Fly cutter face angles

There are loads of diagrams showing how to grind lathe and milling cutters. However, the tool in a fly cutter is held at a (fixed) angle from the horizontal. The angle specified for the bottom face always seems to ignore the tool mounting. Is this because all fly cutters have the same mounting angle?

Funnily enough, because the work has a flat surface, it's very convenient to see and measure the actual bottom clearance so why isn't that angle quoted (in my experience)?

I have always found that the face angles for a fly cutter are actually easier to appreciate and make more sense than angles on other cutting tools.

I find the biggest problem is to appreciate that it is cutting sideways and not downwards (in a mill). It seems to be easier to think of lathe tools so I describe it as a lathe tool held for normal longitudinal turning but ground to take a facing cut.
However if you make a flycutter block that is a disc with a hole parallel to the axis or angled for a round tool bit (instead of the angled slot type block) the equivalent tool grind is as for a facing cutter.

I don't think it's that critical unless you have a massively different angle seating to the holder, main thing is to make sure nothing rubs particularly as the swung diameter reduces much like you do with a boring bar.

I used various lathe tools, round button tools, commercial ground flycutter bits and many home ground ones and all work

This is my go to fly cutter !

As far as the cutter is concerned it is treated as though it was a left hand lathe tool. Its on a 20 mm shaft and the disc is 20 mm thick. Depending upon material and spindle speed I can hog mild steel off with a 0.5 mm DOC, 1 mm in aluminium. I recently tried a carbide cutter and whilst I could up the spindle speed the surface finish was little different from the HSS cutter I use.

I won't go into the pros of this cutter unless anybody asks.

I did wonder about that. They don't seem to sell them, though. But, go ahead; wax lyrical.

I don't generally use flycutters for flat surfaces, but I use a homemade one for curved surfaces:

I've never thought about angles; I just grind as per a lathe tool, in all cases, and it seems to work fine.

Andrew

Hi Guys,

No I've never seen ones like mine for sale either ! Probably because they are too easy to make, primarily a simple turning job, with a press fitted shaft, and with only a single tapped hole for an M6 grub screw and a simple drilled hole for the cutting bit. Obviously the shaft and the disc have to be square to each other.

After turning the shaft and pressing it in, I faced the disc whilst holding the shaft in the lathe chuck. This ensures that there will not be any wobble and the whole lot will be concentric. The hole for the tool bit was drilled and the grubscrew hole drilled and tapped before pressing the shaft in place.

I use short pieces of 1/4" inch square HSS tool steel, though I did have a small piece of 8 mm round carbide rod that I put a cutting edge onto and tried.

The original idea was to take advantage of the flywheel effect and making the shaft and disc out of 20 mm thick material offers both rigidity and balance ! I can spin this fly cutter as fast as the mill will go, just over 2650 rpm without any detectable vibration, which you can't do with the conventional angled cutter types.

It also reduces the edge hammer that you get with narrow work and large DOC.

In use I tend to run at around 250/300 rpm and 20 thou DOC in steel. The actual cutting face has the same rounded corner as a lathe tool but the cutting face is about 2 mm wide. I do this so that it reduces the tram lines that you get if the feed rate moves the work forward before the next cutter sweep.

In the picture the cutter has a round edge, I no longer use this shape ! While it works it produces a high spindle loading and looses its edge very rapidly. A more left hand lathe tool shape is much better and actually easier to sharpen.

Unless I'm losing it (always possible) I think you have a normal right hand tool profile in John's setup.

Added this to my list of to-do jobs

Hi Duncan, Thankyou for your comments.

I called this a left hand cutter because if it were in a lathe looking down at it, the tool cutting edge would be on the left side !

But I'm probably wrong to call it that, after going and looking at some lathe tool pictures. It seems that handedness is as viewed from the front rather than down from the top. But I can't go back and edit my post to correct it.

I have sometimes wondered why RH lathe tools (ie for working to the left towards the headstock) are so called.

Apart from the suggestion by John Baron, it occurs to me that a woodturner cutting to the left would typically be using his right hand as the dominant hand to control the tool; could this be the origin of the conventional nomenclature?

I don't think it matters if the disc is at an angle. As the tool rotates it doesn't move vertically or horizontally relative to the disc, and so it's height above the work doesn't change either. Suppose the disc was canted over by 45° and the tool was perpendicular to the disc. Now take the disc away and we have the tool in the position it would be in a conventional flycutter, ie, at an angle to the axis.

The important constraint, apart from the spindle axis being perpendicular to the table, is that the axis of the flycutter is parallel to the spindle axis. Concentric is good too, but not essential.

Andrew

It’s called a right hand tool because it cuts from the right, and leaves a shoulder on the right of the workpiece ( when in the machine)

Yes the spindle axis being perpendicular to the table is important to achieve a flat machined surface but the axis of the flycutter does not have to be parallel to the spindle axis to achieve good results.

Tony

You often get a better surface finish if the spindle axis is ever so slightly not square to the table, as the cutter is then clear on the second sweep. However, this then produces a very slightly concave surface if the job is of any width. You pays your money.......