Leadscrew flex

[cpcp]

Hi,

I purchased a homebuilt gantry type mill and bought the problems that came with it!

I notice that on milling hard acrylic the x axis leadscrew flexes and straightens giving the cut edge machining marks.

The threaded rod used is about 16mm diameter by 1200mm long.

Are there any tricks to reduce this problem?

So far I can think of: 1. Apply a stretching tension to the leadscrew 2. Get a new Acme type made of harder material and perhaps thicker -although how thick I don't know.

Thanks for any ideas

[svenakela]

First of all, is the existing leadscrew acme or ball?
It all depends on the overall design, not only the screw. As a start you can start out to tension the screw (if possible). Of course you can always slow down and cut with a lower feed rate, but if you want to maintain or get to a higher speed I think you have to upgrade to a 20 mm screw or a ballscrew (16-20 mm, 20 is better but more expensive). You can also consider a rotating ballnut, a little bit more complex to fabricate but it solves a lot of problems.

Regards,
Sven

[cpcp]

Hi Sven,

It is a rolled Acme with a simple nut.

The more I think about the more I think I can't apply tension to the leadscrew as it has to be able to shrink/expand with temperature. So it looks like as you suggest I will have to go with a thicker diameter for better stability.

[Oldmanandhistoy]

Hi,

Is the screw straight and what RPM are you turning it?

I was wondering how easy it would be to flex a screw that was dead straight. You don’t hear of your problem that often and a lot of people use screws that are only 12.7mm in diameter. I’m guessing the screw is bent or you are going past the critical speed or a combination of both. If you are planning a new screw it might be a good idea to remove it before you go and buy a new one just to check.

Just my thoughts,

John

[Madclicker]

My guess is that it's not properly supported on both ends.

[cpcp]

Thanks for the suggestions.

The problem happens at even very low revs (5 rpm) -I have noticed a "whipping" at higher jogging rates.

Over Easter I will remove the screw and check for straightness and if ok re-install and check that the end bearings are sitting firmly and perpendicular.
Looks like if that doesn't work the idea is to invest in a new screw.

Thanks again for the advice -will get back with results.

[svenakela]

If the screw flex at low rpm you have a serious problem.
As the other guys says, check the straightness, the end blocks (check the bearings) and not to forget, what's the condition of the nut and how is it mounted?
If the nut is clamped bad so it forces the screw to bend it can cause a lot of problems. Make sure everything is tight, but straight.
To check the screw, put it on a flat surface and roll it back and forth by hand. If you don't visually can see any flex, use a leefer gauge and test for clearance.

Regards,
Sven

[cpcp]

Pulled the leadscrew and checked for straightness. Result: bent. Overall there is approximately a 0.6mm maximum deflection along the 1200mm length. When turning the screw by hand I noticed it binding slightly every revolution and I guess that under load that bind force increases greatly giving the stuttering motion I was getting.

I don't imagine I can straighten the screw so will have to find a source of a new one here in Australia and find an engineer to turn down the ends for me as I don't have a lathe large enough. As mentioned by previous posters I will also redesign the end bearing mountings.

Thank you very much for the help everyone

[Oldmanandhistoy]

Originally Posted by cpcp Pulled the leadscrew and checked for straightness. Result: bent. Overall there is approximately a 0.6mm maximum deflection along the 1200mm length. When turning the screw by hand I noticed it binding slightly every revolution and I guess that under load that bind force increases greatly giving the stuttering motion I was getting. I don't imagine I can straighten the screw so will have to find a source of a new one here in Australia and find an engineer to turn down the ends for me as I don't have a lathe large enough. As mentioned by previous posters I will also redesign the end bearing mountings. Thank you very much for the help everyone

Every lead screw I have ever bought has been bent. If badly bent I straighten them before I machine them and then do it again after if necessary. This can be a little time consuming but not too difficult if you have a good flat surface. What I do is turn the screw on the flat surface until I have the high spot at the top. Place me hand over the high spot, lift one end and check for straightness, repeat if needed. On some occasions I have had to use a vice and 3 wooden blocks, care needs to be taken not to over compensate. Would be interested to know if people use other methods that may be better than the ones I have given.

John

[cpcp]

John, I was a bit doubtful about your "bending" suggestion but figured I didn't have anything to loose. Well after gently bending it back as straight as I could and re-seating and adjusting my end bearings my machine has never cut better! The machine just sounds more taut and the chips look so different. I am getting a better cutting finish at 10 times the previous speed

Thanks to all once again!

[Oldmanandhistoy]

Thanks for coming back on this and letting us know how you have got on. Very pleased to hear of the improvements. I have very nearly finished my new machine which uses a 14mm screw at 1.5m long. I have done the calculations on the screw and it should be good to over 580 rpm before whip. Different end fixity makes a lot of difference to the max rpm of a screw. If it is of any help there is a lead screw calculator here http://www.nookind.com/acme/AcmeCalculators.cfm , click the link in the critical speed section.

John

[svenakela]

Great work guys, with a small amount of support from Mr. Oldman and you're good to go. Jeeze I like this place.