"The Cutting Edge on DTF"

The Cutting Edge!

In my last Article. “The Ruptured Duck” I was posed a question that I really hadn't given much thought to...... until now! Noobi 1951 asked if I used a laser to cut my parts. I had always thought they were really kind of bad myself, and I really wasn't able to give him much of a response at the time. To be honest, it really didn't seem to be much of an issue to me until I began cutting parts for my next design. Then it came to me.... I have a specific way to cut each part, and a specific tool for each cut. I also have limited space on my work bench so I try to keep as few tools on it as possible. That's what prompted me to write this article, and I hope it helps those of you who are new to the hobby, and even some of you vets of the hobby that are just looking for better way to cut more accurate parts from DTF.

Before I get started though, let me say something about safety!

Always be sure to follow the general rules when working with any type of knife, blade, or cutting tool. Always cut away from you, and always replace safety guards or retract the blades when laying the knife down. Never leave blades exposed on a work surface, and dispose of used blades properly!

If you think ahead a little bit, and have a little patience. You can cut smooth accurate parts with just a hand full of inexpensive tools!

Choosing the right tools for any job is something we all have heard and know quite well, but is that all we really need to think about before starting a job? No... Far from it! Think about it this way. If you are going to build a porch. Do you just grab some wood, a saw, a hammer and some nails and start building? Absolutely not! You need a plan, the right materials and the right tools and the skills to use them properly! You wouldn't choose a 5 Lb sledge hammer to drive the nails, or a hack saw to cut the planks now would you?  I know you're probably thinking how hard can it be to cut some foam board, but still struggle with making descent cuts, to some it doesn't really matter, but to others it does, and how many people want to spend a bunch of money on the latest greatest cutting tool from ACME like Wile E. Coyote with the spring loaded blade and GPS / AM FM receiver and God knows what else to save money on building with DTF.

Like this!

I purchased this Ultra steel 16 piece Precision knife set...... (Not from ACME)...... to do my modeling  about 5 years ago. I wanted a good knife set to work with and thought it was worth it at the time. I can't even remember how much I paid for it. I'm just glad I didn't have to pawn my car title for it. LOL I've designed and built seven aircraft over the past year, and not once did I even use it!

This is the most expensive knife I have used for all the designs I've created and posted here on FT in the past year!

 It's the Fiskars 627381 finger tip control craft knife available from Wal Mart.com for $11.82. I highly recommend this tool as it works very well for cutting intricate curves, slots, and tabs found on most DTF deigns. It uses the standard #11 Exacto blade shown below. 

This is the only other knife handle I use, and the least expensive one. It's the standard utility knife from Dollar Tree. Along with the three blade replacement pack. $1.00 each. This tool is extremely handy for cutting longer lines, Trimming the template sheets, and the adjustable blade helps when the depth of the cut matters!

Standard utility blades that I never even use a handle for. 5 pack from Ace hardware for $2.00. these I usually only use when trimming bevels for leading edges and hinge lines. They last for ever that way!

An average pair of Scissors and a steel ruler. 

Sanding blocks from Wal Mart 150 and 220 Grit $3.50 each. Optional

Your choice of adhesives and or tape.

And that concludes my long list of tools and supplies I use to create my designs, but like I said. It's not just a matter of the right tools.

Think ahead a little bit and use the right tools to get the job done, but don't forget that the right tools alone won't guarantee accurate parts. In order to cut accurate parts you need to have accurate templates,  and be able to transfer them to the foam board without loosing something in the transfer. This has been one of the biggest problems for most modelers. I think we've all tried just about everything imaginable short of witchcraft to transfer them. Even beyond that, the use of proper cutting techniques will help you cut better parts. I'm going to share what I have found to be workable for me, and I hope that it will help at least some of you to have a more enjoyable experience in building and flying model aircraft made from DTF. 

 Working with templates!

To create accurate parts you must have accurate templates. Depending on the templates you have access to. This can be very difficult. The grid system I have developed for my designs allows me to line up the grids accurately. Each sheet has a dashed line around the perimeter with nodes at each corner as shown in the picture below

Using a straight edge and a utility knife I remove the section that would overlap the adjoining grid sheet by lining the straight edge up between the nodes at the corners and trmming. This now becomes the edge of your template sheet.

This allows you to use glue rather than tape to join the grid sheets by overlapping the sheets rather than butt joining them. 

Remove the edge of only one grid sheet!

Simply add glue to the adjoining sheet outside the perimeter along the edge to be glued.

Align the nodes at the corners and keep the edge of the sheet as close to the perimeter line as possible.

This allows the most accurate templates possible!

Multiple sheets can be connected to create accurate templates of any size. 

Depending on the templates you have in hand you will need to find the best way to attach them in the most accurate way possible.

The Dreaded Transfer!

This has been the most difficult  portion of any build, even before DTF was found to be a suitable material for RC aircraft. There are many methods that I'm sure we have all tried, but most seem to be very inaccurate, and we end up with parts that don't quite fit right, and usually have to be re trimmed, or even discarded and cut new ones. I'm going to share the method that seems to work best for me, however it requires the destruction of the templates themselves. If you are working from Tiled PDF plans that you printed of your home printer then this is not much of a problem. You can always print more. Paper and ink are relatively cheap  compared to the savings of using DTF as a building material. As a designer I go through a lot more paper and ink on a single design than you will after building about ten of these. 

I start by cutting the templates from the sheet leaving from 1/6” to as much a 1/4” perimeter of paper around the template as shown below.

Note! : Do not cut directly on the template line itself!

Also note that the center section on F-2A and B in the photo above is not part of the actual template, but doesn't really need to be cut from the template at this time. This makes it easier to work with the templates at this stage. 

I then use Loctite Spray Adhesive to affix the templates directly to the foam board. It dries permanently, but the actual cure time is over 24 Hrs and the template can be removed, and or repositioned as many times as you like without losing it's tackiness. Don't over do it though! A very light misting of the board holding the can about 12” away is plenty for the job. This allows you to cut directly on the template lines and preserves the accuracy of your cuts.  Most don't approve of this method due to the sticky residue left behind, but there are ways do deal with this also. For the most part the surface that has been sprayed will be on the inside of the aircraft and won't even pose a problem. 

 A little Prep Work

 I have found the FT method of using Minwax to waterproof the aircraft can actually be used to resolve this. When I start a build. I start by pre coating several boards of DTF with Minwax on one side only. Note: only coat as many sheets as you plan to use in about a 72 Hr period. After this amount of drying time the minwax becomes brittle and folding or bending the board becomes difficult. After about 30 minutes of drying time I sand the surface  with a 220 or higher grit sanding block to create a smooth paintable surface. I then turn it over and lightly dust the other side with Loctite Adhesive, and affix the templates I need to cut as seen below. Once you have decided on the correct position of the templates press them down firmly to prevent them from moving when you make your cuts. 

Note: In this shot, the only template that requires both sides to be exposed after the aircraft is complete is the vertical fin. This can simply be treated with Minwax after cutting as shown in a step later in this article.

Plan your cuts and use the right tools!

 For the sake of simplicity I've chosen to only demonstrate a couple of pieces for you, but planning your cuts are as important as  any other aspect of building the aircraft. Start by cutting the board into manageable sized pieces. As you can see from the previous photo there is plenty of space between each template to dissect the sheet into more manageable pieces. This is done with the utility knife! 

Let's start with this former. As you can see there are several curves and some strait lines as well. Most about 1” long more or less. The two longest being on the sides, so that's where I'll start. 

Using the utility knife and a strait edge I'll trim the sides since they are the longest. Do not use your  Fiskars, or hobby knife for cuts like this. Cutting longer lines against a steel edge dulls the blades much faster. 

With the sides removed you now have the curves to cut.

Now using the  Fiskars knife,or hobby knife. Gently trace the curve only cutting through the template and scoring the first layer of paper on the board. Do not try to cut all the  way through in one cut! 

As you can see only the first layer is cut. I chose to remove this portion of the template as it can sometimes hang up on the blade during the rest of the cut. This tends to drag the blade off course and results in ragged cuts.

Now holding the blade at a 90 degree angle to the surface of the board make several cuts on the same line until the scrap piece separates from the part. Continue this process for the rest of the curved portion of the part.

Curved surfaces need the extra attention given when cutting. Patience is the key!

Remove the bulk of the portion in the center by cutting along the lines at the end of the vertical, and horizontal tabs untill your cuts intersect. These lines do not have to be accurate but the length of the cut requires a straight edge. Note: I never cut lines longer than 1” without a strait edge.

Once this section is removed you can then use the  Fiskars, or obby knife and basically free hand the cuts between the tabs as they are only about an inch long. As shown below

Accurate cuts can be made on lines 1“ or less without the aid of a strait edge. Just continue to make several passes for each cut. Do not cut all the way through on the first pass. 

Once the tabs are trimmed. The upper portion F-2A in this case can be separated from F-2B

Once again switching to the utility Knife and using the strait edge. These lines do need to be accurate because these pieces need to butt against each other during assembly of the aircraft. One of the reasons for thinking ahead, plus the two lines are separated by a portion of the part that has already been removed. This type of cut should always be done with a strait edge.

The piece is now complete! You may even chose to lightly sand the part before removing the template for even more accuracy. Removing the template is not necessary at this point unless your parts will be sitting for extended periods of time before building. Remember... Loctite Spray Adhesive is permanent after it cures long enough.

Now let's take a look at another piece. A vertical fin. To demonstrate inside curves, bevels and intricate curves.

As before it's best to cut all the strait lines around the perimeter first and this is done with the utility knife. 

After the perimeter has been cut away. Note: the portion forward of the leading edge and above the streak cannot be removed yet due to the curve that blends the two together. The strait cuts have been made far enough to meet the curve. 

Use the  Fiskars, or hobby knife as you did before connecting the two strait cuts first cutting through the template and then cutting through the rest of the sheet at a 90 degree angle to the surface. Making several passes. 

Notice the line left on the fin at the curve itself! This is intentional to allow for some sanding.

The arc at the top of the fin is gentle and can be cut freehand  using the method prescribed earlier.

Cutting in intricate areas like this relief arc for the elevator.  Always cut the arc first! Since you are cutting an arc there is a chance the blade will drag. 

Cutting the strait portion after saves a messy relief cut.

This one could still use some sanding.

Some curves are just too small to use a knife. Just sand it!

This one's ready for the hinge to be cut! Notice I provide a cut line aft of the hinge line. This will provide an accurate hinge line using the paper on the board as a guide for the bevel cut.

I start by drawing the blade in the utility knife back to the last notch leaving about 1/4” of the blade exposed. 

Holding the knife at about a 30 degree angle to the surface I make my cut that creates the hinge line.

I then break the hinge free, and prepare to make my second cut on the other line. Note: this line is 3/16” from the hinge line. 

Now holding the blade at a steeper angle I make my second cut on the bevel line. The blade only cuts through the first layer of paper on the DTF.

The paper strip is then removed providing a strait line for a perfect 45 degree angle cut.

This is the only time I use the utility knife blades , and I don't recommend using them in this fashion! I only do this because it gives me better control. 

The results are pretty impressive.

Sanding is another option for this, or a combination of both. This is especially good for the double bevel at the leading edge of the wings. An inaccurate cut there can lead to inaccurate, or uneven curvature of the camber.

The template can now be removed, and as I said earlier it simply peals right off!

Now that the template is removed. Using the same method you used to apply the Minwax to the whole sheet. Apply it to the untreated side of the fin.

When this dries. You can sand it as you did the other side, and it's ready to be assembled. 

 Note: If you don't plan to paint your aircraft but still want to eliminate the tacky residue left by the Loctite. Simply dust the surface with baby powder and lightly sand. 

Building aircraft from DTF is fun, inexpensve, and challeging, and with a little bit of forethought, patience, and some simple tools. You can produce models you can be proud of!

 I certainly hope this article has been helpful to some of you, and maybe even an inspiration to others to share some of your ideas and techniques.  No matter how trivial they may seem to you. They may just be the diamond someone else is looking for!

 Visit my wepsite at: Flyin Foam RC

Visit my FB page at: Fly'n Foam RC on FB