Z axis - how many starts 1 or 2

[RicknBeachcrest]

Greetings everyone.
First let me say a sincere thank you to all who have posted their invaluable experiences on this forum.

This is my first post and as many who have come before me, I have been reading and rereading the posts on this forum. I have gone from the planning stage to the parts acquisition stage. My plan is for a 24" X 24" 8020 router. I am getting a K2 5" linear slide acutator and have a choice between a one start or two start leadscrew. I am leaning toward the one start since z axis will only be moving 5" at the most.


Any thoughts?
Thanks.
Rick

[cncserveng]

I think thats a good decision because the ballscrew will last a lot longer and the axis will be more controllable during reversals. Speed for the sake of it is not always better.

"Better to be a snail than a rabbit".

[cyclestart]

Originally Posted by RicknBeachcrest I am leaning toward the one start since z axis will only be moving 5" at the most.

I have a ZA500QR Z from K2 with 10 tpi single start.
The 100 ipm rapids (80 ipm feed) advertised are optimistic without frequent application of lube. It is possible mine was made on a Monday
Even at 100 ipm it was the slowest axis on my machine and was is the active word. It's been replaced with a ball screw assembly. OTOH, if a reliable 70 ipm is fast enough it's an okay unit. Speed is a drug

[hemsworthlad]

A better thing to ask your self is what will you do with the machine.?
If mainly 2D-2.5D then speed are not so important but if you plan on doing lots of 3D work then speed/resolution becomes important.

If you have the abilty to build the rest of the machine why pay for an expensive and from reading previous post slighly inferior sounding product.

Why not use Chinese ballscrews that come pre-machined with end bearing blocks which you basicly just bolt on to the back plate are cheap and much better than lead screw.
Combined with decent rails which again just bolt on It's not difficult to make your own for a fraction of the price thats much superior in strength and accurecy.

[cyclestart]

Originally Posted by hemsworthlad and from reading previous post slighly inferior sounding product.

At 80 ipm the axis would squeal due to friction in the anti-backlash mechanism. There was black debris proving the top nut was being shredded. The weight being lifted was ~12-15 lbs. I contacted k2 support and won't be revisiting that route any time soon.

If anyone can suggest a fix it would be much appreciated. Alternatively if anyone wants it for $200 plus shipping via Canada Post, it's yours Total run time on it is less than an hour.

[ger21]

Getting the fastest possible acceleration should be your main goal here, as it will determine your max feedrates when doing any 3D cutting. Actual speed isn't as important, imo. Motors and drives play a big part in this.

I have a 1/2-10 single start, though, and would go for 1/2-8 2 start next time.

[RicknBeachcrest]

Thank you everyone for sharing your opinions. I won this z axis on ebay last week and K2 gives the option of either 1 start or, for a little more money, they will put a two start screw on it. From the pics it looked like a very well built piece. I guess time will tell. As this is my first cnc machine, I thought I would try and keep some of the unknowns to a minimum and buy a ready to go sub-assembly. I plan on using acme screws for the x and y axis so I doubt the z axis will be the slow part.

I do a lot of woodworking and thought that a cnc router would be an excellent addition to the shop. So my anticipated use was basically 2d routing. But experience has taught me that an anticipated use of an item is rarely what happens in the end.

Rick

[hemsworthlad]

Originally Posted by RicknBeachcrest I plan on using acme screws for the x and y axis so I doubt the z axis will be the slow part.

Like Ger says speed is not so much an issue with Z axis it's acceleration your looking for and if you use correctly sized motors/psu/drives etc you wont have any issue's for 99% of the work you plan. Intense 3D work came tax a weakly matched Z axis.

I'm confident it will be fine for your intended purpose and you'll love cnc machineing. . . . It's very addictive and rewarding.

[cyclestart]

Good luck with your purchase. Apply some lube to the screw and start out with slow settings. Someday I'll tear mine apart to see what's wrong, it's likely just a bit of misalignment of the nut.