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Mach Discussion => General Mach Discussion => Topic started by: Katoh on June 14, 2011, 03:43:31 AM

Title: Lead or Ball Screws (lathe Conversion)
Post by: Katoh on June 14, 2011, 03:43:31 AM
Friends
I am venturing into converting a lath from manual to NC, I have read prior post and peoples experience's in the conversions and there's one thing that puzzles me.
Why do they change the Z and X axis lead screws that are already part of the lathe with more expensive ball screws and have the hassle of fitting and trying to make them work. Is there a reason why the original lead screws do not make the grade? and how is it they are good enough for manual work and not for CNC?
Katoh
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Tweakie.CNC on June 14, 2011, 04:06:54 AM
Katoh,

I think it all comes down to Backlash.
Under light loading ballscrews can exhibit zero backlash whereas adjusting the double nut on a trapezoid to obtain the same result produces much increased friction.

Tweakie.
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Hood on June 14, 2011, 06:24:11 AM
Yes, backlash is the big one, on a manual lathe you do the majority of cutting in one direction and you tend to pull out and move back in the same direction, on a CNC often the motion is both ways when cutting. Friction is another reason as Tweakie says and lastly ballscrews tend to be more accurate but that will depend on the grade of the ballscrew and the grade of the original leadscrew, some leadsrews are very accurate.
Hood
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: RICH on June 14, 2011, 06:27:49 AM
Accuracy,repeatability, wear / longivity of the screw and of course backlash removal. Additionaly one may desire to have a speciific resolution or turning rate of the screw.
The original screw may have good lead accuracy but backlash exists ,and even if adjustable for backlash,you just can't maintain it without constant adjustment.

RICH
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Katoh on June 14, 2011, 08:20:21 AM
Gentlemen
I agree 100% and I see my questioned has been answered, but just to through a spanner in the works. For my own curiosity.
One, backlash easily can be adjusted for in Mach! Yes ball screws can be more accurate but a lead screw that cuts threads is not to bad as well you have to admit. The other point of friction Why don't we simply use a larger motor with a reduction to the screw, solve that problem.
Please don't take me wrong, I'm not trying to be difficult, its some answers I would like to know. Its OK doing something because you have been told that's the right way, but can the wrong become the right way?
Cheers
Katoh
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Tweakie.CNC on June 14, 2011, 08:37:01 AM
Good points you have raised there Katoh.

I would like to know the answers as well. I must admit I know nothing about lathes but Hood is an expert, he will know.

Tweakie.
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Hood on June 14, 2011, 08:41:06 AM
Backlash comp works but is no substitution for lack of backlash in the first place, backlash comp cant take out the issue of the axis being pulled. Yes more likely in a mill but still relevant on a lathe.

I think you may find that the accuracy of your leadscrew is not that great, especially as its a relatively cheap import if I recall correctly. Threads done on a manual lathe of that size  tend to be short so accuracy is not so important as its a short distance. Using CNC requires accuracy over a distance when turning and you may find your leadscrew is not that great.

Bigger motors will overcome friction but you also have to consider friction causes heat which will also affect accuracy.

People do use the leadscrews and are happy, I wouldnt be and wouldnt consider it but you may be happy.

Hood
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Katoh on June 14, 2011, 08:56:19 AM
Good one Hood answered to a "T"
One thing I would like to bring up has anyone played around with the Igus polymer lead screw nuts? They claim zero backlash, dirt and dust resistant, and a long wearing life. I use The Igus Polymer linear s on my router and they are impressive, not cheap buy any means but work under a ton of dust and work well, maybe this could take another twist.
Katoh

Sorry Should have added this for anyone interested.
http://www.igus.co.uk/_Product_Files/Download/pdf/41_GL3_UK_DryLin_TR_RZ3.pdf
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Hood on June 14, 2011, 09:12:42 AM
Plastic compresses and lathes can have high loads. Granted its a smallish lathe you have but still would be my concern.
Hood
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Katoh on June 14, 2011, 09:21:47 AM
Hood Again I thank you for your patience in answering my questions, I can now easily conclude accuracy and reliability can only be achieved by using ball screws, well that's the way I have to go.
Many Thanks to All.
Katoh
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Hood on June 14, 2011, 09:30:16 AM
Also remember there are ballscrews and there are ballscrews. Ground better than rolled and even then there are different classes of them.
Look at the linear accuracy of the ones you are intersted in and see if the accuracy is as good as you want. Again with a smallish lathe  it is not so much of a concern but still worth noting.

Hood
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Katoh on June 14, 2011, 09:42:19 AM
Yes your dead right there. I remember when I built the router the 1.8m screw on the y 3/4"  and the 300mm on z same size were only available from limited sources, I got mine out of the states and again weren't cheap, but I have noticed the market is flooded with all sorts and at no were near the cost of the old stuff.  I cant say how good or bad they are, a lot of people bag the Chinese stuff but some of it is pretty darn good. Its hard to tell any more, personally I think it all comes out of the same shop just re-badged.
Katoh
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Jeff_Birt on June 14, 2011, 10:52:07 AM
IMHO, you are much better off with a really good quality lead screw than a cheap ball screw. In the hobby CNC world 'ball screws' have this magical aura about them and many think that you can't build a good machine without them, which is non-sense. It all depends on how you are using your machine and what you want out of it.

If you are using a machine in an industrial situation than the money spent on a good quality ball screw is money well spent. You are using your machine all day long and time spent tweaking things is money down the drain. As your machine size goes up then the additional efficiency offered by ball screws is also a big benefit. A more efficient drive train means small motors, smaller drivers, smaller power supplies, a smaller foot print, and less power used. The cost of energy to run machinery is huge factor on an industrial scale and should not be ignored. (Since Hood depends on his machines for a living then ball-screws are a great investment.)

For hobby use or small scale commercial use a ball-screw may not be as good an investment. Most hobbyist spend more time tweaking their machines than running them (as it is fun), so a few minutes spent every now and then to adjust the backlash nuts is not a big deal. Also as I mentioned before a good quality lead screw is much better choice than a cheap ball screw. A cheap, inaccurate ball screw is a waste of money.

Open loop backlash compensation, like Mach does, has limitations and it cannot make a sloppy machine work like a precise machine. Think of it like driving an old truck that has a lot of play in the steering wheel. Your brain is really good at trying to compensate for the slop in the steering but you'll still be darting around in your lane on the highway more. What happens when you hit a pot-hole though? The truck is pushed around by external forces that you have a much harder tiem trying to compensate for and you cna loose control. The more slop in the steering the worse the situation can be.
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Hood on June 14, 2011, 11:02:08 AM
Jeff
Problem is  we are talking about re-using the leadscrews on a small import lathe so they are not likely going to be of high precision.

Katoh

The Chinese can make as good as anyone else, just depends what you are paying that normally determines the quality. I have Chinese  ER collets that are as good for runout as any UK or USA made ones and heat treatment seems right as well as they dont seem to be losing their spring and have not cracked yet.

I got ballscrews from a place in the UK, might be worth contacting them. Also worth contacting  Dan13 to see what he has to advise as I know he also got some from them but had some issues but think he got them sorted. Mine were 32mm Dia class 5 ground and I am happy with them. This is the place
http://www.motionandbearing.co.uk/Home.html

Hood
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Jeff_Birt on June 14, 2011, 11:19:13 AM
Hood, yes the stock screws are likely junk. Good quality leadscrews will likely be a less expensive alternitive to ballscrews and may well fit the application of the OP.
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Katoh on June 14, 2011, 11:35:57 AM
Jeff Cant really understand your saying that lead screws are better than ball screws in some circumstances but to use ball screws? What about my lead screws are they really that bad?
Hood you get what you pay for 90% of the time. The other 9% I buy good name products and they end up being absolute CRAP! and then 1% I buy cheap stuff and its better than anything you ever bought? Go Figure.
I should make something clearer, this is no industrial lathe nor is it going to be used in an industrial situation, it was only bought to make parts for a boat project, still none the less if your going to do something then might as well do it right.
A friend  of mine laughs at my 11x30 lathe, and only places it on turning pieces 10mm dia or less and its far to small and weak for an NC work, you really need to use something like his 15hp 5ton beast for anything substantial! I would dearly love to prove him wrong!
Katoh
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Hood on June 14, 2011, 02:04:37 PM
Obviously if you want to take 20+mm off in one pass then you need something a bit bigger but even the wee Sherline lathes can make good parts. :)

Hood
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Jeff_Birt on June 14, 2011, 03:28:50 PM
Quote
Jeff Cant really understand your saying that lead screws are better than ball screws in some circumstances but to use ball screws?


No that is not what I am saying. If cost is a concern you will likely be better off buying the good quality lead screw than a cheap ball screw. Think of it like buying a new flat panel TV. Your friends might be telling you that you 'have' to buy a 50" model. When you look at the various TVs in the price range you want to spend you find that the picture on the 42" model is better. So you can buy a cheap 50" TV with a horrible picture or a slightly smaller 42" TV that has a fabulous picture.  The cheap 50" TV, or a cheap lead screw, does not really get you much.

Quote
What about my lead screws are they really that bad?

No idea. You will have to measure them to know for sure. Chances are if it is a cheap import model than they are not really useful for CNC purposes.
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Promech on June 14, 2011, 07:41:19 PM
The question should be asked the other way around.  Why do manual lathes not use ball screws? That is because lead screws are self locking ball screws are not.  But in the case of CNC where steppers or servos (instead of your hands) do the work they do the self locking for you.  In a manual lathe the operator usually takes a measurement before the last cut, so he compensates for most of the lead screw problems.

If you want to consider a ´half way´ solution then you could go for a ball screw only on the X axis.  In lathe work the diameters usually need to be very accurate, lengths not that much.

Jorge
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: RICH on June 14, 2011, 09:38:33 PM
Interesting to get a variety of opinions on the matter.

Katoh,
You can get a lot out of a "system" and get good prefomance and accuracy for some dollar amount.
As the desired accuracy and preformance increases you pay for the increased system. So only you can decide and define the cost, effort, and end
system quality for what you want or may ever want  to do. You can't compare systems of different quality ie; hobby to  industral. Your's is what you have or will have should you desire to improve it.That my friend is just a fact of life!

As my music teacher said....YOU CAN HAVE ANYTHING ON THE SHELF BUT ONCE YOU DECIDE YOU WANT IT THEN YOU MUST PAY THE PRICE
Then he said .....NOTHING EVER STANDS STILL IT  PROGRESS OR REGRESSES AND THAT IS JUST NATURE

What you see is what you get , but, maybe what you see is not necessarily what you think it is. Ask someone how much  it cost to replace their bearings in a say a Hardinge lathe, or it's lead screw. Can the lathe cut a class one thread, a class two thread, or does the nut just go on the thread?

Practical info on screws:
They are not all created equal. The ones removed from both my Atlas lathes and mill were ground.
You are not going to get ground lead screws for cheap and certainly not on the imports for their machine pricing.
There are lead screws in my engraving machine and the nuts have backlash removal and provide for 20 micron accuracy over their travel 
( yes that's .000020", but i could only verify to .000040", so i will believe the certification papers).
 I know of friends that have cnc lathes using all thread  / crappy cheap threaded rods along with .080" backlash and they have done some nice work using backlash comp. I wonder why they changed them out to ball screws?

I have a Sherline lathe with the anti backlash attachement, the lead screw is quite accurate, but, even with the backlash adjustment, as compared to ball screw it sucks in accuracy. Not bashing Sherline but it depends on what your trying to do with it.

My Rockwell manual lathe will produce threads that rival my cnc converted Atlas lathe.
But the Atlas is very reliable on producing the part over and over even if this dumbo defines it incorrectly.

The "system" is only as good as the sum of it's components and installation. A high end ball screw with poor  / improper bearing installation
 could be just as bad or no better than a higher end screw.

So should you replace the current lead screws with ball screws?
Only you can decide as stated above. ;)

BTW, A non-backlash accurate cnc machine is just pure joy to use.  ;D

RICH
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Promech on June 14, 2011, 09:54:44 PM
Just want to take the opportunity to ask, does backlash compensation in Mach lathe work well?  Has anyone done tests using dial indicators?

Thanks,

Jorge
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: RICH on June 14, 2011, 10:00:24 PM
I used it on both the lathe and my mill. Yes it does work quite well for the intended purpose.
I don't use it anymore as both machines no longer have a backlash problem.

RICH
Title: Re: Lead or Ball Screws (lathe Conversion)
Post by: Katoh on June 15, 2011, 02:07:55 AM
The backlash in Mach works well, on my router I have set it to compensate  backlash in the y screw and the x rack but playing around with ,05mm in timber is just stupidly, so I just keep it off, unless I'm doing picture imaging on timber and then its on. What you will notice it makes the machine jerk and bump as it compensates but you can smooth that out with shuttle speed if I remember correctly.
Yep for the price of things might as well put ball screws in, and be done with it, then I've still got all the original screws untouched if ever I plan to sell the machine.
Thanks Everyone.