[ger21]

But the cured end result is (again, as far as I know) quite safe

Yes, cured epoxy is essentially just a plastic. You don't want to breathe in any dust, but it shouldn't be toxic to the touch.
Uncured resin and hardener is another story.

[Voyager77]

As far as I know the individual components of epoxy are toxic to some extent and can indeed cause allergic reactions that can build up over time.
Always wear gloves when handling and mixing and clean spills from your hands using vinegar.

But the cured end result is (again, as far as I know) quite safe, as long as when they are mixed in the correct proportions!!!
If you can point to information that shows the opposite, I'd love to read it.

Health Effects from Overexposure to Epoxy

......Never breathe the sanding dust of partially cured epoxy. Epoxy chemicals remain reactive until they have cured. Serious health problems can result from sanding epoxy before it is fully cured. When you inhale these dust particles, they become trapped in the mucus lining of your respiratory system. The reactive material can cause severe respiratory irritation and/or respiratory allergies......

Some epoxies can take days or even weeks to fully harden. So it's good to err on the side of caution and wear protection when sanding this stuff after its been cast.

[CaptainVee]

Some epoxies can take days or even weeks to fully harden. So it's good to err on the side of caution and wear protection when sanding this stuff after its been cast.

That is a good point, I knew about it but forgot to include it in this context.
Link also good to read.
Thanks.

[castinite]

The epoxy is not the big problem. The aggregate dust, if it has quartz in it can cause silicosis. Always wear a mask approved for the dust that might occur.

[petruscad]

You guys have been very patient all this time while I got lazy and did not post anything even though I have been making progress with the build. If you still remember, the next step was actually grinding the epoxy granite (EG). This is the most critical part of the build, everything hinges on grinding the EG flat to mount the rails to it. At first, just out of the mold, the top of the base was irregular by about 1 mm: pretty good for a wood mold! I ground one side of the base flat by "feel", until I could place the straight edge on it and have it sit securely in place without any rocking about. Then I started measuring the surface with the surface gages and grinding the high spots away. This worked for a while but at some point the rigidity of the surface gage's arm is not sufficient to repeatably measure the surface. At this point the flatness was about +/- 0.010" over the whole surface. To go further I had to come up with a much more rigid setup for holding the dial indicator. Here it is, bellow:

Attachment 284180

Attachment 284182

Attachment 284184

On top of the ceramic straight edge I laid the steel straight edge and on top of that there is an aluminum extrusion beam with a flat steel plate attached to it. The indicator's height over the EG can be adjusted by loosening the screw. I used this as a X-Y slide, sliding the steel edge over the ceramic one longitudinally and the aluminum beam over the steel edge for transverse movement. The dial indicator rides over a precise steel block to measure a small patch of surface rather than a single point. This contraption allowed me to get the surface flatness to within +/- 0.001".

After one side was finished I switched the straight to it and started grinding the other side until that was within plus or minus one thousands. I then tried to improve the surface further by using the blue dye method with the surface place and the straight edge. You can sort of see the blue spots on the EG is this picture:

Attachment 284186

I kept grinding at the spots, cleaning the surface, rubbing the surface plate/edge again maybe several dozen times. This improves the surface somewhat but one really has to use a surface plate larger that the base itself for this to work well and to have confidence that the surface flatness is improving not just locally but also overall. So I gave up on this way of doing it.

But then I realized the obvious, there is no point in making the whole surface flat because the rails mount to a tiny fraction of the surface. So I suspended the straight edge in the middle of the base on three adjustable contact points and adjusted them until the top of the edge was as co-planar as possible with the two sides of the EG base (unfortunately I do not have a photo of this). Then I used a surface gage with a 0.002mm precision dial test indicator attached to it (0.000078", yes less than one ten-thousands), such that the gage runs on top of the straight edge and the dial indicator needle touches the EG surface at a single point. Because the straight edge sits rather stably and securely on three supports and is in the middle of the base it makes the setup a lot more rigid. This allowed me to check and grind every point on the base where the rails will actually be mounted to a precision of +/-0.0002" (two tenths). Also, because The two sides of the EG base are ground relative to the same surface, in the same position, they should be as co-planar as possible.

I call +/-0.0002" good enough and move on to mounting the rails.

[ger21]

What did you do the grinding with?

[petruscad]

What did you do the grinding with?

Diamond plates of different sizes, shapes and grits!

Look at the first photo in my post from 04-20-2015.

[louieatienza]

Very cool! Now, you want to grind the seats of the bearing blocks parallel to the rail bed. Or will you mount the rails first then use them as a datum for the bearing block surfaces?

[petruscad]

Very cool! Now, you want to grind the seats of the bearing blocks parallel to the rail bed. Or will you mount the rails first then use them as a datum for the bearing block surfaces?

Do you mean grinding the linear rails bearing block tops parallel to the bed? I think not. I do not intend to do that!

I will mount the rails and then move a dial indicator attached to a block to scrape the steel plate in the middle of the base parallel to the rails. The ball screw will mount there.

[louieatienza]

Do you mean grinding the linear rails bearing block tops parallel to the bed? I think not. I do not intend to do that!

I will mount the rails and then move a dial indicator attached to a block to scrape the steel plate in the middle of the base parallel to the rails. The ball screw will mount there.

Sorry, I meant the bearing blocks for the ball screw fixed and floated end!

[johnh]

Looks like high craftsmanship dude!

It just occurred to me that if I do one of these in the future, I'll find some large surplus granite plates and use those as the cast surface for the rail beds,

Auto flat, square and parallel



jh

[CaptainVee]

It just occurred to me that if I do one of these in the future, I'll find some large surplus granite plates and use those as the cast surface for the rail beds,

I've been pondering along those lines too.
But I am worrying about how well it would release.

I did do some undercasting with epoxy to set my gantry straight, on a polished granite plate for facing buildings and even though it had just 4 smallish areas where it contacted, I had a hard time getting it to release.
And that was with a release agent that had not given me trouble before...

With a big cast and big surface area you get a lot of super stiff weight that can not be peeled, not be hit with a hammer and such...

[ger21]

You can use a polyethylene or mylar sheet on the surface plate. Epoxy won't stick to them.

[louieatienza]

I've been pondering along those lines too.
But I am worrying about how well it would release.

I did do some undercasting with epoxy to set my gantry straight, on a polished granite plate for facing buildings and even though it had just 4 smallish areas where it contacted, I had a hard time getting it to release.
And that was with a release agent that had not given me trouble before...

With a big cast and big surface area you get a lot of super stiff weight that can not be peeled, not be hit with a hammer and such...

Or, just have a piece of granite cut and drilled to spec. Having a mold with some give makes it easier to release. Maybe make the mold with ground aluminum tooling plate, then set that on a granite plate...

[CaptainVee]

I doubt that PE sheet is flat enough as to not change the flatness the surface plate could provide.

Mylar may be better in that respect but all of this is theory that needs to be tested.

Then again, if curing of the epoxy adds distortion this talk of uneven thickness of foil is irrelevant

[ger21]

I doubt that PE sheet is flat enough as to not change the flatness the surface plate could provide.

.004" HDPE film has a tolerance of ±.0005"

[gary_808]

Simple releasing agents are available for this type of application.

I use some often, when I replace turcite on a slide I flip it and put it in to its normal orientation and place of on the base it'll be living on so it's closest to the shape needed and won't need much scraping/ even just a tickle. The issue is is the glue can stick it to where you don't want it, we use a special releasing agent but I'm pretty sure it's watered down pva glue.

[ger21]

If you use a vibrating table to vibrate your epoxy granite mix, isn't it likely that the vibrating aggregates can compromise the release agent?

[gary_808]

I can't see it beings an issue. It's not hugely aggressive.
Or polish and wax the base granite just like they do for fibreglass molds etc.

[johnh]

understood, 0 flex means the whole surface must release at once so that could be a problem. You could cast in a few plastic wedges that stick proud from the side, give them a whack to get the delamination started? suran wrap is probably pretty consistent in thickness, stretch that over the plate, epoxy will weight it down flat, plate would fall off under its own weight, that is my educated experience guess.

jh