[_Phil]

I'm in the early stages of converting a TM45 (RF45 clone) to CNC. I have read many threads on this but cant quite decide if I should purchase quality used ground ball screws or new Chinese rolled and double nut. Both are close price wise and I'm leaning towards the Chinese with double nuts. As this should give me close to zero backlash? Id appreciate some opinions or if I'm overlooking anything.
Cheers.

[AVRnj]

I recently looked into both as well, and chose to go with the Chinese rolled double nuts.

I was not seeing a comparable price though, the used ground seemed to be quite a bit more money when I was looking.

I was willing to spend the money, but my concern was how do you assess how used they are, and how good they are.

[cyclestart]

I was willing to spend the money, but my concern was how do you assess how used they are, and how good they are.

It's a crapshoot.
I won a good used THK C3 from Singapore on ebay, considering it came with the original mounting blocks and bearings it was a steal. The new Cinese screws on ebay don't come with bearings anywhere near that quality. Another ebay find from a Canadian seller was trash, so I don't consider the seller's location to be any guarantee.

A C7 screw won't have the lead accuracy of a C3 even doublenutted, but if the C3 is worn to within an inch of it's life ?

I don't know what will be best for you, but if a used screw has mounting blocks you can use don't throw them out of the equation.

btw this old post may be of interest
http://www.cnczone.com/forums/linear...html#post67528

[_Phil]

Thanks for replies and the link, lead accuracy was something i was overlooking

[arizonavideo]

Finding a long ground screw for the table will be a lucky find. there is a lot of good ground shorter screws for the Y and i would get one if you find one. finding a 33" 25mm ground screw is a bit hard.

[Zekyx]

Hi Guys

First, sorry for bumping this old thread. I'm in the same boat here. What I plan is to build a machine with 350 mm travel on x and y, epoxy granite construction, 20 mm Hiwin rails. It will be used mainly for cutting my own produced carbon fiber parts, but I expect the machine to make molds from 7075 aluminium.

Do I really have to worry about lead accuracy, since I don't do precise machine elements. What I need is repeatability, to have as smooth mold as possible (we always hand polish them after machining, so I know what kind of finish to expect).

So, will I get good results with chinese double nut ballscrews, or ground is a must.

Thx

[Fastest1]

IMO if you are going to polish anyway, double nuts would work. Single nuts if reballed with larger balls would work well too. There are a few here who claim .001 or less on single nuts.


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[109jb]

Lead accuracy can be compensated depending on the control software. One user on here used rolled screws for a G0704 build and then he mapped the screws in LinuxCNC using a glass scale for measurements.. He did a complete write up to show the lead accuracy of the rolled screws before and after mapping. In a nutshell, the rolled screws after mapping can have compensated lead accuracy as good as a ground screw. That is if the control software allows screw mapping. I know that LinuxCNC does, and have heard that Mach3 had an experimental version of screw mapping. I don't know if it was ever finalized and proven though. Don't know about other control software that may be able to do it.

[lcvette]

You can also get ground double nuts from china, they are a little bit more but I have seen them in action with less then .0001" backlash when properly shimmed and the lead error is significantly less so shimming doesn't create the binding issue that rolled screws do and allows for a much closer to zero lash shim be installed.

They grade on the ground screws is P4 and C5 I believe and want to say the cost was around 350-400 for all 3 screws with double nuts. Definitely the way to go if you are running a bigger machine.

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[lcvette]

Also, the biggest issue with cheap ballscrews is the backlash in my opinion it is way worse then the minute amount of lead error as it wreaks havoc on circle pockets and any time the acis changes direction even with backlash comp turned on. Always leaves a dent at the 90's

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