Finding Centre Height

Hi Michael,

I may attract an outburst from some ME members when I explain my method of finding centre height.

Like Topsy, it just growed.

First, I have to say a few things which will actually take me longer to write than it will for you to set the tool height.

1. My eyesight is not the best either, requiring a 4" binocular magnifier to handle most of my small work, especially when making 1mm diameter dowels for the skeleton clock.

3. Even though I had a four-way tool post on the Myford, and an (upside-down) rear tool post for parting off, I adopted a `one-tool- suits-all’ for general facing and surfacing, and even parting off. The latter was a little bit time consuming in operation but often saved time in setting up. More shudders!?

Unlike the `height-gauge’ shown by Terry which will allow you to not only see but `feel’ the correct height, my method does require close-up inspection.

For a few quid, (I’m not sure of the price), you might also choose to buy yourself a `centre and edge finder’ normally used in the milling machine to pick up hole centres and edges. I’ve never used one in the lathe, but I don’t see why not.

Have fun,

Sam

Hi Michael,

Here’s one which will fall into the category of `grandmas and eggs’, and which is already generating in my mind, "What have I started here?".

However, I have to ask "How important and how accurate is setting the cutting tool to the centre of the workpiece?" Do I hear growls of resentment?

I must also preempt and simplify my comments by suggesting that we consider a basic lathe setup for surfacing, ie. reducing the diameter of a short length of bar projecting unsupported (cantilevered) from the chuck.

To start with, in every situation when metal is being removed by the `cutting’ process (which is not really cutting anyway), stresses act upon every part of the machine, the tool, and the workpiece. These stresses translate into strain (deflection/movement) of every component involved. They will also to some degree, take up any slack in the machine.

Clearly, the stresses generated will be in opposite directions, starting at the point of contact between the cutting tool and the workpiece. For simplicity and my own limitations, the actual direction of the resultant force is a combination of three forces generated at the cutting tool tip. In other words, there is the (main) downward force on the tool, there is a sideways force mainly from the feed, and a `backward’ force pushing the tool away from the workpiece. By definition, there will be an equal and opposite resultant force in the workpiece.

For all sorts of reasons, I believe it is important to note here that, relative to the cutting forces, the deflection of the projecting bar (at its tip), will be proportional to the cube of the projecting length, and to the fourth power of the diameter. Similarly, the overhang of the cutting tool (also cantilevered), will add to the deflection in proportion to both the length of tool overhang cubed, and the vertical `thickness’ of the cutting tool cubed. Don’t be surprised if the workpiece is somewhat thicker at its tip than close to the chuck.

With these factors in mind, it is easy to see that the question of centre-height accuracy is controversial.

So, relative to the style and sharpness of the cutting tool, the depth of cut, the surface speed of the workpiece, the feed rate of the cutter, the material being cut, and the presence (or otherwise) of lubricant/coolant, (there could be others), we have a situation where the workpiece will lift by some amount, and the tool will dip by some other amount.

And by how much?

Suffice to say that IF the tool is always too high under any conditions, the front face of the tool will rub against the workpiece. The converse is perhaps preferable and, as a result of the above comments, will take place regardless.

Anyway, keep your `cutters’ sharp gentlemen!

Sam

Chris wrote :-

Hi Chris,

You’re spot on with regards to the nub, the pip or the remaining tit. There was just a hint about this in my post yesterday, timed 20/07/2010 10:49:52 (ie. third paragraph from the end). I wouldn’t agree with your comment about the tool being a bit too high `cutting perfectly well’ unless of course the resultant forces `bend’ everything into line. See my last paragraph in my posting timed 21/07/2010 02:22:56

As for diameter, you are correct again. It didn’t make the grade because, as you say - it is so obvious.

Michael, from all the responses both detailed and simple, it looks like you have lots to choose from. Clearly, it will be what’s best for your needs. The little extras (like the `power’ laws I mentioned), are a bonus for anyone interested.

Best regards to you all.

Sam