Roller Screw for Bridgeport

[bbutcher85710]

Thanks to everyone for the great new ideas for DIY ball screw replacements. I decided to add my ideas for a roller screw that should fit into a Bridgeport mill. I have not actually built it yet, but plan to try it as soon as I can find time. I decided to use the Acme screws that came with the Bridgeport, 1.25-5 single start and add three rollers. I decided to use rollers with an Acme profile but straight cut, i.e. no helix to keep the 0.200 inches per turn linear motion. I located some needle thrust bearings capable of high thrust loading at MSC for $2.40 each, plus two thrust washers at $0.87 each and plan to incorporate these into the design along with ball bearings for the roller axles. I also added a simple method of adjusting backlash along with a Belleville washer spring to handle slight wear in the lead-screw. This will allow a few thousandths of backlash, but the spring will apply a force against the backlash and should not be an issue on finish cuts where the axial loading is fairly low. To avoid a few thousand more words I am attaching images that explain the details. I am not sure how to add a thumbnail image to the post.
BP Roller Screw.pdf

BP roller screws 2.pdf

BP roller screws 3.pdf

BP roller screws 4.pdf

BP roller screws 5.pdf

[BillTodd]

You do not appear to have any means of adjusting the radial clearance. Are you just going to rely on axial preload to remove backlash? (Lucien [rokag3] and I both found that radial clamping worked well)

Glad to see someone else trying out roller-nuts

BTW to post attachment use the 'Advanced' button then use the manage attachments button to open the attachment window.

[bbutcher85710]

Hi Bill,
I considered using a radial adjustment to take out the backlash, but it seemed simpler to use an axial adjustment. I still may need to have some method of adjusting the rollers radially as well, but if I get the diameters right the axial adjustment should work. I am sure you noted that there is only one thrust bearing on each of two rollers, and none on the third roller. The three rollers are offset axially to match the thread on the lead-screw as determined by the thrust bearings. The top roller just floats axially to the correct location. It looks OK on the Autocad, but I will learn more when I build it.

If I include a jpg attachment will it show up as a thumbnail?

Bob

[BillTodd]

considered using a radial adjustment to take out the backlash, but it seemed simpler to use an axial adjustment.

One thing to watch out for is the tendancy for the lead screw to flex away from the rollers, so whether you pre-load axially of radially, you need to ensure the lead screw is constrained by the rollers (i.e. it would be a good idea to include some adjustment).

Does the floating roller need to be grooved; after all it will either bare only on the edges or flanks, so why not use a plain roller?

You will need to make the gooves very precisely and with the correct profile to fit deep into the thread. If you have access to a cnc lathe use it


[edit] yes, jpeg attachments work OK

[bbutcher85710]

Hi Bill,
Good point, the third roller could be flat, it is only to prevent flexing of the lead-screw. The other rollers will be full thread Acme profile with perhaps a slight angle change to fit the screw. No CNC lathe yet, so I will need to be precise. I am considering using brass for the rollers, but am concerned it might wear too fast. I need to do some experimenting on this.

Bob

[BillTodd]

Make yourself a roller first. See how well it fits the leadscrew. If you can get it to 'bottom' in the thread you'll be on your way.

(This is a subtle hint to include some radial adjustment in your design )

As to wear rates of brass that'll depend on the condition of the lead screw, mine was rough and pitted. I see brass deposits in the nut's lubricating oil, so I know they are wearing into the thread (that was allowed for in the design). Ian (handlewanker) suggested that brass would be subject to excessive fretting if used under continuous pre-load.