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Author Topic: eliminating taper on deep cuts  (Read 2725 times)

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eliminating taper on deep cuts
« on: June 23, 2010, 05:25:40 PM »
I have a CNC machine that is made out of mostly wood and will cut thin aluminum (0.1" or less) fairly accurately. I want to cut a piece of aluminum (T61) that is 1/2 inch thick. The cut is 2D and I need to use a 1/8" end mill. The problem is that if I cut clockwise (CW) around the piece, it is larger than I want, and if cut counter-clockwise (CCW), it is smaller than I want. To make things worse, the part is tapered toward the direction of the error, that is, the bottom of the part has more error than the top.

I am aware that this due to the lack of stiffness in the CNC machine design, and in part, to the flexibility of the small sized tool.  

I was wondering if I can resolve much of this error by first cutting each pass CW and then following with a CCW finish pass (same tool path same depth). It occurred to me that the CCW pass might not take out all of the error, and it may be beneficial to lower the tool a small amount into the material on the CCW pass to help hold the tool on course while it is finish cutting the part.  

Is there anyone who knows the best way to do this kind of cut with this kind of problem?

If I need to use a different tool path for the finish cut, is there an optimal finish tool path offset (% of tool diameter) for the CCW cut?

My primary goal is to get a part with a vertical side and around 0.010 tolerance on overall dimension.

Thanks, Bill

Offline RICH

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Re: eliminating taper on deep cuts
« Reply #1 on: June 23, 2010, 10:00:02 PM »
Bill,
- max depth of cut should be 1/2 the cutter dia say, so no more than .050" ( if you have backlash reduce it even more )
- the end mill should be one that will plunge ( cuts to the center of the end mill)
- clear out the bulk and leave say, .010 for finish cuts
- make one finish cut at .004" , .003", and finaly do a .002" at max depth
- use appropriate speeds and feeds

Now before doing the above, i would test the machine and just see what happens when you reduce, say the od of some al scrap.
This way you can fool with depth of cut, rough and finish passes, and gain some experience on what your machine will do relative to holding
the diameter and strightness along the side.

Just some rough suggestions,
RICH
Re: eliminating taper on deep cuts
« Reply #2 on: June 24, 2010, 04:36:04 PM »
Rich,
I rarely cut aluminum deeper than 0.010 or I tend to break tools. I judge the path speed (usually around 35ips) & motor speed by the chip size, not too small or too big. I usually have the run the spindle speed as slow as I can get it to run smoothly. I use a 1.5" long 1/8" 2-flute end mill (looks to have the outer edges longer than the center).  If I try to use a 3" long tool (drill-point and not carbide), I can see it wander all over the place. 
If the tool consistently cuts too large in the CW direction, do I need to allow for a finish cut in the CCW direction? In theory, the finish cut will not try to pull away sideways from the ideal tool path as much as it does in a bulk cut. Do I want to finish cut only at near full depth or on every pass?
Thanks, Bill

Offline RICH

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Re: eliminating taper on deep cuts
« Reply #3 on: June 24, 2010, 09:04:53 PM »
Quote
outer edges longer than the center)
- they all have a ground relief to the center of the end mill, when plunging down into solid stock you want a center cutting end mill

 
Quote
0.010 or I tend to break tools
- You are saying you have run out at the end of the end mill, you have what you have and that will affect the cutting, couple that to
lack of rigidty and backlash and you will break them
-So just do shallow depth of cuts and plenty of them ( what more can i say ?)

Quote
run the spindle speed as slow as I can get it to run smoothly
- good spindles with appropriate holders run just as good slow as fast, like i said you have what you have and need to fool some to gain
experience with what you have. minimize the extension of the tool from the holder.

Like i said, take a piece of round stock and machine the outside trying different depths of  cuts, feed rate / spindle speeds leaving say .010"
for finishing to some diameter. Look at results of  machining to a depth of 1/2", is it square to the face, out of round, plunge cuts coming out differently?..........experiment with finish cuts cw and cw at different finish cut depths and look at the result

If you had say a normal mill i would just give you some recommended info that you can find anywhere, but with your machine you need to experiment and see what you can do.

Can't give you any better advice than the above,

RICH