Build Your Own Grit Blasting Cabinet
By George Hartman
A Little Background
Like many people, when I first began my restoration project
(my 1956 Ford F250) I discovered that a grit blast cabinet, also called a sand
blast cabinet, is a great tool to have around.
It cleans many parts faster and better than any other method. If you do a little research on the subject
of grit blasting cabinets you will find that there are a number of top-quality
units available from various vendors. If
you want to blast larger parts, though, you get into the industrial-strength units
that are usually beyond the budget of typical restoration or rebuild
project. Costs for larger cabinets
usually start at over $2,000 and go up to well over $5,000. I found I needed a larger cabinet to handle
many parts, but I just couldn’t afford to shell out the kind of money that a
commercial unit costs. The only option
was to begin experimenting with various designs of my own. The idea was not to redesign the available
commercial units with all their capabilities but to provide a simple, workable
cabinet with enough interior room to handle bigger parts.
Since this was my first time building a grit blasting
cabinet it was quite a learning experience.
Hopefully I can pass on to you some of the lessons that I learned the
hard way building the first few cabinet versions. I think it’s important to let you know the reasoning behind
certain features since some of them are not obvious but came into use through
hard experience and tinkering. Here are
a few of the biggest issues I dealt with.
From my experience, the single biggest issue with blasting
cabinets is visibility. If you can’t
see, you can’t clean. Keeping the dust
level low and providing sufficient lighting and a good-size viewing port make
any blasting job a whole lot easier.
The visibility battle began with my very first cabinet design. That first version was nothing more than an
18" x 18" x 28" wooden box with a door, a small viewing window, armholes, and
an exhaust fan. Boy did I learn a few
things from that experience I could
blast for about two minutes tops before I couldn’t see anything at all. I tried a number of dust retention methods
including a larger exhaust fan and a variety of filter types including a
homemade water scrubber stack. That
water scrubber was actually pretty neat and it did work but it was too
cumbersome and messy to use in the long run.
I also looked into electrostatic filtration but that was prohibitively
expensive.
I finally reached the conclusion that the plain sand I was
using was a major part of the problem.
It just created too big a mess inside the cabinet for proper
visibility. All grit breaks down with
use but the plain sand tends to break into small, rounded particles. Those little rounded particles are no good
for blasting but are great for making a big cloud inside the cabinet if you
want to obscure your vision. In
addition, inhaling this fine silica dust can cause serious health problems such
as silicosis. After a lot of reading
and research, I found a material called CrystalGrit from Virginia Materials,
Inc. (http://www.sandblaster.com/index.html). CrystalGrit is specular hematite, which has
a number of advantages over plain sand including:
- It
doesn’t break down quickly, - It’s
just about as cheap as plain sand (except the shipping), - It is
very dense so when it does break down, it tends to settle and not form a
dust cloud, and - When
it does break down, it shears leaving sharp, angular surfaces that still
cut rust very well.
The difference between the grit types was like night and
day. With the CrystalGrit I found I
could actually see what I was doing and I didn’t need an elaborate dust
retention setup. I still made a few
changes to improve the visibility such as better light placement, inlet and
exhaust baffles, and a better viewing window system. I’ll describe these in detail later.
Another big problem is that grit bounces off the part being
blasted and strikes the viewing window creating small pits. The process is the same one used to etch
glass. I was using a polycarbonate
window with a scratch resistant surface but after a few hours it would still
need replacement. After wearing out a
number of windows on my box, I decided that there had to be a better way. After a little thought my brother suggested
I use the same system they use for racing helmet visors and, I found out later,
for commercial grit blast cabinets – oops.
I need to read more closely next time I do research I guess. I now use tempered glass for the window and
attach a replaceable polycarbonate film on the inside with masking tape. When the film gets pitted, it’s easily
replaced and the glass window stays like new.
I have been using my grit blaster in its current form for a
couple of years now and I have yet to find any major problems with it. There are things that could be improved, to
be sure, but I am happy with the current design. Now let’s look at the design details and how to build it.
Designing A Large Grit Blasting Cabinet
Before
starting the design, I needed to determine what features are important and what
I could afford to leave out. If I tried
to include every option the cabinet will end up costing more than an industrial
unit. Here are some of the main design
considerations.
First, many people prefer a funnel shaped bottom on their
grit blaster to automatically recycle the grit to the grit bucket. I’m willing to load up with a bigger amount
of grit and stop every couple hours to manually recycle the grit back to the
grit bucket. Since automatic grit
recycling isn’t critical to me I’ve simplified the design and used a flat
bottom on the cabinet. This flat bottom
also lets me easily put different height and shape tables inside to support the
parts while I work on them so I can handle both small and large parts. Second, I’m willing to run an exhaust pipe
outside so I don’t need a 100% efficient dust retention system although it has
to keep the dust down as I discussed earlier.
Finally, I dislike is standing for hours on end while blasting, so the design
has to accommodate a seated operator in reasonable comfort.
The simplest cabinet design is a big, square box and the
least expensive and easiest material to build it out of is either oriented
strand board (OSB), particle board, or plywood. I chose particle board for my project but the other materials
should work just fine too. All of the
materials come in 4′ x 8′ sheets so the obvious cabinet size to build would be
a 4′ x 4′ x 4′ cube big enough to handle all my parts, small enough to fit in
my garage, and the right size to minimize construction effort. I chose �" thick board to provide the
strength needed at the joints without reinforcing. When I first designed the box, I hadn’t considered it, but the
thicker material also allowed me to cut side ports for longer parts without
worrying too much about the sidewall strength.
This was a great bonus when I needed to clean a differential during the
cold winter months. The photo in Figure
1 shows the side port with a cover in place since I’m not using it right now. You can probably use �" board to save money and decrease
the weight but you will have to provide joint reinforcement (2" x 2" or
similar) and you might not be able to cut side ports without sidewall
reinforcement.
The following paragraphs contain a description of the
cabinet I built along with some of the reasons I did things the way I did. My approach is far from the only way to do
this project and I encourage readers to experiment, substitute components, or
make other improvements to meet their individual needs. As far as construction goes, a friend or two
to help with the main cabinet assembly is strongly advised. You need to be comfortable with either a hand
or power saw and with other common hand tools to do the assembly. You will also need to do some minor
electrical wiring for the lights and fan.
If you don’t feel you are qualified to do this wiring please get a
qualified professional or knowledgeable friend to help out.
Please read through the ENTIRE process carefully before starting. I’ve tried to make sure there aren’t any
gotcha’s but a little study on your part could save you a lot of aggravation in
case I’ve missed something. A parts
list for the cabinet including vendors and estimated costs is included at the
end of the article.
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Building The Cabinet
The 4′ x 8′ sheets get cut to the sizes shown in Figure
2. Note that the front door is just a
section cut from the 4′ x 4′ panel for the cabinet front DO NOT cut
out the front door panel yet! We want
the front panel in one piece during assembly to make sure we get things square
and true. I’ll describe how to cut out
the front door later.
It’s easier if you cut the round ports in the back and
bottom panels after you cut the panels to size but before you assemble the box. It’s also a good idea to attach the inlet
port baffle to the back panel before you assemble the cabinet. Center the baffle over the 10" diameter
inlet vent hole and attach it and the spacers with four screws in the corners
of the baffle as shown in Figure 3. The
baffle keeps flying grit from going out the inlet port and into your workspace
but allows air to enter since the spacers hold it off the back panel just a
little. We’ll deal with the cutouts in
the front door after assembly of the main cabinet.
We needed two people to assemble the box, but I suppose you
could figure out a way to do it yourself if you needed to. If you are assembling it outdoors make sure
you have a door big enough to get it back indoors when you are finished putting
the box together No point in providing
comic relief for the neighbors, eh?
Stand the two sides, the top, and the bottom panels on their front edges
on a flat, hard surface (garage floor, driveway pad, etc.). Clamp them in place with a strap clamp or
several long pipe clamps. Place the
back on top of the rear edges of the four panels and make sure everything fits
square and true. Make any required
adjustments (trimming, etc.) now. Pay
attention to which side is the outside of the back and bottom panels or the
holes won’t be in the right places when you’re done.
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With the back, two side, top, and bottom panels still
clamped in place pre-drill holes for the screws that will hold the cabinet
panels together. I used six or eight #8
x 2-1/2" particle board screws in each edge.
Now let’s put these five panels together.
1.
Remove the back panel.
2.
Unclamp the top and one side panel and remove them as well.
3.
Keep the other side and the bottom panels standing on their
front edges and apply construction adhesive to the joint between them.
4.
Install the screws in this joint while holding the panels at a
right angle.
5.
Place the second side panel on its front edge, apply adhesive
to the joint between it and the bottom panel and install the screws in this
joint.
6.
Place the top panel on its front edge, apply adhesive to the
joints between it and the two side panels, and install the screws in these two
joints.
7.
Finally, apply construction adhesive to the back edges of the
two side, top, and bottom panels, replace the back panel, and install screws in
all four joints.
Note that the construction adhesive is both
to provide strength as well as to seal the edges so fine grit doesn’t work it’s
way through. Don’t be afraid to use a
little extra and wipe the excess off the outside after you’re done assembling
things. If you’re really a neat freak
you can wipe off any excess on the inside as well.
After waiting for the adhesive to cure, carefully turn the
cabinet over so it’s sitting flat on the back panel. Place the front panel on the edges of the sides, top, and bottom
panels and check for a good fit. We
don’t want to attach the section of the front panel that will be our door so
carefully mark the top and bottom edges of the front panel door cutout on the
two side panels. Avoid putting adhesive
or screws between the marks and you’ll be able to remove the door after we
attach the front panel and make the door cuts.
Pre-drill holes for screws in the front panel-to-box joints, apply
adhesive, and drive in the screws.
At this point you should have a big cube with some round
holes in it. I strongly recommend
letting the adhesive cure according to the manufacturer’s instructions. You’ll need to move the cabinet around a
little and there’s no point in tearing a joint apart at this point because
you’re too impatient to wait a little for the adhesive to cure.
With the cabinet sitting on it’s back we’re ready to cut out
the door and take care of a couple other details.
1.
Cut two, three-inch-long pieces of the 6" diameter PVC drain
pipe and smooth all the edges with a file or sandpaper.
2.
Rough up the outside surfaces of the pipe with coarse
sandpaper so the adhesive will bond properly when we apply it later.
3.
Use one of the pieces of pipe to trace the armhole cutouts on
the front panel. You want the holes
just big enough to accept the pipe so trace the OD of the pipe closely.
4.
Mark the square cutout for the viewing window.
5.
Cut out the viewing window and armholes with a saber saw.
6.
Test fit the armholes with the pipes and, if necessary,
enlarge the holes a little until they fit.
On to the front door cutout and hinge installation.
1.
Set your circular saw so it cuts just a hair over �" deep and
run it across the front panel along the upper and lower door cutout lines. If you’ve been careful with the adhesive and
screws the door panel should come right off.
2.
Decide which side you want the door hinges on right-handed
folks will probably want them on the left side and vice-versa.
3.
Cut a 47" long piece of 2×4 and attach it with adhesive and
screws to the front edge of the side where the hinges are going. See Figure 4.
4.
With the door panel off, place four Popsicle sticks on the
front edges of the side panels and replace the door so it is sitting about 1/8"
off the sides. This gap is for the
weather strip seal we’ll install later.
5.
Place two of the three hinges about 1" from the top and bottom
of the door opening and center the third one.
Use the long arm of the hinge flat against the side of the cabinet and
the tab flat against the door. The
hinge arms have to be rotated backwards so the pin is on the outside of the
corner. If the hinges don’t rotate
quite far enough use a file to remove a little material where they bind.
6.
Attach the hinges with #8 flat head particle board screws.
7.
Remove the Popsicle sticks, attach the door handle opposite
the hinges, and test the door to make sure it moves freely.
8.
Cut two 45" long pieces of 2×4 and, with screws and adhesive,
install them horizontally at the top and bottom edges of the door opening as
shown in Figure 4. The 2x4s should
extend about �" past the door cutout to provide a place for the door weather
strip to seal against. Choose 2x4s with
smooth and straight surfaces where the seal will contact.
9.
Install the weather stripping around the inside edge of the
door so it contacts the 2x4s on the top, bottom, and hinge side and the front
edge of the side panel on the door handle side.
10.
Insert two or three flat washers behind the turn buttons and
attach two turn buttons along both the top and bottom edges of the door opening
so the door can be held closed. The
washers should hold the turn buttons off the front panel just enough to allow
the turn buttons to pass over the raised door surface. Adjust the number of washers to get a good,
snug fit all around the door.
11.
If necessary, fill any gaps in the 2x4s or side panel edge
with construction adhesive and flatten the surface toward the door with a putty
knife. Wait until the adhesive has
cured to try the door fit again.
Time to cut the grit strainer hole, so tip the cabinet onto
its top panel. I found swimming pool
strainer baskets work just fine as grit strainers. They have holes about the right size for the grit but small
enough to trap any parts you drop. They
also catch any crud that might plug up the siphon tube or grit gun. They’re made of durable plastic and are
cheap to replace if you break it or it eventually wears out. In the parts list I’ve included the part
number of the one I use with my abrasive but you may have to try a couple
different ones to find the right size strainer holes for different
abrasives. Cut the hole in the bottom
panel for the grit sieve as shown in Figure 1.
The hole should be just a little smaller than the inlet opening of the
strainer you choose. With the hole cut,
place the strainer over it and drive #10×3/4 pan-head sheet metal screws into
the bottom panel so their heads catch the lip of the strainer and hold it in
place. I prefer round strainers and I
put four screws evenly spaced around 190 degrees of the circle as shown in
Figure 5. That holds the strainer
firmly but allows me to squeeze it and slide it out without removing the
screws. Remove the grit sieve for now
to avoid damaging it.
The Cabinet Base
The base should provide solid support since the cabinet,
parts, and grit can add up to substantial weight. The base design shown in Figure 6 has worked well for me. It provides knee space for the operator,
support on three sides and across the middle of the cabinet, and is stiffened
to avoid rocking. Other designs are
possible just make sure the base can fully support the cabinet and handle the
load, which can reach 1000 lbs or more with a large batch of dense grit and
some heavy parts.
Start by cutting all the 2x4s and the particle board
gussets. There should be sufficient
particle board left from the last sheet to make the 8 gussets. Assemble the five 2x4s that make up the top
of the base exactly as shown in Figure 6.
Use adhesive and the #10×3-3/4" wood screws. With the top frame assembled, attach the eight gussets using
adhesive and #10×1-1/2" wood screws. Be
careful to stagger the joints exactly as shown in the figure. Use 4 or 5 screws into each 2×4 to ensure a
strong joint. Finally, attach the legs
to the gussets and the top frame using adhesive and wood screws. Use the #10×1-1/2" wood screws to attach the
legs to the gussets and the #10×3-3/4" wood screws to attach the legs to the
top frame through the gussets. Let the
adhesive in the base cure according to the manufacturer’s instructions before
performing any more work on the cabinet.
Once the adhesive in the base is cured set it up in the
location you intend to put the cabinet.
Keep in mind that you need access to the cabinet front door and some
space at the rear for the air inlet and exhaust ports and exhaust hose. The corrugated exhaust pipe needs at least
18 inches to turn 90 degrees unless you use an elbow to attach the hose to the
cabinet back panel. With the base set
up get a friend or two to help lift the cabinet onto the base. Line up the rear and sides of the base and
cabinet but do not fasten them yet. Sit
down at the operator’s position, put your arms through the armholes, and
determine if the cabinet is at the correct height for you. It’s still relatively easy to put blocks
under the legs or cut them off at this point.
Make height adjustments as necessary.
Once you are happy with the setup drive three #10×1-1/2" wood screws
through the bottom of the cabinet and into each rail of the top frame of the
base. The cabinet should now be solidly
in place on the base. If there is any
tendency for the legs to rock use a small wood or metal spacer under the short
leg to make the base solid on the floor.
Exhaust System
The exhaust system consists of a baffle, exhaust pipe, fan,
and dust retainer. The exhaust must be
sent outdoors since the dust retainer is not 100% efficient. If you exhaust the dust into your garage or
workshop everything in the space will become coated with a combination of fine
grit and rust. Don’t say I didn’t warn
you. Plan where you will route your
exhaust pipe and where you want the exhaust fan, dust retainer, and their
housing to be when operating the cabinet.
I run mine out the garage door to the back yard. Once you’ve got your plan, attach one end of
the corrugated plastic exhaust pipe to the exhaust opening on the rear panel of
the cabinet. I inserted the pipe so it
extended about �" inside the cabinet and then drove six of the #8×3/4" particle
board screws through the pipe and into the cabinet back panel. I then used the construction adhesive to
seal both the inside and outside seams to produce an airtight and strong
connection as shown in Figure 7. If
desired, you could use an elbow to attach the pipe to reduce the rear clearance
required for the cabinet.
The other end of the exhaust pipe gets
connected to the fan housing. I used
the simple box shown in Figure 8 for my housing. I connected the pipe to the housing rear panel using the same
method I used for the cabinet end. The
fan gets mounted inside the housing front panel as shown in the figure. There are markings on the fan to indicate
the airflow direction. Make sure to
install the fan so the air blows in the direction indicated in the figure or
you’ll have a mess in your garage the first time you turn it on, since the grit
will be blown back through the inlet and grit sieve ports instead of out
through the fan.
I did not use any adhesive when assembling the housing since
I want to be able to service the fan if necessary. The housing parts are small and it’s not too difficult to cut
them accurately enough to minimize air leaks.
A little duck tape around the outside takes care of any small leaks that
remain. I realized that having the fan
in the dust path would probably shorten its life but I wasn’t sure how bad it
would be damaged. I’ve been using the
same fan now for over three years and it’s still working. I figure if I have to replace it every few
years it’s not too big a sacrifice for a simple and effective dust exhaust
system.
The wiring to the fan is straightforward.
1.
Drill a 1-1/2" diameter hole through the center of one side of
the exhaust housing.
2.
Install one of the cable connectors in the center rear of one
of the electrical connection boxes.
3.
Install a second cable connector in one of the sides of the
connection box.
(Make sure the screw clamps for
both connectors are on the outside of the box.)
4.
Cut off the female end of one of the 25′ extension cords.
5.
Feed the cord through the connector on the side of the box and
then through the connector on the rear of the box.
6.
Tighten the screws on both connectors to clamp the cord in
place leaving about 12" of cord extending out the rear connector.
7.
Position the connector box with the rear connector in the
1-1/2" hole you drilled earlier in the housing.
8.
Fasten the box to the side of the housing using #10×3/4" pan
head screws.
9.
Strip the three wires inside the housing and connect the black
and white wires to the two fan terminals.
You can solder them or buy the special crimp-on connectors that fit the
small tabs on the fan. The green wire
gets connected to the metal fan housing with a small machine screw and nut.
10.
Put the cover on the electrical connection box.
11.
Plug the cord in to turn on the fan and test your
installation.
At this point you’re probably wondering about this "dust
retainer" thingy I’ve mentioned a few times.
Well, it’s a large pillowcase placed over the exhaust housing. I hold it in place by tying a loose knot in
the open end of the fabric behind the housing.
When the fan starts up the pillowcase balloons up and is forced up
against the back of the housing. It
can’t get free because the hole I’ve left with the knot is big enough to fit
around the exhaust pipe but not big enough to fit over the housing.
A couple tips are in order here. NEVER try to wash the dust retainer in your everyday washing
machine. You’ll be very sorry for
several months afterward. Even sorrier
if you’re married and your spouse finds out.
The best way to clean the retainer is to turn it inside out and hand
wash it in a bucket of warm water with some detergent. Let it air dry and you’re ready to go.
Next, insert the armhole PVC pipes so that about 2" extends
into the cabinet. This leaves a lip on
the outside for adhesive and a short length on the inside to attach the
gloves. Apply adhesive to both the
inside and outside of the joint between the pipe and the door so provide a
strong attachment as well as an airtight seal.
Don’t get adhesive all over the inside pipe extensions since that’s
where we’ll be attaching the gloves.
Just a solid bead that leaves an inch or so of clean pipe for glove
attachment is fine.
While that adhesive is curing, lay a bead of adhesive around
the window opening and drive two small screws to the left and right on a line
where the lower edge of the window will be.
Install the glass centered on the opening and resting on the small
screws to support it. Use duck tape on
the window edges to hold it against the door while the adhesive cures.
Completing The Cabinet
The last phase of construction is to connect and attach the
remaining auxiliary items. First,
install the exhaust port baffle as shown in Figure 9. Use adhesive to seal the top and side seams and use three
#10×1-1/2" wood screws along each side seam to hold the baffle to the cabinet
back and side panels. The baffle allows
air and dust to be drawn up from the bottom of the cabinet to the exhaust port
but the heavier grit falls back down and stays in the cabinet.
Prepare for the lighting installation by cutting off the
female end of the second extension cord.
Cut off approximately 10 feet of the cord on the opposite end from the
male plug. We’ll use this spare cord to
connect the lights inside the cabinet and the remaining 15 feet with the male
plug will run to an outlet to provide the power. Connect one end of the spare cord to one of the light
fixtures. The ground wire is not used
here since the fixture has no exposed metal parts. Just trim the green wire off and connect the black wire to the
gold (brass) contact and the white wire to the silver contact. Install this light fixture on the inside of
the cabinet centered directly above the door opening. Snug the mounting screws down but don’t over-tighten them or
you’ll end up with a broken fixture.
Drill a 1-1/2" diameter hole through the cabinet where you
want the power cord to exit. It should
be near the door hinges but in a location where it won’t get caught in the door
or otherwise damaged. There should be
sufficient room around the hole to mount the second electrical box the same way
the first box was mounted on the exhaust housing.
Install one of the cable connectors in the
center rear of the second electrical connection box. Install a second cable connector in one of the sides of the
connection box. Make sure the screw
clamps for both connectors are on the outside of the box. Feed the cut end of the 15-foot cord through
the connector on the side of the box and leave about 6 inches of cord inside
the box. Tighten the screws on the side
connector to hold the cord firmly in place.
Strip the outer insulation off the wires inside the box.
Route the wire from the light fixture you’ve installed
across the top of the cabinet and to the 1-1/2" hole you drilled. Hold the wire firmly in place every 4 to 6
inches with the wire mounting clips.
Run the wire through the hole and then through the rear cable connector
on the electrical box. Hold the box in
place over the 1-1/2" hole and cut the cable leaving about 6 inches inside the
box.
Mount the two remaining light fixtures on the inside of the
front door about 6 inches to either side of the viewing window. Use the remaining length of the spare cord
to connect these two fixtures in parallel and route their supply cord to the
1-1/2" hole in the same way as the top light fixture. Read the paragraph below before routing the cord to avoid getting
it in the way of the light shields. Run
the cord through the hole and then through the rear connector on the electrical
box. Now, carefully position the
electrical box over the 1-1/2" hole and leave a loop of wire for the door
lights so that the wire will not be damaged as the door is opened and
closed. Too much loose cord invites
damage from parts and too little will cause the cord to be stretched or pinched
with door motion. When you have the
cord adjusted correctly, pull the box away from the hole just enough to tighten
the rear connector screws and hold the two cords running through it firmly in
place. Now attach the connection box to
the cabinet, strip and connect the three black wires together, the three white
wires together, and the three green wires together. Use wire nuts on the black and white wires but attach all three
green wires to the metal connection box using a small machine screw and
nut. Put the cover on the electrical
connection box and install three 100W bulbs in the fixtures.� Plug the cord in to test your installation.
Next, install the light shields on the door to avoid having
the door lights shine back in your face through the viewing window. You can see some older versions of the
shields in Figure 10. Cut the aluminum
sheet in half so you have two pieces each 6" x 12". Bend a �" strip along the 12" side of each piece to a right angle
and drill three evenly spaced 3/16" holes through it for mounting screws. Mount each piece vertically between one of
the door light fixtures and the viewing window so that you can’t see the light
bulbs as you look left or right through the glass. Use the #8 x �" particle board screws to mount the shields.
With the adhesive cured on the armhole pipes take each glove
and pull the cuff over the inside of the armhole pipe. Slide a cable clamp over the glove and
tighten it down to connect the glove cuff to the pipe. I use a strip of felt between the clamp and
the glove cuff to avoid cutting or tearing the glove with the clamp. Make sure to get the correct glove on the
correct side and make sure the gloves are rotated with the thumbs up and in for
a comfortable fit.
Time to install the air and grit pickup hoses and the gun. Note the two holes shown in the front of the
cabinet for tubing feedthrough in Figure 1.
Choose one to be the grit pickup and one to be the air supply. Drill the grit pickup hole so that the
plastic grit tube that came with the gun will just fit through. Insert the tube so that about 24" of tube
remain on the inside of the cabinet with the gun attached.
I wanted to keep my compressor air hose intact and use the
gun both inside and outside the cabinet.
The easiest way to do this was to make a short air line with a male
quick-connect on one end and a female on the other. Connect the female quick-connect-to-hose-barb fitting to one end
of the 6′ piece of �" polyethylene tubing.
Drill a �" hole through the cabinet front panel for the air supply. Feed the cut end of the polyethylene tubing
through the hole from the inside until there is about 24" of tubing left inside
the cabinet. Connect the female fitting
to the male quick-connect on the grit gun.
Finally, connect the male quick-connect-to-hose-barb fitting to the cut
end of the tubing outside the cabinet.
You are now all set to connect your air line and test your installation.
The last step is to modify the grit bucket so it catches the
grit that falls through the grit sieve.
I’ve included a grit blast kit in the parts list that has a plastic
bucket with the appropriate funnel shape and suction tube at the bottom. Unfortunately, the manufacturers and vendors
tend to change these molded plastic units fairly often. I don’t know why they do this, maybe to keep
up with the fast-paced world of grit bucket fashion? Anyway, I cut off the top of the grit bucket that comes with the
kit in the parts list and it worked fine catching the grit. You may have to make some other
modifications to your bucket if the style has changed since then. The bottom line is that you need a bucket
with reasonable capacity, a funnel-shaped lower section, and a suction tube at
the bottom. The only other requirements
are that the opening at the top is big enough to catch the grit from the grit
sieve and the unit is short enough to fit under the cabinet.
Figure 10 is an interior and Figure 11 is a front view of
the cabinet after much use. This
particular cabinet is a little older and has some minor variations from the
description above but it is pretty much what you should see when you are done
with your project.
Using the Cabinet
As I mentioned at the beginning of the article, I wanted to
be able to put various tables, part holders, etc. inside the cabinet to make
the work easier. The flat cabinet
bottom doesn’t allow for automatic grit recycling but it does make it easy to
handle items and put tables or fixtures inside. The easiest way to recycle the grit in this cabinet is to remove
any items inside and use a short end of 2 x 4 and your hands (with work gloves
on!) to push the grit to the front corner and into the grit sieve hole.
The heavy rubber gloves are designed to keep the skin on
your hands when you’re blasting.
Unfortunately, they also make holding small parts difficult. I’ve found that clamping small parts in a
vise grip before putting them in the cabinet gives you have something to hold
on while blasting. You’ll have to stop
and change the clamp position to get the entire surface blasted but it’s better
than searching around in the grit pile trying to find that little bugger after
you drop it. Another tip is to put a
nylon wire tie on the gun trigger to keep your trigger finger from permanently
freezing in the "squeeze" position. I’m
too cheap to buy a foot-operated air valve so I just disconnect the
quick-connect air supply fitting to the gun to stop the airflow.
Even with the exhaust system the fine dust in the cabinet
gradually builds up on everything inside.
I periodically pull the grit suction tube off the gun port and use the
straight compressed air from the gun to kick up all the dust on the cabinet
sides, top, and especially the lights.
After a minute or two with the exhaust fan on, the inside is clear again
and I reinstall the grit tube and continue blasting.
I’d strongly recommend that you use a compressor of
sufficient size to avoid the frustration of low air pressure and incomplete
surface coverage. There is also the
possibility of burning out either the motor or pump from overwork, I learned
that lesson the hard way. The minimum
I’ve been able to get away with is about 20 cfm at 80 psi for heavy-duty
work. I do use lower pressure for more
delicate items but I wish I had something like 25 cfm at 100 psi to really make
the hard jobs easier.
As I mentioned before, I’ve used the large cabinet to blast
the rear axle from my F-250 by cutting a port in the right side and putting
each end of the axle inside the cabinet in turn. I supported the outboard end of the axle with an engine hoist
although the �" thick cabinet side panel took a fair amount of the weight as
well. Some duck tape and a plastic
garbage bag sealed the opening between the axle and the cabinet side panel when
the axle was in place. An oversize
scrap piece of particle board with weather stripping attached to the edges
closes off the port when I’m not blasting longer parts that don’t fit in the
interior.
One change I would make to the current design is to use a
larger armhole diameter for better comfort.
I’m sure there is 8" diameter PVC pipe available somewhere and the glove
cuffs would probably stretch to go around it.
If anyone would like to try this you might try looking at sewage pipe
supply houses, since they tend to have larger diameter plastic pipe.
Optional Parts And Tips
The polycarbonate film in the optional parts list comes in
24" x 24" squares. I use a utility
knife to cut each 24" square into four smaller squares that fit in the viewing
window cutout in the door. Four pieces
of masking tape seal the edges of the film to the window. As I mentioned before, the film takes the
abuse from the flying grit rather than having it pit the glass. When the film gets too hazy I just replace
it with a new sheet. That’s a lot
better than having to replace the glass.
Again, as far as grit material, I highly recommend the
specular hematite product called CrystalGrit from Virginia Materials, Inc. (http://www.sandblaster.com/index.html). I’ve included information on it in the
optional parts list. It does tend to
wear out gun nozzles faster than normal, but the advantages make it worthwhile,
in my opinion. To handle the extra
wear, I’ve included some alumina tube in the optional parts list that is
exactly the same ID and OD as the nozzles for my blasting gun. I cut the tube to length with a carbide hack
saw blade and make my own nozzles for about $0.75 each instead of $12
retail. McMaster-Carr (http://www.mcmaster.com) has a good range
of alumina tube sizes so if your gun has a different nozzle diameter you may
still be able to save some money using this method.
Another option to strongly consider is an oil/water removal
unit right at the air supply entry to the cabinet. I have one on my compressor but the air is still warm there and
it holds quite a bit of water vapor. By
the time the air reaches the cabinet it’s cooled down and an oil/water removal
unit takes a lot of water out so it doesn’t go on your parts. I’ve included a reasonable unit in the
optional parts list.
The most important issue when using the cabinet is
safety. ALWAYS use a particle mask to
avoid breathing any residual dust that might leak from the cabinet and ALWAYS
wear safety glasses in case the viewing window gets broken while you have
you’re peering through it.
That pretty much covers the procedure for building the large
grit blast box. I hope yours works as
well as mine has. Remember, if you
think of a better idea or don’t like something about the design, change
it! This is your grit blaster and you
have to use it. That’s what building
your own tools is all about.
