Bridgeport Z axis - my design concept and a review of the existing parts

My design concept for the Bridgeport Z axis:

Most Bridgeport conversions connect the Z axis ballscrew to the quill by means of a yoke (bolted to the side of the quill) which slides up and down in a slot in the quill housing. Due to the shape of the head casting and the various features littering the front of it (associated mainly with the downfeed trip mechanism), the yoke has to cantilever out a fair way from the quill. This is a weakness in the design concept (literally), as there is only a small bolt (around 3/8" or 10mm) to hold the yoke to the quill. 

My cunning stunt uses a 16mm ballscrew that occupies the space taken by the feed trip adjuster screw. This concept was one of the first parts of the conversion I designed, as the X and Y axes seemed fairly straightforward. So when this was first modelled up (when I was living in Canada in 2013), I was using Solidworks. Here's what I came up with. I think this assembly view may have been from a subsequent trial in Onshape when I was beta testing it. But the design is unchanged.

The main features are:

• 16mm ballscrew ("SFU1604", 260mm length) replacing the feed trip screw, supported with thrust bearings at top and bottom. I bought this from AliExpress in March 2014.
• Leadshine closed loop stepper with integrated driver ("Integrated Easy Servo"). I selected the biggest version (iES2320 - 2Nm) which should be more than adequate for my machine and application. 
• Toothed belt drive underneath the quill housing. Sounds dodgy but in fact it won't compromise the quill movement or clash with the toolholders. The alternative (belt drive at the top of the ballscrew) would be tricky and lead to a loss in quill movement which is already a problem with these turret machines to start with.
• Jockey wheel belt tensioner, rather than the usual slotted bolt holes. There's no reason not to fix the motor and take up the slack if you design the belt linkage carefully. One rather nice feature in Solidworks that hasn't been implemented in Fusion 360 (yet) is the chain / belt assembly tool (see Pootube video below). I used this to set the precise position of the motor relative to the ballscrew once I had selected the initial belt length. As toothed belts only come in fixed increments, you have to design your system around the nearest belt length rather than the other way round. The result is better than I got using the more commonplace "pulley centre calculator" tools.
• The original DRO scale sits in front of the assembly (its original position didn't seem to clash with the new system, so it would seem rude not to retain it).
• Steel yoke with split / pinch bolt to allow the ballnut to be disconnected from the quill. Wasn't sure how useful that would be but it also allows a small yoke, unencumbered by the need for fastening bolts. I machined back the original (hardened) flange on the ballnut to minimise the size of the thing - in the process the fixing holes disappeared in a cloud of sparks.
• Omron proximity switches for limit and homing signals. I bought these off ebay and discovered that they were actually a mixture of NC and NO versions. Given the pricing (from China), that wasn't a problem - I'd bought 20 of the buggers, so had plenty spare. Not convinced they will be repeatable enough for use as the homing switch but if that's the case, I will replace that one with something more suitable. But they will almost certainly be fine for the "shit or bust" limit switches.

Solidworks belt / chain tool:


Here's a good demonstration showing how to use this tool.

Cross Section View of Z Axis Assembly:

Here's the section view:

Front right view: 

Underside view (with DRO scale and proximity switches hidden): 

Front left view: 

And as this was the most interesting / challenging part of the build, it was also the first assembly I machined up. 

"Here's one I made earlier" - yoke

Although I'd actually made the parts by the end of 2014, I imported them into Fusion 360 last year so I could remake the yoke in steel using The Shiz. My original yoke was made in loominum using manual operations. The machining of the steel version is documented in this very same blog. It's already fitted to the quill.

The finished SFU 1604 ballscrew:

The main bracket - underside:

Yes, it had to be sawn in 2 and welded back together after machining, as my lathe isn't big enough to swing a part this size - doh!! The slot is for the sliding tensioner wheel.

The main bracket - topside:

The welding looks particularly shite here but it's largely due to the lighting, honest.

Ballscrew with upper and lower bearings in place:

This brass thing is the means for fitting or removing the ballnut from the ballscrew without all the balls dropping out. Its diameter is the same as the minor diameter of the ballscrew.

The male thread fits into the similar length of brass that is currently in the ballnut. I remove the dome headed M6 fastener / washer and screw in my brass adaptor instead. That allows me to slide the ballnut directly onto the ballscrew.

The main bracket fits under the feed trip mechanism housing:

And the motor sits on the main bracket....

...something like this:

This is the lower bearing, viewed from the top side:

There's a spacer to ensure the bearing is firmly clamped against the casting:

Like this:

Finally, I made up this tool for tightening the square nut to the end of the ballscrew:

There are a few other parts in the kit (proximity switch targets, proximity switch bracket, DRO scale spacers etc but certainly I seem to have made up most of the parts - and not lost any of them.

Next - slap it together!