cnc for marble

[phill05]

Hii All i am trying to build a big cnc router to use for stone engraving ,achieving higher speed and higher precision building a rigid machine i will be working on a 3m *1.5 m machine

You will not do much good with the type of bolt together frame that you are showing.
You do NOT need higher speeds, but you do need a RIGID machine and plenty of water, rough out leaving 4 to 6 mm for finish cut, using Diamond and carbide cutters depending on the type of stone your cutting, marble / limestone is soft and will cut well with carbide, for granite use PCD (poly crystal diamond ) cutters, sandstone use brazed diamond tooling or pcd.




Head being cut is Limestone and cut with carbide to a depth of 16mm, the eyelet is in sandstone and is cut with PCD.

Phill

[yasser]

You will not do much good with the type of bolt together frame that you are showing.
You do NOT need higher speeds, but you do need a RIGID machine and plenty of water, rough out leaving 4 to 6 mm for finish cut, using Diamond and carbide cutters depending on the type of stone your cutting, marble / limestone is soft and will cut well with carbide, for granite use PCD (poly crystal diamond ) cutters, sandstone use brazed diamond tooling or pcd.




Head being cut is Limestone and cut with carbide to a depth of 16mm, the eyelet is in sandstone and is cut with PCD.

Phill

excellent work just how long did it take to finish the head nad what is the shape of the tool used ...these are the results i reached till now
the hexagon tile 50 cm * 50 cm took around 3 hours maximum depth 5 mm step down 5 mm

[Rufe0]

The distance traveled per revolution with rack and pinion is the pitch circle of your pinion gear diameter * pi, for example 1inch 25.4mm pitch circle diameter pinion gear, 25.4*3.145 = 79.883mm distance traveled per revolution. Divide that by your stepper motor step count which is normally 200. 79.883/200=0.399415. So the resolution is 0.399415... so to get that 0.001 resolution I was talking about you'd need a 300:1 gear ratio, which is crazy. 0.001 might be a little optimistic... but the point being that the problem of smooth slow feeds will be much worse with rack and pinion.

Don't get me wrong rack and pinon is great for soft material routers that need high speeds but I don't think it's correct for your application. Of course you could probably make it work but it wouldn't be the ideal solution.

[yasser]

The distance traveled per revolution with rack and pinion is the pitch circle of your pinion gear diameter * pi, for example 1inch 25.4mm pitch circle diameter pinion gear, 25.4*3.145 = 79.883mm distance traveled per revolution. Divide that by your stepper motor step count which is normally 200. 79.883/200=0.399415. So the resolution is 0.399415... so to get that 0.001 resolution I was talking about you'd need a 300:1 gear ratio, which is crazy. 0.001 might be a little optimistic... but the point being that the problem of smooth slow feeds will be much worse with rack and pinion.

Don't get me wrong rack and pinon is great for soft material routers that need high speeds but I don't think it's correct for your application. Of course you could probably make it work but it wouldn't be the ideal solution.

ok then i will stick to the screw system but i am worried about the length 3 m , and i think there is a problem in my setting of the driver i am using ,this is the setting i am using on the driver

[Rufe0]

What specifically do you think is 'wrong' with it? As is do you think there is malfunction with the driver or are you wondering if another setting would be more appropriate?

[Rufe0]

Just to reiterate on earlier 0.001 resolution was a bit optimistic. 0.01 is more realistic and would be fine.

[yasser]

i was wondering if another setting would be more appropriate

[Rufe0]

Maybe, try a few out. It depends on your steppers, drivers and how well they work together, there is probably a sweet spot. I would stay clear of the really high ones though, mircosteps aren't always as good a thing as you might think they would be for a number of reasons. Have a read about it http://www.micromo.com/microstepping...-and-realities . Obviously remember if you change your microsteps you will need to adjust the settings on your computer otherwise it will come out the wrong size.

[JAZZCNC]

Don't get me wrong rack and pinon is great for soft material routers that need high speeds but I don't think it's correct for your application. Of course you could probably make it work but it wouldn't be the ideal solution.

Sorry but your wrong R&P is the perfect choice in this application. Just about every industrial Stone cutting machine uses Helical R&P.
Your also Wrong about the Resolution and Micro steps because no one who runs steppers would run at FULL step 200ppr. WHY.? . . . Because Resonance and the affects it has on the motors is crippling to motor performance when full stepping. Infact you'll be hard pressed to find decent quality drives that provide Full step mode. Half Step(400ppr) is usually minimum. In real world use then between 800 and 2000ppr are used to give best balance of performance/smoothness.

In this application cutting hard material and Steel framed machine the resonance will be high so using higher micro steppping would be used to help smooth motor performance. Micro stepping however shouldn't be thought of as way increasing resolution, though it does to some degree dependant on motor quality.

Resolution should be calcualted based on pitch. R&P will minimim ratio of 3:1 (typical 5:1) dependant on several factors like Pinion size, motor Rpm, desired Rapid speeds etc.

Lets look at it again with realistic figures using same pinion size. Lets start by applying a minimum Ratio 3:1 with minimal 400ms setting and seeing the speeds, resolution this would give.?
With an affective pitch of 79.833 / 3=26.611mm pitch which based on usable RPM of typical motor used being 700rpm this would give 18627mm/min or 18.6Mtr/min which is ridiculous speed for cutting stone that would never be used.
The resolution would be 26.611 / 800=0.066 which is still more than enough resolution.

Now lets go to more likely setup that would be used. 5:1 ratio still giving a speedy 15.966 Pitch and using 1600Ms which is often optimal for motor smoothing.

Rapid speed: 15.966 * 700= 11,176mm/min or 11.1Mtr/min still a nice healthy speed which you'll never use cutting stone other than for positional rapid moves.
Resolution: 15.966 / 1600= 0.009 far more than needed.
Now the Resoultion figure isn't exactly true figure but rather theoretical figure and in reality there will be some mechainical loses but the higher it is to start with the higher it will balance out to in the real world.

Also the higher ratios have increased torque which will be needed with R&P because it's quite inefficient compared to ballscrews so will need more power.

Helical R&P it's better than straight cut and provided you protect it from debris and keep pinion tensioned into rack properly then it's the perfect solution for long machine like this.

[Rufe0]

Just about every industrial Stone cutting machine uses Helical R&P.

That doesn't necessarily mean that R&P is 'the ideal solution'. Hate to say the obvious but if everyone jumped off a cliff would you join them? R&P is great for a number of things, I think primarily where you have a very long travel. Is it ideal for a machine where you primarily need smooth slow speed movement in a travel length acheivable by a screw? I don't think so but thats just my opinion.

Your also Wrong about the Resolution and Micro steps because no one who runs steppers would run at FULL step 200ppr.

Yes sorry if it came across as me suggesting you use fullsteps. Of course you should use microstepping. What I should have mentioned is... referring to my link http://www.micromo.com/microstepping...-and-realities

"The real compromise is that as you increase the number of microsteps per full step the INCREMENTAL torque per microstep drops off drastically. Resolution increases but accuracy will actually suffer.

The consequence is that if the load torque plus the motor’s friction and detent torque is greater than the incremental torque of a microstep successive microsteps will have to be realized until the accumulated torque exceeds the load torque plus the motor’s friction and detent torque.

Simply stated, taking a microstep does not mean the motor will actually move!"

A microstep doesn't guarantee movement. For my point about wanting a high resolution for smooth slow speed to be valid and useful the resolution has to be real world accurate resolution.

So how many microsteps can you guarantee? Well ofcourse your motor could stall completely, but lets put that to a side for a moment. 1 microstep/full steps you can guarantee them. 2 yeah thats fine. 4 probably OK. 8 maybe I wouldn't be confident. More than that I would say you definitely couldn't call that guaranteed accurate movement.

From the point of view that over doing it is better than under doing it, using 200steps/rev for this calculation means you definitely will get that accuracy. However realistically 400/2microsteps or 800/4microsteps is probably the right thing to base the calculation on.

When the machine is built you can still run it at whatever microsteps you like, nothing stopping you doing that.

I'm suggesting something like 20mm pitch screw with 1/3 belt drive. 20mm/3 = 6.6mm / 400 = .016mm or / 800 = .0083. 0.016mm - 0.0083 real world accuracy. Or better yet a 10mm pitch screw with a belt drive.