Hi Lads abd lassies, taken delivery sabel lathe this is fitted with a dewhurst switch at the rear of headstock, operation is by lever mounted on front of headstock.
Started lathe up Forward o/k but when switched into reverse a blue flash RCD tripped.
reset rcd forward o/k but did not tyr reverse.
the motor is mounted down and back,the flash was higher up where I think the switch is mounted.
I have not looked at it yet closly as i am disabled and will have to work up to it. any thoughts on matter appreciated JM
Doesn't sound good. Dewhurst switches are pretty simple items, but may look a little confusing when the cover is first taken off (after completely isolating from the mains).
Now I am no electrician, so this is not professional advice, but I think you should have an NVR on/off switch upstream from the Dewhurst and then use the Dewhurst to decide which direction the thing rotates, rather than use it as an on/off switch. The contact blades will last forever if used that way. I bought a neat little NVR from Axminster recently. The NVR bit means the switch reverts to OFF if the power is cut (it has an internal relay).
If the Dewhurst is used to switch load the contact baldes can burn through and break into pieces. I have some spare used blades left, and can help out if you are stuck, but I would take the switch off - and then have a look inside and see what is going on. You don't need all the baldes for a normal single phase, so may be able to shuffle around what you have.
I put some diagrams on a website, the link is in the earlier post.
A few more thoughts. RCDs are fairly finicky so might just be upset by the switch especially if switched under load slowly so it arcs (I think this is what Andrew Mawson is pointing at). So as I mentioned before, use a separate power switch and the Dewhurst for direction (while power is switched off). I don't have an RCD anywhere - but I have seen some in offices at work that were so sensitive that the surge on a computer monitor switch could throw them out. So the Dewhurst may be fine.
Second is that there are complete switch assemblies available that do the job. On my day-to-day lathe I have one from Chester UK which was not a fortune (something like £15 to £20). It has an on/off switch and a reversing switch for the odd time you want to reverse the direction. So you don't need to repair the old switch if you would feel happier with something newer.
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