Hobby Machinist Article ...by Dale Pynenberg
Learn how to make your own with hobby grade machining equipment...
The
T-Bar is a relatively new product used to hold your stuffing tube in place with
out having to fiber glass it in. Another
benefit is the fact that you can adjust the drive shaft from side to side by
simply loosening a set screw. This way you can adjust the stuffing tube to the
exact place you need it.
There
are no set dimensions for a T-Bar to work, but I will include some measurements
that you can use as a starting point for building your own. It would be very easy
to modify this design to make a cool looking custom mount.
First
I started off with a piece of 1/2" by 3/8" bar stock aluminum for the top piece
which is the exact size that I want for the finished product. As for the bottom piece, I had to mill a half
inch off from one side to get the size that I wanted. I make the bottom piece 1/2" by 5/8" and 1
1/2" tall.
I
don't have a band saw yet so I cut the top and bottom pieces out using the
mill. The cutter I used is a roughing
mill and works great for removing large amounts of material in a hurry. It leaves a pretty rough finish though, so I
cut the pieces a little long and cleaned them up later.
Make
sure to de burr your edges every time you make a cut. A few passes with a file can make a big
difference in accuracy when clamping parts in the vice.
After
the pieces were cut with the roughing mill, I switched over to a four flute end
mill and cleaned up the ends. This is
the time to bring them down to the final dimensions. As you can see I'm making four parts at a
time and it's much faster than doing them separately.
The
top piece needs to be 5" long to fit inside standard engine rails. I make the bottom piece 1 1/2" tall for my
boats, but you may need to make this piece a little shorter if you have a model
with really low engine rails.
Next
I cut the slot in the bottom piece. Once
again I'm cutting four at a time for efficiency. The top part is 3/8" wide so I cut the slot a
few thousands under 3/8" of an inch. I
like to keep mating parts a little tight so I have some material left to remove
while sanding out the machining marks.
This way the finished parts will go together with a very snug fit.
I'm
looking for a finished depth of 1/2" so the top piece fits in flush with the
slot. The easiest way to do this is to
set up a 1" indicator on the mill head so you can see exactly how much material
your removing. The dials on the mill
aren't accurate enough to trust, and this saves you the time of measuring the
part over and over again while milling it.
Just bring the end mill down until it starts to leave a mark in the work
surface. Once it does, zero out the
indicator and start milling. I always
assume that at least one thousand of an inch was removed when the bit first
touches the metal. So I mill the part
one or two thousands under the desired measurement and check it for fit. A cheap caliper can double as a depth gauge
and takes pretty accurate reading.
Here
are the parts fitted together after machining.
After
the slot us cut in, I flipped the part around and drilled a 5/16" hole in to it
for the stuffing tube to go threw.
(Issue #14 shows you how to drill a hole in the exact center of your
work.) I left a 1/4" of material between
the hole and the bottom of the part.
This is to leave room for a smaller hole that needs to be drilled on the
other side for a #6 bolt. This bolt will
be used to clamp the stuffing tube into place.
In
order for the clamping action to work I needed to make a small slot so the bolt
could be tightened and compress the metal around the stuffing tube. I used a 1/16" end mill to cut the slot.
The
other side of the part gets drilled for a #6 bolt too, but it's drilled under
sized so it can be tapped. This bolt
will hold the two parts together and allow for adjustment.
After
the hole is drilled I kept the part in the vise and used the mill the help tap
the part. The end mill keeps the tap
handle square to the hole and helps prevent the tap from breaking. As the tap enters the work, I lower the mill
head to keep the end mill inside the tap handle. A live or dead center would have been the
best tool to use here, but I don't have one that fits my mill. Now the bottom part is done and I can finish
the top piece.
I
have to make a slot in the top part for the #6 bolt to go threw. I centered the work and scribed some starting
and ending points on the metal. I made
the slot around 1/2" wide in the center of the piece.
You
don't have to do this next step, but I think it looks a lot nicer if you do. I
mill the slot so that the head of the bolt is flush with the top of the
part. Since the part is already centered
in the vice, all you have to do is switch to a larger end mill and boar it
out. Measure the height of the bolt and
cut the slot down to that depth. Once
again I put the digital indicator on the mill head to see exactly how deep I
have gone.
As
you can see the bolt is perfectly flush with the top of the bar.
After
that was done, I scribed some marks on the part to indicate the areas that I
could remove to shave down weight. I
left a 1/2" at the ends and a 1 1/4" space in the middle. The rest was removed with a 1/4" end mill. A shop vac works great to prevent the chips
from pilling up inside the deep cut.
This way I can see the bit better, and it prevents the cutter from
regrinding the chips over and over again.
The
last step is to drill and tap the ends of the T-Bar. These holes are used to mount it to your
engine rails. I use #8 screws, but there
is enough room for larger ones if you want.
The easiest way to drill and tap the exact center of the part is with a
lathe. I centered the part with a
independently adjustable four jaw chuck and a little help from my digital
indicator.
I'm
starting to tap the hole in this picture.
You have to keep the tail stock loose so it can move freely across the
bed of the lathe. You can tap the part
by turning the chuck back and forth by hand, or run the lathe at a very slow
speed and use the forward / reverse switch.
I usually power the lathe up and have yet to break a tap. You could also use a tap handle to hold the
tap, and live center in the tail stock to keep it aligned with the hole.
The
only thing you have left to do is sand off the milling marks and put it
together. Make sure to use a lock nut on
the bottom bolt and some lock tight on the other bolts so they don't vibrate
loose.
Well
there you have it, now let's see some of your designs!
Dale Pynenberg
