Grinding in a CNC mill.

[Kenre]

Working on a project where I will need to ID grind an oval ish shape, that will be hard chrome plated.

I'm fully aware of the grit issue, will have flood coolant. I'm thinking of building a box of sorts to contain everything into a self contained area as parts are only small. I will have a separate coolant supply within the box to avoid using the mill coolant.

Have 15000rpm spindle so should be usable. I can mount a diamond point within the box.

Not sure how I will go about programming for the wheel dressing and corresponding diameter reduction.
Luckily the final dimensions are not critical, + - .002 does not affect operation at all, just need a smooth surface
Any suggestions welcome.

 

[eKretz]

What is the intended diameter of the grinding wheel? If it's very small, 15,000 may still be low. As far as programming, I expect you'll have to enter an offset every time you dress the wheel. Just measure it and treat it like a milling cutter. You can use a mic if you hold a piece of paper around the wheel. Just use a light touch and measure the paper so you can subtract it. Chrome can be a bugger to grind sometimes, especially on a machine not made for grinding - rigidity may be a problem. Do everything you can to optimize that. Aside from that, you're just going to have to see how it goes.

 

[michiganbuck]

Eric's concern about the wheel is very valid. A Dimond point to do its best work should be one with a larger/largest shank if possible.
*You might give the dimensions of the oval so the PM guys can better think about the job.
Making a box and a separate coolant supply is a great plan. Even a waterproof one made of plywood for a one-up job would be suitable/ok.
4000 to 7000 SFPM is a ballpark, too slow and you are honing too fast and balance can be a problem.

Re: The pressure to penetrate a harder material is best served with all the rigidity you can provide.
Think about how hard one pushes stock into a bench/pedestal grinder.
For some jobs bulls of guys push with two hands.

 

[memphisjed]

Will be chrome, not is chrome.

 

[Matt@RFR]

My first thought for comping your wheel after grinding. You don't say what control, so this would work for a Brother:

Give the point a work offset. Call this as a sub so you can call it at any point in your program. I'm assuming the grinding wheel is the only tool in use, and that it's 1" diameter. Put your pass width for dressing in #800:

N1000
#5301 = [#5301 + #800] (shift G58 X by whatever is in #800)
#12001 = [#12001 - #800] (adjust T1 diameter by the same amount as your dressing pass)
G0 Z2. (clear all work on table)
G58 X-.5 Y0. (position your wheel at the diamond)
Z.05
G1 Z-1. F10. (dress the wheel)
G0 X-.6
G54 Z2. (call next work offset and clear all work on table)
M99

 

[FamilyTradition]

I agree with Matt@RFR on using variables to automatically comp the wheel after each dress. Makes it a lot easier on the operator and less chance of error vs. doing a manual comp every time one needs to dress.

 

[eKretz]

I agree with Matt@RFR on using variables to automatically comp the wheel after each dress. Makes it a lot easier on the operator and less chance of error vs. doing a manual comp every time one needs to dress.

Make sure the diamond is dead on centerline of the wheel. More of a pain in one way equals less in the other...

 

[Kenre]

Thanks all.

More info, as im still designing the part i dont have any pics to show, sorry!
It is the rotor housing for a small Wankel engine. Approx 50mm in X, 38mm Y, and Z 16mm. May be even larger when i nail the final dimensions.

I have 20mm Dia wheels i already use for hard chrome that work well. I could use a much larger diameter for this job.
Mill is a Robodrill T14, Fanuc 16iM controller. Should be stiff enough for this job. Recently rebuilt spindle so its nice and rigid.

Planning on a thick steel subplate to bolt the part and diamond point onto. Aluminium walls surrounding it, sealed up water tight. small pump for the coolant within.


Programmming side using fusion 360, and i am very green with mill code. variables, subs, macros, all foreign haha.

 

[Kenre]

Will be chrome, not is chrome.

The part will be hard chrome plated, need to grind it smooth.

 

[Terry Keeley]

Cool project, please post pics when you can!

 

[Kenre]

Something different to burn nitro Terry!
In theory should be good, as usual downfalls of the wankel don't affect us, lubrication of rotor seals, emissions and fuel consumption.

 

[jccaclimber]

Not CNC, but I’ve used the “put it in a box” method quite successfully using small diamond hole saws through ceramics on a Bridgeport. The other methods I’ve done for things that are not through holes is build a clay wall or super glue in a heavy steel ring and seal the edge with hot wax to form a pool. I’ve mostly relied on cutter swirl to keep things moving, but I’ve never had a picky surface roughness callout doing this.

If you do build a box and are going to be making several of these, or take that part in and out multiple times, have a pedestal in the box. That way you can drain the coolant and swarf below the mounting surface.

 

[Kenre]

I will make just the one box, with a pedestal, this will be hollow to allow coolant and swarf to drain, and allow the grinding wheel to extend past the part bottom.
The whole box will be only about 150mm square so no need for drains.

I will set it up and grind a few parts at a time, they will be located by dowel pins and bolted down through the part.