Large Epoxy Granite Vertical CNC Mill

[LeeWay]

This is true with just about any inserts in many materials. Goes double if you have to remove the screw or bolt very often. I find that the smooth inserts will loosen up over time. Even ones designed for plastics. The best type are the Hex type with barbs. Then size those accordingly.
For this application though with very little repeat installations, it should be okay. No real torque required there either.

[dwalsh62]

epoxy with an aggregate is a material that doesn't retain bonding to steel well over time but the aggregate does help in this case, something that has a smooth finish from machining, if using round-stock a helix pattern is the way to go as the insert will eventually release over time.

The frames I've been casting lately require no machining, from cast to assembly,and using a polymer based binding agent (epoxy is just a name brand for a polyester based binding agent) is easier to work with as it has a viscosity close to water (or super thin maple syrup), keeps the mixed material fluid enough that it still runs, doesn't require degassing and trapped air is not a problem if you mix from the bottom up at a slow speed.

As far as using epoxy goes, West Shore Marine sells some stuff that isn't polyester based, resin #205 and harder #209 and a thinning agent (12:1 ratio) they will order for you, is still fluid with aggregate mixed in (not like mashed potatoes), wont need a vibration table and has a work life of about 3hrs (the thinning agent doubles the 90min cure time so natural settling occurs) and 1/2 gallon of resin plus a 1/4 gallon of hardner (0.75gal of mixed resin plus 7.5gal aggregate = 10:1 ratio which is optimal) makes 8.25 gallons of mixed material which can make a fairly decent frame with reasonable size.

If you spend the time making your mold with critical surfaces precision processed you wont need any post processing but of course as the OP has run into, missed inserts and after-thoughts cause problems requiring precision processing to correct.

Attached are some basic 3D drawings of a mill and lathe stripped down to the bare minimum in the event someone would like to try their hand at making a frame and you can scale the design to fit your needs, I only caution you that you will get out of the mold the precision you put into it and not all surfaces require precision so if you think using MDF or countertop material on critical surfaces will give you a good result you should not bother as the end product will only disappoint you.

[Chazaxl]

epoxy with an aggregate is a material that doesn't retain bonding to steel well over time but the aggregate does help in this case, something that has a smooth finish from machining, if using round-stock a helix pattern is the way to go as the insert will eventually release over time.

The frames I've been casting lately require no machining, from cast to assembly,and using a polymer based binding agent (epoxy is just a name brand for a polyester based binding agent) is easier to work with as it has a viscosity close to water (or super thin maple syrup), keeps the mixed material fluid enough that it still runs, doesn't require degassing and trapped air is not a problem if you mix from the bottom up at a slow speed.

As far as using epoxy goes, West Shore Marine sells some stuff that isn't polyester based, resin #205 and harder #209 and a thinning agent (12:1 ratio) they will order for you, is still fluid with aggregate mixed in (not like mashed potatoes), wont need a vibration table and has a work life of about 3hrs (the thinning agent doubles the 90min cure time so natural settling occurs) and 1/2 gallon of resin plus a 1/4 gallon of hardner (0.75gal of mixed resin plus 7.5gal aggregate = 10:1 ratio which is optimal) makes 8.25 gallons of mixed material which can make a fairly decent frame with reasonable size.

If you spend the time making your mold with critical surfaces precision processed you wont need any post processing but of course as the OP has run into, missed inserts and after-thoughts cause problems requiring precision processing to correct.

Attached are some basic 3D drawings of a mill and lathe stripped down to the bare minimum in the event someone would like to try their hand at making a frame and you can scale the design to fit your needs, I only caution you that you will get out of the mold the precision you put into it and not all surfaces require precision so if you think using MDF or countertop material on critical surfaces will give you a good result you should not bother as the end product will only disappoint you.

Thanks. What material would you recommend for making these 'critical' surfaces?

[steve123]

This is the first time I have heard of this problem with end machining and ball nut quality. Please post the name of your ebay ballscrew source so we can possibly avoid them. Please PM me if you don't want it public.

[petruscad]

Some advice based on experience with epoxy, polymer and concrete machine castings, when making inserts use hex-stock of the appropriate size, make the serration's just as you did in the round stock so when you epoxy it in there is no way it can turn or pull out and you wont face a problem where you are unable to tighten or remove a bolt because your insert turns.

I guess I did not think about the insert rotating vs. the epoxy: I could have machined a flat on one side. I don't think it rotated at all and as LeeWay mentioned there is little torque needed here so I don't expect it to fail. If they do fail I can make some hex inserts and replace the existing ones.

[dwalsh62]

Thanks. What material would you recommend for making these 'critical' surfaces?

Your question is incomplete but I'll answer you in the way I believe your question is intended to be about the mold and it's critical surfaces.

The material critical surfaces is made from is not really important as long as it can maintain the critical dimensions/tolerances required for the surface and be rigid enough to support the weight of the aggregate material.

We've used granite countertop material with excellent success and it's wax/PVA friendly.

If your mold is small (ex. 16in x 24in x 5in as needed in the base) you can make the mold bed from two pieces of countertop granite, cut them at the same time so they are the same width/length, drill one face up (the mold bed), the second face down (the mold base) with the holes exaggeratedly over-sized (so you don't need extra long bolts to attach the inserts) and then epoxy them together and when the epoxy starts to gel, clean up the holes and sides and push washers into the oversized openings to aid installation/removal.

In this small size the countertop material has a flatness of 0.054mm per 1000mm length (worst case) and your working with 60% of this so expect 0.0028mm (0.00011in) to 0.025mm (0.000984in) max.

We have used this material several times now and after CMM'ing we have found it to be around 0.008mm (0.000315in) and three tenths of one thousandth in flatness is more than acceptable and at 2.5in thick more than strong enough to resist any flexing (and yes the material will flex so using a flat surface to join the two slabs is important).

We've also used 2.5in thick 1018 steel stock and blanchard ground it to get the surface flatness to acceptable tolerances, the sides are not really critical and the majority of the base is purely cosmetic so you can get away with a lot of deviation and as long as you can't see it no one will complain.

The sides of the mold can be 1/4in thick aluminum stock and you can make a wood replica of the base in wood so you can make a silicone plug to give the base top the angles, cut-outs and contours you want and trim the silicone flashing after it's set to keep the cast product detail clean.

[Chazaxl]

Your question is incomplete but I'll answer you in the way I believe your question is intended to be about the mold and it's critical surfaces.

The material critical surfaces is made from is not really important as long as it can maintain the critical dimensions/tolerances required for the surface and be rigid enough to support the weight of the aggregate material.

We've used granite countertop material with excellent success and it's wax/PVA friendly.

If your mold is small (ex. 16in x 24in x 5in as needed in the base) you can make the mold bed from two pieces of countertop granite, cut them at the same time so they are the same width/length, drill one face up (the mold bed), the second face down (the mold base) with the holes exaggeratedly over-sized (so you don't need extra long bolts to attach the inserts) and then epoxy them together and when the epoxy starts to gel, clean up the holes and sides and push washers into the oversized openings to aid installation/removal.

In this small size the countertop material has a flatness of 0.054mm per 1000mm length (worst case) and your working with 60% of this so expect 0.0028mm (0.00011in) to 0.025mm (0.000984in) max.

We have used this material several times now and after CMM'ing we have found it to be around 0.008mm (0.000315in) and three tenths of one thousandth in flatness is more than acceptable and at 2.5in thick more than strong enough to resist any flexing (and yes the material will flex so using a flat surface to join the two slabs is important).

We've also used 2.5in thick 1018 steel stock and blanchard ground it to get the surface flatness to acceptable tolerances, the sides are not really critical and the majority of the base is purely cosmetic so you can get away with a lot of deviation and as long as you can't see it no one will complain.

The sides of the mold can be 1/4in thick aluminum stock and you can make a wood replica of the base in wood so you can make a silicone plug to give the base top the angles, cut-outs and contours you want and trim the silicone flashing after it's set to keep the cast product detail clean.

Thanks, suspected something like granite or similar.

[petruscad]

I do not have a separate thread for the tool post grinder. The construction is very simple. The motor and the shaft are mounted to a piece of aluminum extrusion which directly attaches to the tool post of the lathe. Here are three views of it:







The motor is a Bosch palm router that drives the shaft through a belt! The pulley screws onto the router in place of the nut. The shaft is rotating on two radial bearings. The router mount was originally made for another purpose (which did not pan out) and since it was lying around I just reused it here.

[petruscad]

On the precision mold topic.

Sure, making a very precise mold would be nice. The problem with it is that its difficult, slow and expensive. And only a few surfaces need precision anyway. A simpler way of making precise bases used in industry is replication. They basically cast the base with a cheap, imprecise mold and then add precision to the few surfaces where it's needed: to bolt linear rails for example. But they do not do that by grinding the granite the way I do it here but by molding another layer of epoxy to the base in a precise way (Note though, that grinding of EG bases after casting was used in industry in the 60s and 70s). The way they create precision is by applying a thin layer of epoxy-based paste (e.g. moglice) to the place where precision is needed and then lowering a straight edge or surface plate on top of it in a very precise way. Injecting liquid filler between the rough base and the precise surface is another way. After curing the reference plate/edge is removed revealing a precise surface behind (this is why its called replication). Cincinnati Milacron used this technique on some of their their grinders for example. Of course this still requires a precise reference surface, which is expensive but at least you need a smaller one than a whole mold. In any case for this build I will stick with grinding rather than switch to the replication method because I know how to do it now, it seems to work and I only need to grind a few surfaces.

[gtrdude]

What sort of bearings did you use on the grinder shaft?

[CaptainVee]

On the precision mold topic.

The way they create precision is by applying a thin layer of epoxy-based paste (e.g. moglice) to the place where precision is needed and then lowering a straight edge or surface plate on top of it in a very precise way. Injecting liquid filler between the rough base and the precise surface is another way. After curing the reference plate/edge is removed revealing a precise surface behind (this is why its called replication).

How do they remove the surface plate, or rather, is there a trick to easily removing a surface plate like that from the cast surface?

So far my experience suggests that separating two super stiff objects that have one surface cast together are very hard to separate.
No matter how well a separation wax has been applied, there is zero room between the two surfaces and since bending is not possible the entire surface has to release more ore less at once.

[LeeWay]

Compressed air works great for this.

[CaptainVee]

No, it does not.

Compressed air only works when one surface is flexible.

[LeeWay]

It works on two rigid surfaces as well. Consider an air bearing. It creates an air cushion, so space between two surfaces. I do see your point though.
It may require some testing to see how to best inject the air so that it does cover the surface between two plates rather than blow right through. Shear volume might handle it. Sealing tape around the joint before the casting might help it. The right amount of contained pressure can overcome stiction I think.

[CaptainVee]

Have you actually tried it?

With an air bearing there already is air between the two surfaces.
With a casting there is no air (and I mean no air and no space whatsoever) between the two.

As both parts are super stiff, neither part will bend and so can not be peeled, no matter where and how you try to apply air pressure, the parts will not release.

There may be a difference between casting and molding. In casting there is so much material that no air can get through the material, with a molding (I am experimenting with super dry epoxy/sand mixes), air can get through and the surface area is a lot smaller and releases more easily.

So far, with mdf as mold material I have had no problems.
But my moldings are thin anyway and thus can easily flex and peel.

[LeeWay]

I haven't tried it in a casting like this, but I have seen it done on large boat hulls at the Ship Yard. A few wedges around the perimeter and an air blast pops it out. They did have to do it in several places around the mold to get all the parts to pop free.
It was a large covered life boat. Neither surface was all that rigid though.
What might take a ton of pressure to lift to get separation might only take a few pounds of side pressure. If you use some thin rubber walls around the casting, then moving it nearly any amount sideways would pop it loose.

I do use air quite a bit around here for various things. More than just blowing things off and filling things up.
It is a great tool to use sometimes. Can be a dangerous one as well.

Ever seen two 5 gallon buckets that were locked in a stack? Compressed air will easily separate them. That is just a shop tip.

[CaptainVee]

With moulding a boat both the boat and the mold are super flexible if you compare them to casting a surface plate.

Try putting two pieces of thick glass together with water in between.

The only way you can separate the two is by moving them sideways, and that is possible because the water lets the parts float.

With two surface cast together, there is nothing in between and the surface is too rough for sliding as again, the entire surface needs to release all at once, it dos not allow peel.

And those buckets? There is air between them already (microscopic amounts) and they are super flexible.

[LeeWay]

Cast in screw jacks to start with.
Use resonance from vibrating it.
Use an air hammer combined with air.
Thermal shock. Use an infrared heat lamp or bank of lamps on one side or stick it all in an oven and hope it has different expansion properties on each side.
Or a block of dry ice on one side.
Or plan ahead of time to use wedges around the perimeter. Think splitting granite.
Cast around a rather large fire cracker.
Play tug of war with two bulldozers.
Drop it out of an airplane.
Oh it's coming apart.
If not, then drop it off a boat and leave it to start an artificial reef.

[DICKEYBIRD]

The motor is a Bosch palm router that drives the shaft through a belt! The pulley screws onto the router in place of the nut.

Cool! Great minds work together....I'm building a very similar set up to yours. I picked up a Sherline mill spindle off ebay & milled a dovetail slot in the housing to mount it directly onto my QCTP. Mine will be powered by a Porter-Cable trim router & belt/pulleys.

I never even thought about machining the motor pulley to thread on in place of the nut. Much better than mounting the pulley on a 1/4" shaft in the collet. Thanks for that! I'm looking for a CBN wheel so I won't have to deal with as much abrasive dust.

What kind of belt is that & does it work OK?

Thanks!

[petruscad]

What kind of belt is that & does it work OK?

I think its a 3L280 belt and it does work pretty well. Not much force is needed so even a 2L belt will work. With the router motor though, make sure you do not exceed the RPM rating of the abrasive wheel (by much!).