Steve's 3x4' Gantry Mill

[steve123]

I got some metal cut by BK Customs. Some of it was water jet, the rest was laser.


Here is one of 3 spreaders for the gantry beams ready for welding.

Here are the plugs for the ends of the beams. The ones with holes are to allow filling the beams with concrete. I had to grind the chamfer for welding.

I also got a 20 foot length of 3x5x3/8" hot rolled steel tube from Industrial metal supply. It cost about $280. They cut it up for me. This is for the two gantry beams, and the two longer 5.5' X beams. The gantry beams weigh about 65 pounds each. The X beams are about 100 pounds.

Here is a beam with a plug fitted in, ready to weld.

[steve123]

Here is a beam after welding in a plug.

Truing up the ends
The two beams are within .005" in length. I THINK that's good enough.

First test fitting! I had to file some high spots on the spreaders to get everything flat with no rocking

I decided to weld the spreaders onto the bottom beam first, then worry about aligning the two beams later. Here it is after grinding the welds a bit (I'm not a very experienced welder).
I welded a little bit here, a little bit there until all of the spreaders were on and complete. I was trying to keep it from heating up too much. I still woke up in the middle of the night worrying about having warped the beam.

The next day we got permission to use the 2'x3' granite surface table at Vocademy. We set four 123 blocks on it and carefully set the two beams down. We used a builder's square and feeler gauges to line up the ends. The first beam was not warped! I was very relieved!
We got some criticism for this careful alignment at this stage with low-accuracy structural steel, but I figured we should try for accuracy from the start!

All clamped and ready to move and weld.

Tack welds done, clamps removed.

We then did small 2" welds all over the place until it was all done. This is the very nice welding area at Vocademy.

I am happy that my brother and I can still pick it up! The gantry beam is ready for heat treating / stress relieving! After that will be filling with something like concrete (probably no-shrink concrete from Home Depot). Then I will slather some kind of slow-curing but thick epoxy on the bearing surfaces and lay it down on my 3x4' surface plate. That should make a good surface to mount the linear rails on.

That's it for now. Happy new year guys!

[Master_Arvist]

Any updates?

[steve123]

Any updates?

As a mater of fact, yes!

I made 8 brackets out of 1/2" x 4" angle. I welded in triangles to eliminate flex. I then machined the two faces flat and perpendicular. I got a 1/2" carbide end mill for this. That was a pleasure to use!


I then started in on the bushings that are the upper attachment of the two main X beams to the granite surface plate.
First I made 8 bushings on the lathe. These are 1" steel.

Those fit in 1" holes I drilled in the beams. My brother and I welded them in.

[steve123]

I finished up welding the gantry legs. Those are 3/8" and 1/2" steel now.

[Master_Arvist]

Nice

[steve123]

I made 3/4 x 1.5" stainless (303) inserts. These pretty much look like the ones I have seen on the web. These will be epoxied into holes in the granite plate. I will drill and tap the shorter ones after they are in place. The longer inserts will stick out 1/2". The brackets I made earlier have grub screws to allow vertical adjustment of the X beams.

[steve123]

Drilling Granite!
I wasn't sure how this would go, and it took some doing. I have 20 holes to drill! I start with purchasing a very high quality water swivel and diamond drill bit. As I recall, they cost about $220 together. Diamond Core Drills, Universal Application - Call (661) 257-2288 I also got a 1 gallon pump sprayer and some tubing to connect it to the water swivel from Home Depot.
I really abused this water swivel and bit, but they just kept working.


I laser-cut a template from 1/4" plywood. Here I have it clamped to the side of the table.


I did a small test spot with the cordless drill with the water going, and found that I really couldn't keep the drill straight. It has a (very slow) bubble level on it, but I would wander up and down anyway!
This was NOT working. I wanted to use something like what I used to know as a portalign AccuDrill Drill Guide | Toolmonger I couldn't find one that had a 1/2" chuck. They are just too small and whimpy. I decided to make something. Here is what I came up with in about 20 minutes.

The drill guide is made from a scrap piece of 3/8 plate, with a piece of 3/4 rod welded into it. I welded a piece of angle to a heavy wall pipe. I drilled holes in the angle spaced out like the grease nipples of the water swivel. As you see, the water swivel is attached using the grease nipples. The pipe was a little larger ID than 3/4", so I added some grub screws. Now we have something that I can clamp on. It won't go anywhere, and the drill will stay straight!

[steve123]

How to finish drilling before I am an old man?

I started out just pushing on the drill. I couldn't keep that up. I next tried this:


The string trimmer line runs across to... A half full 5-gallon bucket.


I drilled that way for 15 minutes with very little progress, even with me leaning on it. I need more pressure!


Even with the extra weight, it took 26 minutes to drill the first hole! OK I will be an old man before I finish 20 holes!

This is what I ended up with. A clamp! This is just before starting a hole. I just turn the handle slowly, and the water coming out of the hole turns to milk! Stop turning and it goes clear again.


How long to do a 1.5" deep hole now? 5 minutes. Fastest time was 2.5 minutes. I kept it to 5 minutes, hoping the bit would last for 20 holes.

Getting the core out is just a matter of inserting a screwdriver and tapping it lightly with a hammer.

If it didn't break off at the very bottom of the hole, just reach in with the screwdriver to the bottom edge of the hole, and twist. I started to realize just how brittle granite is.

Results! A total of about 7 hours drilling. Faster next time!


How does it fit? Great!


Now for hole cleanup and epoxy!

[steve123]

I blew the holes out with compressed air, and sprayed them with the pump sprayer, and blew that out. I also got a tubing brush from Harbor Freight and used it in a drill. I am going to stuff some damp cloth in there and spin it around also.

What epoxy to use? I found out that they make anchoring epoxy. One of the good ones is Redhead G5. It comes as a double caulk tube. You screw a mixing nozzle on the end.


I had a hard time finding these at a good price. Grainger wants $230 for the gun! Amazon sellers wanted $5 each to ship each nozzle.
I ended up buying the G5 epoxy for $25.76, The gun for $40, and 6 nozzles for $20 from Ebay. Tomorrow I will do the final cleaning, and give the epoxy a try.

[steve123]

I decided to explore ways to get the X beams flat. I started with hand scraping. My first objective was to get any rock/twist out. I purchased a scraper from A&W precision, and Dapra water-based spotting ink. The beams weigh about 100 lbs each, so I couldn't keep lifting them on and off the table. I put down paper towel to catch scrapings, and had to clean up and re-apply the ink from time to time. The ink was really nice to work with. It cleans up with Windex really easily. One beam had the high spot in the middle, the other had the high spots on the ends. The beams are also longer than the table, that's a no-no with scraping, but I have little choice.



Twist finally out of one beam:


I spent 6 hours on each beam. Scraping steel is a real pain! At least both beams are basically flat. I need a faster way to do this! Hmm... I have a flat reference...

[steve123]

I figured I would try making a flat surface on the beams by coating them with epoxy and laying them on the granite surface plate. I was really worried about getting the epoxied beam back off the surface plate.
I looked into mold release and discovered the some of them are just carnauba wax. I got some carnauba car wax and some big tubes of JB-Weld (the original slow kind).

I first used some very rough sand paper to clean up and roughen the beam. I stoned it lightly after that so that no burrs were sticking up in the high spots. I then cleaned the beam with acetone.
I put one light coat of wax across the diagonal of the surface plate, waited 10 minutes, then hit it with one more coat. I didn't wait for it to dry after that. Here we go!


I buttered about 14 inches of the beam with JB-Weld. You can see the sheen of the wax on the table. I only put the JB-Weld on the side where the linear rail will be.


Contact! The JB-Weld squished out. It's runnier than I thought it would be, and very sticky. I'm really hoping it will come off.


I waited 24 hours, and came back and lifted on the end of the beam that was sticking out. I had to lift pretty hard. Suddenly there was a loud krrraaaak sound and the beam came free. I think the sound was the wax releasing.


The result looked quite nice. It was NOT smooth to the touch though. There were many small points sticking up where the JB-Weld had pushed down into the pores in the granite. I scraped those off with a very light stroke of a cabinet scraper.

So now I have part of one beam very flat. I know I can do more this same way!

I am worried about my surface plate though. I measure a dip in one corner of over .0015" using my .0002/10" machinists level. I'm looking for someone in southern Calif. to calibrate it. But first I need to clean out the garage...

[joe90023]

Great minds think alike!

Here are a few pics from a router frame I built in 2003 but never finished. It has been used as a desk since then. I got restarted on the project this week which is how I stumbled on this thread.

The frame was welded with cross bracing and then the whole table inverted over a granite plate and an epoxy slurry sandwiched between the plate and the table. The plate was waxed and I also used a thin coat of PVA for to assure release if I remember right. I was thrilled with the result at the time since both sides are flat and coplanar. I got the rails bolted up and then it sat for 13 years.

Now for the problem which I discovered this week after getting back to the project. I wanted to see how the epoxy layer held up after all this time so I unbolted a rail to see. Sadly, the bolts had pulled the rail down into the epoxy layer by several thousandths. I tried to show the indentation in the pics but I only had a low res camera. The problem is actually aggravated because the rail got pulled down more at the bolt locations and not as much in between. So, there is kind of roller coaster effect with a valley at each bolt and a peak in between the bolts. This probably only amounts to a few thou difference but still not what I had hoped for.

My epoxy layer is about .03" thick. I could have probably made the epoxy layer thinner but I wanted to ensure I could get the entire 80" length flat. I had to put the epoxy on in three batches due the granite being only 48" long and the frame 80". The first batch was with the frame centered on the granite and about 16" overhanging each end. I let it cure in place several days each batch. The second and third batches were with the frame shifted to get epoxy on the last 16" inches at each end. This worked very well with the entire 80" flat and co-planar, assuming no flex in the frame.

My epoxy recipe was totally unscientific. MGS laminating resin reinforced with milled fiberglass. Cabosil was added to make it non runny and a little carbon powder to make it black. The cured mixture seemed very hard and abrasion resistant. I'm now hoping to come up with a recipe that will result in more compressive strength. It may be possible to use less torque on the bolts holding down the rail. I had them torqued very tight.

Here are the pics. The dates shown are not accurate. Actual date is 5/22. I can get better pics this week with a much better camera if there is any interest. Also shown is a new experiment with an 18" piece of steel tube with some rail bolted to it. This has a thin layer of epoxy. The tube was pushed straight down on the granite as much as possible. I'm guessing the epoxy is about .010" thick. The rail is actually two short pieces butted together as an experiment in making longer rails but that is another subject for a diffrent thread. I will unbolt this in a few days to see how the epoxy held up. This was oven cured at about 140F for several hours. The epoxy responds well to post curing but I can't do that with the big frame.

[steve123]

I wonder if your epoxy didn't actually cure all the way. I had some (old) epoxy that I tried to fix a car mirror with. The mirror fell off after a week, and I saw that the epoxy was still a little gummy.
I started looking for the compressive strength of JB Weld and found it here: J-B Weld Weld-8265-s - The Home Depot
3,960 psi tensile strength
10,733 psi compressive strength
7,320 psi flexural strength

My rails will use M5 bolts. They are torqued between 1.3 and 5.7 ft-lbs, depending on the bolt class. That gives a clamp load (from the table linked below) of 538 to 2319 lbs. I calculate that the max value of 2319 on a 20 mm rail would give 2318 / (.787 * .787) = 3746 psi. Not close to 10k.

https://www.google.com/url?sa=t&rct=...,d.cGc&cad=rja

If I were you, I would strip off the epoxy you have, plug the holes, and use a pre-mixed metal-filled epoxy like JB Weld. The layer is thin, so you won't use/spend that much. What do you think?

[joe90023]

Yes, I plan to strip off the old epoxy and try again after some research. I'm going to scan through the massive epoxy granite thread and see what additives they use. This surface plate strategy can't use the larger size aggregates they use but some of the finer powders could be useful.

I used MGS brand epoxy which is very high quality and has always given flawless results for composite work. My 20mm THK rails use 6mm bolts. I torqued them by feel instead of using a torque wrench. I'm thinking maybe they were torqued more than needed and perhaps much more than your 5mm bolts.

I look forward to seeing your machine completed.

[joe90023]

I looked up the specs on my resin. Assuming proper cure, the bare resin is rated at 17,400-20,300 psi. I'm not sure what the effect of the various additive powders is.

I'm pretty committed to using this resin since I already have it on hand for composite work. Typically composite parts are heat cured in a hot box or an autoclave for the high end stuff. Room temperature cure does not give full strength for my resin but has always been fine for non critical parts. I have a big stash of insulation foam board I will slice up with a hot wire and make a really big hot box and cover the whole frame on the next try.

[joe90023]

After experimenting with some short sections of rail and tubing I got the idea that my rails did not get pulled down into the epoxy layer but that the steel tube got deformed around the bolt and pulled up against the rail, distorting the epoxy. To test this theory I put a layer of epoxy on a 2" thick slab of steel and cured it against the granite plate. After an oven cure I bolted down an 8" section of rail as tight as possible by hand with the hex key. After sitting about 5 days I pulled the rail off. The epoxy looks good so I think the theory is proved. You need a solid base that won't be distorted by the pull of the bolts. My original frame had .25" wall tubing which apparently is not adequate for 20mm rail with 6mm bolts.