26286RE: [GrizHFMinimill] How would YOU do this?
- May 19 7:12 PM
I am assuming that you have a Real Bull like most Europeans, and not a Sieg like most Americans. While I have not had the opportunity to do a side by side comparison, from what I have seen, the Real Bulls are somewhat better in most areas. The original Sieg models with the small table and still sold by Harbor Freight, were especially bad. The Y axis gib had only two setscrews. That meant that under some operating conditions, the stresses transmitted through the carriage to the base were carried by only two 6mm screws. It was nearly impossible to work a piece which was over 6 inches above the table, say for example when I had my rotary table and 3 inch chuck mounted on the mill. Now the lever arm working against those two screws was relatively large, and it was impossible to keep the table from flexing. The improved Sieg mill sold by Little Machine Shop has a much larger carriage and 4 setscrews. That fixed the table flex problem, and made using the rotary table much easier.
The original Sieg column is thin. I remember reading the story one person who lightly compressed the bottom of a Sieg column with a C clamp while measuring the compression with a caliper. It easily compressed a few thousandths of an inch in the area where the pivot bolt goes through. That is flexible, and any flexibility at the base is magnified by the distance the head is up the column from the base. It is this flexibility that has prompted many users here to try things like filling the column with epoxy.
Finally, we now have a Sieg mill comparable to yours in the LMS Solid Column Hi Torque mini mill. So, in the future, please consider these differences before making generalized statements which imply that I do not know how to operate or adjust my mill.
I can only think that Robert's mill was a particularly bad example and poorly assembled and adjusted to boot.
I have had my mill for several years and other than the commonly discussed tweaks and adjustments is fundamentally stock.
I made a belt drive conversion primarily for the noise reduction (bedroom workshop) and to give a degree of protection against breakage due to the increased use of fly cutters to surface stock relatively quickly and cheaply.
I do not find the flex to be an particular issue with appropriate management and technique and due regard to the size and power of the machine
Your advice is good, and I do not disagree with a word of it. However, the problem needing to be solved was rapid removal of material, not quality of the cut.
Everything you said about the tendency of an end mill to flex in a slot is correct, however I have found that to be the lesser of two evils, with the greater evil the considerable flexibility of the Mini Mill.
I ran a test to see how much material I could remove rapidly, in response to the original question:
“So I started a little project today that involves taking a 1.150 x 1.850 inch piece of aluminum I had lying around, and sizing it to make a widget. Only problem is that it was almost 1.500 inches thick, and I need it to be 1.000 inches thick. So I thought, "Nah, I don't need to use the band saw! I'll just mill that puppy down.". Almost 2 hours of continuous milling later (while wearing hearing protection, because I haven't made a belt drive conversion), it is now down to 1.250 inches.”
I used a 1/2 inch 4 flute carbide end mill, and removed 1mm of aluminum at a speed of 240 mm/min with a minimum of vibration. I used the full with of the end mill. 1.25 mm deep cuts might have been OK, but 2 mm was definitely too much.
It took me 4 minutes to remove 5.5 mm from a 64 x 19 mm block. At that rate, I could have milled the 29.2 x 47mm block mentioned above down 12.7mm in about 12 minutes, instead of 4 hours.
The Mini Mill is so flexible that end mill flexibility is only a factor below ¼ inch. I have now modified my Mini Mill with a solid column and large table. Now it is maybe 10 times better than my original mill.
The other day, I was surfacing a piece of aluminum with a ¾ inch carbide facing tool. The aluminum had an imbedded steel screw flush with the surface. The screw was 2 mm in diameter. I was only taking off maybe .02mm at a time, yet the screw deflected the cutter enough to cause it to dig deeper on the opposite side. Not much, just enough to ruin the appearance of the surface. No matter how small a cut I tried, it happened every time. Then I put light pressure on the head of the machine while the cutter was turning. Even though I h ad the Z axis locked tight, and I run my gibs tight, a few ounces of pressure was all it took to tilt the head back enough to raise the fly cutter away from the surface it was cutting. That is how flexible these machines are.
I realize that at times I break some long held conventions when it comes to milling, but what I am doing is adapting my techniques to accommodate the limitations of the mini mill.
Use a 3/8 end mill, and make 3/8 wide cuts. I think you will find that the machine is quietest when the end mill has a large contact area with the material being cut. The reason is that there are always at least two flutes in contact with the material, while your method of using 1/3 the diameter likely has just one flute in contact, creating an alternating cycle of high loads going to near zero load, and back.
I would take a different view to that expressed above - depending on the type of cutter you are using.
Before I start a word about my terminology - I'm in the UK and in this part of the world an end mill is never able to centre cut, what is called a centre cutting end mill in the USA, we call a slot drill. Slot drills usually have two (sometimes three) flutes, end mills have four or more.
For slot cutting, or cutting the full width of the cutter, you should not use an end mill - the four flutes make it almost impossible to cut a smooth side to the slot.
To get a idea of the problem, imagine a four flute cutter cutting a slot (so it is cutting the full width of the cutter). Picture the plan view of the job, the cutter rotating clockwise and call the direction of the pass over the material the "front".
As one of the cutter teeth cuts away the front of the slot, it will tend to bend the cutter to the left (looking at the plan view) and push the left hand cutter tooth into the side of the slot. This causes a very rough, lumpy and poor quality side to the slot - this is why you should not cut a slot with a end mill (or if you do, use a cutter smaller than the width of the slot that you wish to cut and cut each side of the slot separately). The two flute slot drill does not suffer from this problem as when the front of the slot is being cut and the cutter flexes, there is no other tooth in contact with the slot sides.
The usual recommendation is to limit the width of the cut for an end mill to less than 1/4 of the cutter diameter of the cutter, thus minimising the sideways flexing of the cutting tool and the associated problems.
Attempts to cut both sides of a slot at the same time with an end mill often results in the corner of the tooth chipping off the cutter (due to the flexing/digging in problem) - this can be avoided by stoning a small bevel on the tip of each tooth. It need not be much, just 20 to 30 thou or so.
All the best,
My experience is that the Mini Mill is not nearly stiff enough to use a 5/8 end mill. It does not matter that you are only using 1/3 of the diameter, what matters is that you are putting a considerable side load on the machine with every pass the cutting edge makes. I’m willing to bet you would make twice as much progress with a 3/8 end mill. Also, I have found that even with the smaller end mill, there is far less vibration when using the full width of the end mill, rather than the side. Here is how I would handle your problem. Use a 3/8 end mill, and make 3/8 wide cuts. I think you will find that the machine is quietest when the end mill has a large contact area with the material being cut. The reason is that there are always at least two flutes in contact with the material, while your method of using 1/3 the diameter likely has just one flute in contact, creating an alternating cycle of high loads going to near zero load, and back.
As for climb milling, I think that has been given a bad rap. I read a good paper on the physics of it a while ago, and climb milling is actually less stressful to the cutting edge. The problem arises when the machine is too loose, and allows the end mill to pull itself deeper into the material. I do all my finishing cuts as climbing cuts, and I have my machine set very tight. When using the full width of the end mill as I suggested above, the climbing forces are held in check by the resistance the other side is experiencing. However, with very small end mills, the mill will try to climb sideways by bending, so one has to watch for that, at 1/8 inch diameter and below.
While I've done a number of small projects on my lathe, my mini-mill is definitely underutilized. So I started a little project today that involves taking a 1.150 x 1.850 inch piece of aluminum I had lying around, and sizing it to make a widget. Only problem is that it was almost 1.500 inches thick, and I need it to be 1.000 inches thick. So I thought, "Nah, I don't need to use the band saw! I'll just mill that puppy down.". Almost 2 hours of continuous milling later (while wearing hearing protection, because I haven't made a belt drive conversion), it is now down to 1.250 inches. I'm face milling with a 5/8 inch end mill, only engaging about 1/3 of the diameter, never climb milling, and cutting in all four directions. (Essentially cutting in a counter-clockwise rectangle.) I'm running the mini-mill fairly slowly (maybe 600 RPM?, though I tried it nearer to 900 RPM and didn't pick up any noticeable benefit except for greater swarf-throwing range). I tried cuts from .020 to .030 inches and settled on .025 inches as maximum material removal without excess noise.
I have a fly cutter set (though I've never used it) but fly cutters are primarily for finishing cuts, I understand, so I don't think that's the solution.
So my question; how would YOU more experienced guys do this? Is there a technique I'm missing that would allow me to do this faster? Since this isn't a finishing cut, could I use the full end mill width? That would certainly speed up the process but I read that it's bad for the end mill (specially a four-flute) since one side is conventional milling and the other side is climb milling.
And yes, if I can't find a better way, I'll probably go back to the band saw and then make a couple of passes with the mill to finish the surface.
Times like this, I wish I had a Bridgeport. But I don't think I'd be able to carry it down the cellar steps...
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