OT - Building a jig and need to size a Gear Rack

[SoCalHobbyist]

Hello everyone, first post.

I'm building a jig for jeweling metal and need a gear rack to work as a linear indexer. I will run a linear guide in front of the rack and set up an arm on the pillow block that will rest at the top of the teeth via spring loaded bearing. I need a gear rack that is 5/8" (.625) between the top left of each tooth. Not the Circular pitch, which I understand is taken from the point between the Addendum and the Dedendum. I believe the rack I'm seeking uses a 25 degree pressure angle. The rack should have an overall width and height of 1" and a length of 6'.

I've tried specifying a rack on a few manufacturer sites but this measurement seems to be outside the normal ones since obviously these racks are designed to work with gears, not as linear indexes.

Thanks in advance for your help.

 

[johnoder]

Racks are available commercially in Diametral Pitch and Metric Module - so you can choose from among those variations, or make / have made one that suits your custom requirements

Nearest to .625" pitch will be 5 DP at .6283185"

 

[3t3d]

Are you familiar with these devices?

Nexen Group

 

[SoCalHobbyist]

Thanks John. Wouldn't the pressure angle effect the measurement at the top of the tooth? So the top left corner of the teeth of a 14 degree pressure angle would be larger than the same DP with a PA of 20 degrees, correct?

 

[SoCalHobbyist]

3t3d, I'd like a 1" wide rack over the roller pinion rack, but thank you for the suggestion! Also, I'm looking for a single 6' length..

 

[johnoder]

Wrong. Feature to same feature next tooth will be circular pitch away. Only way to not have this is to have random PAs tooth to tooth, or other random dimensions, such as depth.

Thanks John. Wouldn't the pressure angle effect the measurement at the top of the tooth? So the top left corner of the teeth of a 14 degree pressure angle would be larger than the same DP with a PA of 20 degrees, correct?

 

[Finegrain]

You'll have to get someone to make what you need if you want precise 5/8" spacing.

Regards.

Mike

 

[Mud]

Thanks John. Wouldn't the pressure angle effect the measurement at the top of the tooth? So the top left corner of the teeth of a 14 degree pressure angle would be larger than the same DP with a PA of 20 degrees, correct?

No, you're thinking of the difference between the bottom corner and the top corner

 

[SoCalHobbyist]

OK I'm clear now that the Circular pitch is what I should have been relying on all along. And the difference between .625" and .6283185 is only 1/128th or so, right? For what I am doing that will work.

I really appreciate the help guys. I'm going to spec a 5DP rack. The DP was the number I was missing.

Do you guys know where to find these used?

 

[Lumberjack]

Pardon for asking but whats the reason for having it one inch wide? that is one hefty rack. Is it some very heavy application?
I´m thinking otherwise maybe you could water or laser cut one or two sheets (2-5 mm depending on your loading) and if needed attach them to a square bar?
Then it wont be so darn xpensiv to change if you like to do some intermediate pitch either.

 

[Heavey Metal]

#50 roller chain is 5/8 pitch.

 

[SoCalHobbyist]

Pardon for asking but whats the reason for having it one inch wide? that is one hefty rack. Is it some very heavy application?
I´m thinking otherwise maybe you could water or laser cut one or two sheets (2-5 mm depending on your loading) and if needed attach them to a square bar?
Then it wont be so darn xpensiv to change if you like to do some intermediate pitch either.

This is true and I am open to suggestions. It didn't appear that I could get a rack thinner, although going back and looking at the link from 3t3d it looks like that could work. I'm going through their site now to try and spec one of those.

 

[johnoder]

.625" and .6283185 is only 1/128th or so, right?

1 / 128th is .0078125" more than twice the difference

 

[Gary E]

#50 roller chain is 5/8 pitch.

Heavey beat me to it.....
use what ever pitch you like... and chain is a lot less cost than a rack.

 

[emcmike]

I did this years ago making VW Bug dashes. The gear on the rack was fitted with a dog ratchet so it would only move the distance the release was set at and catch at the desired spot, very fast and simple. About 20min. Per dash. X and Y racks.

 

[Robert Campbell Jr.]

#50 roller chain is 5/8 pitch.

Chain in tension could be as solid as a rack under forces generated by jewling and be considerably less expensive than a 1" wide 5DP rack, not to mention that it's an easy way to get your desired 5/8" increments, exactly.

In addition, the chain rollers are 0.4" in diameter. 0.625 - 0.4 = 0.225, the width of a rectangular peg that will fill the gap between the rollers so that ramping would not be a factor. That's just the gaging apparatus, I'd add some form of simple pneumatic clamping between moves.

Speaking of pneumatics, if production warrented it, mechanically automate the process, clamping, repositioning the work and driving the tool quill vertically with air pressure, that has the potential to improve uniformity.

The cool thing here is that no heroics would be needed for accurate advancement, well confined pegs between the rollers would complete the accuracy of repositioning. They could even be on a pivoting arm that registers the pegs exactly perpendicular to the chain on closing and include the advancement op. Slightly radiused shoulders on each side of the peg could uniformly remove any clearances in the roller/pin assembly.

And unless you intend only a single row of engine turning, ("jeweling") heed emcmike's X and Y note, but you knew that and now that your using chain, it's affordable.

Bob, distant nephew of Rube Goldberg

 

[emcmike]

My Jewler was 60in x 18in , that was what I had avalible in flat stock ( like 8020 rail ), used in hobby cnc machines. the micro switches were double type (surplus center-1.83ea). Threaded rod 3/8's fitted with switch cam (controls spaceing exact), driven with pittman 20-1 ratio gear motor. timers are ELK boards , very accurate. the up /down on the spindle (motor /spindle one pice) was from the spooler section of an old deep sea fishing reel , so never have to reverse the up/down travle on the motor (the spooler goes back and forth in the real) . the micro switch opperates the spooler in conjunction with the timer board. every thing time properly it runs by its self till turned off. Basicly it's a CNC machine run on micro switches and 2 timer boards , no computer required or programs. Lost it the fire back in 92


Forgot to add at the end of the run is a DTDP switch on each end that reverses the drive / spacing cam connected to the 3/8's threaded rod , same on X &Y axis , just the cam you make may be different for design you desire .




if the image loaded , see the fishing line guide spooler, this is the secret of how this machine works without all the hassel of reversing up down spindle motor. A microswitch at top and bottom of the spooler travel operates the timer board , the timer counts down and starts the feed motor to the down switch and the cycles starts agan ........ no reversing switch .... the line spooler shuttles runs back and forth.

 

[emcmike]

Guess he disapired ....poof.........lol

 

[SoCalHobbyist]

Nope! Actually, I've been looking at the chain solution because I already have a chain at home (#50 motorcycle chain). I'm working on how to secure it to a straight edge, and then I think it might work. I assume I can use tags so when I get something mocked up I'll let you know.

I did miss the responses over the weekend, and just caught up on those. Thanks for all the input so far guys!

:)

PS here's a shot of what I created with a jig like the one I'm building.
[ATTACH=CONFIG]107034._xfImport[/ATTACH]

 

[emcmike]

Not to be a nay nay , lol I went through all that and found it would only do one scale pattern and changing was a pain in the neck . With the screw all you do is change the cams , sooooo much easier. And it's a self runner no fiddle around.