Good Rack Choice?

[DeviousMW]

So I am considering dropping $400 bucks on some
rack from stdsteel.com.

I am planning on buying (2) 4ft and (3) 6ft sections
of 20 PA, 20 Deg pitch, 1/2 face by 1/2 wide steel
gear rack.

The 4ft and two of the 6ft sections would be machined
to matchup by the company to form two 10ft lengths.

All the rack would be drilled and tapped by them with 3/16" holes
1" from each end and spaced evenly across the rack with no
more then 12" between holes. The holes would be
coutersunk so flathead machine screws could be used.

What do you think? Good buy? I want to make the right choices
when building this table, as the wrong ones cost a lot of money

Thanks,
Mike

[Al_The_Man]

It is not completely necessary to machine-match the ends, if the ends are cut just short of the bottom of the trough, a small gap can exist because the gear does not touch the bottom, a short piece is used to straddle two lengths while fixing to keep the pitch identical, I usually use counter-sunk allen head, why the tapped holes by the way, if you are counter-sinking?
Al.

[DeviousMW]

Originally Posted by Al_The_Man It is not completely necessary to machine-match the ends, if the ends are cut just short of the bottom of the trough, a small gap can exist because the gear does not touch the bottom, a short piece is used to straddle two lengths while fixing to keep the pitch identical .

The are only charging me a few bucks to match them up.. might as well take advantage of it. I dont have an decent metal working equipment at home besides a chop saw and welder.

Originally Posted by Al_The_Man I usually use counter-sunk allen head, why the tapped holes by the way, if you are counter-sinking?

Good point.. I dont know what I was thinking.. I'll skip the tapping and just have them drilled and countersunk. That should save a few bucks

Thanks,
Mike

[Torchhead]

Before you bite off on the 20 pitch rack make sure there are off-the-shelf pinion gears to match. My catalogs have lots of choices for 24 pitch but 20 seems to be less plentiful. Maybe I am just looking in the wrong place..........

Tom Caudle
www.CandCNC.com

[Al_The_Man]

Boston Gear carry them with 1/2" face from 12 to 200 teeth.
Al.

[DeviousMW]

Originally Posted by Al_The_Man Boston Gear carry them with 1/2" face from 12 to 200 teeth. Al.

I just called them up.. they recommended a 30 tooth 1/2" face 20 deg pitch 1/2" bore gear based on the torque it needs to handle... supposedly about $20 each from mcmastercarr (a distributor).

This would be for my X axis with a max 100lb gantry.. for the Y axis
I am thinking maybe a smaller gear since it will be driving a much lighter
load despite having the same torque (Got a deal on those 640 oz steppers).

Mike

[Greolt]

If you are going to direct drive the pinion you may want to go smaller. Less teeth.

If you are using a reduction drive then disregard what I said.

[DeviousMW]

I am planning on a 3 to 1 belt drive reduction.

Thanks,
Mike

[jcc3inc]

Sir,

A good source of gear rack is http://www.stdsteel.com/gear.htm
They have in stock 1/2 x 1/2 x 6 ft rack, 20 DP, 20 degree PA for a price of $23.20 each in quantities of (2). The pricing is about a year old.

Earlier comments about fitting rack end-to-end were correct. And as to drilling, make clearance holes for #10 screws and do-it-yourself. If you are building a machine, mounting the rack is easy for you to do.

Regarding pinions, for 20 degree gearing, 18 teeth is the min number to avoid undercutting; for 14 1/2 degree rack the min number is 32 teeth to avoid undercutting. Boston gear lists 20 DP, 20 degree pinions, steel, which should be OK.

If you spring load your drive pinion into the rack, you need to provide sufficient force to overcome the separating force from 20 degree pressure angle; this would be [Drive Force x Tan 20 degrees].
If the drive force is to be 50# max, spring force = 50 x .3639 = 18# spring load.

Regards,
Jack C.

[DeviousMW]

Originally Posted by jcc3inc A good source of gear rack is http://www.stdsteel.com/gear.htm They have in stock 1/2 x 1/2 x 6 ft rack, 20 DP, 20 degree PA for a price of $23.20 each in quantities of (2). The pricing is about a year old.

Yep.. thats the exact rack I am buying.. but the price has jumped quite a bit.
I am paying $80 for this segment with the drilling.

Originally Posted by jcc3inc Earlier comments about fitting rack end-to-end were correct. And as to drilling, make clearance holes for #10 screws and do-it-yourself. If you are building a machine, mounting the rack is easy for you to do.

I'd rather have it drilled at the factory.. my metal equip consists of a
chop saw, a welder, and drill press. I am sure the factory can make much more precise holes vs me.. I will have enough dilling to do when mounting
it to the table.

Originally Posted by jcc3inc Regarding pinions, for 20 degree gearing, 18 teeth is the min number to avoid undercutting; for 14 1/2 degree rack the min number is 32 teeth to avoid undercutting. Boston gear lists 20 DP, 20 degree pinions, steel, which should be OK.

I bought 30 tooth 20 deg pitch pinions. They should work well given the torque.

Originally Posted by jcc3inc If you spring load your drive pinion into the rack, you need to provide sufficient force to overcome the separating force from 20 degree pressure angle; this would be [Drive Force x Tan 20 degrees]. If the drive force is to be 50# max, spring force = 50 x .3639 = 18# spring load.

Thanks for the spring calc.. I do plan on spring loading the pinion to the rack.
How do you caculate the drive force?

Thanks,
Mike