CNC Mill - Swarf Guarding I

Much of what follows will be a bit disjointed, and not of the most logical sequence, but I felt that as of this date, March 16, 2005, I needed to get some additions up to 5 Bears. The text on this page is partially copied from what I had previously posted as news on the Home Page. For this, I apologize. Subsequent installments will have some fresh commentary.

Following the successful installation of tool length sensing onto the mill, I once again practiced a few cuts with the KaVo spindle. And once again, I was amazed at the swarf this spindle generates with a typical 1/8" carbide end mill at 35,000 RPM, and even worse, a 1/4" EM at 25,000 RPM. At these speeds, the finish left behind on a decent aluminum alloy is beautiful; so is the powdery swarf, which showers everything in the vicinity of the mill with an aluminum dew-fall. Similar to party glitter, but finer. This stuff went everywhere! Without proper shielding, my linear motion hardware would soon be rendered polluted and useless.

There are a large number of vendors which manufacture shielding for both ballscrews and for linear rails, such as the THK HSR series. These are expensive, and due to the custom nature of most work with rails, they are typically engineered at the factory, using a customer-provided print. Lead-times are unacceptably long with work of this nature. I was determined to engineer my own shielding system with off the shelf components.

First, I must emphasize the need to plan for, and execute, effective swarf guarding while your mill is under construction. This means creating rail to rail distances which provide space for the chosen bellows, tapped holes for mounting brackets, etc. Think about the configuration and apply solutions while you engineer. If done in this way, the swarf guarding will be relatively painless, and importantly, you will not lose as much travel distance as I did.

	As usual, MSC came through once more. Pictured here is a 24" / 600mm long Gortite bellows of special synthetic rubber, immune to oils and other typical cutting fluids. The interior dimension is 6" / 152mm square; the outside is 8" / 203mm. Even though this is not a custom product, these bellows are expensive, and unlike much of this mill's hardware, searching for way protection on eBay was a bust. Why a square shape? I could have purchased a pre-cut bellows section which was roughly one half of what you see here, but the cost of such a section was perhaps 75% of the entire square. By buying the square shape, and cutting it myself, I ended up with two 24" bellows at a significant savings. This technique also allowed me to customize the shape of the bellows, as needed, during the retrofit.
	Hard to believe it, but this compact bellows, which naturally compresses here to about 20mm high, less than an inch, will open to a full 24", or 600mm long! I was impressed with the apparent quality of this product. It is a tough, high-quality synthetic material, with fabric woven inside, quite unlike the cheap, all rubber way covers one often sees on "mini-mills" and similar. My plan, then, was to cut this square bellows into two separate "U"-shaped bellows. Each of these would in turn be further cut to create a total of four bellows sections. Two of these would occupy the Y axis, in front of, and behind, the saddle area. The other two would take care of the Z.
	Because I did not pre-plan, one of the issues I had to deal with was the fit of the bellows over the THK rails, specifically the Y-axis. Gortite square bellows are available in increments of 2", with the inside dimension jumping from 6" to 8", and nothing in-between. A 6.5" internal dimension would have been perfect. To determine the needed dimension, a bit of math was called for. My Y-axis rail to rail (measured from centerline) dimension is 135mm or 5.3". Adding the actual width of the rails to this, the required inside bellows dimension to correctly guard the Y-axis was 160mm or 6.30". I knew that the 6.30" would be a touch large for the factory 6" bellows, and before I ordered, I was hoping that the bellows would be flexible enough to deform a bit and handle this additional space. The only other option would have been to order a 8" bellows, and that was too big. When the bellows arrived, I sadly discovered that no, they would not flex enough! There was no way that a 6" cut bellows section was going to fit over 6.3" of Y-axis. My two options were simple... I could modify the mill by altering the rail-rail distance on the Y axis, or I could alter the bellows. The former was definitely out of the question. However, when I examined the adhesive, or heat seal, between the individual bellows pieces, I found that the width of the adhesive contact between each individual bellows piece was about 0.250". This meant that I could cut the interior of the bellows section, opening the inside width from 6" to the required 6.3", without causing failure of the connection. Working rubber compounds is often tricky and a bit unpleasant, but in this case, a Dremel sanding drum worked wonders. I first applied some masking tape to keep the bellows stack tightly compressed. Next, I simply drew by hand a rail-shaped female form onto each inside edge of the bellows. The Dremel coarse sanding drum was carefully applied to the interior edges up to my drawn limts, using great caution as I came close to the final form, as there was not much adhesive left to keep the bellows together at the rails' widest points. Sanding too far would create a hole, allowing ingress of swarf right up against the rail at the hole's location.
	Please, use a dust mask! There's quite a bit of smelly rubber dust generated using a Dremel sanding drum, and I cannot help but think that fine, synthetic rubber dust in the lungs isn't a health tonic. When the sanding was completed, the section of bellows shown here easily fit the rails in a very satisfying, custom-fitting manner. This section is for the front of the Y-axis. The aft bellows section was likewise modified to handle the rails' form. At this stage, I had two custom Y-axis sections, and two unmodified sections for the Z axis.
	This process took quite a few hours, mainly due to the required care needed. If even one bellows section was ruined, I'd be forced to buy another square bellows, and at over $100 U.S., I had no desire to do this! My next step was to create a set of sealed mounting brackets for the ends of the bellows. When I ordered the square Gortite bellows, I also ordered a rather expensive set of aluminum mounting flanges. May I suggest, do NOT do this! The factory mounting flanges were cheap and soft 0.064" thick aluminum, and can easily be replicated using a $2 piece of flat aluminum sheet stock and a jig-saw. Each section of bellows (there were four) required two flanges shaped as shown. These are inserted into the bellows, and clamp the very last bellows vane against any flat surface. In this photo, you can see the aft aluminum flange inserted correctly into the bellows, where it secures the vane to the Y-axis saddle section. The Y-axis saddle plate end was drilled and tapped for a number of 8-32 button-head cap screws. The aluminum flange was temporarily held against the bellows vane, and the location of the holes noted. These were created in the rubber using a small punch.
	The front mount was trickier, as there was no handy vertical aluminum structure at that location. I needed to create one, and while doing so, took the opportunity to also enclose the Y-axis servo coupling section. A piece of simple aluminum angle stock of 1/8" thickness was cut, drilled, and tapped appropriately. Here, an angled cut is being executed in my big mill with a slotting saw.
	As part of the construction of the forward mounting bracket, the bracket itself was mounted in my mill vise as shown, and the necessary 8-32 tapped and clearance holes were drilled through both the bracket and the mounting flange at the same time. Doing so in this manner will provide for accurate hole locations.
	In place on the mill base, I was pleased with the shape and functionality of the front Y-axis bellows section + bracket. Before this picture was taken, the Y-axis servo mount was removed, and it too was drilled and tapped for the two 8-32 SHCS shown. From front to rear, we have the bracket, one aluminum flange, the bellows section itself, another mounting flange, and the Y-axis saddle plate. The forward limit switch was also repositioned to reflect the loss of Y-axis travel. You can see that there is quite a bit of work involved in correctly mounting these bellows! Total travel lost was ~3/4". There are certainly methods which one might use to guard for swarf without loss of travel. Whatever you decide, I'll say it again, plan for it during the mill's construction. Overall, though, so far a neat and effective installation.