Hi Jonathan,
Thanks for contributing.
Do you use the screws for machines you build at the Uni? or do you use them for home use?

2 out of the three screws I had were bent in about 4 places along the length (think sine wave) so something must have bounced on them repeatedly. I also haven't said they are rubbish either. If an individual that doesn't have too much money, like a lot of people building their own machines on this forum do, then they may have to save up for a long time to buy them. I just hope they aren't disappointed "IF" they are bent. The ones I bought also came from Linearmotion Bearings on eBay. I will still buy from them if the need arises which probably will when I build my 5 axis router next year. The good thing about Linearmotion Bearings is they will make any length of screw you need as long as the pitch is 5mm or 10mm. Maybe they have some newer machinery now and can make other pitches but when I asked them, about 1 1/2 years ago, they said they only do those 2 pitches.

I would love 25 to 30 millimetre pitches on the smaller screws so I don't have to spin them as fast for positioning. I have a 12 start 1" on my X axis on the router and it's great. It's an anti-backlash polished acme thread. I've had it for about 3 years and hasn't seemed to have worn any noticeable amount in that time. I lube it every time I use it with a couple of drops of 3 in 1 oil.

A few years ago I bought a ballscrew from the USA from McMaster Carr ( VERY inferior to the Linearmotion ones and which would probably be better used for a jack screw) and when it arrived from FedEx it had a 90 deg bend in it at the top 1 foot down. This was a hardened steel 5/8 screw. It must have had a car fall on it or something. I was able to return it for a replacement which took about 1 week to arrive. Maybe that was the case with the other ones. I'm not a clairvoyant so I can't answer that. I can see Jazz's point too if he is selling the product. He obviously wouldn't want any negative things said about them which would impact on his profits.

The easiest way I have found to measure the run out of the ballscrews is to use the nut as a gauge and keep it from turning so it rides along the ballscrew and select the pitch of the leadscrew on the lathe to follow the same pitch of the ballscrew and use an indicator on the ground part of the nut. There will be some minute discrepancies with the pitch's but measuring where the balls ride it is much better than measuring across the outside where all the deformation is after the rolling process. Sounds harder to explain than to do it

To Robin,
I use chain drive in my plasma table build and have found that if you go over about 18tpi then the cogging effect is reduced tremendously and seems pretty smooth. I gear down the stepper with 5/8 wide TDH belts 3 or 4 to 1 then drive the roller sprockets with the chain. It's actually pretty smooth and I only have problems with chain stretch or bounce if I go over about 8meters/min in speed. You have to adjust your acceleration to suit as well.
I used the chain drive selector on the Renolds Chain selection chart and mine will last 35 years without lubrication. I guess it will be a lot less than that after all the plasma dust works it's way in. It's very cost effective compared to rack and pinion. If you can afford rack and pinion then I would use them as there is zero bounce with that setup. It's just so darned expensive.

Richard.