Scarfing 1mm birch plywood

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Tom Busey

Member
Joined
Aug 28, 2019
Messages
9
I'm planning how I might cover the horizontal stabilator of my Gazaile build. The plans call for 1.2mm okoume plywood, although 1mm birch can be substituted. However, the birch is only sold in 4'x4' or 5'x'5 pieces, and I need a full 8' piece to cover the stabilator. Is it structurally safe to scarf two 4'x4' pieces of 1mm birch together, with maybe a reinforcing strip on the interior? How are thin pieces of plywood joined end-to-end?
 

Rljj

Well-Known Member
Joined
Nov 7, 2021
Messages
49
I am looking to build a celerity and it calls for 1mm ply in places and their recommendations for scafing is 16 to 1
 

Rljj

Well-Known Member
Joined
Nov 7, 2021
Messages
49
I am also looking for 4 x 8 1mm birch ply in Ontario
 

TiPi

Well-Known Member
HBA Supporter
Log Member
Joined
Aug 25, 2019
Messages
497
Location
Peeramon (AUS)
Scarfing ply is a normal practice, usually at a slope ratio between 12:1 and 15:1. 1mm ply is very easy to scarf:
  • make a scarfing tool for your desired slope ratio. I used 15:1 for my 0.8, 1.0, 1.2 and 1.5mm ply
  • put the edge to be scarfed along a straight and solid bench edge
  • use a straight section of timber or alu rail and clamp it to the bench, set-back the correct amount for your scarfing tool
  • run the scarfing tool along the rail until the ply is sanded down to the edge of the bench
I use a Unirac solar mount rail (any other extruded profile with some slots at the right height will work as well) and a sanding block to give me the 15:1 scarf slope. As you sand the edge, the changing colours of the different plies give you a very clear guide on where you need to sand a bit more and where you have reached the far edge. Use a good quality 40 or 60 grain sanding paper (I use Wurth) and you get your scarf very quickly. Don't forget to check that you put your scarf on the "right" side of the ply :)
1641699524638.jpeg
 

Tom Busey

Member
Joined
Aug 28, 2019
Messages
9
This incredibly helpful. Thanks. Do you just epoxy the seam? Any fabric reinforcements?
 

TiPi

Well-Known Member
HBA Supporter
Log Member
Joined
Aug 25, 2019
Messages
497
Location
Peeramon (AUS)
The SD-1 Minisport that I'm building has a G40 or G80 glassfibre cover over the ply to protect it a bit and add a bit more strength. The scarf joint is just epoxied. On the fuselage, I have rounded some joints that were on a bulkhead and then added a strip of G80 glass to make up for the removed wood.
1641731996268.png
 

Gsport

Active Member
HBA Supporter
Joined
Mar 7, 2013
Messages
40
Location
Colorado Springs, CO
Had to cut over 50' of scarfs for the wing skins alone (.0625"; 1.5mm), so I made a power tool set at 12:1 to rough it out, then refined it by hand with a setup like TiPi. Steel brush the joints to remove dust before gluing. Let the glue (T-88 in my case) soak in for 15 min and apply more if necessary before gluing together.

Do all the scarfs in the 'rough' shape of your piece. When dry, scrape/plane the joints with a heavy piece of steel with a sharp edge (sanding will likely lead to undercuts), then cut to final shape. No additional reinforcing used.

.Scarfing Tool.JPG Scarfing.jpg Scarf.JPG
 

Vic Bottomly

Member
HBA Supporter
Joined
Nov 24, 2020
Messages
22
Location
Clarkston WA
Yes, scarfing is straightforward. The only thing I’d add is that the joint is stiffer than the surrounding plywood. On curved surfaces I like to bend so the joint is in line with the curve. Can’t always do that, but if you can it acts similar to a batten to make a fair curve.
 

Jsample40

Well-Known Member
Joined
Jan 18, 2020
Messages
83
Location
Western North Carolina
In the mid 1960's while on the North American Rockwell Apollo launch team I was building a 38' Trimaran... Needed to scarf the marine plywood for the sides, top deck, and bottom full length. Designed a similar setup to the roller sander at appropriate angle but used a router instead. Could adjust the height of the rotating cutting blade and thus vary the speed of the cut. Used Genepoxy 190 for base and Versamid 149 catalyst for all joints in the craft.
Jay Sample
 

n45bm

Well-Known Member
Joined
Feb 20, 2019
Messages
149
Location
Seguin
I'm planning how I might cover the horizontal stabilator of my Gazaile build. The plans call for 1.2mm okoume plywood, although 1mm birch can be substituted. However, the birch is only sold in 4'x4' or 5'x'5 pieces, and I need a full 8' piece to cover the stabilator. Is it structurally safe to scarf two 4'x4' pieces of 1mm birch together, with maybe a reinforcing strip on the interior? How are thin pieces of plywood joined end-to-end?
Veeeery, veeeery carefully.
 

sming

Well-Known Member
Joined
Apr 10, 2019
Messages
222
I'm pretty sure the plans say you can butt joint with a small backing strip. At least for the fuselage, iirc.
 

Geraldc

Well-Known Member
Joined
Nov 12, 2011
Messages
736
Location
nz
I'm planning how I might cover the horizontal stabilator of my Gazaile build. The plans call for 1.2mm okoume plywood, although 1mm birch can be substituted. However, the birch is only sold in 4'x4' or 5'x'5 pieces, and I need a full 8' piece to cover the stabilator. Is it structurally safe to scarf two 4'x4' pieces of 1mm birch together, with maybe a reinforcing strip on the interior? How are thin pieces of plywood joined end-to-end?
There is also the option in the plans to only cover the front and ribs and cover with fabric like the Alpi which would leave a small scarf at the front middle .
1642054725540.png
 

cluttonfred

Well-Known Member
HBA Supporter
Joined
Feb 13, 2010
Messages
8,806
Location
World traveler
FWIW, I have done epoxy butt joints in 1/4" or 6mm Okoume marine plywood reinforced with a strip of fiberglass tape on each side in boatbuilding and in testing the joint was stronger than the surrounding wood. Note that I mean an epoxy butt joint...the two sheets joined at the edges by the epoxy as well as reinforced by the fiberglass strips over the joint, say a 25mm strip cut from 4 oz fabric on each side. I would test the technique first with your own materials before using it in anything structural, but I would be very comfortable with that solution. If that wasn't strong enough in testing, then I'd add a 50mm strip over the 25 mm strip, but I doubt that would be necessary with plywood this thin. In this particular application, if you can locate the joint over a rib so that each piece is epoxied to the rib, then fiberglass on just the outer surface would do the trick, the inner surface would already be reinforced by the rib.
 
Top