Author: AlanD
Date: Wednesday, 31 October 2007
Title: An Economic Z-Axis Quill Feed on BP2JB Head
Project Objective:
This project aspires to conceive a suitable (simple) method and design a low-cost CNC drive for the "Z" axis on a Bridgeport 2JB milling head. Wide utilty may be quite readily achieved, however, at the least, the design needs only have precision and power suitable for point-to-point and basic pocket-machining typically in 3~5mm thick aluminum panels for prototypes and small batchs. Typically the depth-of-cut does not require high precision, indeed, 2-1/2 axis processing covers most jobs BUT full 3 axis controll will be achieved within the limits of the final design.
Interference with general functions and head adjustments needs to be minimised. In particular, access to the head fixing and adjustments should not compromise the normal readiness of the versatility of the 2JB machine head. Furthermore, the "Z" axis drive componets and housing should be compact and not conflict with typical work-parts nor the manual operations of the quill. 
Our prime use for the CNC functions include the economic productuion of custom parts where the alternatives of punch-press set-up is too costly or tooling does not exist and the expediency of other methods, including laser or turret-punch, may be un-acceptable.
Review and Rationalising of Options:
US$1000 retrofit offerings seem an expensive overkill. One suggestion for mounting a stepper centred on the head pivot, which involved tapping into the pivot casting and compromising head rotation etc.
The Emerging Concept:
With a few basic components in hand (belts pulleys and motor), I visited the 2JB and mocked up a NEMA34 motor (stepper or servo) which I conceive can be mounted on 5mm thick steel plate attached by through/sandwiching the quill return-spring boss and a toothed belt pulley . Perhaps 2 extra fixing holes in the boss will be applied and/or the 3/16" screws replaced with larger HT socket-head cap screws?

Note - I've post some pictures of the mock-up and some PDFs of the CAD drawings (and/or DXF files) as I progress so that others can comment.
Subject to the preferred position, the motor may be mounted extending/projecting away from the machine body or into recesses available behind or under the head.
If an extending motor is acceptable then a shorter belt (and plate) may be permitted - so  there are at least 2 basic mounting arrangements (at differing angles and pulley centres). These may be choosen to minimise the obstructions to critical head adjusting regions and can be oriented to best compromise depending upon the user's operational requirements. Making the plate such that it can be readily adapted to evaluate either of these arrangements and may be readily trimmed down for the projecting motor arrangement is a design consideration.
Drive Gearing:
Calculation of the gearing can be done based upon the specific values of these parameters:
1) stepps/rev of motor - a 144step/rev motor is ready
2) drive~driven pulley ratio - 15:60 is currently being assembled
3) spindle linear motion in mm/rev of pinion - yet to be measured
Spring & Backlash:

Motion loss should only occur in these transitions:
1) rack-n-pinion 
2) belt-n-pulley tensionioning
3) pinion support boss/sleeve bushings
Based upon the expected gearing that will be achieved, it is considered that only the rack-n-pinion backlash will be a potential issue for critial applications.
In assessing the back-lash and lost motion in the quill rack & pinion, I believe this will not be a serious issue for our typical work and the quill-spring boss bearing arrangement will be quite serviceable if greased. A more propper bearing arrangement may be facilitated in the future if this prooves more desirable.
While I have not yet measured the lost-motion, it feels less than 0.5mm BUT clearly it will vary based upon the state of wear in the rack-n-pinion and this may also vary through the quill's range. However, the specific amount should be no less, or greater, than what the Power-Quill gear drive would encounter and no more than what is added by the fitting of the new hand-lever stepper/server drive. On this basic assessment we accept this "Z" drive consept.
As for its elimination, I suspect that it can, to some extent, be addressed in software (if back-lash is facilitated, enabled and calibrated). Or, by driving against the quill-stop, a single definitive depth can be set. The original, factory-retrofitted NC air-over-oil "Z" axis drive had a screw-adjustable 6~8 station capstan-depth-stop.
Note - If a servo motor is used then even PID can not address this backlash. Indeed, depending upon the gearing and gain etc., PID may encourage/exacerbate bounce and chatter. Of course, proper tuning/damping of the servo will minimize this. We have accepted that this back-lash is the compromise of this cheap "Z" axis motion and that, in our application (predominantly 2-1/2 axis CNC), we expect to be able to live with it.
Coupling Pulley and Pinion:

The quill pulley will be mounted on the quill handle indexing boss and keyed to the flange by a similar 3/16" dowel as used in the quill assembly.

Some extension for the quill lever to permit manual operation is being considered, however, it is estimated that it will only take about 1 minute to change the driven pulley back to the quill lever. However, an optional small pinion-shaft extension can potentially facilitate continued use of the hand-quill-feed and all this secured by a substitute 5/16" screw and clamping washer (or a combined spigot-boss with internal recess for a 5/16-UNC socket-head cap-screw) to minimise the extended overall length of the pinion-shaft.
Motor Position/Orientation:
After some worrying about obstructing head swivell/tilting clamping bolts etc. It has been decided that there are 2 reasonable motor options if quill-motor centre distances of 100~150mm are deployed with 180~200XL belts:
1) 235mm plate (150mm centre) with motor below the ram and behind the spindle with the motor projecting into the recess (to the left - fixed to the inner plate)
2) 210mm plate (100mm centre) with motor behind the quill-lever and projecting outware (to the right - fixed to the outer plate)
Note - The outward projection motor will inhibit maximum manual quill-lever rotation to about 270 degrees (nominally 50~70mm of travel), however this is only a limitation when the quill-lever is fitted. Of course, Under CNC control the quill lever can/would/should be removed for safety reasons.
A pair of 5mm steel plates are being prepared to trial both options. Thinner plates and/or alternative material (eg. aluminium) may be reviewed after the initial unit is assembeled on to the machine and testde under operating loads.
Electrical Connections:
The vulnerability of electrical wiring from the stepper motor may deserve attention. Fitting IP-rated or other suitable industrial quality connectors to the top/bottom or end panels can be readily done if their selection in flange size is a comfortable fit on these panels, otherwise, fitting to the inner or outer plates may be possible.
To be Continued:
Note - the image and file resources on Yahoo Bridgeport Group will be used to publish this desgn for criticism and contributions.