Bridge vs. Portable CMM - Advantages and Disadvantages

S

skuziej

Considering these two types of coordinate measuring machines and get a quick idea of the strength & weaknesses of CMM's

We are in an assembly type mfg environment & we have what we consider large parts up 90" long by 36" wide & then of course have small component parts. Tolerance will get down to +/- 0.005 on some parts.

Articulating arms seem great for portability into the plant mfg area but their ability for small repetitive measurements at tighter tolerances is not so good.

The bridge style seems great for repeable measurements, escpecially the small parts but the unit is stationary.

I'm trying to write justification papers for each type of CMM and I'm looking for any type of help listing out advantages / didavantages of both types. Even listing applications of both. Any help is much appreciated. :bigwave:

Thanks,

Steve
 

Wes Bucey

Prophet of Profit
Considering these two types of coordinate measuring machines and get a quick idea of the strength & weaknesses of CMM's

We are in an assembly type mfg environment & we have what we consider large parts up 90" long by 36" wide & then of course have small component parts. Tolerance will get down to +/- 0.005 on some parts.

Articulating arms seem great for portability into the plant mfg area but their ability for small repetitive measurements at tighter tolerances is not so good.

The bridge style seems great for repeable measurements, escpecially the small parts but the unit is stationary.

I'm trying to write justification papers for each type of CMM and I'm looking for any type of help listing out advantages / didavantages of both types. Even listing applications of both. Any help is much appreciated. :bigwave:

Thanks,

Steve
For what it's worth, when I ran a contract machining business, we found that the world of inspection rarely has a "one size fits all" product that was more efficient (labor often costs more than the instruments) than using a variety of instruments specialized to the task, especially when many repetitions are needed (manufacturing production quantities rather than model shop one-offs), thus we sometimes found it was more cost-effective to buy a specialized gage or instrument for a long-running product than to use a "lab technique" (we would buy ring gages rather than use three-wire set up for checking external threads, only resorting to three-wire from time to time to confirm the continuing accuracy of the ring gage.)

So I guess one of the factors you need to consider is the relative economic effectiveness of having two separate machines which could complete the necessary inspections in a lot less time than one "all-purpose machine" using a number of "work arounds."
 
T

True Position

The stationary unit also is likely CNC so if you tend to run the same parts you'll be able to check more parts without someone there full time, check more features regularly, and usually output them right into excel for your capability studies.
 
J

JRKH

Without knowing the particulars of your situation, I would lean toward the Bridge type unless portability is crucial. Accuracy, repeatability, calibration are all factors that, in my mind, give the Bridge machine the edge.
Plus as True position has said the Bridge is likely to be CNC controlled which offers additional advantages.

Another thing to consider though is the relative tolerances on the assemblies and the component parts. If the assemblies are relatively "open" in tolerance and the "small componenet parts" are tighter, you might consider a relatively small and inexpensive bridge type for small part checking along with a portable arm type machine for larger, more open tolerance assemblies.

One final thought (for your write-ups) is that the portable machine is not as restrictive in the size of parts it can check, though it can get tedious and time consuming to check large parts/assemblies. If you commit to a bridge type machine based on current requirements and later are building larger assemblies.....The Bridge machine won't stretch....

Just my 3cents worth.

Peace
James
 
T

True Position

I'd agree with that, I'd get a larger machine then you absolutely need now. Also, remember you will need a lot more table space then your part. For example, if you have a probe 100mm long, it needs to be completely able to retract from the part so you're going to need 100+mm more travel then the size of the part. This is especially an issue if you get a fixed probe type machine for the greater accuracy/scanning potential since you'll have a big star probe (usually) trying to work it's way around the parts.
 
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