I have been studying several sources dealing with milling surface finish using a mini-mill.
There is a school of thought that does not recommend fly-cutters. The reasoning is that if they catch somewhere it is likely to lead to breaking the gears of the mill. The same source recommends indexing mills using carbide inserts.
Another source advocates use of fly-cutters to cut no more than 0.001" for the reason mentioned above. They, however, do not recommend indexing mills citing insufficient rigidity of the mini-mill.
I assume (I have not found this stated explicitly anywhere) that if an indexing mill catches the carbide insert breaks before the gear. The indexing mill is more expensive than a fly-cutter. Otherwise I am not clear of the benefits of one over the other. Is rigidity really an issue with the indexing mill?
----------- Remember that a mini mill is a [very] light duty machine. As long as you don't attempt to remove too much material [width and/or depth] you should have good luck.
Fly cutters are an old tool developed to allow the use of inexpensive and easily resharpened lathe type tools in lightly constructed milling machines [as that was all that was available at the time]. They actually impose less load on the part and spindle that a conventional cutter because only one cutting edge is engaged at a time and the cuts tend to be narrow and shallow, albeit with a large sweep.
While fly cutters can produce very nice finishes with the proper grinds and feeds/speeds/depths, in many cases a much smaller diameter end mill will produce a flatter surface, if this is of any concern.
It is doubtful that a mini mill has the power to use a true inserted face mill.
There are a hybrid type of end mill that takes a single or two carbide lathe inserts that can be helpful in that you can get a sharp edge at low cost by simply rotating the index, and the inserts these use are TPU or TPG that are among the least expensive of all carbide inserts. These come in a verity of grades for both steel[c6] and aluminum[c2], several nose radiuses [e.g. 1/64, 1/32 -- generally bigger is better] and several different coatings, which may or may not be helpful, given the limited power of most mini mills. The TPU is unground and slightly cheaper. I have never been able to tell any difference between the TPU and TPG inserts.
For mini mill use I suggest a smaller fly diameter cutter with HSS lathe tool bits and one of the single insert end mills, which is just another type of fly cutter using carbide inserts.
For some examples see:
?PMPAGE=173&PMCTLG=00 If you watch for the sales you can do much better on the price, and 10 [std pckg] should last the typical HSM a very long time.
Just remember that a mini mill is a light duty hobby machine, and if you want excitement with tons of blue chips with heavy feeds and doc you need another machine.
Unka' George [George McDuffee]
------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).
If this is one of the machines with plastic gears, then I think you are really pushing things too far.
Absolutely, do not believe this, in the case of the machines with plastic gears. The insert will NOT break. It will likely chip a bit, but not completely break off. The
If you are going for heavy stock removal with either cutter, then a 150 Lb mini-mill is way too small a machine. If the "mini-mill" you are talking about is one of the 300+ Lb bench top machines, it may still be a bit on the light side, but more capable. Just for reference, when using a fly cutter on my Bridgeport, I usually use a .010 - .020" depth of cut. With the indexable mills, I usually do .025 - .050" depth of cut! Carbide inserts usually do not perform well on very shallow cuts like .001". You need a dead sharp cutter, I prefer Mo-Max cobalt HSS cutters for this.
Hi, Mike. Several years ago I discovered shell end mills and use them as often as possible. I have not had need to sharpen one, yet. They are much better than a fly cutter in that they have many cutting surfaces. My experience with fly cutters is they will always have a little bounce when they encounter an edge.
My shells and holders are 1/2 inch. The holder is an R8 holder. I machined a similar shell end mill holder with a 1/2 inch shank to use at my home shop milling head on the Emco lathe. Haven't used it, yet.
I got the shell end mill holder and a bunch of shells, some new, on Ebay several years ago. I don't see them mentioned much either here on this group,or in the home shop magazines.
Don't knock plastic gears out-of-hand. I edited a report a few years ago for Hoescht Celanese on a compact plastic planetary gearset, about the size of a grapefruit, that ran reliably while delivering 110 pound-feet of torque. They've come a long way.
I use a mini mill with an indexable end mill for surface finishing and for milling the hollows in straight razors. Here is the tool I use:
it's a great little number. Inserts are about 3 bucks each, or a buck a point.
If you're using a more recent Central Machinery mini mill, you shouldn't need to worry about catching and breaking gears. My mill's motor shuts down immediately under loads that are too heavy, including the occasional full-on stop situation. Haven't broken a gear yet in the four years or so I've been using it. A lot of my work is annealed carbon and stainless steels, some titanium, along with wood, bone, ivory and MOP.
You've got to keep the mini mill tuned for this. No, it isn't rigid like a Bridgeport, but keep all slop out of it, take light cuts in hard materials, and you shouldn't have a problem.
My Clausing mill has a vee belt drive tightened by sliding the motor by hand, ie the belt isn't usually very tight. Generally when I screw up the belt will slip without breaking anything.
A freshly sharpened fly cutter gives me the best finish, possibly because it's so easy to keep sharp compared to other tools. The cutting tip extends only a little past the holder to reduce the bounce when it makes contact with an edge. It starts cutting more smoothly if it barely overhangs the edge and enters at a low angle..
The one small bit is hard to see and thus easy to jam into the work. I lower it down slowly until it makes scraping noise, add no more than
0.005" depth of cut and lock the quill feed. (Raise the knee, actually, but same effect.)
If they weren't so expensive I'd use HSS shell mills whenever possible. 6 and 8 flute end mills with integral shanks work very well, too, but they are even harder to find cheap. They remove the most metal with the least banging noise from the pulley / quill splines. I think end mills leave a poor finish when a chip catches under the cutting edge near the center. Shell mills seem to have less of a problem.
I have a 1-insert carbide mill but don't use it much. It's tedious to hand-feed slowly enough to get a smooth finish, and I rarely mill hardened steel because I can anneal it or use the surface grinder.
I only used a face mill once. It was dull and chipped but still the best cutter in that shop. After grinding the bits I set them to height against a scrap of sheet aluminum sitting on a vise jaw. It left the most and deepest tool marks.