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Author Topic: enlarging an existing pocket  (Read 2394 times)
lloydsp
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« on: December 16, 2011, 19:23:37 pm »

I thought I knew this, but I cannot figure out how to do it...

I have a piece of stock that already has a 1.25" hole in it.  I want to enlarge and finish the diameter (full depth) to 1.314" (and with a bit substantially smaller than the hole... say 0.375")

I have done this with a large bit, close to the hole's finished diameter, but this one escapes me.

How can I do this without "pocketing" the entire space (and cutting air for most of the time)?  I tried an inside profile and a tangent leadin, and the leadin started OUTSIDE the desired toolpath, rather than inside it?Huh  If I don't give it a leadin, it just plunges into the work at the finish diameter, and that will give a rough finish.  I can give a roughing clearance, and that works... but it still just plunges in.  I'd like to do it in several passes (with deference to DesertRunner!)

LLoyd
« Last Edit: December 16, 2011, 19:30:14 pm by lloydsp » Logged

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blowlamp
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« Reply #1 on: December 16, 2011, 19:43:48 pm »

Try putting a minus sign in front of the Tangent Radius setting.
Are you sure the polyline is closed?
Maybe move the Start Point as well.


Martin.
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lloydsp
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« Reply #2 on: December 16, 2011, 19:55:47 pm »

Martin,

That does work with a pocket, but not a profile... with a profile, a negative leadin radius starts the tool from outside the inside profile! <G>

However... I did it with a pocket, fooling with the finish stepover figure... (.03) only I'm really not clear how that will control the _actual_ depth of cut, since nothing that works that I set it to results in more than just two cutter paths.  I'd like to just "peel" the diameter out, with four or five passes at full depth.  The material is prone to hogging.

LLoyd
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blowlamp
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« Reply #3 on: December 16, 2011, 20:06:02 pm »

Lloyd.
If I understand you properly, it seems to work OK for me.

Can we see the file - I'm intrigued as to to shape of the pocket?

Also, what about a Profile with a Spiral Lead In move?

Martin.
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lloydsp
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« Reply #4 on: December 16, 2011, 20:13:50 pm »

Ok... the outermost circle is the stock size.

The middle circle is the desired new bore size.

The inner circle is the existing bore.

I want to take off the difference between the inner and middle circle with multiple thin cuts (in a single MOp).

I can do it in two cuts, but cannot control it otherwise.

Oh.. that target depth should be -2.0... I was fiddling, and didn't correct it, but it makes no difference to the toolpath.


Thanks, Martin.

Lloyd

* Half-nut hub sample.cb (6.09 KB - downloaded 94 times.)
« Last Edit: December 16, 2011, 20:26:03 pm by lloydsp » Logged

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lloydsp
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« Reply #5 on: December 16, 2011, 20:31:24 pm »

AHAH!  I can do it with a profile, if I interact the cut width and stepover figures.  Tangent leadin still comes in from the wrong side of the profile, but other than that it works.

Lotta arithmetic, though.  I have to solve for the distance, fraction of stepover...

Lloyd

* Half-nut hub sample2.cb (8.78 KB - downloaded 73 times.)
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blowlamp
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« Reply #6 on: December 16, 2011, 20:33:41 pm »

Here's my attempt using Profiles  Grin




Martin.

* Half-nut hub sample profiles.cb (11.5 KB - downloaded 81 times.)
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lloydsp
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« Reply #7 on: December 16, 2011, 20:43:21 pm »

Yes!  And thanks.  That's the way I came up with it, also.  But I wish I had not to do all the arithmetic.

Although it's not a burden for one hole, it could get tiresome with a lot of different bit and hole diameters.

Take the difference in radius of the holes.  Add that to the cutter diameter to get the cut width.  Then divide the desired cut depth per pass by the cutter diameter to get the stepover.


Thanks, Martin!
( I think there should be an easier way, but I guess not Huh)

LLoyd
« Last Edit: December 16, 2011, 20:52:00 pm by lloydsp » Logged

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blowlamp
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« Reply #8 on: December 16, 2011, 21:08:57 pm »

I didn't think it was too bad, Lloyd.

1) Set the Cut Width to a bit more than Cutter Diameter (say 0.5 for a 0.375 cutter).
2) Max Crossover Distance to zero so you always get a Lead In move.
3) Step Over to equal the required 'cut width per pass'.

Apply one MOP to as many pockets as needed (and that the cutter will fit into).

 - Job done!


Martin.
« Last Edit: December 16, 2011, 22:37:58 pm by blowlamp » Logged
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« Reply #9 on: December 16, 2011, 21:19:27 pm »

Tangent leadin still comes in from the wrong side of the profile, but other than that it works.

Hi Loyd,
The tangent radius is too big, CB is doing its best to do what you tell it  Grin
Make the radius smaller and it is just fine.
Don
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lloydsp
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« Reply #10 on: December 16, 2011, 23:18:58 pm »

Step Over to equal the required 'cut width per pass'.

-----------
Martin,
Stepover is a fraction of the cutter diameter, not an absolute distance -- yes?

LLoyd
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blowlamp
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« Reply #11 on: December 16, 2011, 23:51:26 pm »

Agreed Lloyd, but if you're using the same diameter cutter for various sized pockets I think the 'bite' should be consistant throughout.

So to my way of thinking, a 0.375" diameter cutter @ 0.1 Stepover = 0.0375" cut width per pass - but remember that maths isn't my forte  Wink

And CamBam will calculate the finishing pass automatically.


Martin.
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lloydsp
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« Reply #12 on: December 17, 2011, 13:41:01 pm »

That;s correct, Martin.  But what it will NOT do is calculate what the "width of cut" needs to be.

That varies as a function of the difference between the desired hole and the existing hole, and would be different for every combination.

  ((D2-D1)/2)+Dc, where D2= desired diameter, D1=existing diameter, and Dc=diameter of the cutter.

So, no... you can't just re-use the MOp, unless you're willing to cut a lot of air.

LLoyd
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Majorstrain
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« Reply #13 on: December 17, 2011, 14:01:03 pm »

Tangent leadin still comes in from the wrong side of the profile, but other than that it works.

Hi Loyd,
The tangent radius is too big, CB is doing its best to do what you tell it  Grin
Make the radius smaller and it is just fine.
Don


I'm with Don on the tangent lead in issue. Here is a quick formula for working out the Maximum tangent lead in that can be set before it starts to come in from the wrong side.

Maximum Lead In radius value =  (Final hole diameter - Cutter diameter) / 2  

The resulting answer causes the tangent lead in tool path to be the same as the profile path ( i.e. no tangent).
So the lead in radius value you enter needs to be smaller than that. LLoyd for your hole the value must be smaller than 0.46955"

Cheers,
Phil
« Last Edit: December 17, 2011, 14:02:48 pm by Majorstrain » Logged
blowlamp
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« Reply #14 on: December 17, 2011, 17:49:00 pm »

LLoyd.
I took it for granted that the finishing allowance left from the previous operations was the same in all of the pockets.
For instance: a Roughing Clearance of 0.1" was left on all pockets.

Doesn't the included (metric) example file do what you need?

Martin.

* lloyd.cb (7.27 KB - downloaded 90 times.)
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