Rack and Pinion questions..

[adam_m]

Making my first run at building a cnc and I'm going the rack and pinion direction as I believe it will work best for me.

My plans are for a 4 x 4 cutting table and for the following motor and controller.

G540
KL23H2100-35-4B 381 oz motors
KL-350-48 48V/7.3A power supply

Can someone help me understand rack and pinion matching and what pieces of information I need to make an educated decision?

I've been eying CNCROUTERParts.com design (http://www.cncrouterparts.com/produc...products_id=50, http://www.cnczone.com/forums/showthread.php?t=77372) and want to understand how to come up with the gearing required, why the need for a 3:1 belt drive?

I found this site with some handy info. on belts and pulley calculations (http://www.sdp-si.com/Cd/default.htm) does this help or make it more confusing?

What is standard rack or is there such a thing? How do you determine the pitch, PA of the rack required?

Do I need a helical pinion? (http://www.atlantadrives.com/racks.htm)

Sorry for all the questions, just want to try and get a grip on what I need...

Thanks for any input you can give.

VB,

[Aryantes]

I can't answer all of these questions but maybe I can help with some.

CNCRP's system uses a 1" pitch circle.

This means 1 revolution of the motor will move the gantry 1" x 3.14 (pi) = 3.14" per revolution.

This gives you a pretty rough resolution of 0.0157" per step (based on 200 steps on a stepper motor) and will make your steppers run very slow for the most part. It might also give you choppy movement depending on how well microstepping works.

By gearing this down 3:1, you reduce your movement to about 1" per revolution. This will be much smoother as the steppers will be running a bit faster but still maintaining a lot of torque.

Your resolution will be tripled, to 0.00523" per step.

Gearing down more (4:1, 5:1 etc) , would give you more resolution, at the cost of speed, and possibly torque as stepper motors lose torque as RPM goes up. Vice versa for gearing up.

There's no real standard rack.

You can get pinions with x number of teeth and y pitch. You can get racks with the same x teeth and y pitch to match.

The reason why they chose the pitch is probably from personal experience. You want a certain number of pinion teeth to always be in contact with the rack to avoid backlash issues and irregularities in movement. I would think you would want the smallest pitch possible, that works without flaws, for cost and space issues.

Hope that helps, I eye the CNCRouterParts/FineLineAutomation stuff every day. I hope to convert my machine to an R&P later. Probably attempt like a 4'x8' build.

[TSJ]

I cannot explain the reason but the people on the forum that are knowledgeable state to use a 20 degree pressure angle, not 14.5 degree. Take a look at the Standard Steel web suite, they are reasonably priced and were easy to work with. The rack I purchased is 1/2" X 1/2", not very easy to mount but I plan to clamp and weld it to
a 1/2" X 2" piece of steel that can be bolted to the frame.

http://www.stdsteel.com/gear.htm

[adam_m]

Thanks for the reply's.

I think I understand now that your trying to get better resolution without the sacrifice of torque...??? and the gear must match the rack.

Using a 1" pitch circle how does one determine then the number of teeth to use?

If you are trying to achieve a 3:1 ratio does that mean you would use a 3" gear?

TSJ - I too think I might go with stdsteel since it's considerably cheaper. Did you go with #200010 or #200011, how tall is it? what did you use for the pinon gear?

Adam,

[freecncplans]

I will be building a new machine this weekend and I will have detailed information about the rack and pinion setup in a build log. I hope to have everything done and uploaded early next week. This new machine will be using most everything from CNCRouterParts.com

Sean

[TSJ]

I purchased both #200010 or #200011 as my machine will be ten feet long and to get a ball screw the cost is out of sight. I believe the rack is cut from 1/2" X 1/2" steel so it may be a little less than 1/2" on the gear height but not much. I have not purchased the pinion gear yet. I plan to drive the pinion gear with a servo motor attached to a 10:1 gear reducer.

Some math:
motor running at 1500 RPM (motor max is 2000RPM)
10:1 reduction gives 150 RPM
with a 1" diameter pinion gear I would get a travel of 471 inches per minute

Accuracy:
1024 count encoder giving 4096 counts per revolution
1 motor revolution is .314" traveled divided by 4096 is 0.0000767" per count
smallest error count I have found on a controller board is 64 so final resolution is 0.005

If I am making any errors can someone with more knowledge let me know...

Thank you in advance

[adam_m]

Originally Posted by freecncplans I will be building a new machine this weekend and I will have detailed information about the rack and pinion setup in a build log. I hope to have everything done and uploaded early next week. This new machine will be using most everything from CNCRouterParts.com Sean

Since you picked up the rack from stdsteel what is your plan for mounting since it appears that it's only 1/2" wide?

Adam,

[TSJ]

I have purchased a 1/2" X 2" steel rod/rectangle 10' long. I will mill one of the 1/2" sides flat and square to the 2" face, drill holes on the flat face on 2 1/4" centers for mounting, and tack weld the rack along the machined edge. How did I choose 2 1/4" centers for mounting, it is the spacing on the linear rail mounting holes I am using.

I looked for some time to find a rack with a rectangular profile that I could just bolt on. They are out there with mounting holes and everything but were much more expensive.

[freecncplans]

I actually got my racks from Moore Gear in Missouri since they are closer to me and accepted a credit card over the phone. I will have a complete list of what I bought from who and what size I purchased. For the rack, you need a 20 pitch (the teeth sizing) 1/2" by 1/2" by however long you want the machine to move.

Ahren at http://www.cncrouterparts.com/ has mounts for the racks. This will all be detailed in my build log with videos, pictures and all. This system looks confusing, but it really isn't.

Diane Spackman
Sales/Estimating
Moore Gear & Manufacturing Co., Inc.
(573) 486-5415
(573) 486-3487 - Fax
diane@mooregear.com
www.mooregear.com

[ksanalytical]

Plus 1 for Moore Gear and Diane. They did a great job of working out the best options from their product lines over the phone and was even willing to source the pinions from Boston gear and include them with my order.

Ken

[adam_m]

I'll take a look at MooreGear also and contact Diane, thanks.

Back to the question about the pinion and resolution and gear reduction...

Using a 1" pitch circle how does one determine then the number of teeth to use? If you are trying to achieve a 3:1 ratio does that mean you would use a 3" gear?

Anyone using Ahrens (CncrouterParts), gear that can elaborate on his setup?

Adam,

[freecncplans]

Ahren's part comes with the pinion you need already sized and the gear and pinion is a single unit. The pinion is 1 inch diameter and is 20 pitch to match the rack from Moore or STD.

If you look at this link, you will see the part that includes the pinion/gear unit.

I believe the 14.5 teeth are smaller and do not engage as good? I read it one time on Mechmate's website.

Sean