I Need a Yoke (and/or dims) for a Bridgeport

[mxtras]

I am looking for a heavy XY nut bracket (yoke, dog bone) for a Bridgeport. I would also like to get a dimensioned print for same.

Any ideas? I have checked E-bay - they always seem to be available until I need one.

Do any of you have one you'd like to unload? How about a print?

Scott

[jdelaney44]

Scott, did you check out Elrod machine? They have one for their screw retro fit. I have been working with my Rockford retro yoke and it seems to be OK. They have a funky bolt pattern on the Y axis though. Three bolts and one is off like 30 degrees...? Odd.

I was initially concerned about it. But, if you have seen the pics of my parts, things are working well. I raidused a .1875 r arc with a .25 dia cutter and it's smooth. Repeated the feat six times.

I was also initially concerned about the rolled screws, but again, all is well so far.

I want to keep my old yoke, sorry.

Hope that helps.

-jd

[mxtras]

Thanks for the reply, JD. I recall you installed rolled Rockford screws, right? Do they offer ground screws - just for price comparison - or only rolled? I am not stuck on ground - depends on the cost difference.

If you have gone through the ball screw thing for a BP - please read this and help me choose a direction, would ya? Many, many thanks.

I realized last night that a stock yoke would not work for what I was planning so now I am in a serious debate here.

I have a perfect NSK ball screw assembly to fit the table but I would have to shorten it a few inches and machine and grind one end, then modify the table mount and yoke. I have the capacity but it would tax my equipment and set-up would be a challenge. I also have a very stout THK ball screw assembly for the saddle but the same holds true - lots to modify and machine. These screws were not made for the BP mill - they are out of industrial equipment that was recently scrapped where I work but both are .200" pitch and appropriately sized for BP use - I lucked out, but they are not drop in replacements. I have three screws total - all are complete with thrust bearing mounts on one end (very similar in size to the standard Bridgeport mounting arrangement but with 4 bearings in a self-contained housing - nice) and appear to be in very good to excellent condition - they were in a clean room environment.

After reviewing all that's involved, I think it's going to be too much time and too much trouble to make these screws fit the BP correctly. I would have to fabricate a yoke from scratch and while this would be possible, it's a lot of high tolerance work and requires tearing down the mill and getting measurements then re-assembling the mill to do the machining with some outsourcing likely. The cost of getting the screws I have to work would likely be more than just buying a set or a kit ready to go.


So - I am searching for a ball screw retro kit.

The players for a complete kit are:
Rockford
Hiwin
Elrod
Nook
Roton (I don't think they make a kit, though??)
McMaster (looks like Mcmaster sells Rockford screws)
Wedin

Rockford is the only one that has replied to me so far. Currently waiting for a quote from my local supplier - Motion Industries.

Hiwin stuff sounds good but you have to jump through hoops to get info - why cant these people make their products easy to understand and purchase? Should I bother with the tricks to get info? (you have to fax them on a form that has absolutely nothing to do with Bridgeport retro-fit kits - why can't they handle a simple E-mail? Is that really all that difficult?)

I saw the Elrod yoke but I think their website stinks for info and I can't gather enough info to feel warm and fuzzy. They have not replied to my direct questions, either. I think I would really like to have the Elrod yoke - it appears MUCH stronger - and they offer ground screws with an attractive accuracy claim - but their sluggish response and cruddy web site has me a bit concerned. I do think they are the front runner right now, but I want to see their quote. I am leaning towards going with them. Maybe I should place a call....

I have requested info from Nook - no response at all yet.

It looks like McMaster carries Rockford - is that correct? $650 for a set of rolled screws and a yoke - ready to bolt in.

Wedin - claim to have a kit, but no info on their site. I e-mailed them but no reply yet.

Are there others I should investigate?

Rolled for me would likely be fine - as long as I can get rid of backlash so that I can profile and pocket without all the issues from backlash. I intend to use the machine to cut injection molds in the very near future (if possible) so accuracy is a requirement.

If I am going to spend close to $1000 for screws, I would be tempted to throw a bit more in and get the best stuff I can at the moment.

Thanks for the shoulder, guys. I needed to get all of this off my chest! I feel better now.....



Scott

[Hood]

Modifying the yolk to suit may not be a huge problem, in fact you may not have to modify at all. I havent had time to read your post through as I am going out for a while but I will read when I get back in half an hour or so. In the mean time if you have pics of your ball screws it would be good to see what the nuts look like.
Hood

[NC Cams]

HIWIN offers JPG's with full dimensions for their screws on their website as well as lead specs of rolled vs ground. Call them at their office (phone is on the website) and ask for tech support. I got thru to an engineer at first call and he didn't have me jumping thru hoops.

For backlash, simply install true, machine tool grade ball screw support bearings. Pricey but well worth it. You do have to make some shims but it is easily done with some lazer cut shim stock (oem BPT bearings are oddball width) but easy to deal with.

We've got nearly 0.000 baclash comps and measure roughly a net 0.0001 "blip" at direction changes when outside circle cutting. We we measure the deviation with our cam measurement equipment

Our screws are OEM supplied rolled threads from HIWIN on a BPT "EXTRAK". Briian at BPT Machine in Carol Stream Ill did the servo tuning.

[mxtras]

Ah - two of the guys I wanted to hear from. Hood and Cams!

Cams - if the stock BP bearings are loose when I do the retro, why could I not fit a quality set of tapered roller bearings in place with adjustable preload via a nut or a pre-ground spacer? Do you think this would be a bad way to go? The screw's max speed will be well within the rating for the bearings - what are your thoughts on using taper roller bearings for this?

Hood - the screws I have are fitted with nuts that are larger than the yoke will permit. They are substantially heavier than the BP nuts - I am sure that I would have to make the yoke myself. I was looking mainly for a few of the dimensions from a reliable source so I would not have to take my mill apart just to get a few dimensions.

Thanks for the posts, guys! Looking forward to your reponses...

Scott

[Hood]

Originally Posted by mxtras Hood - the screws I have are fitted with nuts that are larger than the yoke will permit. They are substantially heavier than the BP nuts - I am sure that I would have to make the yoke myself. I was looking mainly for a few of the dimensions from a reliable source so I would not have to take my mill apart just to get a few dimensions. Thanks for the posts, guys! Looking forward to your reponses... Scott

Still not read your post yet but when you said the nut was larger than the yolk I thought I would post a pic of what I did. I basically made a part that was the same OD as the original brass nuts, keyway in it and thread on the end, on the other end I had a lipped flange to accept the ball nut. Slipped it through the yolk and screwed a nut on to lock in place, put the yolk in the mill and then from the front I fixed in the ball screw and secured with the cap screws. Not very good at explaining but the pic should help you follow what I am saying.

[NC Cams]

Re: tapered roller option or several reasons for NOT using tapers as a ball screw support bearing.

1. friction
2. package size
3. axial runout

Some of the taper axial capacity comes from rolling resistance of the tapered rollers. BUT some also comes from sliding friction of the roller sides against the flanges. Common trick in racing axles is to replace tapers with A/C balls - reason, to reduce fricion albeit at a loss in fatigue life due to lower capacity.

To get ZERO to negative axial clearance with a preload in a taper, you WILL have more rotating torque that with a preloaded ball. In my case the OEM ball bearings (modified 6204s) had something like 75lb preload. We replace them with 480 lb preload A/C true ball screw bearings. The A/C bearings take LESS torq to turn the handles than the OEM's.

Although you might find a taper that will fit (ID and OD) in place of the ball type, the axial width is probably gonna be wider - getting two with comparable radial runouts is gonna be even tougher. They may tend to radially "fight" each other when packaged in DB mode.

Yes, you can mess with different bearing retainers and other mods to fit T/R's in place of the balls but compare that to a "bolt in" ball bearing DESIGNED to do what you want/need to do. No comparision.

You are looking for minimal AXIAL RUNOUT in a ball screw bearing. Most tapers are NOT machine tool grade. Since radial runout of a taper does affect axial runout, it is another issue to have to deal with. Yes, there are M/T grade T/R bearings but they are not easy to find.

When you add up all the compromises, it doesn't make sense (to me) do do anything but install ball screw bearings.

NOTE: having been involved w/ball bearing engineering for a number of years, I know more tricks than the average bear about how to cheat up and use bearings in unique ways.

When it came to trying to tune up my BPT/Eztrak, I simply bit the bullet (teeth still hurt) and bought the proper bearings.

When you buy the best, you are seldom disappointed.

[mxtras]

I agree that you get what you pay for. I only know enough about bearings to be dangerous - it sounds like you have your act together with a lot more experience with precision rotating things than I.

Elrod responded - no yokes are available at the moment. Their screws are ground and are quite a bit of cash. I need to contact Hiwin. Hopefully today. - - - UPDATE - Elrod screws are made by Hiwin - confirmed by Elrod.

Cams -

My current screw arrangement (factory BP installation) has Fafnir 6204 KTT restrained on the left side of the machine only - the right side HAD an aftermarket power feed and the bearing was not captive. Should I conserve the arrangement and leave the float in the right end of the screw? If so - it places a lot more emphasis on the 'play' in the only thrust bearing on the other end of the screw, which concerns me.

What 6204 is highly pre-loaded? I ask because I am in the midst of designing and machining the right side drive bracketry and am currently working on the bearing retention. I could use your design guidance here.

Thanks for the experience, Cams.

Scott

[mxtras]

OK - One issue resolved!!!!!

I called HIWIN and spoke with Jason in engineering. He was extremely helpful, answering all but one of my questions.

I have decided to get ground screws direct from HIWIN. The price of the ground screw kit was substantially less than I expected!

They do not, however, offer a high quality bearing option. He did clarify that the factory installation was held on one end and floating on the other and he seemed to think this was acceptable.

So - I guess that I need to find a really good bearing for the left side of the table screw and I am good to go for the table. HIWIN does not offer the bearing and Jason did not offer a recommendation.

Scott

[NC Cams]

Scott: Here's what to do:

Buy two sets of 24TAC47BDFC10PN7A bearings from NSK. Hold your breadth when you see the price but they are worth every penny/yen.

Replace left side X axis dual bearings - the other end is OK as is. You make have to make an OD shim to make up the difference 'tween the OEM and the above ball screw bearings. Insert shim between bearing retainer and outer race of bearing that would touch the bearing.

Repeat operation for Y axis.

Reset backlash and lead comp if you have a computer hooked to the system.

If you get preloaded ballscrews, you should have no appreciable b/l to deal with. Ours is under 0.0002 and if we take our care, we can see less than 0.0001 of a "flat" at direction changes.

Don't forget to spend a number of hours adjusting your gibs, too, as this will also affect B/L and lead slightly.

Drive wise, drive on the side of the ball screw suport bearings (left side of X axis and you don't have a choice for Y). Provides for more rigidity as you can't stretch the screw as easily.

[mxtras]

Excellent - I will dig into obtaining these bearings.

* * * * EDIT * * * * * * * * * * * * * * *

The recommended bearings: NSK# 24TAC47BDFC10PN7A
The bearings I was looking at: NSK# 20TAC47BDFC10PN7A (direct replacement)

The recommended bearings appear to be over $475 each. OMFG! I was expecting maybe $150 tops. Can't do $950+ for two bearings. I know there is a difference, but I can not justify it based on the relatively minute benefits.

I will have to deal with .0003" backlash* from the set of angulars that came on the mill. I will replace them but it will be with a new set of the same bearing.

*I will measure the actual bearing slop in my current set of used bearings tonight and report back.

Thanks for the info and for the help. It's much appreciated. I will post any remaining info either in this thread or in the thread about the mill conversion.

I may start a new thread about the ball screws, yoke, interchange details and what I have learned from this experience as well as what I did to achieve whatever results I end up with.

Scott