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Cutting gears

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LJM

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Hi,

Is there a tool, or standard method for cutting teeth onto flat bar to create a flat, straight rack? I’m after low tech! I’m not dealing with high loads or speeds, so precision and accuracy need not be 100%

Ta
 

Cheshirechappie

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There are several methods of cutting racks, varying from marking out with dividers and shaping teeth by filing, using a shaper or planer with a form tool, using a milling machine with appropriate tooling, or specialist gear cutting machinery.

For a one-off not needing close accuracy, I think I'd be tempted by option one - mark out and file. A bit of research into tooth dimensions, and perhaps make a profile gauge, a bit of patience, and job's a good 'un.

There is another option - such things can be bought, for example;

HPC Gears Products - Racks
 

LJM

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Great, that’s all within my initial thoughts on how to go about it. How crucial is matching the profile of teeth on the pinion?

Thanks very much for your input
 

Jelly

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How crucial is matching the profile of teeth on the pinion?
It depends what you're using it for, how much force is being transferred, what kind of speed it's running at, and how much/little backlash you need in the mechanism...

It's hard to say for certain without knowing the application, but as a rule of thumb it works something like this:

Slow Speed
Profile Not Critical
High Speed
Low Forces
↑←
→↓
High Forces
Large Backlash Allowance
Profile Critical
Little/No Backlash Allowance



It's important to know that for any of the "Profile Critical" areas, you don't actually want to match the profile of the teeth on the pinion per se, you want to achieve a matching "involute" so that the pinion teeth will have a constant contact area with the rack as the pinion turns.

So if you're hand filing, it would be easiest to print out a template of the involute tooth of a rack of the appropriate module/DP as laying it out geometrically using dividers etc would be inordinately complicated.



Personally I'd consider if I thought I could buy one for a reasonable price and if that was a no, then go to Milling it using a No. 1 Involute cutter of the appropriate size (or for a rack to long to fit on the mill use a form tool in the slotter)...

If you have access to a Mill, Shaper or Slotter (or a friend who does), I would 100% pursue that option before hand-filing the profile, (there's also potential to mount an involute cutter on an arbour between centers in a lathe with the workpiece on the cross-slide table, which would work but be faffy to set up), as it will be substantially faster, run much nicer when meshed with the pinion, and substantially reduce the wear on the mechanism in use.
 
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LJM

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Thanks Jelly, that sounds comprehensive! I’be now got a great starting point for some reading up.
 

Cheshirechappie

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The tooth profile of a rack for involute gears is straight-sided; rack teeth look like triangles with the pointy top lopped off. That should make filing one a bit simpler. Cycloidal profile, as used in clocks and some small mechanisms - not quite so simple, unfortunately.

Speaking of reading up, a useful source of suitable information without breaking the bank (about £8 or less new, plus p&p) or delving into the mysteries of high precision manufacturing is 'Gears and Gear Cutting' by Ivan Law, one of a series of paperbacks published for model engineers in the 'Workshop Practice' series. Should give the necessary theory, and how to (simply!) calculate the necessary dimensions.

Gears and Gear Cutting: 17 (Workshop Practice): Amazon.co.uk: Law, Ivan R.: 9780852429112: Books
 
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Jelly

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Thanks Jelly, that sounds comprehensive! I’be now got a great starting point for some reading up.
This Info Sheet from Boston Gear is a great starting point...

There's also a link in this thread to the online version of Machinery's Handbook, Pages 2125-2135 (2131-2141 if you're typing the page number into to skip to them) will give you a perhaps overly comprehensive overview of everything you need to know about gears and gear tooth forms.

It also includes all the reference data you'd need to identify your gear, and then lay out a rack which matches it (either on a drawing you photocopy or in CAD to print) as a template.

For most (but not all) gear systems, the involute profile of an ideal rack is wholly straight sided, with those sides set at whatever the pressure angle for that gear tooth is (normally 20 degrees), which will simplify your task substantially if working by hand.



Edit:
The tooth profile of a rack for involute gears is straight-sided; rack teeth look like triangles with the pointy top lopped off. That should make filing one a bit simpler. Cycloidal profile, as used in clocks and some small mechanisms - not quite so simple, unfortunately.
Whilst this is effectively always true, it's worth being aware that there are a couple of older (now depreciated) gear standards which (for reasons I cannot fathom) have rack profiles which aren't straight sided, the "14.5 Degree Composite" form specified by ASA B6.1-1932 is probably the most prevalent of these...
 
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Cheshirechappie

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Whilst this is effectively always true, it's worth being aware that there are a couple of older (now depreciated) gear standards which (for reasons I cannot fathom) have rack profiles which aren't straight sided, the "14.5 Degree Composite" form specified by ASA B6.1-1932 is probably the most prevalent of these...
There are all sorts of complications once you get into high spec, high load, high speed, special equipment - but the OP said he wanted low tech, so I stopped at just past that!
 

LJM

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Thank you both, this is all good food for thought; all I could find on hand cutting gears was particular to watch and clock making; hand working small pieces of brass (and potentially being paid handsomely for it) is a different proposition to working with relatively big bits of steel.
 

Boatfixer

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For an easy to understand explanation of gear teeth This Old Tony is worth a watch -
 

deema

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If it’s a modern pinion, ie MOD then you can buy gear rack easily. Here is one source, but just type gear rack into google

 
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