OK， that makes sense but I am afraid my toy mill does not have enough power to drive a fly cutter that big .....
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Basic question on face milling
- Thread starter compact8
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OK， that makes sense but I am afraid my toy mill does not have enough power to drive a fly cutter that big .....
You do have a small mill. Plenty of people on this forum have small mills. You should still be able to get a good cut within the limits of the fly cutter. You should be able to cut 3-4 " with good results. The reason people use fly cutters is because they do not need the power of larger cutters. Something I did not see in the posts. The machine needs to be locked down solid in all axis except for the one being used. Also you need to adjust DOC, feed speed and RPM to minimize chatter. Also, just brought up on a recent post is that somebody was getting deflection because the tool bit was too small. He re-milled the groove to take 1/2" square tool bits and solved his problem.
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OK, I tried again with the following changes :
1) a much shallower depth of cut - 0.03 mm
2) reduce the overlapping between adjacent passes to 0.5 mm
3) use cutting oil
The finish is a lot better although my major concern is still on the flatness of the surface. The only flat surface reference I have got is the mill table so checked by inking the table surface and rub the workpiece against it. The result shows that :
1) the workpiece is basically flat over the majority of the area except for the part on the right. May be the mill table was moved so much to the left that the weight of the table plus the power feed unit on that side tilted to table so this part of the workpiece was raised hence cut deeper ?
2) in each pass ( tool always moved from top to bottom of the workpiece ), the right side is always cut deeper by a very tiny amount. This is not consistent with the expeceted deflection direction of the tool so I think deflection was not happening. May be the X tram is still imperfect.
Another interesting thing learned in this work is that thermal expansion of the mill is really something requiring attention in high-precision work. In one of the attempts I milled the workpiece for only half of the area and went away for a beer before coming back to finish the rest. What I got was a step betwen the two areas. It can be clearly felt by hand and my DTI indicated that the height was about 0.005 mm. The cause is thermal expansion of the spindle. Then I redid it in one go and the step did not appear again.
Fly cutter is not an option for me because I don't have one and this work will require one with 120 mm diameter. Base on the awful experience of cutting stainless steel with a 40 mm facing head, I have some doubt if it will work on my small machine. Cutting aluminium is no problem though. I have got a 63 mm facing head which can produce very good finish on 6061.
1) a much shallower depth of cut - 0.03 mm
2) reduce the overlapping between adjacent passes to 0.5 mm
3) use cutting oil
The finish is a lot better although my major concern is still on the flatness of the surface. The only flat surface reference I have got is the mill table so checked by inking the table surface and rub the workpiece against it. The result shows that :
1) the workpiece is basically flat over the majority of the area except for the part on the right. May be the mill table was moved so much to the left that the weight of the table plus the power feed unit on that side tilted to table so this part of the workpiece was raised hence cut deeper ?
2) in each pass ( tool always moved from top to bottom of the workpiece ), the right side is always cut deeper by a very tiny amount. This is not consistent with the expeceted deflection direction of the tool so I think deflection was not happening. May be the X tram is still imperfect.
Another interesting thing learned in this work is that thermal expansion of the mill is really something requiring attention in high-precision work. In one of the attempts I milled the workpiece for only half of the area and went away for a beer before coming back to finish the rest. What I got was a step betwen the two areas. It can be clearly felt by hand and my DTI indicated that the height was about 0.005 mm. The cause is thermal expansion of the spindle. Then I redid it in one go and the step did not appear again.
Fly cutter is not an option for me because I don't have one and this work will require one with 120 mm diameter. Base on the awful experience of cutting stainless steel with a 40 mm facing head, I have some doubt if it will work on my small machine. Cutting aluminium is no problem though. I have got a 63 mm facing head which can produce very good finish on 6061.
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Yes, the table itself may tilt a couple of tenths if the weight gets too far off center.
But we don't really know if the table is flat to a couple of tenths yet in any event.
As a clockwise cutting tool is moved from top towards bottom, it is cutting on the right towards the front and will be deflected such that the right side will be deeper than the left even with a perfect tram. You hog out metal with deep cuts and fast movements, you finish cuts with shallow cuts, slow feeds and higher spindle speeds with lube.
Interesting......
A face mill has a number of cutting points (3-7), a fly cutter has exactly 1. This reduces the forces needed to keep the spindle turning at a constant rate.
Also note: mild (cheap) steel does not machine as well or as easily as some other steels with similar properties such as 1118.
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Observation #2 suggests that your X tram is imperfect. But....is the "tiny amount" you referenced more than you can tolerate? How much will it actually impact the work that you to do? Maybe it's good enough as it is. From my own experience, it's all too easy to get sucked into making ever-finer adjustments to our machines rather than using them to make useful (or fun) things.
When I'm machining something I always try to get as close as I can to the designed dimensions, even if they're not all that critical -- I view it as an opportunity to improve my abilities in that direction. But if I'm off by a few thou I don't start over again. In the vast majority of cases I still have a perfectly usable item, and maybe I learned something along the way. It's all good
That's exactly what I have been doing. I always measure the dimension of the work piece after cutting even when high precision is not required. If it is off by more than what I have achieved in the past, I will try to find out the reason and improve the method. So far the following source of errors have found. I have managed to fixed some of them and prepared to accept the rest unless there are fixes doable by me.
1) The X and Y rails are not perpendicular to each other. It's off by about 0.02 mm over 90mm.
2) The vertical column is slanted backward by about 0.02 mm over a distance of 90 mm.
3) The tram along the Y axis is off by 0.05 mm over 150 mm if the quill is locked. Increased to 0.1 mm max ( but in different direction ) if the quill is lowered by operating the quill handle
4) 2 & 3 added together, the tool tip shifts by 0.01 mm in the Y direction for every 18 mm increase in the length of tool / measurement device.
5) The quill moves forward by 0.1 mm when pressure is applied to the quill handle.
6) Play in X Y Z rails with that along the Z rail having the most prominent effect.
7) Strange drift in Z depending on the direction of rotation of the spindle ( details here ).
8) Tool / head flexing : 0.01 mm to 0.018 mm depending on the material of the workpiece.
9) The Y position shifts by 0.03 mm and Z by 0.005 mm when the milling head gets warm.
10) Occasional glitches in my brain.