Engraving/Milling Spindles

I think I am going to pull the trigger on a CNC engraver. I like the looks of this one. http://www.k2cnc.com/shop/proddetail.asp?prod=KT-2514S-09&cat=26

I will get it as a turnkey system with a Bosh Colt router as the spindle to start. I want to engrave and do light milling of aluminum and stainless steel. I don't think the Bosh is up to the task of stainless steel.

I've been looking at fancier spindles but I don't know enough to be sure I'm doing the right thing. I don't have much metal working experience. I think I should get a better feel for it before shelling out for better tooling. I have a few very basic questions that I am embarrassed to ask on CNC Zone.

Is the rotation speed of the cutter for a given material just dependent on the feed rate. In other words if I am working with aluminum can I use any spindle RPM as long as the feed rate matches? Or does a specific aluminum alloy have a an ideal cutter RPM and feed rate.

I'm unclear on the relationship between speed and torque. Is it a fixed relationship or does it vary with motor design?

Do you think a spindle like this http://hptdrivesystems.thomasnet.com/viewitems/z-spindles-2/manual-tool-change? would be good on a desktop system? Better finish, more accurate?

get ready to go through an expensive process.... I've found that spindle speed, depth of each pass and speed of XY movement are different for each material, and each bit.

I've had to do a lot of experimentation, and I think I'm almost at the point where I'm confident (or confident enough to be dangerous!).

You definately want some spindle speed control. I thought faster the better, but then, I was cutting Delrin (supposedly the easiest plastic to mill) and found that it was melting around the bit. Not good. Advice was to slow the cutter down.

Good luck.


/R

Had a very quick look at that link. Very high quality spindle motors.

Micky-mouse routers like cheap Bosch etc. won't last. Most of our work used to be on plastic and urethane and we used to go through them like chewing gum. The bearing mechanisms fail very quickly.

I won't lecture on the rest of your questions. Internet is your friend. Start with below links. But fundamentally as in any discipline, practice, practice, practice. Until you start messing up work pieces and break tools you'll never learn.

http://its.fvtc.edu/machshop2/Speeds/RPMcalc.htm

http://en.wikipedia.org/wiki/Speeds_and_feeds

Thanks. How do you control the spindle motors that are frequency controlled variable speed? Say the motor is 5000-30000 rpm, 300W, 75V, 1000Hz-6000Hz. Do I need variable oscillator and power amp that can output 75v at 300W 

not sure, in my case, I have a DC spindle, that can run from my fixed 36V supply.

Slowing it down in my case means that I'll use a PWM signal to vary the "on" time duty cycle (through a power switch mosfet). (i.e. a 30% duty cycle will mean it's on 30% of the time, and off the rest, in a 150kHz switching frequency).

The spindle should then (if I understood correctly) average that itself to a 12V input.

Gold said:

Thanks. How do you control the spindle motors that are frequency controlled variable speed? Say the motor is 5000-30000 rpm, 300W, 75V, 1000Hz-6000Hz. Do I need variable oscillator and power amp that can output 75v at 300W

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Pretty much yes. However this is a crude and primitive method. Under load the speed will vary and you'll have no control over it. If you are going to invest into a high quality spindle motor such as in that link then you should  just  bite the bullet and invest into a proper var. freq. speed controller too.

I was essentially going to make a circuit like this:

http://www.dprg.org/tutorials/2005-11a/index.html

but with a whopper of a Mosfet that could handle the switching.

--- Sahib - it sounds like you have more experience than I do -- what do you think?

I am not very sure about that but I do my best. I have been involved in robotics and motion control in the past fifteen years.

As the name suggests that is a simple pwm control, which is fine. In fact I have built similar circuits so many times and it works reasonably well for non demanding applications.

However, it is nevertheless open loop. When the motor goes under a load the speed will drop, and when it is free of load it will speed up. The controller has no way of knowing it to increase/decrease pwm to compensate. There are two ways we can let the controller know. We couple a tacho generator onto the motor which generates a voltage proportional to the speed that we can compare. Or, we read the back emf generated by the motor itself.

At the end of the day it all comes down to the demand of the work. If you have small passes say like .25mm at a time on aluminium I don't think you would be bothered if the speed goes down a bit. But if you are churning out chunks like 2-3mm at a time you want to maintain the speed as it is proportional to your feed rate.

Part of my business was in prototyping and modelmaking. We did not machine metal on the flat bed CNCs very much, mostly plastic, timber and polyurethane. Occasionally aluminium or brass sheet, but always with small passes. We air cooled the cutter as these machines are not designed for wet cut.

Ahh, okay a VFD is necessary with a variable speed frequency controlled spindle.

The industrial style spindles that K2 carries are big and expensive. 2.2HP - 5HP. I don't think I need something that big. I would like the ability to engrave stainless steel and nickel. I would also like to be able to cut some stainless steel. I don't need it to happen fast.

Am I correct in thinking that if I use a less powerful spindle I have to make sure it can turn slow enough to cut the material at a practical feed rate for the given power? If I use an 850W motor which is approximately 1HP and I specify the feed rate then I can determine if the spindle is up to the task by its RPM?

you only need low rpm/high torque for bigger tools - face mills and roughing tools - for the task of making holes in sheetmetal and engraving 3mm or 1/8" tools are commonly used, so a HF spindle like the Jaeger is a nice fit. But IMO the quality of the router (no ballscrews, unsupported round rails, bridge looks a bit whimpy..)  doesn't quite fit to the state-of-the-art spindle. I'd say ballscrews are a must for fine engraving since we want very low backlash (could be possible with delrin nuts as well, but it wears faster)

Do you plan to use Mach3 software?

ioaudio said:

you only need low rpm/high torque for bigger tools - face mills and roughing tools -

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I guess I would like the ability to do some machining of stainless steel if possible.

for the task of making holes in sheetmetal and engraving 3mm or 1/8" tools are commonly used, so a HF spindle like the Jaeger is a nice fit. But IMO the quality of the router (no ballscrews, unsupported round rails, bridge looks a bit whimpy..)  doesn't quite fit to the state-of-the-art spindle. I'd say ballscrews are a must for fine engraving since we want very low backlash (could be possible with delrin nuts as well, but it wears faster)

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I believe it does have a ball screw. Isn't a ball screw the same as a lead screw? It has a lead screw with 0.002" pitch and the rails are 20mm support rails with micro switches for servo motor positioning.

Do you have an example of something you would recommend?

Do you plan to use Mach3 software?

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Yes. I haven't decided on  CAD/CAM software yet. I'm leaning towards V Carve Pro. I like the software that is tailored for engraving rather than a general CAD program and a CAM plug in. I don't need or want to learn software as sophisticated as Solidworks or Rhino. I also like the look of Type 3 but it is over three times the price. It looks like it has more tools for converting images to vectors. I will want that capability but I don't want to invest in an expensive software package until I have a better idea of my needs and preferences.

just for reference, i use CAMBAM for the CAD/CAM and Mach3 to drive the CNC machine.

I'm thrilled with Mach3. Very very good.
Cambam, pretty good - but difficult to get your head around from time to time.

the learning curve is pretty steep with this stuff.

Gold said:

I guess I would like the ability to do some machining of stainless steel if possible.

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re-reading the description of the K2 machine i see that they are quite upfront about it:

Applications:
MDF, Maple, Balsa, Corian, Circuit Board, Hard Wood, Plastic, non ferrous metals.
Special misting, tools, proper feed rate, proper technique required if used to cut aluminum.

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this machine could be up to the task: http://www.k2cnc.com/shop/proddetail.asp?prod=KG%2D3925%2DGTHK&cat=26

The machine you linked to has a larger table top than I need. This one http://www.k2cnc.com/shop/proddetail.asp?prod=KG-1414-G&cat=26 looks like the same construction style in a smaller size.

That construction style bumps up the price a bit, of course. I couldn't afford that machine and a fancy spindle. Do you think I would be better off starting with that table with the Bosh router and upgrading the spindle later rather than getting the lesser table with a better spindle?

Machining stainless steel is not an easy thing. But if you really want to include it then first invest into a table which is designed for that task. Get a cheaper router and practice on plastic based materials first. 

Absolutely do NOT buy an expensive router right off the bat...you're going to need a lot of practice and trial and error, and you don't want to be doing that with a fancy expensive motor. Just get the bosch and learn first...and lots of bits.

Also...at first glance that machine does look nice, but you may want to consider not getting servos instead of stepper motors. Servos on this size of machine offer no real benefit, cost more, and you can hurt them a lot easier than steppers.

More info here

http://torchmate.com/resource_center/editorials/stepper_motors_vs._servo_motors_vs._intelligent_motors_-_the_facts/

Ptownkid said:

Also...at first glance that machine does look nice, but you may want to consider not getting servos instead of stepper motors. Servos on this size of machine offer no real benefit, cost more, and you can hurt them a lot easier than steppers.

More info here

http://torchmate.com/resource_center/editorials/stepper_motors_vs._servo_motors_vs._intelligent_motors_-_the_facts/

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I disagree. You hurt steppers a lot easier.

Stepper motors on these machines work open loop. Under normal conditions there is nothing wrong with that. However, imagine a scenario that the gantry is jammed/slowed down or the motors start to lose steps (due to dirt or other factors) which can happen easily.

The controller keeps sending pulses to the steppers as there is no feedback to realise that something is wrong. You'll only realise that when your circles start to come out as ovals and/or the centers of holes are disclocated. 

Whereas on servo if something goes wrong the controller will halt the operation. Saving your work piece and even the motors.

I see what you are getting at...but you will not hurt the steppers by getting stuck, they will lose steps though.

Seems to be some pretty widely varying opinions on this topic...

Ptownkid said:

http://torchmate.com/resource_center/editorials/stepper_motors_vs._servo_motors_vs._intelligent_motors_-_the_facts/

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I hadn't run across this company in my search. Looks well built. The ground bar stock rails look nice. Do you have one of these? I do want to buy a turnkey system. I'll trade the money for the time. Then I'll work my way backwards though the setup over time.