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John, I don't think you've offended anyone with your advice. It's good advice to suggest servos in the right context. I think the problem with servos though is that the price really shoots past what is affordable for a "hobbyist" machine. For what I'm planning to do, I'll never need the speed servos aford. I'll be happy with 120 ipm cutting speeds, since I won't be trying to make money with mine. I think the folks discussing it here are in a similar position, unlike yourself who couldn't run your business without tremendous speeds. Some of them will be happy with a 24"x30" machine, and will never care about cutting at high speeds.

Using steppers can be a good starting point to gain exposure into small-scale CNC. Once a person gets some experience with it, they will be able to determine just how far they want to take it, and can upgrade or do what you did and get a newer, better machine that meets their future needs.

As far as repeatability, you can always re-zero on occasion just to avoid any position error. Again, the hobbyist can afford the time to recalibrate to maintain accuracy, since they probably won't be cutting 100 pieces at a time.
That's a good point you raise though, and should be a consideration when making purchase.

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

Don, I agree about re-homing being a good solution. I used to do that with my stepper machine and it got me through. But, how about this scenario; you know your machine loses a certain amount of position over time for whatever reason, usually harmonics, let's say. You've been running an inlay job on a piece of Ablam for 3 hours. Now the cutter breaks. If you just back up a little and resume the program, you'll go right back to where you left off, position errors and all. But if you re-home, you'll go back to where you should have been when you left-off, which could result in problems; most notably that your tenth depth pass is now a single full depth pass because your cutter is going into virgin material instead of that channel you've been whittling at, which means another broken cutter instantly. What do you do? It'll drive you crazy.

I don't think speed is really a factor in choosing stepper vs. servo, however. For example, my first machine topped out at a nervous 800ipm and had 150w steppers. The 1/2" screws would whip like licorice at those speeds and there was barely enough travel to get up to speed and back down again without slamming into the end. My Techno will do 1200ipm as a matter of course all day long with 450w servos; My third machine does about 400ipm with the same 450w servos as the Techno. The only difference really is the pitch of the ball screw. If my third machine had a pitch like that of the Techno, it would do 1200ipm also. It has the same bearings, same size, etc. I think if you put some 450w steppers on the Techno, it would probably have similar performance. The difference is in the torque curve. The Servo would be weaker at 1,000, stronger at 1,400 and the stepper would be the opposite. A stepper has the most holding power at a stand-still. That's why servo machines more often have Z-brakes.

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John Watkins
CNC Guitar Parts

[QUOTE=John Watkins] Don, I agree about re-homing being a good solution. I used to do that with my stepper machine and it got me through. But, how about this scenario; you know your machine loses a certain amount of position over time for whatever reason, usually harmonics, let's say. You've been running an inlay job on a piece of Ablam for 3 hours. Now the cutter breaks. If you just back up a little and resume the program, you'll go right back to where you left off, position errors and all. But if you re-home, you'll go back to where you should have been when you left-off, which could result in problems; most notably that your tenth depth pass is now a single full depth pass because your cutter is going into virgin material instead of that channel you've been whittling at, which means another broken cutter instantly. What do you do? It'll drive you crazy.
[/QUOTE]

Yikes....great point. I'm sure I'll hit that scenario at some point. I hope not...
Still, I'd rather risk a small part than spend the extra $2000 up front right away. It's a risk, but the whole dang thing is a risk. I'll not rule out servos down the road for sure.

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"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

John is right, zero'ing is a bad thing when working on a single piece. We are talking machines that have precision up to .001 in some cases and when you zero you don't *always* get the level of precision you would think, when I do bridges I actually mill the bottom to the arch of the top and then flip the piece over, becuse it's one piece I avoid homing in that situation so I don't lose any precision.

In fact I wrote my own tool change macro just to avoid that situation, as well I use the tool table offsets in Mach so I never have to home the machine while working on a job. Some jobs require more than one bit and that can be a challenge.

But it took me a while to figure that out, CNC is a learning process, like I said before. CNC is easy in theory, but it's incredibly difficult to master.

-Paul-

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-Paul-

Patriot Guitars

Just to put in my .02...I'd also recommend servos. I looked into all the servo vs. stepper stuff and decided servos were the best investment in terms of accuracy and life.

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http://www.PeakeGuitars.com

[QUOTE=Marty M.] Somebody that has been in business for 10-20 years probably has a good shot of being there when you need replacement parts.

I was referring to commercial grade machines, not hobby machines, and magically picked those numbers(10-20) out of the air :-).
mm[/QUOTE]

Makes more sense. I was just referring to the fact that there hasn't been a hobbiest CNC market long enough for anyone to have been in it for very long. Going with a company that's been around in the commercial market, and who's dipping into the hobby market is a safer choice if you're willing to pay the premium for it, though. I'd like to say they're also less likely to make critical design mistakes, but I've seen some pretty big companies make some pretty poor decisions.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services