In general, as Andrew says, keeping the tool rigid is important. It improves accuracy and reduces the risk of chatter.

Using the largest possible tool holder shank, subject to maintaining centre height of the cutting edge may mean shortening the shank, to reduce overhang, maximise rigidity, and in a four way toolpost, avoid interference with other tools on the turrent.

My Diamond tangential turning tool was shortened for all those reasons.

Based on some of the comments on here, a QCT tends to be less rigid that a 4 way, More reason for the largest practicable shank.

Howard.

Hi Dell,

If your Multifix QCTP has enough room for adjustment you don't need to pack the toolbit. If you already have HSS toolbits for your old lathe, a holder something like this may be used, or you make one yourself. I use several toolholders for my home made QCTP with different tools in each holder, then I can quickly change to another tool and it is on the the lathe centreline.

Thor

Large tools are primarily to make it easier to get to centre height on large machines without stacks of packing. Comapare the size of the tool bit with the size of the shank on J&S and similar toolholders to get a good idea of how big the tool needs to be for sufficient rigidity to cut efficiently.

Basically not very!

Main disadvantage of small toolbits in holders is lack of heat capacity so you have to resort to coolant earlier when doing heavy cuts.

Small bits in holders makes a lot of sense for home shop work. In your case I'd seriously consider 1/8" bits in U shape holders. Parallel slot is easy machine but making at an angle J&S style reduces grinding waste and gives easy height setting without resorting to a QC post. Due to the overhang QC posts are inevitably less rigid than block types where the loads go down direct to the slides. Small sizes are weaker than large ones so the cross over point where they aren't stiff enough to stand the cutting loads or, usually more important, damp out vibrations comes lower. Maybe too low.

I use L sections welded frm flat bar ('cos I'd go some the right size) to space up small tool shanks in my Dickson T2 holders. Tool needs to come out as well as up to keep it central under the clamp screws. The upright leg of the L gives something to push back against making it easy to maintain the alignment. You may need a packer above the tool so the whole of the screw end bites. Light alloy better as it reduces risk of damaging the screw end. Especially on HSS. Welded shank tools have a softer shank to help protect the screw ends.

Clive

As you have already got a QC system the easiest way is to pack up the tools is to find / make / buy some suitable L section sharp internal corner angle. Make the upright thickness such that the tool is centralised under the clamp screws when both tool and L section are firmly pushed to the back of the QC system tool carrier.

Base thickness to taste. Its not important so long as the QC adjustment lets the tool tip come onto centre. Ideal is to have the tool carrier fairly low so its well supported on the post. You can always put packing in or under the L if need be but extra bits are a pain.

Make sure the clamp screws clear the upright on the L, trim down if need be. It is said that you get a better grip on the tool with a thick aluminium alloy shim, 1/8" to 1/4" perhaps, interposed between clamp screw and tool. It certainly protects the ends of the screws from mushrooming but needs regular replacement as the screws inevitably damage the top surface. I'd rather chnage a bit of ally than source good quality hard screws for clamp duties.

Clive

Even if you don't pack up smaller tools its still best to put a spacer behind the tool so the screws are centralised and the tool can't twist sideways.

Ruins your whole day when it does!

Bit me once and took ages before realising the 10 thou taper in a 6 inch cut was the tool swivelling a touch under cutting loads.

Back when I didn't have enough QC holders to keep a decent selection of tools ready mounted using L packers made for a lot less up'n down shifting on the two holders left over after I'd got the normal use tool set mounted. Those two got to hold everything else so had to cope with a variety of sizes.

In those days HSS tooling was relatively much more expensive and being able to standardise on shank sizes was a dream. Take what you can get / find / rescue from the scrap bin then figure out how to mount it was the rule.

Clive

Riding a hobby horse, a Tangential Turning tool (Diamond if you want to buy ) is very good for turning and facing. But every time that you resharpen the toolbit, it has to reset in the holder to centre height.

In general, the object is to make everything as rigid as possible So a 8 mm shank is 4 times as strong (Therefore, all other things being equal; rigid ) as a 5 mm.

I know which horse I would back, if it fitted my lathe.

Howard