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Lead Screw Recomendataions

Posted by ElmoC 
Lead Screw Recomendataions
December 15, 2016 12:26AM
I'm looking at changing out the lead screws on my printer and was wanting to get some recommendations and suggestions for what to get. They need to be 8mm x 500mm. I could cut longer ones down. The ones I have now have a slight bend to them that I have not been able to get out. They are, I believe, 2mm pitch with 4 starts. I remember reading somewhere that single start screws are better.

So where do you get your lead screws from? Suggestions for what to look for?
Re: Lead Screw Recomendataions
December 15, 2016 05:23PM
I got mine off amazon. I have 400mm 8mm 4 start screws that are fairly straightforward from drillpro.

What's better for leadscrews depends on what you want, speed or resolution. An 8mm 4 start moves 8mm in one revolution. 8mm √∑200steps√∑16 microstepping = .0025mm per step A 2 start moves 4mm in one revolution so the resolution is twice as fine, but speeds are slower. I don't remember off hand the numbers for speed, but basically the screw can only spin so fast because it starts to vibrate. So, fewer starts will make finer details, but print a lot slower.
Re: Lead Screw Recomendataions
December 15, 2016 05:30PM
One other advantage of a smaller number of starts is the pitch is not as great and when the power to the motors are turned off, a heavy bed tends not to cause the screws to spin and drop the bed.

I'll probably stick with the screws I have now for the time being.
Re: Lead Screw Recomendataions
December 15, 2016 07:06PM
My heavy x axis does not drop with 4 start 8mm leadscrews.

All I can say is, buy good ones. I skimped out on mine and they work great but as I get my quality up I can definately see repeatable banding which my best guess is from the screws. Lesser printers may never see this, but it's something that bothers me.
Re: Lead Screw Recomendataions
December 15, 2016 08:09PM
I currently don't see banding from z wobble, but I can see the vibration from the ball bearings, possibly motors too, which is annoying. I am going to sample out some igus filament and see if I can use it to finish my i3 upgrades. My screws were $12 each and came with a bronze nut. I also have them supported by bearing blocks on top and bottom and driven by one motor using a belt. I was just reading that if they are mounted with 2 bearings on each side spaced at the diameter of the screw it allows for a lot more speed.
Re: Lead Screw Recomendataions
December 16, 2016 03:56AM
1 bearing at each end of the screw can cause more banding if your screw is not true. This is where it is handy to have a screw with no trapped end.

My set up is 2 acme (4 start) screws, driven by a closed loop belt. so that the belt does not force the screws out of alignment, I have 2 bearings on one end, with the pulley in between. I tried capturing the other end of the screw, but the mechanism started to bind and quality decreased. With this set up I get almost completely smooth boxes at 100 microns. I still get some Z banding, but this is me being picky. I would have forked up the extra cash for decent lead screws if I could.

also, Why do you need extra speed out of your Z? It can be as slow as you like. Aim for torque, not speed.

Re: Lead Screw Recomendataions
December 16, 2016 09:20PM
I am building a leadscrew only printer currently. Just have to finish up the enclosure design and get the parts made. I also use z hop during retraction and disable combing so my surface finish is nice and smooth and I don't lose details due to pressure drop while coasting. A faster z lowers print time in that situation. I am still tinkering to see what kind of speeds I can get.
Re: Lead Screw Recomendataions
December 21, 2016 11:49AM
See previous discussion here: [forums.reprap.org]

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