Fly cutting

[larry phelan 1Participant @larryphelan1]

Good morning fellow metalbashers,

I have a question regarding flycutting which I do not understand.

I have been given a number of 10mm plates,150×150, some a little bigger, with quite a lot of surface rust and some pitting, but quite good underneath. When cleaning them up with a flycutter, I find that the surface does not end up flat all over. There tends to be ridges and hollows across the surface, even when I tried to overlap the sweeps of the cutter. I ,m sure there is a simple reason for this and someone knows it. Pray share !

I did clean off most of the heavy rust with a wire brush and a flap disc,out of respect for the cuter, so what,s wrong ?

Not sure what I will use the plates for, but too good to scrap them , and there are more to be had !

🔇

[JasonBModerator @jasonb]

Are the ridges and hollows circular eg the shape that the cutter is swinging. Or are they at the edge of where cuts overlap and therefore parallel to the direction of feed?

[larry phelan 1Participant @larryphelan1]

Good morning Jason,the ridges are along the line of the feed.

[larry phelan 1Participant @larryphelan1]

Good morning Jason, not sure if I got my reply right. The ridges are along the line of the cut, in the direction of feed.

[Martin of WickParticipant @martinofwick]

You can only take light cuts with a FC. Assuming tool is good and mounted ok and non active slides are locked off, seems like the feed is too high in relation to cutter speed (and or cut too deep).

I dont know your set up or what you are machining but say if at .005in depth of cut you aim to take off 0,0015 to 0.002in each pass of the cutter and your max calculated cutter rpm is 100 your feed rate would be about 1/4 in min.  Raise rpm to surface speed for material in question and adjust feed accordingly, if you can go at 500 rpm then the feed rate can increase to 1 in/min for the above example.

[Martin of WickParticipant @martinofwick]

I may have misunderstood your question. If the ridges are at right angles to the sweep of the cutter, then it is likely that the workpiece is not normal to the axis of the cutter sweep, ie is your mill trammed correctly? The wider sweep of a fly cutter will show up any errors much more that a smaller diameter milling cutter.

Always useful to post a photo

[larry phelan 1Participant @larryphelan1]

Thanks for that Martin, I can go at 500 rpm or more if required.

Perhaps my feed rate is wrong, so will try different rates.

Dept of cut was very little, keeping in mind the state of the surface.

[Martin of WickParticipant @martinofwick]

Is that 500 rpm based on the calculated surface speed for the diameter of the cutter vs material  concerned, or just the minimum speed of your machine?

With fly cutting, you need to respect the fundamentals of feed and speed calculations. If using HSS cutter the surface speed for free cutting steel maybe 25m/min. As you probably don’t know the type steel, you may have to vary around calculated values until you get the best result. A sharp carbide cutter can do about 3 x HSS rate on a hobby mill.

Other things you might ned to consider – the way the work piece is held ie clamped in a vice and non parallel to the machine table or grot and rust interfering with the mounting or is the work piece itself deformed possibly not parallel?

[Martin of WickParticipant @martinofwick]

calculate rpm from (1000xSurface speed m/min) divided by (Pi x cutter diameter mm)

ditto feed (imperial inches min)  from rpm x no cutter teeth x tooth load in thou inch eg 500x1x.002

Sorry about the mixed units, just the way my brain is wired!

[JasonBModerator @jasonb]

If they are in the direction of feed and at the edge of the overlap then I would say you have a tram problem, if feeding with X axis then it is Nod. The column leaning backwards or forwards.

 

If the ridges were curved then that would be more likely the cuter loosing it’s edge due to too fast or mill scale taking the edge off if it is hot rolled plate then the blunt cutter tends to ride over the work or push the unsupporte dmiddle down.

As for speed with an HSS cutter swinging say 80mm dia you want 100rpm to get Martin’s 25m/min and a snails pace feed to go with it. Changing to a solid or insert carbide cutter then you could run faster. I tend to run a bit faster with a HSS and cobalt cutter as it is hard to hand feed any slower, this if it starts half way through is 300rpm with an 80mm swing dia.

 

[larry phelan 1Participant @larryphelan1]

Thanks very much to both Jason and Martin.  A number of good points there to check out.Just for the record, my mill is a Chester Lux, so if the column is tilted I may need to shim it, not a handy job !

I will need to study the speeds and feeds ect since I did not pay too much attention to them. Thanks again for the feedback, such a lot to learn.

[bernard towersParticipant @bernardtowers37738]

dont forget if you have only machined 1 side it could be stress related

[Howard LewisParticipant @howardlewis46836]

Two thoughts

1) Feed Rate.

A Flycutter is effectively a one tooth end mill, so the feed per rev should not be more than 0.002″  (0.050mm)

2) Tool Form.

(Assumes that the tool has been ground with the right clearance angles, say, 5 to 10 degrees, and is running atb the cirrect surface speed. If in doubt, go a bit slower.)

It would be beneficial for surface finish to have a small radius on the nose of the tool, so that it cuts a series of radii.

With the correct feed, each radius will cut away the ridge left by the previous rotation.

HTH

Howard

[JasonBModerator @jasonb]

Alternatively ditch the flycutter and treat yourself to a 5 or six insert face/shell mill and some non ferrous inserts. Not only will the carbide allow you to run the spindle much faster but the increased number of cutting edges will allow a faster feed for the same given chip load.

So instead of plodding along at a feed of 25mm per min which will take an age to clean up your 150mm square plates assuming it will need three overlapping passes. Go with the shell mill and feed at upto 450mm per min as per the video half way down this article.

[larry phelan 1Participant @larryphelan1]

I dont need to machine faster, just better.

If the column is tilted forward, as Jason suggested, I dont see how a shell mill would make any difference, since it would still have to make more than one pass.. when the workpiece is small enough for the flycutter cover in one pass, the result is good, no problems.

I will try inserting a few shims, since I have some 1 thou and 2 thou strips. Nothing to lose, might even improve !.

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