The choice of mass finishing processes includes vibratory finishing, centrifugal barrel finishing, centrifugal disc finishing and barrel finishing. The subject of this article is selecting the right machine size once the process has been established. The basics of selecting the right size are common to all four mass-finishing methods. A mathematical approach is often used when sample parts are not available or when an actual demonstration is not possible for whatever reason. If a formula approach is your only choice, use it very carefully. If you specify, and your company buys, a machine that is woefully inadequate for the task, you can be in big trouble.
Here is the simple formula for determining the size machine you need for a batch operation: C=A/B, where A equals the parts needed per shift, B equals the number of parts in each cubic foot of machine size and C equals the number of cubic feet (parts and media) required in one shift. Another formula you need is E = C/D, where D equals how many loads you can get per shift and E equals the machine size required. Select the next largest size from the manufacturer you favor. Five simple steps.
Here is the simple formula for determining the size machine you need if you buy a continuous process machine: C = A/B, where A equals the parts needed per hour, B equals the number of parts in each cubic foot of machine load and C the cubic feet (parts and media) required per hour. Another formula you need is E = C/D, where D equals machine loads per hour and E equals the machine size required. Select the next largest size from the manufacturer you favor. Five simple steps.
The formula approaches are useful but have led more than one novice down the primrose path. There are several pitfalls in the mathematical path to machine selection. First, estimating the parts/cubic foot of machine load can be very inaccurate. If you calculate the cubic inch volume of a single part and divide that into 1,728 you get a number that is supposed to be the parts in a cubic foot without media. Then, multiply that by the ratio of parts to total volume, such as 25%, and you get an estimate of parts/cubic foot of machine. The problem here is making a valid estimate of the parts to media ratio. Long parts, odd shaped parts, parts that tangle, delicate parts—these all require great care in estimating the ratio. Just to be safe, figure a 1:6 parts to media ratio, unless you know for sure that a higher number will be safe.
Getting a good estimate of the number of parts/cubic foot of load is extremely important. Get a demonstration with the actual parts in any size machine, if possible. Make sure the demonstration is in the style of machine—tub, bowl, barrel or centrifugal disc—that you plan to buy. If you don’t have parts, get something close to the size and configuration of the proposed parts.
The next pitfall is that there is no industry standard for stating machine sizes. Some machines hold no more than 60% of their stated size. You don’t want to find that out when you are attending the machine runoff prior to shipment!
With batch machines, you can be fooled into allowing less time between loads than will be the actual experience. This will change the loads you get in a single shift and can lead to a significant production shortfall.
The level of media in continuous process machines will significantly affect the time cycle. It is common to find these machines running with an inadequate load (parts and media) and time cycles that are far greater than intended.
Other factors change the time cycle in all finishing machines. For example, high compound flow rates or poor drainage can easily add 50% to the time cycle. Media selection, changes in burr quality, maintenance of drive systems and operator practices can degrade the initial operation, and whoever specified the system might have to answer for it.
What about the small shop with irregular production runs? There is a good selection of 5-cu-ft or smaller vibrators and demonstrations are readily available.
While it is possible to estimate the machine requirement with a formula, it is always best to get a production demonstration in the machine you plan to buy. If possible, do this with a production load of parts and with the media and compound you plan to specify.