Newbie- need help in selecting parts for cnc router

[sahilkit]

hi guys

this is my first post here (about cnc mech. stuff),i want to build a CNC router...this is what i have come up with

1. machining area(X x Y x Z) 500x500x240 (mm)
2. exclusively for cutting aluminum
a) maximum 6mm aluminum plate
3. spindle = from wood router motor 2hp-3hp ??
4. galvanized iron will be used to make x-axis n y-axis structure
5. Granite base with iron/steel frame
6. use readily available z-axis units from ebay

now here come the difficult part

i have decided to use steel hardened chrome plate rods with aluminum supports and THK rolled ball screws and i need to know whether these will do or not ????

specs

THK MTF Steel Rolled Ball Screw MTF1202-3.7+500LT, 2mm Lead, 12mm OD , 500mm L

a) two screws for y_axis
b) one for x axis

linear guides

a) Y-axis linear rod - 30mm dia x 600mm LT with aluminum support - 2 number
linear bearing 30mm open type - 4 number
b) X-axis same as above but 25mm once


so what do you guys think i have not at decided on the motors n electronic stuff,please help n advice

thanks in advance

sahil

[sahilkit]

oh i forgot to mention if the machine can cut a A4 paper size 4mm aluminum sheet under 40 minutes then its fine !

sahil

[CNCRouterm]

Greetings,
Cutting aluminum is certainly feasible with a router. You will need to select the proper tooling, and if you can afford it, get a micro-lube system such as Accu-Lube to cut down on chip welding. You will also want to get the chips out of the cut as much as possible either through extraction or air blast. The important factors here are work holding, machine (and spindle) rigidity, tooling, feeds, and speeds. A light duty spindle, say a PorterCable hand router, could be used, but only with light cuts, and expect to rebuild it often. A Perske or Columbo spindle would be much better, much more expensive, and not really designed for cutting aluminum, but with conservative chip loads, and fairly shallow depths of cut they will work a lot better than a hand router. I assume you can't justify an HSD cartridge spindle or equivalent, so you just have to stay within the capabilities of the spindle you DO choose.

As I use vacuum hold down for almost everything, including aluminum and copper, I can only say that granite base should give you a "rock solid" table, now you just have to figure out how to keep the parts just as "rock solid" while machining them. Holding on to the parts effectively is one of crucial aspects of getting a good quality finish in routing, along with rigid spindle control (ie: rigid machine structure, tight motion control), and spindle quality. Tools are important, as are feeds and speeds, but they can't compensate for bad spindle bearings, part movement, or a spindle mount that easily deflects under load or vibration.

Regarding the rolled screws. They will have a certain amount of backlash inherent, so if budget allows, put two ball nuts per axis with some preload or a preloaded single ("zero backlash" ball nut) to reduce/eliminate backlash. Keep the screws clean, enclosed if possible, and well lubed.

My second machine (that I was responsible for, not owned by me) used rolled ball screws, two start, with the ball nuts having just one circuit, there was enough play in the system to make proper tuning a true nightmare, and when maintenance missed a few lube schedules, it trashed one of the screw starts, so, fortunately, we were able to re-thread the nut onto the other start as a temporary fix. Moral of the story is - keep the screws lubed and clean!

Regarding the linear motion choice. Sounds workable, the old style,(as in 1980's vintage) Thermwood 5 x 8 gantry machines used round ways. Since the table is granite, I am guessing your design is a moving bridge type with two screws for the bridge/gantry (Y- axis?) and one for moving the head assembly. Spread the bridge/gantry mounted rails as far as practical.

If my rough math is correct, to get 300ipm or 7620mm/min rapids, you will have to spin the 2mm/rev ball screw at 3810 rpm. I foresee real problems there, check the mfg. specs on critical speed for your bearing to bearing distance. Figuring out a whip damping mechanism adds a lot of complications and aggrevations that can be avoided by a larger diameter, steeper pitch, or a combination of both.

Of course, my expectations on speed are heavily biased by my experience, and the fact that I make my living by routing components for customers, so speed is probably a bit more important to me that it is to you with this project. By comparison, I do cut aluminum in 1/4" thickness, single pass, usually around 120 (3050mm/min) to 180 ipm ~4500mm/min. With 1/8" stock, I may get over 200ipm. With plywood, particle board or mdf in 1 inch (25.4mm) and under, depending on finish quality specs, I may push 900ipm (22860mm/min) to 1200ipm (30480) on a regular basis. HPLam runs a bit slower and eats tools. Plastics are usually under 700 ipm. Your selection of lead, screw diameter, and accuracy should be driven by what you intend to cut, and what can your spindle sustain in terms of feed rate. Feed rate is determined by rpm and chip load, chip load is determined by material and tool first, finish second, and spindle torque third, although if the spindle can't handle optimal chip loads, the first two points are secondary to the spindles limitations, so then you go back to tool choice, such as a smaller diameter tool, or less flutes, or shallower cut, multi-pass, rough and finish stratagies. Rpm is determined by tool diameter, tool geometry, and desired surface speed, again, spindle choice will play a major roll here.

All in all, I think you have a good start, though I would double check on desire feed/rapid rates and if the selected ball screw will support those criteria. Remember that off the shelf hand routers usually spin upwards of 21000 rpm. Those that are variable speed are probably constant torque, so as you drop the rpms, the total power is less, though the torque should be nominally the same, net result, it is likely that the 2hp router at 21k or 23k rpms will by only 1hp at 10500 or 11500 rpms.

I suppose you wonder what difference it makes if you take the "optimal" chip load or take a lighter chip load. The short answer is tool life.
The slightly longer, but still incomplete answer is each cut made by each flute generates heat, and has a certain amount of abrasion or "dulling" effect on the flute, more heat means the amount of abrasion or dulling effect is increased each cut of each flute, so the hotter the cut, the faster the tool dulls. The heat generated in the cut has to go somewhere, it can go into the tool, the air, the work, or the chip. Small chips mean that more heat has to go into the other three heat sinks, the tool being the one that affects tool life the most. In practice, most of the heat goes either into the tool or the chip, with only a small portion going into the work piece, so if you see the work burning, you know that the tool is really suffering. By carrying a "proper" chipload, you move most of the heat generated in the cut into the chip, which then is cooled by the air, the heat absorbed by the tool then conducts the heat to the collet, tool holder, and spindle, some of it is cooled by the surrounding air, more if you use an air blast on the tool, or use coolant or micro lube.
So, besides tool life, why worry about "optimal chip loads"? Answer; quality of cut. Different materials behave differently (no surprise there I'm sure), but having too light a chip load in aluminum will lead to premature tool failure, chip rewelding issues, and possibly an aluminum lollypop where your tool used to be. Under feeding in solid woods, such as maple and birch lead to burning, squealing screeching noises sure to bring all the neighbors and maybe the authorities or PITA wondering why you are putting small creatures in a blender. Oak will burn too, and the tanic acid will quickly degrade carbide tools abused in this manor. Plastics are a fickel bunch. Feed too light, and you get melted plastic, feed too aggressively, and you get cratering, crazing, or outright failures, depending on the type of plastic and it's age.

[sahilkit]

hello Eric Neuman

thanks for the nice n informative reply

1) yes i will be using some sort of oil/lubrication system and lubricant will be ethanol or kerosene and will also add an air cooling/chip blowing system.

2) what would a good industrial spindle cost and what all required with it ???
wattage or hp etc. ?

3) i think will go for Hiwin linear rails as they cost slightly more then what i was thinking of using and also what size rail do you think will be good ??? will a 20 or 25mm rail do ?

may 30mm rail on y-axis (base) n 20mm or 25mm for X-axis or is this a overkill ?

4) okay will get a preloaded nut and also what about lead screw with anti back lash nuts are they okay ?? how do i select the diameter of the screw ???

i am building this for a small machine shop and will at the most us it for 5 day in a year (cutting)

thanks for you time

sahil

[sahilkit]

I am guessing your design is a moving bridge type with two screws for the bridge/gantry (Y- axis?) and one for moving the head assembly.

yes thinking of building some thing like this

http://cgi.ebay.com/3D-CNC-ROUTER-MI...d=p3286.c0.m14

Spread the bridge/gantry mounted rails as far as practical

did not get it ??

sahil

[Rally]

What "Machine shop" are you building this for?

[sahilkit]

rally i meant for my small work shop, will be cutting parts for a book cradle !

sahil

[chrbau]

That link to Ebay; the shown machine does not use ball screws? Does it not matter?

[Rally]

Sorry, I was being sarcastic.
Whatever it is you are trying to accomplish I can you that you are going to be disappointed going the cheep rout.
Here is what I would do!
1.Get heavy linear Rails
2.Use Rolled Ball Screws and zero backlash nuts
Also, personally I hate steppers if you go with a servo system you'd be better off.
That being said, more than likely there will be many people to tell you that steppers are just fine. They do work! no argument there but none of them can hold water to my servos.
I personally had steppers on a mill and then switched to servos.

[sahilkit]

hello rally

can you suggest some specs one the heavy rail n ball screw ?? what are u using ?

yes i was also thinking of going with servos may be 200 watt once ?

thanks

sahil

[Rally]

Originally Posted by sahilkit hello rally can you suggest some specs one the heavy rail n ball screw ?? what are u using ? yes i was also thinking of going with servos may be 200 watt once ? thanks sahil

My settup don't apply to what you are trying to achive.
I'm about to head into work but can look up some Items for you later.
I think you will be much happyer with a servo system. Biggest advantage is the simple fact that the servos will not lose position.

[Walbac]

The chrome shafts are definately a "NO NO", the chrome will peel off in no timeand render your machine useless, rather use Hiwin linear guides, for your size machine and what you want to do, use at least 20mm guides, you won't be sorry !!