Colchester Bantam power feed problem

This is my first post so I would like to say hello to everyone.

My lathe is a Colchester Bantam 800 mk1. The power feed drives either the saddle or the cross slide and works fine.

The problem I have is if I use a bed stop, when the saddle touches the stop lots of pressure builds up before the feed disengages, to the point where it has actually broken off a couple of teeth on the bed rack.

From looking at the parts list in the manual I can see that this pressure can be adjusted by altering the tension on two springs inside the worm wheel assembly.

As this adjustment can only be done by removing the apron I would like to know how to set this tension other than by trial and error.

If anyone has made this adjustment or knows how to make it, could you please help out.

I think this was covered in MEW 187 but I can't find anywhere to buy a back copy.

Thanks Stew

Hi StewA,

Check your messages.

John

My lathe lacks such a facility. Having run against a stop, under power, and damaged the rack and ruined the pinion and shaft, I do want to spend another week curing the self inflicted problem.

My technique is now to disengage the power feed a few thou from the stop and to feed, gently, the last few thou upto the stop.

The turning is done to length, and the machine remains unstressed and undamaged, so everyone ends up happy.

Howard

Hi Howard

Sorry for not replying sooner but all my notifications from this site are going into my spam folder.

I am indeed doing as you suggest but as my lathe has the facility to auto stop I would like to have it working if it is not too difficult a job.

Cheers Stew

Hi John

Email sent

I have the same issue on my Chipmaster, which shares a common apron I think? From memory the article in MEW implied it was quite a task to resolve? If I test it by winding the chuck manually feeding up to the stop, the pressure builds up and then the feed drops out with a hell of a bang. Under power it just tends to drive the stop along the bed. I am just living without the feature although it would be a nice to have.

Hi Stuart

One of the members on this site is going to email the details of how to sort it. I'd be happy to forward them if you can message me with your email adress.

Stew

Hi

I have a Bantam, would love to know how this problem is resolved.

Eric

Hi if possible I would like the info on feed stop adjustment. I’ve got an apron from a Bantam in pieces at the moment and would like to adjust the feed “knock off” pressure before I reassemble.

Ashley

The original post is in error when it states that the apron needs to be removed to do this adjustment.

All you need to do to access the adjusting screws is to remove the interlock bar. This allows the part to pivot downwards exposing the adjusting screws. Set them on the loose side and test. Adjust maybe 1/8 of a turn at a time.

In principle, the knock-off works with the saddle moving in either direction so you have to adjust both sides.

I'm intrigued, how is the interlock bar removed?

There is something that looks like a core plug just below the halfnut lever. Prise that out and the female threaded end of the interlock bar is revealed. The bar is held in place with a grubscrew bearing on a flat on it, accesible from underneath. Slacken the grubscrew, screw something into the end of the interlock bar (possibly No. 10 UNC thread) and pull it out.

The knock-off on my Bantam appears to have the original factory setting, since I doubt that the original technical college owners adjusted it. It works OK, apparently without excessive force, though there is a fair clunk on disengagement. I do use it most of the time. However, it is possible to set a sufficient depth of cut to cause spurious operation, but I just accept that as a compromise in a useful but somewhat mediocre design.

Edited By Macolm on 25/01/2023 20:47:59

I removed the interlock bar this afternoon, it is easy if you have the apron on a bench but probably a bit more difficult if on the machine. It did pull out easily, I threaded a 3/16 whit screw into the end of the bar to enable it to be pulled out I think it’s really a unc thread but they are same pitch. Yet to adjust the knock off il waiting for a warmer day.
I forgot to mention on heavy cuts or parting off I hold the lever up to prevent frequent “knock offs”

Since you are able to apply that description to it, you must have in mind some superior methods of achieving the same thing.

Please do feel free to share these methods and the machines on which they appear.

One might say that there is a 'fair clunk' when you pull the trigger of a firearm to discharge the weapon. In that contect, it is hardly a fault.

I’ll try. The apron power feed knock off works by releasing the worm from mesh with its pinion. During this, the gear teeth are nearly out of mesh while still transmitting the full maximum torque. As others have confirmed, it can also release under high, but not excessive, cutting forces. This suggests a less than ideal compromise.

Some owners seem to have increased the release force to the point of causing damage. Unfortunately there do not seem to be instructions from the makers as to how to set this up. A much better design would incorporate a dog clutch, since this allows a more robust release interface. In addition, it would be best actuated directly by contact with the stop, not indirect by the build up of force when baulked.

Another annoyance is that the cross slide has a two start lead screw, resulting in the need for quite eyesight challenging setting of the dial. This may have been done because the cross feed rate is already only half of the surfacing rate, and more appropriate feed gearing could not be fitted in.

However, the manufacture is undoubtedly to a high standard, and the smooth controls and minimal backlash (when unworn) make for very satisfactory use.

Realistically the thing is best considered an overload safety device to disengage the drive before serious damage occurs. As such it's a little kinder than the common shear pin. One of which is also fitted.

Although it can be adjusted so the release force is reasonable such adjustment limits the cut that can be taken before the loads become great enough to trigger the release. Sufficiently so that hefty roughing cuts become impractical.

There is an inherent mechanical delay between the carriage stopping and the mechanism running to the end of its travel to release the drive. This delay doesn't affect accuracy because the saddle has to be hard stopped before the release mechanism starts to drive. However it does mean that things are engaged under full drive pressure as the mechanism moves them out of engagement. Less than mechanically kind tho'.

The single tooth dog clutch arrangement used by Holbrook, Pratt & Whitney et al is operated by a spring loaded trigger so it only has to disengage against normal drive loads rather than maximum stall force. The spring gives an inherently fast release too. However the trigger permits some positional scatter as its release point cannot be absolutely defined. Never enough to be an issue on machines of such quality when in good order but in old age, especially if the machine has been neglected, the trigger point can become undesirably variable. One day I shall dig into my P&W to fix things but needing to lift the headstock is a little daunting.

Clive

Edited By Clive Foster on 26/01/2023 14:19:09