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scubasteve_911
08-24-2007, 01:38 AM
Hello,

Does anyone know about the precision rating of the Taig Leadscrews? It seems to be a pretty unpublished / unknown specification. I'm thinking of replacing them with some 10 to 20 TPI 1/2" 0.0001"/1" leadscrews, but I am having a bit of trouble trying to find a nut that is such low profile. At least if I replace the screw, it'll give me a chance to use the helical couplings I have lying around.

Has anyone tried replacing these with better screws?

Also, has anyone improved the bearing support? I was thinking of using two Abec-7 deep groove bearings preloaded to give that extra rigidity.

I just haven't been too happy with the machine's precision lately and am trying to cook up methods to improve it without breaking the bank. It's probably the gib play and preloading the nut better..

Any suggestions on how to gain that extra bit of precision? Besides buying a new machine :(

Steve

doanwannapickle
08-24-2007, 10:22 AM
The Taig ships with some pretty good screws even if the fine thread makes it a bit slow. Any screw will have some lead error. I'm one of the few people that have managed to shoehorn in ball screws and there is still some lead error. If you are using Mach, you simply change the steps setting until you get accurate movement.

Forget the ABEC7 deep groove bearings. Use angular contact.

Regards,
Walt

Fixittt
08-24-2007, 12:22 PM
If I remember right, the taig has acme 1/4-20 TPI lead screws. Same as my maxnc. If you have alot of miles on the machine, then it might be time to replace the backlash nuts. As they are plastic (At least mine are)

scubasteve_911
08-24-2007, 01:01 PM
Walt,

I have been fairly impressed with the Taig leadscrew since they are a decent 1/2" diameter. I guess the 20 TPI is okay since it is best to choose a resolution that yields about 10 X your desired accuracy for motion tracking purposes.

Leadscrews come in a lot of accuracy variations and you pay exponentially for improvement! I have seen published specifications from 0.001"/" to 0.000025"/" of travel. One leadscrew will be out 5 thou in five inches and the out will be out a respectable 0.125 thou... That's a forty-fold relative loss :(

Judging by the poor motor couplings, Taig did spare some expense and I wouldn't be surprised if they cheaped out on the leadscrews. I understand that they must draw the line somewhere because it is easy to spend 10000$ on a machine..

It's really tough to find angular contacts that will work with a 1/2" leadscrew. If you can find one, please suggest it! I found a rare NSK unit, but they're super expensive..

Fixittt,

Yes, the Taig uses 20TPI 1/2" leadscrews with brass nuts with adjustable preload.

Steve

cartertool
08-24-2007, 03:22 PM
They grind the screws (or rather, send them out to be ground) out of cro-moly. There is some lead error although you can map it in Mach. Overall I have to say the screws are pretty darn accurate, but don't know what the actual specs are. I'll ask the next time I talk to them.

doanwannapickle
08-24-2007, 03:55 PM
Like Nick said, the leadscrews are quite good. The lead accuracy is moot as long as it's consistent for it's entire length and you can adjust it out. I didn't keep the screws that came with my machine. They were too nice to just throw in the junk pile so I traded Taig the screws for some other stuff.

I wouldn't know about the quality of the couplings. I started with a manual machine and stripped it completely right out of the box. The parts I didn't use went back to Taig.

You're right about the angular contact bearings, you won't find them to fit 1/2".
I see some lathe work in your future. Don't worry about mucking it up, replacement parts are very reasonable. :)

Walt

scubasteve_911
08-24-2007, 04:01 PM
Thanks Nick!

Ground screws can be very precise. I found some screws at http://www.universal-thread.com/ that would fit the taig. They offer standard precision of 0.0001"/" and at most 0.000025"/". I would be curious to know the spec. of the Taig screws, maybe they're better than anticipated.

You can map it out in Mach3?

Does that mean you can add or subtract steps at certain points in the screw in order to get the overall accuracy you want? I guess it is just a matter of finding a way to calibrate it after that.

I'm working on a webcam-based center / edge finding algorithm in Matlab, maybe with a good optical target I could calculate actual movement. Very interesting stuff :)

Steve

scubasteve_911
08-27-2007, 02:24 AM
Like Nick said, the leadscrews are quite good. The lead accuracy is moot as long as it's consistent for it's entire length and you can adjust it out. I didn't keep the screws that came with my machine. They were too nice to just throw in the junk pile so I traded Taig the screws for some other stuff.

I wouldn't know about the quality of the couplings. I started with a manual machine and stripped it completely right out of the box. The parts I didn't use went back to Taig.

You're right about the angular contact bearings, you won't find them to fit 1/2".
I see some lathe work in your future. Don't worry about mucking it up, replacement parts are very reasonable. :)

Walt

Walt,

I checked out your CNC site! Brilliant work, that was gutsy to do what you did with the ballscrews. I bet part of your motivation was personal because people saying it couldn't be done.

I'm getting an 8X12" HF lathe soon, so you are correct about the lathe work :) I will see about getting the precision screws and creating my own supports and coupling. Maybe I can give them the poor student excuse and they might pity me enough to give a discount on the ultra-precision screws. I guess the only thing left is tons of gib adjustment and oiling. I really just want to machine some 1 thou tolerance parts :(

Steve

scubasteve_911
08-29-2007, 05:30 PM
Well, I thought it would be neat to find new leadscrews that I knew the specs on, then machine down the ends to fit some shaft couplers.. until I called around to find prices on these leadscrews..

Univeral Thread grinding wanted nearly 2000$US for two 10" screws and a 20" screw of their standard 0.0001"/" tolerance. They can make three grades better tolerance, down to 0.00001"/" and 0.0001"/ft. With each step of tolerance improvement, the price doubles! So, if I wanted the most precise leadscrews, then it would cost a whopping 16000$US!

Are these people absolutely insane? Remember, this is for the screws without any end machining, nuts, or bearings. I have some Kerk 0.0001"/" leadscrews that were given to me, but the nut diameter is massive and I don't think it will squeeze into the Taig.

Steve

toastydeath
08-29-2007, 08:10 PM
There are more kinds of error in a screw than fall into a pure linear error category - there are also a variety of periodic errors, which don't accumulate over distance but occur within a revolution of the screw. The error map of a leadscrew looks like a high-frequency AC wave, that drifts in a direction. The long range drift is the per foot, but that high frequency noise is the periodic error.

Those screws, for $16000, are a steal compared to the cost of the machine they go in. They're not "just" accurate over .000025"/1', but also what isn't advertised - the periodic error is almost nothing, which becomes a huge limiting factor in screw accuracy. What good is a .000025/1 leadscrew if it is in any old random place in that .000025" tolerance anywhere in the travel? Plus, those screws require temperature and vibration control in order to use them. Nobody who balks at the $16,000 dollar screw can afford the what it would cost in environmental control systems to actually work to that tolerance. Setting the thermostat to 68 degrees no longer qualifies as "temperature control" when someone starts talking about 25 millionths per foot.

Plus, it requires a ridiculous amount of time and hand work to produce a screw of that accuracy. You're talking many, many months of time, because a screw of that tolerance is manufactured with principles very similar to a gage block. The screw is a gage standard at that tolerance level and can be used as such.

scubasteve_911
08-29-2007, 11:23 PM
Thanks for your input. I am aware of periodic error and other factors that effect true linear precision. I just think it is a completely silly route to spend that much on a leadscrew. The only way that I'd ever justify it is if the application was extremely time-critical or if I was trying to duplicate it's precision that exceeded any linear feedback measures.

Nonetheless, I am very surprised that it cost that much. I had some Kerk leadscrews with the same tolerances as the mentioned ground leadscrews and I am sure they aren't even close to the quoted costs.

Electrical engineering is my field and I suppose I might not have the appreciation for precision mechanical wonders as I should.

Steve