[flyhigh]

I lack tools for tapping, and don't have a ton of cash. I was thinking of using 1144 steel rod from onlinemetals.com, clamping it down on both ends, and then using epoxy to hold it in place on a home-grown continuous support. The rails will be a little over 8 ft in length, and I will buy open pillow blocks for them. Viable or a pipe dream?

[dfmiller]

Do you have a sketch or drawing to show what you are planning?
My WAG is it might not work.
I seeped the top ofmy head several things to consider.
Will you have enough bonding area to not exceed the holding power?
Over 8 feet will you have any differential temperature expansion issues?
How will you contain the epoxy while it cures.

If I were doing what I think you are proposing I would drill some holes in both the base and rail and use the epoxy as a rivet instead of just a adhesive.
I would look at using a mechanical fasteners unless you did the engineering to ensure it would work.

But please post some diagrams so we can look at your idea. They are using adhesives many places these days and they are better than mechanical fasteners in many places.

Interesting concept.
Dave
Dave

[giz]

why not just purchase a tap? I can't imagine spending the money on a CNC machine with 8' rails on not being able to afford the $5 for a tap and handle...

[louieatienza]

What might be even less viable is the tolerance of the rod versus the that of the pillow blocks. Drill rod is ground to closer tolerances, but I don't know if it comes in 8', and it costs way more than regular round rod. Plus I would think the epoxy would cost way more than a few nuts, bolts, and washers!

It might be more cost effective to use flat bar and CNC Router Parts carriages?

[ssutton]

I used 3/4 fully keyed shaft for my linear rails. The key slot is what I used to align all of the drilling and tapping operations with on my mill. This was a LONG and TEDIOUS job, but it came out well. You could do the same thing except use a drill press to accomplish the drilling with if you do not have access to a mill. I would have serious concerns about using epoxy, I think that it would work great for compression loads but side loading is where I think that it will fail.

Scott

[flyhigh]

Thanks for the replies. The rest of this post is basically brainstorming.

Yes, the tap is not expensive, but the equipment to drill accurately in metal is. I have nothing right now, and paying for tapping would cost more than a linear rail system. The key grooved shaft is an interesting idea, but it seems like it is close to the cost of pre-tapped shaft. I was trying to avoid specifically cnc markups (ouch!).

I have been thinking about this, and I came to the same conclusion that it would be better to try to punch some holes in the bar, leading right back to the cost issues. Another solution might be a chromoly tube that I cut a wide groove in with a grinder - I do have that!

The tolerances and coefficients of expansion are definitely something I didn't think about.

I am out of town right now and can't crank out a drawing, but I will try to if my hope does not fail me when I look into material specs. However, I did have another idea, which might make your reading this post worthwhile!

Place two rods top and bottom inside one side of an i beam (or some similar situation), and use concrete to wrap them to the same extend as an open pillow block would. Pick up some v or u bearings, and run in between the two rails pressure fit to both sides. with a decent width wrap, it should be quite solid, and I think, since the concrete doesn't expand much, that it would force the rod to elongate, just need to cure it for a long time to get to an almost hardened concrete level.

At any rate, these setups, if the materials will permit it, allow one to avoid the tedium and risk of attempting high precision holes -- even if some drilling might be necessary.

Thanks for your ideas, and please keep them coming!

[flyhigh]

Finally, it is true, the CNC Router Parts are easier than this. I guess I balked at the idea of 1/4 steel plate determining my precision, although if one is going 8020, it makes more sense. (Yes, I may be dreaming to think I can use epoxy or hardened concrete and maintain my precision, although I have used adhesives quite a bit).

[hjcraver]

If you do decide to use round rod for your rails do not use 1144. It is much better to use T & G (Turned and ground) 1045 rods which are readily available. Try Speedy Metals Online Industrial Metal Supply and you will not be dissapointed. Hubert

[ger21]

I didn't read your first post earlier, but open linear bearings will quickly wear grooves in the steel rod. For it to last, it MUST be hardened, and it needs to be ground as well, if you want it tight.

When you're talking about an 8 ft machine, the only real viable option imo is to spend money. Or you'll have a big machine that's very slow and/or not very accurate.

You have 4 options.

1) CNC RouterParts and CRS.
2) V-Groove bearings on hardened track or ground angle ala Mechmate.
3) Real linear shafts. At $300 a set, it's hard to beat these. Glacern Machine Tools - Linear Rails and Bearings
4) Profile rails and shafts. Hiwins at Automationoverstock will set you back about $450-$500 for a set of 8' rails and blocks, but it's the best option.

[rocketflier]

http://www.cnczone.com/forums/epoxy_..._concrete.html

Epoxy Granite - CNCzone.com-The Largest Machinist Community on the net!

[johnohara]

ger21~

Didn't know Glacern sold linear rails too. Thanks for the post.

VXB Bearings offers a similar product

[louieatienza]

ger21~

Didn't know Glacern sold linear rails too. Thanks for the post.

VXB Bearings offers a similar product

Yes, but Glacern's is dirt cheap! And no need to wait from a China shipment...