Harold Hall QTCP, MEW 50, any one using it?

The brief answer to Goran's original question is Yes, I have, but looking at my name that should be obvious. Those who read my articles in MEW will have seen very many illustrations of it being used.

I do not though think mine is the be all and end all in QCTH design but as Bill Pudney comments it shows that there's more than one way to skin a cat!!

A few comments about the design may be of interest to those coming to this thread.

The original was developed to limit the amount of milling required firstly to minimise the time taken to make them and secondly making it practical for those who only have a lathe and drilling machine.

The time element should be obvious as drilling a hole through two holders clamped back to back will take no more than a few minutes, milling dovetails on two holders would take considerably more, perhaps a half an hour. If one intends to make a large number of holders that is a considerable saving. I would though recommend the hole to be reamed as drilled holes can be a little barrel shaped, though mine work adequately.

A technical aspect of the design is that the length to width ratio of the half round location is around 4 to 1 whilst most dovetail holders this will be around 1to1 maybe less. This should make it pull into place more precisely but there are other considerations. If you are unsure, consider a longish piece of 10mm diameter held in the three jaw to a depth of 10mm, then place it in 40mm, I know which will be running more accurately at its end.

Of course, with commercially made dovetail holders having a good finish, may be ground and hardened then they will move more freely over the two mating surfaces than ones made in the average home workshop. Of course if power feed to the milling machine table is available then it will be less of a problem but hand feed will cause furrows if the feed is slowed or even stopped.

It was also suggest that a square key to be used but the two curved surfaces coming together act as a mini inverted dovetail whereas the corners of a square would tend to dig in.

If I where making a set again with the experience gained I would just ream my drilled holes that make the locations and increase the throw of the clamping cam by just a little, though I think I have included this in the drawings on my web site.

I do not normally visit the forum, I do not have the time, but will keep my eye on this thread should any comments need answering.

 

 

And a half inch tool

 

 

Both at dead centre, and no shimming required because the tool is upside down and cuts from the back, just swap 'em over.

Those of us with a reals mans lathe, like a Drummond with a slotted cross slide, can do these things.

Edited By ady on 15/12/2010 03:46:57