Dear Friends,
This morning we got more than 4 inches of snow dropped on us. I wish it would stop doing that. These are the days of winter in Wisconsin that seem the longest. March and April are usual miserable weather months for us. Ice and mud, with a side of low gray clouds for weeks unending grace our days. It's not all bad news here on the farm though.
The reason this is late this week was yesterday I was putting the finishing touches on the new fixture for the mini bars. Since we have a way (albeit a bad one) to make medium bars, it became apparent that we would need to have the mini bar fixture made before I reattempted to make the medium bar fixture.
And thus Sisyphus began his climb...
As with all these fixtures, the first step is to make yourself good flat, parallel rectangles. This is unironically one of the hardest parts of the whole process. One, you have no where to hold onto the rectangle (when its this large). Two, you have to elevate it at some point off the table so you can cut off the sides and put holes in it without cutting holes in your table. Finally, with this particular set of rectangles, I was taking a 0.75" plate down to 0.6" in thickness.
That doesn't seem like a lot to remove, but when you are removing it from a plate that is 8"x18" the volume of metal chips is quite a lot. This time around I used a new tool called an "indexable face mill" to do most of the material removal. This is an enormous cutter (the one I was using is 2" in diameter) where the actual cutting edges are little pieces of hard carbide that are removable.
It's really a genius solution. You need a big tool to be able to remove a lot of material quickly, however a full tool this large made entirely from tungsten carbide would be both extremely heavy (which would require a lot more power to spin it) and would be hundreds of dollars in material cost. One little chip on the corner of it and the whole tool would be trashed. This so this gets us the best of both worlds and the little carbide tips can even be rotated when they chip since they have two cutting sides.
One dilemma I kept running into was that anytime I took a cut deeper than 0.01" thick the cutter would stall in the machine. This puzzled me greatly. I have been using a tool called GWizard to do my speeds and feeds calculations now. They actually have the ability to plug my specific machine into the system and it can adapt the recommendations to fit it better. All in all I have been happy with its suggestions.
To understand this problem a bit better I need to cover a bit of how these speeds and feeds are calculated. The materials have a number called their "Surface Feet per Minute" (SFM) variable. This is dependent on what type of material the tool you are using to cut it is made from but for the most part is a stable number. Now bear with the math for a second, you take the SFM and multiply it by 3.82 (I honestly couldn't tell you why its 3.82, it just is) and then you divide that number by the tool diameter to get your spindle RPM (what I have referred to as "speed" previously.)
So for those astute arithmetically gifted among you can see, the smaller the tool is, the faster it spins.
However, when you have an enormous face mill, you are spinning the tool very slowly.
This is where the electronically gifted individuals begin to see my issue. Motors when spinning very slowly have high torque (which is the rotational force), but their effective horsepower drops precipitously. Interestingly, gWizard shows both the required torque and horsepower for the cut you are putting into it. To better illustrate my issue, I will provide an example.
A 0.375" diameter 4 fluted endmill doing a full width cut (0.375") at a depth of 0.75" traveling at a feed of 5.5 inches per minute requires 356 oz/inches of torque and 1.13 horsepower.
A 2" diameter 6 fluted index face mill doing a full width cut (2") at a depth of 0.1" traveling at a feed of 5.5 inches per minute requires 1112 oz/inches of torque and 0.8 horsepower.
I know this feels like homework. Stick with me...
The smaller endmill removes 1.54 cubic inches per minute while the facemill removes 1.1 cubic inches.
This is about the time I realized I needed to contact Tormach and get their graph data for RPM to Torque and Horsepower. The smaller endmill is running at 3200 rpms and the large facemill is running at 727 rpms.
They confirmed my suspicions. When the machine is running at 727 rpms it barely has enough torque and HP to keep cutting. Any deeper of a cut and it stalls. On the other hand the higher RPM for the 0.375" endmill means that there is twice the torque and 1.5 times the horsepower it needs to do that cut.
So if the smaller tool cuts another 40% more material than the face mill in the same time, and doesn't have the stalling issues, why use the face mill?
Deflection...
A long, skinny tool, doing a deep cut will bend while its cutting. A much thicker tool doing a shallow cut will not.
All this to say I learned a lot of new things this week.
After getting my rectangles finished (one tenth at a time), I was able to begin cutting into them the grooves and holes that allow me to make the bars. This new fixture had quite a few new elements to it. Beyond the new slot ideas and the addition of the pins, I also made a small change to operation 1 that was designed to provide more space for chips to be put. This minor change has proved to be a mistake, but more on that later.
My first hurdle was when I found that my pins (despite being correctly sized) would not fit into the holes in the plates. My guess is that it has something to do with the tapping process and how that may slightly change the hole geometry. To fix this I decided that rather than having the pins go all the way through the plate into the top fixture, they would just go up a little bit into the chamfered part of the hole (which looks like a cone). This has some advantages and disadvantages. It doesn't work as well to hold them firmly, but it also makes them less likely to be broken off.
In the end, making the fixture out of steel does seem to give it a lot of good heft. It is very heavy to lift, and hopefully with the thicker plate, it will be stronger.
But I haven't even got to test the slot part of the fixture yet...
In the first version of my fixture, I had used slightly larger holes in the lower plate to go behind where the holes were drilled and threaded. This meant that there wasn't much clearance between the cutting tool and the walls once it actually cut through the plate. Since I was changing the procedure a little bit with operation 1 this time around I thought a slot would be better. I was adding twice as many holes with half of them being 0.25" in diameter. This would create a lot more chips. So the advantage of the slot was that it would help prevent those chips from compacting in the smaller holes below.
I have broken about 5 taps in as many minutes this afternoon trying to test this new fixture. I don't know for sure why the keep breaking, but my current theory is that the pressure of the cutter going down is enough to allow the material to bend downward slightly since the holes are suspended in the slot rather than supported below.
With another week and hundreds of dollars in materials sunk into this fixture already, the best thing I can do is to try to make an insert that will go in the slot in the lower fixture plate, that will have holes in it like the first fixture. It's worth a shot at least.
This all goes back to the same issues I have been having this whole time. The devil is in the details.
But I press ever onward since that's all that can be done.
Thankfully our early international sales have been solid, and that demand has ensured that I haven't needed to spend as much of my time on marketing. I do long for the days when I can get back to my usual insanity instead of this bonus insanity.
Stay tuned for more letters on Thursdays in the coming weeks and be sure to go subscribe to our YouTube channel. If you like and watch the videos it helps us get promoted more by the algorithm to people who may never have heard of us.
Ever your servant,
Colin Murdy
CEO/Owner
Murdy Creative Co.
Cell: 414-434-9001
MurdyCreative.Co
