Surface grinding attachment WIP

[Hubert S.]

I started building a surface grinding attachment and thought I would document my progress as I go along. This build is likely going to take a little while, as I have not really made a plan or ordered any components other than the linear rail.

My first step was to make the chuck. I am not sure if anybody here has made one out of steel before, but I don't recall seeing anybody post about it in the Home Built SGA thread. In case anybody wants to attempt this on a mini mill, I'll add a bunch of details here describing how I machined the chuck. I started with a piece of 2.5" wide by 1" thick cold rolled mild steel about 12.5" long and immediately ran into trouble. I wanted to mill the width down to 60mm to match the length of the magnets and reduce the thickness by 2-3mm to lose some weight. I first tried using a 2.5" face mill for this, but my mini mill (PM-30MV) was not able to take a decent bite out of the material without excessive vibration. After switching to a 5/8" end mill, things improved a lot and I was able to get the piece machined to 60mm by 23mm by 316mm in less than an hour. The length of 316mm was chosen because it was the closest even number.

I am using 10x5x60mm magnets and decided to use 19 of them with 6mm space in between them, and 9mm left over on each end. After some quick back-of-the envelope calculation, I figured milling 19 slots 7.5mm deep and a bit over 10mm wide is way too much hand cranking, so I looked for a way to align the slots with the x-axis, which thankfully has a power feed on my mill. My vise came with a rotation base that has been collecting dust since the day I got it, so I cleaned that up and used it to rotate the vise ninety degrees and then indicated the fixed jaw with the y-axis. I milled the first 11 slots yesterday and used a 3/8" carbide 4-flute end mill at 2,800rpm with a mist coolant system. I did the first slot by hand to get a feel for the depth-of-cut the mini mill could tolerate. While I could get a 1mm depth-of-cut without any sign of trouble, I decided to only take 0.75mm per pass. I guess @J. Keeton's video where he fried his mill machining a magnetic chuck was stuck in my mind... To get to the 7.5mm depth I needed 10 passes, plus two passes to widen the slot to a bit over 10mm, so a total of 12 passes per slot. Since I do not have enough travel on the y-axis, I had to flip the part after the first 11 slots to machine the remaining 8 slots, which I did today. To align the slots with the ones I machined yesterday, I briefly looked at the edge finder and then just put the cutter in the last slot, moved the y-axis until it touched the edge of the end mill and zeroed out the DRO. On the first pass, I added 0.1mm to the offset and then measured the ligament to make sure it was 0.1mm larger than that of the other slots, just in case I needed to make an adjustment. There was a small difference (0.02mm), but I didn't trust the calipers enough to bother making an adjustment on the DRO. The total time spent machining the slots was 75 minutes yesterday for 11 slots, and 45 minutes today for the remaining 8, not including setup and cleanup.

After some deburring, I made sure the magnets fit in all the slots. They fit very nicely and I am very happy with the result. Without the magnets, the chuck weighs about 6.5 pounds, so it's probably around 4 pounds heavier than an aluminum chuck of the same dimensions. I think the chuck is probably a bit thicker than it needs to be.

I want to build the translation stage for the feed adjust next. I am thinking about something similar to the one K Ken H> built and described here, but maybe with a dovetail. It's probably time to make some sketches and figure out what type of wheel to order. Based on REK Knives 's comments, I'm leaning towards a metal wheel, maybe a 5" tracking wheel turned to remove the crown.

 

[Ken H>]

You got that milled out nicely, with waaay more slots than I do for magnets. I think I have perhaps half as many, but I do have to put a short brass pin to help hold small sections of steel for grinding to prevent it from sliding off.

On the feed adjustment - yes I agree, a dovetail is the way to go. I've planning to rework my feed adjustment with dovetails, but what I've got is working and I've not gotten around to rework yet.

Keep thread updated as you go along. You gonna LOVE that SGA.

 

[Hubert S.]

Thanks, Ken. I'll see how strong the chuck turns out with half the magnets first, I guess. I put six magnets in there yesterday for testing and it did not feel too strong, but the magnets are recessed by 2.5mm. One thing I have been wondering about is whether every other magnet should be flipped over for maximum holding power. If anybody knows, please chime in.

I have a rough idea for a dovetail slide and will update the thread once I make some progress.

 

[Nick-D]

I just purchased a wee mill, this will be my first project I do on it.

Any reason why you went with steel as opposed to alu?

 

[Hubert S.]

I just purchased a wee mill, this will be my first project I do on it.

Any reason why you went with steel as opposed to alu?

Congratulations! You will love having a mill, it is incredibly useful. What type of mill did you get?

To be honest, the main reason I went with steel is that I did not want to have to glue the magnets into an aluminum chuck. And I have this vague idea that I want to bolt the chuck on from the front side, which requires the magnets to be removable. I doubt that I will ever wear out the chuck from repeated resurfacing, but if I do, it will only take an hour or two to mill the slots a bit deeper and put the magnets back in. I guess the downside of the steel chuck is weight and rust. Other than that, I really don't know if there is a functional difference, it seems like everybody who has built an aluminum chuck is happy with it.

 

[john april]

looking good. i have the tw-90 surface grinder attachment, i took out every other magnet and it still holds fine. with all the magnets it was very hard to position the blank and move/remove. also be carefull of your fingers between the blade and magnets. with all the magnets in there, it pulled so hard and fast i got a blood blister on my fingertip. now i use a magnet to place the blade on the sliding table so my fingers are not under the edge.

 

[Hubert S.]

looking good. i have the tw-90 surface grinder attachment, i took out every other magnet and it still holds fine. with all the magnets it was very hard to position the blank and move/remove. also be carefull of your fingers between the blade and magnets. with all the magnets in there, it pulled so hard and fast i got a blood blister on my fingertip. now i use a magnet to place the blade on the sliding table so my fingers are not under the edge.

Thanks, John. I showed the chuck to a friend who came by yesterday and installed a few more magnets. It did seem pretty powerful and dangerous. And that's before grinding the chuck down. I seem to recall Travis Wuertz mentioning in one of his videos that you have to grind the chuck back to where the magnets are about 60 thou below the surface for it to hold well. Mine are still recessed by over 80 thou, so I think it will get a good bit stronger and I might have to leave some magnets out.

 

[Ken H>]

pinching a blood blister - I've done that with my chuck. With my aluminum chuck I wound up filling the slots with epoxy so the top if fully smooth. This allows removing the grinding dust easier. That grinding dust really holds to the magnetic chuck.

 

[Natlek]

looking good. i have the tw-90 surface grinder attachment, i took out every other magnet and it still holds fine. with all the magnets it was very hard to position the blank and move/remove. also be carefull of your fingers between the blade and magnets. with all the magnets in there, it pulled so hard and fast i got a blood blister on my fingertip. now i use a magnet to place the blade on the sliding table so my fingers are not under the edge.

Well John , what I don t understand in all this SGA thing is why they use that kind of magnets ? Why not round magnets ? So simple to install them , just drill holes in right place ? Ten minutes job ?

 

[A.McPherson]

N Natlek didn't you use long flat magnets on your build?

I wonder if round magnets would interfere with each other's magnetic field, or if you could rearrange them in a way to reinforce the field, so you could use fewer...

Anyway, just my imagination.

 

[Natlek]

N Natlek didn't you use long flat magnets on your build?

I wonder if round magnets would interfere with each other's magnetic field, or if you could rearrange them in a way to reinforce the field, so you could use fewer...

Anyway, just my imagination.

No , I have 10 x 10mm round magnets , strength 4 kg each one and about 770 grams displacement force...........
. From this side of pond everyone use round magnets , especially in Russia . Watch this from 1 , 40 min. There are hundreds of different video clips, all using round magnets .

 

[Hubert S.]

I made some parts for the feed mechanism today. First, I made the base plate, which took a lot of time because I did not have any 4" wide stock, so I had to cut it out of larger material. The base plate of the slide mechanism will mount to the bottom of the tool arm. The base plate also doubled as a fixture plate to hold the stationary half of the dovetail during machining. I used a dovetail cutter with three carbide inserts which was slow going. I went in increments of 8 thou and it took quite a lot of passes. This picture shows the bottom dovetail being machined.

The moving part of the dovetail slider took quite a while to machine as well, as it too came out of oversize stock and I had to hog out a lot of material. Dovetails are tricky and I tested the fit a few times to get the top and bottom pieces perfectly flush on the side. To check the clearance for the gib I used a piece of 1/8" brass rod. Here is a picture of what I got so far.

Excuse the comically long gib screws, that's all I had on hand. The four holes in the base plate on the left hand side are for mounting the plate that will be threaded for the lead screw. The bearing block is not fastened yet, I did not have any M6 flat head screws, and I am not sure if I have a countersink with the right angle. I seem to recall metric screws are different, so I did not want to drill the mounting holes without having the hardware in hand.

I did not have any 1/8" brass for the gib, but I had to try the thing out, so I cut a piece of HDPE to fit in there and it works really well. I will replace it with brass eventually and use steel balls to retain the gib.

The next picture shows the front of the slider where it mounts to the tool arm. I will cut the corners off to make it look a little more streamlined. I never picked up CAD, so there is no model or proper drawings for this. If anybody is interested, I can post the sketches I made and used to machine this.

I have to purchase some hardware before I can do too much more on this. I think I'll use a 1/2"-20 fine thread screw and maybe add some way to mount a dial indicator. Then I'll have to figure out the hinge to allow bevel grinding. I am leaning towards something similar to the arrangement on the Reeder SGA, but with a lot more room between the two plates, maybe 3/4". This will allow tilting in the other direction, which I think would be very useful for tall nakiris (or anything tall, really). I'll likely use shoulder bolts and press fit bushings for the hinge. I made the tilt mechanism for my grinder that way and it has no play, so I think it will work well for this also.

 

[Ken H>]

That looks really good. You did a good job on design and machining. Keep posting

 

[john april]

Well John , what I don t understand in all this SGA thing is why they use that kind of magnets ? Why not round magnets ? So simple to install them , just drill holes in right place ? Ten minutes job ?

the difference is the round ones cannot removed, unless there is a hole thru the back to push them out. the bar magnets can be slid out by pushing them out with a screwdriver, and you can alter the magnetic strength to suit the job you are doing. once a year i color all the aluminum on the face with a back sharpie and with a 400 grit belt i just go lightly enough to surface grind off all the color. this lets me know the surface is flat. make sure to use a belt with no bump or splice, like a green gator belt.

 

[Hubert S.]

A little bit more progress today, I squared off the tool arm and drilled and tapped it. I also knocked the corners off the base plate. The rotary base for the vise came in handy once again. I barely had enough travel on the y-axis, but it worked out pretty well and I did not run into the vise or a parallel. After I milled the first side to the scribe line, I set the DRO to zero and flipped the part over, making sure it stuck out the other end of the vise by the same amount it did when milling the first side. Then, I just milled it down until the DRO read zero. Still a little bit of work left rounding the front, I just deburred it and called it a day. Mounted to the tool arm, it now looks like a paddle.

 

[Hubert S.]

I forgot to mention, I added some UHMW-PE strips to the slide to make it glide even more smoothly. And I was thinking about the gib some more and started to wonder whether brass is such a good idea, since it is harder than the aluminum. I had an off cut from a handle I made a while ago using paper micarta, which just happened to be pretty close to the thickness I need for the gib, so I cut a piece to fit in there and it seems to be working really well. We'll see how it holds up over time, I can always make a new one later.

 

[Ken H>]

I like the way you think - that UHMW stuff is pretty slick and works good.

 

[Hubert S.]

I've been using it for years on various jigs like table saw sleds, and it works really well and lasts forever. I've never used it around grinding dust or in a wet environment, so I can't say how long it will last for this application.

The tape I have is about 0.3mm thick (0.0115"), and putting it under the slider moved the top over to the left by a hair, but it is not really noticeable unless you look for it. I had actually taken the tape out of the drawer and put it on the workbench before machining the slide, thinking I would put a piece in there when test fitting. But then I forgot about it until I saw the tape still sitting on the workbench today. I still have aluminum to aluminum contact on one side of the dovetail, but I'm probably going to leave it as is. It is very smooth and with the micarta gib, it has no play and locks well. I just have to secure the gib, I think I'm going to drill some shallow dimples and put steel balls in there.

 

[Hubert S.]

UPS dropped off some 18mm long M6 flat head screws and I got the bearing block mounted to the slider. It feels pretty solid and does not seem to have any play in either the rail or the feed mechanism. For the lead screw, I decided to go with a piece of 3/8"-24 all thread. I got the missing pieces for the feed mechanism cut and machined to size, but did not have enough time to complete the mechanism. With the linear rail on the bearings, it is starting to look like a surface grinding attachment.

 

[Ken H>]

That looks good - I wish I'd got the 5 bearing version but the 4 bearing version was what was available when I build a few yr back. It works good, but just feel the 5 bearings might be better? You're sure making progress.