The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)

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awemawson:
I 'think' that I can eliminate movement error in the Beaver Partsmaster because the 'error' in bore diameter is exactly the same in X as in Y, but just for elimination purposes I set up a test.

Mounting a long travel digital plunger gauge magnetically on a set of hold down clamps firmly bolted to the bed I brought the collet of the spindle up against the plunger, over travelled a bit to compress it, and zeroed the gauge. Then I set the TNC355 controller to move 20 mm in the X axis and noted the indication - 20.002 mm and those 2 microns at the end are probably introduced error as I pressed the zero button.

So I believe that eliminates movement and scaling errors in the TNC355 controller and Beaver servo system.

So what the heck is doing it ?

Any suggestions would be very welcome !

djc:
Your disappointing end product is the result of running one cutter once through a program. I think you might need a larger sample size to draw any conclusions and eliminate the problem.

Use the same cutter in the same material and do it again. See if anything changes. Blue up the sides of the hole and run the program again (effectively cutting air) and see if any of the blue is removed or the hole changes diameter. It might just be deflection, which could be removed by a spring pass. Try the same hole with a 6mm cutter, with a different number of flutes. Experiment with tool offsets to see if they change anything. Try the same cutter and program on a different material (change feeds as necessary) to see if anything changes. Do a series of five holes from a cold start and see if anything changes - maybe the machine needs to warm up.

Is it climb cutting or conventional cutting on the final orbit? Is there any issue with coolant? A 22mm cutter in a 25mm hole does not leave much space for chips - maybe air blast them out as they form.

awemawson:
DJC thank you for your comments and suggestions.

I had already run the program a second time to see if a clean up cut was needed. (It is climb milling)

To eliminate cutter flexing (or maybe identify) I had decided to run the program again with the feed rates drastically reduced from the recommended levels, and went through the program first thing this morning editing them down to just 10% and had just run it when I read your comment!

The results are rather surprising (to me at least) as best as I can measure it at the moment is am getting:

Diameter in X Axis 24.914 so Delta X = 86 microns

Diameter in Y Axis 24.934 so Delta Y = 66 microns

So two odd things - firstly that material IS still being removed and secondly that there is a small discrepancy between X and Y

As the spoil plate is now fixed and centred I'm loath to dismount it for too many more experiments. I'll carry on with the machining of the adaptor as the critical dimensions will be finished machined on the lathe rather than the mill, and return to experiment when that is done.

But I will do a further run of the exceedingly slow feed rate version and see if anything changes !

(Later Note: second run of 'slow feed' program removed approx 10 microns from both X & Y diameters ending up as X=24.922 Y= 24.944)

I guess at this feed rate there is a bit of rubbing going on - cutter feels dead sharp but I don't have another 22 mm finish end mill, just roughing ones, as a substitute.

awemawson:
OK OK enough of this messing about with oddities - time to get some work done !

So I turned up a stepped plug for the hole in the spoil plate, aiming for the mid point of the two measured diameters of 24.930 and 24.948 being 24.939 guessing that this should give me a fairly light press fit, which seems to be the case. Used a bit of unknown scrap that was an absolute joy to turn - I suspect that it's EN24T. Then I gave it a nominal 10 mm spigot 10 mm long - I aimed for 9.99 to go in a 10 mm reamed hole and actually achieved target!

Next to sort out the billet. It is a nominal 170 mm 60 mm sawn off the bar, so both faces were not necessarily parallel or square. Mounting it in the three jaw with jaws reversed I pressed it firmly home with the tailstock while tightening it - don't want that chunk leaping about the workshop - and hand spun it to estimate how far out the exposed face was. Answer, a tad over 1 mm. I skimmed it down by 1.1 mm which just left witness marks barely visible, then took a final cut of 50 microns to a fair finish.

Reversing it I hoped that the newly exposed face would be running reasonably true, and as expected I only had to take 150 microns off to achieve a fair finish loosing all the saw marks.

Then it was just a case of centre drilling, drilling out to 9.9 mm with jobbers drills, and passing a hand reamer though (supported by a tailstock centre) to finish at bang on 10 mm. The plug spigot fitted nicely. I took the precaution of drilling all the way through the billet so that were the spigot to bind I can knock it out. This plug / spigot jobby is sacrificial - it will be machined away as the adaptor is milled and I will probably have to drill and tap it to remove it finally from the spoil board!

awemawson:
So at long last I've started machining the EN19T billet for the collet chuck to spindle adapter.

I've been holding off waiting for a 22 mm long series end cutting end mill to arrive, but I got feed up waiting, and re-jigged the program to use a 25 mm NON end cutting 6 flute end mill instead - this involved ramping down into pockets rather than plunging. I still used a 22 mm end cutting roughing mill to start off the pockets then the 25 mm for finishing.

However despite being very careful to set suitable speeds and feeds, (even to the extent of confirming the actual hardness of the billet) after a short bit of cutting disaster struck, the roughing cutter stalled the spindle and the spoil plate was pulled off the bed of the machine when I raised the spindle :bugeye:

Very close examination of the roughing cutter, which is a Sherwood Tools HSCO one, showed a tiny chip off one of the end teeth but no further damage so I really don't know why it happened. Flood cooling was washing chips away as they formed, but something jammed. It's not nice stuff to mill, EN19T, though I must say that it turned OK when I faced it.

As I had a reamed 10 mm hole in the spoil plate I was able to re-align it and re-fix it. Alignment being with two precision dowel pins, one the the spoil plate and the other in a collet, with a precision 10 mm brass collar between them. (Tap the plate about until the collar slides down!)

Having done this I was back in business, but I took the precaution of halving the feeds but I did use the same (one tooth chipped) ripper cutter - heck it has four other teeth :lol:

After that things went OK but rather slowly due to the low feed rate.

It's a compromise - too fast too high a chip load and danger of breaking, too slow and danger of rubbing and not cutting, (and this material work hardens).

Anyway, the billet is now ready to be inverted for the other side to be milled, but that's a job for another day.

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