I thought I'd check out the Bridgeport conversion now, given that I seem to have all axes working, the various limit and homing switches installed and the controller set up with sensible parameter values.
I've got some engraving laminate (white outers, black inner) which I'd like to try out and some crappy 1/8" engraving bits from ebay or Aliexpress (I forget which). Cut a piece off with the cordless circular saw and bolted it down to the table. So far so good.
Slight issue-let here. I can't compress the pullstup drawbar, as it requires a significant force to overcome the supey dupey die spring I am using to hold my ISO30 / BT30 tooling - certainly more than I'd designed the Z axis to achieve. I recall doing the sums when I bought the springs and double checking later on when applying the preload (achieved by compressing the spring with the toolholder mounted). The requirement on the drawbar is to avoid the toolholder being loosened or pulled out (even slightly) during operation.
The mechanism simply required me to lift the quill up against a fixed stop at the top of travel (using the hand feed lever). Further movement would then compress the spring and release the tool. Works well. It's mounted on the top of the head:
Possible solution:
There's a "chuck tighten" button on the Newker controller which toggles on and off each time you press it. I've checked it works as intended by observing the "M10" ouput which appears at connector CN3 pin 21. This is an active low output using a ULN2803A darlington array (open collector, active low), rated at 50V / 500mA. In practice, 300mA may be pushing it. However, they are ok for driving small relays directly due to the inbuilt flywheel diodes.
Here's what the Newker manual shows. Seems to fit with what we know:
So it looks as if I could create some sort of pneumatic piston controlled by a solenoid valve. This would allow me to (manually) change the tools using the controller panel.
Had a cast about in the workshop and found this. I used it to transport the ballscrews when we moved back from Canada in 2014. The ballscrews are now in use, so there's little need to preserve it. It's a nominal 3 1/2" pipe with an actual ID or 77mm (3").
Looking more closely, it's actually rated at 320kPa ie 3.2 bar. I could probably use this at 5 bar or so, although of course I only need a short piston movement, so there would be a fairly limited amount of pneumatic energy stored in it if it ruptured:
I'd need to ensure the ends are flat and perpendicular, suitable for sealing against an o-ring at each end (particularly the driving end). It just fits into the fixed steady, rather like the spanner. The steady is required due to the limited grip possible with the chuck. I can drive it with the chuck but not support it.
Face off one end so that it can fit up square against the chuck. The steady fingers damage the surface but it doesn't matter if I ensure the piece I finish up with hasn't seen contact.
Parting off:
Messy!
Finally - square, smooth faced and deburred. 40mm length:
Also found some 3/8" x 3" loominum bar stock from which I bandsawed a couple of 3" lengths and faced off manually in The Shiz using the MPG. Came out nice, bang on square, which in this case is 101.8mm each side.
Fusion 360 design:
So, having made the first parts, I then created an assembly in Fusion. That's a bit arse about face but I'm probably going to struggle to make this on the lathe, given the lack of grooving tools and inserts which would be required for the o-ring and cylinder bores.
As shown, this gives a movement of 12mm. With the quill at home position, I will set the piston position so that that 12mm is enough to open the pullstud mechanism without exceeding the required movement. It wouldn't damage anything but there needs to be a positive limit somewhere in the system.
There will be a return spring under the piston, hence the gap. I'll need to figure that out later, making sure it can't get trapped between the piston and the housing.
The concern:
It's all very well launching into the design and making bit - but I need to convince myself it's actually going to work before getting too far down the road. "GutCAD" doesn't always deliver the required result so let's do some simple tests:
I've got several "luggage scales" that I acquired when I was thinking about making a tool load / deflection ("stiffness") gauge. You can get these from pound shops and they typically have a load range of up to 40kg or so. Made of the finest Chinesium but adequate for this sanity checking. Firstly, reassemble it back to how it is supposed to be:
Refit the manual feed level and get some numbers:
The luggage scales indicate ~10kgf to open the pullstud mechanism.
The effective radius of the manual feed lever is about 180mm.
By zooming in on the drawing, I was able to achieve a 1:1 scale factor (so all dimensions were approximately life scale) and make simple measurements of component sizes. This suggests the downfeed pinion is around 24mm PCD ie an effective radius of around 12mm. So the mechanical advantage seen by the luggage scales is about 15:1, suggesting around 150kg required at the drawbar itself, aka 1500N force.
The piston has an effective diameter of 77mm, resulting in an effective area of ~3/4*(77^2) ie 4400mm2. So I require 1500N / 4400mm2 which is 337kPa or 3.4 bar air pressure.
Quick sanity check from the other end, that's 3" ID, so a CSA of about 7 square inches. At 8 bar / 120 PSI air pressure, that suggests about 840 lbf or 380 kgf would be generated. As I only require 150kgf, that's 150/380 * 8 bar ie 3.2 bar or so. That agrees. It's all the same numbers, just thrown around slightly differently.
Conclusion:
Looks as if my piston should work happily with 5 bar or so. Given that the pipe is formally rated at 3.2 bar working pressure, I reckon it may just about survive if I am careful to avoid creating stress concentrators. Good.
Let's continue with making the parts now....
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