Author Topic: Experiment Engraving Machine Dials (Read 29278 times)

awemawson · « **on:** February 21, 2016, 07:08:09 AM »

This barely qualifies as a project - just a bit of experimentation.

I wanted to add a calibrated dial to the infeed screw of my Dormer 108 Drill Sharpener - later models have it and it's a relatively simple and worthwhile addition. So I just need to engrave lines and numbers around the circumference of a doughnut of steel.

Recently having bought CamBam (very impressed for the money) I drew up a linear version of the scale from which to generate G code

awemawson · « **Reply #1 on:** February 21, 2016, 07:18:03 AM »

Now available with CamBam is a 'plugin' that enables you to 'wrap' G code around a cylinder. You specify the cylinder diameter, the angle you need to wrap around, and it does the maths to convert all the (in my case) Y axis moves into appropriate angles for the A axis to move - (there are several programs 'out there' on the web that do the same).

As original this plug in wasn't quite suitable in that it generated too many decimal places after the point (5), and embedded comments in brackets - neither of which are acceptable to my Heidenhain TNC355 control which chokes on more than 3 decimal places in metric mode. The nice author of the plug in tweaked it so the latest version on the CamBam site now has a box to tick to supress comment, and a selection for the precision ie figures after the point

So now we need something to engrave. It just happens that I had a length of 2" 'hollow bar' amongst my 'come in useful' bits - so a slice was cut off, bored to 30 mm i/d and skimmed to 50 mm o/d

awemawson · « **Reply #2 on:** February 21, 2016, 07:26:14 AM »

Now the eagle eyed amongst you will notice that I'm using a 3 jaw chuck - should really be a 4 jaw to get it spot on - in practice the mandrel run out was only a couple of thou, and this was an experiment rather than the finished item, so it was the three jaw

The mandrel set is rather nice - can't remember where it came from but was made by the Armature Manufacturing Co of Oldham - no doubt they don't exist any more

They have a taper sliding arrangement that expands and grips a hole in the part, and you press them in to expand, then tap them with a soft hammer to release.

So it was duly mounted in the 4th Axis on the Beaver Partsmaster, a few careful setting made to origins and I had at it with my diamond engraver

awemawson · « **Reply #3 on:** February 21, 2016, 07:32:06 AM »

Now it started off really nicely - sharp cutting - crisp engraving - then it all went down hill

Turned out that I'd lost the tip of the diamond

... no spare

So I decided to pull it out of the 4th axis - skim a few thou off and start again with my air turbine powered rotary engraver with a 90 degree carbide cutter running at about 20,000 rpm.

This went ok but the engraving width is too wide and nothing like as nice as the diamond drag engraver produced - replacement diamond on order

awemawson · « **Reply #4 on:** February 21, 2016, 07:33:33 AM »

Now contrast that engraving with a bit I did as a test earlier - nothing like as nice

awemawson · « **Reply #5 on:** February 21, 2016, 07:41:45 AM »

It's been a useful exercise in that I've learned a few things :

... diamonds don't conduct electricity so no use trying to use my LED 'Z Height' setting gauge to set tool length

... I must have used ceramic bearings in my air turbine all those years ago - the spindle & collet are electrically isolated from the body so again no point in trying to use the LED Z height gauge !

... my lubrication / misting system for the air turbine that passes oily air through the bearings to keep them alive works, but using WD40 is messy and needs refining somehow

... my HUGELY expensive Heidenhain TS511 automatic probe does (JUST!) clear the 4th axis tailstock in use on automatic tool changes without breaking it's nifty little ruby precision ball off

When the diamond arrives I'll make another one - but it's been a learning exercise

So where does this dial go - here:

philf · « **Reply #6 on:** February 21, 2016, 08:11:48 AM »

Andrew,

I'd forget the diamond and use tungsten carbide for most materials. I chipped my diamond (forgot to set Z zero!) and thought I'd try a point ground onto a broken carbide end mill. I've never had to regrind it and I've done a lot of engraving with it - much on to stainless.

I've got plenty of 1/8" shanked broken or worn out cutters so I can grind up different angles for different hardnesses. (They can also be ground into rotary engraving tools but that involves a lot more work.)

Drag engraving doesn't seem to take paint or filler as well as rotary engraved. Maybe because it's very shallow and the surface finish in the engraving is too smooth.

My diy tool height setter doesn't rely on conductivity and works very well.

Cheers.

Phil.

awemawson · « **Reply #7 on:** February 21, 2016, 11:06:23 AM »

Phil, thanks for that tip

So encouraged by your suggestion I steamed up the Clarkson T&C grinder, found a green grit wheel and modified a broken 1 mm carbide end mill that had a 3.17 mm shaft.

That completed I made another steel doughnut, went through the origin set ups again (LED sensor OK on carbide

) and engraved the new disk.

Oh so much better than the rotary engraver, but obviously shallower. I need to experiment with tip angle - this one was about 40 degrees included angle as far as I can measure - and was completely arbitrary.

I suppose the sharper I make it, the deeper it will cut but the more likely to break

philf · « **Reply #8 on:** February 21, 2016, 11:36:33 AM »

Andrew,

I usually use a 90 degree included angle on hard materials and 120 degree on soft. You may find that 90 degrees will give you a wider mark than 40 even at the same pressure although I've not done any scientific comparisons varying angle and pressure vs width of mark.

This example was a test in a piece of brass before I risked it on a trophy.

Have you got some backlash in your dividing head? In the 4th image the gap between 50 and 51 looks smaller than the rest. Or is it an optical illusion?

Phil.

awemawson · « **Reply #9 on:** February 21, 2016, 11:47:56 AM »

I'll do some experiments. I've no idea what the steel is that I was engraving, but it's certainly not plain mild steel. Cleaning up after boring, my normal de-burring tool wouldn't touch it - I think that it had work hardened - or maybe it was pre-hardened stock

mattinker · « **Reply #10 on:** February 21, 2016, 12:44:05 PM »

That was nice to see! I think it counts as a project.

Thanks Andrew, regards, Matthew.

BillTodd · « **Reply #11 on:** February 21, 2016, 01:08:42 PM »

Snap Matt,

very interesting :-)

what about grinding the carbide to a sharp V D bit? (as used in the gravograph engravers)

Bill

philf · « **Reply #12 on:** February 21, 2016, 01:35:27 PM »

Quote from: BillTodd on February 21, 2016, 01:08:42 PM

Snap Matt,

very interesting :-)

what about grinding the carbide to a sharp V D bit? (as used in the gravograph engravers)

Bill

Bill,

I've done just that for my PCB isolation routing cutters.

The most difficult part is grinding the carbide to half the diameter. A diamond wheel helps considerably.

I found a useful webpage with clearance angles for different materials:

http://www.pilotltd.net/engraving.htm

Phil.

awemawson · « **Reply #13 on:** February 21, 2016, 03:06:00 PM »

Have you got some backlash in your dividing head? In the 4th image the gap between 50 and 51 looks smaller than the rest. Or is it an optical illusion?

Phil.

Well spotted Phil - I hadn't noticed that

When I first drew the scale I'd intended to have it read 50-0-50 and started drawing the lines at the middle out to +50 then back to the middle and out to -50. I then changed my mind and re-numbered them 0-100 (well 0-0 really as the 100 corresponds to 0)

When the program generated tool paths and G code it did so in the order that I drew the individual entities, so started in the middle to +180 degrees, then back to the middle and out to -180 degrees. This later version of CamBam allows you to optimise the cutting order, which I've now done so it engraves all the lines going clockwise, then re-winds and engraves all the numbers clockwise - so cutting will all be in the same direction and backlash eliminated. It does beg the question though why doesn't it show in the numbering, as any one number is being cut in both directions. It is just possible that the figure "0" on all the numbers has a more curved left side than right side, but the difference is nothing like that between the 50 and 51 lines that you pointed out.

At least my final version should be uniform. Nice to get it right, but in this application it doesn't matter a jot - the feed screw dial is 'zeroed' when one cutting edge of the drill is sharpened, then backed out and returned to the same zero for the other cutting edge. The scale calibration is just to show how far you have to go to zero.

Just shows - this is all a useful learning exercise

RotarySMP · « **Reply #14 on:** February 22, 2016, 05:11:22 AM »

Did you grind a number of facets on that 1/8th Inch carbide or is it conical with half ground away?

Mark

awemawson · « **Reply #15 on:** February 22, 2016, 05:48:52 AM »

Mark, the last version that you saw was 'drag engraved' ie the cutter was not rotating and was just a 40 degree conical point pushed and pulled across the work. In the earlier example I used a high speed rotary engraver with a commercial carbide 'D bit' engraver which is 90 degree included angle.

RotarySMP · « **Reply #16 on:** February 22, 2016, 10:15:14 AM »

Thanks. Looks really nice.

Mark

awemawson · « **Reply #17 on:** February 23, 2016, 06:23:16 AM »

The more I look the more I find

Watching the machine engraving another version I was looking at the displayed angle of the 4th axis incrementing away - 100 divisions over 360 degrees so why is it incrementing by 3.575 degrees and not 3.6

So I looked at the G code that it was obeying, and sure enough if you look at the difference between successive values for A they are 3.575 degrees apart as you can see in the attached file of G code

awemawson · « **Reply #18 on:** February 23, 2016, 06:28:44 AM »

Much head scratching ensued

Much coffee consumed

But it wasn't until a period of awakeness at 3 o'clock this morning that it dawned on me. The wrapping program works out the extents of the code that it is wrapping in terms of maximum X and Y then applies it's clever mathematics. The lines to be engraved are fine and in the right places BUT THE ZERO DESCENDS BELOW THEM

This means that the length of the wrap is wrong as half of the width of '0' is added to the Y length as you can see in this picture:

awemawson · « **Reply #19 on:** February 23, 2016, 06:31:11 AM »

The solution is trivial. add some lines below the zero, remove some lines at the other end of the scale and all should be hunky dory.

As per these pictures:

awemawson · « **Reply #20 on:** February 23, 2016, 06:34:39 AM »

So now the 'length of the wrap' is dictated by the lines and not the numbers - so did it work?

... well I've not cut metal to prove it, but if you look at this version of the G code you'll see that the A values are incrementing now by 3.6 degrees. 100 x 3.6 = 360 = one revolution which is what it should have been all along.

... as I've said before, it's a learning exercise - and I'm certainly learning things that I wouldn't previously have considered

raynerd · « **Reply #21 on:** February 24, 2016, 02:48:29 PM »

Cracking post Andrew, learnt a lot. I may invest in cambam and will be trying the carbide pointed engraver. I currently use a diamond but I see the carbide works well.

Chris

DavidA · « **Reply #22 on:** February 25, 2016, 07:20:02 AM »

Andrew,

I was really surprised when I read your line stating that diamond doesn't conduct electricity.

So surprised I looked it up. And it seems..

...The electrical resistivity of most diamonds is on the order of 1011 to 1018 Ω·m...

That should read 10E10 to 10E18 Ohm per square Cm.

As diamond is carbon and carbon is used in conductors (Brushes etc) it was unexpected.

We learn something new every day.

Dave.

awemawson · « **Reply #23 on:** February 25, 2016, 08:57:40 AM »

ARGH I Have a PROBLEM

I've been chasing tiny errors in the engraving line spacing for what seems ages - convinced I got the wrapping program set wrong, or something in my original drawing like too many or too few lines, so in desperation I decided to to a manual test:

a/ I skimmed off one of the 'doughnuts' to 48.5 mm o/d and mounted it up on the 4th axis

b/ I sent the 4th axis to -10 degrees, then to 0 degrees to eliminate any back lash

c/ I then manually drove the engraver into contact and scribed a line then raised it again.

d/ I then drove the 4th axis to 360 degrees then manually drove the engraver into contact and scribed a line then raised it again.

e/ I then drove the 4th axis to 720 degrees then manually drove the engraver into contact and scribed a line then raised it again.

f/ I then drove the 4th axis to 1080 degrees then manually drove the engraver into contact and scribed a line then raised it again.

Now all those lines that I scribed SHOULD fall on the same place and over lay each other BUT THEY DON'T

As you can see there is an increasing cumulative error. To try and quantify it the gap between lines is of the order of 0.833 mm on a circumference of 152.367 (Pi x 48.5) so of the order of 0.833 x 100 / 152.367 or 1/2 % which is far too large to tolerate !!!

Back lash has been eliminated by travelling in the same direction all the time so what the heck could cause that

BillTodd · « **Reply #24 on:** February 25, 2016, 09:38:58 AM »

Your 4th axis settings must be wrong too many (or too few ) pulses/steps per rev. (has someone swapped encoders by any chance) . What make of 4th axis BTW?

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Author Topic: Experiment Engraving Machine Dials (Read 29278 times)