VMC as CNC lathe

[mattpatt]

Just thought that I'd share this.

I guess quite a few of you have a CNC lathe at your disposal, but unfortunately I don't. A reasonably local shop has one, but getting a job done there means joining the queue, and if it's only a one off, or a couple it means a lot of expence too.

Well, just for laughs I thought that I'd try to use my VMC as a sort of vertical lathe!

I needed to machine the gasket face and combustion chamber of a two stroke motorcycle cylinder head. Could do it using 3D milling, but it'd take forever to get the finish I need. So what I did was make the 3D model of the head in SW and slice it down the middle, so that I have the combustion chamber profile in the XZ plane (Y axis stays on 0). Now, I wanted the machine to cut on the left side (as I face the machine) of the head, so I then generated a "3D contour" toolpath on the rightside of the 3D model, running in the X+ direction. All good so far.

I set the cutting tool as a 0.01mm diameter Ball nose cutter, so that I could stay almost exactly on the profile. And used the "Tip" selection as the tool reference.

Now the fun bit. Made up a mandrel to hold the head (using the central spark plug hole) with a 25mm shaft. The mandrel fits nicely into a tool holder. The head is screwed onto the mandrel and then the whole assembly stuffed into the spindle (now my DIY lathe chuck).

For the cutting tool I ground up a piece of HSS I had and jigged this up in the vice. The datum was set on the exact cutting point. Bit fiddly but got there. I ground the tool sharp, and then put a very small rad on it with a stone by hand.

It was a bit scary at first, but worked like a charm. It only took 5 minutes to reprofile the head with this method once set up.

It's probably of little use to anyone but curious me, but I was quite please with my ingenuity!

I've since thought that it would be very cool if I could play on this new idea. Is it possible to machine a candlestick holder shape? Possibly!

I just made a 3D model of a simple cadlestick holder. Again cut it in half so that I can get profile geometry. I then set my origin so that the profile is on the XY plane and again used the 3D contour strategy to get an XY toolpath.

I then took the generated code and switched the axis designation. So that all X's became Z's, Y's became X's and Z's became Y's. I did this in the G code editor. When I then simulate the toolpath the Y axis stays at zero, while the X and Z axis move together so that if I cunningly set up a lathe tool I could (holefully) cut the profile of a candlestick holder. I've only just run the sim, so I have no idea if it'll work on the machine, but can't see why not.

It would seem to me that as long as you can hold the stock securely in the makeshift Chuck (in the spindle) you can do pretty much anything (within weight limits of course). Imagination seem to be the only other limiting factor here.

[mattpatt]

Been fiddling a little more. Some of the above about the "candlestick holder" may not be totally accurate.

Just tried with another job that I' could actually do with making.

Got it worked out a little better. I set the origin at the centre of rotation of the 3D model. The 3D model is cut so that the profile runs in the ZX plane. The Y axis will not move from zero when the profile is chosen in "3D contour". Now I changed the X's to Z's and the Z's to X's in the editor.

All looks good to me.

[Brakeman Bob]

Matt, that's ingenious! And of course with careful use of (if my memory holds good) G46 & G47 you can apply tool offsets to the profile. I think the only thing to worry about is the weight of the part hanging off the pullstud.

What size is your spindle taper? I have seen HSK tool holders users as workholding either as a collet chuck or as a fixture.

All the best

Bob

[mattpatt]

Thanks Bob!

Yes, my worry is about the weight, but in the case of the heads, they're aluminium, and off a 150cc engine, so not very heavy.

My spindle is BT40. I'm not expecting to be loading anything particularly heavy in there.

I'll be playing with this some more as soon as I can. Getting new ideas :-)


Matt.

[mattpatt]

YouTube - VMC Cylinder Head
Just picking up on this ancient thread. In case anyone is interested I've just done a bunch of heads so I thought that I'd make a video and put it on youtube.

Not exactly rocket science, but I think it's quite cool, and cheaper than buying a dedicated CNC lathe to do it.

[Brakeman Bob]

Good video Matt. What is the profile in the head - is it a simple dome or do two smokes have something more fancy these days?

By the way, there should be a video of one of my parts being cut going up on YouTube soon.

All the best

Bob

[mattpatt]

Just took the video on my phone, so I was a little surprised that it actually came out ok!

I've got some other pics somewhere showing the before and after. Hang on and I'll try to dig them out.

[mattpatt]

The head on the left is the stock combustion chamber shape (2 Stroke). On the right is what I re-cut it to.

The actual volume of the two heads is identical, but the characteristics of the engine is very different between the two.

The Left head gives good bottom end and midrange power, but suffers from a lack of over rev, whereas the head design on the right loses at the very bottom, but does gain throttle response in the midrange and over revs nicely at the top end.

In the case of any budding 2 stroke fanatics out there, my design was developed with help from TSR 2 Stroke software, which is a quite indepth, but very accurate tool for engine development. That gave me the basic parameters and then I just made 3 or 4 designs close to that and tested them for the best performance.

These heads are Honda NSR150SP heads. I use these, and NSR150SP cylinders as the base for my Honda NSR300 big bore kit. As well as one off designed Wiseco pistons and other components.

[mattpatt]

Oh, and with this head cutting I don't need to change anything in the editor. Just generate and run.

[mattpatt]

None of the cylinder heads I work with are made from granite, so I don't think I'll be needing those tools right away!