Lathe Help

Level with the chuck/workpiece centerline. What type of tooling/toolholder? What type of steel? Speeds - feeds ? Bunch of other questions others will think of.

Regards. Ken.

You should buy a copy of "How To Run a Lathe" originally published by South Bend in the '40s. It's a good primer on turning metal and being a newbie myself, I find myself referring to it often. E-bay is filled with folks selling various iterations of this book.

Peter

Unless you are either facing to the center, or cutting a taper or profile, you should give the tool greater relief by setting it .005 to .010 from center. For turning and facing set the tool below center. For boring set the tool above center. The rule is - Turn below, bore above.

I belive the answer you're looking for is how to get the tool up to it's proper cutting level... I can be shimmed, or better yet, buy yourself a QCTP that allows use of several sizes of tool bits, all of which are infinitely adjustable as to height. You won't regret the purchase.

Bill

Vince,

Cutting edge must be exactly on the centreline of the work.

Easiest way to check: (with lathe turned OFF!) gently pinch a 6" ruler beteeen the cutting tool and the work by feeding the tool forwards slowly. If tool is on centre height the ruler will be upright. If too high the top will tilt away from you. If too low the top will tilt towards you. Amazingly easy and quick method.

It may be that you have an adjustable tool holder - if not you need a smaller tool bit or grind the top to lower the cutting edge the 45 thou that you mention.

Good luck.

Andrew Mawson Bromley, Kent, UK

Not if it starts out above center, as he has stated is the problem. :-)

For a HSS bit, the option would be to grind the top down to bring it close or below, and use shims to fix that. Or perhaps to use smaller tools -- though a properly made toolpost should have the top of the tool mounting slots at or slightly below the center height. In other words, it should not be *possible* to put in a tool which would have its cutting height much above center.

The problem *may* be coming form a QCTP (Quick Change ToolPost) which is too large for the machine -- or even a turret toolpost.

It would help to have a bit more detail about the machine (in particular, its size) and the style of toolpost. Maker might help. (Is it an import or a US-made one?)

Let's start with the size of the lathe -- swing (maximum diameter which can be turned above the ways) and maximum distance between centers (distance between the point of a center in the headstock spindle and the point of one in the tailstock with the tailstock at the very end of the bed, and with the tailstock ram cranked as far in as it can go without ejecting the center).

As to the toolpost, if you are asking such questions, perhaps the best bet is to photograph it and put the image in the dropbox (visit

and read on how to submit things to the dropbox. Then, once you have your image there, post the name of the file, and the URL given above.

Whatever you do, *don't* post the image directly to the newsgroup. That is a no-no, and it won't even reach some, because many news servers automatically drop images posted to discussion newsgroups.

Once we have the image, we'll probably need to have some more information. For example, if it is a quick-change toolpost, you will need to look for markings which give the size. If a turret toolpost, there need to be two measurements:

1) Height of center above the compound. 2) Heights of bottom and top of the tool slots above the compound.

For the quick-change toolpost, the height of center above the compound will be needed. The other figures can be looked up for the common sizes. (My 12" lathe has a quick-change toolpost whose size is called "BXA" by some makers and "Series 200" by others.

I presume that if it is a QCTP you have already found the height adjustments on the tool holders.

Good Luck, DoN.

He says it is already 45 thou *above* center, Bill. His problem is to get the cutter *down* to center. He hasn't yet told us what toolpost style he has, what tool bits he's trying to use, what toolholders he has, etc. So we can't yet give him any real help.

Gary

The tool height isn't itself so important as you might think - it depends on the tool's top angle. The _combination_ of the two is crucial, but you can play one off against the other.

You do need to have quick and easy control of the centre height. Either an adjustable toolpost (screw up and down), a rocker toolpost (tilting) or something like a four-way toolpost that's adjusted by shimming. Thick shims are fine (hacksaw blade is traditional) - we're not talking tappet-setting accuracy here.

Ideally the tool height is 5° above the centre height - i.e. about 20 thou high for 1/2" bar. If you set if too high you'll get rubbing under the tool's edge, an overheated tool and very poor cutting. If you set it too low you'll get chatter.

The tool is generally a horizontal bar. Naturally it will deflect slightly under load, so for small deformations of a horizontal bar, this will be vertically downwards. If the tool contacts on or below the centre height, then this deflection downwards will tend to reduce the cut depth, reduce th eforce and reduce the deflection - the tool now bounces upwards, repeats the cycle and you see chatter.

If the tool is at its optimum height (slightly above centre), then it will still deflect downwards but this is always tending to _increase_ the cut.

Yes, the optimum height varies with diameter. You generally leave this set for a particular tool, but you might well set it lower with a fine tool for small work. For facing work it's OK to run on center. A quick facing cut is also another way to measure tool height - a centre pip means you're off centre, a cylinder left is low, a cone is high.

With your tool height chosen, you can check the most crucial of all settings - the tool top angles. There's a top slope back along the length of the tool and a side slope across it. These vary hugely for the shape of the tool, the type of work you're using it for, and the material you're turning. For rough steel try a top slope of 8° and a side slope of 15° - go "blunt" for harder materials, "sharper" for heavy work in free-cutting materials. Some materials (like titanium) are awkward - you have to use an extra-sharp tool. At this point you'll be checking the references, like Machinery's Handbook.

The clearance angle is the angle at the front or side face of the tool. It doesn't affect the cut much, it just needs to get out of the way and not rub. So try twice the centre height - about 10°.

If you're turning brass, then that's a different material altogether. It's more of a scraping cut, so use a tool that has minimal top slope and is set on the centre line (but watch out for that chatter).

Wow! What a great explanation. This is why I lurk here... to learn this type of stuff. Thanks for the info.

I disagree. If the tool is set high, any deflection pulls it deeper into the cut and makes it worse. If the tool is set on center, any deflection pulls it out of the cut and reduces the load. I always set mine right on center.

That's the whole point...

It's a lot less trouble if you have a magnetic base for the surface gague and stick it to a metal rolling shop cart, especially on a very long lathe bed...

JHbs ;-}