Four Times is Twice as Big

Just one more quick comment on CFM, nozzle size and blasting...

I discussed a little about the differences and relationship between CFM and PSI in a previous post. PSI is basically the amount of force you blast with. CFM is the volume of air being used during blasting.

It is sometimes suprising to realize how much air (CFM) is required during blasting. At a given PSI setting, the amount of air needed is directly related to the nozzle size being used. The larger the nozzle, the more air that is required. This also means that if you want to use a large nozzle, you will need a large compressor.

For example, at 80 psi, a 1/8" nozzle will require 20 CFM. At the same pressure a 1/4" nozzle will require 85 CFM and a 1/2" nozzle will demand 340 CFM. A basic chart that shows the air requirements for different nozzles at different pressures is on the PPB Series page.

Rule of thumb: double the nozzle size, quadruple the CFM required.

Blasting - CFM vs PSI

There are a few variables that need to be addressed when determining the blasting system that will work best for your application. Two critical issues are the blasting pressure (PSI) and the air requirement (CFM).

PSI {pounds per square inch} indicates the force that the blasting media is being discharged from the nozzle. The PSI measurement is independent of the nozzle size being used and is directly related to the pressure setting on the air compressor and/or the blasting pot. The most common abrasive blasting pressure used is 80 psi.

CFM {cubic feet per minute} is the volume (cubic feet) of air that is being sent threw an opening per unit time (the minute part). The CFM required during blasting is based mostly on the nozzle size being used. A larger nozzle will require more air just like a larger hose will need more water. Abrasive blasting is using air continuously so the compressor used must be able to supply enough CFM to 'keep up with' the blaster. For comparison, pneumatic (air powered) tools only use air in small increments so much less CFM is required.

Side note: For a given nozzle size, the CFM requirement will go down with lower blasting pressures (PSI). Using the garden hose comparison, more water will come out of the hose if the spigot is turned to a higher pressure, less if the spigot is turned down.

Dust Control

We recently had a post on our ShopTalk Forum about how to control dust during vibratory tumbling. Either Walnut Shell Grit or Corn Cob Grit can be used quite successfully to burnish parts with a dry tumbling operation.

As these organic media break down (or if a fine grit is used initially) the vibrating action can send dust all over the place. Adding water to reduce the dust is not really an option (muddy Walnut Shell doesn't burnish very well).

Something as simple as draping a rubber mat over the tub or fitting a similar mat over a bowl vibe will keep the dust to a minimum. Using a thick, heavy-duty sheet of rubber of foam and bracing it to the machine will give good dust control and keep the mat from vibrating off.

These 'covers' also have the added benefit of reducing the noise level of the process.

Mass Finishing Basics (Part 1)

There are two basic types of mass finishing - barrel tumbling and vibratory tumbling. Each of these mass finishing methods have pros and cons. There are also a number of sub-varieties which we will discuss in future posts.

Barrel tumbling might be more familiar if described as rock tumbling. Inside the tumbler the parts and media are lifted by the corners of the barrel and allowed to bump, scrape and slide against each other. The media used will depend on whether you are trying to deburr or polish your parts. Barrel tumbling is also good for part-on-part tumbling (another future post - so many posts and so little time). While typically used wet, barrel tumbling can also be a dry tumbling process. The big down side to barrel tumbling is the loading and unloading of parts.

Vibratory tumbling is a little different. Similar to barrel tumbling the parts and media are sliding against each but in more of a 'filing' motion. The tumbling action is occuring in 100% of the load with a vibe versus only 20-30% of the load in a barrel tumbler. This allows a vibrator (you know what I mean) to typically offer shorter cycle times. Vibratory tumbling can abrade and smooth a surface with minimal affect on the edges while barrel tumbling loves to round the edges. Vibratory tumblers can offer very simple and even automatic separation of parts and media.

Generally, barrel tumbling is good for durable parts that require heavy deburring or high polishing and for processes that use a lot of weight (i.e., tumbling with steel media). Vibratory tumbling is great for general purpose deburring, surface finishing and for easy loading/un-loading of parts.

Corn Cob Grit hard as Iron!

All solid materials can be measured for hardness on the Mohs scale. The scale was created in 1812 by Freidrich Mohs and is based how easily one substance can scratch another. Diamond is the hardest rated at a 10 while talc is a very soft 1.

Suprisingly, Corn Cob Grit and iron have the same hardness at about 4.5. Even more amazing is that Corn Cob Grit is harder than brass (3-4), copper (2.5-3) and aluminum (2-3).

The really cool thing is that blasting with Corn Cob Grit won't frost or pit glass and can be used to absorb grease, oils and water from metal surfaces. Corn Cob Grit is also great for blasting on wood surfaces (more to come on that). I wouldn't recommend using iron grit near glass or on wood!

Deburring Aluminum Tube

A client asked us for a process that would remove the burrs from an aluminum part consisting of an assembled tube within a tube. The part was made by inserting one tube into another and then bonding the assembly together. The outer tube was 4 inches larger in diameter than the inner tube. Holes were then drilled through both tubes of the assembly. The holes could only be drilled after assembling the two tubes together due to the critical tolerances involved. There were a total of about one hundred holes in each part. The challenge was to remove the burrs from the inner as well as the outer surfaces of both tubes.

The problem was solved by attaching the part to the vibratory tub in a fixed position so that it would move along with the tub, but still be able to rotate on its axis. By clamping the part down and forcing it to vibrate with the tub, the amount of energy transmitted to the part was greatly increased.

A deburring, ball-shaped Precision Ceramic Media was used to minimize the chance of having media get stuck inside the part. The tub was slightly under-loaded with media to increase the flow rate through the holes and to aid in rotating the part. The compound flow rate was kept at eight gallons per hour. A mild acid cleaning compound (Kramco 1030) was used to speed up the cutting rate and maintain good color on the aluminum.

Due to the elevated cutting energy, the part was completely deburred with a 0.005 edge break in a cycle time of ten to fifteen minutes in a tub-type three cubic foot capacity vibratory machine. The inner tube was deburred thoroughly, exhibiting a 0.003 edge break. The parts were originally being deburred by hand, which took over three hours per part. The vibratory finishing system developed not only dramatically reduced the cost of finishing the parts, but also produced a more consistent part than when finished by hand.

Glass Beads for Auto Restoration

We have been receiving lots of calls from auto restoration companies and hobbyists looking for recommendations for stripping engine parts. A major concern is removing old paint, dirt and grime from aluminum parts that need to be repainted.

While each customer has a preference for surface finish they want to achieve, we have found that Medium Glass Beads offer the best balance of strip and clean rate with surface finish. This mesh size of glass beads leaves a satin-matte finish (if that makes any sense) which is perfect for painting or powder coating.