Inch Vs. Metric Calibrations on Sherline Tools

All Sherline tools are available calibrated in either inch or metric divisions. The system you choose to work in will most likely be determined by your cultural background, training and requirements of the work you do. The system we were raised with usually remains the most comfortable, but there are also some other considerations when choosing a system that you may not have considered.

Despite the current movement to standardize the world on the metric system, my own preference is the inch system. Here's why. When working in metal, .001" (one thousandth of an inch) is a tolerance that can be achieved with cutting tools, and .0001" (one ten thousandth of an inch) is a tolerance that can be achieved by grinding. The numbers don't come out quite so neatly in the metric system. 1mm equals .03937", .1mm equals .0039" and .01mm equals .0004". The tolerance of ±.1mm (.004") is too coarse for most work, and ±.01mm (.0004") is too fine. Therefore, you end up with tolerances too tight or too loose because the draftsman often calls out a tolerance of ±.1mm when it should be ±.25mm.

The inch system was developed by craftsmen who took into consideration the materials they were working with as they developed their system. Did you ever watch a seamstress measure cloth from her nose to her fingertips? That human dimension is normally about 36" or a "yard of cloth". It would take a basketball player with arms over 3" longer to measure meters that way. If you have been using the inch system all your life, I wouldn't consider buying a metric machine unless the requirements of your work force you to do so. You will have enough new terms to learn without having to learn a new measuring system at the same time.

The difference between inch and metric models of the lathe and mill are not just the handwheel markings, but include the leadscrews, nuts, inserts, handwheels and tailstock spindle. (It is important to remember that a "nut" can be a complicated machined part, not just a simple hex nut. The tailstock spindle is an example of this.)

The handwheel on the inch models moves the slide .050" per revolution and has 50 graduations of .001". The metric model moves the slide 1mm (.03937") per revolution and the handwheel is graduated into 100 divisions. Even though the metric handwheels have finer graduations, this doesn't necessarily make them more accurate, as you don't have to stop exactly on an engraved line. You can interpolate between lines.

I recommend using the system you are most familiar with and for which you have the measuring tools. Don't buy a metric tool and plan to use an inch micrometer with it. Converting dimensions is a constant headache and a source of errors.

If most of the drawings you will be working with are dimensioned in metric measurements, it might be wise to buy a metric tool, even if you have to buy all new measuring tools to use with it.

Parts are available to change any Sherline tool from inch to metric or vice versa. Included is a list of the parts you will need to make a conversion. On the lathe, the tailstock spindle will be replaced as an assembly.

As you can see from the parts list below, quite a few parts need to be changed to make a conversion. The time spent deciding which system to use before purchasing a machine is "time well spent" if it eliminates making a change later on.

Joe Martin
President and Owner

(Items shown in parenthesis are upgraded or optional items which would be used instead of the standard item which may be also listed.)

Instead of purchasing the individual parts listed above, you can order prepared kits for most machines. See below.

* Does not include engraved base or table.

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