newbie: proxxon mf70 machining aluminum

[khrtt]

Hi everybody!

Allright, so I got me a new shining proxxon mf70, and converted it to cnc -- congratulations me :-)

Now, I'm trying to machine some flat parts out of a 3mm 6061 plate. I'm using a 4-flute carbide 3mm cutter, cutting through the plate at 20k rpm in 3 passes, and then shaving off 0.2mm for the finishing pass in climb direction -- the feed is about 1mm/sec.

I get WOBBLY CUTS!!

I get a nice mirror finish but -- here's my problem -- the cut is about as wobbly as you can imagine, every time I cut in a diagonal direction the side of the cut has a 0.1mm or so of wobble. The wobbles are about 1.5mm long, which is coincident with the 1mm lead screw pitch on the mf70. I tried adjusting the gibs, tried changing speed and feed -- nothing helps!!

WHAT AM I DOING WRONG?

Now, the thing gives some wobble when machining plastic as well, but it doesn't seem to be nearly as much.

I suspect that I am not adjusting the gibs correctly, they are either too tight to too loose, and I just can't figure it out. I've never used a mill before; I really don't know how to deal with this.

Any ideas?

Similar Threads:

• Proxxon MF70
• Proxxon MF70
• Proxxon MF70
• Need Help!- PROXXON MF70
• proxxon mf70 cnc

[serriadh]

Couple of things immediately spring to mind...

• Misaligned motor
• Eccentric motor/leadscrew driving wheel/gear
• Dodgy leadscrew

A poorly aligned motor without a sufficiently forgiving coupling seems like the most likely culprit to me, especially if the error is indeed occurring once per screw revolution. Try cutting a bunch of straight lines lines parallel with the suspect screw (so only one axis is doing any work) of various lengths, and see how close the actual length comes to the commanded length. This should help identify the problem axis if you didn't already know.

You're not using gears to couple the motors to the leadscrews, are you? How about timing belts or similar? If they were mounted eccentric you'd get periodic errors, I reckon.

It is possible that the leadscrew could be damaged but I don't know whether a screw that is bent enough to cause problems yet straight enough to run nicely in a CNC cycle would cause this sort of error. It would be once per cycle periodic though, however it manifested. The severity of the error would likely differ depending on how far along the axix the nut was at the time. Are the ripples the same size regardless of how far along the axis thes bed was during a cut?

You could try sticking in a new leadscrew, though that may end up requiring some new bearings and a little bit of engineering. I've got some new trapezoidal screws for my proxxon KT150 table, but once i'd turned down the threads to get a smooth OD for bearings it'll only be 8mm dia instead of the current screw's 10mm OD. I imagine the KT70 screws are similar, though at least the nuts on the KT70 are so chronically prone to backlash! Possibly you'd need to get some acme screw and make a new nut; no idea how easy it is to get hold of trapezoidal thread leadscrews in the US.

Doesn't sound like a gib issue to me, but I could very well be wrong. Are the ways well oiled? I'd expect a dry way/overtightened gib strip to manifest as random stick/slip rather than a relatively steady 1-twitch-per-rev, though. As you changed gib tightness without any effect, I'd say that probably rules them out as a cause.

[khrtt]

I have lovejoy couplings on all axes, tapped to fit the stainless M6-threaded rod that I've replaced the stock leadscrews with (stock screws are also M6). Each screw is attached with 2 thrust bearings, jammed between two nylock nuts in this order: nut/bearing/mounting plate/bearing/nut. The free end of the screw is unsupported at all. The free ends do wobble a little bit as the screws turn, which indicates that the screws are a little bit bent, but so what? They should still pull/push correctly.

Granted, I did a far-from-perfect job tapping the lovejoys, but it runs ok, i.e. doesn't bind :-).

And, I get much more wobble than the play in the rubber spider in the lovejoy would account for, even if they did bind somewhat. And then, the amount of wobble I get would probably depend on feed rate, and it doesn't.

In other words, I'm, like, dumbfounded.

Couple of things immediately spring to mind...

• Misaligned motor
• Eccentric motor/leadscrew driving wheel/gear
• Dodgy leadscrew

A poorly aligned motor without a sufficiently forgiving coupling seems like the most likely culprit to me, especially if the error is indeed occurring once per screw revolution. Try cutting a bunch of straight lines lines parallel with the suspect screw (so only one axis is doing any work) of various lengths, and see how close the actual length comes to the commanded length. This should help identify the problem axis if you didn't already know.

You're not using gears to couple the motors to the leadscrews, are you? How about timing belts or similar? If they were mounted eccentric you'd get periodic errors, I reckon.

It is possible that the leadscrew could be damaged but I don't know whether a screw that is bent enough to cause problems yet straight enough to run nicely in a CNC cycle would cause this sort of error. It would be once per cycle periodic though, however it manifested. The severity of the error would likely differ depending on how far along the axix the nut was at the time. Are the ripples the same size regardless of how far along the axis thes bed was during a cut?

You could try sticking in a new leadscrew, though that may end up requiring some new bearings and a little bit of engineering. I've got some new trapezoidal screws for my proxxon KT150 table, but once i'd turned down the threads to get a smooth OD for bearings it'll only be 8mm dia instead of the current screw's 10mm OD. I imagine the KT70 screws are similar, though at least the nuts on the KT70 are so chronically prone to backlash! Possibly you'd need to get some acme screw and make a new nut; no idea how easy it is to get hold of trapezoidal thread leadscrews in the US.

Doesn't sound like a gib issue to me, but I could very well be wrong. Are the ways well oiled? I'd expect a dry way/overtightened gib strip to manifest as random stick/slip rather than a relatively steady 1-twitch-per-rev, though. As you changed gib tightness without any effect, I'd say that probably rules them out as a cause.

[serriadh]

Me, I'd have got leadscrews in a threadform more suited to linear motion... eg trapezoidal or acme. I'd have turned the threads off the end and mounted bearings and couplings on a smooth, true shaft. I don't think plain old threaded rod is made to any sort of precision that you'd care about, though presumably the stock screws were.

Have you worked out which axis is causing problems? Or is it both?

I have lovejoy couplings on all axes, tapped to fit the stainless M6-threaded rod that I've replaced the stock leadscrews with (stock screws are also M6). Each screw is attached with 2 thrust bearings, jammed between two nylock nuts in this order: nut/bearing/mounting plate/bearing/nut. The free end of the screw is unsupported at all. The free ends do wobble a little bit as the screws turn, which indicates that the screws are a little bit bent, but so what? They should still pull/push correctly.

[khrtt]

Can't agree more about the leadscrews. Unfortunately, the only machines I have access to is the little proxxon, and a big 3-axis grizzly, so I can't turn down anything with any kind of precision :-)

I got the idea of using the regular threaded rod from some project description on the internet; that guy apparently either had better luck than me, or wasn't trying cutting aluminum :-)

if the leadscrews turn out to be the problem, I'm going to either try fitting the original ones back onto the machine; and perhaps thinking about some adjustable A/B nut arrangement... which would be difficult, given how little space the proxxon has inside of its crossslide.

I'll try re-doing the gibs once more first, though...

Me, I'd have got leadscrews in a threadform more suited to linear motion... eg trapezoidal or acme. I'd have turned the threads off the end and mounted bearings and couplings on a smooth, true shaft. I don't think plain old threaded rod is made to any sort of precision that you'd care about, though presumably the stock screws were.

Have you worked out which axis is causing problems? Or is it both?

[serriadh]

It has been suggested that the stock screws and nuts on the MF70 aren't too bad; there are a few conversions about that keep them. One I looked at some time ago: http://www.indoor.flyer.co.uk/millingmachine.htm

As for installing small antibacklash nuts, will be tricky in that small space but I'm sure with a little ingenuity something can be worked out ;-) The lazy option is of course just to tap a little lump of delrin and use that... it'll squeeze the screw quite tightly and may require a few retaps until you can move the bed without needing too much force. Use it til it starts getting a bit loose, and swap it out. Depending on how hard you'll be working your mill and how precise things need to be, this may well be a perfectly acceptable approach.

[serriadh]

I have lovejoy couplings on all axes, tapped to fit the stainless M6-threaded rEach screw is attached with 2 thrust bearings, jammed between two nylock nuts in this order: nut/bearing/mounting plate/bearing/nut.

If the bearing mount is sloppy or misaligned, that too can cause periodic error. Just reading up on a related problem elsewhere. Do you have anything to stop radial movement of the screw? Are the thrust bearings in a recess, or anything? Or are you just hoping that between the motor bearings and the leadscrew nut the screw should stay pretty straight? The lovejoy mount won't help that.