Poor finish using a flycutter

[GreensandsParticipant @greensands]

Using a flycutter in the milling machine I have often noticed that the finish of the cut once the extent of travel along the X direction exceeds the diameter of the ‘fly’ can be ruined due to the cutter making contact with the workpiece on what might be called the return stroke. When this happens, is it an indication that the spindle has not been set truly square with the milling table? It is very frustrating to see a perfectly good finish be spoilt in this way.

 

[JasonBModerator @jasonb]

Try feeding in the opposite direction then if your tram is a bit off the lower edge will lead and the trailing edge nor recut the surface.

A blunt or rubbing tool getting pushed up off the work can also cause the trailing edge to cut as the leading one completes the cut.

[Graham MeekParticipant @grahammeek88282]

When using a sharp tool and a correctly set milling head. A very slight or hardly perceptible back cut indicates the surface is perfectly flat. No back cut indicates the surface is ever so slightly concave. A back cut which actually takes metal off is again producing a concave surface. While the initial passing cut in this instance was slightly convexed.

Whether in the grand scheme of things this does not matter. There might come a day when a dead flat surface is desirable and it is nice to know how to achieve this.

Regards

Gray,

[Nigel Graham 2Participant @nigelgraham2]

I would expect a faint trace of back-cut as that removes the microscopic ridges between in each sweep in the forwards direction.

Too much though, and I would suspect the mill slighly out of vertical or the tool itself not cutting properly so perhaps springing.

I don’t use fly-cutting much, but try to cover the entire surface in the full travel.

[old martParticipant @oldmart]

I made an extra for the flycutter that I made from a 20mm boring tool which takes WNMG08 and WNGT08 inserts. It is to swing an indicator for tramming the head as well as possible. I don’t have a picture of the indicator being used at the moment.e.

[PeteParticipant @pete41194]

What type / size of mill and the usual depths of cut your taking with your fly cutter Greensands? HSS or carbide tool tips, and how are you sharpening them if it’s HSS? Are you also using cutting oil covering your part on the last cut? Sometimes that can improve the surface finish by a lot with aluminum or steel even during that back cutting.

As usual Gray and others are 100% correct. To expand a bit on what’s been mentioned, any cutting process such as fly cutting is going to cause deflection within the tool and the machine it’s being done on. In some areas and depending on there location, the amount may be immeasurable during operation with what most of us might have available. There still present though.

For example, I once spent an inordinate amount of time and effort with a .0001″ reading DTI tramming my Bridgeport clone head into as perfect head alignment to the table as I could manage over roughly a 12″ distance in the X axis and not quite 9″ in Y. Any of my fly cutters would still show a visual trace of back cutting. Very little and I’d guess less than .0002″.

Using a bit of logic after would indicate due to that machine and any cutting tool deflection. Or Newton’s Third Law. “For every action (force) there’s an equal and opposite reaction”. With a fly cutter and it’s usual extended cutting tool tip, a possible slight bending backwards away from the direction of cutting and upwards of the tool shank just like Nigel’s mention of the tool springing. Plus the tight but still required clearances within the spindle bearings are likely causing the most deflection. As the tool tip exits the cut, those forces or cutting loads are relaxed and allow the tool edge to lower a tiny fraction until the rear of the tools cutting edge advances enough during it’s revolution to start that visual back cutting.

And any mill head tramming no matter how perfectly accurate it’s done is still only a static condition for alignment. It doesn’t and can’t account for what happens during those dynamic loads as the cutting starts and progresses. The Turret, ram, head and motor assembly on my mill would probably weigh over 300 lbs. / 135 kilos. I’ve tried checking it’s head deflection a few times with an indicator tip against the side and then rear of the motor. Depending on the tool diameter, depth of cut, and feed rate, it’s easy to see upwards of .005″-.008″ or more of movement. Larger diameter drilling does the same, but that’s always forcing the head and spindle to lean towards the rear in the nod direction a bit.

With something like a fly cutter it would of course be less at the cutting tool tip that’s much closer to the heads center of rotation for tilt and nod though. That’s why for the best accuracy, a light finishing cut to final size are always recommended. A few basic indicator checks against what we might think can’t or doesn’t happen can sometimes be educational.

For any mill most of us have the room and available funds for. I’d suggest that back cutting where a fly cutter is being used is inevitable and should be expected. Off setting the tram slightly to prevent that while fly cutting may work if the partially dished surface can also be tolerated. But every other cutting tool being used is then angled at least  slightly to the table surface as well. Depending on what your mill is usually used for and what your projects happen to be, that might have differences of opinion about the negative’s involved.

An overly pointed or dull tip on your fly cutter tip may also be affecting the surface finish. And Jason’s thought about trying both directions to see which might be better is a good one. Unfortunately there no longer in production, but I bought a specialty fly cutter that uses the round RCMT replaceable carbide tips, mine have a inside bevel towards the center mounting hole inside the carbide tip, and a further but slight back lean in the tool shanks tip pocket making them very positive cutting. While it still back cuts a tiny amount, the cutting action is almost always smooth and even.

It’s design is efficient in cost as well since that round tip can be rotated a bit each time it dulls, or the few times it’s rarely chipped a cutting edge. With the normally light depths of cut a fly cutter is designed to do, a larger radius on the tool tip doesn’t hurt as long as there are no other protrusions or features on the part. Although and depending on edge sharpness, that extra radius might or could increase the back cutting effect as well.

I can say the tiny little round column Emco mill I started with, then the Sieg X2 mill was a bit less, but both always showed a far larger amount of back cutting than my Bridgeport copy does. So that extra mass and much more rigidity also makes a difference. A lot of what happens during cutting and those inevitable machine deflections does depend on your mill design and it’s mass as well.

 

[JasonBModerator @jasonb]

Couple of examples of the pattern produced when the trailing cut intersects with the leading cut that Graham mentions both on lightweight flexible far eastern machines.

[larry phelan 1Participant @larryphelan1]

Try tilting the head, you might be surprised !

[Pete]

On Pete Said:

Unfortunately there no longer in production, but I bought a specialty fly cutter that uses the round RCMT replaceable carbide tips, mine have a inside bevel towards the center mounting hole inside the carbide tip, and a further but slight back lean in the tool shanks tip pocket making them very positive cutting. While it still back cuts a tiny amount, the cutting action is almost always smooth and even.

It’s design is efficient in cost as well since that round tip can be rotated a bit each time it dulls, or the few times it’s rarely chipped a cutting edge.

I made two of these for my fly cutter. One takes a polished RCGT0602 insert for cutting Aluminium Alloy and the other uses a larger insert for steel.

[Howard LewisParticipant @howardlewis46836]

The higher the feed rate, the greater the deflections of tool machine and workpiece.

A flycutter is effectively a single tooth end mill, so any feed greater than 0.002″ per rev could be considered excessive. Moulded inserts are not sharp, as we interpret a freshly ground HSS tool to be.

Ground inserts are sharper, so are likely to give a better surface finish.

Finishing cuts should be shallow with a fine feed rate, even with carbide inserts, probably with higher speeds to suit the carbide.

[VicParticipant @vic]

I have on one or two occasions desired a shall we say, less than smooth finish on alloy parts, and achieved this by feeding the workpiece under the cutter and an increased rate. The result can be most pleasant. Sadly I don’t have a picture to hand.

[old martParticipant @oldmart]

I remembered to take a picture of the fly cutter arm which holds the lever type indicator for checking the tram.

 

 

[PeteParticipant @pete41194]

I don’t know the optical reasons that create it Jason, but your first picture shows a very slight hint of a rainbow coloring or shading. It doesn’t always show up, and the surface seems to need to be angled just right to the light source. But I’ve seen the same a few times when the surface is extremely flat after fly cutting with a very fine and consistent feed rate.

Thanks Vic, those mostly intended for aluminum lapped and polished inserts have been something on my list to try with my own fly cutter. I thought they might, but nice to hear it confirmed they do make a noticeable improvement even with a fly cutting tool.

Fwiw, the one I bought was made by a North American company called New Dimension I think? Unfortunately there’s no brand name on mine and it was bought long enough ago I’m not fully confident I even got the name correct. Not important since they seem to have disappeared about 15- 20 years ago anyway.

But or in case it’s of any use, the measurement dimensions I just did on mine. It seems to have what appears to be a pressed in 5/8ths dia. tool shank, 2-1/2″ diameter main body diameter that’s 1-1/2″ long with two flats created by removing 1/4″ of material from each side to keep it balanced. And with the usual 3 set screws holding it’s 1/2″ square tool shank in place. That insert holding tool shank is 4.925″ long on mine.

Unlike most fly cutter designs, the body is slotted horizontal for the insert tool holder and not angled down towards the work. I also should have mentioned that insert holding tool shank is double ended. The one end takes those RCMT inserts, and the other is set up to accept the usual TCMT triangular inserts with a center mounting hole. If I was wanting to use inserts on any fly cutter and since my version doesn’t seem to be available any more. I’d definitely copy there design even if I had to reduce it in size a bit for smaller mills since there’s also RCMT inserts that are as small as 6 mm in diameter instead of the much larger mine uses.

And I’d want to have the ability to use the two different shapes of inserts either by having it dual ended like mine, or probably better, just making up a second insert holding tool shank. I’ve even used mine a few times with home ground 1/2″ square HSS to do form milling. There’s really nothing on mine that couldn’t be duplicated almost as well by anyone wanting to make there own using my imperial or converting to metric dimensions, or even reduced to more suitably sized for smaller mills.

[VicParticipant @vic]

I just used a piece of mild steel to mount the insert. There’s no relief needed (but you can add some extra if you want) as it’s built into the insert. I just relieved the back of the bar a little.

[JasonBModerator @jasonb]

I used to use my flycutters quite a lot to square up sawn stock ready to be carved into something else but since getting some face mills the flycutters have hardly come out of the draw. I can do a similar cut in 1/20th the time that a HSS flycutter would and get a good finish including the rainbows with the non ferrous inserts

[PeteParticipant @pete41194]

My insert pocket is around 25% of the insert depth. I can’t say the angle they cut the pocket to lean the insert back a bit more does or doesn’t make much difference. Without setting my insert holder up on something like a sine bar, it’s difficult to estimate by eye how much that angle is. Maybe a further 2-3 degrees back at most. But if yours already works fine, I sure wouldn’t bother changing anything.

[bernard towersParticipant @bernardtowers37738]

The word Shaper comes to mind

[GreensandsParticipant @greensands]

Following Jason’s views on the use of a facemill in preference to the use of a flycutter for squaring up  sawn stock has got me thinking that this might be the way ahead for me. Initial searches for a Morse No. 2 face mill return a number of relatively expensive items but before taking any decisions I would be interested to receive any recommendations from users given that I have a bench mill with a 300mm travel in the X direction and 150mm in Y

[JasonBModerator @jasonb]

What sort of diameter are you swinging with your current flycutter?

With the small X travel them maybe

 

Holder

Facacemill

Inserts

 

You might want to look at these two articles I wrote

 

Part 1 and particularly Part 2

[DiogenesParticipant @diogenes]

Should that be a link to 1135 inserts?

[JasonBModerator @jasonb]

Thanks, I’ve altered it.

[VicParticipant @vic]

This is an interesting article on Flycutters.

https://tormach.com/articles/science-face-milling-fly-cutter?srsltid=AfmBOoof-L-JPQSlz024pQQ1JHiZH-epG1GC4j5YNtY6KpltAFfyGSO0

[JasonBModerator @jasonb]

Couple of things about that article,

Opening statement. Some people may well think of facemills being the big one they show. But if you look at the link I posted to a suitable facemill for Greensands small machine you can see that I at least don’t think of them as that big. Smaller diameter means less inserts cutting at any time so less power needed and smaller diameter means better mechanical advantage for a small machine. That superfly is certainly not swinging the same radius as the big facemill.

As they say a flycutter is slower due to only having one cutting edge so for the same given chip load the feed needs to be slower than a facemill. This is OK of their CNC machines where the feed is powered but it is not easy to feed at a constant slow speed by hand. They also have faster spindles than a lot of manual machines which again will mean feed rates have to be reduced further on manual machines unless fitted with power feed.

I don’t get the bit about insert facemills being expensive. Their Sperfly also uses a milling insert. Once again for the same chip load and piece of material a single insert will have to remove 5 times as much material as each insert in a facemill that takes 5 inserts. Common sense should tell you that the single insert will wear at 5 times the rate of the flycutter inserts so you would need to replace it 5 times more often. So cost is equal.

As for running out of insets and waiting for replacements. That is simply down to the user not thinking ahead. If you can’t plan to have a replacement set to hand then you are just as likely not to have the single one for the flycutter to hand either. Is that really even a problem apart from sales talk as you can always run a facemill with less inserts or you can dig your old flycutter out and run with HSS or a lathe tool.

[Graham MeekParticipant @grahammeek88282]

I use 40mm diameter HSS face mills on the Proxxon mill to square up stock, insert cutters do not do well on this machine.

I only ever use a fly cutter to sweep a large surface that needs to be flat and with no steps in the machined surface.

Regards

Gray,

[VicParticipant @vic]

Not included on the positives of a Fly Cutter on this list is that I can make a finishing cut on a fairly large workpiece in one pass. Something not possible with a face mill that most of us on here are likely able to run on our modest size machines? I’d quite like say a 50mm face mill but at over £50 plus regular insert replacements I can’t justify the cost. I’m a hobbyist so if a job takes a few more minutes using a flycutter it’s no an issue for me. At least it hasn’t been over the past 20 odd years.

Fly Cutting Explained: The Fly Cutter as a Face Milling Tool

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