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Ok, I got the mill...and the required buyers remorse. I'm hoping that is temporary. I knew going in the thing was never set up or tuned properly, but as someone pointed out the DRO's, Camtronics, steppers, software, four jaw, and other basic tooling would cost more than what I paid...so, I guess I can't complain to loud about having to put some time and a little money into my free mill
The backlash is ridiculous (like more than a half turn) so I think I'll start there:
Where should I look for ballscrews? I'd prefer close to zero backlash but depending on price could probably be ecstatic with .001 or settle on no worse than .0025-.003.
Specifically what type, size, and style fits the Bridgemill (model before Tri-Power)? Preferably without having to do any machining. I have spoken with one company, they said they can mill the screws for the chosen end bearing and the coupling end. I'm sure that costs something...but probably not as much as me messing it up.
What should I expect to pay? To me, it seems that the Shoptask X-Y ballscrews are pricey and some posts reported more than .008 backlash in spite of the claims of less than .003. Since I don't really know what to ask for, it is kind of hard to price them out.
Thanks in advance! By the way I did look through the old posts and they are several years old and didn't really give any specific details. I'm hoping for some fresh insight and current sources.
Before I got the Patriot, I had an earlier model Shoptask with quadra lift, which is similar to the Bridgemill. Your machine has adjustable acme nuts on X and Y, so I would suggest you look into the adjustment before spending big on ball screws. Shoptask used to offer a torrington bearing upgrade on the Z axis to eliminate lash as well. With proper adjustment and the upgrade you can probably get into the 0.004" range ( tighter will create a lot of drag on the screws). Also check the gib adjustments, as loose gibs will translate into big backlash. However, if you are set on ball screws, then you could look at the Nook industries rolled thread units with select fitted nuts. Nook advertises these to be at about 0.003-4" per foot. If you want tighter, there are 2 options-
A. The rolled thread screws with double pre-loaded nuts- these can be set to near zero lash. However, due to the length of the double nuts, you will lose about 3" travel on your carriages.
B. Go to ground thread screws and nuts to get into the 0.001" range, but be ready for some serious sticker shock.
Nook will machine the ends for you if you send them a sample or a drawing. Price is about 200.00 per screw.
Thanks smallblock. I will screw around with current acme's and look into the Z mod.
With all the parts for CNC conversion coming with the mill, I don't want to put a large amount of effort into overcoming the backlash...only to turn around and tear it apart to put in ballscrews.
Check out the Shoptask web site. JT sells ballscrew sets already made up. They are rolled and I know some folks have complained about them, but they are bolt in.
I'm looking at going to ball scres myself and was wondering if anyone has drawings for them. I don't want to have to strip the machine to reverse engineer from the acme screws, then have to reassemble to machine to machine the ball screw ends, disassemble again to install the ball screws.
I am still not to sure how the Z screw gets installed. If anyone could put up some detailed pictures that would help.
My unit had a Shoptask with 4 axes servo control. Where we were stationed, it was hot, dry, dusty and the wind blew all the time. The stuff got into everything, so we set up a scheduled maintenance on the Shoptask. I suggest you do a thorough maintenance before jumping into CNC or buying ball screws. Disassemble all the screws and carriages and clean them carefully. Check all your gibs for flatness, and use fine sandpaper to polish all the moving surfaces. The Shoptask has thrust ball bearings on all the screws, so clean them and re-grease every thing. Put all your carriages back on and adjust the gibs until you have no rocking motions, but they still slide freely along the ways. Now replace the screws and adjust the double nuts to as little backlash as you can get without getting a lot of drag. You will be pleasantly surprised how smooth the machine will be after a Saturday afternoon of basic cleaning and adjusting. When you have the screws off, make some measurements and drawings in case you decide to buy ball screws and have them machined. Once this is done, test run the CNC and setup the backlash compensation in the software. You may find that the speed and accuracy are fine for what you do- if yes, no money spent- if no, then consider the various ball screw options.