Flycutter and indexable bits question.

[Twmaster]

I have never been able to get a flycutter to make a good smooth finish for me ever. I've asked loads of what must seem like retard questions in other venues but still in spite of the help offered been able to achieve anything that does not look like poop.

I saw John's (Bogstandard) radiused flycutter bit and have attempted to replicate that. Still no joy.

Now I just had this idea hit me...

An indexable tool holder with a round cutter like the RCMT style. Like the one on the top of this page:

http://www.glanze.com/indexable-tool-holders/mc-tc-prdnn-prgnr.html

Have I perhaps come up with an idea or have I yet again hit a daft notion?

Thanks!

 

[bentprop]

I hate to rain on your parade,Mike,but the radius on that might be a bit much.
Then again,Bogs swears by them for general work,so it's always worth havin' 8)
A flycutter needs to be sharp,turning slowly,and at minimal depth.
What material are you trying to cut?Some are better than others for this process.
Speed?If you're using HSS,it should be ground as for a right hand turning tool,and fitted with the point facing down.
Sorry if I'm teaching you to suck eggs,just trying to cover all bases ;D

 

[Blogwitch]

Not a daft notion at all.

I can use mine because I have a very rigid machine, if you don't have one, you just might struggle to get a good finish, no matter what shapes you try.

People seem to think that flycutters are the be all and end all of getting a good finish, nothing could be further from the truth.

The first thing you must do is find out how rigid your machine is, and what the maximum diameter of flycutter you can swing before vibration starts to rear it's ugly head. It just might be you flycutter holder is way out of balance and causing the problems you are getting. Without being there it is very difficult to give exact advice.

The cutting edge of the flycutter must be razor sharp and have plenty of clearance angle on all faces.

Start off with a bit of ali in your vice, and set the max diameter of cut to about 1.5". Put a depth of cut of about 0.005" then take a slow cut across the face, using some sort of lube, either parafin or WD40. See what you end up with. If it is a good finish, move the cutter out say 1/4" and take the same cut again. Keep doing that until you start to get chatter marks on the face of the cut, just under that will be the max diameter you can use. Also remember, the more you move the cutter out, the more unbalanced the setup becomes, plus you reduce you machine speed the further away the tip is from the quill centre. If I am cutting say a 1.5" diameter, I would be running at about 600 RPM.

The cheaper range of flycutter holders do suffer quite badly from being out of balance. The way to counteract the worst of it is to have much longer tools than are required and have it sticking out of the back, sort of balancing things out.
Doing that does raise a safety issue as there is a lump of spinning tool steel in an area there is usually nothing to be afraid of, so care must be taken.

Another way requires you to remove metal from the side of the flycutter that the tool sticks out of. I can't tell you how much, as I have never had to do it, but if you look at a few of the more expensive types, you will see how they have been shaped to counteract the imbalance.

That is of course if it is the holder or your mill causing the problem, if it isn't, then you can ignore all this bullsh, but still carry out the trial cuts.


Bogs

 

[shred]

I'm not sure an RCMT will be at all happy with the usual flycutter interrupted cut. I know the lathe ones sure don't like it.

 

[Florian]

Hi twmaster, bogs

I Also use a flycutter, selfmade, and it works really fine!

Has been made from an old gear-wheel (the teeth are turned down now, but i couldnt do it when i built this flycutter)

The cutting edge has a small radius (something like 0.2 or 0.3 mm) and in about describes a radius of 35 mm when rotating. The Actual cutting bit has been made from a hardened cylinder bolt.
Clearance angle to the face thats going to be milled flat is in about 5°, on the side in about 10°.
The cutting angle also has something like 5°.

And the most important clearance angle is the one of the cutting edge which is (almost) parallel to the surface that will be milled and points to the rotation center of the cutter!
That one should be between 1 and 3 °! If you dont have that one, there will be chatter marks because the cutting bit cuts on its full width! (And this requires a rigid machine)

I can get really good surfaces, in contrast to bogs, i am using quite a lots of rpm!

Depending on the material, i use in about 2000 turns per minute for brass,
2500 for aluminum and around 1000 for mild steel!
I rarely have to resharpen the cutting bit, thoug one could think it should be resparpened quite fast with such high rpm. The trick is that almost all the heat goes with the chips when fly cutting with high rpm.
So, i recommend to try what rpm you need for your flycutter! It may be a lot more than you thought!

When milling aluminum, i use some denatured alcohol and a good cutting oil for mild steel.
Brass is being milled dry.

Usually i put a cutting depth between 0.25 mm and 0.5 mm when milling brass. Same with aluminum and something around 0.1 mm for mild steel.

Then i feed the table very gentle until the first cut is being taken. Then i try how much it can take by steadily increasing the feed rate. If its too much, you will feel immediately, just reduce it or take a slighter cut and then complete the "step" with another cut.


Oh and the most important thing is to have a extremely sharp cutting bit!
After grinding the cutting & clearance angles, i use an oil stone to make the ground surfaces better and also make the slight radius on the cutting edge.

Good luck with yournext try..
Cheers Florian

Ps: If i take the last cut, i somethimes (only with brass or aluminum) rise the rpm again and only take a very slight cut (must be something like 0.05 mm or so). Then its in about 3000 rpm for both... (and never forget to cool when milling aluminum)