The surface gage or transfer stand is one of the indispensable precision measuring tools, found on nearly every granite surface plate in injection mold making shops everywhere. Knowing how to choose the best surface gage for injection mold making will make life in the shop much easier and inspection more consistent.
Like many tools, they are generally taken for granted, until something goes wrong. Not much can really go wrong, except the lack of repeatability, which can be maddening to work with. Life in a mold making or tool-and-die shop is considerably easier when the surface gage does what it is supposed to: faithfully transfer a reading on a dial indicator to the height gage.
If you have a good electronic height gage, you might be able to get along without a surface gage or transfer stand. Nevertheless, there are many situations when you just cannot manage to reach the area to be measured with an electronic height gage.
Surface gage or transfer stand?
Nobody seems to know, or possibly even care, what the difference is between a surface gage and a transfer stand. However the transfer stands are generally more robust, heavier and expensive. Surface gages are usually rather small and do not work well in the .0002 in range of measurement.
There are numerous high quality models available, some more expensive than others. The Mitutoyo series 519 is an excellent and very reliable tool. The Fowler Workshop transfer stand is similar to a surface gage, but more robust and heavy. The Mahr Indicator stand 815 GN is a well made stand which also resembles a surface gage, though this model is much heavier than your typical gage.
Hermann Schmidt seems to have developed just the right combination of accuracy and price with his surface gage that is more like a transfer stand. This medium sized tool performs very well and is popular in shops everywhere. It costs much more than a basic surface gage, but repeats extremely well and is worth the extra cash outlay, if you need precision in the .0001-.0002 in range.
Choose a surface gage if your measurement needs are in the .0005-.0010 in range and if your budget prevents the purchase of a more accurate transfer stand.
Choose a transfer stand if your measurement needs are in the .0001-.0002 range. Life will be much easier if you have the right tool for the job.
For many applications the basic surface gage works just fine. If you work in the .0005-.0010 in range this is all you really need. Models such as the Starrett no.57 series or the Fowler Xtra-Surf have been around for decades, which means that they must work! You can still find old Lufkin. Eclipse or Brown and Sharpe models in use that still perform well.
Starrett makes a small version that is handy for limited areas, such as the parting line on an injection mold. These little tools are great for checking shut-offs and parting line sizes. When use with gage blocks it is possible to reach very difficult areas and obtain a precision measurement.
What to look for
- The tools should match the measurement requirements. If you need a high level of repeatability, choose a transfer stand, or the Hermann Schmidt surface gage. If you only need a basic surface gage, get one and save your money!
- The gage or stand should not “chirp” when sliding over the smooth surface plate. A lapped 3-point stand is ideal for this purpose.
- Should be flexible and the arms should be able to rotate nearly any possible direction.
- Fine adjustment is essential, hopefully with a large headed screw for ease of operation.
- One edge should be precision ground to facilitate sliding along a ruled edge.
- Should be somewhat ergonomic, or at least comfortable to the hand.
- The attachments must be sufficient to accommodate a range of dial indicators, including a drop indicator.