Epoxy-Granite CNC Lathe Design (and Build)

[romihs]

Hello,

I have spent, what feels like ages, reading (and contributing a little to) the Epoxy-Granite Machine Bases thread, slowly putting ideas together of what I would like to build. Initially I wanted to build a mill, but have realized that a lathe is probably one of the first machines one needs, so I have decided to try build a CNC lathe with Epoxy-Granite (EQ).

My initial ideas were to use full length inserts for the linear bearing beds (and all other components that need to be bolted to the base) which could then be ground/machined true once the base/part was de-molded.
Looking at components manufactured by the pro's, I saw that they very rarely had large metal inserts. The reason for this could be any of the following:
- Differences in the linear coefficient of thermal expansion between different materials.
- Time saving.
- Cost saving.
- Something that I have not thought of yet...
Considering that I don't have access to any, well equipped, workshop, I decided not to use full sized inserts. Instead, I will spend more time building an accurate mold and use ground plates for the areas where the linear components and other hardware will be bolted to the base. All that will be cast into the base are steel treaded inserts.

Initially I planned to build a flat bed machine, but one thing kept bugging me; EQ is probably better at handling compressive forces than tensile forces, and the EQ will probably creep most when exposed to forces other than compressive (I do not have proof of this, it is only based on my understanding of the material and its composition). So I looked at the slant bed design and saw that this configuration would transmit most applied forces into the base, and not out of it (i.e. compressive).

Here is what I have come up with so far.


I decided to use a 60 degree angle.
The total length of the base will be about 1 meter.
The linear rails are 15mm wide, and I plan to order types with half-pitch bolt holes. I came across an SKF product range designed for use on these types of machine bases, who's mounting hole pitch is half that of the regular rails. The hole pitch will be 30mm, which will allow me to mount any standard 60mm pitch rails too.

The 'green' parts are EQ. As you can see I plan to use EQ for the cross slide base too.
I'm considering adding chopped fibers into the EQ mix for the cross slide base to increase its mechanical properties.

I won't be building much for a while yet as I do not have any suitable work space in which to do this, and a quick estimation puts the mass of this machine at about 500kg, so I need a pretty solid workshop in which to build this machine.
We are in the process of building a house, with a workshop in the basement.

I will keep updating this thread as I continue with the design.
Any suggestions and info would be appreciated.

Best regards

Sandi

[Gizmot]

something to consider:
how to do you plan on making an accurate 60 degree angle with what I assume would be an mdf mold?
the base is goign to be 1 or 2 pieces? if, how do you plan to join them accuratly?
-will you vibrate the base?


anyway, like the design

[Belding]

What are you thinking for a spindle. Looks like a A2-?. Where are you going to find this. Dunham tool has some cartridge spindles that might fit the ticket. Pretty expensive though. How are you going to keep the ground steel under the linear rails flat when you cast the EG? Unless it is of a substantial thickness, it will not hold itself true when you poor the EG in. It would have to be ground after cast into the EG.

Looks good,
Zac

[romihs]

Gizmot,

Yes, the 60 degree angle is a bit of a problem, that is one of the things I have on my mind quite a bit lately. I plan to construct the mold with a combination of wood (MDF or plywood) and steel reinforcement.

An idea I had yesterday was to use accurately machined steel plates which have a similar profile of the rail and car (use a plate that is the same thickness as the hight of the car on the rail) and build the mold for both the base and the cross slide together. I this case, if I mess up the angle a little, both the base and the cross slide will have the same angle even if it's not exactly 60 degrees.

I plan to build the base in one piece.

I feel that vibration is a must.


Zac,

I plan to build the spindle myself, I plan to make it (at least the face of it) according to DIN 55028 A6. I want a swing over the 'bed' of at least 150mm (300mm workpiece), so I want a 250mm or 310mm chuck. But this is not final. I also like the idea of the larger bore through the spindle.

As I was explaining above, I will probably use machined steel plates as thick as the rail and car is high (24mm). I then plan to build a steel structure around these plates that will reinforce them and position them correctly.
This will apply to other sections of the mold where other parts will be fixed to the base or cross slide. The rest of the mold can be built of anything.
Regarding the weight carrying capability of the critical parts of the mold, I could design the mold such that it is turned in such a manner that these parts are not carrying too much of the EQ mass.

The base will not have the steel plate embedded in it, the steel plate will just produce accurate surfaces on the EQ base for the rails to be bolted onto. At least, that is the plan....


Best regards

Sandi

[Belding]

Sandi,

I have been in your shoes before in terms of thinking about making your own spindle. The more I looked into it though, the more I realized how hard it would be. Even with the right tools, I think it would be very hard to do right. On that type of spindle nose, the gauge line on the short tapered section has to be ground in such a manor that when a chuck is bolted on, the taper becomes tight just as the flat faces come into contact. Then you have to think about the bearings....

Not trying to discourage you, but a spindle is hard to make right. Check this out http://cgi.ebay.com/ws/eBayISAPI.dll...TODAY.m238.lVI

Good Luck,
Zac

[romihs]

Zac,

You are not discouraging me, don't worry.

I am aware of the fact that building your own spindle is difficult. I have never built one, but I am willing to try.....
I have also thought about just buying it, but I know that new ones are damn expensive.... The one you sent me the link of, looks cool, but is typical of many ebay items in the USA, they don't ship outside the USA... I should look around Europe a bit and see what I can find...

Something that I do keep thinking of every now and then regarding building a spindle; what if I finished the spindle off in its final place? All that would have to be machined accurately is the bearing surfaces.... Just a thought I had....
Probably is not feasible for some reason or another... I have very little experience machining anything.

Thanks for your comments.

Regards

Sandi

[digits]

Hi guys - this is an interesting looking project IMHO.

I don't know much about EG - I haven't read all 500 pages of that monster thread, but am I right in thinking EG is self-leveling? If so, can't you cast the ways using an angled mould, so that the bit you want at 60 degrees is actually horizontal during casting? Presumably you don't need the back side of the casting to be precise?

Just a thought - I'll get back to my bolts and aluminium extrusion now!

Good luck!

[romihs]

Hi digits,

The main problem I see with that approach is how to position the inserts in the EQ while it is setting.

The freshly mixed (unset) EQ is not liquid at all, so it will not self level. Self leveling is only achieved using clean epoxy with additives that thin it out.

I must apologize for not having replied sooner... Haven't had the energy to spend time on the forum... Busy preparing for the house build... This process is a long winded one in Slovenia, the paper work (red tape) can also be a bit depressing..... but we will get there. :-)

Best regards

Sandi

P.S. I think you are doing a great job on your build.