A tombstone with fixtures on all four sides set up on a Tormach CNC Mill…
I was reading an article about Tombstones in the Tormach blog recently and it got me to thinking about how Tombstones on VMC’s can make a job more profitable.
Tombstones are par for the course on Horizontal Machine Centers, but they’re seen a lot less often on Vertical Machining Centers. Perhaps it’s because they’re just built into a lot of HMC’s at the factory whereas you have to set one up using a 4th Axis on the a VMC. The question for this article is how to get a handle on their value?
There’s an old saying that you’re not getting the most return out of your CNC Machine until you have fixtured a job up so that every square inch of the table’s accessible area is being used to make parts. Put as many parts as possible down for each setup, in other words. Relying on just a single machining vise, for example, can mean a lot of available table area is wasted versus putting two vises on the table (so you get 2 parts per setup), or two double station vises (so you get 4 parts per setup). It’s all about how many parts the machine can “chew” on for each setup. The more the merrier because your setup time divided among all the parts will invariably be less on a per part basis.
With that in mind, why not think of that Tombstone in the 4th Axis as being all about creating more addressable table surface area?
On a Tormach PCNC 1100, you get 18 inches of X and 9.5 inches of Y travel. So, that’s 171 square inches you can utilize for making parts–fit as many in as you can!
Now consider a tombstone like the one pictured above. It’s like having 4 smaller tables available. In fact the width of the tombstone is 4″and the length is 13″, so counting all 4 sides, you get 4 x 4 x 13 = 208 square inches. That’s actually 22% more area than the travels suggest would be available on the table. So, for about every 4 parts we can fit 1 part gets a free ride. That’s not bad!
In addition, we can machine 3 faces on each part. Ordinarily, that would take 3 setups which would be a lot more time consuming. Pretty cool, eh? Depending on how much additional setup time is required for this arrangement versus just using the table, we’ve probably made our job more profitable. Perhaps by quite a bit.
Can we do better than that?
Suppose we had a tombstone that was a full 18″ long to match the Tormach’s travel. Yes, we might need a big fixture plate so the 4th axis and tailstock have room on the table, but that’s doable. With the same math, we’d get 4″ x 18″ x 4 sides = 288 square inches. Dang, now we’ve got 68% more available area and we’re really starting to cook with fire. 68% more parts each time we press the green button get finished.
Suppose we create our own “Tombstone” with a piece of 1″ thick cast iron. We can go 2″ if we want it to be more rigid. Now, let’s further assume it is the width of the table’s diameter, which for the 8″ 4th axis is 7.9″. We’ll mount it dead center of the axis, make it a full 18″ long, and use the two sides. Our total area is now 7.9 inches x 18 inches x 2 sides = 284 square inches. That’s almost as good as the earlier tombstone example, and gives us 66% more area.
What if we have a triangular tombstone that just fits in the inscribed circle of the table? No problem. The math says each of the 3 faces of the triangular tombstone will be 6.84 inches, so we have 6.84 x 18 x 3 = 369 sq inches. That’s 116% more area than on our table–twice as many parts per run. Winner, winner, chicken dinner!
When you look at it this way, it’s not hard to see how a 4th axis with tombstone can increase the available area for parts in each setup and make available more faces of the part to be machined.
Do you use a 4th Axis and Tombstone on your VMC like this? Tell us more in the comments.
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