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Spindle cooling and Depth Cutting consistency

1434 Views 13 Replies 5 Participants Last post by  JayKay777, 
J
I am currently using a benchtop CNC router (solid, German build) with an air cooled spindle.

I need to maintain reasonably tight tolerances (<.005").

Primarily, we are cutting two types of material:
1. Gravoply (for making nameplates to go onto our products)
and
2. 2.5D toolpaths in 1/8" ABS plastic sheet.

We are finding that when the machine starts cutting and the spindle warms up that we are losing considerable cutting depth accuracy, up to .015". The accuracy loss, surely due to heat expansion, is causing considerable issues for us, especially with the Gravoply.

I must find a way to keep the spindle and tooling cooler. An oil mister is out of the question so I am looking for solutions that are based on either water cooling or blown air.

I am assuming that having an "air cooled" spindle is only meant to prevent the spindle from overheating and not tool expansion (as is definitely the case for us).

Any help or advice will be very appreciated.

Jim.
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difalkner
Welcome to the forum, Jim!

What bit, feeds, speeds, depth of cut, etc. are you using? The chips should carry away the heat from the cut. If you are feeding too slowly then the bit and work piece stays hot and this could very easily affect the cut.

Photos always help, btw.

David
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J
2
My cutting info (for example in attached photo) is as follows:
Cutter is 1/8" solid carbide V cutter.
Feed: 100 ipm
Speed: 9000 rpm
Depth of cut: .25mm to 1.5mm

We have experimented with various feeds and speeds with no improvement in results. I have confirmed wasteboard surface flatness to .002" with the use of our dial indicator.

I have attached two images.
1. Gravoply and cutter.jpg show the cutter used (but this is an issue with any of our cutters) and the progression of the increasing cut depth deviation from bottom (best) to top (worst).
2. CNC Router.jpg is our machine.

Thanks,

Jim

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difalkner
Are those seven pieces cut like that on the machine or do you cut one, remove it, then cut the next in the same spot? What happens when you increase the spindle speed? I haven't cut Gravoply but I do use cutters like that in Walnut and Maple and I cut at 18k rpm.

David
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J
They are cut sequentially as part of an array and then broken out.

I have cut as fast as 12000 rpm and as slow as 6000 rpm. The issue remains regardless of feed and speed.

I will try cutting at 18k and bump the feed up to 200 ipm to see what happens. I have never attempted this file at that speed and feed before.

Get back to you in a few hours.

Jim
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J
UPDATE: I cut the same file with a faster feed and speed. 18000 rpm, 200 ipm.

As you can see in the image, the faster speed had an even greater depth cut error than my 9000 rpm attempt. The spindle was considerably hotter as well. I am still convinced the problem is thermal expansion of the spindle/collet/cutter.

Jim

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difalkner
Just for grins, can you cut this in the reverse order - top to bottom instead of bottom to top?

David
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Semipro
Hello and welcome to the router forum Jim
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G
Looks like it could be the set screw(s) on you Z ball screw is slipping. Same thing happened to me. When the code orders the axis up, the slipping prevents Z from going up high enough. Then, when it goes negative, it goes too deep.
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J
difalkner said:
Just for grins, can you cut this in the reverse order - top to bottom instead of bottom to top?

David
Click to expand...
Looked into it David. No easy way to do it.
Jim
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J
Semipro said:
Hello and welcome to the router forum Jim
Click to expand...
Thank you!
gmedwards said:
Looks like it could be the set screw(s) on you Z ball screw is slipping. Same thing happened to me. When the code orders the axis up, the slipping prevents Z from going up high enough. Then, when it goes negative, it goes too deep.
Click to expand...
Well, that May be it! I managed to get a 1/8th turn on the Z axis set screw. I will try cutting the file tomorrow and post the results.
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MEBCWD
Welcome to the Router Forums Jay.

On the left you are cutting 9000 rpm and 100 ipm, on the right 18000 rpm and 200 ipm, so you are doubling both rpm and ipm. That almost tells me you have a lot of play in your machine or a lot run-out in your spindle. You might also check the collet and make sure it is in good shape, if not then taht might be the problem. Of course some or all of the problem might be over-heating the material and causing problem. Does the manufacturer of the material give any cutting data that might help calculate the proper speeds and feeds?

As far as changing cutting parameters you can hold one setting and leave the other one alone to see if that makes any difference. Like hold the 9000 rpm and change the ipm in 10 ipm changes.

The cooling on your spindle should be more efficient at the higher speed so if it is over heating when running it at the 18000 rpms there might be a problem there.
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difalkner
Jay - if you cut one and then wait a few minutes to cut the next and keep doing that does the last one end up as good as the first one?

David
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J
gmedwards said:
Looks like it could be the set screw(s) on you Z ball screw is slipping. Same thing happened to me. When the code orders the axis up, the slipping prevents Z from going up high enough. Then, when it goes negative, it goes too deep.
Click to expand...
That was it! I managed to slightly tighten the Z axis set screw and now the Z axis cutting stays true to depth. I am happy!

Thanks guys!

Jim
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