Timing belts

Another step toward finishing the CNC of the big mill arrived today. These are timing belts and pulleys. The largest one is ~300mm in diameter. I'll use this to run the Z-axis (aka the knee) on the mill, and the smaller ones to run a 2:1 reduction for the X and Y axis.

Like most solutions, they come with a problem. The holes are just guide holes, I'll need to bore them out to size. That's easy. The hard bit is cutting a keyway in each one. I don't have a keyway broach, and don't feel like spending ~ $200 just for this. I'll probably try and grind out out of a bit of high speed steel. We'll see how well that works....

I'm making slow progress on putting the table for the table router back together, but I've gotten mildly distracted by cleaning up the workshop.

The need to put a dust case around the table router has stirred up areas of the workbench that haven't been touched in months. So now I'm moving the PC under the bench, re-running the ethernet and power cables, tidying up and putting away, and generally making ~ 2 meters of bench space useable again. The long slow cycle continues!

Tedious machining

Sorry, more machining. I'm still on my quest to rebuild/improve the mini table router.

The previous table supports were hacked together from a bunch of aluminium box, drilled and bolted at one end. It wasn't fastened at the other end, and the structs were overlength (I had never cut them to the right size). The ho

Today I drilled and tapped holes into both end brackets for the table support structs to bolt onto. This was drilling 14 holes, each 5 mm diameter, 35 mm deep, and then tapping said holes. Suprisingly, the bulk of the time went into drilling the holes. I suspect all my 5mm drill bits are getting blunt (I use 6mm bolts almost everywhere) so it may be time to invest in a drill sharpening tool. It took almost half an hour to drill the 14 holes. Part of the time was fiddly adjustment (release table clamps, wind table to x+70mm, fiddle back and forth until it was exact on the DRO, clamp table) but it just wasn't cutting very fast either. I get bored and wound up using the quill auto-feed and auto-stop to do the bulk of them.

The tapping by comparison went very fast. I love the VFD I fitted to the mill. It's just so nice to dial up 40 rpm, and let the tap just thread straight in. I press reverse when it's in far enough, and the VFS nicely slows the mill to a stop, and then winds it up into reverse. Makes tapping (at least in Al) very quick.

I followed this lot of tediousness by milling slots into the ends of all the table structs too. 28 slots (12mm slots on top, 6mm slots on bottom). Man, I'm glad I don't do this for a living.

Boring

I had noticed earlier that there was a little too much slop in the mini table router. The Y-axis was rotating, being insufficiently restrained by the Z-axis carriage.

So this afternoon I got around to doing something about it (or maybe it was just an excuse for some boring action.. :)

I decided to add another pair of linear bearings to the top of the Z carriage so that a moment about the Y-axis would be carried by the bearings on the same rail instead of working to seperate the rails. This should (with a little pre-load) be much stiffer.

And while I had the boring set out, I enlarged the motor mount opening. The top photo is the boring bar in action, on autofeed. I love the finish it leaves! This is using a carbine tipped bar in aluminium which isn't ideal, but it was the bar I had to hand.

Nest was making a bracket for the linear bearings. This is a simple piece with 2 x 22mm holes for the linear bearings, and 2 x 7mm holes for attaching it (and 1 x 6mm holes in the center for the threaded rod to pass through). The digital readout on the mill makes this a piece of cake: Just touch off the piece to find the corner, set it to (0,0) on the DRO, then just drill 6mm pilot holes for the 2 x 22mm, enlarge to 16mm using a roughing mill bit, then use the boring bar to enlarge 1mm at a time to 21.8mm, then enlarge 0.0.05mm at a time to get to 21.99 which leaves the linear bearings a press fit.

Last photo in the new bracket installed. It's the top bracket on the Z carriage (the carriage on the vertical rails). There's a little preload from the Y rails, so it's now nice and stiff. Well, not up to cutting metal stiff, but it should now be adequate for wood and plexiglass. (I'd previously just used it for XPP foam and wax).

Modifying ACME nut on mill.

Taking another step toward converting my mill to CNC control, I decided this to take a closer look at the nut on the X axis with a view to seeing how easy it would be to replace/augment. (The idea being to remove backlash out of the system).

This is a little trickier than it would normally be because my mill has an X-Y digital readout, with the optical encoders bolted to the mill. These needed to be removed as the table was being moved beyond the normal limits to expose the nut.

Being an idiot, I look at the end with the power feed and said "Hmm. Looks complicated, let's leave that alone".

So I started with this end. After about 30 minutes of mucking around, it become abundantly clear that this was the end with the thrust bearings; I.e. the complex end. The end that's relatively high precision. And with the benefit of hindsight, it was obvious that the power feed would be fitted to the end that's easiest to fit to. Doh.

So back to the powerfeed end, it disassembled much more easily. This is after removing the retaining nut, handle, and feed cog. Nice simple mechanism.

The power feed just sits on roller bearing (no thrust bearing required) and so is very easy to remove after clearing the keyway.

And there's the acme nut. Piece of cake. Interestingly, it turns out there's two of them (another on the other side), so removing the backlash looks very easy. Just mill some slots in the nut, and turn it to adjust the degree of slop.

Nut removed.

Preparing to mill the nut. It's just centered and clamped onto a small rotary table. I'm using a 4 flute 6mm bit. The slots will be 7mm wide, but I didn't have a 7mm to hand, and I can just cut one side of the slot at a time.

All done. The slots I left slightly under size to improve the centering. It was the work of moments to reassembled everything and measure the nut backlash. (Well, many, many moments. About an hour actually).

CNC for Milling machine.

Making intermittent progress on converting my knee milling machine to CNC. This is random notes on my researches to date

Those that have gone before: Hafco HM-52 CNC Conversion

Torque goals:

1. Maximum handle pressure in normal operation is probably under 5 kilograms.
2. Handle radius is ~ 3 inches.
3. This is ~ 3.3 newton-meters of torque.

Speed goals:

1. Table is ~ 1200mm
2. Would like to be able to go one of the table to the other in substantially less than a minute.
3. Say 12 seconds?
4. Speed circa 6000mm/minute.
5. Leadscrew is ~ 3mm/turn from memory (8 tpi)
6. So need around 2000 rpm at the leadscrew for this speed.

Which gives around 600 rpm/Nm required from motor. So something around 5000 rpm @ 1.5 Nm peak torque would be ideal. (Using timing belt to gear it down to 2k rpm @ 3.3Nm).

Would like to use brushless DC motors for efficiency, heat, ease of sealing et al.

Can use of-the-shelf brushless controllers but they're expensive, and require hall-effect sensors on motors.

Can use Takao Shimizu effort, and modify slightly for sensor'ed operation. This would require doing boards, a fair bit of testing and mucking around, but probably substantially cheaper, particularly if I do a bunch of such boards.

Looking to use EMC2 for motion control, so the Mesa boards seem popular and vestitile. Particularly the 7I43-U which is USB or parallel port @ $USD90. Mesa also sell the 7I39-LV which will drive 2 BLDC motors at up to 250 Watts. ( ~ $USD300 for 4 axis). This is currently ~$AUD600 plus shipping. Hmmm.

On the other hand, running brushed motors enabled using the 7I30 which does 100W/channel over 4 channels for ~$USD90. This would effectively drop the top speed to ~2500mm/minute for the X and Y axis, and something much less for the Z-axis (which will need to be geared way down anyway).

Still need to find a suitable source for servo motors (i.e. cheap!).

GCAM is looking for more useful for simple CAM tasks.

Need to change over to anti-backlash nuts for X and Y axis (can probably rely on weight to handle Z-axis). Simplest thing is probably to use delrin split nut? May need to make/buy an suitable ACME tap for nuts (or find an already made one on ebay?)

APT looks like an interesting thing to play with for CAD/CAM.


Cheap rotary encoders are available for $USD25 ea. Example of one being fitted is here.