BP Bracket Design for CNC Conversion

[Cartierusm]

Here are some drawings I've done for my Bridgeport Clone (First brand) Mill CNC Conversion. I would like any suggestions or just feedback whether you think it will work and be accuate? Thanks. I'm all for design but I just want to get it working.

I would just copy some other conversions that are out there but none show or talk of the bearing setup. Most use the original brackets and just add on to that, but I can't as my mill being an import has much larger bearings from what I can tell. I can't find very many specs on 6204 Bridgeport bearings but I found some info at NSK and they seem to be 20mm I.D. bearings, my bearings are 1" I.D. Although the weird part is my mill is called a First and most have never heard of it but I have a Servo Products Power Feed that came with it and the bracket I'm sure is for a Bridgeport and of course fits, plus the internal non removeable bearing on the Servo Power Feed is for the 1" ballscrew. I just don't see this being a custom made Servo Power Feed, Servo is the brand, not servo motor. So I'm going with this way as the drawing shows as I don't want to order parts thinking they will fit a BP clone and have to fight for returns.

On the pulley side I will of course have the 2" aluminum bracket hold the motor, as well and the angular bearings will be shimmed it needs be. On the pulley I will have a flat on the ball screw and use a set screw, I know not the best but if the machininst that's cutting my screws can do a key I will do that or once cut should be soft enough for me to cut on my mill.

[Torsten]

Looks good exept on the X-Axes you only want Angular contact Bearings on one side, use a regular radial bearing on the other.
This is because if the heat expansion between table and screw is not exactly the same it could lead to binding or even warping of the screw.
So one side should be free to grow.
Allow for some adjustability for the axial preload on the Bearing.
Set screw for the Pully is fine, even better to have the clamping center style.
You do not want a key on there, they have some backlash and are not ideal for cnc.
Good Luck

[Cartierusm]

Thanks Torsten and Himy (from a PM). So here are the revised drawings whereby I added another thread to load the bearings and a separate thread for the pulley.
I decided not to have the other end of the X axis to be machined. If it's to be floating then I see no need to make it fancy as the machinist I'm going to use charges per side. Let me know what you think.

[HimyKabibble]

Originally Posted by Cartierusm Thanks Torsten and Himy (from a PM). So here are the revised drawings whereby I added another thread to load the bearings and a separate thread for the pulley. I decided not to have the other end of the X axis to be machined. If it's to be floating then I see no need to make it fancy as the machinist I'm going to use charges per side. Let me know what you think.

I still don't like the way you're supporting the thrust bearings. The plate should clamp the outer races securely together. The one nut on the end of the screw is sufficient to clamp the inner races together, and to hold the pulley in place (along with a keyway and/or setscrews - I used both). Pre-load on the bearings should be set by either using a matched pair of bearings, or shims between the bearings, to space the inner or outer races are required to achieve the desired pre-load. Using a nut to set pre-load will be very imprecise, and the nut could come loose, leaving you with axial play. Or, you could over-tighten it, leading to rapid bearing falure. If you do it right, you'll never have to touch those bearings again.

Regards,
Ray L.

[Cartierusm]

Did you look at the new drawing? I just thought that's what I was doing I probably didn't make it clear. So I now have two nuts on one side. The plate to hold the bearings in will hold the bearings tight, and the nut will tighten the inner race to the ballscrew and then a separate nut will tighten the pulley against a step down on the ball screw.

If this is not the way it should be done please let me know. Thanks.

Here's a link, I'm sure you've seen, to elrod's design and I think mine's the same except the extra nut, but I could of course be missing something. http://www.elrodmachine.com/images/IMG_0214.jpg

[HimyKabibble]

Originally Posted by Cartierusm Did you look at the new drawing? I just thought that's what I was doing I probably didn't make it clear. So I now have two nuts on one side. The plate to hold the bearings in will hold the bearings tight, and the nut will tighten the inner race to the ballscrew and then a separate nut will tighten the pulley against a step down on the ball screw. If this is not the way it should be done please let me know. Thanks. Here's a link, I'm sure you've seen, to elrod's design and I think mine's the same except the extra nut, but I could of course be missing something. http://www.elrodmachine.com/images/IMG_0214.jpg

Unless the bearings are a matched pair, you're still using that inner nut to control pre-load - not a great idea. In fact, it may not work at all. Two random bearings will, in general, either have slack or pre-load when simply bolted together - so you could tighten the nut fully, and still have axial play in the shaft. The only way around this is to place a shim between the outer races, to move them further apart. On the other hand, you might find that when you tighten the retaining plate, the bearings have very high pre-load, with the shaft not even installed, and the nut does absolutely nothing. A matched pair will be ground so that when stacked together properly, the outer races can be firmly clamped together, and the inner races can be firmly clamped together, and you will have proper pre-load, without having to adjust the clamping bolts/nuts/whatever. Tighten everyrhing up, and your' done. No adjustment needed. Barring use of a matched pair, you have to accomplish the same thing by inserting shims between the inner and/or outer races, to achieve the same result. In either case, the nut is doing nothing more than holding the shaft in place - it plays no part in determining bearing pre-load, as long as it's tight enough to ensure all the races are clamped firmly. At that point, there's no need to have two nuts. One nut can hold the shaft in place, and hold the pulley and spacers in place.

Regards,
Ray L.

[Cartierusm]

OK, I'm understanding but you said a lot. Let me summerize. If I have 2 matched ground pair of bearings, then one nut will do because no matter how tight it get them (to a point of course) the prelod will already be built into the bearings?

Aside from removing the nut to have the drawings as in the fist post, my design is good to go?

[HimyKabibble]

Originally Posted by Cartierusm OK, I'm understanding but you said a lot. Let me summerize. If I have 2 matched ground pair of bearings, then one nut will do because no matter how tight it get them (to a point of course) the prelod will already be built into the bearings? Aside from removing the nut to have the drawings as in the fist post, my design is good to go?

Yes, as long as you use either matched bearings, or shims, the rest of the design should be fine.

Regards,
Ray L.

[Cartierusm]

Don't get me wrong I know engineering and I'm not dense but I always want to make sure I've got everything down correctly and since this a forum and somtimes words and pictures are hard to convey...

So the plate to keep the bearings in will be tight agains the outer race and a sleeve being pushed by the pulley which is bolted on will push on the inner race.

Will these work http://www.vxb.com/page/bearings/PROD/Kit1087 it is 2 in a set so I'm assuming they are matched? If not what and where do you suggest?

I ordered my ballscrews Hiwin direct $540 for rolled X and Y BP standard machined ends, yoke and zero backlash ball nuts. Pretty sweet.

I also replumbed the lube fittings. I took all of them off and tapped them some 1/8 pipe and some 10-32. I used my pneumatic cylinder fittings which I have a ton of, they are quick disconnect. Came out pretty good will post pics tomorrow. Got the Y cradle back on all ready to accept the screws.

[Cartierusm]

A couple more quick questions. I know to use a 2 to 1 pulley ratio but what size pulley should I start with on the motor?

Next question i'm going to be using 1200 oz/in. keling stepper motors and a 72VDC power supply what gauge wire should I use. I know wire for low voltage DC and AC stuff but not DC this high. OH and what gauge wire for the wires going to the motor.

[HimyKabibble]

Originally Posted by Cartierusm Don't get me wrong I know engineering and I'm not dense but I always want to make sure I've got everything down correctly and since this a forum and somtimes words and pictures are hard to convey... So the plate to keep the bearings in will be tight agains the outer race and a sleeve being pushed by the pulley which is bolted on will push on the inner race. Will these work http://www.vxb.com/page/bearings/PROD/Kit1087 it is 2 in a set so I'm assuming they are matched? If not what and where do you suggest? I ordered my ballscrews Hiwin direct $540 for rolled X and Y BP standard machined ends, yoke and zero backlash ball nuts. Pretty sweet. I also replumbed the lube fittings. I took all of them off and tapped them some 1/8 pipe and some 10-32. I used my pneumatic cylinder fittings which I have a ton of, they are quick disconnect. Came out pretty good will post pics tomorrow. Got the Y cradle back on all ready to accept the screws.

At that price, I doubt very much those bearings are a matched pair - just a pair. You can get matched pairs from McMaster-Carr, or any bearing supplier, but you might not like the price. But, you can buy bearing shims from McMaster-Carr. Buy a bag with the same OD as the bearing OD, and another bag with the ID the same as the bearing ID. Buy the thinnest shims you can get. When you get the bearings and shims, stack them, and see which races contact first. If it's the outers, then use small shims between the inner races. If the inner races contact first, then use large shims between the outer races. Shim so there is a small pre-load, when both the inner and outer races are clamped together tightly. If the bearing gets tight, or feels rough, you have too much pre-load. If pre-load is too light, you'll have axial play. Check for end-float on the screw after putting the machine back together. You may *think* you've got the pre-load right with the screw sitting on the table, but you really need to test it under load to be sure.

Regards,
Ray L.

[HimyKabibble]

Originally Posted by Cartierusm A couple more quick questions. I know to use a 2 to 1 pulley ratio but what size pulley should I start with on the motor? Next question i'm going to be using 1200 oz/in. keling stepper motors and a 72VDC power supply what gauge wire should I use. I know wire for low voltage DC and AC stuff but not DC this high. OH and what gauge wire for the wires going to the motor.

Use the largest pulleys you can fit, that give you the ratio you want. Use the design tools on www.sdp-si.com to select the proper pulleys and belts. They'll give you the correct center distances, and warn you if you've picked too small a pulley (not enough teeth engaged). At 2:1 you won't have any problems - you can use a large-ish small pulley - probably 24-36 teeth.

Wire gauge is not critical, as the currents are still not that high. Anything over 22 gauge should be fine. But some nice 4-wire cable from DigiKey or Mouser for the motor cables. There are also people that sell suitable cable (search on "stepper motor cable") on E-Bay in 10-foot lengths. Using shielded cable is probably a good idea if you'll have radios/TVs/neighbors around.

Regards,
Ray L.