Milling question

[Gordon]

When milling with an end mill what is the correct procedure to eliminate or at least minimize the fish scale finish? Faster or slower cutter speed? Slower or faster feed? Deeper or shallower cuts? Better tramming of the mill?

 

[ShopShoe]

Gordon,

Pictures might help us see what you mean. I know what you're describing, but there are fish scales and there are fish scales. You have mentioned all of the things involved, to which you can add the choice of the end mill you used and the material you are cutting and whether the cut you are doing needs a lubricant or cooling fluid as well.

I would say:

1. Consider whether the mill is rigid enough. Can you fix something. (By the way, I had your problem very bad a couple of years ago and I found the gibs on my mill's column were loose.) It is good to lock travel on things you are not moving for the current cut.
2. Check the tram of the mill
3. Diameter of the end mill for the cut you are doing. Larger?
4. Length and construction of the end mill: shorter will be more rigid. If it's a small end mill, one with a larger shank size might be stronger.
5. How many flutes for what you are cutting.
6. Large area of cutting? Fly cutter?
7. If you are hand feeding, try to crank consistently.
8. You will have to experiment to find the depth of cut vs. feed rate that is right for your situation.
9. Cutting fluid? You will have to experiment.
10. If you are cutting slots, pockets, etc. You need to get the chips out of the hole as you are cutting.

Unfortunately, I can say that in my own case only experience leads to getting each cut I make to be better: After awhile, I have just developed a feel for what will work in the most-often-used situations. Studying tables and reading these forums helps with this, but I have to do it to get it right.

One last word. I buy end mills of different types as part of my learning. I have found that sometimes one has to spend more to find what works, then I can find my go-to tooling for different situations.

 

[doug gray]

Embrace the swirl.

 

[Philipintexas]

I may be totally wrong but in my experience, and my mill, a perfectly trammed head will allow both cutting edges of an end-mill to "drag". By chance, my mill leans very slightly to the right (I've shimmed until I gave up) and I always cut right to left which causes the trailing edge of the cutter to have the last say. I get very nice finished cuts by sticking to this regimen. I use a fly cutter a lot and get a very nice finish.

 

[deverett]

An eminent model engineer in UK (Graham Meek) used to advocate stoning the corners off the flutes, just a couple of thou. Haven't done it myself, mind you!

Dave
The Emerald Isle

 

[cwelkie]

I've also had success with finishing some materials with a "climb" cut vs. normal.
FWIW
Charlie

 

[Gordon]

I frequently use oversize stock because that is what I have. I am now working on an aluminum piece which will end up 1 1/2" x 2" x 3/4" thick. I have a piece of 1" thick x 2" wide so I cut off a piece about 1 5/8" long and using a 3/8 dia end mill I mill it to the correct size. I have a power X axis feed so I usually cut off about .3" wide path and feed in both directions. Since I am getting the fish scale in both directions I believe that my mill head is trammed correctly. Generally it is not a problem when I am just trying to remove stock but It would be nice to have a better finish on the final cut. Usually I just put a piece of fine wet/dry sandpaper on a flat plate to polish the scales out. As others have stated locking the spindle helps especially on the final cut. I also heard that putting a slight radius on the ends of the cutter helps.

 

[Blogwitch]

Gordon,

What you have asked for is the same as asking how long a piece of string is when you haven't got anything to measure it with.

I have great difficulty producing bad finishes as everything on my machine is set up to the best it can be.

You don't mention what mill you have, if it is a basic X2 with the swivel at the bottom of the column, then you will maybe need a few mods to solve the problem as it sounds like the head is swinging side to side, causing a permanent trailing cutting edge, depending on the direction of cut, whereas with a Bridgy, even with a bluntish cutter the results should be very good. Lack of rigidity somewhere on the machine is a normal cause of bad surface finishes, plus having table and column jibs not correctly adjusted, in fact Shopshoe has covered most of the problems.
See if it cuts any better in the Y axis, if it does, then that really does point to the head swinging in the X axis.
How rigid is your machine vice? If it is like a drill vice, then you may as well stick your metal to the table with chewing gum.

With reference to your asking about speeds and feeds. You can't rely on the charts that are floating about, they are for industrial machines and most probably have no relationship to what you are trying to achieve.

I will tell you what I do.

Razor sharp cutters always, very fast spindle speed with very slow feed speed. The depth of cut doesn't really come into it, but as a rough rule, with everything rigid, 1/2 of the cutter diameter is your max, when either cutting down onto the material or cutting along it's edge, so 1/4" cutter can have a cut of 1/8".

Tell or show us what you have to work with, and I am sure someone will be able to solve your problems.


John

 

[goldstar31]

I'm with John- at least 98.7 or three Standard Deviations:hDe: It may be that the head is secured but tilted and needs to be corrected and locked down again.

As for the so called rounding of tips on cutters, this isn't something new. If you have a Bridgeport in one's proverbial sh1thouse, it doesn't really matter that much whether one accepts sharp edges or not but with the average hobby mill one should expect to do the tool tip as one does it for a lathe which is a milling machine but at 90 degrees!

There is no magic in all of this, it been like that since Tubal Cain turned the hollow brass pillars in Solomon's Temple- or so people tell me.

Cheers

N

 

[Gordon]

Perhaps I am just trying to get too much of a mirror finish. My mill is an Excello 620 so not a lightweight home shop mill.

See attached

 

[bazmak]

You will not get get a good finish using a 3/8" endmill to face a 50mm width
For wide alum I use a flycutter to get the full width in one pass.My mill is trammed well and the return cut just polishes out the trailing cuts.Also get
a great finish on mild steel but depth of cut has to be reduced on
wide cuts

 

[xpylonracer]

Hi Gordon, certainly not the light build of your machine causing the fishscale, are all the gibs adjusted and spindle bearings OK ?

xpylonracer

 

[machinejack]

Perfection was always on my mind when I started making parts for the space station. I didn't make all that many but I did learn a thing or too. You wouldn't believe how many grades of aluminum they use. The first parts I made I polished and took all the tool marks out. The parts came back as scrap. Nobody took the time to tell me that NASA wants the tool marks left on them. I guess the reasoning is so you can't hide a filler weld. If one want's or needs a better finish that is were the grinder comes in.

 

[mc_n_g]

Something that will help will be an end mill with a small radius (also referred to as bull nose end mills).
Something in the neighborhood of 0.025 to 0.050 thou depending upon end mill size.
The radius removes most of the sharp circles you are experiencing.
The problem is cost. Radiused end mill cost more.
If you have a nearby sharpening shop maybe they can do it for you.
It is the same thing as having a small radius on a lathe tool.
If the lathe too corner is too sharp you end up with a record-type finish (scratchy)
You also need to make sure the radius is larger than the feed rate so the same thing does not happen.

 

[Gordon]

The problem is that things like bull nose end mills increase the cost and it is cheaper to just remove the tool marks when I am done. I am retired and this is just a hobby so my time is basically free. I did try using a fly cutter and the finish is good even though I did not really grind the tool bit correctly. I will have to experiment more with this. My main concern in the past has been that I did not think that the depth of cut was going to be very deep so I will have to do some more experimenting with that. Also fly cutters only work well on a flat top surface and frequently stock must be removed in an area against another vertical surface where the cutter cannot just swing out over the edge of the piece.