View Full Version : Cutting a "rack" gear

Lew Hartswick
01-17-2007, 07:29 PM
I am building an adaptor to use the hand torch of a plasma cutter with a
motor driven Oxy Acetylene cutting device. The machine is an Airco Radiagraph
#10 . The rack on the 1 3/8 dia. tube is 6" long 0.240 wide and about a 1/4"
high. It has 51 teeth in 5" of length. (~0.098" pitch) . I finally found a source
of tubing the right size, (what the plumbing folks call 1 1/4 Type L Cu pipe).
Now how do I make somethig that will at least work as the rack ? I don't
think it needs to be as good as a real rack gear since the only work it has to do
is move the torch assy up and down, ie. backlash, even fit probably dosen't
have to be great. Machines available : 7 x -- Clausing /Metosa lathes (8),
Bridgeport vertical mills (4) . A limmited selection of endmill cutters.
Any suggestiond as to how to generate an adequate shaped tooth form that
will "work". ?
Things that have been suggested include casting a replica but I' rather not
have to get into another whole field of work. :-) / (hobby)
Thanks for any ideas.

01-17-2007, 07:46 PM
So are you actually wanting to get a rack gear? If so, you can just buy it from McMaster Carr for to much, ~$25 for 2 feet, ~$50 for 6 feet.
Just search for rack gear on mcmaster.com

Boston gear also sells it.

01-17-2007, 08:32 PM
Rack teeth have straight sides, so making a cutter for them would be simpler than making a cutter for teeth on a round gear. Just make the tooth angle the pressure angle of the mating gear (14.5 degrees or 20 degrees or whatever).

But if McMaster-Carr will sell you two feet of rack for 25 bucks, making your own may be a bit pointless, unless you just want to do it for the bragging rights.

Oh -- by my calculations, you're looking for 32 diametral pitch rack. (pi/32 = .098)

Jim Hubbell
01-18-2007, 02:34 AM
I recently improvised a rack by stretching a length of no. 40 roller chain fairly tight. The pinion was a sprocket of suitable size. Due to the configuration of chain/sprocket there is a small ammount of backlash of no concern to me.
A lot cheaper than rack and pinions.
Works for me.

01-18-2007, 03:36 AM
Lew,a post by John Stevenson a while back might give you a bit more insight into building your rack (http://bbs.homeshopmachinist.net/showthread.php?t=20970)

01-18-2007, 08:42 AM
I think I have seen a toothed belt and sprocket used as a rack.

01-18-2007, 01:25 PM
Until about 5 years ago the shop I work in repaired torches for the company. Since then they decided to outsource and or just replace with new. ABOUT TIME! Anyway, I think I know where there is a couple of pieces of the rack you need. Where are you and how long do you need? I am thinking the longest peice we had/have might have laying in a corner somewhere is about 2 foot long. Probably less ,18"? When I go back to work next week I will look around. Can be that expensive to mail.


Lew Hartswick
01-18-2007, 06:55 PM
Thanks for all the ideas. A lot of them won't work due to having to fit the
current machine. So the actual size has to be as described. Marks offer
looks like a great one. ( I've mailed him ) I will have to check out Johns
link. I was trying to figure a way to cut an aproximation good enough to
fit the pinon for rough positioning.
I checked out the right angle drive for the Bridgeports today and it would
require a 4" diameter slitting saw to clear the body in able to cut a 5" length.
That sounds a little "hairy" for a 1/32 thick blade. :-)

Peter S
01-18-2007, 08:52 PM
For simple racks to raise and lower bits on the machines we make, we laser cut both rack and pinion from 5mm? plate, then weld them to their appropriate parts. Usually done in pairs over a long span, so a single crank handle can raise a long frame evenly at both ends.

01-19-2007, 06:55 AM
I think I have seen a toothed belt and sprocket used as a rack.

Yeah GK, old mate of mine built a monster plotter using toothed belt and pulleys. Used to design boats. He sold kits of full-size mylar templates to amateurs and small pro shops.

01-19-2007, 11:18 AM

I am familiar with the rack you referred to. It is 32DP, I believe, and any local welding supply distributor should have this rack as it is standard on oxyfuel machine torches. Should be easily gotten.

Jack C.

Lew Hartswick
01-19-2007, 06:00 PM
I just found an old South Bend 7" shaper, it's going to require a lot of
"clean up" but it runs. I am going to see if I can get it going for the next
time something like this comes up. If Mark comes up with a peice of rack
I'll use that. Second I'll check with the local welding supply for a replacement
part. THEN if all else fails, it's time to learn how to use a shaper. :-)

01-21-2007, 02:35 PM
Cutting the rack with a shaper is pretty easy if you have a piece already. First you grind your tool to match the tooth profile. Easier with shaper tool than mill cutter by far! Second you clamp your original piece in the vice and figure out how far to move the tool for each tooth. The rack you need does not have to be exacting tolerances! It aint rocket science for what you need it to do!