Tramming a Simple Mill

[jschmidling]

I seem to be missing something about tramming and how it applies to a mill with a fixed head, viz., a re-purposed drill press.

My problem is that when I make a pass, the leading edge takes down some metal but the following edge also does. This is most conspicuous when using a fly cutter.

Not sure but I think this indicates that the traverse of the table is not square with the spindle. I can not change the spindle so the obvious is to shim the base of the table.

If I put an indicator on the table at one end at zero and run the table across to the other end, the delta is about 2 mils. This is probably enough to cause the problem.

I put 5 mil shims between the table and the machine base on the low side but get the same results.
I then tried 15 mil shims and nothing changed.

Am I chasing the wrong spook?

Jack

 

[pontiac428]

Tram in this case refers to the spindle being perfectly perpendicular to the table. It needs to be checked with a dial indicator on a swing arm so that the indicator plunger is coaxial with your spindle and circumscribes a large radius, like 1/2 of your table width. By traversing your table, you are proving the table stays the same height, but the table can be skewed out of tram and still provide zero endpoints.

You could also use a gauge ring or "tram ring" and a tenths indicator by mounting the indicator in your spindle and swinging the tip of the indicator around the height of the circle. Not as nice as using a longer arm, but quick.

 

[jschmidling]

Tram in this case refers to the spindle being perfectly perpendicular to the table. It needs to be checked with a dial indicator on a swing arm so that the indicator plunger is coaxial with your spindle and circumscribes a large radius, like 1/2 of your table width. By traversing your table, you are proving the table stays the same height, but the table can be skewed out of tram and still provide zero endpoints.

Thanks for screwing my head back on. After a few minutes of scratching my head and a few lines on paper, a big "DUH" jumped out at me.

About 15 min later I had made the attached tram fixture from stuff lying around and quickly found that I was 20 mils higher on one end, not the 2 mils I found the other way. The arm could be longer but as it is, I spent several hours futzing with shims of various sorts to get it within 1 mil using a hacksaw blade (about 25 mils) under the low side of the table.

Don't know if it solves my problem but I didn't want to spend the night trying to figure out why not. I will save that for tomorrow.

Thanks for your help.

 

[pontiac428]

You have got it now. I would imagine a drill press conversion would take a little adjustment, but getting it straight is well within the realm of possibility. Let us know how it goes!

 

[RJSakowski]

When you face a surface, you are moving the table, vise, etc. under a fixed spindle. The surface that you cut will be parallel to the mill x and y ways. This is true whether the spindle is trammed to the table or not. If the table surface, vise, etc. is not parallel to the ways, you will cut a tapered part. The surface finish is a different matter. If the spindle is tilted, you will cut scallops and depending on the direction of travel relative to the tilt, you can see cutting on both the leading and trailing edges of the end mill.

To a certain extent, you can expect to see marks from both. As the cut progresses, the end mill is cutting a series of microgrooves. The center of the end mill is slightly relieved so as the trailing edge passes over the previously cut grooves it cuts an intersecting path resulting in the double cut pattern. A good indication of proper tram would be to observe the same pattern from cuts made in both directions.

A head that is out of tram in the plane perpendicular to the direction of cut will cut a saw tooth pattern. This is most obvious when using a facing tool, fly cutter, or large end mill as you are dealing with the tangent of the out-of-tram angle.

Another consideration is cutting forces. The leading edge will do most of the cutting and there will be a a slight tool deflection, the magnitude of which depending upon tool rigidity, machine rigidity, and aggressiveness of the cut. This deflection has the effect of lifting the trailing edge of the end mill. By its nature, it can be less pronounced on light cuts and the double cut pattern can appear whereas it was not present on the more aggressive roughing cuts.

Unless you have previously determined that your table, vise, etc. is dead parallel to the mill x and y ways, any tramming to that surface will be questionable. When I tram my head, I skim cut a ring of the same radius as my tramming indicator swing with a small diameter end mill. This assures me of a reference surface that is parallel with my ways. The head is then trammed to that surface.

 

[tech610]

You can put an accurate piece of a drill rod in your chuck and use a machinist square on the table against the rod to see if there is a gap (light shines through). Do it for left/right and front/rear direction. That will be your starting point and it will quickly give you an idea of what to do next, then proceed with sweeping your DI around for fine tuning.

 

[kb58]

I had a drill press that over time, the spindle bearings became worn enough that I could pull the spindle back and forth and see the tip of the drill bit shift. I can't imagine how bad it would be had I tried using an endmill in it. Put a piece of drill rod (or even a large drill bit) into the chuck, then put your DI parallel to the table and up against the side of the drill. Now pull the chuck back and forth in both the X and Y direction and see what kind of play you have.

The good news is that it provides ammunition for getting yourself a mill. I actually got rid of my drill press when I got my mill. It's truly striking the difference when drilling a hole with a really rigid machine. The only thing you hear is the motor, and a faint hissing sound as the drill bit goes through the material. Doing the same on the drill press would produce all sorts of chatter.