By Graham Howe

The BH600 lathe has a standard leadscrew with a 29 degrees acme thread form. (Previously and wrongly I thought this thread form was a modified square thread with a 5 degree flank) The advantage of the sloping sides, like the acme sloping sides, makes manufacture of the thread easier since there is some clearance.  My guess is that this and most leadscrews manufactured these days are formed by a cold rolling process which has proved to give very accurate thread form. The other thing to note in the case of the BH600G is that the saddle hand wheel is on the left-hand side of the apron. This I think is more common with American designed lathes than English which seem to favour the hand wheel on the right-hand side, but there are notable exceptions.

I am unsure about the reasons the industry makes for right or left handed hand wheels but from personal experience I can see that if on the left side then the operator hands are more likely to get showered with hot chips when turning and if on the right the leadscrew is more likely to get chips caught up in the half nuts since these are positioned nearer to the chuck end! Having two lathes and both are differently anded I can form a personal preference for the hand wheel to be on the left if only to protect the half nuts plus hands can be covered or a shield added if hot chips start flying.

Fortunately, lead screws rarely need replacement unless severely damaged. While this is good news the same might not be the case for the half-nuts which are made from bronze material to hopefully prevent wear in the leadscrew. The thought of making a set of replacement half-nuts is not that simple assuming the bronze material is available and I have read recently that others having to find a solution have reverted to using a ‘plastic’ material such as Delrin and forming the thread form by applying heat and pressure using the leadscrew as  die. I understand that this process is quite successful and if it works out well the thread form produced is more accurate than a machined thread!

The obvious solution is prevention rather than cure in this case and so the best method is to prevent chips and dust particles from getting embedded into the leadscrew and half nuts. It is for this reason I prefer the half nuts to be as far away from the source of chips as possible and this basically means the leadscrew hand-wheel is best positioned on the left of the apron so that 70% of apron length overs the half nuts.  

While this may be advantageous it is not the only prevention method and that is to provide some sort of cover to the leadscrew so that chips and dust etc. are deflected. The current method adopted by the latest designs have a metal spring coil which completely surrounds the leadscrew but unfortunately also restricts saddle travel.

After a lot of thought on this I decided the spring coil was good but expensive. The other method used in modern machines is the use of a retractable curtain-like cover which moves as the saddle moves but these are also very expensive also and probably not available for the smaller hobby-type of lathes.  

The eventual solution came after I abandoned the desire to completely cover the leadscrew but instead opt for a top cover only approach. The idea being that as long as the top of the leadscrew was completely covered then any falling chips or particles would be deflected and fall away from the thread. This was not a perfect solution but it would provide 90% protection and was basically simple. After some experiments with various materials I found a cheap shop in town selling a 10m tape measure having a  tape width of about 1”.  The tape came out of the holder in a curved section which added the ability for the tape to stay flat and of course rewind back into the holder. Sitting a length of tape on top of the leadscrew completely covered it and the curved shape also matched the leadscrew and would deflect falling chips outwards. 

Unfortunately, the size of the recoil spring was massively too big to fit neatly on my lathe but if I could replicate the concept of a retractable tape fixed at the headstock end and the other tape-end attached to the saddle then as the saddle moved the tape would stretch out to keep all of the leadscrew covered. For my lathe the total tape length was around 24” so I decided to allow for 36” of tape and a similar length of recoil spring to wind the tape. 

The remaining problem was the space available to fit the device meant that the tape/coil spring had to fit in a 1” diameter cover.  The finished cover and mechanism was a revolving spindle with the tape winding at one end and the spring coil moved away to the other end having a fixed spindle to cause the spring to tension as the tape was unwound.  The spring in the original tape measure was to big so another smaller tape measure spring was used. 

The mechanism was finally housed with its own cover fitted unobtrusively above the leadscrew with the tape sitting just 1/4” above the leadscrew using a single screw to fix it.  The tape end attached to the saddle so providing an expanding cover which could be removed easily if needed.

This device cost virtually nothing to make and can fit any lathe.  It will provide a high degree of protection from falling chips and easy access to the leadscrew for inspection and cleaning.