Tuesday, February 28, 2012

Final Details Before Paint

I spent some time tonight and wrapped up all the last little details before the frame is sent for clear coat etc.
The first thing I did was drill and slot the back of the seat tube.  The slot may be a little short but it clamps well.

I then drilled holes for the down tube cable stops followed by bonding with DP 420 and two rivets each.

Finally I carefully drilled a hole right through the frame behind the BB to guide the front derailleur cable and seal the frame.  Next time I'll get some titanium tubing to avoid possible corrosion issues.  I'll trim the tube flush once the glue sets.

A final light sanding all over with 600grit, a cleaning with alcohol and it's ready for the painters.

Saturday, February 25, 2012

Finished Sanding

I finished sanding the joints on Chuck's frame and I'm pretty happy with the results.  I also started to layout and size the paint mask files on the frame.  Here is there frame with the mock up mask applied.
Next is the head tube badge mask.
I wet down the sanded head tube joints with water to judge how they'll look under clear coat.
With the larger fillets I think the joints came out looking good.
The next step is to install the derailleur cable guides under the BB and under the down tube.  After that I'll bond in a seat post adapter sleeve so a 27.2mm post will slide in perfectly, then it's ready for paint.

Wednesday, February 22, 2012

Bagged Baked and Sanded

The first step after the joints are wrapped in prepreg is to cover each joint in release cloth.  In this case I use a very fine woven coated nylon that lets excess resin through during cure.

Once all the joints are covered they get a layer of bleeder/breather material to distribute the vacuum across the joints and absorb excess epoxy.  Once that's in place the frame is slipped in a vacuum bag.  I use Stretchlon 800 which is a 500% stretchable material that can withstand cure at 250F.  You can sort of see how I form a "Y" shaped bag to slip the rear stays into.  I actually managed to bag and pull a vacuum with no leaks which is a border line miracle!

Here is Chuck's frame hanging in the oven post cure.  You can see the epoxy saturated cloth at the joints.  I added a support bracket and bearing to the oven fan so it didn't try to self destruct this time.
After the oven the frame gets unwrapped which sounds a lot easier than it is.  It requires pliers, a knife and a lot of work.  I ended up with a pretty good looking joint with only a few creases and ribs where the bag halves meet.

Some quick work with a file results in this.  I just take off the little ridges and high spots.  You don't want to be removing carbon.

Post cure weight is about 26g less than before the oven.  That means quite a bit of epoxy was removed.  The prepreg I use is 45% resin to start.  The cure process brings that down significantly.
Here is the BB joint after it was filed then coated in a thin layer of epoxy, sanded and re-coated.  Once this is sanded the joint should be nice and smooth and ready for paint.

Sunday, February 19, 2012

Joint Wrapping

I finished the fillets on Chuck's frame and prepared it for carbon joint wrapping.  The tubes near the joints are sanded with 80grit to roughen the surface for bonding then wiped down with a lot of isopropyl alcohol.
Here are the head tube and seat tube junction fillets.  I made them larger than on previous frames for smoother tube junctions.

I started the joints with 300g/m^2 uni directional carbon.  I use long pieces that wrap around the tubes from multiple angles.

New on this frame is the 2X2 Twill weave carbon.  It draped well over the joints and will add significant strength.

Here is the finished head tube joint.  The final layer is unidirectional long wrapping pieces with a  few cosmetic bits to fill gaps.
This is the first piece I add for the BB joint.  It has a lot of contact with the tubes spanning the whole joint.

I finish the first layer with some diagonal fibers for bending and torsional strength.

After a few layers of 2X2 Twill I finished the joint with some more unidirectional.
I used significantly more carbon forming these joint than in the past.  This will make for a nice strong frame with rigid tube intersections.  It added a little more weight but some of that will come off  when the excess epoxy is pulled out of the joints under vacuum. Still working though the "cosmetics" with each build.  This should be the nicest yet.  We'll see when it comes out of the oven in the next few days.
Coming up next, the frame gets vacuum bagged...

Friday, February 17, 2012

Weight, Alignment and Fillets

Chuck's frame cured over night and was ready to take out of the jig in the morning.  It is starting to look like a real frame.
Glued together prior to adding the fillets, joint wrapping etc it's quite light.  Even with a titanium BB sleeve and internal top tube cable guide it came in at 880g.
I checked the frame alignment and how the rear wheel sits in the dropouts.  Everything looks good.

I added the fillets to the frame joints using a mix of DP 420 and glass micro spheres.  This forms a thick paste that's easy to spread, stays put and makes a light sand-able but rigid fillet.
Next step is to sand the joints smooth then prepare to wrap them in carbon.

Gluing Chuck's Frame

Below is a series of photos showing how I glued Chuck's frame together.  I use 3M DP 420 epoxy for all the joints.  First is two photos showing the bonding of a titanium BB shell into a carbon BB tube.  I made sure that there was always glue being pushed at the leading and trailing edge when inserted.  I managed not to get any in the threads which was a miracle.

The next two photos show the amount of glue used on the seat tube joint.  I's a small bead on the end of the tube that, when placed in the jig forms a small fillet and seals the joint completely.

When all the joints are set and the dropouts bonded in it's time to let it cure overnight. I used flash breaker tape on the seat and chain stays as well as the dropouts this time to make clean up much easier post cure.

Tuesday, February 14, 2012

Finished Guide and New Carbon

Today I finished the titanium brake cable guide tube flush with the top tube.  Careful grinding to avoid heat build up then finish filing and sanding left me with this.
Which looks like this with some casing installed.
I also received a new batch of carbon fiber prepreg.  It's all being stored in a small chest freezer at -20C.
Here is the 2x2 twill weave prepreg I'll be using some of to help bulk up the joints rather than using all uni-directional.
I did a very quick drapability test on an old test joint.  The 2x2 twill conforms well to the tough contours at the BB so it should not be any problem using this in the other joints.
 Here is some of the equipment being used.  There is a late 1970's First Mill, a Standard Modern 11" Lathe, a Garvin Horizontal Mill from 1910(ish) which needs some work and a King 7" bandsaw.

Monday, February 13, 2012

Internal Top Tube Cable Guide

I decided to try making the next few frames with internal cable guides for the rear brake.  I decided to use some titanium tubing to guide the brake casing the entire length for ease of insertion etc.  I chose titanium because it's light and has a similar coefficient of thermal expansion to carbon composite.

Here is the rather cobbled fixture I used to cut the 20deg entrance holes.  That's a carbide burr to cut through the tubing cleanly (and not get dull).  Once I have my little Garvin horizontal mill set up this will be much easier and less nerve-wracking.
I ended up bending the 1/4" titanium tubing around my 6" rotary table diameter by hand.  A parallel filled the gap nicely and I bent the tube with some cable casing inside.  There was no collapsing of the walls at this bend radius.
It's hard to see but the tube fit quite nicely down the center.  Next time I'll cut the holes 1/2" closer to the ends of the tubes so I can actually reach in with my fingers.  Fishing it through was interesting but running casing through helped guide the tube into place.
When ground and sanded flush the casing should exit at a good angle for the rear brake.
The tube was quite snug as is but a little 3M DP 420 on the outside and some on the inside should hold everything in place. 
I'll let this dry, finish it smooth with the OD of the top tube then everything is ready to bond in the jig.