Projects

Project to design a belt drive fine feed mechanism on the BH600G lathe.

This project started some time ago following the making of a Screw Cutting Clutch unit designed by Gray Meek and based on that used on a Hardinge lathe.  The BH600G lathe originally had a standard tumbler reversing arrangement to select forward and reverse feed or screw cutting direction.  With the introduction of the screw cutting clutch the tumbler unit was removed as it already incorporated forward and reverse operation.  Yet another version has now been implemented. The original design was basically very simple and easy to make in a couple of days   So why change it? With time on my hands plus a desire to alter the design to provide feed in both directions.  This has been nagging me for some time even though I rarely use feed towards the tailstock but when needed my original design failed this requirement. The original design follows on below but you can jump ahead to the new design article here. In its original configuration the BH600G lathe provided forward and reverse operation using the tumbler mechanism for both screw cutting and fine feed operation.  In the case of fine feed operation,  the gear train provided the power to the gearbox input gear.  With the completion of the screw cutting clutch  I decided that my next project would be to attempt to reduce the running noise which emanated from the gears in the gear train which is ever present as power feeding availability is always needed.  Despite several efforts to reduce this noise using oils and even special open gear additives the noise was still unacceptable especially as the machine workshop is immediately below the main living room!  This became a major issue as the screw cutting clutch had additional gears and this would slightly increase the noise but more importantly the gears and bearings in the clutch would wear quicker because the mechanism would be operational all the time due to the need for a fine feed drive.  I had already thought along these lines when discussing the initial design requirements with Gray and asked that he incorporated a lever to easily disengage the clutch gears from the rest of the gear train.  Of course with the lever disengaging the clutch it also disengaged the drive to the feed shaft and removed any fine feed! I prefer to accept a compromise solution when it comes to problem solving rather than build in complex designs which inevitably out-weigh the main user need.  In my case the screw cutting clutch, which normally operates at slow speeds, provides the perfect solution for screw cutting needs and the resulting noise issues are minimal because the speed is low.  This then left the need to design a solution to provide a quite drive for the fine feed especially as it operates all of the time and at faster spindle speeds.  Initially this problem was solved by introducing a simple belt drive from the spindle direct to a pulley on the gearbox input shaft bypassing the gear train and this worked incredibly well, it was quiet and effective. but I was concerned it may cause wear on the gearbox input shaft bearing.  The only other negative was the this new arrangement would provide fine feed in the forward only direction.  This was not a major problem as most times I need forward direction and in the event I should have to use the reverse direction then this could still be accomplished by reverting back to the conventional gear train including the noise implications but for me, this  is rarely needed.  The direction issue could be sorted by introducing another tumbler mechanism into the  mechanism but this would over complicate things considerably and also introduce more noise.    In practice I rarely use reverse direction for feed but in the event it became essential then the screw cutting clutch can provide this even if somewhat noisily!  The other remaining issue was the coarse feed rate which using the original set up with gears and tumbler mechanism gave ae bottom-end feed rate of 0.0047”  and the new belt drive method provided an opportunity to resolve this issue. After some experimenting a final design evolved.  The new arrangement is much the same as before with a belt being driven from  a pulley fixed to the lathe spindle driving another pulley.  Attached to the lower pulley spindle is a small 25t gear (20PA, 1.25MOD) which drives another 25t gear and this finally meshes to a 50t gear on the gearbox input shaft.   The lower pulley and gear mechanism is fixed to a sliding arm which enables the gears to be quickly and precisely engaged to the 50t gear by way of  an adjustable stop to restrict meshing movement . There are several advantages to this new mechanism: • it provides a much slower input to the gearbox shaft providing a range of fine feed rates, the slowest is 0.0017”  compared to the original 0.0047” •  noise level is very low • load on the gearbox input shaft is almost eliminated • the design is simple, elegant and easily engaged at correct mesh • quick and easy to remove giving full access to the main gear train A free set of drawings in A4 PDF format are available on request

Latest revised design (May 2020)

The original design was basically very simple and easy to make in a couple of days but whilst it worked without problems it did have some short comings.  It could only provide feed towards the headstock;  it was time consuming to change over to screw cutting as the fine feed unit needed to be removed;  it required the gear door to be opened to make adjustments; there was no intermediate setting for no feed. These negatives are very minor but I always felt that the fine feed unit should replicate that originally provided with the stock machine.  The reason it was changed from the stock version was initially to incorporate the screw cutting clutch mechanism and to provide a finer feed rate plus of course reduce gear noise.  With the stock version all gears in the train from the spindle gear to the gearbox were in motion at all times and unfortunately the large 120/127 gears were very noisy having a ringing sound which I found annoying and impossible to resolve. The new revised fine feed unit builds on the simple one direction version and adds another gear which enables forward and reverse operation plus a neutral mid position.  The belt drive is still used as this eliminates all the noise and now has a jockey pulley with a spring to maintain tension.  The spring has a tubing surrounding it to prevent any noise.  In practice very little belt tension is needed as the gearbox provides  gear speed reduction to the final fine feed main drive but in the event of a crash situation the belt would minimise the resulting damage by slipping unlike the direct gear drive which is unforgiving as it has no clutch build in.   Belt tension is adjustable.  The new design is fitted once and remains in position however if access is needed to make up a special screw cutting train the complete unit is removed by undoing a single nut centre) and 2 M6 screws that connect to the lever extension(lower centre).  The fine feed assembly is mounted on a shaft which is fixed to a base plate which in turn is fixed to the lathe headstock base casting.  The base plate arrangement enables adjustment of the shaft for alignment as the casting may not be square.  In addition the new assembly also incorporates an adjustable centre of rotation, again to facilitate adjustment of meshing gears but also its design provides extra bearing surfaces from front to back on the shaft maintaining squareness.  This bearing enables the rotation of the assembly and gears to stay in alignment to the shaft axis with no lateral movement. Operation of the unit is done without having to open the end gear casing door and incorporates a similar lever handle in exactly the same way as the levers are used for the gearbox selection of speeds and feeds.  The fine feed lever can be changed whilst the lathe spindle is turning though this is optimal when the spindle speed is not running at high speed.  The reverse direction, towards the tailstock or in the case of the cross slide towards the operator makes use of a 30 tooth gear unlike the forward which has 25 teeth.  This does mean that the feeds are not identical but both considerably finer than that originally available.   In use the belt drive and gears run almost silently needing occasional oil lubrication.  If needed the complete mechanism can be quickly removed and replacement requires no special alignment process as the lever operation determines the position and meshing of the gears. When it comes to wanting to set up screw cutting operations the fine feed driver 50t gear to the gearbox is removed (single M6 screw) and exchanged for the appropriate gear, commonly a 40t gear plus M6 fixing screw, which covers most screw cutting threads.  The fine feed assemble can remain in situation as it is clear of the main screw cutting gear train.  If the casing door is removed then if needed the fine feed drive belt can easily be removed by moving the jockey pulley to slacken the belt tension. Originally I planned for the index plate to be mounted on the gear cover door but in practice found this resulted in significant additional noise being amplified by the door being made from thin sheet steel in a box form.  To get round this I added a new index plate which is attached to its own stand away from the door, which in turn is mounted to the lathe base tray.  This tray as supplied is folded at the sides to improve rigidity and conveniently provides a base on which the new stand is fitted using a a steel square fitted under the folded ridge.  This arrangement ensures the index  plate is clear of the door and so no additional resonance is created and the door can be opened easily.  The base plate of the stand is fixed with two M6 bolts to the top of the front fold with a sturdy square bar under and also has a third M6 bolt which provides a simple jacking method of adjusting the vertical alignment of the stand and index plate so that whilst close to the door it has about 1mm of clearance and the door can be opened unhindered by the stand.  This fixing arrangement has proved to be very effective and the stand is completely rigid allowing the selector lever to latch as needed. Making the unit is easy but takes more time than the previous version as there are more parts to make. If conventional screw cutting is to be done using the 40 tooth gear as a  direct input to the gearbox then the existing 50t fine feed gear can be easily swapped over by undoing the M6 gear attachment screw.  This process is done easily and quickly with the belt drive left in place (unlike the previous version) or if preferred the belt can be removed but this is not essential.

GWH Engineering

creative engineering in a home workshop