4 and 5 Axis Setup - Minimill

Hi there,

I'm pretty new to the CNC and even machining arena. I'm a university student on an internship and I'm having a bit of trouble with our mini CNC mill here. It's a Minitech GX mill (Mini-Mill/GX - MINITECH MACHINERY CORPORATION), with 4 and 5 axis stages(5th Axis - MINITECH MACHINERY CORPORATION). I understand the basics of G-code, and I know how a dial test indicator works, but I'm struggling to understand how to set up the 4th axis, the trunnion table. I've looked online a ton, but haven't found anything that really helps with my issue. Here it is:

So I have the trunnion table on the mill, but it isn't straight on the roll, pitch, or yaw axes. I understand how to run the dial test indicator along the table in the x-axis and y-axis for roll and pitch and how to shim it up, but yaw confuses me. I had the table accurate to under half a thou over a couple inches (we're only making small parts) in the x- and y-directions, but when I ran the DTI along the table at +90 and -90, the runouts are different. So I believe that the base plate on the trunnion table isn't level with the axis of rotation, but I don't know how to accurately level the axis of rotation if I can't count on it being level with the table. I attached a picture of my setup, if that helps:

Any help is appreciated! I've been reading lots of these forums, and thanks to all of you for answering other folks' questions too,

S

I would recommend a bolt in the center of the 1,2,3 block. Not pinching it under a portion of the side of the head like you show it.

If your artifact is moving, you are chasing your tail.

That giant spongy stack of shim screams of slobbery, chewed on tail.

SpencerFTO said:

Hi there,

I'm pretty new to the CNC and even machining arena. I'm a university student on an internship and I'm having a bit of trouble with our mini CNC mill here. It's a Minitech GX mill (Mini-Mill/GX - MINITECH MACHINERY CORPORATION), with 4 and 5 axis stages(5th Axis - MINITECH MACHINERY CORPORATION). I understand the basics of G-code, and I know how a dial test indicator works, but I'm struggling to understand how to set up the 4th axis, the trunnion table. I've looked online a ton, but haven't found anything that really helps with my issue. Here it is:

So I have the trunnion table on the mill, but it isn't straight on the roll, pitch, or yaw axes. I understand how to run the dial test indicator along the table in the x-axis and y-axis for roll and pitch and how to shim it up, but yaw confuses me. I had the table accurate to under half a thou over a couple inches (we're only making small parts) in the x- and y-directions, but when I ran the DTI along the table at +90 and -90, the runouts are different. So I believe that the base plate on the trunnion table isn't level with the axis of rotation, but I don't know how to accurately level the axis of rotation if I can't count on it being level with the table. I attached a picture of my setup, if that helps: View attachment 292917View attachment 292917

Any help is appreciated! I've been reading lots of these forums, and thanks to all of you for answering other folks' questions too,

S

Click to expand...

Someone stop the madness please!

heres a few tips.
your indicator looks to be too far back and looks to be rubbing on the shaft rather than hitting the ball ( couldnt see a ball on your indicator tip either in blowing up the picture, so maybe you dont have one )
your part Flat out sucks for location and clamping. has a chip under it in upper right corner, doesnt look flat against the fixture and in all honest your locating fixture flat out sucks, chewing gum would be a better improvement.
you need to add more shims under the main fixture as I dont think you have enough.

Just for shits and giggles have you tried indication just the fixture in 1st? thats the best way, once that is done then place your part in but not like you have.

get rid of that shit under the 4th axis, indicat the X axis (long axi) should be zero or close, if not check your indexer mounting and remedy. Checking the y axis ( short one) is a matter of small incremental jogs of the indexer untill you get the reading you're lookin for. G92 the A axis ( your indexer) when you get Y to indicate near zero. I don't see any 5th axis on there.

If it was me... those shims under the 4th would be scaring me.

I start by popping the trunnion off the table and verifying basic machine alignment:
1. Clamp a precision square to the table, put an indicator in the spindle, align the square so movement in X does not deflect the indicator. Indicate the Y axis. This should be square. (Checking Y motion is square to X motion)
2. With the indicator in the spindle, traverse the length of the table in both axes. (Checking that the table is aligned in the XY plane)
3. Put a gage rod in the spindle and check spindle runout.
4. Put your square on the table, aligned in XZ. Indicate along the long side. Repeat this with the square aligned in YZ. (Checking that Z motion is perpendicular to the table)

OK, now you should be reasonably convinced that things are square in 3 dimensions. If something here is messed up, stop and fix it before moving on. Now you can drop the trunnion on and move forward. Otherwise you're just chasing your tail....

Thanks everyone for the tips!

I guess that step 1 would be to take off the trunnion again and indicate on the main table of the machine. I did this before, but didn't worry too much, thought I would just fix it as I aligned the trunnion. This was likely the first mistake.
Step 2 would be to mount the trunnion back again, and indicate it in, first in x, then in y.

My main concern is what to use on the trunnion as reference. That ground block on top is just so I have a good surface to indicate on. I don't think that the main base of the trunnion is level/parallel with its axis of rotation. I have a mounting block, a breadboard (black plate on picture), on top of the trunnion, which I think I will have to shim independently of the trunnion in order to truly have a level surface that is parallel with axis of rotation. This could have been a beginner mistake, I will try again today.

GENERALDISARRAY, delw, greggv: my shims are just too loose and floppy? What exactly should I fix?
greggv: you suggest that I get my x perfectly straight, then use the program to align y? Thank you, I was using shims, this will work much better.
delw: I tried to indicate the trunnion in first without the block, but once I got x and y aligned, I would indicate along the table at +90 and -90 and would get different results. I thought that this meant that the table had a weird shape to it, so I tried with the block. Also, I made sure that the indicator ball was contacting don't worry What would be a better part for referencing?
trochoidalpath: thanks for the good overview of alignment. I'll go back and double check this.

Thanks for your quick replies! This definitely gives me something to go off of today! If anyone has anything else, especially in terms of aligning the trunnion, I'd really appreciate it!
S

Just so you know, the threads in the mill are 1/4"-20. This is why my fixtures are a bit weird, I can't just bolt an ordinary 1-2-3 block in, and I work at small facility that isn't a machine shop. Not tons of good options, but can purchase.

Thanks again!
S

So I have an update: I took off the trunnion and levelled out the base table. When I put the trunnion table back on, 0.005" difference left to right (x-axis). This is pretty much the same problem as before. My concern is that the axis of the table's rotation isn't parallel with the plane of the table top, so leveling the x-axis here actually isn't aligning anything. Only problem is that I'm not sure how to measure the axis...

Thanks for any help!
S

From the link, the mill only weighs 200lbs ?. Not sure how accurate your going to get, and will it stay put once machining starts.

Yeah, it's really small, but we also only manufacture really small parts (8mm or less) and not too much material removal. The goal is just to get as accurate as possible now and go from there.
Thanks Wsurfer,
S

Hi Spencer FTO:
You wrote "I don't think that the main base of the trunnion is level/parallel with its axis of rotation."

So you've really answered your own question when you recognized that truth; this means you need to either find or create reference surfaces from which you can indicate the trunnion in and make it's axis of rotation coincident with the motion planes of the rest of the machine.
There are a few ways forward, but the most reliable of those is to modify the trunnion so you have a reference stub shaft sticking out of the headstock end and a corresponding stub shaft sticking out of the tailstock end.
These must be the same diameter, and be perfectly (as close as you can measure) concentric with the axes of rotation at each end of the trunnion.
The tailstock end can be nothing more than a flange with a few bolt holes in it and a stub shaft turned on it that sticks out a quarter inch or so...just enough to get an indicator on.
Make the headstock end stub shaft longer...that way you can quickly find out if the tailstock end axis is even in line with the headstock end axis.

Now you have something you can actually measure.
This is necessary because you have no other references you can trust, and short of taking the whole trunnion apart in order to measure the component parts and find out what you've actually got, you're dead in the water without this mod.
This is the easiest and cheapest way forward if best possible accuracy is your goal.

Once you have these stub shafts accurately located, there are about a gazillion ways to proceed from there.
The best way for future projects is to fix the trunnion so it's decently accurate...remilling the base if needed, milling reference flats to indicate to; whatever you need so you can set it up consistently without having to fuck around too much.

The rude and dirty way to just get through the job is to do as you have done...break out the shims and shim away until you get it as good as you think you need in order to make your parts.
All the criticisms of your shimming protocol that others have remarked on are valid...you need to make some decent shims with slots or holes in them so the shims can surround the hold down bolts.
The way you have it set up now, when you tighten the bolts you'll just bend the trunnion base, so you'll never get it decent.
The shims MUST surround the mounting bolts to avoid this.

So make the flange stub shafts, figure out how to mount them (the headstock end is easy...you already have mounting bolts there of which you can borrow two), tap them around until they clock in perfectly, then throw the whole trunnion on a surface plate if you have one and find out how bad it is with dial indicators.
From there you can make decisions about how to proceed.

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

Hey Marcus,

Thank you so much! I've been waiting for someone to really address my question of the axis of rotation, and you hit the nail right on the head!
Unfortunately though, this wasn't the answer I was hoping for, was hoping to just align it a different way than I've thought of. Making stub shafts adds a bit of work to my project, we'll see if my boss decides this is the way we want to go. If you think of any other (easier) solutions, I'd love to hear them! Even if it's not as accurate.

Thanks again for your response, that really helps to answer my questions and gives me a solution to aim for!
S

Hi again Spencer:
You can try a couple of other things first.
You have semi round features on the top of both ends of the trunnion.
Put an indicator on each of those surfaces and swing the trunnion.
See if they are concentric with the axes at each end.
If you get lucky, you can use those if you assume (and it's a big assumption) that they are the same radius.
Another way, is to take a round bar, put it in a vee block and then shim it until it runs concentrically, but it will be a fair nightmare to get it concentric at both ends.
You can also indicate the flat face at the headstock end to see if it runs true...if it does you can use that flat face as a datum from which you can establish the orthogonality of that face to the mounting face of the trunnion base.

If you walk through a mental picture of what you have to do to get any certainty using any of these alternative methods of interrogating the trunnion, you can see why I made the original recommendation I did... the stub shaft way is easy and accurate because you don't need to infer anything; your measurement is the most direct it can be.

Probably the closest alternative that does not require you to make anything is to check and use the flat face as I described last.
If it does not run out, then a simple indicator will be all you need to get it the best it can be.
Bolt the trunnion to the mill and sweep that flat face with the indicator in the mill spindle up and down and front to back with the Z axis and then the Y axis of the mill.
Shim it and tap it around as you need to
Pretend that all is good once you have it clocked in.

You can then swing the trunnion 90 degrees one way, clock the height of the headstock end of the trunnion, swing it the other way, clock it again, pray the heights are identical, calculate the difference if they're not, measure the dimension between the flat faces, and divide it by two while compensating for the mismatch between the heights of the faces.
That gets you the vertical location of the axis.
Pick up the horizontal location by spinning the trunnion back to zero, and touching off one of the sides of the swinging bracket as before, then go over your compensated distance to get the Y axis location of the trunnion axis.

Long winded, not super accurate, but far better than nothing.

Best of all you don't have to build anything!

Cheers

Marcus
Implant Mechanix • Design & Innovation > HOME
Vancouver Wire EDM -- Wire EDM Machining

One thing to add to the above:
When loaded as it swings the table no rotary axis makes a perfect circle so you fudge for a compromise.
People think a rotary will be circles, they are not and it's worse when gravity loaded as here.
One does not care about table top which could be .050 and who cares, one worries about twist in space to the machine axis travel.
Get the head right first to the machines X-Y-Z which means not just at its face but out there away also, then check and attach the ass end to it.
Shimming as shown you are chasing both at the same time. Good luck.

At this size you should be able to get this to micron level on this rough checking even on this light machine.
Bob

Marcus and Bob,

Thanks for the suggestions for the machine as-is. I'll give them a whirl and see what I can come up with!
Really appreciate the time you've taken and the wisdom you've imparted

Thanks!!
S