Fly cutter

[DaveH]

Hello all,
I’m going to make a couple of Fly cutters to replace the ones that went missing.

One thing I just can’t get to grips with is why the slot for the cutter bit (HSS or whatever) is off set so that the cutter’s edge (the part that does the cutting) lines up with the centre line.

Seems most of the ones I have seen are like this.

Any comments?

 

[Holt]

One thing I just can’t get to grips with is why the slot for the cutter bit (HSS or whatever) is off set so that the cutter’s edge (the part that does the cutting) lines up with the centre line.

Seems like you answered your own question, in that way you wont have negative cut angle.

 

[DaveH]

Hi Holt,

You perhaps couldn't enlighten me a bit more.
I just don't see it.

Dave

 

[tel]

What's not to see Dave? The slot is offset to put the cutting edge on the diametric centre line of the tool, rather than offset one way or the other. Picture a boring bar!

 

[Holt]

It is like the center height of a lathe

 

[DaveH]

Hi Tel,

I'm old and blind

If I cut the slot equi-spaced about the centre line. What happens?

The cutter is in front of the centre line, does it make any difference?

The two I made were off set - I off set it because at the time is was the "way to go"

Now older and wiser I'm not so sure, but I no idea why I am not so sure.

Dave

 

[tel]

I'm old and blind Sad

Ain't we all?

Cutting a central slot would probably work OK but you would have to grind more off the tool bit to get a viable shape.

 

[Holt]

You can get away with making the hole in the center of the flycutter if you make a cutter bit like this

Works really well on both surfaces and in holes

 

[DaveH]

Ok I understand, the cutter, hence the cutting should take place at the diametric centre line. ;D

But why?

If the cutting is taking place in front of the diametric centre line, I can't see why this may not work. However there may be a reason not to do it.

Dave

 

[Lakc]

Both the angle of attack between the cutter and workpiece change, as well as the diameter swept for the amount outsticken (new technical term)

 

[DaveH]

lakc,

Yes I can see that, but is it badder or gooder?

Dave

 

[tel]

Can't say I've ever thought it through (and my brain hurts too much at this hour of the morning to do so) - might be a case of make one and see wot it does.

 

[Holt]

If you make the cutter in the way you describe, there would be a radical change in the cutting angle with different overhang, and you simply don't know how your cutter behaves when you try a new overhang, like the lathe, if your center height ain't right it wont matter as much with a large diameter as it would with a small diameter

 

[Troutsqueezer]

You want the cutting edge be perpendicular to the surface it's cutting as much as possible. If you offset the slot, picture the cutting action as being analogous to the grader blade on a tractor which is set to an angle. As it goes along the road, it pushes the dirt from one side to the other according to the angle. Seems like you would have a varying rake angle along the cutting edge.

My two cents, ok, maybe one cent.

 

[Ken I]

Sure it changes the tool geometry - but do you care ?
My flycutter is central and since I do most of my flycutting A'la facemill the negative influence of the "offset" is effectively side rake.

I normally use a L.H. cranked 45° CT turning tool for this purpose.

If you are cutting in a bore then its a negative influence on top rake.

Can't say that the geometry has been much of an issue for me.

Regards,
Ken

 

[bambuko]

One picture is worth thousand words:

Hopefully self-explanatory?

Chris

ps you can put the cutter in the middle, just have to adjust tool angles to suit

 

[DaveH]

Dennis,

I was thinking along the same lines as you, in that it may have a "slicing action"

Now I don't know if this would be good or bad.

Dave

 

[DaveH]

KenI,

Your right - I don't care.

Any chance of a pic of yours.

Dave

 

[DaveH]

Chris,

Thanks, you're right says it all.

Dave

 

[Troutsqueezer]

Chris, I thought about it from that angle too. The only thing is, the cutting action is on the side of the bit, not on the end.