Metal filings on Gear Rack

[Blisswoodshop]

I recently did some rotary work wrapped around the Y-axis and noticed some metal filings at the X-axis gear rack where the X-axis motion was. Does any one have a thought on what might have caused this?

Thanks,
Be Bliss.

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[DDgitfiddle]

Be Bliss,

Looking at the first picture of the rack, it looks like there might be a couple of broken or damaged teeth on the rack right near where the cable chain is bent around in the image. It is hard to tell from the picture if that is the case or not. If that is the case, that would explain the metal filings. The broken portion of the tooth may have gotten ground up during operation. Alternately, it could be a localized gear tooth surface failure (severe pitting). I don't know what would cause the broken teeth except for possibly some hard debris getting in the gear or rack or the tooth breaking from fatigue. What I am seeing is in the middle of the tooth, so that would NOT indicate that the pinion was out of alignment with the rack. The rack looks a little dry to me, like it could use some grease. Running without grease for a long time could cause premature wear (pitting).

Caveat Emptor: I am a total noob with CNC Routers, but I am a mechanical engineer with a decent amount of gear train and automation experience.

-Robert

[sixstringlz]

I recently did some rotary work wrapped around the Y-axis and noticed some metal filings at the X-axis gear rack where the X-axis motion was. Does any one have a thought on what might have caused this?

Thanks,
Be Bliss.

What rotary is that, I have been looking for one. I get a little of that on my machine also, not sure why though. Happened on my first machine too.

[sybilsurf]

You might want to use a magnet to narrow it down between the rack (steel) or the pinion (aluminum) but it looks like something to address right away.

[ericclinedinst]

Eh.... both the rack and pinion are steel. The pulley that drives the pinion is made of aluminum.

Could you imagine how fast a steel rack would destroy an aluminum pinion? Lol. You wouldn't make it through a single job.

You might want to use a magnet to narrow it down between the rack (steel) or the pinion (aluminum) but it looks like something to address right away.

[sybilsurf]

Eh.... both the rack and pinion are steel. The pulley that drives the pinion is made of aluminum.

Could you imagine how fast a steel rack would destroy an aluminum pinion? Lol. You wouldn't make it through a single job.

Oops, sorry, for some reason I thought the pinion was also aluminum....and I was imagining.....now I feel better about it

Ward

[Blisswoodshop]

What rotary is that, I have been looking for one. I get a little of that on my machine also, not sure why though. Happened on my first machine too.

I followed Digital Fabrication for Designers: CNC Router Rotary Axis expect the mounting system. I simply mounted mine to a plywood for now till I come up with some thing better.


I see lubing the gear rack is mentioned. Don't recall anything on that from CNCRPs. Is that a recommend standard practice and if so, using what?

Be Bliss.

[sixstringlz]

I followed Digital Fabrication for Designers: CNC Router Rotary Axis expect the mounting system. I simply mounted mine to a plywood for now till I come up with some thing better.


I see lubing the gear rack is mentioned. Don't recall anything on that from CNCRPs. Is that a recommend standard practice and if so, using what?

Be Bliss.

Thanks for the info on the rotary.
I think Cory told me all they use is WD40 on the gear racks

[SixOfOne]

I have the 4824 machine, I noticed the same dust on 2 of my 3 gear racks. I have always ran it dry b/c of dust concerns I have read about. So I contacted CNCRP and they got back to me super quick (great ppl over there btw) and said in a nut shell:

Some dust is normal and not a problem. If there is a lot of dust it could be:
- tension spring to tight
- gear rack alignment (new revision of PRO machine last year has a bigger flange "easier to align perfectly vertical")
- accelerating and moving very fast " 1000IPM+ "

They don't recommend oil type lubes b/c of dust, it causes more harm then good. If you want to clean your racks, use WD40 and wipe it clean "no residue" then apply a dry Teflon lube that you typically find in bike shops for bike chain.

In my case, I know my machine isn't going to fast, not sure on the alignment yet but "over tightened" is what I am guessing.

[louieatienza]

On a commercial r&p setup, there is a felt pinion that acts as an oil applicator, and a wiper or brush for the rack. These things are placed such that they're shielded from dirt as best as possible. Commercial machines also do not use tensioners; rather they rely more on super-accurate setup (the racks are adjusted via eccentric bolts) using a test indicator mounted on the LM guide, using either a spring pinion or a slave motor/pinion to take up backlash. The oil film also reduces backlash in such systems.

Barring such sophisticated systems, you may try graphite dust, or if you're super-patient (or have a kid that needs something to do) pencil lead...

[dgage]

Barring such sophisticated systems, you may try graphite dust, or if you're super-patient (or have a kid that needs something to do) pencil lead...

You'd have to trick my kids into thinking it was an app/game on their phone. And since I bought cheap Windows phones for them, I doubt they'll believe there are any new apps. So graphite dust it is. LOL!

[louieatienza]

You'd have to trick my kids into thinking it was an app/game on their phone. And since I bought cheap Windows phones for them, I doubt they'll believe there are any new apps. So graphite dust it is. LOL!

Well... maybe you can whip up a surface comparator app or such and make it a game to see who had the most coverage on their side. Or, better yet, there is a readily available app - stopwatch!

[SixOfOne]

I will buy the spray. Closest thing I have to child labor is a nephew with the attention span of a gnat, lol.

Maybe I could get him to do it if I told him "NOT" to do it?

[DDgitfiddle]

I've been bouncing between designing/building a CNC system and buying one, but I have pretty much decided that R&P drive is the way to go. Lubrication and dust have been concerns, though. I was considering a few different options:

Dry Teflon (as suggested by SixOfOne and CNCRP) like "WD40 Dirt and Dust Resistance Dry Lube PTFE Spray" (https://www.wd40specialist.com/products/dry-lube/).
I found similar products form Dupont, Blaster, and Finish Line (bicycle lube).

Dry Graphite (as suggested by louieatienza). Blaster and CRC seem to have spray versions that might be easier to apply.

Dry Molybdenum Disulfide. Available from CRC, Rocol, and McLube and others.

Dry Silicone Spray.

Any of these would probably help in this application, but the moly lubes are probably the best performing. Surface pressures on gears can be REALLY high, and would be worst under heavy accelerations in this application. Lube adhesion and resistance to pressure are the critical factors. The moly lubes seem to have higher pressure ratings in general, but the info is not that readily available.

One of my concerns is carry-over to the workpiece. Alot of wood parts are finished with some sort of varnish/paint/polyurethane, and silicone causes alot of trouble with adhesion. So I avoid silicone in my shop. The graphite and moly lubes are black and will make a mess when spraying and will stain clothing. They can probably be sanded off of work pieces pretty easily, though. Teflon is probably the cleanest. I debate whether it would cause finishing problems, but WD40 claims it won't. YMMV.

Another thought is that the CNCRP gear train might be under-designed. I haven't done the calculations, so this is a question, not a statement of fact. Material hardness, gear precision and surface finish all are factors. Alignment is also important. Higher end systems probably use hardened and ground pinions and racks, and better alignment as Louie mentioned. They also probably mount the rack to a machined surface to ensure flatness. All of these factors could probably be improved in the CRCRP machines. At the price point of these machines, it is probably cheaper to spray them with a dry lube once a month and replace the R&P every so often. When I win the lottery, I'll buy a high-precision machining center to cut my wood parts.

[ger21]

I don't think that any gear rack manufacturer would say it's OK to run a rack and pinion without grease.
I just looked at the Shopbot maintenance manual, as they've been selling rack and pinion machines for ~20 years.

Every 40 hours:

Clean rack and pinions with small brush and re-grease.
Add a BB size blob of light grease every 6” and run the tool to its extents in all axes about 10 times.
Wipe off any excess grease and recheck X & Y with push/pull test.