benchmark international advanced manufacturing trade show
Published

Mind the Tool Center Height When Turning Small Diameters

Let’s use the Pythagorean Theorem to see how small, turned diameters can vary from nominal when your lathe’s tool center height is off.

Share

Louis Trumpet with Vallorbs Jewel Company in Bird-In-Hand, Pennsylvania, wrote a piece that reminds us how small turned diameters can vary from their nominal dimension when a lathe’s tool center height is off. What follows is what he sent to me (which I’ve only mildly edited).

From Louis: Very small, precision turned parts may have several different diameters, making for complications here and there. For example, when turning these small parts, you may notice that when one diameter is on the nominal dimension while others may be off by several tenths. At first, you might think this is caused by different tool pressure at the different depth of cuts, but that’s unlikely. To be sure the problem isn’t mechanical, check for backlash, flex in the machine/toolholder and the fit of the material to the guide bushing (when using a Swiss-type lathe). If everything checks out, and you’re still experiencing a differential, the likely culprit is that your turning tool center height is off.  The following example shows how being off center can affect the diameters being turned.

First, let’s assume that if your tool was brought to XO, the tip would be dead on the centerline of the bar. Line “A” is how far your tool is off center. Line “B” is your programmed X-axis dimension. Line “C” is the actual distance to the cutting edge of the tool or half the actual turned diameter dimension on your work.

Imagine making line “B” longer and longer (turning progressively larger diameters), and you see that angle “A” flattens out, which in turn will make line “C” shorter relative to line “B.” In other words, the error becomes less the larger the diameter your turn is. So, when turning very small diameters, it is critical to be on center.

The Pythagorean Theorem tells us that a2 + b2 = c2. Using that information, let’s assume your tool is 0.003 inch off center and you are turning a 0.030-inch diameter (side a = 0.003 inch, side b = 0.015 inch). Side “c” is equal to 0.0153 inch because c = √(0.0032 + 0.0152), so your turned diameter will be 0.0306 inch or will be 0.0006 inch off of nominal size.

Now assume you are using the same tool to turn a 0.0125-inch diameter. Running the same math, we find that the turned diameter (rounded) will be 0.1251 inch or 0.0001 inch off of nominal size.

Since the 0.030-inch diameter was 0.0006 inch off of nominal size, we have a differential of 0.0005 inch between the two dimensions. It follows, then, that when you offset one dimension to nominal size, the other dimension will be 0.0005 inch off of nominal. All this makes it difficult to dial in the workpiece without editing the program (bad), or using two separate offsets (nearly as bad).

You can also add a macro variable to the programmed dimension, but when you think about it, all that does is provide a convenient way for the operator to edit the programmed dimension. It’s better to fix the root cause of the problem by getting the tool on center. You can use this information to calculate how far your tool is off center and correct it with an offset, assuming you have Y-axis capability. Some small-capacity Tsugami Swiss-types have a feature built into the control to calculate the tool height using this principle, but you can see it works best at very small diameters where angle “A” and the resulting error are greater.

Gardner Business Intelligence
Kennametal
Koma Precision
Hurco
benchmark international advanced manufacturing trade show
Mazak - Innovative manufacturing for medical
The view from my shop.
Paperless Parts
IMTS 2024
SolidCAM
Starrett 2900 Series Digital Indicator
MMS Top Shops

Related Content

Grinding

Choosing The Right Grinding Wheel

Understanding grinding wheel fundamentals will help you choose the right wheel for the job.

Read More
Milling Tools

A New Milling 101: Milling Forces and Formulas

The forces involved in the milling process can be quantified, thus allowing mathematical tools to predict and control these forces. Formulas for calculating these forces accurately make it possible to optimize the quality of milling operations.

Read More
Turn/Mill

Watchmaking: A Machinist’s View

Old-world craftsmanship combines with precision machining on a vertical machining center and Swiss-type lathe to produce some of the only U.S.-made mechanical wristwatch movements.

Read More
Turn/Mill

Understanding Swiss-Type Machining

Once seen as a specialty machine tool, the CNC Swiss-type is increasingly being used in shops that are full of more conventional CNC machines. For the newcomer to Swiss-type machining, here is what the learning curve is like.

Read More

Read Next

Shop Management Software

5 Tips for Running a Profitable Aerospace Shop

Aerospace machining is a demanding and competitive sector of manufacturing, but this shop demonstrates five ways to find aerospace success.

Read More

3 Mistakes That Cause CNC Programs to Fail

Despite enhancements to manufacturing technology, there are still issues today that can cause programs to fail. These failures can cause lost time, scrapped parts, damaged machines and even injured operators.

Read More
Precision Components