CNC Coolant Filter (0.1)

This is a quick breakdown of my current CNC coolant filter setup — what parts I'm using, what works well, and what needs to change for the next version. I'll do a separate post about the coolant system at a later date.

Setup

I constructed a crude coolant system for my desktop hobby CNC machine to keep the stock/part and milling bit cool during operation. It's overkill, but it solved a problem I had with the excessively high-speed router bits getting "gummy", due to heat build-up while milling parts.

To clean the metal chips from the coolant before it returns to the reservoir, I needed a reliable catch basin and filter. My first prototype helped prove that the coolant system worked, but it was prone to backups, overflows, and caused a lot of anxiety during operation. I was constantly monitoring the coolant levels because the filters were prone to clogging often and unexpectantly — very stressful.

The current filter setup is a collection of mostly off-the-shelf parts, and the assembly works well enough that I can enjoy long milling operations with minimal supervision.

The setup consists of a plastic laundry tub (legs removed), a small tray rack inside the laundry tub, and two mesh-bottom plant grow trays lined with fine nylon screen. The coolant drains through the bottom of the CNC table into the laundry tub below, where the filter cleans out chips before draining into the reservoir.

Parts

You can find most of these parts at your local hardware store. I had to order the mesh filter and coolant online.

Laundry Tub

The laundry tub is spacious with dimensions of 61cm x 56cm x 38cm / 24" L x 22 W x 15" D. The deep sides are great for containing splashes as the coolant drains through the CNC table onto the filters, and in the worst case scenario of a drain clog, the tub can hold a lot of liquid. I like that almost all coolant in the laundry tub will find it's way back to the drain hole with the sloped tub bottom. The filter trays fit inside the basin with ample room around the perimeters.

Where to buy: Home Hardware

Storage Bin

The storage bin is the coolant reservoir and contains a small submerged fish tank pump. The bin has dimensions of 73.7cm x 50.8cm x 39.4cm / 29" L x 20" W x 15.5" H, and has more than enough volume (102L) to hold all of the coolant. I actually only have the bin filled half way with coolant, because it's quite heavy to move when I need to slide it out from under the table.

A large hole drilled into the top of the storage bin lid was an easy way to secure the laundry tub and reservoir together via the drain. Multiple other holes were drilled in the lid for pvc pipe connections for pumps (spindle and circulation).

Where to buy: Rona

Seedling Plant Trays

The mesh bottom plant trays are perfect for drainage and holding the filter screen. The dimensions are 38cm x 30cm x 8cm / 15" x 12" x 3". Prior to finding these trays, I had tried drilling holes in paint trays, and I even made a feeble attempt at making a tray, but both did not work as well as these seedling mesh bottom plant trays. These are great.

Where to buy: Amazon

Filter Mesh

The main goal with the filter mesh is to clean as much metal from the coolant return as possible. My chip size varies from 20mm to 0.1mm, and probably smaller. When I use 3.18mm bits the metal chips produced are incredibly small. I decided to use a mesh that would filter anything I could see with the naked eye. I read somewhere (reference?) that 50µm (270 mesh count) filter will catch any visible particulate.

Before I discovered that helpful tidbit, I tried many different filters with a wide variety of properties. Metal screens with larger weaves, smaller screens that didn't drain, and numerous others. I found that a 50µm (270 mesh count) nylon filter drains well once saturated. A filter with a higher mesh count (smaller microns), doesn't drain fast enough, and a filter with smaller mesh count (larger microns) lets too much particulate through. This filter screen size is also surprisingly easy to clean with a brush, and fresh water.

I still haven't figured out the best way to secure the filter mesh to the tray yet, I'm just using tape for now. I might try sewing the filter with a drawstring so it can be secured around the tray and easily removed if needed.

NOTE: I'm uncertain if this filter mesh is also cleaning the coolant lubricant out of the water. This thought crossed my mind recently when I noticed my coolant lines looking really clear, when they should be green. I'm not sure if they are clear because I haven't added fresh coolant in over a year (probably), or if the filter is too good, or if the coolant is evaporating. I don't know at this time, I will report back. TBD.

Where to buy: Amazon

Tray Rack

To hold the filter trays, I used 2020 black aluminum extrusion I had laying around, and constructed a primitive tiered rack. Originally it was three tiers with varying filter screen sizes, but in the end settled on two tiers with the same filter screen size on both for redundancy.

I'd like to find an off-the-shelf alternative to the 2020 extrusion rack, perhaps a suspended sink drying rack. Something to hold the filter trays suspended in the air, or raised off the bottom of the laundry tub.

Wish List

I've only used this filter setup for a 3 months. It is significantly better than the filter prototype I was using previously — leaps and bounds better. Now that I'm not anxiously running my CNC worrying about coolant backups in the middle of a job, I've been taking mental notes of the minor drawbacks and annoyances I've noticed during operations.

• Tight coupling between tub and reservoir. Having the laundry tub and reservoir linked together originally seemed like a great idea, but the more I try to maintain and operate the system I see that it is a hinderence and needs to be changed for easier operations. Most of the challenges below are related to this coupling. I would like to move the reservoir to another area.

• Awkward to clean the filter. I need to pull both the laundry tub and reservoir forward to gain access to the filter trays. Then squeeze the trays out a narrow opening between the CNC table and the top of the laundry tub. I was originally planning to cut an opening in the front of the laundry tub for easier access, but it just didn't feel right at the time. I also have to be cautious when pulling the entire assembly out, because the reservoir is connected to the coolant lines and electrical wires. Not ideal.

• Difficult to monitor and adjust coolant PH levels. Opening the reservoir lid is really awkward and not practical at all, for all the reasons listed above. For the first time since I purchased GreenCut cutting fluid, I've noticed that my coolant lines are no longer green, so I ordered more coolant. When I received the new container of cutting fluid it had instructions for how to maintain the PH levels. I'm almost certain the original shipment from a year or two ago also had this, but I misplaced them and forgot. Now I'm trying to measure the PH levels in the tank and there's no easy way to do it. I need to be better at maintaining the coolant PH, and before I can do that I need an easy way to mix, measure and check without pulling the entire assembly out. Not ideal.

• Difficult to see coolant levels. I want to make a coolant level circuit to show what's happening inside the reservoir, but haven't got around to it yet. I have a fuel sending unit installed in the reservoir to determine levels from resistence changes, but I still need to make the actual circuit. At this time, I just hook up a voltmeter and measure the resistence during CNC jobs to periodically check on coolant levels. Not ideal.

• Submerged pump is not easily serviceable. There is a fish tank pump in the reservoir, a remnant from the original coolant prototype, that sends coolant to the spindle. The pump worked well enough that I didn't want to buy another pump when I made this new setup. I dislike having the pump inside the reservoir though. It's not a huge deal, but similar to monitoring the PH levels, I need to open the lid to do any pump maintenance. Lately I've been thinking that with all the other challenges listed above, it might be time to move the pump out of the reservoir — rethink the entire setup.

• Drain plug or shut-off valve. When the system is not in use, I'd like to be able to completely close the reservoir to prevent evaporation.

Try

I would like to try cutting fluid paper at some point, but I've only seen it available in large rolls. I would like a small amount to try before I go all in. It could potentially be a great alternative to the filter mesh listed above based on what I've read.