Hello Guest it is April 23, 2021, 03:41:06 AM

Author Topic: Mirror probing  (Read 1984 times)

0 Members and 1 Guest are viewing this topic.

Offline BR549

*
  •  6,952 6,952
    • View Profile
Re: Mirror probing
« Reply #10 on: March 31, 2019, 03:24:28 PM »
That should be a simple 4th axis probing function. Mount teh barrel in teh 4th center teh probe in Y axis. Then probe teh X axis for the length of the rib. Then rotate 4th say .25deg then probe back along teh X. Repeat until you have probed the area of the rib. That will create a point cloud for you to process for cutting.

Just a thought, (;-) TP
Re: Mirror probing
« Reply #11 on: March 31, 2019, 06:31:32 PM »
Hi Don,
Terry's (BR549, good to see Terry) is the classic method, namely rotate the object you are trying to probe
or measure.

That may represent a challenge, rotating the rifle accurately (between centers?) given that they are so long.
The same result could be achieved if the probe were fitted in a rotating head. It would require making
a probe head assembly that fitted in your spindle and yet could rotate in the Y plane.

Depending on how often you have to do this work it may prove simpler/cheaper in the long run to go back to
your epoxy mold.

Craig
My wife left with my best friend...
     and I miss him!
Re: Mirror probing
« Reply #12 on: March 31, 2019, 07:35:28 PM »
Hi Don,
I have a variation of the idea of either rotating the rifle under the probe or rotating the probe head.

My proposal is to have three probes (plunger micro switches, you know the cheap and cheerful ones!) mounted
on a plate which in turn mounts in the spindle.

To probe at or near the center line of the rifle you would monitor the output of the center (vertical probe)
and move it with Gcodes lke:

G31 Z-1 F4

If you are not familiar with probing G31 is the probe code. The move (in absence of X and Y coordinate data) is
downwards to Z=-1 inch. When a probe event is detected the movement stops and the motion controller reports
the XYZ position of the event.

This is very much the common use of the G31 probe command. It is not however the only way to use it.
for instance if you coded:
G31 Y-1 Z-1 F4

Then the motion would be in the YZ plane at 450 to vertical. If you monitored the probe parallel to
the direction of motion it would record a probe event as normal.

You can see where this is going. What I'm trying to do is arrive at a compromise that does not require either
rotating the rifle OR rotating the Probe head, because such rotation device would require extensive 'making'. This
alternative is an approximation to the rotating Probe head idea.

I will admit also that because Mach4 has the possibility of four separate probes the idea occurred to me to utilize that
ability. This could still be done in Mach3 but would require all three probe heads to be wired in parallel so that if any
one of them triggered a probe event and its associated data point would be recorded. That would work fine provided
that if for instance you are probing with a vertical movement (-Z direction) and either the right or left hand probe were
to strike some part of the rifle that would cause a trigger event erroneously.

Does the idea appeal?

Craig
My wife left with my best friend...
     and I miss him!
Re: Mirror probing
« Reply #13 on: March 31, 2019, 07:56:09 PM »
Hi Don,
I had these switches in mind:

https://nz.element14.com/honeywell/bzc-2rq1-a2/switch-basic-top-pin-plunger/dp/1525197

Note this is the New Zealand site of Element14 so the price is in NZD. Works out to about $13 USD. Element14
I think might be called Newark in the US.

I thought that if you could drill a small hole in the plunger and glue in a 1/8 bit of wire as a probe extension.
All three switches could be attached to a flat plate of some description which would have a shank that fitted
in your spindle.

These switches are pretty good quality and 'guess' you could get 0.001 repeatability with them. One of the features of
these switches is about 5mm of overrun. That is the travel that the plunger can make AFTER the switch event without
bottoming out and damaging itself.

As you can see I'm thinking that you could make a thee axis probe for under $50. I imagine that there would be some
coding required, but that is time and brain power intensive, no dollars involved!

Craig
My wife left with my best friend...
     and I miss him!

Offline BR549

*
  •  6,952 6,952
    • View Profile
Re: Mirror probing
« Reply #14 on: April 01, 2019, 12:38:19 AM »
Simply mount the barrel in the 4th and use a steady rest to support the rest of the barrel. :) Yo can then probe in X/A in either of 2 modes long runs up and down X indexing A or  A with short moves of X.

That would seem to be the easy way using just 1 probe.

 (;-) TP

Offline MN300

*
  •  243 243
    • View Profile
Re: Mirror probing
« Reply #15 on: April 01, 2019, 09:55:21 AM »
If the barrels are round, not oval shaped, I think there might be an easier method which would use only one probe.
The centerline of the barrel would be trammed in to the X axis of the mill before measuring and used as the zero reference.
A single set of measurements would be taken down the centerline. They would be mirrored and adjusted for the tool diameter to create a tool path.  That path would then be mathematically rotated a few degrees at a time for successive passes.
What I am suggesting is in effect measuring the radius at many points and converting it to a tool path. Rather than writing a custom program to do the conversion the values could be transferred to a CAD program and it would do the heavy lifting.
If you already have a method of converting probing data  to G code the set of readings could be mirrored and the rotated passes added before entering into that program.
Re: Mirror probing
« Reply #16 on: April 01, 2019, 02:54:10 PM »
Assuming you'd machine the final part with a ball endmill, make a stylus of solid steel the same dims as the ball endmill. I made one with a .375" bearing ball silver soldered to the end of a .250" drill blank mounted in a piece of delrin. Simply isolate it electrically and do a "contact" probe grid in the size necessary, find and replace all Z-'s with Z+ to mirror. Just like a duplicator, it should come out "dead nuts". Wouldn't it ?
Lemme know if I'm way off, just thinking.
Russ

EDIT: might replace the + and - the other way round.
« Last Edit: April 01, 2019, 03:02:45 PM by Overloaded »

Offline MN300

*
  •  243 243
    • View Profile
Re: Mirror probing
« Reply #17 on: April 01, 2019, 04:29:32 PM »
Bluing on the barrel may interfere with electrical contact.

If the part is not symmetrical in the X axis inverting both Z and Y would rotate the part 180 degrees.

If you use a spherical tip you're really measuring points that are the radius of the sphere away from the part. The surface being measured is not flat so you can't just subtract the probe radius, especially at the edges. The probe radius would need to be subtracted from the measured position of the center of the probe at the angle from the measured position to the center of the barrel. This means both Y and Z would be adjusted. Then the tool radius would be subtracted in the same manner to calculate the center of the tool path. If the probe and the tool have the same diameter, that diameter could be subtracted instead of doing the radius twice.
 
Some additional adjustment might be required if the angle of the tapered section is too steep. A quick calculation says that error from a small taper angle might remove an extra thousandth or two of an inch.
Re: Mirror probing
« Reply #18 on: April 01, 2019, 04:53:12 PM »
Good point. Yes, I see that now, thanks.
Hmmmmmm .... plan b. ::)

Offline MN300

*
  •  243 243
    • View Profile
Re: Mirror probing
« Reply #19 on: April 01, 2019, 05:26:25 PM »
The required compensation problem is one of the reasons why I suggested the approach in the earlier post. The measuring could be done in one pass and then expanded by computation to define the rest of the part. A probe with a bar shaped tip with a V edge aligned with the Y axis should work well.

There are other potential problems with the 3 probe method too.
In the measuring phase how do you measure the offset between the 3 probe tips in all 3 dimensions and then adjust and combine the readings into one file?
The plunger of the microswitch doesn't slide in a precision bearing. Will it be pushed to the side when measuring against an angled surface creating an error?