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Need Advice on Mill Surfacing

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Rickwjenn

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#1
Have my PM-940 CNC up and running with the Acorn Controller and have made two parts from Fusion 360 to finished product. Lots of hand written G-code as well.

Was playing around this AM with a 2" surface cutter trying to make a 5" X 6" block of aluminum as flat as possible.

I get these ridges between passes at exactly same Z cutting depth. I am thinking the head is sagging relative to the table/vise thus the cuts are at a slight angle. In other words the head is not square to table/vise at least in x-axis.

Any advice on next steps?
 

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7milesup

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#2
I would say your head is not trammed to close enough tolerances. I don't know enough about that particular mill to offer specific advice, but it sure looks like, as you mentioned, that the head is not trammed. A test indicator will be your friend here.
 

Asm109

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#3
Tram the head as best you can. If the remaining unevenness is no good, use a smaller diameter end mill.
 

TomS

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Do as others have said and make sure your tram in the X and Y axis plane is as good as you can get it. You may have to shim the column to get your Y tram where you want it. Once that is done you can check the perpendicularity of your spindle to the table using "Rollie's Dad's Method". Hoss Machine has a video on how to do this. My PM-932 spindle is about .002" out of perpendicularity over 7" using this method. Not too bad in my opinion.

Another consideration is "nod". Nod is the sagging of your head in the Y plane due to loose gibs. Check your column gib and adjust it if necessary to minimize the nod.
 

Rickwjenn

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All - thanks for the feedback.

Will get to work on it this weekend.
 

phazertwo

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I have the same issue on my 940... if tightening the gibs doesn't get you where you need to be you can shim the base. It's on my list of things to do.

PZ
 

wrmiller

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#7
Just a suggestion? A counterweight system for the head works better than using the gibs to try and keep the spindle bore and column parallel. The few CNC bed mills that I've seen had a counterweight system, which is where I got the idea.

I made one for my PM25 and my Charter Oak 9x40. The one on the Charter Oak made a huge difference in cranking that big head up and down. :)

I used barbell weights and hung them behind the vertical column.
 

BaronJ

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#8
Hi Rick,

All - thanks for the feedback.

Will get to work on it this weekend.
If you have a dial indicator gauge, put it in the chuck and run it over the work piece. That will give you some idea of how bad it might be.

FWIW I can get my mill head trammed so that I don't have any measurable out of tram, and I still get the shading that you show. I can't actually measure any ridges or feel anything, but there is still a visible marking.
 

Rickwjenn

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I built a tramming gauge last weekend and have 0.008" of either 1) table tilted UP along Y axis or head sagging DOWN along Y AXIS.

Any recommendations? Turning gib screws does nothing. Seems like shimming table is next step....
 

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Winegrower

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#10
This is what I like about a Bridgeport...so adjustable, and so solid. Mine just stays in tram until I intentionally put it out.
 

TomS

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I built a tramming gauge last weekend and have 0.008" of either 1) table tilted UP along Y axis or head sagging DOWN along Y AXIS.

Any recommendations? Turning gib screws does nothing. Seems like shimming table is next step....
I think you meant shim the column not the table. .008" is quite a bit out. Shimming the column will get you where you want to be. It can be tedious but is worth the effort. Then using Rollie's Dad's Method as described on the Hoss Machine website you can check spindle perpendicularity.
 

Rickwjenn

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This is what I like about a Bridgeport...so adjustable, and so solid. Mine just stays in tram until I intentionally put it out.
Thanks -will think about buying bridgeport next time.
 

Rickwjenn

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I think you meant shim the column not the table. .008" is quite a bit out. Shimming the column will get you where you want to be. It can be tedious but is worth the effort. Then using Rollie's Dad's Method as described on the Hoss Machine website you can check spindle perpendicularity.

TomS

Thanks for the clarification. Makes complete sense - shim the column. Will go look at that Saturday. Appreciate any other tips. Thanks.
 

TomS

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#14
Here's the link to the Hoss Machine video that I referenced above.
 

BaronJ

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#15
Hi Guys,

I made a head tramming gauge quite some time ago, very much like yours.
I like you found that my new, at that time, mill had several thou of nod which required a little shimming in order to correct it. I really ought to have taken the column right off and scraped it since it wasn't actually flat requiring a couple of thou more shim on one side than the other.

This is mine !
Tramming_tool-3.JPG
Notice that the two gauges are slightly different. They are both equally accurate, but I did replace them with a matched pair of tenths indicators. I also made the bar at the bottom only long enough to go across the table.

Incidentally, try measuring across the table and traversing it from one end to the other ! You might get a surprise ! :eek 2:
 
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