Fillet Strategies for Inside Corners on Sand Casting (& other) Patterns

Discussion in 'Pattern making' started by Melterskelter, Mar 21, 2018.

  1. Melterskelter

    Melterskelter Silver Banner Member

    I am reworking a wood pattern for greensand with 62 inside corners—-where three planes meet at right angles. Getting any material I have tried so far into those corners with a nice smooth finish has been challenging. I have tried Bondo, POP, latex caulk (too soft an no easier), and Famowood. (Wax from Freeman is on the way but not here yet). I will soon be making a scaled up version of the current pattern. So, getting the inside corner problem figured out is important to me. Bondo has worked the best for me so far. Up til now I have ended up tediously sanding those corners with 220 waterproof paper to eliminate any sand-grabbing irregularities.

    However, tonight I tried applying the stock Bondo with my finger as smoothly as possible, but then doing a finishing wipe with my finger wetted with 70% isopropyl alcohol. That made a huge difference. (I also tried acetone but I think the alcohol worked better for me.) No more micro-ridges of Bondo and a nice smooth fillet that will not require sanding other than just a light hit with 320 prior to lacquering.

    Do other people have similar/better tips and materials for inside corners? Straight inside corners at the intersection of two planes seem to be relatively easy with almost any fill material.
    Last edited: Mar 21, 2018
  2. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    I happed to already have a response for that one :)

    Yes, the corners are tricky. Bondo is my material of choice. The problem is you are always dragging the ball the wrong way for one of the intersecting fillets. Based upon your last sentence, not sure if you meant to post this on the forum or in this message but here would be approach:
    1. You had previously mentioned you build your own fillet tools by soldering a ball on a stick. Make sure the ball is polished to a high luster.
    2. The larger the fillet the more it exacerbates the problems in the corners. So sometimes building the fillet up in two layers, at least in the corners is a good strategy so the last coat is thin.
    3. Here's the technique that occasionally gets acceptable results the first time for me. At worst it makes the second attempt just touch up
    4. First remove the excess bondo shaping the fillet away from the corner on each axis leaving a only a small amount in the corner, making sure you remove all excess from the ball/fillet tool with a paper towel after each stroke.
    5. Dip the ball in acetone and do the corner as you rotate the ball by spinning the shaft between your fingers. I usually make top of the ball spin the opposite direction of travel. You can dip in acetone repeatedly and as needed.
    6. The acetone keeps the bondo from wetting and adhering to the tool and also thins the bondo just a bit so it lays down well and the rotation also helps it lay down on the trailing side as well.
    7. You mention using alcohol and it may do the same thing but acetone actually reduces the polyester, is very thin with little surface tension, so you can just dip and don't even have to shake it off. It also minimally inhibits cure and of course evaporates very quickly.
    8. I don't remove the excess bondo outside the wake of the ball fillet tool until it just starts to go plastic and is rubbery. Then I use the scraper tool to remove it.
    9. Just after that when the bondo has cured beyond rubbery and is semi hard, I wipe the joint with an acetone soaked cloth and this further smooths the fillet and removes any residual bondo not previously taken by the scraper. This will also work on small amounts of bondo after full cure since the acetone reduces the bondo.
    10. You need to catalyze the bondo and work in batch sizes that allows time for all the above. I use small batches and mix it hot because I'm an impatient sort.

    Wax is a different ball game. Working wax fillet is all about controlling the temperature of the wax and the tool to keep the wax plastic and just below the melt temperature. At that happy temp it will be workable and stick to your piece but not puddle and run away from your work and making a mess.
    1. Preheating the wax to about 100F makes a huge difference in how pliable workable it is. You can do this a piece at a time in the stream of a hair drier or heat gun before you apply it. If you have a lot of fillet, laying it on a metal plate sitting on a heating pad works well. If it's Summer time laying a few pieces in a cardboard box in the sun works too. You can reheat a strip already placed with air stream or by rapidly moving a heated fillet tool across it.
    2. I have my heat gun running continuously and heat my fillet tool with it. Too hot and it will melt the wax and make it puddle into a mess. Keeping the ball moving quickly back and forth across the wax fillet can prevent this, and will take heat from the tool and put it into the fillet you are working.
    3. You have achieved the right temperature where you can work the wax plastically without melting it. At that temp you will also leave almost no excess wax to scrape.
    4. Frequently returning the ball for short periods of time to the hot air stream as you are working the fillet helps keep you in the happy zone. Wiping a hot a tool back and forth across a fillet area will take heat from an overheated tool so it doesn't locally puddle wax.
    5. Corners are easier with wax but spinning tool as you hit the corner can help, especially if the tool is a on the high side of the happy temperature.

    -Takes a little patience and practice but as you've noticed, a little technique and experience can save a lot of time and get superior results.

    Melterskelter likes this.
  3. Rocketman

    Rocketman Copper

    Freeman sells leather fillet - it's applied with glue and a round ball fillet too. For rounding tight corners with it you use an exacto knife to make small incisions in the inside or outside edges of the leather to allow it to conform properly.
    It works exceedingly well - it was used extensively in traditional pattern making. Once you get the hang of it it lays down very quickly and makes a very consistent, even fillet. Can purchase it in many sizes, too

    The wax fillet works well for low volume patterns, we used to use it extensively on valve patterns where the flanges were changepieces for different weight flanges. The wax would allow us to change the flanges & re-fillet easily
    Melterskelter likes this.
  4. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Leather fillets are nice and remarkably durable but a little trickier if you are trying to get pretty 3-axis corners that Melterskelter mentions. Pretty tough to find in smaller quantities if you don't have a lot of feet to lay or need multiple sizes. Sadly, the traditional pattern making skills are quickly becoming lost to days gone by.

  5. Melterskelter

    Melterskelter Silver Banner Member

    If a person uses leather for an inside 3-axis corner, is the fillet made by mitering the leather strips much like joining quarter-round in a wood-lined closet or box?
  6. Melterskelter

    Melterskelter Silver Banner Member

    BTW, I do not use only sand paper to contour the inside corners after they have been Bondo’d. This little scraping tool shown in the still thumbnail is approximately .030” thick and removes material considerably faster andwith more precise localization than abrasive paper. Abrasive paper I use as a final shaping and smoothing method.

    The scraper is basically a card scraper as used in woodworking and is sharpened as they are so that it peels off a fine shaving of wood or Bondo. For those unfamiliar with card scrapers and their sharpening here is a link:

    Attached Files:

  7. Like Melterskelter I use scrapers, but made from hacksaw blades ground to specific contours to apply the bog/bondo like a palette knife and then to scrape them afterwards when cured. The edges have to be a sharp 90 degrees even for contouring the bog or it will roll over the blade instead of going on smoothly.

    Melterskelter likes this.
  8. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    For kicks I searched leather fillet and to my surprise this popped it by the foot. Never saw it available by the foot and never bought from them but thought I'd post it here cuz it could be handy for small users. There were a couple other suppliers that popped up too but they looked more like wholesalers.

    I did a quick YouTube and eBay search too but they're too obscured by examples of making leather sheathes for fillet knives. Remember the leather fillet is flat in it's natural uninstalled state. There are at least three ways to handle that 3 axis intersection with leather fillet. The first time I asked I nearly started a fist fight between two Journeyman pattern makers LoL. There were also dedicated one-stroke tools for making the cut and planes for shaving your own fillet stock.

  9. ESC

    ESC Copper Banner Member

    On my small patterns I use the super glue/ baking soda method. It ends up a little coarse, so sanding is needed, but it is very quick and if care is taken when the inside corner is formed with the soda there is no additional build up. I use a Dremel carbide tool to clean up and then sand. An example is the pattern for the crankshaft with multiple small junctions.

    Kelly, nice find with the leather fillets.
  10. Al2O3

    Al2O3 Administrator Staff Member Banner Member

    Hadn't heard of that one before ESC. Is the advantage speed?

    Might be worth offering some commentary on the pros and cons of various materials.
    • Wax is really quick and easy to use, and rework. It's fine for master patterns that you will pull molds from but not very durable for foundry patterns, unless it is a very low use pattern. Ramming hot foundry sand against wax fillet will rapidly degrade them.
    • Leather is much more durable for a foundry pattern but takes a little extra time compared to wax because you need to glue it in place. It's not quite as formable nor easy to rework as wax.
    • Bondo (polyester) is inexpensive, very versatile, and durable as there are many different hardness's, and you don't have to stock multiple fillet sizes so it's a good choice for those that wont apply fillet to patterns often, but it is a bit messy and more tedious to apply.
    • There are also wood fillets. OK for straights and large radii and be blended well on wood patterns but also a bit tedious and not very versatile.
    -My 2 cents.

  11. Another handy product is fumed silica dust, it's a very low density thickening agent you can stir into resins such as epoxy, to get a free standing resin that can be sculpted for an hour or so before beginning to cure. Unlike polyester bog or bondo which you have to work quickly or it'll go off, the epoxy gives plenty of time to get things just right. Fumed silica is a thixotropic agent: it's viscosity is thick when left alone and runnier when being moved around which is a desirable when sculpting it. The product I have is Wacker HDK N20 which I bought from the fibreglass shop, the stuff is relatively harmless, they even add it to edible food products like sauces to thicken them.

    The extra working time lets you get a good neat fillet with no hassles about sudden curing. I also use it to make a fibreglass cast of a MDF core piece, you goop it on all the sharp corners and edges that glass cloth can't get into and then lay the glass cloth over it. It's a very durable material and quite hard wearing.
    Last edited: Mar 23, 2018
  12. HT1

    HT1 Silver Banner Member

    I use Wax fillets, alcohol lamb and fillet irons
    Master Wax Fillets - Rolls (Continuous Roll Form)
    i have 00R and 1R I have the three smallest fillet irons, and have never used the largest of them, so save your money unless you are going big for some reason... trust me real wax fillets and fillet irons are a life saver, you can substitute a candle for the alcohol lanp if you are on a budget

    V/r HT1
  13. Melterskelter

    Melterskelter Silver Banner Member

    Here are a couple pics of my reworked pattern. I simply worked over most of the fillets using Bondo. Then sprayed it with lacquer. One small lesson I learned (and was confirmed by the paint department in my local hardware store---a REAL store not a box store) is that a clear coat tends to lay on smoother as it does not have pigments compared to a pigmented lacquer. I want my pattern to be as slick as possible so this has a couple coats of clear over the white.

    By the way, I did get a sample of 1/4" master fillet wax from Freeman Wax---they were kind enough to send it gratis. I will be reporting on this in the not too distant future and would be willing to share a foot of wax with those that express interest.

    ReworkedPattern (2).JPG ReworkedPattern (4).JPG
  14. Jason

    Jason Silver Banner Member

    Freeman usually sends out free tins of wax for samples. That's a nice looking pattern!

Share This Page