My advise would be to forget dovetails altogether, adjustable or not, and look at an Aloris ring/pin style toolpost. They are best in my industrial experience. Jpeg of this style from a Far East tool supply website is shown.

Edited By Jeff Dayman on 24/09/2017 15:19:09

I would drill and ream holes for 2 dowel pins and make them what locates the dovetail.

I wouldn't massively worry about the material between the hole and the dovetail, I don't think too much force would transfer through it.

Not sure if the general concept is better or worse than the usual methods of toolpost design. I guess if you can't make cams too easily it makes sense though.

Hi Thor, thanks for link. The HH design is an improvement on other dovetail toolholders I've seen.

I don't recommend dovetails in general because they are fundamentally flawed for tool holding – they do not fully restrict all 6 degrees of freedom of the tool, they are bulky, and are more work to make than simpler designs for no real benefit. However, they are traditional and conventional, which a lot of people seem to gravitate to. I suggest a few minutes study with google on jig and fixture design theory, regarding restriction of the degrees of freedom of items for good workholding and good tool design. It may change your way of thinking about many everyday shop operations.

Also a google of "principles of kinematic location" is useful. A link to a good article on the subject is below.

The Principle of Kinematic Constraint

Just food for thought before cutting metal. (A lot of metal, if making dovetails)

With these small machines you are inevitably on the wrong side of the size – strength curve so its hard to get enough material into the tool post and holders to make them strong enough. The overhang and round the corner load path inevitable with any form of QC system doesn't help.

As regular forumites will know I've always maintained that quickly interchangeable block style tool posts are a better approach for a home made QC system. Especially as they give a direct load path from tool to slide, inevitably stiffer than a conventional QC and these small machines need all the stiffness you can get. Shimming the tools to the correct tip height is pretty much unavoidable with any block system but arranging to set-up off the machine with a suitable jig makes it a fairly simple task. Indicators are inexpensive these days so working by direct measurement becomes easy. Especially if you have a nicely sorted set of shims. Probably generally more accurate than setting tool height on the lathe with a QC system which needs a certain extra care to get just so. Close enuf, usually, being easy of course.

The major issue with interchangeable block systems is how to arrange quick release and how to index the base so they always go back in the same place.

For these small machines a simple pin in hole indexing system should be fine. Put the holes in a suitable intermediate plate rather than direct on the topside to avoid irredeemably altering the lathe. Pin should be double diameter with a screw thread at the top end so it can easily be retracted when you need funny angle on the post.

My concept of the quick release system involves a hollow castellated nut in the top of each block fitting over a rotating tool post stud that is cross drilled near the top for a tommy bar. The tommy bar engages in the castellations so a third of a turn or so covers the range between locked down tight and loose enough to pull the tommy bar out. Stud needs to be around 20 or 25 mm diameter at the tommy bar end which may be too big for these small lathes. Rotating bearing part can be much smaller of course. Similar effects can be achieved with bayonet and spline set-ups but maintaining the appropriate timing between the components could be tricky as there is only a limited angle of rotation when engaged. Interrupted threads have similar issues. The uber neat ideal would be a locking lever which could be engaged with the stud and turned or pulled down to lock things. Serious engineering time.

Actually I'd be unsurprised to discover that a simple thick washer with a slot cut through one side so it can be slid out when loose would suffice to hold the tool block down on such a small machine. Hole in the tool post needs to be large enough to slide over the nut so you need a new stud with larger main section and a smaller thread at the end. Loosen the nut by 1/3 or 1/2 turn, pull out the washer, swop blocks, slide washer in and tighten. Sounds fast enough to me.

Clive

On my myford, the simple pin method is employed, this is located to the side of a M10 nut, under the nut that is tightened, although mine is not exactly a quick change toolpost, as it does take some time, the principle of the pin locating, and against the side of a larger nut, work very well for repeatability. I do think that some dowls to the top and bottom of the sliding dovetail will be very beneficial so as to keep the dovetail to run true.

Hi Iain,

Check out Harold Hall's web site, he desinged and made a very robust qctp. **LINK**

Only had a quick look Neil and it looks very functional, can I ask why brass pistons, is it the lower friction?

Brass is just to reduce the wear of having two similar metals against each other.

The little step is in case the corner of the groove isn't perfectly square, so that cam seats properly.

Hardened cams might not grip as well hard=slippy.

Neil

(Plus a bit of brass looks nice).

All I can say is it's been in use for over five years and it's probably the single most-used of all my home made tools. I haven't had it come loose although I have forgotten to lock it a couple of times.

Neil

Either something isn't right or I'm reading the drawing wrong Neil!

So another query, piston drawing shows piston at 9/16" diameter reducing to 1/2" at the end, but the toolpost block drawing shows the piston bore at 0.502"? Typo? Or is the piston really 1/2" diameter? Plus the cam is shown as being only 0.510" width, smaller than 9/16" that the piston is shown at – so only the very last 1/32" of the piston will fit on the cam itself as it comes round, the rest sits on the bit of the cam shaft above and below the cam?

I am querying as I am having to enlarge the toolpost block – and probably everything else – as my lathe has a bit bigger toolpost currently. To me I supposed the piston would always sit on the cam whatever position it was in?

Apologies to Iain if I seem to be sidetracking this thread, but if Iain is following this design it will be relevant to him too!

Chris

Well good on you Iain for having the courage of your own convictions. Shall expect to hear great sounds of boasting from your direction in due course,

all the best, Chris

Many thanks Neil – all is now clear! I must admit also that I did not study the web pages closely. merely skipped through them to look at the drawings – sorry – and so missed the flange. I saw a 9/16" piston reduced to 1/2" for the last 1/32", not the other way round. Your sketch all makes sense of it all now! I do like the concept of this QCTP.

Chris