New Machine Build- Couple of questions on converting a bridgeport to cnc

[jawsoflife]

Where can I buy brackets for mounting servo motors to x, y, and z axis of my bridgeport?

Why does everyone mount the rotary encoder on the motor shaft instead of on the leadscrew? Why not use linear encoders on the table for feedback?

Seems to be a big price difference between hiwin ground ballscrews and Rockford. What is the difference?

What brand servo motors do you suggest? I have belts and pulleys from a anilam retrofit. The pulleys have a 1 to 3 ratio and the small pulley has a 5/8" i.d. There are also some sem brand motors. Should I try using these motors or is it more trouble than it's worth?

I'm not interested in wiring up a power supply and I have my heart set on gecko drives. Is there a box out there that is reasonably priced that would be ready to rip out of the box?

Thanks for any guidance that you can give me.

[keebler303]

I think most people make their own brackets, after all you do have a bridgeport, the more you use it by hand, the more you will appreciate it once it is under power.

The encoder is generally mounted on the motor because there is backlash between the motor and screw or motor and table if using linear encoders. This backlash is small, but enough to make some servos misbehave a bit. The control will not be so tight because the motor moves before the encoder knows it. Its also generally much easier to mount to the motor versus the screw. It could make for a more accurate machine but it can also create problems in the feedback loop which makes the servos dither more (oscillate between encoder counts)

rockford ballscrews are rolled, not ground with a tolerance of .003" per foot. I didn't find the specs on the hiwins with a quick google search but if they are ground, they are probably a bit more accurate. Look at what you are getting in the package, rockford seems to offer a ready to drop in kit. Is the hiwin offering that way as well? is the end machining and size ready to drop in?

motors should be picked with the drives in mind, if you want to go with gecko's, you need to stay under 80V and 20A peak. The motors also have to be BRUSH DC motors. Gecko's cannot drive brushless type motors so that is a consideration as well. Gecko's might not give you the ass you may want for motors but they will work. I would suggest looking at what others have done to get an idea of what motors and drives people are using. There should be plenty of info on cnczone if you search around a bit.

Shop around for a wired up box. I am not sure if one exists for servo's, I know there are several for steppers. It all depends on what you call "reasonable" and what you want it to include. I am sure you could find someone to put together a nice box for you if you don't want to mess with it yourself, although its not terribly difficult, depending on your skills.

no one will tell you how to do it, look at what others have done, ask questions and get those gears spinning upstairs. cnczone will help tremendously in researching what you need and how much it will all cost.

have fun
Matt

[keebler303]

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[jawsoflife]

Originally Posted by keebler303 I think most people make their own brackets, after all you do have a bridgeport, the more you use it by hand, the more you will appreciate it once it is under power. ]B]After spending another day yesterday looking for brackets on the web i have decided the same thing. I will make the brackets. Elrod machine makes a very well designed and complete setup but it is just too pricey for me. If I had a tight machine the Elrod setup is exactly what I would do. I figure I will leave the bridgport iron bearing brackets where they are and build off of them. I doubt that I have the expertise to bore the bearing counterbores (even if I had an adjustable boring head). The encoder is generally mounted on the motor because there is backlash between the motor and screw or motor and table if using linear encoders. This backlash is small, but enough to make some servos misbehave a bit. The control will not be so tight because the motor moves before the encoder knows it. Its also generally much easier to mount to the motor versus the screw. It could make for a more accurate machine but it can also create problems in the feedback loop which makes the servos dither more (oscillate between encoder counts) On prototrack machines(http://www.southwesternindustries.co...retrofit.shtml) they offer have glass linear encoders. Perhaps prototrak control has a way of supressing the dithering due to the linear encoder. rockford ballscrews are rolled, not ground with a tolerance of .003" per foot. I didn't find the specs on the hiwins with a quick google search but if they are ground, they are probably a bit more accurate. Look at what you are getting in the package, rockford seems to offer a ready to drop in kit. Is the hiwin offering that way as well? is the end machining and size ready to drop in? http://www.hiwin.com/bs/xy.html this is the link to the ground ballscrew setups however the link isn't working right now. Perhaps their server is down or something. motors should be picked with the drives in mind, if you want to go with gecko's, you need to stay under 80V and 20A peak. The motors also have to be BRUSH DC motors. Gecko's cannot drive brushless type motors so that is a consideration as well. Gecko's might not give you the ass you may want for motors but they will work. I would suggest looking at what others have done to get an idea of what motors and drives people are using. There should be plenty of info on cnczone if you search around a bit. With SEM and Glentek motors the continuous torque is rated on the motors. With less expensive motors only the max. holding torque is specified. It makes it tough to compare apples to apples. I will proceed as you say and try to find others who have done servo retrofits on j heads. [/B] Shop around for a wired up box. I am not sure if one exists for servo's, I know there are several for steppers. It all depends on what you call "reasonable" and what you want it to include. I am sure you could find someone to put together a nice box for you if you don't want to mess with it yourself, although its not terribly difficult, depending on your skills. I probably will build the box myself, I'm just a bit weary of messing around with 70-80 VDC. no one will tell you how to do it, look at what others have done, ask questions and get those gears spinning upstairs. cnczone will help tremendously in researching what you need and how much it will all cost. you didn't tell me how to do it but you did get me pointed in the right direction. Thanks for the reply, much appreciated. have fun Matt

-jawsof life