The cutting tool on fly cutter is giving me problems.
|andrew lyner||24/11/2021 22:54:45|
|254 forum posts|
I have been hunting for definitive information about how to grind a fly cutter tool. The cutters I want to make are for a mini mill, which has more flex in it than a 'proper' mill, so things may be different.
The recommendation is to put a radius on the tip of the tool - like a lathe tool. That makes sense but then what happens to the tool when it's passing 'round the back'? The patterns we see of good finishes have that moiré pattern which has all lost equal contributions from cuts in both directions. That implies there must be something left for the tool to cut after the first pass.
Is this just because of the flexing of the machine?
Either way, I should have thought that the tool should not ideally look like a Lathe tool at all but it should have a profile that's an arc that's symmetrical fore and aft. All the arguments for a radius at the front should apply to the rear ofd the tool.
Are all the fly cutters that we see for sale actually not optimal? Is that 20 degree slope what we want, actually?
|Martin Connelly||24/11/2021 23:53:21|
2181 forum posts
For general facing I use a small insert tool using Ø6 buttons. Usually called profiling cutters. The inserts have a sharp edge and can be rotated to a fresh position to use all of the periphery. I think the holder is 8mm square. Intermittent cutting does not seem to be detrimental to the insert but it is usually only used for finishing rather than taking off large thicknesses where a normal milling cutter makes more sense.
23050 forum posts
The "double cut" is often a result of the mill being slightly out of tram, you will soon find which feed direction on your mill either cuts on the return or only cuts on the leading edge. Depending on the look you want feed accordingly. If you get a machine trammed well you get the holographic looking rings.
To some extent lack of rigidity, blunt tool, poor geometry that rubs, etc will also cause the tool to take a second lighter cut on the trailing edge.
A tool without a radius will load the machine less and give an acceptable finish, the larger engagement of a radiused tool can load the machine more but often gives a more mirrored finish.
One of the hardest thing sis keeping a constant feed at the slow revs that an HSS cutter needs to run at so a left hand insert tool can help by allowing a higher spindle speed and therefore feed rate for the same cut per tooth.
I know of a book that has a whole page showing how to grind a flycutting bit that will work on a benchtop mill like this
|Martin Connelly||25/11/2021 08:27:10|
2181 forum posts
You can see my profile tool being used to flycut in a post in this thread. Machining acetal thread
The banging noise at the beginning is the spline drive backlash being taken up at first contact of each rotation.
|David George 1||25/11/2021 08:34:10|
1873 forum posts
This is my favourite fly cutter. A piece of silver steel turned down shank then cross drilled for a piece of tool steel or solid tungsten carbide and a grub screw to hold the cutter. You can regrind the cutting edge to suit the material to be cut like a small radius for aluminium or cast iron but sharper for harder steels. It is made from a 1 inch diamiter bar and you car also use it for a boring bar in a pinch.
|andrew lyner||25/11/2021 13:54:49|
|254 forum posts|
I did wonder about of tram but the effect works in both directions so the tram can't be too bad. As Boadicea said when she drove her chariot in reverse "It cuts both ways". When only small parts are involved, you can avoid the back part of the rotation hitting the work piece - that always gives a better look and feel. But some things are just too big for that.
But my basic question is about symmetry or asymmetry for the cutter. The mention of 'buttons' suggests that the regular shape is not the best. But most of the world seems to be doing it the regular way. I have tried grinding the bottom of a tool to give a radius back up the other side but the HSS piece I had was sort so everything was tight up against the holder. I have some longer HSS square section now so I can experiment soon.
Steel on a mini mill is a bit of a big ask but I keep wanting to make useful stuff for lathe and mill and that calls of steel, mostly.
|Howard Lewis||26/11/2021 12:38:05|
|6310 forum posts|
A Flycutter is effectively generating a series of arcs, as it traverses the work..
The heavier the cut involved, the greater the likelihood of the tool and its holder flexing.. So there may well be a second (Back ) cut on roughing, but a very light finishing cut should minimise spring and just take off the tops of the ridges left from the roughing cut.
If the spring on the roughing is great, it may need a second, fine, finishing cut to remove the marks left from roughing. Hopefully without affecting dimensional accuracy.
The larger the diameter that the flycutter traces, the more likely it is that spring will be present, because of the increased leverage exerted upon the tool and its holder.
|Tim Stevens||28/11/2021 14:48:25|
1622 forum posts
The Moiré pattern you describe can be caused by vibration of the machine, and the pattern is likely to change at different speeds as the resonance causes more or less vibration in the various parts. Another cause is a very careful set up (so the trailing edge cutter follows the path of the leading edge exactly) used with a small cutter radius or a large feed. the cutter leaves grooves, and on the return sweep cuts into the peaks between the grooves.
You can also get rainbow effects if the cut surface is shiny (rather than rough or black). This is especially the case with a fine feed, and is a bit complex to explain, although it is vaguely related to soap bubble colours. The effect is greater if the light source is small such as one bright LED. It will disappear altogether if illuminated by a laser beam, though.
Hope this is helpful
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