just to rule it out. the TB6560 chip based drivers seem to be the most available and very cheap. Max 3.5A, but I see there is a thread on this chip elsewhere here, that suggests using them under this.

I am now becoming more inclined to build a small machine out of one of these cheap kits just as a learning experiment. (I can always sell it to fund the bigger one) Then I can be up and running. learning how to program , get sorted using cad/cam/ whatever software I can find to run on Linux.
I could even make a 5 axis machine quite cheaply - just to see what 5 axes can do ! LOL !

Maybe even use the small machine to make the parts for the bigger one I eventually make. ??? I could definitely use it to make the circuit boards for a driver if I went that way.

Thank you all for taking the time so far to help me. I am sure every new beginner asks these same questions and goes through this process. I'm really quite impressed with this forum so far !

The biggest thing for me, is it is unknown territory, so jumping in the deep end and spending all that money worries me a little.

I have no doubt at all about my abilities to make a cnc machine and make it well. So I will do a little more reading and then dive in.

I'll take plenty of photo's and video along the way.

Rich

[CarveOne]

Some of the features that Gecko drives have that make them a leader in driver sales is automatic morphing, midband resonance reduction, idle current reduction, and output short and open circuit protection. Motors run very smoothly over the full rpm range. Stalling normally won't happen except under unusual binding conditions.

You'll probably need to order them directly from Gecko Drives or from resellers like cncrouterparts.com or kellinginc.net and others in the US.

Linuxcnc.org has what you need for running the machine at no cost. Download the EMC2/Ubuntu ISO file and burn a CD to install it, or try it without installing it. I went that way with my first build, and still use it on one machine.

The beta trial version of CamBam Plus will give you what you need for generating gcode from DXF drawings. 40 tries before it expires, but if you open it it will run all day as "one try" long as you don't close it.

Build a small machine with MDF on the cheap as a lot of others do. Don't use unsupported rails of any kind, and you'll do fine with the 270 oz motors. It will carve anything you want, within its size limit, and do a respectable job of it.

CarveOne

[TrickyCNC]

I'm thinking about this one
eBay - The UK's Online Marketplace
as a good start,
or this one
CNC Kit 3 Axis TB6560 Driver + 1.86 Nm Stepper Motor | eBay
with a PSU for a cheaper 1st build "learning" thing ???

anyone think it's a definate no no ?

Rich

[Walky]

TB6560 is a no-no, AFAIK they're terribly overrated (some say they get terribly hot at 24v even when the supposed max voltage is 36v) and have a high tendency to failure.

I have no idea about the other drivers (PM542), but I'm sure you can find some info on the forum of Google.

I agree that the safest bet is the Gecko G540 with a 48V PSU.

I've had good experiences with the Probotix Probostep drivers, too, which are a little cheaper but have less features.

Regarding the motors, remember that bigger does not always mean better. Not all 380 oz/in (or so) motors are the same, there are some variables involved (inductance, voltage rating, amp rating) which are key to decide a good match for your drivers/PSU. The geckodrive website has some info about selecting motors that would be a good read.

[bearwen]

I am an electronics tech by trade I work on multi million dollar radar systems and I will tell you. I built a jgro for my first machine years ago and I built my own boards ALL of them even the BOB and my own power supply. Heres the thing after only 2months of running the thing I built a new power supply and bought the gecko 203 drives and have never looked back the Geckos are AWESOME!

Now as for speed and torque etc etc.. if you do a search for posts by me you will find a few conversations that I have had with others as well as Mariss(spelling?) who is one of the engineers/ designers of the gecko drives (FREAKIN GENIUSE) any ways you will find in the posts power in watts to torque and screw direct drive to gantry torque etc etc.. all the figures you will need unless you go belt drive then there are differnt formulas but PM me I have belt drive stuff as i am building a 4' X 8' router with 1200+IPM rapids with small (300+oz-in)motors.

Last as for the pressure on a bit at set depth going set speed blah blah blah. There are way to many variables to calculate this HOWEVER from personal experience (and asking your same questions) the best answer I ever got was this if you are using a router bought from the hardware store and not a spindle this is your limiting factor. The router has crap bearings and can only hold up to so much abuse so what I was told is take your router by hand and set the bit in question in it and set the depth you want to cut and route by HAND as fast as you could go go and keep that pace for 10 minutes or so and that is about as fast as you want to go as the bearings in the router are designed for HUMAN power.

Now that i have said this I will tell you that speed is your friend I can run my machine at break neck speeds and do a 1/4 inch deep cut and make 4 passes to cut through 1 inch wood with a 1/4 or 1/2 in bit faster than you can probablly cut @3/4" deep in a single pass with the same bit due to loads on motors, router bearings and bit. Think about multi pass fast versus plowing through in a single cut.

Hope this helps and go with the geckos you wont be sorry and you will be able to grow and expand later.

Good Luck!

[stevespo]

I can't comment on the drivers, but the PM542 looks like a reasonable product. However, the 425 oz-in motors it is paired with will perform better at 60V, which is beyond the drive's capability.

The triple stack 425s all seem to fall off the cliff pretty rapidly, so you aren't left with much torque above the 7000 PPS mark. Compare this with the 270s and 380s and you can see where those curves are much flatter and still maintain respectable torque at higher speeds, with lower voltage.

My own 425s were strong at 72V (higher inductance), but really should have had 2 TPI screws and not the 5 TPI I was already using. Even so, one motor could accelerate my 100 lb gantry to 250 IPM with a Gecko G203. I could NOT reliably manage 90 IPM with the Xylotex and 24V PSU with the same exact motors. That's why the drive/PSU/motor decision needs to be matched to the machine and your performance expectations.

The point we're making is that the Gecko w/380 oz-in motors is a solution that works for a WIDE range of DIY machines and performance needs. It's a great hedge. You might very well find you CAN get by with less, but it's unlikely you will need more for the small or mid-sized machine you have described.

Steve