Thread: Knee Mill Gas Spring Rating

How about bathroom scales? They go up to 120kg or more. That should be more than enough to measure the springs either in or out of the machine.
Two bath room scales would have to be able to measure the knee wouldn't they?

The following is some rambling as I have no CNC Z axis experience.
As for "what proportion of the load on the Z-axis lead screw should be supported by gas springs". I'd say the more the better so 90%? most times you will either add a heavy vice or a heavy job on the bed anyway. Sure there will be some lifting force with some cutters. But surely it couldn't be more than 10% of the total Z load. Assuming your CNC accelerates/decelerates I can't see you throwing the knee very high

If the struts have leaked out you can have them regased.
I played around with gas struts on my mill when I fitted the knee power feed, but couldn't get enough travel for the given space. I am going to use a air ram as Chich on cnc zone has done.
For using them on the knee you don't want to take all the weight off it as this will give you backlash problems unless you have a ball screw on the knee.

Below are some scans of my Ovesco catalog for their range of struts that might help out. There phone number is at the top if you need it and I hope it's readable.

Dave

Try Stabilus, you will find they make them for everyone else.




Products | STABILUS GmbH | Worldwide manufacturer of Gas Springs and Damper Systems

Hi
John this is how we solve our z axis.
We have a Hare and forbes hm45 that we cnc. We cnc the z axis using a air cylinder and tank system.
In the attached photo the brass cylinder is mount down the back of the column with a wire going from the head up and over the column around the end of the ram and back to the head the red fire extinguish is the tank.
We run this so the head weight (100kg) is equal to zero so the EMC does note have any issues with having different load going up to down. But this can be adjust by changing the air pressure, this might work well for you. We could not get this from gas spring we believed.
Our drive has the smallest amount of backlash being direct drive sevos and double ballnuts with digital scales(1um) no encodes used.
Hope this give you so ideas toward you z axis

All the best
Russell

Thanks for all the comments.

The balancing system by Russell is particularly interesting, this is what I am considering for my small manual vertical mill - just so that I can confidently wind the head down without worrying about backlash.

Dave provided a link in the thread "Bridgeport Clone Lubrication & Swarf" to a chap that overhauled a Bridgeport and added CNC.
The gas springs used on this machine were 2 x 200#.
In the US, the '#' symbol is used frequently to denote lbs weight.
The gas spring rating would then be about 180 kg = 1800 N total.

I will search the CNCZone forum in the US for more information on gas spring ratings.

I will also make sure the Z-axis stepper motor acceleration and speed settings are not too severe for the leadscrew.

John.