Advice on Bridgeport converted to CNC for sale

^^Jim is a smart cookie and definitely knows this end of CNC conversions better than I do. Hopefully we can all help you figure this out
 
All the above. I had to help him try to start it. Managed to start it once before it froze and then no amount of un-powering would get it to do anything by make that beep sound from the board. 20 years of dust and grime.
This is easily cured by a good wash and scrub (don't tell him that). Done it many times. Even a good airhose shot can do wonders.
So if you wanted to get going, and who doesn't? that would probably solve your onboard problems and leave you with only securing G-code from any of a dozen possible programs or sources. You can work out the quirks down the line.

If this were me, and I'm old and crotchety, I wouldn't even think about refitting motors and control. The hardware is as good or better than anything you can get today (in terms of ruggedness and longevity). Cure the 486 (with a nice bath) and it's ready to rock.

BUT, my gut tells me it's worth every penny of $4500... in 1995. But today? Notta chance. The number $2500 seems more comfy considering the risk. And every used machine comes with risk. Heck, even a brand new machine has a little risk.

You might figure any time you buy ANY machine, you're looking to set aside some budget to cure the hidden gremlins. If you bought a big, simple machine, you'd be wise set aside over a thousand just because. This is a small, complex machine, so you should do likewise.
 
Wrat/Jim - Thank you.
Here is where I am at - and it has been super helpful to post here and get some perspectives.
- There is no question that a CNC set up speeds wear. This machine is still in reasonable shape - I've seen worse - but I agree with the crowd that it is a $2500 machine not a $4500 machine.
That is the oldest Centroid system I have ever seen
- That caused a chuckle. I'm honored to have posted it...
- For $2500, I could strip off the CNC and restore it to manual with X axis feed while I get my milling teeth. If/when I decided to go full CNC I could decide what I would want to keep. He has the original handles and I assume the DRO would still do the job. If it came with the tools and the rotating table - it might start to feel like a pretty good deal. Maybe if I threw in some hours of organizing/cleaning his shop, I would find some extra bits and bobs worth keeping - and he would feel like he got a proper value.

Questions:
- If I went back and ran the program a few times in a row, what are my tolerances for returning to zero that should make me feel like I can do some nice work with this thing? At what tolerance do I walk (or is this a poor way to judge)?
- Pic below of the ways. There is some wear - but I'm a poor judge based on lack of experience. Views?
- It's 3 phase 1HP. Can someone post a link or two to a basic converter option so I am sure I am grasping what I would need (I hav 220 single phase in my garage)

Thanks again all. Appreciate everyone helping me get to speed - even if its a crawl.
 

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Questions:
- If I went back and ran the program a few times in a row, what are my tolerances for returning to zero that should make me feel like I can do some nice work with this thing? At what tolerance do I walk (or is this a poor way to judge)?
- Pic below of the ways. There is some wear - but I'm a poor judge based on lack of experience. Views?
- It's 3 phase 1HP. Can someone post a link or two to a basic converter option so I am sure I am grasping what I would need (I hav 220 single phase in my garage)

I expect a machine to repeat to +/- 0.0001'' or better. When it doesn't repeat then you have to ask why? Loose belt, servo drive not tuned, controller needs tuning? Are any of these show stoppers? No, just minor tune up problems, proper tuning can even overcome backlash in the ball screws to a degree. With that antique controller you may be at the limit of it's resolution, but I'm betting on a tuning issue.

From the pictures the ways look pretty good for a 40 year old machine.

What you want to convert from 240 single phase to 240 3 phase is a VFD. For a 1HP machine these are about $75 and up.

Here is a good one with good support and documentation https://www.automationdirect.com/ad...requency_drives_(vfd)/micro/cfw100c04p2s220g2

Teco and Lenze are good options also. Stay away from the really cheap Chinese units, however I have had good luck with the Huanyang units and they are well documented.
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No, just minor tune up problems, proper tuning can even overcome backlash in the ball screws to a degree.
Thanks Jim.
One question this ^ raised.
When I operated it manually - the X axis had a full .05 backlash.
Though I cringe as I type these words: is that a lot of backlash?
(5 degrees on the X axis knob's scale).
He noted to me that he had upgraded the ball screws when he did the conversion - and another response here identified that (from the picture I posted?).
Is there any adjustment to be made in these upgraded type screws? (apologies - for any butchering of the terminology).
Or would that back lash only be addressed with some snugging of the gibs?

Otherwise - I think I have what I need now to have a fair negotiation.
Thanks all.
 

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0.05'' backlash would be some serious backlash for a ball screw, but 5 degrees is more like 0.010'' backlash, still a lot for a ball screw. 0.05'' is even a lot for an Acme lead screw with a worn nut. 0.0018''/degree with a 0.2'' pitch screw. I have about 0.004'' in my X ball screw and concider it worn out and in need of a regrind and repack. I would be very surprised if there were 0.05'' backlash in the ball screw it self. That is almost impossible short of a bunch of broken balls and you would feel those crunching if it were in that bad shape.

Having said that, backlash can also come from the thrust bearing, may just need to be shimmed, or even a loose tensioning nut. If it has double ball nuts then those can be adjusted to remove backlash. I can't tell from the picture if it has double nuts, but the area of the ball screw I can see in the picture looks good but they don't wear on the ends so that doesn't tell much. I can see that is a ground screw rather than rolled so a very high quality screw.
 
I was loath to type it - knowing that I might be misstating.
To be clear - I could move the X axis dial this much without re-engaging in the opposite direction.
 

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I was loath to type it - knowing that I might be misstating.
To be clear - I could move the X axis dial this much without re-engaging in the opposite direction.

Yeah, that's about 0.005''. So the question is where is it coming from, the ball screw or the thrust bearing? Putting an indicator base on the table and the indicator on the end of the ball screw will tell you if the thrust bearing is loose.
 
Questions:
- If I went back and ran the program a few times in a row, what are my tolerances for returning to zero that should make me feel like I can do some nice work with this thing? At what tolerance do I walk (or is this a poor way to judge)?
The only really reliable test that i've seen used is the "circle-square-diamond" test. (sometimes known in a different order like circle-diamond-square, etc.) Couple of big companies of my employ used that test for many years.
In short, you bolt down (near the middle - you'll be using the perimeter) a piece of material - about 12" square and an inch thick, yes, aluminum.
Then you cut a 12" circle about .250 deep. Then you cut a circumscribed diamond (square at 45deg) at .500 deep. Then a square (oriented XY) at .750 deep. Each layer clearing out excess material.
This gives you about a dozen points to carefully check alignment and repeatability. Is the circle exactly tangent to the square and diamond? Is it exactly 12" diameter in every direction? Is the square and diamond exactly 12" across in both directions down both sides? Are the steps exactly .250? Etc. etc.
It's a very thorough and telling test of tracking, columnation, and repeatability.
Obviously, it can be done with any sized piece, but the bigger, the better.
Trouble is, you probably won't get to run anything like that prior to purchase, plus there's a little bit of G-code involved that has to come from somewhere. But were I to get it, that'd be the first chips I'd make so I'd know what i have and not be chasing it all over.

After all, you don't care if it returns to zero faithfully. You care if it cuts where it says it's gonna cut, faithfully.
 
I was loath to type it - knowing that I might be misstating.
To be clear - I could move the X axis dial this much without re-engaging in the opposite direction.

.005" of backlash is not crazy bad, but it's also not great - you can still make a lot of good parts with that much backlash. New style motion controllers have the ability to compensate for backlash, but the caveat is they work best on mechanical systems that have as little physical backlash as possible. I highly doubt the centroid controller that is currently installed has this ability. My ground screws have around .0006-.001" of backlash and I've had experience with rolled screws that have as much as .004" from new. If the ballscrews pictured have double nuts, it's possible to eliminate most of the backlash in the nut (if there is any) by shimming. It would be a good idea to check where that backlash is coming from - rule out the thrust bearings first.

As Jim said, those ballscrews appear to be ground screws which is good. A ground screw will have a lot better accuracy compared to a rolled screw (that is, the lead error of the "threads" will have a much tighter tolerance over its entire travel than a rolled screw).
 
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