Building CNC Surface grinder - whole lotta questions

I'd ask this same question on yahoogroups DIYCNC (not instead, but as well).

Adam Smith Midland, ON

If I was going to spend that much time on a project I'd use servo motors and stay away from steppers. For lotsa good reasons. However, for good advice call Gecko Drives. They sell both stepper and servo drives. Their servo drives take step and direction commands and uses them to drive a servo with encoder feedback. That way you can use cheap or free stepper software. ERS

You will have a substantial shatter shield around that fast-spinning wheel, right?

O.K. With reed making, you have my attention. (Though my focus is on concertina reeds, not accordion reeds. They are different in the mounting (concertina reeds mount on individual dovetailed carriers, sometimes called "shoes" from the shape.

O.K. More width taper than a similar concertina reed.

Lower concertina reeds may be similarly thickness tapered along the length, or may be thinner at about the 1/3 point from the root to the tip, depending on which specific reed, as about four consecutive reeds use the same physical dimensions of reed and reed shoe.

Agreed.

O.K. First suggestion -- design in ways to keep the grinding swarf out of the ball screws. It will *kill* them fairly quickly. Grinding is dirty and nasty. You'll want bellows around the ball-screw

-- both sides of the nut, or some other protective shape. There are some which look like a clock spring which has been stretched out from the center. You want to mount those so the cracks are on the bottom, so there is less likelihood that grit will work its way to the inside.

Even Acme screws have special protective housings around them of some form or other.

Ball screws take an amazingly low amount of torque under normal circumstances.

That sounds good.

Again -- look for ways to enclose it to protect it from the swarf. My Sanford grinder (manual) has the table running on one inverted V way and one flat way, held in place by gravity only (not much lifting force in a grinder), but has a lot of overhang in all directions, so the ways are never exposed to the swarf.

Note that you can get mag chucks with either a fine pitch pole spacing or a coarse one. The coarse one is fine for holding big chunks of metal, but for something as thin as your spring stock, you will want a fine pitch one. And you will want a permanent magnet one, not an electromagnet, to allow you to use coolant while grinding without opening a shock hazard (Of course, a well isolated power supply will do that too, but a permanent magnet just does not *have* any high voltage to worry about.)

And in terms of the travel -- understand that when you mount the chuck, you will have to grind the entirity of its top surface to assure that it is truly parallel to the travel of the table. This is important to me because the nice fine-pole Permanent Magnet chuck which I got from eBay is about 1/2" longer than the X-axis travel, so I will have to take it to the milling machine to remove some of the ends to allow a proper complete dressing. So -- for now, I still have the electro-magnetic chuck in service -- and *no* coolant.

Agreed.

I disagree here. You will want to take multiple passes at right angles to the reed stock, with the rim of the wheel dressed to a gentle curve to avoid steps in the thickness of the reeds, which could act as stress concentrators, leading to fatigue cracks after hard playing. The X-axis could carry the wheel over a whole row of reed stock, and the Y-axis is changed with the Z-axis in small steps to make the contour you wish.

And -- you will want both axes to be capable of moving a greater distance than the dimensions of the mag chuck, so you *can* grind it flat at the start. (Just after you mount the chuck on the table, and just after any time you remove and re-mount the chuck.

One way around this, at least for production of a single reed profile, would be to dress the wheel (a wider wheel) to produce the profile in a single path over the reed stock. But that has two disadvantages that I can see:

A router motor has the disadvantage of lots of bulk too close to the mag chuck, so it might limit your travel.

And I'm not sure that the bearings in any router will be sufficiently precise for surface grinding.

My Sanford, and other surface grinders that I have seen, have a long spindle with precision bearings at both ends (well sealed, to protect them from the swarf, again), and a belt drive to a motor set behind the column and much lower, so the weight distribution is better.

I think that the motor on my Sanford is a 1HP -- but it is driving a wheel of 4" diameter.

The time to spin up is not a serious problem. The ability to drive a full width wheel dressed to the profile would be, however -- even if you downfeed by only 0.0001" per pass. You can use lower power if you dress the wheel to a '(' profile, and cut only a narrow path per pass -- but that needs Y-axis automation in addition to the X and Z which you were planning.

You have the things which have occurred to me above -- some of them based on my experience running the Sanford as a manual surface grinder, which helps to get a feel for what is happening.

Harold Vrodos (or should that be Howard?) will be able to offer lots of information on grinding, but he has managed to avoid the CNC side of things, so perhaps some of my thoughts will be of help.

Good Luck, DoN.

P.S. FWIW, I think that my Sanford might be a good candidate for CNC conversion on the X and Z axis (with ball screws), but the Y-axis is a rack-and-pinion drive, which would take a bit more motor to drive it.

Not exactly what you are asking and no where near as cool as CNC but , why not make up a reverse die and set that on the mag base , then just use a regular surface grinder ? Luck Ken Cutt

Ken -

do you mean use a pantograph style device to transfer dimension of a die to the target ? Can you elaborate a bit here ?

I did look around on the Web and have quite an extensive collection of metalworking literature @ home, so I do have a pretty good understanding of a what an industrial SG looks like and how it works etc.

My whole setup is in a garage and I can not get a used SG from Ebay in there, unfortunately. But then again, my work envelope is tiny (1x3"), compared to even the smallest industrial SGs (6x12")

I thought about making a belt driven spindle for the grinding wheel and it does look like something I can tackle (I have a mill and a lathe and some spare time). Just might go that route if the router spindle is too bad. I am trying to keep the size and weight of the Z assembly down.

In my dreams :) I see myself loading a (pre-cut) strip of blue steel onto the chuck and carefully aligning it with the X -0 point, loading particular reed's profile into Mach2 and hitting "go". Minute later I have a complete reed, tuned to within 50 cents of target or, may be, dead on :)

I don't really have to spin @ 4000rpm, can probably go much lower.

About X-Y travel: just might have to go that route. Ideally as long as I am doing it, I'd like to build a X-Y-Z CNC assembly that I can use for milling or grinding. One immediate problem is the grinding dust - it will be offly hard to absolutely keep it out of the ballnut.

Another is price: my very limited grinding envelope doesn't require a whole lotta of travel, there is no forces to speak of - and it means I can go with short

Building milling-capable assembly is a whole different ballgame - 3-4 times more expensive. Much longer and beefier rails and ballscrews.

Try the Sanford, which is 4x7" envelope. It is a benchtop machine, not a floor-standing one. I just checked on eBay, and they don't have one at the moment -- but one would be an excellent starting point for your CNC machine.

That level of accuracy is possible, but I don't see it happening in 60 seconds -- especially with your low power spindles.

Probably not. The cutting behavior of the stones is a function of the SFM (Surface Feet per Minute). A smaller stone needs to go

Remember my comments in another followup about needing to grind in the entire top of the magnetic chuck prior to putting it into service.

Good Luck.

But -- no grinding swarf to destroy your ways (linear rails). Grinders are *nasty*.

Good Luck, DoN.

Sorry I missed the part about space being an issue . I was thinking conventional surface grinder . Still I think it would be cheaper to make up the die and have them ground at a shop or a friends grinder . The time and expense of building a " high tolerance " machine would cover the cost of a box load outsourced . Still pretty cool to have your own CNC set up but business wise I think other solutions might be more profitable . Luck Ken Cutt

I supposed he meant: Cut out a cavity in a block, like a mold for the reed. Put your stock in the cavity, hold both down via magnet, and grind top surface flat (on ordinary surface grinder), leaving proper cross-section.

With this approach, the hard parts are making the die and getting the stock to hold down well enough. To see if the process would work, I think you could make up a quick-and-dirty die by a couple of cuts on a mill, with a fairly arbitrary profile rather than an accurate one, and take that die and some stock to a "shop or a friends' grinder" (as Ken suggested in later post) to see if the process would work, before going to the trouble of making up an accurate die. If it worked ok, then you could make an accurate die and a miniature SG with manual downfeed and motorized X axis.

-jiw

Hmm -- accordion (and concertina) reeds are very sensitive to the contour of the bottom surface. Your method would be forcing the flat bottom surface to curve to allow the removal of material from the top side. When you release it, you may have a curve which will make getting the reed to "speak" quickly (or at all) more difficult. I can see you spending the time saved in the machining carefully bending the reed back to the correct profile. (A curve near the tip will result in a shift in pitch as the reed goes from a dead stop to a stable pitch -- and more of a shift with each increment in bellows pressure. There is always some shift as a function of bellows pressure, but it is beneficial to make that as small a shift as possible.

And aside from that -- at least some concertina reeds (I know -- the original question was about accordion reeds, but there are similarities) will start out at a constant thickness where the reed is clamped into the carrier, then a fairly sharp step, curving down to a minimum thickness at about the 1/3 distance from the clamp to the tip, and then gradually increasing in thickness towards the tip. I don't know if any accordion reeds do this -- though I would expect it more of the bass reeds on the left-hand end. Anyway -- such a shape would require a rather massive magnetic field to achieve the initial pull-down, and then a lot of work straightening the back when the grinding is done.

And -- the contour of a reed may need to be changed from instrument to instrument (as well as from note to note), so the number of dies would quickly get out of hand, I fear. (Unless this is for production -- the manufacturing of a single line of accordions, instead of making replacement reeds for instruments from here and there.

Another consideration is that while the die and hold-down field might work for the shape, it would still need quite a few passes of gradually increasing depth to avoid burning the temper of the reeds with the grinding heat (thus making the reeds duller and less responsive).

But -- it still might be an interesting experiment.

Enjoy, DoN.