Cutting a new rack with 3/8 circ pitch...

Hi All,

Was hoping for some sage advice from the gear cutting cognoscenti who haunt this forum

My old circa 1900 GA Gray planer has a toothed rack that the vertical feed advance uses. While cleaning, I recently noticed a section of the rack was notched out and replaced with a new section with very roughly cut teeth. It works as is, but up a ways where joined, the teeth don't mesh well. So was wanting to source a piece of gray or ductile iron bar and cut a new replacement rack sometime in the future.

I measured the original teeth on the rack and 8 teeth span exactly 3 inches... so that would make this a 3/8 circular pitch... or pi/CP = 8.3776 diametral pitch... an oddball to find an involute cutter for I suspect? Anyone have one in their involute cutter collection?

So was wondering if any such involute cutters were made for this 3/8 circular pitch? I lack old cutter reference catalogs to search, but I do see 3/8 CP listed in the B&S rack cutting tables.
Planing and milling; a treatise on the use of planers, shapers, ... - Full View | HathiTrust Digital Library | HathiTrust Digital Library

... and similarly in the B&S "Practical Treatise on Milling and Milling Machines" for my No. 2 universal mills.

I'm thinking, worst case I use a "close" cutter ie. 14.5 PA No 1 8DP at less than full depth and come back for a finish pass with a shop made "ground to fit" fly-cutter.

Thoughts and comments?

Thanks again,
Jeff P

I think (??? so please don't shoot) this could be what you're looking for, Ash Gear & Supply Corporation - Rack & Worm Milling Cutter 3/8CP 14.5PA M2 Material

But give Ash a call, they're the gear cutter folk your side of the pond.

PS Just in case you don't know, .....it will not be cheap

I could be completely off-base, but I strongly believe that the rach (infinite teeth gear) has plin triangular teeth with angles equal to the pressure angle.
Therefore, you would just need to make a tool with that angle and you're set.
The only issue I see in using an involute cutter for larger teeth (i.e. smaller DP) is that the root of the tooth is not cut deep enough.

Paolo

Limy and others, Ash also RENTS involute cutters, hobs, gear shaper cutters, etc. It's quite affordable.

Before jumping the gun on possible mistakes in pitch calculation, I would check how Jeff measured the pitch.

Here is the sketch of both what I guess Jeff has measured and what (I believe) John thinks is in reality:



Jeff: what of the two cases is: the top or the bottom?

Paolo

Thanks for asking the first question - How was this measured? .. It's the bottom case as in Paolo's picture. Here's an actual picture so you can see the teeth and the spliced in section as well... Trust me that the tips of the teeth are centered over the 1 and the 4 on the tape measure... the close in camera viewing angle doesn't make that so obvious!

I have repaired straight tooth racks like grinding teeth in a flat broach..Come in with a parting wheel or straight grinding wheel to depth.. then dress the wheel to angles and finish sides..often a shim can be set under the rack and the whole rack can be restored.

Yes having he right mill cutter makes the job easy on the mill.

That looks like a hand hack job from here..but you might blue it up and see how many teeth do touch..weld up 5 or so teeth might make a job easier..but yes not many can not build up CI... think I only knew one welder who could do that Bobby Dodah, an old blacksmith turned welder.

Sounds like I need to talk to the folks at ASH about renting a 3/8 CP 14.5 PA cutter... unless someone comes along with a used one?

You never know what's out there until you ask!

Sourcing gray or ductile iron bar stock?

Any recommendations on where to source a suitable piece of ductile or gray iron bar stock? I live in Idaho so West coast is probably my best shot?

Thanks again,
Jeff P

Speedy Metals 3/4" x 1-1/2" Class 4 Cast Iron Flat

unfortunately their next larger size is far larger than you need 1-1/4" x 2-1/4"

Looks like I need to go get some measurements... Price seems reasonable enough for the 3/4" x 1.5" bar.

I emailed an inquiry to Ash Gear Co on that .375 CP 14.5 PA cutter as well... we'll see where that comes in.

Thanks again,
Jeff P

You can cut the rack with an endmill. First pass is to gash the teeth full depth. Tilt the head equal to the pressure angle and cut one flank. Tilt the head the other way and cut the other flank. Ideally the end mill would have a corner radius. Getting the flank cuts in the right place may take a few trial cuts so you should plan to make the rack a couple teeth longer than needed. Do the trial cuts on the extra teeth and cut them off when finished.

AFAIK, the walls of the teeth on a rack are straight. Tilt head of mill and mill one side, the tilt the other way and mill the other?

Probably not the best choice here, but I've made "racks" out of round, in the lathe, using a simple straight sided tool with the right included angle. Much less contact area than a proper rack, but fast and easy for low load situations like instrumentation.

edit- dang, Illinoyance beat me to it!

Grinding an old rack repair you do much the same.. take out the square cut, then wheel-feel a few of the good teeth with angle dress to get the spacing plus wear.

With a end mill feel a few good teeth to get angle and spacing at the wore in/wore out condition and run the row....

i have milled many racks with a piece of hss or cobalt ground to the profile of a good tooth, its kind of slow compared to a gear cutter or endmill though

For others reading this thread in the future...
I talked with someone at Ash Gear & Supply today about a 3/8" CP 14.5 PA No 1 (rack) involute gear cutter... They have import M2 TiN coated cutter for $99 and US made M4 cutter for $360 IIRC. Minimum rental fee is $80 ...so the push to buy the import.

Another data point for this project...
JP