unusual rack and pinion size of clausing colchester 11" bantam

lowCountryCamo · May 17, 2020

So I am repairing a rack on a clausing Colchester 11". This was a school lathe and it was crashed. Rack has 4 teeth missing. I measured the rack on the comparator and the total depth is .190 and .224 pitch. The pinion is a 12t and measures 1.085. I calculated the DP n+2/diameter give 12.9DP. I have some small knowledge of gears only making a dozen or so over the years. This is not a metric lathe but could this be metric numbers? Could someone shed some light of this subject.

Thanks,

Steve Austin

PDW · May 17, 2020

lowCountryCamo said:
So I am repairing a rack on a clausing Colchester 11". This was a school lathe and it was crashed. Rack has 4 teeth missing. I measured the rack on the comparator and the total depth is .190 and .224 pitch. The pinion is a 12t and measures 1.085. I calculated the DP n+2/diameter give 12.9DP. I have some small knowledge of gears only making a dozen or so over the years. This is not a metric lathe but could this be metric numbers? Could someone shed some light of this subject.

Thanks,

Steve Austin

I'm pretty sure it's the same as a Chipmaster. The rack should be 14DP 14.5 PA and the pinion is oddball but essentially 14DP IIRC. It's been many years since I made a replacement set for mine and I made both the rack and pinion. I recall having to shim the rack down to get proper tooth engagement with the pinion which was a PITA and likely due to a mis-measurement on my part as the old parts were totalled.

Anyway the rack was definitely 14DP. I bought a set of gear cutters to do it myself as this pitch rack off the shelf was unobtanium.

PDW

Dan from Oakland · May 17, 2020

Pinions with low numbers of teeth are often cut on an enlarged operating pitch diameter in order to avoid excessive undercutting. These types of gears are referred to as long addendum. The number of teeth and outside diameter formula to determine the DP only works for gears cut on a standard pitch diameter where the addendum is 1/DP.

john.k · May 17, 2020

Break rack and pinion by using bed stops and expecting feed kickout to work......bang ,crunch ,clunk..........I might add ,every gear Colchester ever made is non standard ,may look like 10DP or whatever ,but dimensions wont fit any formula......The geared head of the Bantam is full of gears that look like 10DP ,but they aint.

PDW · May 17, 2020

john.k said:
Break rack and pinion by using bed stops and expecting feed kickout to work......bang ,crunch ,clunk..........

I use the bed stop all the time, but NEVER power-feed into it. I've made one set of replacement rack/pinion parts, no desire to do it a second time. The pinion wasn't difficult but the rack was a PITA.

Frankly if I was doing it again I'd swap them to 16DP and adjust the rack to get the engagement I wanted with the pinion. I did the calcs at the time and IIRC I'd have had to go to a 14T pinion which wasn't too bad. It was going to make a minor difference to the feed per rev but that's only a noise level thing.

PDW

bmw77 · Aug 25, 2020

PDW said:
I'm pretty sure it's the same as a Chipmaster. The rack should be 14DP 14.5 PA and the pinion is oddball but essentially 14DP IIRC. It's been many years since I made a replacement set for mine and I made both the rack and pinion. I recall having to shim the rack down to get proper tooth engagement with the pinion which was a PITA and likely due to a mis-measurement on my part as the old parts were totalled.

Anyway the rack was definitely 14DP. I bought a set of gear cutters to do it myself as this pitch rack off the shelf was unobtanium.

PDW

This is very useful info .. thank you.. I have A colchester Mk2 Bantam 2000 which has a very worn out Rack Pinion ( which i have managed to source a new original for) and the rack is badly damaged / worn .. in fact looking at it i think it has has the incorrect rack fitted for the pinion at some time in its life.. hence the wear to both parts.

I too am having difficulty finding a 14DP rack .. but just wanted to check that you found the original rack to be 14 DP with a PA 14.5. I cant trust the rack thats on the machine to measure from as i am sure its the wrong size and seems to have straight cut teeth with no PA.

Any help any one can offer on confirming the DP and PA of the rack for this machine would be very much apriciated.

PDW · Aug 25, 2020

bmw77 said:
This is very useful info .. thank you.. I have A colchester Mk2 Bantam 2000 which has a very worn out Rack Pinion ( which i have managed to source a new original for) and the rack is badly damaged / worn .. in fact looking at it i think it has has the incorrect rack fitted for the pinion at some time in its life.. hence the wear to both parts.

I too am having difficulty finding a 14DP rack .. but just wanted to check that you found the original rack to be 14 DP with a PA 14.5. I cant trust the rack thats on the machine to measure from as i am sure its the wrong size and seems to have straight cut teeth with no PA.

Any help any one can offer on confirming the DP and PA of the rack for this machine would be very much apriciated.

Yeah it was definitely 14DP 14.5deg PA. I spent a lot of time cursing that fact.

The pinion was pretty easy to make (provided you've a mill, dividing head and proper gear cutters that is). The rack for a Chipmaster was a PITA as its cross section is an asymmetric pentagon not a nice square or rectangle. I hope for your sake that the Bantam is different.

I used a shaper with a temporarily rigged single axis DRO to cut the rack teeth. Slow. Very slow. Worked though.

Frankly if I had to do it again I'd cut it at 16DP and get over it.

PDW

ratbldr427 · Aug 25, 2020

The last time I contemplated cutting a rack with the equipment I have I determined I needed a much larger diameter gear cutter than standard.Low and behold they make just such a cutter.Chinese of course and reasonable.They would allow me to use the ra head on the Webb mill with the necessary clearance.I believe they were made for a type of rack cutting machine.
I ended up finding some 2mod and 2.5mod racks really cheap, just had to saw the width to match the oe parts.
I even used the end pieces that were left over to repair some racks that had only 5-6 stripped teeth by milling out a section and inserting the new tooth section in and welding.
Worked good and something you might consider if you can find the correct rack cheap enough.

1953chevB · Aug 25, 2020

lowCountryCamo said:
So I am repairing a rack on a clausing Colchester 11". This was a school lathe and it was crashed. Rack has 4 teeth missing. I measured the rack on the comparator and the total depth is .190 and .224 pitch. The pinion is a 12t and measures 1.085. I calculated the DP n+2/diameter give 12.9DP. I have some small knowledge of gears only making a dozen or so over the years. This is not a metric lathe but could this be metric numbers? Could someone shed some light of this subject.

Thanks,

Steve Austin

do you want to keep the same set or do you want to change it to a different diametral pitch?
yes agree about the pinion could have been profile shift (enlarged) to prevent under cutting at the root

if you want the same setup measure the angle of the rack tooth space, one side only it is straight and not an involute.
base pitch = circular pitch x cos Pressure Angle
circular pitch = (Pi/DP)/2 there fore (2*Pi)/2 =DP diametral pitch

watch this video for the necessary info on cutting a gear, about Measurement over wires, and the back lash required.
027 DIVIDING HEAD & GEAR CUTTING, MILLING 101 MARC LECUYER - YouTube

google base pitch tables by William Janninick for the spur, for the unknown gear.

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unusual rack and pinion size of clausing colchester 11" bantam

lowCountryCamo

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PDW

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Dan from Oakland

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john.k

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bmw77

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ratbldr427

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1953chevB

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