Shower Pan

16 gauge stainless steel shower pan

One of the primary reasons I purchased the Clipper was the desire to have a bathroom with toilet and shower. The Boles Aero I renovated earlier has what I think of as an emergency toilet. It's a porta potty located on a slideout platform under the rear bed. Because it's in the way when deployed and also used the limited open floorspace, the only time I'd consider using it would be if it were the middle of the night and it was raining cats and dogs outside. Under any other situation I'd use a campground facility.

So I've been thinking about the bathroom and it's driven many, many of the Clipper design decisions.

I'd decided on a small wet bath design a long time ago. More recently I decided that I'd make it very small to reduce the footprint as much as feasible. From a practical point of view this means the floorspace needs to be no less than about 32" x 24". Because of the rounded roofline extending the width past 24" even a small amount yields a great deal of headroom making taking showers more comfortable for someone above average height (ie me). But there's a hard limit of about 29" because of interference with the shower sump floor access panel.

In the end I settled on dimensions of roughly 32"x28" as the maximum.

There are a lot of options to sourcing a shower pan. There are a lot of inexpensive plastic type pans available. There are few standard stainless steel pans available, most at 3+X the cost of a plastic pan. You can have a pan custom made for 5+X the cost of a plastic pan. Or you can make your own.

I decided against the plastic pan simply because they do break occasionally and the entire bathroom is built around the pan. If the pan breaks then replacing it is a monster repair.

Bespoke stainless steel pans are really expensive ($1000s). And the off-the-shelf stainless steel pans require making a lot of compromises and they are still quite pricy (~$700), so I investigated making my own. Eventually I decided to make my own.

I designed a stainless steel pan using FreeCAD and then using a fabricator called OSHCut to cut it out and bend it to my specification. Arranging this is an online process: you upload the digital design, select a bunch of options including material, they automatically run a bunch of design checks and then, assuming it passes the checks, you pay. They then fabricate the pan and ship it to you.

It sounds a lot simpler than it really is, but all things considered it's amazing that you can actually do this. It took me about 3 days part-time to iterate the design before I hit the "order" button. Most of that was the learning process. Now it'd probably take me about 2 hours.

The design includes a threshold that's wide enough to accommodate a retractable shower door that are popular in RVs (one brand is Nautilus). The threshold also includes a front return that will provide a front finish. And the design has vertical returns on 3 sides with 1" offsets to accommodate shower walls and water runoff from those walls.

Freecad Techdraw Spec for Shower Pan

Shower Pan from OshCut.com

One note on the shower pan design. You'll note that it doesn't include a slope towards the drain. They recommend a 1/4" per foot slope to channel water towards the drain. I chose to ignore this to simplify the design. I saw many stainless steel pans that used a X-patten break centered on the drain. I think this approach may help drainage but I couldn't design this in (forming the X-break is a manual operation). I also think it has more utility as a means to prevent "oil-canning" than it does to facilitate draining since it doesn't really add a meaningful slope. For my application it would actually introduce an installation complexity since I intend for the pan to rest directly on the floor and an X-break would prevent a flush fit.

In reality I'm not really sure in practice whether the 1/4" per 12" slope (about 2% or 2 degrees) will be inside my ability to level the trailer. As it turns out from what I've read this is around the same slope where humans can detect an uneven floor. I guess this is one of those decisions that time and usage will judge.

A few notes on the design/process:

• You really need to select the material thickness up front in the design process. this brings with it a bunch of material characteristics that you will likely need to input into your design tool.
• The fabricating service (in this case OSHCut) will have some processing considerations that you need to account for and input into the design. Things like bend radius (influenced by the choice of bending die, default or other) and bending gap tolerances (how close together seams can be).

OSHCut has a really nice tool that simulates the bending process on your uploaded design so that you can see how it will be fabricated. You don't specify the bending process, the automated process does. The simulation will also show you when something can't be bent the way you've designed it and then flag it as an error. While this tool is really cool it is a bit frustrating because the error messages are often not specific enough to point out the specific flaw in your design. For me this led to a great deal of iteration guessing on design adjustments, uploading the modified design, and running the simulation to see if your adjustment removed the error condition.

After a bit of trial and error I began to see a pattern of design flaws that led to particular errors flagged by the simulation. After many, many iterations eventually I had a design that worked with their process. The flaws in my design were mostly centered around allowable seam gap tolerances resulting from interferences (collisions) in the folding process, i.e. edges coming into contact with one another. I also needed to adjust the bend relief allowances. This latter influences the size of holes left for the welder to fill. This is an area I'm sure I could improve if I ever do this again.

There are a few more steps required to get a functioning shower pan. This first is to seal the seams. OSHCut does the sheet metal cutting and bending but this leaves some open seams in the corners of the pan. These seams need to be welded and OSHCut doesn't provide a welding service. (I have found subsequently that other services do provide welding and charge by the lineal inch, RMFG is an example).

I could have probably welded it but I don't have the skill to make it look good and I also don't have the equipment (100% argon tank). So I reached out to the welding instructor at the local community college and he welded the seams for me. Following welding the seams required some grinding to clean up which I accomplished with a Dremel and carbide grinding bits. The seams didn't turn out as pretty as I'd dreamed but I'm probably the only person that will notice. I think mostly people's eyes will be drawn to a difference between the brushed surface finish and the weld surface which ends up looking more polished than brushed.

The next piece of the shower pan puzzle is swaging the recess where the drain assembly sits. This allows the drain assembly to sit slightly recessed so that water can more fully drain out (otherwise there would be a small lip). It also keeps your feet from constantly brushing the edge of the drain assembly trim.

Swaging is done by pressing the pan between two parts of a die set. Because there aren't a lot of people making shower pans these die sets aren't readily available. The ones that can be found are very expensive. But I found a couple online forums where DIYers had tackled the challenge in slightly different ways. I took the option more suited to my skillset.

I used FreeCAD to design a shower drain swaging die set. I then used a 3D print service to print the dies using tough PLA+. The PLA+ material is not a durable approach but it's really only a single use item for me. If I were planning to use the dies repeatedly I'd probably have a machinist turn the dies from steel on a lathe. The 3D print is fairly cheap and has a quick turnaround time for PLA+ material (~$20 + S/H, 1 week).

3D Printed PLA+ Die Set

Die Set with Backing 1/4" Steel Caul Plates and Test Stamp in Scrap Aluminum

I designed the dies with a "close fit" (0.6406") center hole for a 5/8"-11 grade 8 bolt. (With hindsight I should have gone with 20mm close-fit, ie 0.827", to be compatible with commonly available hydraulic punch press.) The intent is to use an electric impact wrench to tighten the bolt and stamp the dimple into the steel. I ran some calculations based on 0.060" 304 stainless steel material and assuming a standard 3" dimple die shape and it showed that 150 ft-lbs (203.4 Nm) of torque should be sufficient. This is well within the range of a typical impact wrench. If the impact wrench approach fails then I can use a 10 or 15 ton hydraulic punch press but I'll need to drill out the center hole to 20mm since that seems to be the standard for hydraulic punch tools.

The biggest questions in my mind was: 1) will the PLA+ die stand up to the abuse; and 2) how bad the metal pan will be distorted. I've read that if the die "bottoms out" then there should be little or no distortion.

I borrowed an impact wrench from a friend and I was able to use the PLA+ 3D printed dies to swage the drain recess. I used a liberal amount of anti-seize grease on the 5/8"-11 grade 8 bolt and the die where it interfaces with the shower pan. I think it turned out well. There is very slight distortion in the bottom of the shower pan, basically none that's obvious to a casual glance. The PLA+ die also showed little sign of fatigue/wear and I'd say it could easily be used again. If I were to start over I'd probably tweak the die to provide for a slightly larger bend radius and also provide a little more offset to accommodate the material.

Laser cut 2.1" diameter drain hole in pan

3D printed die with backing plates inserted in pan 

Drain recess after swaging process

Drain test fit (shipping plastic still on drain cap)

The final step was to grind the welds, remove the weld oxides, and re-passivate the areas heated by the welding process.

I ground the weld crowns (not flush) using a combination of a new 120 grit flap disc on my angle grinder and some tungsten carbide die grinder bits with my Dremel rotary tool. The flap disc works well but the inside corners are the big challenge and the primary reason I needed the die grinder bits. The new flap disc is necessary so that the ground area isn't contaminated with addition iron that might be stuck in an old disc.

The grinding and flap disc remove much of the weld oxides but I needed to use new stainless steel wire cups using both my right angle drill and the Dremel tool to get at the inside corners. I also used some grinding stones with my Dremel to smooth out the finish left over from the sanding disc and die grinding bits.

Following all the sanding and grinding I cleaned the weld areas using acetone to remove any residual grease.

Welding can result in stainless steel losing it's non-corrosive properties so you need to re-passivate it after welding. For 304 stainless steel the standard way to do this is to bath the item in a 10% citric acid solution at 120-150 degrees for about 20 minutes, rinsing off the acid, and then letting it air dry. This removes the surface iron and allows a protective oxide layer to form. The chromium atoms will also eventually migrate back into the metal lattice to help prevent corrosion.

The alternatives to a acid bath are to use "electro polishing" or use a citric acid gel that clings to the item. While there are advantages to using electropolishing it requires additional equipment. So I chose the acid gel approach.

I made up a 10% acid gel using distilled water, citric acid powder and xanthan gum paste (87%, 10% and 3% respectively by weight). All the ingredients are inexpensive and available at Walmart. You can easily google the recipe. I made the gel a little thicker than normal to cling better to vertical surfaces. I used the gel to cover the corner seam areas that had been heated during welding, dabbing on a thick layer and then covering it with cling wrap to keep it from drying. And let it sit for about 60 minutes as my google search recommended.

10% citric acid gel paste

acid gel paste passivation of welds

After rinsing I let it dry for a day and then did a "distilled humidity wrap test" to test the passivation. Basically you wet paper towels with distilled water, wrap them on the area, wrap the area in cling wrap to keep it from drying out, and let it sit for 24 hours. If there's no rust then the passivation worked.

Distilled humidity wrap test

The results of the first test showed a trace amount of oxidation (some faint rust colored staining showing on the paper towel) on one weld area so I cleaned them all up and reran the acid gel passivation process (which is what Google suggested). This time I let it sit for a few hours (vs 60 minutes), then cleaned off the gel, and reran the distilled humidity wrap test with no visible oxidation.