Lathe tool size

Is it a case with lathe tools that bigger is better , the reason for asking is I have just purchased a QCTP for my new to me Pultra 17/70 and it will take up to 12mm bits and only having used 5mm bits on my other lathe it got me to wondering.

Dell

No, in a word. A bigger tool will be more rigid. But controlling overhang is as, or more, important. It also depends upon the job. Big tools can get in the way, may need more overhang or make the cut impossible. On my 13"x40" centre lathe I use 12mm and 16mm shanks for carbide and HSS 5/16" to 1/2" square depending upon what is easiest to grind. My repetition lathe takes 5/16" square as standard. It will easily take a 1/4" depth of cut in steel. When I get my Pultra up and running I'll be using HSS 1/8" to 1/4" square tooling.

Andrew

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.

Thanks for the replies folks

the reason for asking was that the Multifix QCTP that I ordered from PeWe tools will take up to 12mm, on my Derbyshire the largest was 5mm and they were easy to get centred, if I use a smaller tool when it can take larger won’t I need to pack it to get it centred on work.?

Dell

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

You may find you need to pack very small tools if the height adjustment of the multifix is not enough, I need to do it when using 1/8" sq bits in my Dickson.

The main time you may want a thicker section tool is when access to the job means you have to use excessive tool stick out but even then a holder for small bits can be used. Hence why boring bars tend to get thicker as they get longer.

You also say "bits" rather than holders which suggests you may be thinking HSS in which case a 12mm sq one will take a lot more grinding and waste most of the toolbit as your smallish lathe is not going to be taking massively deep cuts so why have 75% of the cutting edge unused. I've shown on here before that I can take a 0.250" (half inch off dia) cut in steel on my hobby lathe with a 1/4" sq HSS tool and similar with a 6mm shank insert holder so why would you really need much more on what is a well made second ops lathe?

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

I bought a QCTP holder that can hold up to 20mm, and it was indicated as correct for my small 330 swing gearhead Lathe.
-I have used up to 16mm so far no problem(HSS).
-I bought a 12mm set of carbide tools, but unfortunately, or actually it does not matter with QCTP not all of them line up with their tips at same height.....
-I also bought a set of 10mm tools and in this case they all are equal height in tips even thread cutting & part off.
In this case I made my own small toolholder for the 10mm set, so if the other one pose reaching problems, I just switch to this toolholder, and no adjustment the height is always correct as well.
(Note:I do not have to space up the 10mm in this tool holder)

I think the bigger can be better, but you still limited by the Lathe capability in making a deeper cut, power, total sturdiness etc, so if your bigger tool does not result in a total sturdier setup, maybe it does make a difference in smaller cuts with bigger tools space permitting.

Edited By Chris Mate on 11/12/2021 13:18:23

Thanks for all the replies so the general feeling is stick with the smaller 5-6mm so the next question is do I use a tool holder or pack the tool bit up if use a tool holder what to use.

Dell

These work well and have built in height adjustment.

**LINK**

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

Before bothering to make up packing or holders simply see what adjustment your Multifix has, depending on it's size and your topslide height there may be no need. While you are at it check how low the holders go as although the slot can take 12mm tooling it may not drop down far enough to allow it.

I only need to add additional packing for 1/8" square HSS and tend to put a bit of flat bar under it, I don't use that size much so have not bothered with anything fancier.

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

Having the biggest tooling that fits is the best idea, but a few smaller ones with shims would be good for the smaller work which crops up. Boring bars are best sized from the maximum your machine will hold down to as small as you can use, as they work best when they have just enough clearance to allow for swarf. You will probably be able to use a boring bar of greater nominal size than ordinary tooling, because of the flats top and bottom and the fact that the tip height is likely to be half the size. I can bore holes down to 5mm but also have up to 25mm bars which would need a start hole of 40mm.

Edited By old mart on 12/12/2021 16:38:56

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