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Stan Dornfeld

A Philosophy For Purchasing A CNC Milling Machine

Among other common considerations, a strong thought must be given to the table travel area. This factor has the most impact on ROI (return on investment). Since the table travel area is the only place where work can be done, lets study it in detail.

Past technology (hand crank mills) required the machinist to "dial in" and "weld" the setup to limit the number of human errors. These setups were designed around holding usually only one part at a time to do only one operation at a time. The natural concept which developed was to use the smaller machines for the smaller parts and the larger machines were reserved to accommodate the "big stuff". This idea was great when the fear factor was the almighty human element.

The human element, during machine operation, has diminished considerably thanks to "computer control". The computer allows the human to be more creative in tooling design while it does the tiresome repetitive work of machining close tolerance parts.

The new technology has a few unique advantages: * The computer does not "whine" if it is instructed to crank all the way across the table at 400-inches a minute to drill a hole. Try that with your hand crank mill! * The computer can also position the table anywhere within the table travel to an accuracy of 0.00005 inch or better. It does this feat while traveling at "warp" speed.

These two factors alone allow us to program and use the entire table travel area--safely, confidently, and profitably. Imagine, take that little part and toss it on the table anywhere, hold it and machine it right where it is.

Think about it...ANYWHERE!

Remember the old geezer that would tell us to put the vise on the other end of the table for a while to even out the table wear? Now, by mounting parts all over the table, we can even out the table wear and at the same time, be far more productive. And we can do all this while we are out having lunch, because a table full of parts can run a long time.

While holding this thought, an interesting idea comes to mind. Divide the total table travel area into the machine's price and we establish a cost per-square-inch of table travel. Now we have one valid "yardstick" to use.

Comparisons between machines will generally yield a lesser cost per-square-inch for the larger bed-type mills.

Two examples will show how:

A Fadel VMC 40 machining center with a table travel of 20" x 16" has an approximate CPSI (cost per-square-inch) of $205.00, while the Fadel VMC 6030 with a table travel of 60" x 30" has a CPSI of around $62.00. The CPSI on the big machine is 30 percent of the VMC 40.

A Hurco KM3 knee mill with a table travel of 24" x 14" has an approximate CPSI of $117.00, while the Hurco BMC 40 with a 30" x 40" table travel, has a CPSI of around $93.00. And they throw in a tool changer! For just about 80 percent of the price and you can do the bigger parts too.

Using all the available area requires imagination, but the rewards are many. If the work is long run and steady, then large fixtures can be built to hold large numbers of parts. If, however, the work is short run and varied, then perhaps the table can be divided into sections. One section might have two vises set up. Another section could have an indexing head. In the rear section, a subplate used to mount various fixtures could make this machine quite versatile.

The main thrust, of course, is to limit the downtime needed to setup tooling.

In any case, remember, anywhere within the confines of the table travel is fair game to machine a part. Use it advantageously.

Stan Dornfeld is the owner/operator of a metalworking job shop in San Diego, California that specializes in small parts machining.

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