[Nicedesigns]

Interesting, thank you! I asked the seller of the low profile assembly for more info on the Kirk lead screw.

The first listing I posted is for a much cheaper chinese assembly. It says it uses a ball screw. Are there any other obviously disadvantages or concerns about that assembly?

The person managing that ebay seller profile is not helpful.

I called Avid CNC more more info about their assembly (it's missing from the product page). It's seems pretty awesome, it's got dual 15mm linear rails with two self greasing bearing blocks per rail. A 10mm pitch ball screw. A dust cover. And it's designed to handle their 8.7 hp spindle. What do you guys think about it?

[machinedude]

with this actuator the servo is only a 100 watts which is not very big, with the 2:1 ratio the torque is better on the drive screw but half's the rpm of the servo motor which i think was rated at 3000 rpm so you end up with a working speed of 1,500 rpm with a 10 mm lead screw which would probably still give you decent rapids and feed rates on a z axis. the big concern would be does the small servo has enough power to lift the head? torque speed curve charts are helpful for all this. so you would need very specific data from the servo manufacturer to make a determination for power requirements.

taking a wild A$$ guess i would tend to think it would be underpowered. the screw pitch has a lot to do with it too so you need to factor that into your decision making process as well.

i can only offer basic considerations to take since i'm not an expert by any means.

one other thing that comes to mind is with a timing belt you introduce backlash from the timing belt into the system. so you end up with more backlash as a result. timing belts probably have .002 to .003 of backlash. this is not a huge amount but in a cnc application it's enough to add problems if you want things to be accurate.

[awerby]

There are lots of ball screws on the market. Some can legitimately be called "precision" ball screws. Others can't. Ball screws are made for use as skylight openers, etc.; they work fine and stress their motors less than other types of screw, but can't be counted on to produce a certain position at a certain degree of rotation. This latest linked ebay candidate seems closer than the previous ones. but that presupposes that the seller's assertions are true.That's possible, but not certain..

[gfacer]

with this actuator the servo is only a 100 watts which is not very big, with the 2:1 ratio the torque is better on the drive screw but half's the rpm of the servo motor which i think was rated at 3000 rpm so you end up with a working speed of 1,500 rpm with a 10 mm lead screw which would probably still give you decent rapids and feed rates on a z axis. the big concern would be does the small servo has enough power to lift the head? torque speed curve charts are helpful for all this. so you would need very specific data from the servo manufacturer to make a determination for power requirements.

taking a wild A$$ guess i would tend to think it would be underpowered. the screw pitch has a lot to do with it too so you need to factor that into your decision making process as well.

i can only offer basic considerations to take since i'm not an expert by any means.

one other thing that comes to mind is with a timing belt you introduce backlash from the timing belt into the system. so you end up with more backlash as a result. timing belts probably have .002 to .003 of backlash. this is not a huge amount but in a cnc application it's enough to add problems if you want things to be accurate.

Backlash on Z is sometimes less of an issue as the weight of the assembly will always pull one way.

My big Chinese CNC has I think a large +.010" backlash on the Z but it never really an issue except maybe when setting up my tool heights I want to make sure not to back up a few thousandths before entering values, as those would include the backlash and make the whole value off.

Weight of the Z assembly (ie light) versus any friction or static friction would be the issue that would allow backlash to show up in more traditional ways.

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[Nicedesigns]

Hey cnc people! Im hoping to get some more help. I ended up going with the avid z assembly. It's pretty sweet and has totally solved the lost z steps issues. I'm really happy I went with it cause it was a complete assembly and my old tiny motor bolted right up and has been working well.

Anyway, I've had to slow my cutting speeds down a lot, to around 40 ipm. Otherwise I get a ton of vibration. Even at slow speeds I still get a lot of vibration moving along the x axis. I cleaned and adjusted the v wheels and rails and lubed them with paste wax. Don't anyone have any tips for me to eliminate the vibration and maybe speed up my feed rate?

[routalot]

Additional bracing seems to be necessary.Those long rails on the sides may well be oscillating and the gantry beam is a bit scant on both depth and height.The extrusions may seem very stiff in isolation,or if you try flexing them using muscle power.This can change in use if you have anything going on that is at some harmonic of their natural frequency and then it can compound if the vibration in one section gets picked up by the extrusion it is connected to.

You have a frame that would be very good for a 3D printer or a laser,but its carrying a fairly powerful spindle with steppers that can exert quite a force.You need to chase the problem and it might be more than one area that needs attention.You might begin by finding a way to mount a dial gauge to something other than the machine structure and about half way along the long upper extrusion and run a little machining.Look for both vertical oscillation and then move the gauge to check for horizontal oscillation.Keep in mind that any movement may be amplified by the distance from the top of the rail to the tool tip.

[ger21]

What kind of drives does the machine have? It could be resonance, and modern, better drives could eliminate it.
Those motors are way too small for a machine that size, btw.

[Nicedesigns]

What kind of drives does the machine have? It could be resonance, and modern, better drives could eliminate it.
Those motors are way too small for a machine that size, btw.

Really? They're old school, but seem plenty strong, given the machine is so flexy.

Motors: Z-AXIS: P22NRXB-LDF-NS-00 M Series POWERMAX II Hybrid Step Motor manufactured by Pacific Scientific 214 oz in

X-AXIS and Y-AXIS*: M22NRXB-LDN-NS-00 M Series POWERMAX II Hybrid Step Motor manufactured by Pacific Scientific 253 oz in



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[Nicedesigns]

Additional bracing seems to be necessary.Those long rails on the sides may well be oscillating and the gantry beam is a bit scant on both depth and height.The extrusions may seem very stiff in isolation,or if you try flexing them using muscle power.This can change in use if you have anything going on that is at some harmonic of their natural frequency and then it can compound if the vibration in one section gets picked up by the extrusion it is connected to.

You have a frame that would be very good for a 3D printer or a laser,but its carrying a fairly powerful spindle with steppers that can exert quite a force.You need to chase the problem and it might be more than one area that needs attention.You might begin by finding a way to mount a dial gauge to something other than the machine structure and about half way along the long upper extrusion and run a little machining.Look for both vertical oscillation and then move the gauge to check for horizontal oscillation.Keep in mind that any movement may be amplified by the distance from the top of the rail to the tool tip.

That makes sense. I can imagine the amplitude of the vibration growing by the time it gets to the tool tip. My goal for this machine was to have something like an Avid 4x8. I thought with a beefier gantry extrusion (3060?), Linear rails and avid's rack and pinion I would basically have the same machine. But is that wishful thinking? Would it be a waste to spend that kind of money upgrading this?

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[routalot]

I think that short term you need to find out where to apply a solution.If you can identify the area that permits movement,there may be relatively simple ways to improve things.It could be that by just bolting on an additional section in the right location you would change the frequency that is causing the problem.Which would allow you to focus on the next most severe area and on it goes until you have a perfect machine.The first thing is to find the source of the flexing and you have to be rigorous in checking for movement all the way from the tool tip to the legs of the machine.I would delay any new parts until you have identified the location where the biggest improvement would result.