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darryl
12-15-2012, 07:43 PM
Some here have made their own machines, whether lathes, milling machines, custom belt sanders, sharpening machines, etc. There has also been interest, though not lately as far as I can tell, in making epoxy/granite machine bases. I have read almost halfway through that tome on this subject, which I think qualifies me for a medal of sorts :) I'm not sure if there were any realized projects to come out of that, but I have my own ideas anyway. In that regard, I am proposing this-

It's just an idea, just for the sake of stirring the grey matter, so take it for what it is. This would be for a milling machine. Take two granite surface plates and stand one up on its end. The other lays flat. Place the vertical one centered along a long side of the other, and at the edge. In an actual construction, these would be epoxied together with a set-up to ensure a perfect 90 degree relationship between what will be the Z axis and the Y axis. They would then be fastened into a made up frame that would help to keep the 'stones' aligned and support their weight, plus the frame would be the interface to the stand.

So, at this point we have a 'granite machine structure'. The ways would be hard chromed rod, induction tgp rod, or whatever suits the accuracy/cost ratio of the builder. Two rods would be placed vertically and two horizontally, each being epoxied directly to the surface plates, being set up perfectly parallel, etc. The bottom of the two vertical rods would nest into holes in a steel piece that is also epoxied into place. This would keep the rods locked into position at their bottoms- a similar arrangement would constrain the top ends. The two X axis rods would be epoxied into place similarly, but these rods would extend past the ends of the base surface plate by an amount determined by the builder. The extension would be enough such that a suitable steel anchor plate could be attached there as well. With the rods laying in a strip of epoxy across the surface plates, and anchored at or near their ends, this should provide a solid and inflexible pair of rails upon which the head and base carriages could ride.

The 'riders' would then be made in suitable fashion to give an adjustable closure onto the rods for elimination of play and ease of movement. I've used .030 poly sheet in this application with very acceptable results, but this could be teflon, it could be air-support bearing- whatever suits. Each rider would be machined in the same setup, so would become identical to all others. They would all have a flat machined onto them as a mounting point for a 'base plate', which would become the foundation of the carriage, with it's Y axis of course, and the head structure. I'm thinking that the Y axis would be a fairly typical type of dovetail slide, to keep the height profile fairly low. The head baseplate would give the option to rotate the head side to side if desired, but probably not a nod.

These are the basics of the idea. The lead screws, etc can be whatever suits an individual design- could be a hydraulic cylinder with a fluid pressure source and valving, with a readout of position, or as simple as threaded rod with dials.

The idea achieves rigidity, mass, and durable parallel ways pre-aligned to the precision of the surface plates. You take it from there.

I also had the idea to extend the vertical rods through holes in the base surface plate as an extra measure of integrity in the basic structure. This would of course require making two through holes in the base plate. I don't know how this might affect the accuracy of the base surface plate, but it shouldn't make a difference since this stuff has been aging for millions of years- The holes would be made oversize for the rods, so the resulting gap could be epoxy filled, thus the holes would not have to be that accurately drilled. With the rods extending through it would give another point for the framework to attach to, further enhancing the rigidity of the entire structure.

Ok, have fun with it :)

Mcgyver
12-15-2012, 08:37 PM
I like the use of a surface plate makes a good base to attach rails to - its flat. All too often these imaginary machines use rails to avoid grinding and scraping mating bearings ....but the daydreamer doesn't address how a flat surface to mount the rails on is obtained.

Past that, granite iirc isn't particularly rigid. As I said at the start of the tome, epoxy granite because of its vibrating damping properties ( because of how much boundary layer area there is between two different materials, you will not get this solid granite) would be a great filler for a fabrication machine tool structure (that had been stress relieved). A fabrication on its own is no good for a home shop sized machine (ring like a bell) nor is epoxy granite (not very rigid) but combined it could really be something. Those proposing to build a machine tool just of epoxy granite are imo barking up the wrong tree imo given its poor Young's modulus.

I've not acted on it because unless you had some very special function machine, its going to be cheaper, quicker and easier to buy old iron and recondition or even a new high end machine...or for that matter new low end one and recondition it :D

J Tiers
12-15-2012, 11:10 PM
I think there is at least one case for making a machine, or at least modifying something very severely. I don't see that granite is a good candidate for what I have in mind.....

One would not choose to make unless the item is actually unobtainable, or unobtainable in a practical sense at a price one would want to deal with. But I think these items fill the bill.

The first item I have in mind is a "mini Bullard"...... a VTL in the 18" or so class. Big enough to be very useful, but never (AFAIK) made in the "real world". I think the smallest commercially made were 24". They are not common, rather heavy and bulky, and probably still command a very respectable price.

Seems excessive, in some ways......However, a VTL gives you the capability to do lathe operations on large work diameter-wise, and in fact, much of the time the workpiece to be dealt with is large in diameter, but not in length. So a VTL would be quiet practical for a lot of the things that need bigger diameter capability, without ending up requiring a huge footprint in the shop.

A mini HBM is another possibility..... I think I recall a link to one that someone in Germany made... Could be very useful for many sorts of work which don't swing on the lathe, are not suitable for a regular mill, and need facing, boring and surfacing work.

Jaakko Fagerlund
12-16-2012, 03:41 AM
I remember one university group making a horizontal wire EDM with two surface plates, one for the machine base and the other as a riser block for Y axis.

MrFluffy
12-16-2012, 04:12 AM
Is granite up to handling shock loads? While I know its much harder and more stable than other stones, I can still get it to shatter with a hammer when struck in the right place, while I'm doing stone wall stuff.
You wouldn't recycle a old granite surface plate into a hammering block for the same reason.

Cast iron yes, its native to what this class of machine is made from anyway, but please before you all rush off and drain the supply of old surface plates, can you give me chance to get one for myself first :)

lazlo
12-16-2012, 12:40 PM
Past that, granite iirc isn't particularly rigid.

I've seen quite a few commercial CNC machines that use a granite bed. These are smaller prototyping machines, but it's a great material for the base -- fabulous dampening properties and cheap per pound.
The problem for a hobbyist is drilling and tapping the mounting holes, which is why cast granite (epoxy granite) is so popular on CNCZone.

Mcgyver
12-16-2012, 01:12 PM
I've seen quite a few commercial CNC machines that use a granite bed. These are smaller prototyping machines, but it's a great material for the base -- fabulous dampening properties and cheap per pound.
The problem for a hobbyist is drilling and tapping the mounting holes, which is why cast granite (epoxy granite) is so popular on CNCZone.

what do you have on the damping properties of solid granite?

obviously polymer concrete (granite) has excellent vibration damping. Its my understanding this is because of the massive boundary layer surfaces existing between the two very dissimilar materials. Perhaps the same happens between the feldspar mica and quartz? Don't know but the dissimilarity of materials is important to damping - IE between the plastic and rock in epoxy granite.

Epoxy granite's or even solid granites suitability depends on the machine; low/no load....ie a wire edm or cmm isn't going to have the same issues as a mill. I agree with you as to why its so popular (so are ghosts and alien abductions :) )I've not tuned into that thread for awhile but my sense of it is there are some knowledgeable people really advancing the subject. There is also a lot of wishful thinkers ignoring its low Young's modulus, bedazzled by its castability.

KiddZimaHater
12-16-2012, 02:01 PM
I've always wanted to build a homemade EDM for tap-zapping.
It's still on the 'to-do' list.

darryl
12-16-2012, 06:44 PM
It would seem then that cast iron has vindicated itself once again. I wonder though if a 3 inch thick slab of granite would compare favorably when you look at what might be available in an iron casting. I look at my mill and lathe and find that the thickest parts of the castings are far less than one inch. I can hook up an indicator and with hand pressure only I can see deflection of a few thou- somewhere along the line there has to be a point at which solid granite would give more rigidity. I can't speak to the ultimate breaking point of either material, but it would also be interesting to compare these strengths based on sheer volume of cross sectional areas. Maybe it would take more peak shock loading to crack the granite slab than it would a cast iron casting with the typical thicknesses in the walls and webbing. I don't know.

One of the advantages of the granite slab construction method is that you can easily buy the 'parts'. Two surface plates, plus several feet of hard chromed solid rod and enough epoxy to set the whole thing would set you back less than $400. With this you have the bed, the tower, and the ways- could you equal this in cast iron for the price and the effort? Importantly also, as many have pointed out- can you get an equal or better performance in terms of strength, rigidity, and shock loading survival capability?

Note that none of this proposal involves casting an epoxy/quartz or granite mixture. However much better the damping might be by using these materials, and however less rigid it might be on its own, it's a different ball game when using solid slabs of granite. I don't know whether studies have shown that solid granite is not 'up to it'. My gut feeling is that for the ultimate in precision, the slab construction might win, and for brute strength in high speed material removal rates the industry standard cast iron construction would win.

Mcgyver
12-16-2012, 09:24 PM
Note that none of this proposal involves casting an epoxy/quartz or granite mixture. However much better the damping might be by using these materials, and however less rigid it might be on its own, it's a different ball game when using solid slabs of granite. I don't know whether studies have shown that solid granite is not 'up to it'. My gut feeling is that for the ultimate in precision, the slab construction might win, and for brute strength in high speed material removal rates the industry standard cast iron construction would win.

you didn't my idea of stress relieved steel fabrications (using say heavy walled rectangular tubing) filled with epoxy granite? Easily made (compared to a casting) and I would bet would have the highest strength and damping per pound of any of the mentioned choices

darryl
12-17-2012, 12:18 AM
Mcgyver, I do like your idea, and it's one that would make a great base support structure for my idea :) My use of the two surface plates would give a means of aligning and maintaining the alignment of machine ways, but would still require a support structure to help keep the two plates together in alignment, and to interface with the stand. Damping the support structure using E/G would perhaps be even more important in this application to prevent possible resonances from compromising the integrity of the surface plates. I'm also thinking that because this idea uses round rod for ways, they would need critical support over their full lengths. Epoxying them directly to the surface plates gives them this.

In theory, it would be possible to use the connected surface plates to align the way rods while the rest of the structure is fastened to them, then remove the plates. This would be a little backwards compared to the normal method of assembling something, but the desired result is the same- accurately aligned ways. Why not then keep the surface plates in place to add mass and rigidity to it all. The result is your idea supporting my idea. I think it would provide an uncommonly found accuracy and rigidity in a relatively small machine, and it should be capable of repeatable high precision work.

You mentioned stress-relief of the presumably welded up frame structure, and I agree that this must be done, or assured via the method used to fabricate it. I'm also of the opinion that the interface to the stand should not put any twisting forces on the frame- this then becomes a matter of how to create the stand and figure out mounting points. In my mind it has come down to creating within the framework a section of steel tubing that is vertical- this then drops over a vertical post which is the stand. If this steel tubing is centralized under the weight of the entire machine, then the framework does not receive any twisting forces from the stand. I have long thought that is important to de-couple the precision alignment of a machine tool from the distorting effects of its mounting. I have mixed thoughts on how much extra rigidity should be provided by the stand itself- the good possibly being outweighed by the bad-

philbur
12-17-2012, 03:31 AM
I think if you are going to assemble a machine from a set of precision parts the real issue is how and with what instrumentation will I achieve the necessary alignments. The available instrumentation may ultimately dictate the machine design.

Glueing/casting precision parts together leaves little room for subsequent adjustment.

Phil:)