Did I get a bad Proxxon MF7?

[DennisH]

Hi All ---

I just received (from Amazon.com) my new Proxxon MF70. This is my first experience with milling machines so I don't know if what I'm concerned about is normal or if my particular machine might have some damage.

The X-Y table movement is solid with no backlash in the hand wheels that I can feel. But the z-axis hand wheel has 1/4 turn of play. So, for example, if I lower the motor assembly (turning the z-axis wheel clockwise) and then stop and reverse direction, it will take 1/4 of a turn in order to take up the lash and start pulling the motor upward. There is an adjustment screw that can be tightened which, according the the manual, is for adjusting this play, but when I tighten it all the way, all that does is prevent the motor from being able to move up or down, the 1/4 turn play is still there. With that screw loosened, the motor has about 1-2mm of vertical play, and only the weight of the motor keeps it at the bottom of its range of play otherwise.

The inside mfg's box from Amazon had a couple nice gouges in it --although I couldn't see any damage to the machine-- but the box wasn't sealed and there were some scratches on the base as if someone pulled off the x-y table without completely removing the hold down screws.

I can't wait to start cutting stuff up, but I want to make sure everything's right before I start going at it. This btw is the 2nd MF70 I got from Amazon. The first one had a major dent in outside of the box like it had been dropped from 4-5 feet, but Amazon was super about letting me return it for this one.

Thanks for any input anyone can offer on this.

Dennis

[neutronics]

There will always be some backlash in a normal ACME leadscrew.

The Z lock must be engaged before making any cuts with your machine. The leadscrew will move Z up and down, but will not hold the spindle against the cutting forces (since the only opposing force really is just gravity and the weight of the head).

Just completely loosen the lock screw before making a change to Z, lock it back down and then make your cut. If you’re converting this rig to CNC you’ll have to address the lash in Z, but this is normal for a typical hobby benchtop manual machine.

[DennisH]

OK, now I get it. Thanks. But why wouldn't the x and y axes have the same behavior? Just the slightest turn in either direction causes movement of the x-y table. There is no play I can feel there at all. If I put a counter-balance on the motor to render it "weightless" as far as the lead screw is concerned, would it behave as the x-y axes do?

In researching this whole business much more, I changed my mind about the Proxxon mill, and returned it. Unfairly now it seems. But I'm going to be getting the small Taig mill instead. It surely costs more, but I'll have more flexibility if I decide to get more into machining for projects in the future. I think the first thing I'll do is put a counterweight on the motor. I'm actually surprised that isn't a normal factory-supplied feature because it seems so logical.

[awerby]

I don't think you really need one. The normal Taig motor is heavy enough to put permanent down-force on the spindle head, which usually is enough to eliminate backlash in the Z axis. A counterweight would eliminate that, so it would bounce around more in response to transient cutting forces. It would make more sense to hang weights on the X and Y axes, since that would tend to eliminate backlash, but that would also increase the load on the motor in one direction, which could cause lost steps. Unlike many of these retrofit kits, Taig CNC mills are engineered to work correctly right out of the box(es). Try it first, anyway, and if you have a problem, start thinking about modifications then...

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[DennisH]

I was thinking about the ease of using the z-axis hand wheel to make it feel as non-backlashy as the x-y wheels do because on the Proxxon, for the short time I had it, manipulating the x & y was like silk but moving the z-wheel was cumbersome --you always have to think, "Am I coming in or going out?". Your observation about it being less liable to bounce around given the motor's weight sure makes sense. On the Taig mill, if you know, how much backlash do you actually feel in z-axis movement?

[awerby]

I've always used the Taig in CNC mode, so I haven't actually felt any backlash. But there's about .001" to .004" on each axis - nothing to worry about for most things you're likely to make, and necessary to provide clearance for the screws to work.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[DennisH]

On the Proxxon I had, one revolution of the hand wheel is 3/64" (1mm). So 1/4 turn would be 0.003" --and that's right in the same ballpark of what you just mentioned for the Taig. At the time I had it, I would have sworn that a very small turn of either the x or y hand wheel (5 degrees or so) in either direction (one direction immediately followed by the other), I could see movement of the table. But if the amount of play on the them was the same as for the z-axis, the sense of movement must have been an illusion. How weird. I've just done woodworking up to now, and I think this is my initiation into the brave new world of machining!