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Change gears - pressure angle

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RWL

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#1
A friend bought a 10" Rockwell lathe but discovered that two of the change gears, a 16 tooth and it's mating 32 tooth gear were completely stripped. A previous owner must have had a heck of a crash. My gage says these are 20DP gears. I've got the equipment to cut gears and even have a 20DP cutter that will cut the 16 tooth gear. We'll have to buy the involute cutter for the 32 tooth gear. My cutter is unmarked, and since gears with a 20º pressure angle are more common than those with a 14.5º pressure angle, I ASSUME my cutter has a 20º pressure angle. I've read that some older lathes (not necessarily Rockwell) have change gears with a 14.5º pressure angle. Does anybody know what the pressure angle is of the change gears in a 10" Rockwell lathe? How do you tell the difference between gears with a 20º pressure angle and those with a 14.5º pressure angle?
 

RWL

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#2
I determined that the gears have a 20º pressure angle by two methods. Someone on the net pointed out to me that a gear with only 16 teeth and a 14.5º pressure angle would show undercutting of the teeth. The small gears don't show undercutting, so that suggests these are 20º PA gears. Second, I measured the base circle. The calculated base circle of 20DP gear with a 14.5º pressure angle is .152" The calculated base circle of 20º pressure angle is .148. I then measured the distance between a group of three teeth and then four teeth to get the difference, which is the base circle - the distance of one more tooth. The stripped worn gear gave a very low number that didn't come close to either calculated base circle. I checked an unworn change gear and the difference between three and four teeth was .148" which matches a 20º pressure angle. Now that we know the PA we know what 20DP involute cutters we need. My friend and I talked about cutting some extra gears since these apparently are not available. We'll see. I may play with a third method of determining the pressure angle by rolling a gear in clay to make a rack and measuring the angle of the created rack teeth, which I'm told matches the pressure angle. I uploaded two photos of measuring the gear teeth, but they're not showing up in the preview.
 

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RWL

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#3
Well, I stand corrected. The change gears in the 10" Rockwell have a 14.5º pressure angle. I placed an inquiry on another board about what pressure angle an unmarked 20DP gear cutter was and learned that it's unreliable to measure the pressure angle just using calipers. Those more experienced with making gears said you could tell just by looking at them that they're 14.5 degree gears. In particular the root of the gears is flat, in contrast to the more rounded root of 20 degree gears. We did a trial gear in aluminum the other day and it went well. We're going to make 414-02-351-5005 and 414-02-051-5034. I still need to get a cutter for the 32 tooth gear. Below is a photo from another site illustrating the difference between 14.5 and 20 degree gears.
 

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brino

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#4
Hi RWL,

Years ago I picked up this gear tooth gauge from Boston Gear:
1.jpg

Here's the 20DP leaf, showing the two pressure angles:
2.jpg

I tried to get a hi-res close-up just in case you needed it.
As for scale it's actually 0.980" measured across the teeth.

-brino
 

RWL

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#6
Thanks for the photo brino. My gage is a simple circular one. I can identify a limited range of diametrical pitches with that but not pressure angle. The ones like you illustrate are pretty expensive. If I were doing more gear work, that's what I'd get. I was surprised by the prices those things were actually selling for on ebay.
 

Nogoingback

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#7
IF you can get your hands on a Boston Gear catalog, they had scale drawings that illustrate the differences: very useful.
 

RWL

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#8
Comparing tooth profiles to determine the pressure angle is not as easy as it sounds. I think the teeth of the known gear fit better in the 14.5º pressure angle profile than they do in the 20º pressure angle profile. The profiles are from the Boston Gear catalog.
28d 32 tooth gear matched to 14.5 degree tooth profile (Large).JPG 28e  32 tooth gear matched to 20 degree tooth profile (Large).JPG 28c Rockwell duplex gear w PA outlines overlaid - cropped (Large).jpg
 

Reeltor

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#9
Thank you for the photo contrasting the difference of the gear tooth roots between the 14 1/2 and 20 degree pressure angles.
I bought a gear tooth gauge on eBay, marked Boston Gear but the set I received is 14 1/2 only.

RWL what cutter are you using to cut your gears; standard cutting wheels for a horizontal mill or are you cutting them using a vertical mill. If you are using a cutter for a vertical mill can you post a close-up photo of the cutter?
 

RWL

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#10
Standard involute gear cutter in a Bridgeport on a 7/8" arbor. For those not familiar with gear cutter nomenclature, this one is No.7 (in a series of 8) with 20 Diametrical Pitch (abbreviated as P here and as DP on other gear cutters) and it cuts gears with 14 to 16 teeth. The full depth of cut is 0.108". The pressure angle is not marked on this cutter. We had to cut a gear with the cutter to determine what pressure angle the cutter was. We got lucky and this one was the correct 14.5º which is what we needed. In my collection I just happened to have this single cutter, acquired some time over the last 30 or so years from an unknown source. The arbor holding the gear blank is unusually long because there is a bad bearing in the dividing head causing runout. By using a long arbor and keeping the cutting near the tailstock, we've reduced the runout of the arbor. I'm still shopping the used market for the #4 cutter in the 20DP series to do the 32 tooth gear my friend needs.
01 Front of cutter (Large).JPG 07 The first cut in aluminum blank  (Large).JPG 08 Gear almost done (Large).JPG
 

Reeltor

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#11
Standard involute gear cutter in a Bridgeport on a 7/8" arbor. For those not familiar with gear cutter nomenclature, this one is No.7 (in a series of 8) with 20 Diametrical Pitch (abbreviated as P here and as DP on other gear cutters) and it cuts gears with 14 to 16 teeth. The full depth of cut is 0.108". The pressure angle is not marked on this cutter. We had to cut a gear with the cutter to determine what pressure angle the cutter was. We got lucky and this one was the correct 14.5º which is what we needed. In my collection I just happened to have this single cutter, acquired some time over the last 30 or so years from an unknown source. The arbor holding the gear blank is unusually long because there is a bad bearing in the dividing head causing runout. By using a long arbor and keeping the cutting near the tailstock, we've reduced the runout of the arbor. I'm still shopping the used market for the #4 cutter in the 20DP series to do the 32 tooth gear my friend needs.
View attachment 227603 View attachment 227604 View attachment 227605

Thank you for the photo, from the shot that you had of the gears sitting on your mill it looked like some type of endmill was being used. Do you have any issues using the (milling) arbor with the unsupported end? When I cut some 4DP gears on the horizontal mill, I had to do everything I could to increase rigidity. On the 4dp cutters you are going just over 1/2" depth of cut.
 

RWL

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#12
The cutter is mounted on a stub arbor held by 3/4" end mill holder. If the cutter had had a bor of 1" instead of 7/8" I could have mounted it on my 1" R8 stub milling arbor with less stick out. With the 20DP cutter I've had no vibrations with this setup. It was a rather smooth cut. When we go to cut the 32 tooth gear, which is larger in diameter, we're wondering if we'll get some vibration from the arbor holding the gear blank. Time will tell.
 

lowpass5

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#13
Nice to see you are able to make what you need to repair the lathe. When I needed to replace the transfer gear on my Rockwell 11", I bought a stock unhardened gear off ebay and machined it to size.

I too had a couple of stripped gears in the QCGB and was able to find used replacements through the Rockwell Lathe Yahoo groups. Would recommend joining the group and posting your parts needs. There are a fair number of knowledgeable folks on the forum that can help you with the machine.

The Rockwells are great lathes.
 

RWL

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#14
Within the past day or two, the gear that my friend needed showed up on ebay. A 20 DP #4 gear cutter also appeared. My friend bought the gear cutter and we anticipate making some of the 32 tooth gears next. We'll make an extra or two of the 32 tooth combined gear in the hopes of recovering our expenses.
 
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