Hello, I am building a DIY CNC mill, and have lead screw question: I have a set of 3 ACME lead screws, and need to select a nut. I have two chices (from Kerk Motion), either their ZBX or VHD series. Their chart is at the following address:
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The screws are 1/2" dia. and 1/2"/revolution. ZBX is rated at 25lbs. and VHD is rated at 150lbs dynamic load. Since there is almost 3 mites price difference, I'd like to go with ZBX, but i don't know if this is enough, or should I bite the bullet and go for the expensive stuff. I will be milling aluminum and may be mild steel. Motor is 1/3 HP, steppers are 180 oz/in.
I've used both on different type machines over the years, and both have their place. You have already identified those differences yourself. Load is the primary factor. 3 times the price gets you more than three time the load capability....
In any event, there are some things you can think about. How much "Push" will your 180oz motors actually push ? To get that info, you would have to refer to all the documentation from the motor manufacturer and then the motor driver manufacturer, and take all that into consideration along with the voltage and amperage levels you will be applying. Each piece of that puzzle can alter your end result drastically. Voltage level alone can affect your end result drastically.
180oz motors are not very large though, and because of that, I assume your not working with a large mill. Think about how much actual load / pressure you might put on a sharp bit. Likely you will never put more than 25lbs of pressure on a bit when cutting with a small machine unless of course the bit is really dull and likely beyond useable anyhow.
So, some rambling thoughts.... Go with the less expensive one, then you'll be able to purchase two more NEW ones when and if the first one fails or wears out......
Come to think of it, I have some 180oz motors driving 1:1, some .5" lead (1/2" per turn) screws WITH ZBX's..... Runs about 120ipm max on a small engraver I built a few years back. I have used ZBX nuts on machines that ran two 8 hour shifts a day for many years before they deteriorated.
Some more things; If the Nut/screw is not covered, chips will get into that ZBX and subsequently come into contact with the screw easier than with the VHD.......
The ZBX takes less power to turn by design
Mcmaster-Carr sells the ZBX style but not the VHD (They also have kerk screws BURIED under the name of "precision modified acme screws")
The VHD is more adjustable by design whereas the ZBX is controlled by the single spring pressure. The VHD has a ring that can be turned to put more or less pressure on the opposing nuts. The ZBX does not have opposing nuts.
The VHD's take up more space.....
Being a mill, I would jump out and run with the VHD because there is a good chance your going to end up running larger motors unless it is a really small mill. if you build around the smaller nuts and then later decide you should have went with the VHD's, you might have clearance issues.
Grummy, Thank you for the reply. There is an old proverb: "Advice is the thing we seek when we know the answer but don't like it" I'll go with VHD, so the mill is somewhat future proof.
One last thought. I assume your statement about the screw you were using indicated a "1/2" 'LEAD', meaning 1/2" travel for each turn of the screw.
IF that is the case, and if you happen to run 1:1 with your motor, at full stepping, your resolution is going to be kind large for a mill (.0025). Of course, microstepping brings that down, but microstepping is not really a good way to ensure tight resolution.
You may already know this and plan to run those screws at a slower speed than the motor via belt or other drive mechanism, which of course would be your best bet if your stuck with the screws.
Grummy, Do you think 3/8" x 24" ball screw with 1/8" lead would be a good choice? I think I'm going to end up just getting ball screws on e-bay, since those ACMEs are going to run me not a whole lot less than a set of "least expensive" ball screws.
Is there a formula or a rule of thumb regarding screw diameter?
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