[Pat2000]

on 'modern' cnc vertical machines they seem to be able to give very high figures for rapid movements and they hang great heavy spindles on them, I guess they must achieve this by using a significant amount of acceleration (also read deceleration) also powerful motors and drives give it the ability to do this in a smallish distance? is that right?

Ok rapid feed rates don't figure too much when it comes to machining metals.... so I guess the rapid movements can be kept for machine movements when not machining (bear with my bumbleing ;o) so with the feed rate called for, the distance travelled while accelerating ain't very far and so feed rate marks on the finished surface look nice and consistant and the actual feed rate is really the feed rate for the most part.

If we are into plastics and wood - soft stuff, I figure we are not so concerned about feed-rate marks- yes melting (plastic) and burn (wood) are of course significant probs so acceleration distances can be pretty big me thinks?

Ok well this is what I'm getting at, It is comparatively easy to make - as I guess a lot of us round here have or are making,a largeish beam type of structure which is in its self a moving component and hang our Z axis and router assembly on that, making that 'moving beam' rigid is quite easy IF we also end up making it heavy... but I worry about the weight!

If I haven't lots of motor power can I use a largeish value for acceleration in my software? so that the motor has time to accelerate the larger mass of the beam (disregarding any cutting forces) and 'get away with' my heavier beam and lowish power motor?

slightly interesting perhaps on my Bridgeport EZtrak at work I do quite a bit of engraving work, I sometimes use a non-rotating spring loaded point for fine lines - well wacking the feed rate up was interesting to see cos the large cast-iron table just cannot handle the rapid changes in movement and if those movements are small the old table gets right-old funny!!! trying to move such a large mass in rapidly changing directions was an interesting eye-opener the m/c just could not do it! or the software is not confuigured right?

Pat

[ger21]

If I haven't lots of motor power can I use a largeish value for acceleration in my software? so that the motor has time to accelerate the larger mass of the beam (disregarding any cutting forces) and 'get away with' my heavier beam and lowish power motor?

It probably depends on the software a bit. If you're using Mach3 and constant velocity mode, you'll get rounded corners, which will be more severe as acceleration decreases.

Other than that, I'd think if the motors are powerful enough to give your desired top speed, the only issue would be if the accelaration rate is acceptable to you.

[Dan Fritz]

You're correct that most CNC controls use controlled acceleration and deceleration for rapid traverse moves. The rapid feedrates are usually quite high, and frequent short rapid motions can be a problem unless the CNC gives the servo a "ramp" to accelerate at a rate that does not require the motor to constantly hit its current limit.

High inertia is a two-edged sword in machine design. High inertia makes for a very stable machine, where outside forces (like intermittant forces from a cutting tool) have little effect on the moving axis. High inertia also makes acceleration and deceleration more difficult, requiring motors and servo drives that can deliver more torque.

Keep in mind that much of the inertia in a typical machine is from the ROTATING mass of the ballscrew, coupling, bearings, and the armature of the motor itself. The sliding (linear) mass has intertia too, but you'll probably find that the rotating (polar moment) inertia is the big factor. The rotating masses act like a flywheel, which takes significant energy to accelerate and decelerate.

For some good examples of low-inertia machine design, go to a local machine show and look at some punch presses and laser or waterjet machines. These machines have large tables (to handle 4 x 8 foot and larger sheets of steel), and have very low inertia positioning systems because there aren't many outside forces trying to knock the axes out of position during the cutting process. These machines have relatively small (low inertia) motors, small diameter ballscrews, and fast-responding servo drives. Some machines do away with rotating mass of the ballscrew altogether and use rack & pinion drive to move the X-Y axes. These machines can accellerate and decelerate VERY quickly, and they do it with rather small motors. The downside is that rigidity must be compromised. This type of positioning system would definitely NOT work on a typical mill or machining center.

For wood routers, the axes travel envelope is usually quite large (like punch presses), and the intermittant cutting forces are usually rather small because the tools are cutting soft materials at high RPM. I would think that a wood router with a low inertia positioning system like a punch press would give the best performance.

Generally speaking, if the intertia of the positioning system is high, then the CNC will need a long-ish Acc/Dec ramp for rapid motions. A low inertia system would not need much of a ramp, making short rapid motions much quicker.

[Al_The_Man]

Also Machines like the CNC sheet metal punches not only have the low inertia motors but generally also have very high lead ball screw in the order of 1" to 2" lead!
As I have pointed out in previous post, there are many free graphicaly oriented inertia calculation programs on various servo sites, where you can plug in your various machine mass parameters and calculate the inertia at desired accel/decel rates and detect wether your machine falls below the recommended inertia maximum for motor to load of <10:1 .
Al.

[Geof]

Pat;

Perhaps you have seen mention of High Speed Machining, HSM? This is a development to handle the problem you have seen that big chunks of metal are not very nimble; it is not easy to decelerate/accelerate them quickly. HSM handles this by 'looking ahead' in the G code program and preparing for abrupt changes in direction by decelerating earlier so the machine does not overshoot the corner. Without HSM it may not be possible to fully utilize the machine's capabilities because the feed has to be kept low enough to handle the quickest abrupt direction change; with HSM the feed can be pushed up toward the limit set by the cutter, rpm and material type. In a sense HSM puts in the "ramp" that Dan refers to, where it is needed.

[Pat2000]

Gerry,Dan, Al & Geof, Thanks very much for your replys, I can see that I should knuckle down and do some homework on these things, thanks I appreciate your experience and info. the HSM has always interested me, I realise that it is 'out of my league' so to speak but the lessons learned from these applications like the high-leads on CNC punch m/c's is all interesting, I used to work a punch for a while.

what I prefer is a moving tool design or should I say beam and tool design where the masses are a known factor or pretty much constant.

interesting with the CNC puch press is the change fron some tiny bit of ally sheet to a great sheet of say 3mm steel !! not much of a constant there - I suppose its 'engineer for the heavy load'.

Gerry - yes I do hope to use mach3 - I note the exact stop/Constant velocity mode, I hope I grasp the application of them!

so rotating mass is my prime concern and I can stiffen up my beam - probably an ally beam, with out too many worrys and at the end of the day 'shell out' for the bigger steppers in my case if really needed

Edit - then we could drive the 'nut' and greatly reduce that rotating mass and stop the 'shaft whip', I kind of realise the complexity of the nut/bearings/belt/motor assembly but I'm a little surprised that we don't see more of those designs?

Pat