Measuring concentricity and flatness on diy CNC machine

[Goemon]

I've reached the part of my build that I was dreading the most - figuring out how to square up all the axis and making sure the spindle is positioned properly in relation to the table and the axis.

Please can someone tell me what tools or measuring devices you guys use for this? For example, should I buy this:

https://www.ebay.com/itm/Pro-Tram-Br...EAAOxyVLNS6tfr

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[Jim Dawson]

You don't need a fancy tool, sometimes simple is better. I use a welding rod on my router.



The welding rod is brazed into a 1/2 inch slug that fits in the router spindle. In this case I was using a piece of paper as a feeler gauge. Paper works better because you can feel the drag, a feeler gauge is too slick to properly feel it in this case. First get the tram as close as you can, then take a light skin cut on the table. That's just to get the table level relative to the rest of the machine. Then tram again to get final tram. Mine is < 0.002 in 36 inches, that's close enough for any work.

On my milling machine, I just swing an indicator in the spindle. That's the old school way of doing it, but I'm old.

[ger21]

Imo, those are a waste of money. You only need one indicator, and you can make something very simple with a rod to chuck in the spindle, and a piece of wood to attach to it, and hold a cheap indicator. Probably $20 at harbor freight, and just as accurate.

As for squaring up the axis, a good square is all you need, and something to check for level, or straightness.

[Goemon]

Imo, those are a waste of money. You only need one indicator, and you can make something very simple with a rod to chuck in the spindle, and a piece of wood to attach to it, and hold a cheap indicator. Probably $20 at harbor freight, and just as accurate.

As for squaring up the axis, a good square is all you need, and something to check for level, or straightness.

I like answers like this that save me money as well as being helpful!

If I use one dial indicator then am I right in assuming that you check the other side by simply turning the shaft until the indicator is on the other side and checking it produces the same reading all the way up and down both the x and y axis?

On squaring, I have an old Machinist square and a bubble level in my toolbox. I was thinking of buying a laser level for better accuracy.

[Goemon]

You don't need a fancy tool, sometimes simple is better. I use a welding rod on my router.



The welding rod is brazed into a 1/2 inch slug that fits in the router spindle. In this case I was using a piece of paper as a feeler gauge. Paper works better because you can feel the drag, a feeler gauge is too slick to properly feel it in this case. First get the tram as close as you can, then take a light skin cut on the table. That's just to get the table level relative to the rest of the machine. Then tram again to get final tram. Mine is < 0.002 in 36 inches, that's close enough for any work.

On my milling machine, I just swing an indicator in the spindle. That's the old school way of doing it, but I'm old.

Nothing wrong with old school if it work. Please can you explain why you don't use the same method in your CNC router as you do for your mill? I.e. If you have the indicator then why not use it so you have a reading to work with?

[Jim Dawson]

Nothing wrong with old school if it work. Please can you explain why you don't use the same method in your CNC router as you do for your mill? I.e. If you have the indicator then why not use it so you have a reading to work with?

A feeler gauge and a stylus works just as well as an indicator. On the router I can swing a 36 inch circle (18 inch radius) so I use that method. 0.002 or so at 18 inches from the spindle is roughly equivalent to <0.0005 at the 4.5 inch radius that I can swing with an indicator on the mill. That's close enough for me ( and NASA ) It's physically easier to use the stylus because you don't have to get eyes on the needle of the indicator, you can take all the measurements without trying to see the indicator dial.

[louieatienza]

Imo, those are a waste of money. You only need one indicator, and you can make something very simple with a rod to chuck in the spindle, and a piece of wood to attach to it, and hold a cheap indicator. Probably $20 at harbor freight, and just as accurate.

As for squaring up the axis, a good square is all you need, and something to check for level, or straightness.

The reason there are two indicators is because they're made to traverse the gap on a vise, since the primary intended use of this is for milling machines, which normally have the vise installed semi-permanently. Trying to tram a Bridgeport, moving an indicator from one side of the vise to another, is not fun. Also used for setup and transfer of angles on, say, a sine plate or vise....

I made something simple with a piece of drill rod, and some aluminum stock that has hole to fit rod on one end, and the indicator on the other end. But I don't operate a busy machine shop. For the home guy (who doesn't have a mill) it's way overkill.

[wizard]

You have some good suggestions here, here are a few more.

1. Considering your previous posts a good dial test indicator will likely be a good investment for you. You will need it for setup of the work piece prior to machining. That gage actually will need a gage stsnd at some point.

2. Gage stands are widely variable in usefullness and rigidity as such you are better off buying a better quality unit.

3. You can not trust off the shelf carpenter tools! This has bitten me more than once. Thankfullly you can check carpenter squares and levels yourself. It is important not too assume you refrences are good.

4, I have too question the accuracy of a laser level for this sort of work.

[NIC 77]

So not really understanding the machine set up.......

Do you have a spoilboard? Mount a bit in the spindle or a piece of straight rod, and get out a good square to adjust it perpendicular to the table. Try it in a few different places on the table to see if the table is noticeably warped, then surface your table using your spindle.

Then check your table with a long straight edge to see if you have any bows in the table due to the surfaces your rails are attached to having bows in them. Or use the simple tool Jim suggested. Not a bad idea Jim!

OR

No spoilboard?

Then you have to shim your table or adjust your gantry with either bolts or shims. Search for "shim tape" or epoxies you can use. Or adjust your gantry on one side to be higher / lower.

There is a macro in mach 3 that someone posted on the forum long ago and it is great for setting zeros. You can also use it to check your table. The machine stops when electrical contact is made.....I'm guessing you know how it works? Well, yes, you can use this to spot check your table height at different locations, and it doesn't require purchasing a dial indicator. You just need to make simple electrical contact from a piece of metal of known thickness that plugs into your BOB and have the other contact at your bit. You can reuse this later for setting your zeros.

[louieatienza]

Being a carpenter, the ol' carpenter's trick for trueing a rafter square is:

1) Using a known straightedge, draw a perpendicular line with the long leg of the square against the straightedge
2) Flip the square horizontally that the long leg is facing the opposite direction, line the corner up against the bottom of the line just drawn and draw another line
3) If the lines match, your rafter square does indeed measure 90 degrees (at least to the accuracy of your pencil line), and you can use it.
4) If the short leg of the square overlaps the first line, then the square is slightly obtuse. Take a nail set and a hammer , and strike close to the outside corner of the square; that will slightly stretch the metal in that area, closing the angle
5) If there is a gap between the top of the short leg and the line, and the corner is touching the line, then the angle is slightly acute, and you'll have to strike close to the inside corner of the square, which will spread it just enough.

May take more than one strike to true the square.

The fastest way to check skew of a table is to take two known straightedges, and stand each up at opposite sides of the table, and sight the top edges of both. You can surface your table with your spindle and it won't guarantee a flat surface, but a surface normal to the Z axis. Basically surfacing will "telegraph" the topology of the rails of the axes. If you use a piece of ground aluminum tooling plate for your table (and for the size of your table it should be relatively inexpensive), you can be assured that for your size it will be accurate to a max .002"-.004" of flat, then you can put an indicator on your z and use the table as a datum to adjust the gantry. Otherwise you'll be chasing your tail.

[Goemon]

The reason there are two indicators is because they're made to traverse the gap on a vise, since the primary intended use of this is for milling machines, which normally have the vise installed semi-permanently. Trying to tram a Bridgeport, moving an indicator from one side of the vise to another, is not fun. Also used for setup and transfer of angles on, say, a sine plate or vise....

I made something simple with a piece of drill rod, and some aluminum stock that has hole to fit rod on one end, and the indicator on the other end. But I don't operate a busy machine shop. For the home guy (who doesn't have a mill) it's way overkill.

I'm not sure what category I fit into for what is considered overkill. I'm not going to be manufacturing product but I will be making tools for work out of aluminum.

To some extent, a CNC machine is a CNC machine regardless if it cuts with a high speed spindle or a low speed milling head. I guess I don't understand why the process of measuring squareness and flatness would be different. Are you saying that the accuracy requirements for metalwork are greater than woodwork? Or is it based on the column instead of gantry design?

[Goemon]

You have some good suggestions here, here are a few more.

1. Considering your previous posts a good dial test indicator will likely be a good investment for you. You will need it for setup of the work piece prior to machining. That gage actually will need a gage stsnd at some point.

2. Gage stands are widely variable in usefullness and rigidity as such you are better off buying a better quality unit.

3. You can not trust off the shelf carpenter tools! This has bitten me more than once. Thankfullly you can check carpenter squares and levels yourself. It is important not too assume you refrences are good.

4, I have too question the accuracy of a laser level for this sort of work.

I have an analogue and a digital dial indicator. Based on one of the suggestions here, I was planning on mounting one on the spindle using a cylindrical rod. I have some very rigid carbon fiber rods that will fit one of my collets. Is there a reason to buy one of those dial indicator stands too?

If a laser level is not accurate enough, what should I buy instead? I currently have an analogue bubble level and a laser level. The reason I thought about using the laser level is that is gives a digital reading while the analogue device relies on me eye-balling it.

If I can't rely on my Machinist squares, what should I buy to check their accuracy?

I have a decent laser tape measure. I was thinking about mounting it vertically next to the spindle to see if it gives the same reading to the table across the X and Y axis. I guess this would have the same issues with accuracy.

I don't mind investing in the right tools (within reason), I just need to know what they are. I don't do well with analogue methods that rely on eye-balling it. My preference is usually to work from a digital readout so I have something specific to aim for and definitive evidence that I have got it right (or not).

[Goemon]

So not really understanding the machine set up.......

Do you have a spoilboard? Mount a bit in the spindle or a piece of straight rod, and get out a good square to adjust it perpendicular to the table. Try it in a few different places on the table to see if the table is noticeably warped, then surface your table using your spindle.

Then check your table with a long straight edge to see if you have any bows in the table due to the surfaces your rails are attached to having bows in them. Or use the simple tool Jim suggested. Not a bad idea Jim!

OR

No spoilboard?

Then you have to shim your table or adjust your gantry with either bolts or shims. Search for "shim tape" or epoxies you can use. Or adjust your gantry on one side to be higher / lower.

There is a macro in mach 3 that someone posted on the forum long ago and it is great for setting zeros. You can also use it to check your table. The machine stops when electrical contact is made.....I'm guessing you know how it works? Well, yes, you can use this to spot check your table height at different locations, and it doesn't require purchasing a dial indicator. You just need to make simple electrical contact from a piece of metal of known thickness that plugs into your BOB and have the other contact at your bit. You can reuse this later for setting your zeros.

Thanks. I was going to ask about this next.

I am not planning on using a spoiler board for my main use as I'll be making mold from flat stock and won't be milling through to the table.

I have a fixed gantry with a table that moves for the X-axis only. Mounting the X and Y rails so that they are perfectly square is the biggest challenge for me....

[louieatienza]

I'm not sure what category I fit into for what is considered overkill. I'm not going to be manufacturing product but I will be making tools for work out of aluminum.

To some extent, a CNC machine is a CNC machine regardless if it cuts with a high speed spindle or a low speed milling head. I guess I don't understand why the process of measuring squareness and flatness would be different. Are you saying that the accuracy requirements for metalwork are greater than woodwork? Or is it based on the column instead of gantry design?

It's overkill because you don't really need the two indicators. Like I said, it's designed that way because the mill vise has a gap, so trying to indicate both sides of the gap while lifting the indicator over the jaws of the vise is next to impossible. You don't have this problem on a router because you work off the table, thus you can sweep your indicator around the table with no obstructions. If you want to buy it for the hell of it, I don't care.

The main point of the ProTram is speed of use primarily for the circumstance I mentioned. The secondary use would be to indicate or transfer an angle for a sine plate. One can argue making your own swing arm is better because you can sweep a greater area than the Pro Tram covers. But in response to your question, generally, yes; because wood will move just from relative humidity, let alone temperature far faster than aluminum, by a factor of about 10.

[Goemon]

It's overkill because you don't really need the two indicators. Like I said, it's designed that way because the mill vise has a gap, so trying to indicate both sides of the gap while lifting the indicator over the jaws of the vise is next to impossible. You don't have this problem on a router because you work off the table, thus you can sweep your indicator around the table with no obstructions. If you want to buy it for the hell of it, I don't care.

The main point of the ProTram is speed of use primarily for the circumstance I mentioned. The secondary use would be to indicate or transfer an angle for a sine plate. One can argue making your own swing arm is better because you can sweep a greater area than the Pro Tram covers. But in response to your question, generally, yes; because wood will move just from relative humidity, let alone temperature far faster than aluminum, by a factor of about 10.

That makes sense. I had misunderstood the point of the two indicators. I had assumed it was a built in leveling feature for the guages to make sure the guage and spindle were not mounted at an angle before measuring.

Have you ever tried using a digital depth guage for part of this process? The one I have in my toolbox is accurate to 0.01mm. Maybe I could use it with one of the methods described below?