Best approach for tramming nod on PM-728VT

[svideo]

I'm a total novice and have been monkeying with my 728 for just under a year. On initial setup I used the tilt adjustment to tram the X axis within a half thou over 6". The Y axis is out by about 2 thou over the same 6", so now I'm trying to figure out what I can do about that. Goal is to adjust the nod forward (top toward the user).

A common approach I observe elsewhere in the forums is to place shims between the column and the base. On other bench mills like the PM-25, the 4 bolts attaching the column to the base are accessible from the top and as a result this seems reasonably straightforward. On the 728VT, these bolts are underneath the base. I think that means I'd have to lift the entire mill off of the base, try and loosen those bolts, insert some shims, and re-tighten the bolts, all while the mill is hanging from my shop crane.

The manual offers this guidance:

If more precise tram is called for, it can be adjusted by sandwiching a metal shim between the headstock and its sliding base (saddle) on the column. It is more likely that the headstock is nodding forward rather than leaning backward, so start with (say) a 1 mil (0.001") shim in the bottom quadrant of the machined mating surfaces between headstock and saddle. This is a temporary fix that may not be satisfactory if the headstock is subsequently rotated.

Based on this, I'm thinking I should do the following:

1. Loosen the two 19mm bolts for the tilt adjustment
2. Insert shim stock at the top of the interface between the headstock and saddle (red arrow in image below)
3. Tighten tilt bolts
4. Check Y tram and repeat 1-4 as needed

Questions:

1. Does the approach above seem about right? I'm pretty sure this is what the manual is suggesting, but the manual also suggests that this isn't a great idea which doesn't fill me full of confidence. Is there some other approach that might be better?
2. I'm not sure if that surface is precision ground, but it is an adjustment interface for tilt and I feel like I want to avoid scratching it up. Would using aluminium (kitchen foil, soda can, etc) be a reasonable solution here? Thought process is that it's softer than steel and might not scratch that surface. I have access to conventional steel shim stock if that's a better plan.

 

[mksj]

Is the knod with the gibbs locked? It is a temporary fix shimming the head as noted, usual wear is nod of the head toward the column so shim would be on the bottom pushing the head out. I would probably use some brass shim stock if trying that approach and assume the head is not swiveled. I also would check the gibbs, and maybe tighten them up if a bit if loose. They eventually will wear on the lower part due to the weight of the head, people that adapt these types of mills for CNC usually add some form of head counter balance system but not for manual milling.

 

[svideo]

The Z column travel is locked while I'm taking measurements with the handle visible in the picture above. The manual makes the same note about wear usually resulting in nod heading in the opposite direction. In my case, the mill was like this at first setup which makes me think it's probably not a wear issue (coupled with the fact that it hasn't seen a heck of a lot of use yet).

I suppose one solution would be to use it for many years until that wear pattern happens

The brass shims sound like a good idea, I've ordered a set from the big river which should be here tomorrow, thanks for the tip @mksj!

 

[davidpbest]

First I recommend you watch a video I made about checking the tram. There are two aspects to this: 1 - does the head move up/down perpendicular to the XY table, and 2) does the quill move up/down perpendicular to the XY table. You have to check both component movements for NOD and for TILT. Here's the video:

Now, some comments on how to correct the tram.

1. First, make sure the gib between the head sliding base (head saddle) and the column dovetail ways is properly fitted and adjusted. Many of these Asian-built machines have gibs that are not flat and not properly fitted. Remove the gib, clean it, stone it, remove any burrs and check that the surfaces the ride against the dovetail ways on both the head saddle and the column are flat. If your gib is shaped like a banana, you will continuously get inconsistent results when you reposition the head.
2. Next, make sure you use the head locking levers consistently when repositioning the head. If you have a well fitted and adjusted gib, that will eliminate most (but not all) of the slop in the head-to-column connection, and you could possibly get away with leaving the levers loose enough to move the head without effort and still maintain reasonable consistency of registration when the head is moved up/down. In most cases, best practices will be to always tighten down the locking levers when the head is re-positioned. Locking the levers is best practice at attaining consistent alignment of the head in both NOD and TILT. Locking the levers will also significantly improve the rigidity of the machine when milling.
3. The next step is to tram the column such that the head moves up/down perpendicular to the XY table. Use the technique outlined in the video to make the checks. Any required correction to bring the column into tram by adding shims between the base of the column and where it bolts to the mill base. You'll have to break that seal, get both surfaces cleaned up first, then by trial-and-error start fitting shims and securing the mounting bolts between each check for proper head movement along the column. Be sure to check and correct for alignment in both the X and Y directions. (Be prepared to employ shims as thin as a cigarette wrapping paper to get this alignment properly adjusted. I have found it helpful to have a collection of really thin brass shim stock on hand for this with thicknesses between 1 and 10 tenths. Here is one example. But you can also use flattened soda can material, aluminum foil and even cigarette papers. This process is tedious and time consuming, but worth the effort. Poor column tram will mean your cutting tool will change position when the head is re-positioned, often by a few thousandths of an inch or maybe more. Here is a video that goes through the process. )
4. The final step is to get the quill movement aligned perpendicular to the XY table. The TILT aspect of this is straight forward, since you can loosen the bolts that secure the head to its sliding base (saddle), then tapping in the tilt and re-securing the mounting bolts. (A spindle tramming square like the Edge Technologies Pro-Tram can be helpful for this, but there are other methods as well. I prefer using the Noga Centering NF holder NF1018 with a test indicator with 0.0005" resolution, since the Noga has more utility than just tramming the head, and you'll want a DTI with that level or resolution for other purposes anyway.)
5. If the quill movement is not perpendicular to the XY table (has evidence of NOD), that can only be corrected by shimming (or scraping the interface) between the head and its sliding base (head saddle) in a trial/error method, similar to shimming the column.

Hope this helps.

 

[addertooth]

svideo,
On an unrelated notation: Wow, your mill is clean!

 

[svideo]

@davidpbest - man that's a helpful reply! Just what I needed, thanks friend!

@addertooth - That's a combination of being fairly new, not having seen much use, and cleaning it all down prior to an attempt to deal with the tramming issue. Gotta get this sorted and get some chips flying again or nobody will take me seriously

 

[Christianstark]

@davidpbest - man that's a helpful reply! Just what I needed, thanks friend!

@addertooth - That's combination of being fairly not, not having seen much use, and cleaning it all down prior to an attempt to deal with the tramming issue. Gotta get this sorted and get some chips flying again or nobody will take me seriously

Maybe you will be taken more seriously if you change your name to HDMI?

I kid...I kid...

 

[RodSME]

There is a PM 30 thread on tramming that mill. https://www.hobby-machinist.com/threads/pm30-tram.94465/
Check it out. I added a reply discussing what I did for my PM 940V. One thing to consider is where the "nod" comes from. If the nod is due to tilting of the column, putting a shim between the headstock and headstock support doesn't completely solve the problem. Hope this helps.

 

[6061T6]

Use a fixture plate and shim it instead. Get it trammed to whatever accuracy you need. Mine is accurate to around 20 microns in X and 10 microns in Y.
Mount your vise on a mini pallet and tram it once. You can take your vise off and put it on and it will repeat within 1/2 thousand.

Once you have a fixture plate and use it, all these techniques for tramming seem so old fashioned.

 

[svideo]

Oh wow @6061T6 - I never knew fixture plates were a thing (told y`all I'm a newb ) and I can see immediately how this could help me out. There are, of course, a zillion options in the market to choose from. Do you have any experience with a solution you like that would be suitable to a PM bench mill?