# Fly cutter



## DaveH (Mar 14, 2011)

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?


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## Holt (Mar 14, 2011)

DaveH  said:
			
		

> 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.


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## DaveH (Mar 14, 2011)

Hi Holt,

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

Dave


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## tel (Mar 14, 2011)

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!


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## Holt (Mar 14, 2011)

It is like the center height of a lathe


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## DaveH (Mar 14, 2011)

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


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## tel (Mar 14, 2011)

> 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.


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## Holt (Mar 14, 2011)

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


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## DaveH (Mar 14, 2011)

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


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## Lakc (Mar 14, 2011)

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


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## DaveH (Mar 14, 2011)

lakc,

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

Dave


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## tel (Mar 14, 2011)

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.


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## Holt (Mar 14, 2011)

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


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## Troutsqueezer (Mar 14, 2011)

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.


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## Ken I (Mar 14, 2011)

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


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## bambuko (Mar 14, 2011)

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


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## DaveH (Mar 14, 2011)

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


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## DaveH (Mar 14, 2011)

KenI,

Your right - I don't care.

Any chance of a pic of yours.

Dave


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## DaveH (Mar 14, 2011)

Chris,

Thanks, you're right says it all.

Dave


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## Troutsqueezer (Mar 14, 2011)

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.


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## DaveH (Mar 14, 2011)

Dennis,

Do you think it will work being in the centre? the slot that is.

Dave


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## Troutsqueezer (Mar 14, 2011)

Dave, I'm the furthest thing from being the expert on this forum. :shrug: Your question intrigued me so I did earlier what Chris did and drew a quick reference drawing in AutoCAD. I offset the bit and rotated it about the center axis and watched how it behaved. Having just adjusted the grader on my tractor yesterday, I immediately saw the parallel. 

I'm guessing it would work. But is it optimal, for heat, rake angles and such? Probably not. 

Is it harder to make one with the slot offset? Is that what brought it up? You know, those fly cutters are dirt cheap at Grizzly's although I can understand if you feel like making one.


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## DaveH (Mar 14, 2011)

Dennis,

If I explain a bit might help to see where I was coming from.

As an apprentice I had to make a fly cutter, probably as every apprentice did at that time.
Fly cutters were widely used in the company I worked for, I have seen many weird and wonderful ones. However over the last 20 years or so their use has declined. 

I quite like them especially for facing, a lot of people in the home workshop don&#8217;t like them, and that is because they don&#8217;t use them correctly. ( I am not being rude it just tends to be the case).

Now some of you may laugh at this, I thought they offset the cutter, because they mill a bit off the side for the &#8220;holding screws&#8221; and by off setting the slot helped with the balance. :big:

Now like I say, older and wiser, but fly cutters especially in the home workshop have an inherent problem &#8211; that of balance. This out of balance causes vibrations, always a problem in the home workshop milling machine, so I thought &#8211; slot in the middle and no cutting a bit off the side. With a little bit of luck it may be better balanced. ;D 

That is why I was asking about a centre slot.

Any more thoughts, comments, good, bad, indifferent? would still be usefull from anyone.


Just like to say I thank you to everyone for the help so far. :bow:


Dave


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## Ken I (Mar 14, 2011)

Dave,
    No problem - here's a picture and a drawing.

Like I said the geometry is not really a problem as long as you are aware of it and make allowances where required - which is again not an issue when used as a flycutter - this is not a boring head - that's a completely different animal.

Regards,
      Ken


View attachment Dump.dwg


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## DaveH (Mar 14, 2011)

Ken,

You sure have a big'un there, mine will be a lot smaller about half.

That is exactly how I was thinking to make it - even with the back over hang of the tool because again I think that helps with the balance.

Now the only thing I need to sort out should I leave it round, or cut a bit off each side like yours.

Thanks for that, really helpful and usefull. :bow: :bow: :bow:

Dave


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## Ken I (Mar 14, 2011)

Dave,
    Just a caution - the smaller you go, the worse the geometry problems.

The offset is not really a ballance issue if the bulk of the "slot" is filled with a piece of toolsteel in any case.

For use with HSS bits it is quite common to machine the slot at an angle for clearance - then you have to remove materal from the opposite side to restore some degree of ballance.

That is what I would have done had I intended to use anything other than the off the shelf cranked tool in the photo.

Regards,
      Ken


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## DaveH (Mar 14, 2011)

Ken,

I do agree with your points. Thm:

I'm not going to slope the bottom of the fly cutter either, I really see no point in doing that, only serves to upset the balance. 
Most commercial fly cutters seem to do it, I'm sure they have just copied each other. 

I'm going to be even more sneaky and use something with an indexable insert cutter. ;D

I can but try, the worst that can happen it won't work - won't be the first or last time.

If it doesn't work I can blame you. :hDe:

Just joking Ken ;D

Thanks a lot of useful info.


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## Ken I (Mar 14, 2011)

Boer maak 'n plan !


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## Maryak (Mar 14, 2011)

Just to be different.................

All my flycutters, (all 2 of them), have a hole drilled at an angle on the major diameter of the boss to take round HSS, (3/16" & 3/8") tool steel.

IMHO for fly cutting a flat surface it makes no difference offset or not. It's the tool tip which needs the correct clearances not the holder.

Hope this helps

Best Regards
Bob


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## DaveH (Mar 14, 2011)

Bob,



			
				Maryak  said:
			
		

> IMHO for fly cutting a flat surface it makes no difference offset or not. It's the tool tip which needs the correct clearances not the holder.
> Hope this helps
> Best Regards
> Bob



I would agree with that ;D

I just don't want a holder with a built in wobble :big:

Thanks - Its 2:20 in the morning here, so I'm off to bed.

Best wishes
Dave


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## Tin Falcon (Mar 14, 2011)

products are made of a common design for a reason.sometime the reason is ease of manufacturing. sometimes aesthetics. sometime to keep the cost down. and sometimes just because the design performs.wheels are round for a reason . pneumatic tires are filled with air and made of reinforced rubber for a reason . 
To me redesigning a tried and true design like a basic fly cutter is akin to redesigning the wheel.go with what has been proven to work.
JMHO
Tin


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## DaveH (Mar 15, 2011)

Hello Tin,


			
				Tin Falcon  said:
			
		

> products are made of a common design for a reason.sometime the reason is ease of manufacturing. sometimes aesthetics. sometime to keep the cost down. and sometimes just because the design performs.wheels are round for a reason . pneumatic tires are filled with air and made of reinforced rubber for a reason .



Yes you are correct, and I don't have a problem with wheels being round, or pneumatic tyres filled with air and made of reinforced rubber.



			
				Tin Falcon  said:
			
		

> To me redesigning a tried and true design like a basic fly cutter is akin to redesigning the wheel.go with what has been proven to work.
> JMHO
> Tin



Now this is just down right argumentative what did I do to deserve this.

 I am not really sure what a basic design is but if it is the like ones I have seen, they have a slopping bottom and a piece cut out of the side seemingly to make the drilling and tapping easier.

The slopping bottom seems to me serves no good purpose except to cause some out of balance.
Then just to make sure it is out balance a recess is put in the side.

I thought this post was reasonable and genuine request for information. I know I was wrong in the past but I honestly thought this was sensible and acceptable post.

I have no idea what I have done to upset you, the management or whoever. I am sorry.

I just don&#8217;t know what I should post whatever I do is wrong so I think it is best I resign.

I wish you all well.

Bye

DaveH


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## Ken I (Mar 15, 2011)

Dave,
    I think you are overreacting - Tin often makes comments that state the obvious and I am sure are well meant reminders of sticking to the tried and true.

I would not read too much into it or take offence.

Luister ou siel, as jy te laat gaan slaap en te vroeg uit die verkeerde kant van die bed uitklim, dink voor jy iets onverantwoordelik doen.

Beste wense,
         Ken


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## bambuko (Mar 15, 2011)

Dave, 
You have asked perfectly legitimate question about how and why something works.
Some of us enjoyed trying to help you to understand.
Some stated "bloody obvious", not adding much to the discussion... ;D
It's in the nature of forums (and interweb), that by posting your questions and comments you leave yourself exposed to comments from just about anybody who has time and inclination.
Some of it is not welcome, but the only way to deal with it is by using "ignore button" :toilet:

Chris


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## Lew Hartswick (Mar 15, 2011)

I think the first question that should have been ask is: Do you understand the term 
"rake" as applied to a cutter. If you do then it should be clear that if the cutting edge 
is off center in the direction you propose it will cause NEGATIVE rake. Not a good 
thing for MOST cutting. 
  ...lew...


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## Troutsqueezer (Mar 15, 2011)

Dave, don't fall victim to the Internet Lack-of-Face-Contact Syndrome. It's very easy to misread intended tones of forum postings. You haven't been here long enough to gauge the personalities of most forum members but in time you will see that 99% of us are easy going, congenial old farts. There are some young farts too. ;D


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