z-axis couter weight

[yoopertool]

I have been using my newly built CNC router for a few weeks now and am very happy with everything other than my z-axis. I have a 425 oz-in stepper motor moving my z-axis via a ballscrew and linear ways. The problem is that after I mount my router to my axis, it is rather heavy and my stepper actually stahls out and looses steps intermitantly. There is no binding at all. I applied an upward force (with my hand) on the z-axis while it was moving and that eliminated any stahling. So as far as I know my two options are to either insert a spring in there somehow or rig up a counter weight. Does anyone have a preference as to which one to go with or another method altogether? I would appreciate the help. Thanks.

[ger21]

Look for a constant force spring at McMaster Carr

[yoopertool]

Thanks for the response Gerry. I have never seen a spring like that! That should work quite well actually. Unless anyone has a better solution, I think that I will end up going with that. Thanks again

[zephyr9900]

yoopertool, another option is to reduce the speed, and possibly the acceleration, of your Z axis. Stepper motors produce their best torque at zero speed, and the torque available (to a first approximation) decreases linearly with speed. Even if you are planning to counterbalance the Z axis, this is a bandaid you can apply immediately. (Of course, that will reduce the speed of your whole machine if you are doing 3D contouring (like carved signs) but if you're doing 2.5D work (pocketing etc.) it will not slow things down much.)

How heavy is your Z axis? Constant-force springs have a finite lifetime in terms of extensions/retractions, and can get quite large in order to support a load with a reasonable lifetime.

I have a pair of Ametek (Hunter) Neg'ator SK18P47 springs rated for 13,000 cycles. They support 5.7 lb each, are 1" wide and use a hub of 2.4" diameter. The largest spring in the range, a SK31U68, supports 19.6 lb, is 2" wide and uses a hub of 4.12" diameter.

If you are willing to accept a shorter lifetime, 2500 cycles, the springs are smaller. An SH20S47 will support 19.8 lb, is 1.5" wide and uses a 1.75" diameter hub. I used a pair of SH10K29 (2500 cycles, 5lb each) to counterbalance my Sherline mill's head for a while, but stopped using them since they really didn't make a big difference.

The springs are definitely much lighter than a counterweight, so from that standpoint they will give less load on the X and Y axes if your machine is a gantry.

Best regards,

Randy

[Khalid]

could someone send the pics of their machine with spring mounted..:-)

[zephyr9900]

Khalid, here's a picture of my Sherline with the constant-force springs. It doesn't look much like a Sherline because I also relocated the spindle motor behind the column, to balance the head about the Z-axis ways.

The reason I removed the springs was because, with the head balanced, the Z motion was silky smooth even without the springs. And I replaced the kludgy small idler pulley (bronze bushing that slid up and down on a steel pin) with a pair of proper ball-bearing mounted pulleys when I installed the 10000 rpm pulley set.

Best regards,

Randy

[Khalid]

Wow gr8 job ProtoTrains... i am planning to counterweight my router ... If there is anybody else having spring at Z-axis gantry...

[yoopertool]

Thanks for the responses. Prototrains, your sherline looks great! I have been messing with my acceleration and speed, but it has not taken care of the issue. My router is Fairly large, about 4' x 5', and the z-axis is fairly heavy duty as I plan on doing fairly heavy cutting. I would estimate that the moving part of the z-axis with router attached is between 25lb and 35lb. I have a 4-tpi ballscrew mounted directly to my motor and at the moment am only trying to travel at about 40ipm (160rpm from motor). I would like to be able to travel closer to 100ipm as my other axes travel at 200ipm+. I am pretty sure that my motor is a bit undersized, but until I can afford a larger motor and driver I think that I will have to just apply the "band-aid" and mount springs. Do these springs come with a spool? or do you have to make your own? Thanks again for all of the input!

[ger21]

What type of drive are you using? And what if you drop your accel by 75%? Will it go faster than? 30lbs isn't that much to lift.

[zephyr9900]

yoopertool, the springs come "bare" and you need to make your own hub. The spring in its natural state is coiled tighter than when on the hub, so you are prestressing it a little when you wind it on the hub. The spring generates its force by being straightened from the coiled position, which is why it has linear force. The good news is that your router will have much less travel than the spring is long, so if the spring does break you can just unwind some more, punch a new hole by the end and use a new section of the spring which hasn't been flexing.

Best regards,

Randy

[yoopertool]

Originally Posted by ger21 What type of drive are you using? And what if you drop your accel by 75%? Will it go faster than? 30lbs isn't that much to lift.

I unfortunately am using a cheap AlienCNC drive, but have been surprised by how well it has been working for me. I have the acceleration down about as far down as it will go and it doesnt act much differently than when it is turned up. I will try a bit more with that, but I think that I will end up just adding a spring for safe measure.

[ger21]

If it's a dual shaft motor, a dampener might help. See this thread.
http://www.cnczone.com/forums/showth...639#post256639