Need a bed surface ground.

[Dabbler]

Detecting twist with the table is on the machine accurately presents some measurement challenges, but you can get the idea by using a 1-2-3 block and using a plunge indicator.

Crank your table so that your milling head is at one corner, at the maximum travel in X and Y. Place the block on the table and measure the height using your plunge indicator. Lift the stylus and remove the block. Crank to the next corner and repeat, gently dropping the indicator onto the 1-2-3 block.

Some caveats: you need to stone the table so that there are no burrs lifting the 1-2-3 block or your measurement will be misleading. The better the indicator, the better your measurement will be. I write on the table using a sharpie what the difference in measurement is (I zero the indicator at the first measurement).

This isn't entirely the whole story but it will give you an idea of what your looking at from a practical perspective.

 

[DavidR8]

Detecting twist with the table is on the machine accurately presents some measurement challenges, but you can get the idea by using a 1-2-3 block and using a plunge indicator.

Crank your table so that your milling head is at one corner, at the maximum travel in X and Y. Place the block on the table and measure the height using your plunge indicator. Lift the stylus and remove the block. Crank to the next corner and repeat, gently dropping the indicator onto the 1-2-3 block.

Some caveats: you need to stone the table so that there are no burrs lifting the 1-2-3 block or your measurement will be misleading. The better the indicator, the better your measurement will be. I write on the table using a sharpie what the difference in measurement is (I zero the indicator at the first measurement).

This isn't entirely the whole story but it will give you an idea of what your looking at from a practical perspective.

I wanted to see if the table was flat on my RF clone and this is precisely (pun intended) what I did. Worked like a charm.

 

[Tom Kitta]

I have an 18" surface grinder - I am not sure what is the actual max left to right. The Y axis is around 8"

 

[Smi_tty]

I will take some measurements tonight, and get back to you guys.

I might hit you up on getting the bed surface ground - do you need some measurements from me to see if you’d be able to accomodate a piece this large?

 

[Tom Kitta]

Can you maybe be OK with just grinding the places where T-slots are - not the actual border around?

Sure you can measure it for me and I check the actual limits of the grinder.

 

[PeterT]

I'm not following along here.

Lets just temporarily set aside that the column (and therefore spindle) may not be perpendicular to the ways, because that issue would give you different indicator readings on a 100% perfectly level and true table. But if the table happens to be bowed/twisted like sketch, that means the upper surface, the lower surface and the dovetail is very likely all bowed together. So if you surface grind the table top (red line) that isn't doing anything for the dovetail surface underneath. Imagine the re-ground table is now displaced (blue arrow). The new flat surface is no longer level to the vertical axis anymore. Its being driven by the underside surfaces and (I assume) the bottom surface & dovetails have to also be re-ground parallel once the re-ground top becomes the new datum. Am I missing something?

 

[Tom Kitta]

Yes, table geometry has to be carefully checked to see what is the problem & if it is the table or something else.

All tables do sag with time - on this machine the table is rather small so I would not expect a lot of sag vs. say a BP.

Also sag may only be an issue if it is not just major and you need quality parts but if you are dealing with a part that takes the full length of the table. This is rare & if you did indeed need say full 28" on a BP of travel and had "bad" table then you could compensate for it - done once a year or so.

Any actual burs on the table surface can be dealt with - according to my sources - with a brand new file that is longer then the table width.

 

[RobinHood]

@Smi_tty: do you have any pictures of the mill before the repair? Especially the broken base would be of interest.

 

[Smi_tty]

Unfortunately i lost all the before photos...my phone crapped the bed after the mill was completed.

i will post a photo of where it used to be cracked tonight.

Again, for my needs, this mill is perfect. I would like to try and tram it successfully though, so i know i could use it on some high spec’d parts in the future.

 

[RobinHood]

I doubt your table is bent. That is not how cast iron behaves. Here is a quote from “machine design dot com“ (link below): “Compared to wrought iron or steel, cast iron is brittle, hard, and non-malleable. It can’t be bent, stretched, or hammered into shape. Its weak tensile strength means that it will fracture before it bends or distorts. It does, however, have good compression strength...”

https://www.machinedesign.com/mater...-the-difference-between-cast-and-wrought-iron

What you are seeing is most probably an alignment issue between the milling spindle and the table. Grinding the table will not fix that.
Not saying don’t grind the table top to give it a better finish - especially since it was rusty. You could also scrape the table. Be sure to do the whole table - including the border using whatever method. You will have problems mounting accessories (like a vise) if you have a high border.

Shimming or scraping in the column surfaces to match the table will probably be your only way to success.

I would take the column off, put an accurate dial test indicator with a mag base on the table and probe the base-to-column surface by moving the table in x and y. This should give you an idea of how much to shim/scrape. Shim the column, reassemble and check again - this time spindle to table. You should be very close if the column-to-head and head-to-spindle were reasonably accurate to begin with. Adjust as required.

EDIT: first sentence should read :” I doubt your table bent as a result of the accident”

 

[Tom Kitta]

Cast iron can be bent a bit and can be (especially on abused machines) shaped to "size" with repeat blows of a hammer. It just cannot be bent or shaped even close to what steel can do.

You can test this on a BP mill - move table to one side of the saddle and then to the other - some mills that are rather bad will have difference of around 0.01 - i.e. 1/4 of mm. Even concrete bends a little - everything bends - even glass or tungsten carbide endmill. Just that the endmill bends far less then a same size HSS end mill.

Table could also be fractured or have some other issues.

There is like 100 things that could be wrong - it could be something got crushed or damaged during a drop. Or it is just worn out. Or a factory shim is gone etc. etc. etc.

 

[Smi_tty]

Tom Kitta is 100 percent correct. As a journeyman autobody technician, I know for a fact that every material (being plastic, steel, cast iron) can be bent without cracking or fracture. I’ve encountered it numerous times on vehicles in an accident, or during restoration.

Every part on a vehicle is engineered to how it moves, and behaves. i’ve seen cast iron spindles bent from a collision, but weren’t cracked otherwise. Same goes for pot metal trim, which doesn’t have to crack for it to change shape.

I messed around with it a little more tonight and got frustrated, which means I need some time to problem solve.

 

[Dabbler]

Ok guys, you are sort of right. Everything flexes until it yields. Not bends. If you smash cast iron it won't bend, it will shatter. Maleable Iron bends before it breaks (ref: Durabar) HOWEVER.... a grey cast iron table on any milling machine just doesn't "bend" in a way that is easily measurable. Before you respond with umbrage, hear me out. Cast iron can flex up to its elastic limit, but it is very small. I mean so small that on a surface plate and pressing with many hundreds of pounds, the surface plate is flexing almost as much as the grey iron. That is because it's modulus of elasticity is very large compared to its tensile yield strength which, as @RobinHood mentioned is quite small. You can get a bar of grey cast iron to flex a small percentage of its overhang but that's not what this thread is about.

@Tom Kitta the effect that you mention in:

move table to one side of the saddle and then to the other

can come from a bunch of sources. In a brand new machine, it can be micro-mismatches between the dovetails until the machine wears in. -- In a well used machine, it can be oil film clearance, but the most common reason is.... drum roll please: very bad maintenance and under-lubrication. by not having an oil film, it doesn't take too many passes with no oil to get the effect that you are seeing...

Your hammer example is a way of putting residual stress in the cast iron. That will result, in time, with a crack. I've seen abused cast iron put on a shelf, and when picked up several years later, it was in two or more pieces. There is really no appreciable bending in cast iron, which is why it is used on machines instead of steel or maleable iron or ductile iron, for that matter.

 

[RobinHood]

I think everybody is making good points.

Sorry for not explaining myself very well. I know that everything bends (and basically is “like rubber”). Some materials bend more than others under a given force and same section before they take a “permanent set”. CI bends very little before it breaks/cracks (compared to other metals).

My (and others) main concern with the approach of grinding the table is we don’t know if the table is actually the problem. Aside from the cosmetic aspect, it could well be that the geometry of it (top surface flat and parallel to the dovetails and other bearing surfaces underneath) is correct.

Correct me if I am wrong @Smi_tty, you are looking to improve the tram of your spindle to the table, primarily, and, secondarily, remove any left-over cosmetic issues on the top table surface.

The road to success, in my mind at least, is to work backwards to eliminate the “acceptable“ components from the solution. If we knew the table is good, then we could check the table-saddle combo. Once those two are proven, the only thing that can be the source of the misalignment is the column/base.

Shimming/re-scraping those surfaces should then get you where you want to be.

 

[PeterT]

Probably time to move on. Strength of materials is a subject unto itself. This link might help with basic terminology & quantifying what range we are speaking of.
http://steeljis.com/roymech/mechanics/stress_strain_diagram.php

Visual conclusions. Cast iron (in tension) has a relatively low fracture strength compared to steel. This is evident by it's stress-strain curve abruptly ending at a certain stress value. Mild steel can continue to withstand higher stress, but it typically enters plastic (irreversible) deformation and then also ultimately fails. Notice the humps & bumps along the mild steel curve as other factors enter the picture. Stiffness is the slope, how much strain per unit load. In this example, at 275 MPa, this particular mild steel alloy deforms less than the cast iron for the same load. At 400 MPa the cast iron has fractured, the mild steel may not have failed but deformation increases dramatically. And of course these comparisons are all a function of specific alloys.

So I think we are saying similar things. For 'reasonable loads' CI probably would have broken. Like if the mill was picked up by the table or fell over on its side it probably would not be in one piece. CI is also much less likely to naturally distort (stress relieve) itself over time like steel can because of casting properties. I commonly hear these references that its normal for CI mill table to droop 2 thou per side over a work life but it would be interesting to hear the underlying details. Did it really do that because of its own weight? Even a heavy accessory parked on either end would mean plastic deformation to some degree? I wonder out loud if they are experiencing some wear & measurement issues masking table distortion? Anyways, good conversation! We are all still learning.

 

[Tom Kitta]

https://nvlpubs.nist.gov/nistpubs/jres/22/jresv22n2p191_A1b.pdf

old paper from 1939.

In it we have "The determination of the elastic properties of cast iron is further complicated by the fact that even moderate stresses produce plastic as well as elastic deformation in this material. Attention was called to this fact approximately 50 years ago, by de Segundo [5] and by Bach [6]. The observations of these early investigators have been confirmed by subsequent workers, and it had been definitely established that plastic deformation of cast iron begins under low loads."

I.e. the fact that cast iron is easy to deform even under low loads (which was a bit surprise to me) has been know for almost 150 years now.

By no means I am an expert on metals - the properties of different steels and grades of cast iron have been a topic of research for last few hundred years in a formal setting and last few thousand years in a less formal setting.

Cast iron is used in machines for its dampening properties of vibrations as well as frequently lower price then use of steel (it can be cast to shape & easily machined or even (new to me) processed from rough cast bar without need for cast to shape) also it helps that cast iron is great at wear surfaces - it resists wear well & provides some self lubrication. See: https://www.machinedesign.com/materials/metals/article/21834718/cast-iron

Note that there are many types of cast iron - most popular stuff is discussed in link above.

 

[Smi_tty]

I got distracted and never came back to fill you guys in on my success.

I dummied the process down after thinking about it a little more. I tightened the bolts in the same sequence each time i shimmed it a bit. The key to all of this was keeping the shims placed in the EXACT same spot throughout the entire process ( same depth, direction, stacked evenly, making sure they did not move at all. The readings were finally consistent, and it was a simple process which i could objectively figure out.

i got it down to 0.002-0.003 total deflection!

 

[PeterT]

That's great to hear! So was the shimming in this area where we were speculating - between the bolted faces?

 

[Smi_tty]

Yep.

 

[YYCHM]

What does "between the bolted faces" mean?