How Bad is it Without Ballscrews?

[wallyblackburn]

My first post...great site and forum!

I am thinking of a 2-axis retrofit on my old 1J. I bought Chris Bruno's plans and think I am going to do something close to that. 1200 oz/in or so steppers and Geckos. Probably Mach2/3 for control. Get that cooking and then do the Z-axis...

The thing I am waffling on right now is whether or not to fit ballscrews. What kind of problems can I expect without them? What suggestions do y'all have for Ballscrew kits if I do decide to get them? Anybody make a kit for the 9"x42" BP Series 1? I have no problem fabricating the motor mounts and such, but for some reason, I would like the ballscrews to be as "bolt-on" as possible.

TIA,
Wally

[NC Cams]

I wouldn't even think of NOT fitting ball screws if you are going to do CNC.

Turn the handle on a zero backlash ball screw system and then do the same with an ACME threaded system with similar zero backlash. The friction difference is quite noticeable as will be the accuracy.

Generally speaking, lead screws do not wear evenly. Thus, keeping backlash comp under control is all but impossible with worn screws, regardless of the efforts/compromises made to "adjust" for the wear.

There are several bolt-in ball screw setups out there. HIWIN makes them in rolled and ground versions. Rolled is plenty good enugh for hobby use. Ground screw is the definite choice for precise milling, especially if you plan to do 3D profile milling of intricate details.

I seem to recall that the rolled versions are available for around $600-$700/set. A billet yoke might be extra and will add to rigidity and accuracy while doing heavy cutting.

The HIWIN and other ball screw retrofit sources can be linked off of the ajaxcnc.com website from their components page (I think).

[ger21]

Originally Posted by NC Cams Turn the handle on a zero backlash ball screw system and then do the same with an ACME threaded system with similar zero backlash. The friction difference is quite noticeable as will be the accuracy.

You can get acme that's more accurate than ballscrews, so that's not necessarily true. Either come in many different grades of accuracy.

[NC Cams]

Gerry: I'd expect that to match the 0.0005" or 0.0003" published lead error of HIWIN ground and rolled (respectively), one would have to use a ground acme thread. How costly these are is something I can not discuss intelligently.

Yes, accuracy could be there if the Acme were ground instead of simpy cut or machined thread. But I"d still be inclined to think that friction of an acme thread/nut would be higher than a ball screw, especially if nut preloading were done so as to eliminate backlash.

We just did a tune-up of our rolled thread ball screw in our BPT EZTRAK. With the screw loaded with o'size balls for true zero lash (slight ball preload) and the incorporation of a 480lb preload, true ABEC 7 ball screw support bearings, our turing friction is now as low if not LOWER than the OEM setup and we're seeing no perceptible backlash.

Granted, we didn't have an acme to compare against initially but rolling friction is typically lower than sliding friction. Ultimately, friction (and resultant wear) is a factor with CNC's, especialy as you try to run at high inches/minute.

[jdelaney44]

Wally:

Two cents from a relative Noob with an operational conversion.

I have not actually tried acme screws since I replaced mine. But, I hear that several people use them with CNC and do just fine. You will probably have to fabricate split zero backlash nuts. Does that make sense?

The big question is how accurate do you need your work to really be? I'm of the opinion that a whole lot of stuff can be +/- .005 - .008. I think you can get that with an acme screw and a good mapping exercise with your software. Remember the software can compensate for a lot of this too. Including the backlash. Some creative programming might be able to compensate for the the rest.

On the other hand, this could get old real fast........

Here's some of what you are trying to avoid:

-- Biting into the work and jamming the tool when directions are reversed under CNC control. Tools will get broken, work gets pulled out of clamps, etc. This is probably most pronounced when transferring to climb milling. This will also result in surface irregularities and just plain ugly work.

-- Accuracy issues as you can imagine.

-- Chatter as the work can bounce off the tool as allowed by any slop in the screw. This can be resolved with a different feed rate of course.

-- Irregular drive motor performance / stalling as you move onto less worn parts of the screw.

But these are all kinda worst case scenarios. There are people who have made big router tables using threaded rod and home brew anti back lash nuts.

Yes, I think you can get a kit for your machine. I got mine from Rockford Ball Screw. Rolled, not ground. I have seen better yokes, but this one works fine.

I have heard good things about those HIWIN screws as well.

Hope that helps.

[Eric]

Originally Posted by jdelaney44 W Yes, I think you can get a kit for your machine. I got mine from Rockford Ball Screw. Rolled, not ground. I have seen better yokes, but this one works fine. I have heard good things about those HIWIN screws as well. Hope that helps.

Where did you buy your Rockford Screws from and how much were they? Do you have a picture comparing the OEM Bridgeport yoke to the Rockford yoke? What are better yokes (elrod)?

[ger21]

Originally Posted by NC Cams Gerry: I'd expect that to match the 0.0005" or 0.0003" published lead error of HIWIN ground and rolled (respectively), one would have to use a ground acme thread. How costly these are is something I can not discuss intelligently.

But you went onto recommend rolled ballscrews, didn't you?

[jdelaney44]

I bought my Rockfords directly from Rockford. I think you can probably special order them from MSC or McMaster too.

$350 for the X assembly
$225 for the Y assembly
$75 for the yoke

From the looks of it the Elrod yoke is beefier. Honestly, I don't think mine is giving me any problems. I think it could if I was hogging out steel with a big roughing end mill.

Note that the Rockford yoke is for the Rockford screws. You are not going to be able to use the Elrod yoke with the Rockford screws from what I know. Different bolt paterns on the nuts.

I have pictures of the new yoke but they are on film. When I get a chance to scan them I'll upload. But the Rockford and stock yokes are about the same. I can take a picture of the stock yoke too. It's in the garage on a shelf.

I have a journal of the conversion so far at:

JDs BP J-Head CNC Conversion Blog

[NC Cams]

GER21 posted: "But you went onto recommend rolled ballscrews, didn't you?"

Not really and I appologize if I gave that imression.

My machine HAS rolled screws which is what it came with. Since we didn't have the $$$ to upgrade, we merely tuned it up with oversize balls in the screw and some horribly expensive ball screw bearings.

We cheat with lead error. How so??? We "cheated up" the lead and backlash comp for deadly accuracy in a 3", +/-0.5" Radius where we want to do real precise profile milling - response is hugely distorted beyond that range.

When we want to do work over larger span, we reload a more proper lead and backlash comp table. Real PITA but thats all we can do with the budget we have at this time. You can do it rather easily as it is a DOS based system and we know our way around that system fairly well.

When time and $$$ permit, I plan to upgrade from EZTAK (pretty much obsolete but it works OK) to a more modern system (considering several) that has modern servo feedback, autotuning and higher resolution.

By doing a laser scan of the table and upgrading to a system that will allow laser comp by section, we can "map" the lead of our rolled screw and let the computer take care of any lead error - this will let us keep the rolled thread screw for a bit of a cost savings. Isn't modern technology great???

The alternative is to go to a ground screw which I did consider and would prefer. Problem is, the EZTRAK uses a special end that isn't compatible with any off the shelf ground ball screw kits I found. I just don't want to throw out my drives, screws et al.

HIWIN's are available off the shelf in ground or rolled thread but they bolt in to manual machines and mine differs substantially due to it being an EZTRAK.

HIWIN screws are in mine now. They work great. Only reason to change is that we're trying to turn the BPT into a Haas for a fraction of the cost.

Reason: to machine some master profiles that we use to grind cams with in-house. We're within 0.0001 of a CNC ground part now which you can't see in the finish ground part.

These are the DIY things that a semi-retired engineer does when one has the know-how and access to bearing/ball screw rework technologies. Besides, it is really cool to be able to have someone say "you can't do that" and then see them utter in amazement, "how the %#@@ did you do that with a Bridgeport???" after you do....

Life don't get much better when something like that happens.

[jdelaney44]

Well....

I'm two parts into this now and I can say confidently that the rolled screws are fine. I am not trying to make a Haas out of my 1982 2J either. But it is well within the +/- .003 range or better. That is good enough for a bearing journal so that's good enough for what I see that I might be doing.

So...I am going to come right out and say that rolled screws are a good thing for the average used piece of bridgeport iron and the average tolerances that are really needed. In my humble noob opinion of course.

-jd

[machintek]

Here is a scan of a ROCKford sales flyer for S1STD Ballscrew conversion.
Yes, ground are a lot more accurate than rolled but at a price!

George

[jimbo]

Nc Cams,
How does one map the lead error of the ballscrew? Is there an alternative to a laser inferometer? How can large distances be mapped without accumulating errors in the measurment?