View Full Version : Mill table upgrade

01-03-2010, 11:57 PM
I have been needing a longer table for my CNC mill so I can take advantage of the full X travel when using the 4th axis. I made this sub table using some scrap plate that I bought for 6 cents a pound in a 400 lb lot some time ago. Because of that it has a few marks and dings that weren't worth taking out but it still came up well. It is made of two pieces that are welded together at a 45 degree angle across the middle. After beating it into a semblance of flatness I then sanded off the spatter and trued up the bottom with a fly cutter. Then the real work began. I flycut the scale off the top and then made a couple of more passes to make it reasonably true. This all involved either two or three setups each pass because of the length. After fly cutting it I used my three insert facing head to bring it as close as possible to flat.

The ugly part came next. Using the mill as a surface grinder I took it down another few thou to make it flat to better than .0005". Following that a very complete cleanup of the mill took about half a day. I would have scraped it next but my hands aren't up to the task of holding on to something like that for more than a few minutes so I used a couple of brand new truing stones to take down the high spots. I used cold bluing to mark up the table.

By using cold blue the mess is minimized and the stones are kept free of contamination. While I can't hang on to items like scrapers I can push around a stone for quite a while. I have one that is silicon carbide fine grit about 9 inches x 2 inches that does a good job of bridging the low spots to reveal the high spots. It has turned out fairly flat to within a few tenths over nearly all of the table. The very right rear corner turns down by almost a thou in the last three inches. I must have clamped it on a piece of swarf when I was grinding it. Oh well, it won't make a different to most work.

There are 75 1/4-20 tapped holes in three rows spaced 1 inch apart. They are directly above the Tee slots in the main table so there is about an inch of room below each hole making it easy to find bolts the right length for clamping. All the holes were power tapped using a quality spiral point tap in a battery hand drill.



01-04-2010, 12:11 AM
Looks nice from here.

01-04-2010, 12:13 AM
Cool - how much of that was the machine able to do? Reminds me of using a shaper to square its own table and vise.

01-04-2010, 12:19 AM
It was all done on the mill with the exception of the welding and tapping. The final hand stoning was done with the table bolted in place of course.

01-04-2010, 12:22 AM
Isn't it a nuisance to have to pull all the safety equipment off for these pictures? ;)

Nice as always. I wonder how much g-code it would take to buff it to a mirror finish.

01-04-2010, 12:27 AM
Evan, with those exposed slides and ball screws, sure looks like a Unimat :)

Maybe Millamat/Multimat/Megamat-VM would have been a good name for it..

All screws must be 5mm allen heads to boot...

01-04-2010, 12:44 AM
Evan, with those exposed slides and ball screws, sure looks like a Unimat :)

Maybe Millamat/Multimat/Megamat-VM would have been a good name for it..

All screws must be 5mm allen heads to boot...

I've seen it in person and it was a treat to see it with all it's bits exposed. And knowing too the unusual construction it's quite a piece of work. Somewhere here or on his website he describes the math that went into the dimensions of it to reduce resonances and coupling.

01-04-2010, 12:49 AM
It isn't at all obvious but the linear rails are not the primary support for the main carriage. The lower rails only constrain motion to the x axis translation. Constraint in roll around the x axis and in translation of the carriage in the z axis from downward cutting forces are handled at the front and rear of the underside of the Y axis carriage on flat way surfaces with a cast iron slider on steel at the back and a teflon slider on a ribbon of spring steel at the front.

The Y axis does depend entirely on the linear rails for support and contraint but it is much shorter and stiffer. I calculated the expected deflections before I built it and the results are quite close to the calculations. The end of the original table deflects about .015" with a 200 lb load on the end of the table.

01-04-2010, 01:01 AM
I read your mill build series Evan :)

I have a SB 9, and its nice to know it alone, could make a quite usable mill...

Just has that bony Unimat look ... :)

01-04-2010, 02:46 AM
Nice job, Evan, and a nice accessory for your mill as well. Did you use some kind of guide to tap the holes nicely vertical? Just wondering how well a power tap self-aligns in a drilled hole-

It appears that one thing you need now is a stand alone surface grinder. :)

01-04-2010, 03:48 AM
I don't use a tap guide. I just clamp the part in the vise and tap the holes. I don't even try to eyeball them. Years of drilling out rivets on aircraft skins taught me how to hold a drill perpendicular to a surface. It's a learned skill like riding a bicycle but it is also one you don't forget.


BTW, all the holes added up is equal to tapping through nearly a yard of steel. :)

Your Old Dog
01-04-2010, 08:31 AM
Evan your mill has evolved into a flatout work of art. I envy the hell out of you the ability to do that kind of project.

01-04-2010, 08:59 AM
Again, nice job, you really know how to do it up right.

01-04-2010, 09:04 AM
That looks beautiful!
I am interested in your use of "truing stones" to flatten the surface.
I know they are normally used to flatten a whetstone for knife or chisel sharpening.
The use of cold blue is also very clever. How did you remove the remnants?
Would you explain the stoning process in more detail?

01-04-2010, 09:24 AM
I'll take some pictures of the stones a little later and perhaps demonstrate the bluing process. The only problem there is that I used up the last of my liquid cold blue but I might have some paste left somewhere. I should have taken some pictures of that step. To remove the last bits of blue I polished it with some 220 grit silicon carbide paper wrapped around one of the stones.

[added] The use of stones isn't an easy way out of scraping. It probably takes longer although it does take less skill. I spent about 20 to 30 hours on that table, most of it making it flat.

01-04-2010, 11:42 AM
Nice job as usual, do you build rockets in your spare time? :)

01-04-2010, 11:42 AM
These are the truing stones I use. They are dense silicon carbide grit, coarse on one side and fine on the other. They are also very flat, probably as good as a surface plate. They are hard as heck and don't show much wear at all even after a lot of use. I don't know what they cost as I got them in payment for doing some machining for my wife.


This is a steel plate that I have trued up by draw filing years ago. I use it as a working surface that is pretty close to flat. It's less than .001 out of flat and serves fine as a surface plate when higher accuracy isn't required. It has been blued in this image although that isn't obvious because of the lighting.


I used the long stone on the coarse side (which is still pretty fine) to reveal the high spots by lightly rubbing it around the piece dry for a couple of minutes.

This is how the piece looks with the high areas being the lighter areas. To take down to flat would take the better part of a day. You have to watch for turned down edges. It is almost impossible to avoid that when filing or truing unless you use strips of material around the edges to prevent tilting the file or stone when it overhangs the edge.


01-04-2010, 11:46 AM
Nice job as usual, do you build rockets in your spare time?

Why do you ask? :D


01-04-2010, 11:50 AM
Thats funny! Wish I had the knowledge and skills you have.....

01-04-2010, 01:42 PM

Just curious: how thick was the starting stock? and how much did
you fly cut off? (both surfaces?) at those sizes, I'm surprised it didn't
warp like a potato chip.

Good job.


01-04-2010, 03:11 PM
It started out as 10 millimetre plate. It's now 9 millimetre. That's about .040" removed. There is no particular reason for it to warp, it's hot rolled A36 structural steel. You can buy it blanchard ground from Online Metals. To get a 24 inch long piece you have to buy a 2 ft square piece at over 200 dollars. Add in freight to here and it would be over $300.

01-04-2010, 03:19 PM
Inspirational as always Evan. How is your shop built lathe coming?