Tool post dovetail angle

Hi all.

I'm in the process of making a QCTP for my Warco WM180.

It will be a piston type, with an eccentric cam in the centre, very much like the one made by Ade of "Ade's Workshop" on YouTube.

The main body is cast iron and I'll make the holders from steel. Male dovetails on two faces of the body and females on the holders. I have 45 and 60 degree dovetail cutters but can't decide which to use. Ade uses 60 deg on his and it seems to be very successful but most of the proprietary dovetails on bigger machines seem to be 45s.

Can anyone suggest which angle to go for and why?

Cheers

Gareth

Hi Gareth,

When I made a QCTP for my small lathe I used a 60 deg. cutter for the dovetails. The holder has worked well for the past two decades.

Thor

60 degree dovetail is correct

Tony

I was leaning towards 60, if for no other reason than Ade's example. He seems to know his onions.

Thanks all.

Agree with Tony that 60° is correct.

But alignment is much more important than angle. The dovetails on each component both have to be on the same plane so full surface engagement occurs when the piston pushes out. Anything more than the smallest error will cause things to try and tilt so much of the registration surfaces are in the wind doing nothing to keep the tool holder stable.

Although this co-planarity is relatively easy to achieve with simple equipment given careful execution of a well thought out technique I've never cared much for the piston concept because the stability dynamics are inherently a bit iffy. Everything rides on the mutual alignment of those dovetails which, to my mind, makes the system under constrained. But from a DIY perspective you literally have only one setting to get right.

Clive

Thanks Clive.

I've machined much of the block but have only roughed the "tenons" for the dovetails. I was planning to cut one 60 deg then just move the Y axis and cut the other (rather than cut one then flip the block and cut the other).

I hoped this would ensure the accuracy you describe.

I'll be using a milling slide on the cross slide of my lathe. I know it's very much a poor man's milling machine but everything is very square and surprisingly rigid - certainly capable of light cuts in cast iron. I've tested it a fair bit and been very pleasantly surprised at the results.

Gareth

I would go for the 60 degree also,