Mill Upgrades Roadmap

I figured I’d update this with some of my CAD drawings since I’m still finishing up my spindle motor upgrade. This is kind of a road map of what I want to add to my machine, in addition to being actual CAD parts/drawings that I’ve completed (just waiting on fabrication, testing and improvement). I do most of my work in Solidworks and the rest in Fusion 360, which is also what I use for all my CAM.

1. Upgraded spindle motor with belt drive.
2. Hydraulic power drawbar.
3. 18-tool automatic tool changer (which I’ll talk about in another post in the future).
4. 36″ x 12″ steel fixture plate.

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Most of my CAD parts I try to keep pretty basic. For purchased parts where the vendor doesn’t have a CAD model, I generally I just model mounting dimensions and the outline of the part. For example, the Baldor motor in the picture above is basically just a truncated cylinder and feet. I didn’t worry about modeling cooling vanes, the fan, etc.

For parts that I have to make, I’ll flesh them out complete and usually generate a drawing to work from in the shop.

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The spindle upgrade is pretty simple in theory. It’s just a base to support the new motor and two pulleys, as well as a 10 rib J-belt.

In reality, those pulleys are fairly hard to make. The spindle pulley has to match the spindle splines, which are not a standard broach size. The motor pulley is large, so it was difficult to turn on my tiny lathe (mostly due to limited horsepower), but otherwise fairly straightforward as it just had to fit onto a 1.125″ shaft with a 1/4″ keyway.

In addition, I had originally designed this so that motor position can be shifted using a pair of screws, so the sides are actually rails. This makes installation much, much harder. I have already designed a potential replacement that would not use rails, but I’m hoping I can get this installed by switching out the bolts and being creative with my assembly steps. We’ll see. I’ll do in an in-depth post on it once I’ve gotten further along.

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This hydraulic power drawbar is very simple. It only requires three parts to be fabricated (although the four tubes do need to be quickly faced and turned to length on a lathe as well). Those three parts are very simple: a plate with six holes and a channel, and a plate with five holes (the center hole does require a single point threading operation, but still not very difficult). It’s also pretty cheap — not shown is the most expensive part, which is the hydraulic intensifier. It uses compressed air to pressurize the hydraulic fluid, and allows for a much more compact assembly with higher output force than even a multi-stage air cylinder would provide. I have everything for this ready to go except the spacer used to hold the Bellevilles in place, which I will get to after I’ve completed the motor upgrade.

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Skipping over talking about the ATC for now, I’d also like to add a fixture plate. I generally use a dual position vise at the moment, but long term my projects will mostly be fixtured, and the convenience that a fixture plate gives in terms of change-overs is great. I’m hoping this will be a quick and fun project sometime this spring.

Compressor Refurbishment – Part 2

Time to tear down the compressor itself. Of course, I waited until I got stuck to find some directions, so this may not be the most sensible order of things, but here we go.IMG_2337.JPG

I started by draining the oil and removing the oil filter. This unit conveniently has a length of pipe that takes the drain plug over the edge of the tank deck. I’m not sure if that’s present on all units, but certainly handy (and an easy install if it’s missing).

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Wow that oil looks gross. Clearly has both water and rust in it. The open bottle on the bottom of the picture is underneath the oil drain, I should have pulled a bin out and put it under the filter location, but I didn’t realize how much oil had been forced up into the hydraulic unloader when I ran it.

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Pulled off the crankcase cover — look at that seal. These are all paper seals, and they are tough. You can also see the crankshaft and connecting rods chilling in there. From here I took a winding detour that included pulling off the hydraulic unloader, unbolting the head, rebolting the head back on, putting the hydraulic loader back on, then using this guy’s trick to get the last connecting rod bolt out.IMG_2523.JPG

Everything is out now, except the crankshaft. You can kind of see the bend it in here. While it was still installed, I decided to measure how bent it is. Pulled out my Noga indicator base (possibly the greatest invention since sliced bread) and popped a cheap Fowler indicator into it, resulting in around 125 thousandths of runout — in other words, 1/8″!

There are a few options at this point:

  1. Buy a new crankshaft. I haven’t asked Quincy or one of their distributors for a quote yet, but some quick trips to my favorite sites for this project (and most of my projects, honestly) — Practical Machinist and Garage Journal — suggest it’ll run $800+. Thta’s more than I’m willing to spend, considering some patience would probably turn up a 325 with a good crankshaft to drop into this for less.
  2. Try to bend it back with my hydraulic press. This seems like a bad plan, as it’s a cast iron part (note the texture of the unmachined surfaces). It will probably crack. That also means I can’t…
  3. Weld a new shaft on. I could braze one on, but I don’t have brazing equipment or any experience brazing. This is a fairly tight tolerance application, so I’d rather not try something new this time.
  4. Turn the end of the shaft down so it’s straight, then make a spacer to fit the pulley back on.

I’m going to start by trying number 4, which means checking whether this thing will even fit on my tiny 10″x33″ lathe, and then figuring out how to get it mounted for turning.

Compressor Refurbishment – Part 1

I’ve been looking for the best way to get more compressed air in my shop for a while. In the near term, it’s because my mill can keep my California Air Tools 1HP unit running pretty much constantly when using air blast, but in the long term I’d like to be able to paint or sandblast, run air tools, etc.

The other factor is that since this is at my house (which is next to a public park) it’s gotta be quiet. A truly quiet unit with a rotary screw compressor is out of my budget, even used. Pretty much everyone agrees that the best option for a quiet (and durable) reciprocating piston compressor is a Quincy 325. So I started my Ebay hunt, and after a few weeks turned up the unit above for $180 (plus freight, which ran me about $250).

Once this thing showed up, it was in sorry shape.

  • Air pressure gauge busted.
  • Motor burned out.
  • Rust on pulleys and water inside the compressor air lines (possibly inside the compressor itself).
  • Pressure switch snapped off.
  • Tank is very rusty inside.

The first order of business was to see if I could get everything working, which started with putting it somewhere I could work on it inside. That meant adding wheels, which I welded on with my AlphaTIG 200X, using the stock SMAW setup and 6011 rod, running at about 90 amps (DCEP).

The next step was rebuilding the entire electrical system , putting in a new motor, and trying to turn the unit on (with the tank uncapped, more on that later). Since I’ve only got single phase, I picked up a 4kW VFD which takes single phase input and provides three phase output. This is a cheap Huanyang unit (actually a rebranded or knock off unit made to the same specs, based on the manual and some Google-fu), but I expect it will do the job since I don’t expect to push this unit very hard. I also checked the crankcase oil level and filter, both of which were fine (although the oil is milky, I’ll replace it and the filter later), and replaced the air filter with a low-noise intake from Solberg.

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All together and ready to run. That galvanized box on the right hand side has the VFD and breaker in it, and you may have noticed the shoddy wiring job to my welder outlet. That’s just for testing.

Turns out the crankshaft is bent. Apparently a common issue with this units if they’re shipped improperly, because they’re top heavy. I wouldn’t be surprised if this unit would last for years for my needs running in this condition, but it’ll definitely shorten the bearing life, and probably the belts’ lives as well.

Guess it’s time to tear the unit down the rest of the way. It’ll also give me an opportunity to inspect the internals and replace anything that may have an issue.

Mill Upgrades – Enclosure

The picture above is my mill, a Precision Matthews PM940CNC (940 is the table size – 40″ long, 9″ wide). I got one of Matt’s first batch in 2015 (it has now been completely disassembled twice, first to move into a basement, then to move into the garage of my new house). I paid $7,200 for it, so I think there’s some good value in it (Tormach’s most similar model, the PCNC1100, starts at $8,400). It has a cutting volume of 3.5 cubic feet (X-23″ ,Y-14″, Z-19″), which is a little bit more than twice the PCNC1100’s, although a lot of that is in Z and is often not useful. It has a 1.5HP geared spindle motor and is setup to run at 3,000 RPM max from the factory. The axes motors are steppers, running double nut ballscrews. The machine itself weighs about 900 pounds (not including the control box or the stand).

The downside of this machine is that there is no upgrade support. Tormach offers an enclosure, a power drawbar, an automatic tool changer, etc. The PM machines come with an automatic oiler setup (except on the Z-axis), but other than that you’re on your own.

The first thing I wanted to add was an enclosure, because this thing can move a fairly large amount of aluminum in short order, and for cuts that require coolant, it really helps to keep it contained.

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The enclosure is made with a 1/8″ carbon steel drain pan, 1″ square T-slot aluminum extrusions, and 1/4″ thick polycarbonate. You can purchase the plans on my site.

Plastic Product Design: Part 2

In this installment I talk about optimization and analysis. This applies equally to mass production and one-offs, and is really the critical set of skills that sets apart engineers from the many other people involved in successfully manufacturing a product. That said, anyone involved in manufacturing benefits from understanding how these tools are used, even if they won’t use them directly.

Note this isn’t a how-to for FEA, there are lots of those available on Youtube and elsewhere.

Plastic bending calculations

As I discussed in my first post, I used BASF’s guide to write my own calculator. To test it, I compared my results with with finite element analysis performed in Fusion 360. I also confirmed my results with a hand calculation, however that was just to confirm my math was setup correctly.

Finite Element Analysis (not just pretty pictures)

Any engineer reading this can probably think of a time that they’ve seen an FEA analysis that was really just a pretty picture, either because it wasn’t set it up correctly, or because the thing analyzed was not worth the time. The goal of this post is to help you understand how to do FEA and get useful information, not just pretty pictures.

The first step in all this is understanding what data you want to get out of the analysis, and why you can’t get it by other means. The second step is to understand the importance of boundary conditions, mesh size, and loads.

Identifying the problem

I expect the issue here to be a stress concentration where the clip bends away from the base. This is simply a matter of experience, but anyone can detect potential stress concentrations by looking for geometry with sudden changes in cross section. These are always potential failure points.

In this case, we only have an interest in structural FEA, but there are many other types of FEA for fluid flow, thermodynamics, electromagnetics, etc. Solving problems with them can be approached the same way, but requires a different knowledge base.

Boundary conditions

The boundary conditions are physical limits that we know or can assume are true. These are absolutely critical to getting a useful solution, and are often the most difficult part of any FEA analysis. Today’s example is very simple, mostly because the part is simple. In cases requiring dynamic analysis, or with many components, or with odd physical limits, some or all of any analysis may be garbage regardless of how it is run, and it’s up to the user to identify which parts are useful and which are not. That’s why you pay a professional for this kind of work.

Once I’ve run the simulation and determined the magnitude of the stress, I want to confirm that it won’t cause the design to break. Given that, I need to go back and determine what the stress at the elastic limit is for my material (aka the yield strength). I performed my analyses using ABS, which is a common plastic for both 3D printing and injection molding. One thing to keep in mind is that 3D printed material is anistropic (the strength between layers is significantly lower than the strength of each layer, which is equivalent to injection molded part strength). Basically, it wants to delaminate because the layers aren’t held tightly together.

The yield strength of ABS is quoted at anywhere from 4-6,000 PSI, depending on the test standard (ASTM D638 is the most common) and who performed it. It’s common in the 3D printing world to assume that Z-axis (the direction that layers are stacked in) strength is 30% of the specification yield strength. So I want to stay below 1,300-2,000 PSI in the Z-direction to prevent delamination.

Mesh Size

First things first, a mesh is the structure of points that is being analyzed. It looks like, a mesh net or fence, hence the name. It’s built using a series of polygons, usually triangles but there are other options. The actual math is performed at the locations where the triangles meet (the nodes), and the system is basically iterating through until the change in value gets very small relative to the value. If you’re interested in understanding more of the math behind it, I suggest finding a book on numerical methods or contacting your nearest university to take classes.

When it comes to sizing the mesh, we have some areas of interest and some areas that are not interesting. Large, flat or otherwise geometrically identical surfaces usually will not tell us anything of real value (that can’t be calculated by hand relatively quickly, for example). Usually there are a few features of the model that are really of interest, and those require finer meshing. I’ll just jump straight into an example.

Below is the meshed holder in Fusion360. The software has done some automatic optimization of the mesh size in different areas, so you can see the corners where it has made the mesh significantly finer in order to get useful values.

Mesh-2

The tradeoff in meshing is time versus the helpfulness of the result. You can make a mesh that has tiny elements which takes forever to solve and gives you high granularity, but it will take longer to run and may not help give you better answers. See the mesh below, which has about 10 times the mesh density of the one above, but most of the mesh is now being solved in areas that are not helpful or interesting.

Mesh-1

To make a fairly long story short, the required deflection of this design creates super high stress at that corner. I tried again using relief notches, but they didn’t help much. Ultimately it was quicker and easier to go back to the drawing board and make up a new model.

Mesh-3

This switches the mounting side away from the flexure, and it’s also a little more compact and robust. In addition, it was easier to tweak this to make the holding force closer to the flashlight’s weight. This requires about 1 pound of force to deflect to the point required to allow the flashlight to be retained or removed.

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Something wasn’t quite right about my measurements, as round 1 didn’t fit. So, I made some slights adjustments.

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Better fit, but I wasn’t happy with how difficult the bevel was making it to insert the flashlight. Time for one more revision.

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Much easier to insert, with no retention issue.

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Plastic Product Design: Part 1

Today I’m going to start a short series on manufacturing plastic parts, using a flashlight snap-fit holder as an example. I’ll talk about design with 3D printing in mind, and go into analysis for injection molding and part optimization in later posts.

Design Process

Any time you’re designing a physical part, there are two critical aspects:

  1. What problem does this part solve?
  2. How can I manufacture this part?

There are addendums to these questions, like reliability, ergonomics, cost, lead time, and a host more depending on application, but if you start with those two you will build a good foundation. To answer these, we have to ask two sets of people questions: the customer, who has the problem and the manufacturer, who is going to make the part. If you work in manufacturing, you’ll often hear these referred to as VoC (Voice of the Customer) and VoP (Voice of the Process). That’s just Lean/Lean Six Sigma’s nomenclature.

For this project, the problem is that I have a flashlight banging around in the dash (there’s a small compartment underneath my radio it lives in), and I’d like it to at least stay put and if possible, be somewhere more accessible but still out of the way.

When I go to think about manufacturing, I have a bunch of options readily available to me: CNC machining, 3D printing, buying a bunch of parts online (for a spring-loaded assembly, for example). Ultimately, I’m going to start by 3D printing because it’s easy and quick, and because creating a retention holster requires fewer parts when using plastic than when using metal (depending on reliability, cost, etc. — you can really dig deep when you talk about optimizing products, but we’ll do that in a later post).

A note about workflow

Pretty much any time I’m making something to interact with a known part, I’ll start by making the known part. So in this case, I’ve modeled the flashlight. This gives me a convenient source of dimensions and an easy way to test fitup. In general, building the model gives me better ideas for how to make the part I’m designing.

Tangent on modeling software (mostly for beginners)

I generally design in Solidworks, using a mouse, keyboard, and 3dconnexion SpaceMouse Pro. I occasionally do work in Fusion360, but I primarily use it for CAM. I’ve also used ProE, which is a nice program with a lot of really great features, but not enough that I’m willing to relearn the work flow from Solidworks. There are a lot of decent free CAD packages available right now if you’re a hobbyist, drafter or entry-level engineer who just wants to learn how to design, and some great tutorials available on Youtube for virtually every one of them. Fusion360 is probably the most common, but there’s also OnShape and FreeCAD. I strongly suggest you start in a 3D, parametric (meaning dimensions define the geometry) CAD software. Stay away from SketchUp (it plays poorly with everything else and is hard to get data out of), and only learn AutoCAD or DraftSight if you have to for your industry.

I’ll note that as use most software exclusively as an engineer. I rarely do renders or animations, I’m not interested in making marketing materials, and I primarily work on commercial rather than consumer products. Those of you who are more interested in consumer products may need a different work flow or set of tools, and you are probably going to be optimizing more for cost than quality, which is usually the goal of commercial products.

Starting the design

Now I’ve got the model of my flashlight and I’m ready to begin building the holder itself. If you’re new to all this, then the best place to start is usually looking at designs that are already on the market. Figure out how and why they designed their part the way they did, and you can incorporate it into your own design.

In this case, there are several options for mounting orientation in the car and for how to retain the flashlight. I’ve illustrated these below, but usually I will go through and mentally eliminate all but the one or two designs I think will be best. In this case, I would eliminate the tailcap-holding design entirely for ergonomic reasons: I don’t see a mounting location where this would end up in my hand the right way. Similarly, the dual clip version will get pretty large to get my hand between the flashlight and the base, so I would eliminate it. The center clip might work out, and it’s really simple (which would be a big plus when we get around to talking about injection molding), but ultimately I see the bezel mount fitting this application better.

Chart

Detailed design method

At this point (when I’m down to the final one or two models) I will start fleshing out the details of the design until I see a significant flaw with one or they’re complete. This is initially just basic mechanics: how do you attach two things together? In this case, I know I’m using a flexible joint in advance so that has informed my design to some extent.

Designing snap fits

BASF has a lot of great literature on plastic snap fits (and plastic design in general), as well as a calculator here:

http://snapfit4.cmg.net/SnapFit/workspace.jsp

Personally, I built my own version of the calculator for several reasons: it forced me to learn the details of the physics involved (which aren’t terribly complex), and my application doesn’t fit their calculator. It also allows me to easily scale if I’m using snap fits with changing geometry, multiple snap fits of different geometry, or need to develop a snap fit that falls outside the guidelines BASF provides.

Ultimately for engineers, you should be confirming that you can get similar results to any piece of software you use, and that they make sense. Whether you’re checking it by hand, in Excel, or with an industry standard of some sort, you should never blindly trust any piece of software regardless of source. For hobbyists and others whose designs will not really have an impact because they’re not widely shared, this is a good exercise as well, but more so because it will save you aggravation, time, and money if things are wrong.

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First iteration

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Ultimately everything can (and should) start from simple geometry. In this case, I’m basically building a cap for the flashlight, which will then require an added feature to hold the flashlight in the cup.End-on mount - Rev 2 - 2.PNG

As I go along here adding features I’m constantly trying to think a step or two ahead. I tend to work in a lot of disciplines, often on large projects, and sometimes miss details, which is why it’s important to look back on my design and try to critique it. I think you will find it similarly useful in your own work.

At this point in the design, I’m basically just shaping the retention clip to the flashlight shape. I’m already thinking about how I will need to reshape the overall design to make it easy to insert the flashlight, but I haven’t modeled that yet. For now I’ll focus on the retaining clip.

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I’ve extended the clip length by cutting down into the cup part. This increases the maximum deflection of the clip and can be used to reduce the required force.End-on mount - Rev 2 - 5.PNG

I’ve tapered the full clip length, in order to maximize the deflection it’s capable of. I’ve also added a chamfer along the length of the non-deflecting edge. These will both make it much easier to get the flashlight into the holder.

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Now, we may have some issues with stress concentrations where the clip attaches to the non-moving portion of the holder. I will save detailed discussion of that for a future post, where I can talk a little bit about analyzing the part in both Solidworks and Fusion 360 (and again, checking by hand!).

At this point, I have a functional design (at least theoretically). I could print this and try it out, as it has a mounting point (the flat bottom) and will at least ostensibly hold the flashlight in place.

Optimization

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Switching to the thinner wall saves about 6% in quantities of 1, and about 13% in quantities of 10 or more. For my purposes, there’s not much value in that 6%, so I’ll stick with the thicker wall.

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To improve mounting this, I want to add some better surface area to apply double-sided tape (3M VHB RP25 – I love this stuff for automotive work in particular). Despite having 12% more volume, this is only 5% more expensive than the original thick wall version.

Note that I’ve tied in the mounting point at the top and bottom, but not connected them to the clip — if I had, the clip would be mounted to the surface and it would be very difficult to work with. Putting the mounting point at some other orientation would simplify the design, but in general I think this will work best ergonomically.

Automatic Tool Changer

This is an automatic tool changer I designed for my PM-940CNC. It should be fairly easy to modify for almost any machine, and I’ve designed three sizes (6, 10 and 18 station) which should cover the range of hobbyist machines. Run by a stepper driver with a simple homing switch, and actuated pneumatically to reach the spindle.

I’m hoping to get the first one built (for my machine) this winter, and start offering them for sale in the spring. At that point I’ll probably be looking for 3-5 beta testers who I would give a price break to.