Alliant CNC conversion

[jchili73]

I have an Alliant (Bridgeport clone) knee mill (2 HP, variable speed, Acu Rite II DRO) that I'd like to convert to CNC so I can cut complicated shapes more easily and get some hands on experience with CNC machining. I don't do this for a living, I just make the occasional part when I need something, but I still need something that works within .001" with reliability.

I've read a bunch of posts about steppers vs. servos and with all the conflicting info I still don't know which is a better idea, although I'm leaning toward steppers that are micro-stepped because I don't need high speeds and they seem cheaper, more durable, and easier to work with for a novice. I've also read that starting with a mill like mine isn't the best way to go, but I already have this mill (and I like it- a lot!) so I want to use it.

Here are a few questions-

1) I've read that part of the conversion is changing the screws that move the table (ball screws?). Where should I get those? Is it true that these screws remove all backlash (that would be cool, not that the backlash really bothers me)

2) Will I still be able to use the mill manually after the conversion is done?

3) Can I get this type of mill to cut parts with the precision I want (.001, +/- .0005) after conversion with steppers?

4) What stuff should I buy and where?

Thanks!

[Adamj12b]

Welcome to the Zone! I just finished a retrofit on my second machine, a let Knee mill, and thought I would share my experiences with you.

First about the screws, You can replace the screws such as I did with Roton ball screws. The screws are precision rolled ball screws. They are accurate I think down to .003-.004". If you go with a dual ball-nut setup on the screws you can use one to adjust the backlash out of the screw. I only used 1 ball-nut on mu screws, because I didnt have the room to add 2 nuts.

As for the what motors to use, I would recommend steppers for motors. Servos are great, but to get ones with a high wattage output are expensive, You need some form of reduction for gearing and they do not stall like steppers keeping from damaging themselves. I would use some nema 32 600oz-in steppers for the x and y. Depending of if your going to motorize the knee or the quill, You will need a large nema 42 for the knee or maybe a nema 23 for the quill.

For drivers for the motors, I would say go with geckos. They are the best motor drivers out there. Very simple and very well built.

As for the tollerences that you are looking for are totall achievable, but will require alot of attention to detail. Dont skip on things and your goal should be within range.

-Adam

[jchili73]

Thanks for the response Adam. I read your Jet mill conversion post before I got started (nice work!), and I notice that you went with servos. I've got a few more questions if you don't mind-

1) Why do you think that steppers would be better for my mill? How did you arrive at the specs for the steppers you recommend (e.g. 600 oz. for x and y etc...)? What is microstepping and do I need to do it?

2) I'm guessing I should go with recirculating ball screws; budget is less of a concern here, I just want build something that I'm not going to regret later by not skimping at all, so I'll put as many ballnuts as I have to in there to accomplish the goal. Why does having more than one help? Is two the most one should use? Do I just order the same diameter and TPI/pitch as I have on the screws that came with the machine?

Thanks! I think with a little more help and some time (and money, lol) spent with the folks at Gecko and Roton I should be loaded for bear soon enough...

[Adamj12b]

I say steppers because if I was going to do it again I would go with steppers instead. They are much simpler to setup and maintain. They don't ware-out (brushes) and they are very difficult to burn up by overloading. As long as you run them at their rated current you can increase the voltage voltage 20 times.

Second reason I would go with them is you can directly couple them together with an aluminum helical coupler. This will eliminate some cost and complication of adding a reduction with timing pulleys.

I went with servos on my machine because I got a good deal on them on a surplus sight, only to find out later that they are not good servos at all. Good servos are expensive. I have already come close to burning up the one on the z axis because they are not as strong as I thought.

As for the size of the steppers, 600 oz-in is probably the lowest you should go with now that I look more closely. My servos are 600 oz-in, but I have a 4:1 reduction so its like 2400 oz-in at the screw, but thats peak and if I run my motors at or near peak for any amount of time the motors will burn up. I will be changing the reduction to 2.5:1 soon to achieve higher rapid speeds, because as of now the motors don't even get warm, and I want tax them a little more to get some higher speeds.

As for micro-stepping, basically you start with a motor that has 200 steps per rev. with micro-stepping you take that 200 steps and multiply it by the the number of micro-steps the controller is set for and thats how many steps per rev you get. So if you take the 200 and have an 1/8th step driver you will end up with 1600 steps per revolution.


Ok, I think I covered all of the motor questions so on to the screws. lol. If you went with dual-dual recirculating(meaning each nut has 2 recirculating channels, and there's 2 nuts) ball nuts, you mount them so one is fixed solidly and the other can be locked down, but you are able to turn the nut before locking it down to preload the bearing in it against the inside of the nut and screw to suck up all the backlash. This way when the bearings start to wear you can just turn the second nut a little bit more to re-preload the nuts and eliminate the backlash again.

As for the pitch of the screw, I went with what was available. The larger the pitch of the screw, the stronger motor you will need, but the faster it will be able to move. .25 and .20 are both very acceptable pitches.

For the diameter of the screw, I went the same 1" that was in the machine so that I didn't have to worry about finding bearings to mount the screw. All I had to do was machine the ends down on the lathe and mount the nut and it was all set.

So I think that is everything if you have any more questions feel free to ask.

Also, Here are a few links to some sites about some conversions that I found very helpful.

http://www.matronics.com/cnc2/index.htm
http://plsntcov.8m.com/JetMill/Jet.html

Good Luck-Adam

[jchili73]

I talked with the folks at Roton, and they don't sell precision ball screws; using those screws is not supposed to be able to give you really good accuracy or precision (really good = .001 or better for me). The screws I found are from a company called Hiwin. These are ground and are better than .0005 over the length of my table. Sadly, they ain't cheap. The two will cost ~1200$ with the nuts. I was told that only one nut is needed because the preload is put in at the factory.

I still can't figure out if I should use steppers or servos, but I figure I'll put in the ball screws first because I'll need them anyhow. How do you take apart the table to get at the screws?

[Adamj12b]

Wow 1200 for just the screws. I didn't even spend that much on my total conversion yet lol.

I would still say go with steppers. Some big nema 38's or 42's, and some of Gecko's g203V Drives. I have never used those specific ones, but have read alot. They are said to be the best. Steppers are so much easier to set up, especially on your first build.

Do you have any pictures of your machine? Those would help out alot.

Also Do you plan on converting the knee or the quill?

-Adam

[jchili73]

I'll snap a few shot later. I really like the idea of making the quill the Z; I just think it's a lot less effort and a lot more precision for that to happen. Also, I'm thinking about making the motors direct drive and staying away from the pulleys if I can.

I'll post the pics this evening. Thanks again for your help.

[Adamj12b]

The quill would probably be easier to do but on my machine it was so sloppy that I didn't really have a choice. All of the gears are worn out and you can turn the hand-wheel like 3 times before anything starts to move.

So dose that mean that you decided to go with the steppers? In not you would need a geared servo, which I have never seen a gearbox that had zero backlash in it.

I look forward to seeing the machine.

-Adam

[Adamj12b]

Any updates? -Adam