View Full Version : Faux Thompson linear rails

01-30-2010, 09:35 PM
I'd like to save some $$ on a CNC project by making my own Thompson-type (almost) fully supported linear rails for use with "open" linear bushings. I'm thinking of using 3/4" ground rod 6' long with a support foot about every 6".

No problem making the feet. But how can I drill the mounting holes through the rod so that the holes are all in the same plane? The rod is much longer than the X travel on my mill, so I'll have to move and re-clamp the rod without rotating it, even slightly.

I thought of using the winding-stick method, with a string hung from the ceiling in the center plane of the rod as a reference and a close fitting rod inserted in one of the holes already drilled for comparison. But that's only "eyeballing" it. Is there a better way?

gary hart
01-30-2010, 09:43 PM
If you got or could make 3 (or more) V blocks that are the same height and have all 3 clamped and drill. When it it time to move allways have at least one of the V blocks clamped to the rod. This should make it so up is always up.

01-30-2010, 09:50 PM
Now why didn't I think of that? (Don't answer that!)

01-30-2010, 10:58 PM
I don't know the exact application, but I've sort of come to the conclusion that somewhere during the design process, you should think about making the diameter of the rod as large as possible. It will cost more, both in the rod and the linear bearings, but it should never be made small unless there's no way around the space restraints.

On the other hand, if you can back up the rods pretty much all along their length, it might be ok. Can you fit a piece of angle iron as a continuous support foot? There are ways to allow the rod to remain perfectly straight while allowing for inconsistencies in the angle iron, one of which is to use a fillet of jb weld or similar in a gap between the rod and the angle iron. Once an alignment is made, the gap filled, and cure has taken place, bolts can be fitted to keep the parts together should the bond fail.

Instead of angle, maybe consider box tubing. It will be more trouble to drill through and mount bolts from the inside into the rod, but it will lend more rigidity to the rod and give you flat areas on which to mount substructure, or to mount this to the substructure. It also gives you an easier way to mount it on the mill for drilling and any other machining.

01-30-2010, 11:45 PM
The V-block idea is a good one. If you don't have v-block's you could mill a flat on the rod in an area where in won't interfere with the bearing surface and indicate the flat when you move it.

01-31-2010, 12:08 AM
3/4" ground rod is not a substitute for hard chromed and case hardened rod like Thompson manufactures. The real deal is Rc62-65 and if it is not hard, the balls will eat the rod. If you use Frelon or bronze bushings, you could probably get away with pain ground rod.


01-31-2010, 12:30 PM
I agree with Doozer on the hard rod issue. Good advice. What i do on a deal like that is get a collet that fits the rod, put it in a square collet block and clamp that on one end of the bar. Then indicate or level the top side of the block with the part in the mill vise. Work that area, then slide it to the next part, indicate or level, etc. till finished, then remove collet block.

01-31-2010, 01:05 PM
X's 3 for hard case rods.

Chromed hydraulic cylinder rod works well with the Frelon bearings however and it's cheaper and easier to drill and tap.

Drilling I just turn out a piece of 1/2"od round bar that's nice and straight about 12" long,spin the end down and thread it to match your holes.Drill and tap the first one,screw the rod in and use it to square off the table for each consecutive hole.
Since the holes in the standoffs will be clearance it matters not if the holes are out by a minute or two of arc.It's more critical that all of your stand offs are exactly the same.

02-01-2010, 08:19 PM
I use a shaft collar with a piece of angle iron welded to it. fasten the collar to the shaft using the set screws in the collar. With a machinist level you can set the shaft each time you move it, providing you don't remove the collar until the drilling is done.

02-01-2010, 08:33 PM
I made a little gadget to drill my holes exactly in line with each other. I laid the Thompson rod in a groove (above a T slot) in my mill. I made a piece of steel that was screwed down to the next T slot over. It had a hole in it that was vertical,and exactly over the center of the rod I was drilling. I drilled the first hole,then slid the rod under the jig,and slid a closely fitting dowel down the hole in the jig,and into the hole in the rod. I continued to do this all the way down the 2 pcs of 6' rod I was drilling. I drilled through the case hardened surface first with a straight fluted carbide drill. Then,I drilled the rest of the way with the regular twist drill I needed for threading into.

02-02-2010, 11:32 AM
I don't know how tight your budget is or what lengths of rails you need but Glacern has an introductory special that is pretty hard to beat. Especially with the 4.99 shipping deal.


02-02-2010, 01:35 PM
I don't know how tight your budget is or what lengths of rails you need but Glacern has an introductory special that is pretty hard to beat. Especially with the 4.99 shipping deal.


Holy poop on a stick, I wish I hadn't already blown my discretionary funds, that's a damned good price.

As it stands, I'm working on making a small cnc router, and I found that drill rod is not a suitable substitute for the real deal. At least I've got material usable for other things now, just need to convince SWMBO that I'll have to order some replacements.

02-02-2010, 03:24 PM
Actually when I get around to it, I am going to buy a piece of 1.250" Thompson shafting to make a new tailstock quill for one of my lathes. I might have to hone the tailstock just a bit for fit, which should work out perfect. It is really inexpensive for the product quality you get. I have also used Thompson shafting when I need a shaft to run in needle bearings.


02-03-2010, 02:32 AM
Needle bearings on thompson shaft- reminds me of rebuilding the tranny in the english ford van I used to drive. I couldn't afford the right shaft, so I had one made at a local shop. The guy didn't realize it had needle bearings riding on it, so he made it from cold rolled and called it good. I put it back together and got about a month out of it. Funny thing is, the needles and the gear looked good still, but the shaft got grooved to the point that the gears were skipping.

I blew that tranny three times before I wasn't allowed to drive it anymore.