[Robin Hewitt]

Working on making a y axis gantry. I removed the stepper so I could move the plasma head across by hand and found it's quite noisy...

Noisy movie (640k)

Are your racks noisy like mine? Perhaps my pinion is a bit small. Do you lube them or are you worried the black dust will convert grease to grinding paste?

[Greolt]

I reckon you're moving at about 1m per second there Robin. That's 60m per min.

I don't imagine it will be going that fast when working.

The steppers or servos will make more noise than the rack.

My router can rapid at about 15m per min and I can't hear any rack noise above the steppers.

Regarding lubrication. I have a router so it's a bit different but I run mine dry.

The shopbot forum which has bought together a lot of experience with R&P (on routers) seems to be evenly devided
between those who see lube as essential and those who don't use any.

White lithium spray is often the lube of choice.

IMO on a plasma it would be better left dry.

Others will no doubt have varying opinions.

Greg

[jcc3inc]

Robin,

From the sound of your application, you must have quite fine rack and a small pinion. We used 24 dp, 20 degree PA rack and never had a rack noise problem. Our typical pinion was ~30 teeth to have more teeth in contact at a time. Also, if you use 14-1/2 degree PA rack, you need at least 32 tooth pinions in order to avoid undercutting; 20 degree requires 18 T minimum.

Regards,
Jack C.

[Robin Hewitt]

It's a MOD 1 (25.4 DP) 20 degree rack with a 12 tooth modified pinion having a PCD of 13 (you'd expect 12, not hugely clear on that bit)

The pinion is sprung in to the rack with about 5 lbf. If I ease that it's practically silent, but then I risk drag from the plasma cable bending everything out of alignment.

[jcc3inc]

Robin,

Since you have less than 18 teeth, one could likely expect noisiness from the undercutting of the teeth. The 20 degree is the better choice for pressure angle.

Jack C.

[HuFlungDung]

Pinions are meant to run on a fixed center distance relative to the rack. Gears are always cut with clearance to some extent, so if you force the pinion down into the rack, its going to defeat the qualities of the involute curves. The pinion will actually jump up and down and there will be accel/decel to each tooth engagement which creates noise.

The best way to reduce backlash on a rack and pinion is to use two gears, one slightly out of phase with the other so as to reduce the backlash amount. The two gears can be keyed together or however you like. But, the center distance of the pinion above the rack should be fixed and correct as per the manufactured dimensions.

[Greolt]

Robin

What HuFlungDung is saying is perfectly correct but don't get spooked.

There are many many machines running spring loaded pinions including many big dollar commercial machines.

I recently attended the major trade show we have in Australia. I specificaly went around every router and plasma
cutter on show and looked at their X and Y axis drives and all but one machine was running rack and spring loaded pinions.

Now some of them may be using much more expensive rack but my point is that it will be very adequate for your aplication.

However you need to go for a larger pinion. 20 teeth min.

My router uses the same module rack that you have, and I run 24 tooth pinions

Greg

[Robin Hewitt]

I have a roller sprung against the back of the rack which keeps it tight, maybe too tight.

Today I added a screw adjustor to back the roller off, still allowing the spring to ride up over any debris that might get in the rack teeth.

If I back it off a tiny amount it gets a lot easier to move, in fact it rolls under it's own weight with <10 degrees of slope, but the noise persists.

There is no possibility of fitting a larger pinion without a radical rebuild so I think I just have to live with it