CNC Router Parts Rack and Pinion Development

[ahren]

Hi all,

Many people have been asking about the new rack and pinion system I'm developing, so this thread will be a discussion of the project. Note, this thread will not be used as an advertisement -- just a log for the development of this component. For any updates on pricing or availability of this project commercially, please contact me through my website. Basically, you'll know when it's done when it's for sale.

Alright, with that out of the way, on to the system! I wanted to make something that works with my linear carriage system. Additionally, since a gear reduction is almost mandatory for a rack and pinion system, I decided to make this system compatible with NEMA 23 motors to take advantage of the popular and easy to setup Gecko G540 drive.

Below are some pictures. The first 3 are the CAD concepts for the design. After that are some pictures of the actual prototypes. I've noted some things I want to change already:

1) I need to move the NEMA 23 mounting slots so they don't interfere with the timing belt. I also need to open the NEMA 23 boss mounting slot up a little -- it fits a little tight right now.

2) I need to not drop the plastic and metal gear assembly down the concrete stairs from carrying too much stuff, thereby cracking it and requiring a tedious hour with loctite since I don't want to wait for another timing belt pulley to get here.

3) I need to provide more wrench clearance on the adjustment screw. I'm not even strictly sure this feature is necessary. I'll have to see how easy/hard this is to set up without it, or if it's even feasible. If I have to, I may abandon this whole approach and go to a spring-loaded pinion design, but I'd prefer not to. I'm hoping the t-slot alignment brackets will allow a rigid, backlash-free connection.

4) Longer, simpler alignment brackets for the t-slots. This part works really well, and doesn't really require the set-screw holes I added (you can see these in the CAD -- I don't have a picture of the part in here). Just a little shim between the rack and the bracket, and the rack is held in tight. By loosing this feature, I can simplify the part and its mounting to the extrusion, and afford to make it a little longer.

Anyway, pics below. I welcome your comments, and promise more pictures of the rack, rail, and extrusion soon!

Ahren
www.cncrouterparts.com

[nlancaster]

Hi all,
1) I need to move the NEMA 23 mounting slots so they don't interfere with the timing belt. I also need to open the NEMA 23 boss mounting slot up a little -- it fits a little tight right now.

4) Longer, simpler alignment brackets for the t-slots. This part works really well, and doesn't really require the set-screw holes I added (you can see these in the CAD -- I don't have a picture of the part in here). Just a little shim between the rack and the bracket, and the rack is held in tight. By loosing this feature, I can simplify the part and its mounting to the extrusion, and afford to make it a little longer.

Possible solution: Rotate the Motor 45 degrees around the shaft. That will get the mounting hols out of the way. But may be an issue for the slot getting to close to the shaft opening.

Be carefull not to forget about the "2 degree drop lock" on 8020 tslot. As talked about in the lower video here. That could result in your rack and pinion being out of alignment by 2 degrees.

But otherwise looks like a good compact setup. I have looked at your products and they have been intriuging but I am currently building my first Router out of MDF and using V-rollers.

Keep up the good work.

[rrdiaz30]

Nice work Ahren! How about slotting the plate for the nuts like the examples below?

Rick

[groomden]

Have you considered having the rack on top of the 8020 and using the weight of the gantry to force the gear/rack connection? Why fight gravity.... I could picture an 2" wide 1/4-2 steel rail with a gear rack running parallel on a piece of 1530, and vav-voom, you should have a solid connection to the rack?

[jcc3inc]

Sir,

How will you account for the less-than-perfect alignment between your rail system and the gear rack? We always used a pivoting arrangement and spring loading to accommodate the variations. Thus backlash was always kept to zero. For this type application, the pivot centerline needs to fall on the rack pitchline.

And what pressure angle gearing will you use?

Regards,
Jack C.

[ahren]

Thanks for the feedback all. A couple of notes:

1) I've considered slots for retaining the nuts. I may still do it, but in order to make the parts usable on both sides of the gantry, I'd have to make the slots on both sides, which is another machining setup (adds $). For now, I may just rotate things 45 degrees so all of the nuts are accessible.

2) Weight of the gantry for rack engagement. This is an interesting idea, but I'm hesitant to rely on it for a couple of reasons. First, the rack teeth would be pointing up, so dust and swarf would not naturally fall away. Second, I would worry about "light" gantries that would skip out -- I don't think most gantries are going to be 100 lbs+, and this is about the preload that I'm targeting.

3) Jack, I'm aware of this approach, and may end up going this way. My goal was to see if with some clever t-slot mounted parts, I could make the rack easy enough to adjust that the misalignment could be taken out, rather than compensated for.

Currently using 20 degree PA gears and rack.

Best regards,

Ahren
www.cncrouterparts.com

[BobF]

Ahren,
When I first saw your carriages, I always pictured in my mind using the rail mounted veritcal. I see in your pictures using it horizontal, and I have seen a few builds this way. Is there a reason not to mount it vertical?
It looks like using the 8020 under it gives plenty of stiffness, just not the way I pictured it.

[ahren]

Hey Bob,

Vertical is also feasible -- it just costs more for the wider rail, and makes getting even spacing on your extrusion very important, as the only way to adjust the rails horizontally is with shims (rather than oversized holes with the horizontal rails). Otherwise, it's fine, and as you've mentioned, others have done this successfully.

Ahren
www.cncrouterparts.com

[oldhack]

Hey Bob,

Vertical is also feasible -- it just costs more for the wider rail, and makes getting even spacing on your extrusion very important, as the only way to adjust the rails horizontally is with shims (rather than oversized holes with the horizontal rails). Otherwise, it's fine, and as you've mentioned, others have done this successfully.

Ahren
www.cncrouterparts.com

Hey ahren
Im building a 4x8 table using Joes hybrid 4x4 for a model to build from. Great plans for a dummy like me.(grin)
Id like to use your your R&P design as im at that stage already. the horizontal flat stock looks like a good way to do it. (My table is 2x2 steel tube instead of 8020 )are you going to just use one carriage per side or double it up for rigidity?
Id like to put my flat stock on the table and get it ready. Do you have an idea on the specs? like how far out should the flat stock hang over the table etc...
Looking forward to using this. Ill modify my gantry parts to acomodate your R&P unit.
Isnt this fun? (what would i do without my "free" weekends?)

[ahren]

oldhack,

The flat stock should ideally be 1/4" thick, and should hang off the steel between 1/2" and 5/8" for this system. This may not be ready for a little while though. I'd hate to hold up your build.

Ahren
www.cncrouterparts.com

[NEATman]

Ahren-
Are there any suppliers of rack out there that make a 1/4" thick rack that is "tall" (perhaps 1.5") so that the gear teeth could be on one side of the rack and the bearings could be on the other side? The rack/bearing plate could then be mounted with a row of holes down the middle to the 8020. Also, as the rack was manufactured it should have been referenced off the straight edge where the bearings will ride, the adjustment should be minimal. Plus, the preloading of the gear into the rack would preload the bearings onto the opposite side.

Keith
NEATman

[oldhack]

oldhack,

The flat stock should ideally be 1/4" thick, and should hang off the steel between 1/2" and 5/8" for this system. This may not be ready for a little while though. I'd hate to hold up your build.

Ahren
www.cncrouterparts.com

no problem. if youre not ready then ill "get er done" some other way. Im sure it will be pretty cool when its done though.