Precision Matthews PM-728VT CNC conversion

[adamyq]

I have both. The set screw works for endmills, but ER20 is needed for drills and touch off probe.

Looks like the set screws are available on 4 sizes, but not sure I understand the difference between a 3/8” set screw and a 3/8” ER20 collet. Obviously going the ER20 route for common end mills is more expensive vs set screw. Is the ER20 more flexible (drill bit tolerances), or does it provide improved concentricity?


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[koenbro]

End mills have a flat machined onto their shaft. That is where the set screw connects. 3/8" or 1/2", etc are the shaft diameters. YOu need to decide what cutters you need first, then get an assortment of holders for them. Our mills don't have enough power, so I think 1/2" is max for Alu, and 3/8" for steel IMO. I also have some 1/4". I started with the basic PM endmill kit, and now I use more upgraded stuff from Travers, McMaster etc.

Some tools do not have the machined flat so you use ER collets for them, again matched for their dia (drills, reamers, Noga indicator holder, probes, fall into this category)

To your earlier question: the KP-3 probe has a 1/2" shaft so I use a 1/2" ER collet. An alternative btw is the Haimer, but I have never used that, so no opinion; however, I love the KP-3.

IMO it is important to not just buy the CHEAP!!!! SCORE!!! collets from Amazon or Ebay, as they might have runoffs and other problems. I got stuff made in Japan from McMaster. One ends up with a collection, for example, I use a 5.1mm (.201") drill for the 1/4" holes that are my standard, so I got a matched collet; collets generally cover a very narrow range.

 

[ptrotter]

Tormach has TTS Gauge Holders that are lower profile than Endmill Holders that work well for holding gauges and probes. I use them on my KP-3 and Haimer. Most 1/4" endmills do not have flats on them so a collet holder is probably best for those. Collets "should" have better concentricity that endmill holders since you are pressing the endmill to one side in an endmill holder with the set screw, but I'm not sure it will make much difference in reality.

 

[koenbro]

Most 1/4" endmills do not have flats on them so a collet holder is probably best for those.

Yes, I stand corrected. Although I have some 1/4" endmills, their shank is 3/8" with flats.

And here are my collets:

 

[adamyq]

Mine looked different. I might have an earlier version and don’t recall adding the zerks. Also don’t think they were preinstalled. Finally I don’t think you can access them once installed. For example, neither the z- nor the y-axis double nuts are accessible once installed. The x-axis nut is accessible if removing the motor and the opposite end connection. Lots of work.


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What trick did you use for tightening the bearing lock nut? I’ve had good success on X and z by applying pressure to the threaded side bearing and even just finger tightening (waiting for spanner to arrive).

I’m struggling on the Y, it’s like the the shank is a bit longer that the bearings in the housing so the nut bottoms out. I can’t tell for sure, will disassemble tomorrow and see.


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[adamyq]

There was no easy way to attach the sensors to the casting; tried to drill a hole, but I learned very quickly that that's not going to work. So I decided to reuse the tapped holes for the locking levers on the saddle. I designed a simple bracket in Fusion.
View attachment 356606

Then cut in on the router out of scrap 1/16" 6061.
View attachment 356608


Here it is mounted on the mill. The left sided sensor is already on; have added the second (+ limit ) sensor since.
View attachment 356609


I plan to add two sensors for axis Y and maybe one for axis Z (upper limit). Still haven't figured out how and where I mount the y-axis sensors, as I want to use existing holes in the casting.

More importantly, will need to figure out how to configure the Acorn to stop the motors. Right now, it shows on the screen that the sensor is activated, but the motors do not stop.

Did you ever sort out why the sensor would activate but not stop the system?

I’m debating if I want to add these at all. There will be plenty of times I walk away from the machine.


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[koenbro]

What trick did you use for tightening the bearing lock nut? I’ve had good success on X and z by applying pressure to the threaded side bearing and even just finger tightening (waiting for spanner to arrive).

I’m struggling on the Y, it’s like the the shank is a bit longer that the bearings in the housing so the nut bottoms out. I can’t tell for sure, will disassemble tomorrow and see.


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Please post photos, as I do not understand the shank length leading to bottoming out. I can post photos too if you need them.

If it's about item #6 in the PM drawing, the precision locknut, I use a Knipex Cobra before the spanner arrived.

 

[koenbro]

Did you ever sort out why the sensor would activate but not stop the system?

I’m debating if I want to add these at all. There will be plenty of times I walk away from the machine.


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Yes, the limit sensors worked out in the end and I could not imagine the system without them. Happy to post screen shots of the Acorn wizard. My first experience with CNC is with the AVID CNCPRO system assembled from a kit, that comes with 5 inputs (2 per axis, except axis-z not having a minus limit; due to the slaved dual y, there are 4 sensors on y, 2 for x, and one for z).

Because of this, I thought I will need plus and minus sensors, but Acorn allows settings software limits. So my current setting is x minus, y minus, and z plus, all normally closed (safer than the normally open sensors AVID offers, but also quite a bit more expensive); the matching x plus and y plus are set in software. I also have a soft z minus limit that is not practically useful. When I turn on the system, it homes first to the sensors, then machine coordinates are set, and the soft limits control the range of motion. Seems very safe.

Check out what I wrote earlier about NC sensors.

 

[ptrotter]

On my Acorn setup I put in X+ and Y+ limit switches as well as soft limits so that in normal cases the soft limit is hit before the switches. I did this because sometimes I want to move my home switches to reduce the work envelope so that homing is quicker. I believe the soft limits are set as a distance from home so if I move the home switch inward, the soft limit would be too far. The limit swich will protect me then.

 

[koenbro]

On my Acorn setup I put in X+ and Y+ limit switches as well as soft limits so that in normal cases the soft limit is hit before the switches. I did this because sometimes I want to move my home switches to reduce the work envelope so that homing is quicker. I believe the soft limits are set as a distance from home so if I move the home switch inward, the soft limit would be too far. The limit swich will protect me then.

That's a cool way to do it, in particular when considering that homing z and x is so slow that it takes minutes.