© all rights reserved G W Howe 2017


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.  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 for download - pdf link here

GWH Engineering

creative engineering in a home workshop.