# My Ballscrew Conversion



## DMS (Nov 4, 2013)

I have posted about this in other threads, but I have wrapped up this project, so it has come time to lay my cards on the table so to speak. Nearly two years ago I purchased a used Bridgeport clone with the intention of performing a CNC conversion. I got it up and running in a very basic way within a few months but the X and Y axes were still running the stock acme screws. Earlier this year I started a project to replace the stock screws with ballscrews, mainly to reduce backlash. Rather than going with an off the shelf kit (which I wasn't sure would work), decided to machine everything myself. I purchased some screw stock from McMaster and some cheap ballnuts. The cost was low enough that I figured if I screwed up (haha) that I wouldn't be out much $$, and if it worked, I would have saved about a thousand bucks and gained a fair amount of experience. I'm happy to say things worked out well. What follows will be a brief walkthrough of the parts involved. I didn't take many "action" shots unfortunately.

The first part I worked on was the Y axis mount. This started life as a large chunk of 6061 I had sitting around. I roughed out the shape on the mill and finished the bearing bores on the lathe and got a decent (medium) press fit. The bearings were open, so the mount was designed for double lip seals on both sides. The hole assembly was packed with grease and the retainer on the front holds the whole thing together. Here is a view of the whole thing assembled, with the already machined Y axis screw in place.



Here it is installed on the machine. The motor mounts are the same ones I have been using since I started, just 1/2" 6061.



After gaining a little confidence with the Y axis, I purchased a 5 foot section of screw stock and 2 more ballnuts for the Y axis. I also purchased a big chunk of hot rolled steel and some DOM tubing for the yoke. The original yoke on the machine used bronze nuts, and was not going to accept the ballnuts, so I had to fabricate a new one. I did this in 3 sections, 2 large pieces made from the HRS, connected with a piece DOM tubing using my MIG welder.

Here you can see the completed yoke from the top side. As you can see, it is a pretty simple design. The "bulge" in the middle is there to acomidate the pre-load nut. More about that later. I am using square ballnuts, so the channel is square, and the ballnuts just slide right in. A flange attached to the ballnuts screws into the yoke.



Here is a test fit with the ballnuts and the yoke. The large white piece is the preload nut.



The yoke is installed way down in the belly of the beast. It requires that the table be removed and both the X and Y screws. I got very good at removing the table. I think I had it down to about 15 minutes. Each time I needed to take measurements, or test for fit, I had to tear things apart.



Here is a shot with the X axis nut installed as well. You may notice that 2 of the corners are clipped off. Well, as it happens, the yoke has to be installed rotated 90degrees, fed down into the cavity, and then rotated 90degrees to the final position. I didn't really test this until final assembly and it wouldn't go. I had to pull it back out, dog-ear the corners with a hacksaw and files, and try again. Looks almost like I planned it. In this shot you can also see the felt wipers and their retainers. These are just 3/16" soft white felt with holes cut slightly undersized for the screw minor diameter. The felt is soft enough to conform to the screw, and makes a really nice seal. The nuts are packed with the same grease as the ballscrew supports. Rather than purchase pre-loaded ballnuts from the manufacturer for an obscene price, I purchased to non-preloaded nuts and made my own pre-loaded nuts. They worked out really well. The white piece is a chunk of delrin, threaded to match the ballnut. The opposite end has a step to accommodation 12 belleville washers stacked  6/6 (like this >>>>>><<<<<<) for a total preload of about 200lbs. This was tightened against the adjacent nut until it nearly bottomed out. 



The last part was X axis mount. There are 2 mounts on the table, I re-manufactured the "fixed" mount, and used the stock "simple" mount. Here is a picture of the stock mount. It was cast iron, and used tapered roller bearings with a jam nut and start washer arrangement for pre-load. Not bad, but I wanted better. The stock mount is about 5/8" thick.



Here is my replacement mount. It is 1.5" 6061. It is a very similar design to the Y axis mount, using the same bearings and seals. It is actually 2 pieces; the cylindrical portion screws into the flange, and mates with a shallow taper. Threads are locked with blue lock-tite.



Another shot from the side showing the beefiness of the thing. It is very solid. You can also see the jam nut that fixes the X axis screw.



After initial assembly it took a little tweaking to get things working as I wanted, but after a couple hours of tuning, I made a test pass cutting a circle in aluminum circularity with 0.0015, which I am pretty happy with.


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## dave2176 (Nov 5, 2013)

This is great. CNC is on my list for my mill but too many projects ahead of it for now. I'll keep track of this for ideas when it comes time. 
Dave


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## astjp2 (Nov 5, 2013)

Are your ball nuts providing preload?  Tim


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## PurpLev (Nov 5, 2013)

very nice work!


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## genec (Nov 5, 2013)

Are the ball nuts Pre packed, or are they hooked up to the lubrication system?
  It looks as if you've done a super job on this.


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## bpimm (Nov 5, 2013)

Very nice, Can you go a little more in depth on the anti back lash nut design? I didn't quite follow your description. 
What diameter and pitch screw did you go with?

Brian


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## xalky (Nov 5, 2013)

Very nice work. I'll bet it runs as good as it looks!:man:


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## DMS (Nov 5, 2013)

astjp2 said:


> Are your ball nuts providing preload?  Tim



Yes, I built my own pre-loaded nuts out of 2 regular nuts. If you look at the picture of the X nut in place, there is a description. Basically there is a stack of belleville springs that forces the 2 nuts apart and provides preload. The delrin adjusting nut limits the total amount of movement if loads exceed the pre-load. The pre-load should be around 210lbs (calculated, not measured), so it will need to be a pretty heavy cut to reach that.

- - - Updated - - -



genec said:


> Are the ball nuts Pre packed, or are they hooked up to the lubrication system?
> It looks as if you've done a super job on this.



Thanks for the compliment, I'm pretty happy with how things came out. The nuts are pre-packed. There is also a grease zerk on the front to top them off. I was initially planning on connecting things to the one shot oiler, but after some research found that many folks prefer grease in this arrangement. It was also easier than trying to route the lube tubs to the bottom nut.


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## DMS (Nov 5, 2013)

bpimm said:


> Very nice, Can you go a little more in depth on the anti back lash nut design? I didn't quite follow your description.
> What diameter and pitch screw did you go with?
> 
> Brian



The screw is 1 inch diameter with 0.25" pitch. The main reason I chose it was the cost of the ballnuts that matched. The pre-loaded nut is a pretty simple design consisting of two standard nuts, an adjuster, and a stack of belleville disk springs. The nuts are mounted front to back, with the disk springs and adjust in between. Other implementations I have seen for this type of configuration use a key or pin between the two nuts to prevent them from rotating relative to one another. Because the I am using square nuts, I was able to use square channels in the yoke to serve this purpose. When mounted, one of the nuts is attached to a flange that then mounts to the yoke. The other nut is free to move (if it can overcome the pre-load).


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## Maxx (Nov 5, 2013)

Very nice!
If you don't mind my asking about how much in materials for both axis did it run you for the screws and ball nuts.

I had debated a few years back buying the ball screw package until I saw the price, ouch.


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## DMS (Nov 5, 2013)

I would estimate total cost was between $800 and $900. About $500 for the screws/nuts, $200 for bearings, and other bits and pieces here and there (seals, felt, raw material). The lowest price I got for a kit was $1800, and that used the stock bearing mounts which I wasn't psyched about.


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## Muzzer (Mar 25, 2014)

That's a pretty neat solution. I looked at the McMaster-Carr site and those square ball nuts are a pretty good price ($51) compared to the other nuts there http://www.mcmaster.com/#ball-screws/=r90flj

I'm trying to understand how this goes together. It looks to me as if you drop in the left hand nut with the white Delrin ring screwed on its end and the Belleville washers in the middle. Then drop the other nut into the remaining space - it looks as if it is already screwed into the rectangular plate on the right. Then tighten the bolts on the plate on the right so the springs are compressed and the right hand nut bottoms out against the white ring. It means that the 200lb preload is carried by the threads in the Delrin ring (shouldn't be a problem if it's done right) and the right hand nut is constrained by the Delrin ring and the rectangular plate.

Looks as if you got the leadscrew with the premachined end. Was the whole bar hardened or just the thread surface? If I bought a piece of unmachined leadscrew, I'm worried I wouldn't be able to machine it down without going through a lot of carbide inserts. 

The yoke was a worry to me and this looks easier than boring out a couple of cylindrical holes. Do you have any drawings for what you did or was it made to fit? The McMaster-Carr dimensions are very basic and not much to work from. Like you, I'm expecting to have to remove the table on a frequent basis, as I will be using the machine to make parts for itself! Mine's a Taiwanese BP clone.

Murray


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## DMS (Mar 25, 2014)

The price of the nuts and the screw stock is the main reason I went with this. I figured it was low enough that if it didn't work out, I would have learned something, and not been out too much $$$.

The rectangular plate (the flange, as it is referred to) only attaches the assembly to the yoke. Please refer to this picture




The flange is on the right side. The threads from the right-most ballnut are screwed into the flange, and a set screw in the flange locks the flange to the ballnut (there is some blue loc-tite in there for good measure). This nut is fixed to the yoke by 4 socket head cap screws that run through the flange, parallel to the ballscrew, and into the yoke. 

The second ballnut (left-most) is free to float. The white delrin ring (the adjusting nut) is threaded onto this nut. During assembly this nut was loaded onto the ballscrew first, then the adjusting nut as screwed onto the threads from this ballnut as far as it would go. The disc springs followed (greased), then the second ballnut. The ballnuts are actually arranged "back to front". The tension on the springs is pressing on the back of the right-most ballnut (the fixed one) and the adjusting nut. Once assembled, the adjusting nut is tightened until the gap between the adjusting nut and the right-most ballnut is around 0.004" (the rated accuracy of the screws). This allows for expansion, but limits total movement in the case that we overcome the spring tension.

I purchased the raw ballscrew stock and machined it myself. I annealed the screw ends first with a torch, wrapping a wet towel around the section I wanted to leave hard. You don't want to get this to orange, you just want to temper it enough that you have a hope of cutting it (if you get it too hot, it will self-quench, and get harder than when you started...). After that it wasn't too bad. I made up some soft jaws for my lathe to hold onto it properly, and the rest was just basic turning, facing, threading. Oh, I also had to make a spider for my lathe, as the X axis screw is around 5 feet, and I had to run it through the headstock and support it on the other side.

I have some drawings of the yoke that I did based on my stock yoke. I'll see if I can find them when I get home, but I would recommend you check your own setup before you trust that my drawings will work for you 

My machine is also a Taiwanese BP clone, but as I have found, there are a lot of things in common, and a lot of things that differ. It's not always clear which is which.


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## dave2176 (Mar 25, 2014)

Here's a set of screws with zero backlash double ball nuts. They are C7 1605 and 2005 based on Jumps4 RF45 diameters. Supplier is LinearMotionBearings on Ebay. $189 shipped for this set.
Dave


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## Muzzer (Mar 26, 2014)

I'm thinking of using a 2505 ballscrew (25mm dia, 5mm pitch), for all 3 axes, possibly with a double nut like dave2176 showed. My lathe headstock will take a 25mm shaft but not a 32mm (which is the diameter of the original BP leadscrews). Can't imagine I'd need a 32mm version anyway. 

One issue with using McMaster-Carr is that they don't ship to Canada where I live. It is do-able but adds another level of hassle. I may stick to Plan A, ie take my chances with a Chinese supplier.

That ebay store is based in China and there are actually loads to choose from if you look at AliExpress, somewhere like this: http://bit.ly/1iyRFMA although dave's looks pretty good.

BTW, there is a whole series of "white papers" published by Tormach which are really interesting for people like us. I'd probably buy one of their machines if I had the dosh but in the meantime it's encouraging to see people like them explaining how to size and test the system components, cutting through a lot of the crap and using good engineering practice. Have a look: http://www.tormach.com/engineering.html 

My Taiwanese BP clone has a whole hotchpotch of metric and imperial threads and dimensions. It's a metric machine with 5mm leadscrews but the threads elsewhere are a mixture of UNC, metric and possibly even some BS Whitworth threads. I've learned to measure them very carefully when I'm planning to replace any of them! If you have a DRO and/or CNC system it doesn't matter in the end if the leadscrew pitch is metric or imperial.

Murray


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## astjp2 (Mar 26, 2014)

Muzzer said:


> I'm thinking of using a 2505 ballscrew (25mm dia, 5mm pitch), for all 3 axes, possibly with a double nut like dave2176 showed. My lathe headstock will take a 25mm shaft but not a 32mm (which is the diameter of the original BP leadscrews). Can't imagine I'd need a 32mm version anyway.
> 
> One issue with using McMaster-Carr is that they don't ship to Canada where I live. It is do-able but adds another level of hassle. I may stick to Plan A, ie take my chances with a Chinese supplier.
> 
> ...


You may want to consider using a different pitch on the Y, reason being is that there is more inertia, weight and momentum on the saddle vs. the X only moves the table....Tim


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## Muzzer (Mar 26, 2014)

There's not a whole lot of choice when it comes to pitch at this diameter. It's either 5mm or 10mm, so I'd go for the 5mm, which is just under 1/4". Pitch translates into what torque you need from your motor for a given thrust (reaction + inertia) load on the table. I'm not after very fast feeds so the smaller motor / finer pitch / slower feeds is a better trade off for my needs (including cost) than the bigger motor / coarser pitch / faster feeds option.

Having had this machine in pieces before, I know that the 42" table is a lot heavier than the saddle, so I'm thinking that the difference in inertia won't be a big issue for me, even if it means I have to limit the Y feeds slightly lower than the x feeds. Something like 60ipm will be fine for me. When I worked out the loadings and torques, it seemed to me that a "true"(?) 650ozin would be pretty handy. If the worst came to the worst, I could always go back later and fit a larger motor to clear the bottle neck.

I'll let you know how I get on, although I'm always open to hearing of others' experiences and suggestions.

Murray


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## DMS (Mar 26, 2014)

You will get the most drive force from the 5mm anyway. You max speed will be about 1/2 of the 10mm. Probably not a problem.


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## Muzzer (Mar 28, 2014)

Looking at how you have to assemble the yoke into the saddle then fit the Y ballscrew from the front of the machine, did you have to carefully feed the screw in while removing a cardboard or plastic tube from the nut(s)? And do that the other way round each time you needed to withdraw the screw? I'm imagining the balls getting loose if you aren't careful. You must have done this a few times. Makes me nervous!

Doesn't look as if you can insert the screw and nut (already assembled together) from the front and then fasten them to the yoke, which I believe is possible with a single piece flanged double cylindrical nut. Even so, I think they install it with the flange at the back - which means they need an open yoke to make it possible to assemble (eg Hiwin). 

Murray


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## DMS (Mar 28, 2014)

The ballscrew and pre-loaded ballnuts can be removed wouthout dis-mounting the ballnuts. First the end supports are removed, then the table is removed, giving access to the yoke. 4 Socket head cap screws are then removed from the flange, and the whole assembly just lifts out. Easy peasy (well, relatively easy).

I did manage to unintentionally unload the ball-nuts several times . It's a real pain in the rear, and I recommend against it, but re-loading them is very doable. The hardest part is getting the ball-return tube back on. The trick is to pack the tube with grease, then fill the tube with balls so they stick in the grease and don't roll under the heaviest pieces of equipment in your shop, or into a nice pile of swarf. Unfortunately I never did find two of the balls, so one of the nuts is just shy of a full compliment. Still seems to work ok.


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## Muzzer (Mar 29, 2014)

Sounds as if the yoke comes out with the X ballscrew still in the nut/yoke but you must have to remove the Y ballscrew first? That's where I can imagine the balls flying out and looking for a deep, dirty hole to hide in. I was wondering how you did this. I see the Y nuts go into the yoke sideways. 

Murray


Sent from my iPhone using Tapatalk


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## DMS (Mar 29, 2014)

Apparently my reading comprehension is lacking. You were asking about the Y axis, but I just wen't yammering on about the X axis.

The Y axis just feeds in from the front after then yoke is in place. I drive the ballnuts almost to the far end of the screw, feed it into the knee, then bring it across to slide into the yoke. Actually not as hard as it sounds with a flashlight shining up from the bottom. Once that is in, I use an hex drive socket and a long extension to put the SHCSs into the flange to secure it to the yoke. Ideally I will never have to dismount the ballnuts from the screw again.


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