Close

Thoughts Collection: Processing / Magnetic Sheet

A project log for Sexy Gantry

Documentation of my journey to a commercial enterprise around hobby machining, specifically cnc, even more specifically 3d printing.

daniel-graceDaniel Grace 08/07/2022 at 08:420 Comments

This post is mostly about me getting things out of my head and onto virtual paper.

Stage one is still to produce just the tool plate. But I have now spoiled the fact that stage two is to product a tool plate with an already-installed metal sheet. The logical step after that would be to produce the spring steel sheet and/or to pre-attach the ceramic heater element (I have zero interest in trying to produce those!). But that's not what this is about, this is about my process of both locking down my supply chain for the magnetic sheet and the way I'm thinking about the processing workflow.

For cutting out the tool plate, I am going to need work fixturing to clamp things down and ensure both a consistent cut, and also a cut that is easy to do (easy equals both faster and fewer mistakes, both of which equate to cheaper/more profit). I could easily design fixturing for my current set of constraints and go for it. In fact, I already started that process. You can see an image of that in the previous post.

But ideally I would want the fixturing to work in the future as well. Why make fixturing that is 100% going to be thrown out soon? (Side note: It might actually make more sense to make a throw-away set of fixturing now. It would technically get me to production faster, and the chances of me having to remake the fixturing due to unforeseen issues is high either way. But I'm so far choosing to try and foresee.)

So here are my set of constraints and the possible paths forward:

Constraint One: Physical Size / Transport

The size of the tool plate I buy from my supplier is constrained by multiple factors. In the short term I plan on transporting these in my daily driver car. There is only so big a piece I can fit in said car. There's not a single set of dimensions I must fit in, because it depends on several factors. I can lay the back seats down and stick the piece between the front two seats. This is precarious, but doable, and allows me to carry narrow and long pieces. I can lay both seats down and NOT go between the front seats, and this allows much less length, but much more girth. Rather than treat the car as a single constraint, I will treat it as a last-pass constraint. That is, I'll first satisfy all the others, then see if I can come up with some orientation in the car that fits the resultant plan.

Constraint Two: Weight

The other part of this constraint though is weight. I will very likely be moving these pieces by myself, at least at first. This means that I need to be able to bear the weight of the pieces, as well as the size. As anyone who has moved knows, a weight in a relatively small area is much easier to weild than that same weight spread out over a large area where you can't get good leverage. Tool plate is necessarily flat, which already leans toward awkward shapes.

Constraint Three: Cost

The cost of each piece can be approximated by Volume * N + Cut Price. And Cut Price is a meaningful amount of money. Therefore, there's a very large difference between getting a 1x10 sheet and two 1x5 sheets. Unlike the prior two constraints, this pushes me toward larger sheets. Importantly though, a 1x10 sheet and a 2x5 sheet costs the same. This constraint doesn't really care about shape, only volume.

Constraint Four: Physical Size / Storage

Imagine a world where I own or rent a large transport vehicle and the first constraint is removed. I still have to store the unprocessed pieces for long enough to process them. This puts a different constraint on size that only really becomes relevant at scale, but is still worth marking down.

Constraint Five: Workflow

The previous constraints mostly had to do with the tool plate itself. Now we start moving onto the constraints that affect the adhesive and/or the magnetic sheet.

The perfectly ideal workflow would involve me getting a tool plate slab the perfect size for the other constraints, then applying a pre-cut, pre-adhesived magnetic sheet to the entire tool plate at once, then cut them both at the same time. This relies on an untested assumption that cutting both at the same time leaves a high-quality surface finish. Any deviation from that ideal workflow is a negative for that particular possible solution.

Constraint Six: Adhesive Thermal Properties

The adhesive used needs to be able to take thermal cycling and sustained 120C temperatures. I've been shooting for at least 200C for a nice safety margin. McMaster-Carr offers two adhesives, both with listed maximum temperatures well below this. Despite that, the Acrylic adhesive is the officially recommended one on the Voron Sourcing Guide for the 0.1. Obviously, it works well enough, but that still bothers me and I'm hoping to source a product that's actually in-spec.

Constraint Seven: Adhesive Application Method

The ideal case here is that the adhesive comes pre-applied to the magnet and I can just stick the other side to my tool plate. If that's not the case, other options I've seen include what is effectively double-sided tape, epoxies (one or two part) and a neat substance that you melt at one temperature, spread it out, then cook at a higher temperature to harden. Oddly, I have not seen any adhesives that you apply with a brush or roller that can also withstand these temperatures. These application methods have wildly different implications for the sort of process flow I would have to undergo. Since I'm expecting manual labor to be by far the highest cost in making these plates, I have to be careful how bad I make the workflow.

Constraint Eight: Suppliers

I have my preferred source for tool plate, but there are several other options. Flexible magnets in big sheets are used for signage and there are also several sources for that. Most do NOT have adhesive pre-applied. The adhesives are the real sticking point. Being able to withstand temperatures that high means I'm looking at things that are usually only used in specialized industries, which often don't sell at smaller quantities. When you can get them, they are unique enough to where I won't be able to switch suppliers willy nilly and keep the same workflow. This worries me.

Suppliers also drastically impact my time-to-source. That is, what if I have a sudden influx or orders and I need to product a large amount of plates in a week? If it takes me a month to get an order of glue, and I don't have a huge stock on hand, I'm screwed. I obviously should bias toward products with multiple, local, suppliers. That mostly works for commodity products, but the glue is proving hard.

Constraint Nine: Cure Time/Process

All of the adhesives have a cure time. When people do the application of the magnetic sheet at home they may not follow the cure time recommendations super closely. That's probably fine for their uses. But when I'm going to be cutting and drilling into the magnetic sheet, I want it to be adhered before I do that. I will probably be processing in batches, so I can afford to put the adhesive down, set it aside, and come back the next day. But the longer I have to wait, the more constrained I am.

Some of the more exotic adhesives (none of which I'm actively considering, but worth mentioning) have more extreme procedures, sometimes involving extended heating procedures to make them cure.

Possible Source One: McMaster-Carr Acrylic

McMaster-Carr sells bigger sheets of their glue-pre-applied Acrylic glue sheets just like the Voron 0.1 sourcing guide suggests. The price isn't too bad. The size is the sticking point. The largest width they come in in one foot (two is what I'm aiming for based on constraints 1-4). I also still have that nagging annoyance of being technically under-spec'd for temperature.

Possible Source Two: McMaster-Carr Rubber

McMaster-Carr also sells a Rubber-based adhesive. Those come in two-foot-wide sheets! And their maximum temperature is the same as the Acrylic one. Still under-spec, but if it works for Acrylic, why not for Rubber? Unfortunately, Rubber adhesive is less-good at sticking to metal. And less-good at thermal cycling. How much less good? That's unclear, and McMaster-Carr themselves weren't much help. The adhesive is proprietary to their vendor and they couldn't give more info.

Possible Source Three: McMaster-Carr + 3M

McMaster-Carr also sells their flexible magnets without any adhesive. And 3M sells a variety of adhesives with a wide variety of thermal properties/application styles. Most of the 3M sheets only go to 1 foot wide (is this what McMaster Carr's supplier uses? Maybe.). But they do go up to 200C (and several go higher, but I would probably use the 200C max variant unless there was some other constraint forcing my hand).

Possible Source Four: schallenkammer

I found a German company that sells "magnetic foil." They use proper metric units, and their default width is 610mm, just over 2 feet, so I'd have some wiggle room if I ordered 2 ft wide tool plate. Great! It also goes to 200C. They only list a maximum roll length (15m, probably too much!), and no prices. I've shot them off an email, but I don't know what the geopolitical climate is over there with the Russian stuff, and there's a language barrier, and international shipping, and probably a relatively long shipping time... I'm both excited for this given the good sizes, but also there's so much that can go wrong. I'll see if they get back to me on Monday.

Discussions