Tramming the mill

Got the new machine set up on it's base today , and checking it out ,
cleaning , getting it ready to use . I chucked a rod with a dial indicator
mounted on it in the drill chuck , just to see ...
It's reading zero at the rear and straight left , plus .004 on the right
and at the front . The sweep diameter of the indicator is approximately the
same as the bolt spread on the column/base attachment . I'm thinking I need
to shim .004 at the left front and right rear bolts , .008 at the right
front .
Am I headed the right direction , or am I about to make the error double ?
Reply to
Terry Coombs
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Normally the tramming sets the spindle perpendicular to the table, done by slight rotations of the head about its 2 axes that are parallel to the table. The shimming you describe might be necessary if as you raise the head (or raise the spindle) up the column (if it's that type of mill), indicating on the two vertical sides of a square standing long leg up on the table, you see a tilt of the column from perpendicular to the table - again check 2 axes of tilt. Do this before trying to tram the spindle or head.... Joel in Florida (aka Critter 67 FLH) ==========
Reply to
joelblatt
I presume you are referring to a mill/drill type machine. Someone posted on RCM about a similar problem and it turned out the problem wasn't inaccurate machining but burrs around the holes where the column mounted IIRC. Cleaning up the machining and reassembly sorted the problem.
Reply to
David Billington
You're the second person to point me towards cleaning ... thanks ! Guess it's a good thing I still have my lifting apparatus in place , that column and head prolly weigh nearly 500 pounds .
Reply to
Terry Coombs
I would use it for a while first, to wear in the table ways. I bought a mill-drill that was off by 0.009" IIRC and never noticed a problem in the parts we made with it.
The table slots weren't quite parallel to the X axis either. I made the vise base key a very snug fit in the tee slot and then milled a step in its upper sides to make them parallel to the table travel.
Jim Wilkins
Reply to
Jim Wilkins
I'd suggest you use a collet to hold the DI rod. drill chucks can impart error
Reply to
RB
Terry, Use a true collet, not a chuck. Clean the base and column and torque the bolts with a torque wrench. Measure at least 3 times. Dissassemble and check again. You are looking for consistancy before you make any changes. Lastly you want the front of the table up about .001 to .002". Even new mills measure that way on purpose. This compensates for post deflection when under load Steve
Reply to
Steve Lusardi
Not if he's sweeping the table with the indicator. The indicator point will follow the spindle's center of rotation regardless of how bad the runout of the rod is.
Reply to
Ned Simmons
I have to agree with Ned. If the chuck is crooked the bottom end of the rod will rotate in a circle but that circle will be centered on and perpendicular to the spindle rotation. Try it with a piece of bent gas welding rod (my favorite steel wire / rod up to 1/4").
BTW, if you don't have a good dial indicator you can tram the mill square with just a bent rod and feeler gauges. It's easier if you check height to the top of a parallel so the rod won't catch in a table slot.
Reply to
Jim Wilkins
So much advice and discussion is a good thing ! Many thanks to all who have contributed to this thread . I have learned several things , not the least of which is to have the table a thou or so high in the front . I think I'm going to run it as-is for a couple of weeks , let all the parts get to know each other . I'm just kinda learning how it all works together right now , precision can come as my skill develops . Whoever it was that said there is no such thing as clamping too well is right . The first thing to be replaced is going to be that POS "vise" that came with the machine .
Reply to
Terry Coombs
Or think about where the indicator is attached: to the end of an assortment of rods and joints whose entire purpose is to swing the indicator in as large a circle as is practical. A little wobble in the chuck only serves to make the circle a bit larger.
Reply to
Ned Simmons
Try this : chuck an indicator on a rod into the drill chuck , sweep the table . Now , add a bend to the rod just below the chuck , and sweep again . Bet ya a bottle of Dickel White Label you get the exact same *runout* . The assembly is going to rotate around the centerline of the spindle bearings , even if the rod (or offcenter chuck , whatever) is bent into a pretzel (rigidity issues aside) . One of the suggestions was to lay out the bolt pattern on the table , set the length of the horizontal so that the indicator swept the centerline of the bolt pattern circle . Then find the highest corner , add the exact reading you get at each of the other three bolts . Torquing the bolts all to 70 ft/lbs changed the error ... which tells me that I need to clean those mating surfaces before I can expect to get anywhere with getting it trued up .
Reply to
Terry Coombs
And cleaning the mating surfaces brought the front-rear difference to less than a thousandth . Side error is about .005 difference on a 15" circle (7.5" arm length) which I can adjust out by rotating the head (Of course I really should set up a 1 2 3 block and an indicator , crank the head up n down the column to be certain it's perpendicular ... but I got no blocks yet .). I'm not sure the error is material at this point , using a maximum diameter of 3/4" end mill that's less than .00025" ... I know damn well that's more accuracy than *I'm* capable of !
Reply to
Terry Coombs
I use a brake rotor that has been finely surfaced, and the flange cut off.
Reply to
RB
I think my heart nearly stopped when I read RB's suggestion to use a collet instead of a drill chuck. As an apprentice, I got yelled at for asking the wrong questions - questions which reveal a lack of understanding of the overall idea. Suggesting a collet as opposed to a drill chuck reveals a lack of understanding of what you're trying to accomplish while squaring a mill.
As long as the indicator is securely fastened to the spindle (the part that actually turns), you're doing the right thing.
In regards to cleaning up the mounting surfaces, this is most certainly the right direction. A .001" feeler gauge is your best friend (along with bearing blue) for correctly assembling machines and tooling with precise mounting surfaces (like the column to base connection).
Had you used that feeler gauge, you would have been able to tell that there was a gap. Hopefully you chamfered the bolt holes and the perimeter (circumference) of the mounting surfaces before reassembly. One slight bounce to a sharp edge during reassembly will likely raise a bump on the mounting surface, which will put you in the same position as before.
Bearing blue can reveal the type of contact you're getting between those surfaces. You can do this with a good flat bench stone as well, but it's more obvious with the blue. You'll be able to see where the surfaces are actually touching, where they're close, and where they're not close.
I used to have to manually flatten the bottom of insert blocks in large stamping dies. You want really good contact around your bolt holes, and ideally under heavily stressed areas (if such areas exist). In your case, around the holes and directly under the column. I understand this doesn't matter much now, but if you find you can't get consistent results while machining, you may want to check that. Under machining stresses, anywhere a gap exists, your machine parts WILL flex.
Another poster mentioned his t-slots were not aligned with the x-axis. I would again disassemble and check for burrs and debris. If the builder used good technique (ha!), the slots should be aligned and this misalignment could point to other issues (burrs, debris, incorrectly adjusted gibs, etc.)
I've never set the front of the table .001" up or down or anywhere other than perfectly square. This may work in some situations, but again, it points to other issues within the machine. You can measure how far the head is flexing during a cut by mounting a dial indicator to the head, and having the needle contacting the smooth ground table. You'll need to protect the indicator from chips, but you can see the head move (and move it will on decent/heavy cuts!)
The amount of flex will increase depending on the load. Facemilling, fly cutting and heavy drilling will typically be the worst - all three are operations where you can easily exceed the max horsepower of your machine if you simply continue to push harder. This is why there are finishing cuts, and roughing cuts.
If you get consistant movement under different loads, it is likely that something is not securely connected (loose bolt, burr, debris). You can use the an indicator and some muscle to find that type of issue.
Good luck with the new machine. Now that you have evidence of the builder not finishing the machine correctly (deburring, cleaning), don't be so quick to jump through hoops to fix errors. Frequently the simplest answer (disassemble and clean/deburr) is the correct one.
Regards,
Robin
Reply to
robinstoddart
That was an RF-31 mill-drill from MSC that I bought for an electronics company shop. It was used for drilling and light milling in flat aluminum control panels, non-critical jobs that didn't justify taking time from other projects to clean it up much. I brought in some precision tools from home to check it and found the slots slightly askew from the sides of the table and the X axis, and the spindle tilted ~0.009" over its 5" stroke. The sides of the table indicated true, so I suspect that the sides/dovetails and the tee slots were machined in separate operations.
It only matters if you install a key in the bottom of the vise to position it accurately.
Reply to
Jim Wilkins
I've also heard the .001 high at the front of the table number, but in relation to how the table should be installed, not as a number to shoot for when tramming a mill. I've got three Bridgeport manuals here, going back to one that shows only the M head round ram mill.
That oldest manual says, "When indicating as in figure A, it may be impossible to get a dead zero in back and front of table as machines are fitted to be slightly high in front - usually about .001"."
A J head on a dovetail ram has another axis of adjustment, missing from the M head in the old manual, that allows the .001" difference to be trammed out. (But, of course, that means the spindle is not perfectly parallel to the column ways.)
The next newer manual, which covers the J head, indicates that the front of the table is intentionally high by .0005 to .001, but is ambiguous on the tramming process.
The newest manual also mentions that the operator should be aware of the high front, but is clear that the objective in tramming is to get the spindle perpendicular to the table in both adjustment axes.
Reply to
Ned Simmons

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