My very own Quick Change toolpost

[FatgadgiParticipant @fatgadgi]

Hi Iain

A couple of constructive comments if I may ….

Personally, I do like the Dovetail principle, pulling the holder into a known and repeatable position and I would argue that the piston design is not as accurate. Your design is not quite as stiff as a Dickson type, but hey, practically on our class of lathe, it should be rigid enough.

One thing to consider though is that pulling the dovetail upwards (in your first drawing), there will be big bending and side forces on the screw against the corresponding hole when you clamp it up. This is not the most rigid method because of that, may wear quickly and may also suffer from a wedging effect, where undoing the nut does not release the wedge very easily.

I have seen some designs where a screw or a cam pulls the wedge in a direction 90 degrees to yours, ie the bolt is horizontal in your drawing, pulling the wedge in the same direction as the clamping force, without any side force on the bolt.

But enjoy making it and, of course, there's always the option of a sharp tap with a mallet if it wedges

Cheers – Peter

[Jeff DaymanParticipant @jeffdayman43397]

Posted by Iain Downs on 24/09/2017 20:11:02:

What a diverse set of responses! Excellent.

Jeff. In principle I can see that's a better design, It's beyond my skills and tooling to make, unfortunately.

Hi Iain, if you can do the milling drilling and turning for a dovetail toolpost it is very likely you could make a ring and pin one like I showed. If you break the bits down into their indvidual shapes there is nothing complex about it. The pins for example on the tightening ring do not need to be cut as part of it, they could be screwed or welded or silver soldered onto the tightening ring. Tightening rings could be cut from solid or could be bent to shape after heating to red heat. High precision is not required for most of the tightening rings – if you get the pins at the right location and square to the eccentric tightening cam hole, that's all you need – the rest could be all over the road. The central post could use 90 degree V slots made with an endmill if need be, or could even be drillings to accept a slightly larger dia pin on each toolholder. The eccentric tightening cam could be done with packing one jaw of the 3 jaw chuck if you don't have a 4 jaw. All manner of tricks you could employ to make a really good toolpost designed to good basic tooling principles, and without dovetails. Dovetails are far from easy to make well.

Again, just food for thought. Good luck whatever you build.

[mark smith 20Participant @marksmith20]

I am following Neils design for a quick change tP . But its smaller and for a pultra lathe. Ive been following the drawings but  im not quite following a few things . Ive changed some sizes but looking at the drawings of the cam ,first ,the drawings show a hole going through it of 9/16″ but the pillar has a diameter of 5/8″ howdoes that work?

Secondly the cam again shows a recessat the top of 0.377″ deep and the top collar at 0.375″.Now in the description that goes with the plans the section on the cam states firstly that the thickness of the top flange should be 2 thou greater than the depth of the recess but a fewlines late it says the opposite. Which is correct?

Thirdly , on the side viewof the tool block the hole through the bottom isstated at 0.457″ but the bottom of the clampingpillar  states 5/8″ ,am i misunderstanding how this toolpost works or are these errorson the drawings.

Thanks errors?

[Russell EberhardtParticipant @russelleberhardt48058]

You might find it useful to look at the late great John Stephenson’s design here:

https://www.hobby-machinist.com/threads/a-quick-change-toolpost-by-john-stevenson.268/

I made one to that design about ten years ago and have found that it works very well. It does use a 75 mm block so you might need to scale the drawings for 50 mm. It does pull the tool holder into the block and and the male dovetails on the tool holders are easier to cut than the female ones – you will need a lot of them!

Russell

[HuubParticipant @huub]

Both my lathes aren’t the most rigid. Any tool that hangs over the base of the tool post will compromise rigidity more. So I made a quick change tool post that is “easy” to make, very repeatable (0.01 mm outer turning diameter), easy to clean and change tools in a few seconds.
This is an unedited video of the old version. It will give you an idea on how it operates. The new version doesn’t need to rotate the top to change from inner turning to outer turning.

My quick change tool post

[Charles LamontParticipant @charleslamont71117]

And here is my effort, which has been in use for over 20 years, which explains the not very good film photos:

http://www.charleslamont.me.uk/iqc_toolpost.htm

[mark smith 20Participant @marksmith20]

Well as there was no replies to the measurements errors  above i went ahead and made one for my little pultra, it actually works very well and is solid .Due to the not very heavy stuff i work on i made the holder to go below the compound slide to enable diffferent size toolbits to be used at centre height . The holder is made from en8 scrap round bar i had lying around , piston is bronze. The tool holder isvery rough and i broke a tap in it. But this was just a prototype holder to test it would work. The main block is around 40 mm square by about 35mm high.

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