Custom Dies for the Mini Molder

Since I decided to debut the mini molder at Maker Faire bay area a custom mold was in order. One of the more iconic images for Maker Faire is the Makey robot figure. It has roughly the correct proportions for a mold-a-rama figure, and a simple enough profile for my basic CNC abilities. I found an image of a pin sold in the maker shed.

I traced the above image in solidworks and created a model of the figure.

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The base on the bottom is partly for stability, but also a result of the liquid plastic entry and exit ports. Those ports are not quiet aligned with the legs of the robot, so the flat base ensures the mold cavity is always around the two liquid plastic ports.

Once the figure was modeled I created mold halves using the cavity feature in solidworks and added various holes for screws.

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In most injection molding dies, water channels are used to route chilled water around the mold, this helps cool the part faster enabling a faster cycle time. Mold-a-rama dies had a water jacket cast into the back side of every mold die. The mold cylinder mounting plate on the back doubled as a block off for the water jacket.

The mold dies I designed consist of several parts:

  1. Mold face with part cavity
  2. Water channel spacer plate
  3. Rear mount plate

I decided to make the mold die in multiple parts for a few reason. The first is that more of the mold is reusable should I want to make a new mold design. If I had cut the channel for the water directly in the back of the mold, I would have to cut that same feature in the back of every new mold as well. Making a separate water channel spacer means fewer setups on the CNC mill. The second is that cutting a water jacket in the back of an aluminum mold would take an enormous amount of time on my little CNC mill, the much faster material removal rate of delrin made this an easy decision.

What follows is a mostly complete step-by-step of the machining process for the mold spacers.

After rough cutting the delrin stock and squaring the sides, it is tightened down to the tool plate in my mill.

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Setting the tool height so my mill knows where the top of the stock is relative to the tip of the cutter (center drill in this case)

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Drilling clearance holes for the 1/4-20 screws that hold all the mold sandwich together.

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1/8″ 2 flute carbide end mill creating the o-ring groove.

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This center u-shaped section is where the water flows into and out of the mold cooling cavity. The entire center section needs to be cut out. Since I don’t want to cut into my tool plate, I cut the slot to half-depth, then flipped the part over and cut it again. Flipping parts is always tricky, as any small misalignment will show up were the two cuts meet.

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In the first part I made, I removed all the material bit by bit (machinists call this pocketing), this took a long time. In the second part I used a 1/4″ 2 flute carbide end mill to cut a half depth slot around the piece of delrin that was to be removed. Flipping the part over, re-cutting the same path and the whole center section comes out in one piece.

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The last step on the spacer plate is to drill and tap the NPT threads for the pipe fittings. The drill bit needed to drill this hole was too long to fit in my mill, with its limited z-travel. So I milled out the hole using a 3/16″ end mill.

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Taping large threads like 3/8 NPT take a decent amount of torque, so your part should be firmly mounted. I sandwiched the spacer plate between two backer plates, put them in my screwless vise, and then clamped the vise to my shop table.

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Getting a straight start on your tap can be pretty tough without a tap guide, or performing the operation in a mill. I used a small square to align the tap, going slowly and re-checking after each turn.

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The mold faces started life as a piece of 4″x1″x12″ 6061 aluminum. Like the spacer plate before, they were cut to rough size on a band saw and then squared up with a fly cutter on my sherline mill. I don’t have as many pictures of the milling process, but I did take a time lapse video.

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To remove the tooling marks left by the mill cutters, I used 400 grit wet or dry sand paper and some free labor (thanks Ashley!) It was time-consuming, but gave me a greater appreciation for what goes into achieving a mirror like finish on all the plastic products I see.

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Exploded view of one of the mold dies (minus o-rings).

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Spacer with its two o-rings. I used plumbers faucet grease as an o-ring lubricant as it was what I had on hand.

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Assembled with pipe fittings installed.

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A Pump, a Check Valve, a Problem

During testing I noticed the molder’s plastic injection pump stopped working after 2-3 consecutive pumps. If the molder sat for a few hours it would work for a few shots, but shoot blanks soon after. The path from plastic melt tank to mold goes like this:

1. The injection cylinder retracts, creating a vacuum in the pump body, which draws plastic through a check valve and into the pump body.

2. With the pump body full of molten plastic, the injection cylinder extends, building pressure inside the pump, closing the check valve and forcing the plastic through the pump outlet

3. Hot plastic travels through a copper tube to the shuttle valve, if the shuttle valve is in the plastic injection position, the plastic pass through the valve and up to the opening in the bottom of the mold.

I checked that the shuttle valve in the pump body was switching and that the injection cylinder was extending fully, both were operating correctly. The next place I could see a problem occurring was at the pump inlet and check valve.

Getting to the check valve means removing the pump body, which means draining the tank. I put a drain port on the tank, but the close proximity to the base make draining a bit of a task. A make shift funnel made from aluminum foil did the trick.

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After removing the pump body from the molder I inspected the check valve and found a few particles, burnt plastic and bits of cork insulation, but nothing that would clog the inlet completely.

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While I had the check valve out, I inspected the flow path through the valve. The path consisted of some very small holes (~0.050″), the molten plastic having the constancy of honey, this particular check valve could not possibly have flowed enough to recharge the pump when the piston retracted. It’s clear I didn’t consider the viscosity of the working fluid when choosing the check valve.

I looked for a check valve with the largest flow path that would fit on the 3/8″ NPT pump inlet. That happened to be a swing style check valve. The new check valve is much larger than the old one, so much so that it won’t fit in the same spot, coming directly out of the pump. A 90 degree fitting in between the pump body and the check valve put it in the upright position.

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While I had the pump out, I took this opportunity to fix the threads in the pumps outlet port.

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They had become cross threaded with the fitting that was installed, and was leaking when the pump pressure built up. A few turns of a 3/8 NPT tap and the threads were cleaned up.

Fast forward a few hours and the pump is installed and has a tank of liquid PE wax around it. Cycling the pump’s pneumatic cylinder it was clear the new valve was a huge improvement. You could see a steady stream of plastic flow into the check valve every time the cylinder retracted. Cycling the injection cylinder back and forth yielded a consistent flow from the shuttle valve outlet.

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Sherline 2000 Solid Column Conversion

A while back I finished the CNC conversion of the Sherline 2000 mill I bought off craigslist. Now that I’ve had a chance to use it I can say it fails in a few respects. First is that it is a pain to tram, there are so many axis with movement that it takes forever with a DTI (dial test indicator.) I got a nano tram off eBay, but did not have any luck getting that to work. Checking with a DTI after aligning it showed it was consistently off in both axis by about 0.005-0.008″ or more. The instructions say to use a feeler gauge to fine tune the alignment, but that kind of defeats the point of its supposed dead simple alignment procedure.

The second issue is that the head stock gets knocked out of alignment by taking some pretty light cuts in aluminum (~0.010″ depth of cut). No matter how much I cleaned the mating surfaces and tightened the bolts, the column still shifted.

The adjustable headstock may be useful for certain weird setups, but unless you are cutting exclusively plastics or wood I cannot recommend it.

After ruining one last part on the 2000 mill, I decided to convert it to the solid column of the 5400. Since I reused the existing 14″ base, I ended up with a mill that has 2″ more Y travel than the 5400.

If you want to do this conversion you’ll need to order PN 50050 from Sherline, I paid $48.00 + shipping for it.

The conversion starts by removing the head stock.

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Unbolting the z axis dovetail

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And removing the column assembly

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For some reason the bolt pattern on the base of the 2000 mill is different than that of the 5400 mills, so you need to drill two new holes to mount the column. This guy has a clever way of turning the mill on itself to drill said holes. I followed his method, using pieces of 10-32 all thread and a piece of .25″ x 1.00″ aluminum flat bar.

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Here’s a drawing of the new hole pattern I made. Note that the column bolts are not centered between the dovetail in the base (which is why the circular 2000 column cut out is not even). Basically drill two holes .5″ from the rear edge, spaced 2″ apart and centered between the edges of the base, with a letter F (.257″) drill.

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I scribed lines into the base and aligned the drill using the pointed end of my center finder.

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And drilled with a center drill followed by a letter F drill bit.

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The solid column still will not bolt to your 2000 base at this point as there is interference between the column base and those little pointed ends of the dovetail. Rather than machine those off I made a spacer from a piece of 5/8″ cast tool plate I had leftover from the mounting plate I made for the mill.

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Depending on how your mill base is mounted you’ll either need to counter bore the underside of the column mount holes in the base to accept a cap screw, or use a low head 1/4-20 bolt. I chose to do the latter.

One more thing, make sure you remove the z-axis drop down bracket from the z-axis nut. You don’t need this anymore and it will prevent the head stock from raising all the way up, limiting your z-axis travel. The 2000 mill needed it so that the headstock could lower all the way to the table.

Once the bracket is removed, you’ll need to use a shorter screw to attach the z-axis backlash bracket. Or use a spacer like I did.

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The left over parts.

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I’ll probably keep these around in case I sell the mill, or if for some reason I come across a setup that requires them.

After the upgrade the difference is night and day while cutting aluminum. Much reduced chatter, more aggressive cuts possible, and the column stays in alignment. It’s like getting a whole new mill for $50.

 

 

Over Center, Under Pressure

One of the ways the mold-a-rama process differs from traditional injection molding is the way the part is ejected from the mold. Usually one of the dies has a set of ejector pins that pop the part out of the mold (if you look at most plastic products you will see a series of small circular or square indentations on the back side, these are small marks left from when the ejector pin pressed the part out of the mold.) In this lego manufacturing video you can see the thin ejector pins stick out right as the part falls out of the mold at the 40sec mark.

To keep the machine simple, the mold-a-rama uses no ejector pins. Instead it relies on the part staying in place while the molds open around it. This is possible because the bottom of mold-a-rama die is actually open. This is a picture of the bottom of my mold dies.

Bottom of Molds

The base of the plastic part sticks to the tank lid (which is why all mold-a-rama toys have some kind of flat base.) Traditional injection molding dies form a sealed cavity when they meet, save for the sprue, the opening where plastic enters the mold. Mold-a-rama dies form a complete cavity by sealing against the melt tank lid (the aluminum square below the molds, which also serves as a cover for the plastic melting tank). As you can imagine, it is very important that the dies press firmly against the melt tank lid, otherwise plastic would leak out between the dies and the tank lid.

Below is a CAD screenshot of the mold cylinder assembly I originally made, the die is the blue rectangle. Can you guess if they functioned correctly?

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The answer is no, no they did not. When the dies closed against each other they popped up slightly, instead of being forced downward to seal against the melt tank. The can been seen in the video below. Watch towards the end and you will see the pair of molds raise up slightly.

 

If I were to run this in an injection cycle I would have melted plastic spewing out.  After some head scratching I realized I had the mold cylinders (the grey rectangular part in the CAD screen shot) placed above the pivot point (the grey circular part), this meant that when the molds pressed against each other it tended to rotate the entire mold cylinder and mold counter clockwise about the pivot point, which meant the mold moved in the upwards direction. I tested this theory by flipping the entire mold cylinder and mount upside down. Now the mold cylinder was below the pivot point, which meant the mold cylinder now tended to rotate clockwise about the pivot point, forcing the die downward. This video shows how the molds are forced downward when they meet.

 

Armed with this realization, I redesigned the mold cylinder mounts to match the flipped version I had tested. With the two designs side by side (old on left, new on right) you can see how the mold cylinder location has changed relative to the pivot point.

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I also used this as an opportunity to to redesign the way the dies are constructed and how they interface with the mold cylinder, but that is for another post! Below are some pictures I took while machining the parts for the cylinder mounts.

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The above part was the first part I CNC cut on my mill and boy was it a learning experience. I probably scrapped 3 or 4 parts before I made the first one correctly. Between fixturing, weird g-code bugs, and getting feeds and speeds right I learned so much on that first part.

The final product. The slotted screw holes allow me to fine tune the position of the die at the extended position. The bolts holding the bracket to the cylinder have serrations under the heads which bite into the aluminum.  This prevents the cylinder from slipping relative to the bracket when the dies meet. The other large cutouts are clearance holes for the cylinders air fittings.

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HP Plotter Take Apart and Analysis

A friend of mine found an old HP plotter from the mid 90’s on the side of the road. Knowing I was into electromechanical objects he asked if he should pick it up for me, and of course I said yes. We initially though about repairing it (its drive belt had desintigrated and it needed new ink), but realized that even after fixing it it would have little resale value.  We decided to take it apart and harvest it for parts, while along the way looking for interesting mechanisms and design features.

Printers are chock full of useful parts like motors, linear slides, encoders, and switches. In fact a very small crude CNC router could be made from the parts of two inkjet printers.

First impression based on getting it out of the car: this thing is heavy and built like a tank; the specification sheet says 95 lbs.

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Opening an access door reveals a few MB’s of RAM on standard SIMM cards.

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Removing more covers, and now we can see the linear encoder for plotter’s print head. This encoder sends position feedback to the plotter; once homed, the print head’s exact position along it’s travel is known. Barely perceptible scribed lines are in the lower clear portion. The black square in the center of the picture with white text is the read head.

 The silver looking strip is actually a stainless band that is pulled taught by a tensioner at the end. The considerable tension is needed as any sag in the encoder strip would lead to position errors of the print head and messed up prints.

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A view of the back, main control board on the left, power supply on the right.

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Continue reading

Sherline Mill Oiler Repair

While browsing the sherline website I stumble upon a page that describes a feature added to the CNC mills in 2010. It’s a series of passages in the saddle and an oil reservoir. This oiler system sends lubricant to the lead screws reducing wear. This is very important on a CNC machine as the axis moves much more and much faster than a manual mill ever would. Furthermore, the sherline uses a very simple 1/4-20 UNC threaded rod as the lead screw, which is much higher friction than the ball screw found on most CNC mills. I didn’t see this on my mill when I picked it up, but when I looked closer, what I though was a setscrew was actually the broken off remainder of the oil reservoir.

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The previous owner must have either over tightened it, or run into it with a mill. I used a screw extractor to remove the broken piece. It came out easily, it’s made of aluminum so the tool had no problem getting a good bite.

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I ordered the replacements through sherline, PN 50930 for the oiler body, and 50920 for the cap.

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Installation was as easy as screwing in the body.

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This was one of the easier repairs I’ve ever made, and a really pleasant discovery, as now I know that the mill is at most less than 4 years old.

The Apartment Workshop Series: Mini Mill

I’ve been thinking about getting my own mill for several years. I just like the idea of being able to shape metal. For the type of odd ball projects I like, I end up making a lot of my own parts, or customizing off the self ones. Having a mill allows me to do that easier and with much greater precision. I briefly looked at 3D printers, but parts produced on them have such low mechanical strength they really aren’t suited to my projects. Plus I like the idea supporting subtractive manufacturing (milling), as all anyone ever talks about is additive manufacturing (3d printing) these days.

Picking a mill can be a daunting task. There are so many factors to consider: price, working envelope, CNC or manual, construction, spindle type, etc. Being that I planned on operating this inside my living room, my options quickly narrowed. After much research I found several machines that fit the bill:

Little Machine Shop 3900 Solid Column mini mill

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Taig Micro Mill

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Sherline 2000/5400

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The taig and sherline are a closer match, as the LMS mill is more of a mini mill, while the others are more micro mills. The LMS mill was my favorite due to the much heavier construction, more powerful motor, and standard r8 spindle. However it is just slightly too large, on it’s own it is not that big, but when you factor in that it will need an enclosure (which is kind of a must have if you plane on running a mill inside your house) it just get’s too big.

Between the taig and sherline, I prefer the taig. It’s heavier steel construction make it much stiffer than the all aluminum sherline. Neither one will handle steel all that well, both can easily do plastic, but for aluminum the extra rigidity of the taig helps reduce chatter.

I was all set to buy a taig, but I came across a deal I could not pass up on craigslist. I got a sherline 2000 CNC ready mill, with steppers for less than 1/3rd the retail price. Whoever was using it last was cutting wood, as there are wood particles all over. It will need to be disassembled cleaned and lubed before use.

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It fits very nicely on my workbench, and even when an enclosure is added it should not hog too much of the work surface. It did not have a control box, so I’ll need to start looking at stepper drivers, power supplies, and machine control software. Looking forward to this!

The Apartment Workshop Series: Workbench

Due to a job change I no longer have access to the plethora of tools at my old job.  While there are some local hackerspaces, I really like having access to equipment whenever inspiration strikes, rather than having to wait unil the hackerspace is open (also working in your boxers on a Saturday morning). So I am setting up a small workshop in my apartment living room, were I plan to do everything from soldering to machining. This will be the first in a series of articles showing how I setup  and fill this space with various toys.

Every good work shop starts with a good workbench. My money-is-no-object bench would be a Lista cabinet with a maple butcher block top.

18733s3.tifUnfortunately these are disgustingly expensive when bought new and, unless you get lucky on craigslist or an auction, they are still expensive used. My goal is to replicate the Lista bench, but for an order of magnitude cheaper.

One of the first things you should do when designing a workbench is to think hard about what you will actually be using it for. A bench designed for SMT electrical work is a lot different than one for taking engines apart. I plan to use my bench for tool storage, some soldering/electronics, parts storage, machining (once I get a small mill and lathe), light assembly, and taking things apart. I took each of those tasks and figured out what requirements they would impose on my design.t

For tool storage (specifically, hand tools) the Lista cabinets are great as the many thin drawers allow for an enormous amount of storage in a small footprint. Lista cabinets are very similar to rolling tool carts found in garage shops (minus the caster wheels), so that’s where I started looking. I spent several hours researching rolling tool carts on garage journal and reached several conclusions. If you’ve got the money, tool truck boxes (snap on, matco, etc) are hard to beat. They offer the best construction, but at a hefty price tag. Surprisingly, Craftsman tool boxes were generally regarded as the worst quality, people described them as having thin gauge sheet metal, and really bad drawer slides. Also surprisingly, Harbor Freight tool boxes were said to be the best tool box for your money, decent quality, but still affordable.

I ended up getting Harbor Freight item#67831 and selling off the top box to recover some funds. Make sure you get the 26″ model, the brownish 30″ one is much lower quality.

With the tool storage figured out I started looking for a work surface. I like working on wood, as I can sand down and refinish it when it becomes too loaded up with crud (it also looks nice). I went looking for a low cost alternative for the maple top on the Lista bench, and found the Numerar series countertops from Ikea.

It isn’t as deep as I’d like (25″), but the construction (almost 1.5″ thick beech!) and price were spot on. I ended up getting the longer 96″ version, figuring I could always trim it down and use the extra as a lower shelf.

Next up were finding sturdy legs. I considered using wood 4×4 posts, but since this is in my living room and very visible, I wanted it to look a little nicer. I chose speed rail fittings and 1 1/4″ sch 40 aluminum pipe, as they are very strong, but gave it a slightly industrial look. I later found out that McMaster has a nice selection of pre made work bench legs, some with cut outs for electrical outlets.

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For medium sized parts storage I wanted to utilize the area under the work surface by hanging pull out drawers. Since I don’t have access to a cabinet shop to make custom drawers, I came up with my own solution. In my experience work bench drawers usually end up as a disorganized pile of random parts you don’t know what else to do with. Since the drawers are just one large space everything ends up mixing together.

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My solutions to this was bins with a divider grid system. These bins are dividable down to spaces 1″x1″, allowing for the creation of all sort of odd sides compartments. They also come in a variety of depths and colors, and are stackable.

The drawers slides ended up being one of the harder problems to solve. How do I hang these bins on the under side of the work surface? They have a large lip, and sloped sides so I couldn’t just attach off the self drawer slides. I considered building a self underneath that they could rest on, but interfacing with the speed rail was problematic. I really needed a bracket that the bins could slide on, supported by the lip that runs along the outside. If you know machine tools think of it like box ways. I initially thought about making my own from aluminum square tubing, but that would have been a lot of machining time to cut all the slots and holes (I needed to make about 8-10 slides).

I was browsing McMaster one day and found this aluminum extrusion that is normally used as trim around panels. It has the perfect shape to function as a slide, but still allow me to have a spot to screw it to the underside of the bench.

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With all the parts acquired I could start putting it all together. I first layer out the hole pattern for the leg fittings, insetting them slightly for appearance.

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3/16″ clearance hole for a 1/4″ lag bolt.

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The drawer slides came next, I drilled and counter sunk holes for  #8 wood screws. I had to counter sink them as the drawer would hit any fastener proud of the surface when pulled out.

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Aligning and spacing all the slides.

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Here’s the almost finished bench. I put on several coats of tung oil to act as a sealer, turning it a golden color. After this was taken I also added an additional leg in the center towards the front, as it needed a little more support mid-span.

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I ended up using the full length of the counter top material since it fit in the space and you can never have enough work surface.

If you’re curious here’s a few shots of the drawers filled with parts.

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First Part out of the Molder

I got access to a set of vintage molds from an actual mold-a-rama machine (the macaw) and put them on the machine. After two misfires the third one came out pretty good. A little incomplete on the wing tips. I need to setup multiple regulators on the air supply, that way I can run the injection piston at a higher pressure than the rest of the system, allowing me to fill the molds faster.

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