Way covers

Notes and research on way covers

2014 3 16

http://www.cnczone.com/forums/general-metalwork-discussion/58425-first-home-made-way-cover-bellows.html

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http://www.franksworkshop.com.au/CNC/Bellows/Bellows.htm

Folded polypropylene !

Optimized folding pattern holds its shape maximizes extension vs compression ratio

http://my.net-link.net/~jsmigiel/bellows.html

Update 2013 10 19

I needed a quick solution to get up and running, so I threw together a couple of large scraps of leather and aluminum. Works well enough for now! The only serious drawback is that they get sucked up into the shop vac when cleaning up. A couple of lateral stiffeners would resolve this. Perhaps they could also be U-shaped to force the leather down and around the ways...

Last Z parts

Fixed bearing mounting plate. I fortuitously found a new NSK bearing on eBay for which it would appear my surplus THK ball-screw journal was designed for. The label on the box for the Z ball screw is:

THK
Lm System
Model: BIF2504E-10RRG0+895LC2
Serial  KX07G01373

https://tech.thk.com/en/products/pdfs/en_a15_188.pdf

Although some of the info in that PDF is a little contradictory. The dimensions seem correct for the 2504-10 "semi standard" size, but the flange shape is different...

Motor mounting plate. Note that both have the CNC signature rounded corners that are essentially impossible (I.e. far too much trouble) to do on a manual mill.

A second mounting plate for the top of the column. This is sligtly redundant, but the bearing is thicker than the first plate and the mounting position (which I definitively established only after tearing down the column) would require cutting out an inch or so of the casting with hand tools. The second plate also allowed me to integrate a counter-balance design for the head, which I had neglected to think about when I cut the first plate.

Lastly, the ball-nut mount. The X axis screw came with a nut-mount which I just couldn't use in that tiny space inside the saddle. It is well made and seemed a shame to waste so I came up with a strategy to reuse it. I didn't have any 2" Mic 6 stock on hand, but making the extension for the mount out of two 1" pieces was pretty straight forward. The tube in the middle fits snugly into the hole in the Z slide that was a generous clearance fit for a 13mm bolt. I'm using a 1/2" bolt instead . The tube will do the job that the two taper-pins were doing in the initial design I.e. pin the nut-mount to the slide while the bolts hold it in the correct position. However, because there is now only one "pin", the nut is still free to rotate so it can be aligned perfectly along the axis of the screw. I just didn't see any way (or real need) to get the holes for two existing pins correctly positioned. Instead I'm replacing the pins with a couple of 1/4-20 bolts for extra (and likely superfluous) holding power.

Counterweight planning

Page to collect notes and references for the head counterweight system. The head weighs around 100Kg, which translates to a lot of permanent down-force that the Z axis will have to contend with. A bigger motor is the easy way around this, but there is also the issue of wear on the screw. I am looking at this now so that I can plan ahead while I make the last mounting plate for the top of the column.

Thomas Powell's solution using gas springs and garage door pulleys.





http://imageevent.com/tppjr/mill/rf45dovetailmill/zaxiscounterweight?p=0&w=1&z=2&c=4&m=-1&s=0&y=1&b=10&l=5

The machinetoolswarehouse cnc version of the RF45 which doubles as a training machine for body building ;-).   Its actually pretty serious: Hiwin linear slides, servos, better spindle bearings, VFD 3phase motor etc. All this can be yours for ~$12k... ...which is why I'm doing this the hard way!




I'm not completely convinced by the garage door pulleys, but the gas springs seem more elegant to me. So chain and sprockets plus gas springs it shall be.

Mcmaster idler sprocket:

35

3/8"

20

0.640"

2.59"

0.72"

__

6663K23

21.95

Above drawing doesn't show the Pitch Diameter. List of said measurements:

http://ablproducts.com/35-chain-sprocket.html

Update 2012 9 26

The counterweight project can no longer wait! I installed the Z axis with the column on the work bench to make things easier; way more pleasant than perching on top of a ladder. As soon as I was done I put the column back on the base and the head back on the column. As I was letting the tension off the chain block I discovered that the head will now turn the screw under its own weight - so without power the stepper the head will crash into the table!

Couple of 3/8" threaded stainless studs with flats and holes for #35 chain connectors.

As a temporary fix to get up and running I threw together a couple of pieces of plywood and some sprockets with 1/2" bolts and rod for axles.

The trouble is that I don't have anything heavy enough in a convenient form factor to act as a counterweight. The bucket-o-scraps just isn't going to cut it!

Lapping done

Having finished all the parts I could for the Z without further measurements, I dismantled the entire machine again so I can finish lapping the ways and to figure out how to install the Z screw. Lapping is slow work. Its not so much the lapping bit as the cleaning between grits. It took two days, a small amount of diesel, three entire rolls of paper towel and copious beverages to lubricate the operator to get this done. 

The hole at the top end of the column needed to be enlarged a wee bit to get the flange of the ball-screw to fit. The tools of choice: a round file and an air grinder with an hss bur bit. I also took the time to clean up the inside of the column casting which had some pretty nasty, errr, crap for want of a better word, that could potentially interfere with the ball-nut.

Measuring up the Z slide to design the ball-nut mount. I'm using a dial indicator again to measure the layout of the taper pins and the central mounting hole so that I can match up the new ball-nut mount. Zero the dial against a reference, zero the height gauge, move the indicator down to what needs to be measured. Using the indicator makes it far easier to find the maximum or minimum of a circular part.

One shot oiler plumbing

I bought a Taiwanese one-shot pump on e-bay for $50 - it seems to be pretty good quality. The out port is BSPP 1/8" I believe.

Reference chart for BSPP aka G thread sizing:
http://www.marylandmetrics.com/tech/bsppthreadspecs.htm

From there I want to use 10mm pipe to run to the manifolds because I have plenty of leftover tube kicking around. However all the Parker flow control valves and the other fittings are 1/4 NPT, so this ends up being a bit of a mishmash of standards!

Parts list from MettleAir for what I don't have:

Version 1 - using Parker flow control valves (of which I'm missing one and two are not as advertised on ebay)

1   - MTC 10-G01              10mm x 1/8 BSPP
5   - MTC 10-N02              10mm x 1/4 NPT
10 - 122-B                       1/4 NPT hex nipple
10 - MTM 1/4                   1/4x1/4 push-fit bulkhead
2  - 121-B                        1/4 NPT plug

Version 2 - using pneumatic flow control + check valves instead (I don't have to make the manifolds this way and I don't have to source more Parker valves)

1   - MTC 10-G01              10mm x 1/8 BSPP
1   - MTC 10-N04              10mm x 1/2 NPT
2   - AM30-250-5               2 x 1/2 NPT in 5 x 1/4 NPT out
1   -  122-D                      1/2 NPT hex nipple
1   -  121-D                      1/2 NPT plug
9   -  MSC 1/4-N02            1/4 x 1/4 NPT meter out flow control valve
9   -  MCVU 1/4                1/4 check valve

In addition, I will plan ahead for the air requirements. These are:

Air jet for the spindle
Fog coolant, also for the spindle
Power draw-bar
Line for a blower to clean off the table etc
One spare

So a 2 in 5 out manifold would be a good place to start.

Parts list for the missing stuff for this:

1   -  122-D                     1/2 NPT hex nipple
1   -  AM30-250-5             2 x 1/2 NPT in 5 x 1/4 NPT out
1   -  121-D                     1/2 NPT plug
10 -  MTPP 1/4                1/4 plug (I need five but they are cheap...)
10 -  MTPP 10                  10mm plug (don't strictly need these but...)

Here is the assembled manifold. The check valve plus the meter-out flow control valve are equivalent to the Parker flow-control valve but are significantly cheaper and far more compact. I plumbed in the X axis ways and ball-screw. Everything seems perfectly satisfactory. Having done this with NPT, I will be using BSPP G thread standard fittings for all future pneumatics projects. I think is it far superior to NPT. Firstly, the thread is parallel and therefore requires far less cutting force than the tapered NPT. Secondly, the seal is established by an o-ring not Teflon tape and just works. I have two (small but annoying) leaks in two recent projects using NPT which I will have to take time to fix and which just wouldn't have been an issue had I used BSPP. Live and learn :)

First chips

The first chips have been cut with the temporarily reassembled X and Y axis. I am using the machine to make the mounting plates for the fixed bearing and the motor for the Z axis. Until the Z is finished, CAM files have to be manually edited to add pauses after Z moves; I use tool-change macros as a convenient way to do this. After editing I run through the whole program "cutting air" to make sure I didn't miss anything. It is a bit laborious, but a whole lot better than manually dialing XY coordinates to make a hole pattern (twice over if center drilling first!). And, watching the table cutting out the bore for the bearing is so much more satisfying than futzing around with my Chinese-import-grade boring head!

After drilling, I add counter-bores to some of the bolt holes.



Blasting away the chips from the 1/2" end mill (the hoggers are in the mail). This leaves an acceptable finish, but it isn't the best.

For the perimeter, I hold a chip brush against the cutter to pull the chips off the flutes. This hugely improves the finish.

Two mounting plates in about a day and a half - but that includes the learning curve for the CAM software (I'm trying out Aspire) and setting up Mach3. What took the most time was actually getting the Windows7 cad station to talk to the XP machine that runs the mill so I could transfer the files (thank you Microsoft). The second plate took about 1/4 of the time of the first one. The surface finish on the shoulder of the lower plate in the image is not great. The pocketing program required that the Z be retracted between each corner and it is very hard to hit exactly the same Z depth each for each cut. As the shoulder is not a bearing surface however, this doesn't matter. That is all I can do without further measurements of the relationship between the Z lead-screw and Z slide. Measurements that require dismantling the head and the column. Time for some more heavy-lifting.

Calibrating the X and Y and some numbers

Here is the setup to calibrate the Y axis "steps per". I clamp down a 1-2-3 block, jog up to zero the indicator, move 3" to the back of the block and see if I hit zero again on the dial. The theoretical number for the Y with its 1.8 degree stepper and 4mm screw pitch is:

360 / 1.8deg = 200 x 10 (for the micro-stepping Gecko drive) = 2000 steps per revolution.
Steps per unit (1") is 2000 x 25.4mm/4mm pitch = 12700

After a few trials I settled on 12710 microsteps per inch.

On the X, with its 5mm pitch screw, I ended up with 10159 steps - just one fewer than the calculated 10160.

As far as velocity and acceleration goes, with the admittedly under-powered steppers currently installed, I can't get anything more out of the X- axis than 23 IPM at 100 in /s/s, pretty slow. Why?

The screw is 5mm pitch, so 25.4mm/inch / 5mm = 5.08 rev/inch.
Multiplying the rev/inch by the inch per minute gives us RPM: 5.08 x 23 = 116.84 rpm - also pretty slow.

Update 2013/9/24 - I significantly lowered the acceleration curves to around 5 in/s/s and got better, but still not great, rapids of 50ipm. However, while cutting out the perimeter of one of the Z mounting plates with a relatively conservative cut of .25" depth x 0.5" wide @ 17.4 ipm I lost a bunch of steps in both axes. In managed to salvage the part, but it is further evidence that more torque is required...

If we look at the performance chart for this motor (with a 24v power supply) we can see the torque available is around 330 ozin at 100 rpm, about as much as you can get from this motor.

The supplier for these motors has a fairly good tool for sizing. Having followed that guide, the minimum torque recommendation was 310 ozin - but while we don't need all that torque for the rapids, the motor obviously needs more than the torque than this one has at the target speed (which if we want rapids of 100 IPM is 100 rpm x 5.08 rev/inch = 508 RPM). From the torque/speed curve we can see that less than half of the recommended torque is available at that speed. So some stronger medicine is required here.

Some possible candidates for the motor:

PK2913DAA, Stepper Motor

Somewhat louche X axis installation

Another slight difficulty in my plan for attaching the ball-nut to the saddle is the fact that the saddle casting has no machined surface to bolt to on the side! There was just no way I could machine this surface myself with the temporary el-cheapo X-Y table. So this calls for some slightly louche behavior.

First I have to get the screw exactly parallel to the table before attaching the ball-nut mounting plate. This image shows the setup for truing against the front edge. I also do the same from the top surface.

Here is where the mounting plate has to go. The bolt holes are the same that held the Y gib locks; preferable to making new ones. I've cleaned off the paint with a steel brush and filed down the worst of the casting mess leaving the rough surface that is more or less square to everything else. Then the louche sauce: West System 105/207 epoxy with 403 microfiber filler. There are better alternatives to the filler, such as stone dust or steel filings, but all I'm trying to do is provide a flat and perfectly perpendicular skim coat exactly level with the peaks of the casting. The bolts that hold the mounting plate in place should ensure that the epoxy is under permanent compression, so I suspect that flex will not be an issue.

Tape on the screw to catch any spills.

A clamp is used to push the ball nut up against the saddle with the epoxy in place.

The bolts, also covered with tape are just hand tightened.

24 hours later. Not bad, but I think I need to have another go to fill in the middle section. The little light blue spots are remnants of blue-tack - the stuff used to hold up posters - that I used to fill the threaded holes in the mounting plate.