[Keonfrey]

Hi I'm building my first Cnc here and I've got my table mostly assembled/welded I was planning on mounting the linear profile rails on the outside side but then I read something on here about alignment issues with that... Extensive shimming required etc my question is would it be any easier to align if the rails were on the top surface? The mounting platforms are 3"x 1.5" by 3/16" wall thickness rectangle steel tube 5 ft long and that is what I was going to mount the rails on. Any tips on one top vs outside?

[awerby]

If that's regular hot-rolled steel tube, it's not likely to be flat enough to mount those profile rails on; they're very particular about flatness. It doesn't really matter which side of the tube you choose - none of it is very flat. People have got around that by building a dam on top of the rail and pouring in self-leveling epoxy to provide a surface flat enough for mounting rails on. Another approach is to screw flat aluminum plate to the rail, or better yet, Blanchard-ground steel plate, shimming where necessary.

[ger21]

If you mount them on the outside, then the two outside faces need to be perfectly parallel.
If you mount them on top, then you get one of them straight, and align the second to the first.

But if the mounting surfaces are not flat, and coplaner, you'll need to shim.

[Keonfrey]

If that's regular hot-rolled steel tube, it's not likely to be flat enough to mount those profile rails on; they're very particular about flatness. It doesn't really matter which side of the tube you choose - none of it is very flat. People have got around that by building a dam on top of the rail and pouring in self-leveling epoxy to provide a surface flat enough for mounting rails on. Another approach is to screw flat aluminum plate to the rail, or better yet, Blanchard-ground steel plate, shimming where necessary.

Thanks for your 0.02! I have considered doing the epoxy method because I think I can source that quite easily.. Can I drill a template on another Cnc to drill and tap the holes for rails? Will that be accurate enough? Also debris buildup shouldn't be problem if mounted on a horizontal surface as long as I keep a shop vac close by! I don't seem to be able to include pictures. Uploading isnt working on my phone. I'll get some pictures of the frame so you can see sometime!

[ger21]

Can I drill a template on another Cnc to drill and tap the holes for rails? Will that be accurate enough?

That depends entirely on the accuracy of the template. Sure, as long as the holes are perfectly in line, and the template can't be twisted or bent.

[Keonfrey]

That depends entirely on the accuracy of the template. Sure, as long as the holes are perfectly in line, and the template can't be twisted or bent.

Well they should be in a perfectly straight line as the cnc at work is a large insutrial one. What material can the template be? I don't really think the boss would like me to use aluminum on there. Also the taught wire light method.. How well does that work?

[awerby]

I've never heard of the "taught wire light method", so I can't comment on that. But if you have access to a large CNC machine, why not do the drilling on that, directly? That will be more accurate than using a template and any method for transferring the hole positions.

[ger21]

Any technique is only as good as the person using it.

[Keonfrey]

I've never heard of the "taught wire light method", so I can't comment on that. But if you have access to a large CNC machine, why not do the drilling on that, directly? That will be more accurate than using a template and any method for transferring the hole positions.

I thought of using the machine to drill directly but my boss doesn't want me to drill or machine steel because it is a nest based router for cutting sheets otherwise I wouldn't think twice! I've heard some rails have almost half a mm of extra room for the mounting holes for example have 8.5mm holes and are made for m8 bolts. With a template I can't see it being impossible to get within that if I take my jolly old time! But I'm not positive on too much because of my lack of experience! Thanks for feedback! I'm in here for the ride as much as for business!

[ger21]

I thought of using the machine to drill directly but my boss doesn't want me to drill or machine steel because it is a nest based router for cutting sheets

It probably wouldn't be able to drill steel. The drills on those machines are typically pneumatic, and can't apply enough pressure. The RPM is probably much too high as well.
I use a machine like that every day.

I would not use rails that used an 8mm bolt, they would be far too large.
Typically, you'd want a 20mm rail which normally uses a 5mm bolt, and will have a 6mm hole.

I'd make a template from 3/4" MDF, with plastic laminate on both sides. This will keep it from warping, so the holes will stay in line.
I'd drill 1/8" or 3mm holes, and then drill the same size holes in your frame.
Then go back and enlarge them to the proper size for tapping.
Because you have ±0.5mm adjustment, you should be fine.

[Keonfrey]

It probably wouldn't be able to drill steel. The drills on those machines are typically pneumatic, and can't apply enough pressure. The RPM is probably much too high as well.
I use a machine like that every day.

I would not use rails that used an 8mm bolt, they would be far too large.
Typically, you'd want a 20mm rail which normally uses a 5mm bolt, and will have a 6mm hole.

I'd make a template from 3/4" MDF, with plastic laminate on both sides. This will keep it from warping, so the holes will stay in line.
I'd drill 1/8" or 3mm holes, and then drill the same size holes in your frame.
Then go back and enlarge them to the proper size for tapping.
Because you have ±0.5mm adjustment, you should be fine.

Yes the drill heads are pneumatic and the rpm can be slow enough but I don't want to worry about the machine capabilities. That's why I'm building my own machine. So I can make it for what I want it for! Yeah the rails I was going to use 15 mm rails since they are cheaper and the cap loads should never be exceeded with my size of machine. 8mm was just an example. As for the template yes that is a good idea starting with a small diameter hole and enlarging so perpendicularity is easier to achieve! :-) Another thing... Getting a master rail straight with a taught wire sounds fairly simple in its concept. Make a taught wire parallel to the rail. Apply current to it and use the truck with a razor blade attached with the other side of the circuit by attaching a light to it so when sliding the truck down the rail if the razor blade touches the wire it completes the circuit and turns the light on. You can be fairly precise if you apply the right current so minimal contact with the wire needs to bemade but it shouldn't strike an arc either. Lol apparently it works since I hear/read people talk about it! Should be able to learn something from this!

[Keonfrey]

I got some questions again....

1. If I'm going to pour epoxy to level my rails do I need to do it exactly where the machine will sit when finished? How much would moving it throw it out?
The reason I'm asking is because of the harsh temperatures in Canadian winters will prevent me from achieving a high enough temp for epoxy to cure in my unheated shop . I have Canadian (hockey pucks) leveling feet on it so I can relevel after I pour if that helps!

2. Will I be able to mount the rails right to the epoxy without them sinking in?
If epoxy if fully cured will the rails settle into it? Is the epoxy going to be flat enough to mount the rails without scraping it flat?

3. Can I epoxy level my gantry arm and z axis rail platform as well?

[ger21]

1) That depends on how rigid the frame is. If it's rigid enough, than it shouldn't matter if you move it after pouring.

2) I haven't done it myself, but yes, that's what everyone does.
It depends on the epoxy, and you need to make sure that it's fully cured. It's probably a good idea to put a heating blanket over it for a few days after the initial cure, especially this time of year.

3) Yes.

[Keonfrey]

Well here is a picture of the frame. I think I will put a frame around the bottom to tie all the legs together to make it more rigid and a shelf for electronics.Attachment 337666 Do i need to make it that terribly much more rigid? Everything is 3/16 wall 1.5 x1.5 sqaure tubing and the x rail channel is 3x 1.5 x 3/16 rec tubing. Triangles are the best friend you can get for stiffening up a structure amirite?

[Keonfrey]

OK... so i did some deflection measurements.. On the 3 inch side rails using my own body weight (160lbs) i could only get it to deflect 0.001" on the midpoint between 2 legs. On a single cross section it deflects 0.0035-0.004" and on doubled up cross section (leg weldment top section and table cross section together) it deflects about 0.002". Is this rigitity ample to move my machine after pouring? or do I need a little extra "beef"?

[elfrench]

I got some questions again....

1. If I'm going to pour epoxy to level my rails do I need to do it exactly where the machine will sit when finished? How much would moving it throw it out?
The reason I'm asking is because of the harsh temperatures in Canadian winters will prevent me from achieving a high enough temp for epoxy to cure in my unheated shop . I have Canadian (hockey pucks) leveling feet on it so I can relevel after I pour if that helps!

I'd make it level first. Here's a good way to make an accurate level: Shop Made Tools - Page 258

[Keonfrey]

I'd make it level first. Here's a good way to make an accurate level: Shop Made Tools - Page 258

How level does a table have to be to have an effective epoxy pour? Seems to me as long as I get it pretty close the epoxy will take care of any warp and slope in my table. A thin laser level will get me close enough I thought...

[ger21]

The more level it is, the less epoxy you need to use.
There is no general rule. Just get it as close as you can.

[rowbare]

Check out this thread. It should answer a lot of your questions.

http://www.cnczone.com/forums/cnc-wo...ghlight=canuck

bob

[Keonfrey]

Alright, I did some more alignment/straightness measurements using an aluminum flat bar stock I found as a straight edge it's below 0.01mm deviation in width and I was pleasantly surprised how straight it was.. Measuring the rails I found they were both concave by about 0.004" in the middle. By setting this flat bar on edge and measuring with feeler gauges then flipping bar over and feeler gauging again I could average the two measurments out and that would give me the deviation if it were 100% straight, without factoring in the sag of the aluminum in the span. So 0.004" dip in the middle plus the sag of the flat bar which I couldn't figure out but I can't see it being that much sag over 5 feet if it's 1/4 x 2 flat bar stock. I'm almost tempted to just shim or scrape the rails.

I measured for co-planar part of things, and corner to corner with taught wire the wires touched. I swapped the wire over for under thinking it might spread this time... Nope. So I knew it was less than the diameter of my wire, which is 0.0118"/0.3mm I found a more accurate way and found it was 0.0095"/0.24mm out so shimming 0.005" on the opposite low corners should get me really really close. 0.010" was a lot closer that I thought I could get it in a weldment. I've never scraped steel before, but I either want to try that or shim my rails instead of epoxy. Thoughts about this? Tips would be appreciated!

Cheers, Keon