Changing the rack on a lathe- New ratio?

Impact-vector · Jun 13, 2016

I currently have a lathe with a metric rack and finding an 11 tooth pinion is proving to be hard!

If I changed the rack and pinion to a new mesh ratio- besides my dials being off- would there be any other issues?

Current pinion is shot and the rack has some bad teeth.

Thanks
~J

johnoder · Jun 13, 2016

finding

Pinion is part of apron gearing - not something you "find" - like it was "stock" gearing

A normal approach is to figure out what you have and make a replacement - like post #208 and #209 here

http://www.practicalmachinist.com/v...bs-lathe-277845/index11.html?highlight=moving

Conrad Hoffman · Jun 13, 2016

If the PD of the gear is the same, the ratio will be the same. To change it you'd have to move the rack to mesh with a new gear size. Seems like it would be better to keep everything the same, maybe make a new pinion and flip the rack end for end?

stephen thomas · Jun 13, 2016

To answer the question:

It might confuse a later owner

But functionally, it should not matter if you keep things somewhat close to what the dataplate on the gearbox represents for feeds.

Even if making the gear complete, it may be easier for many of us to use inch type cutters that we have on hand, and stock rack. But it would not be out of the possible to find a common spur gear wide enough and near enough PD to shrink or key to the shaft or mating gear in the apron. The rack, of course, can be planed or shimmed as necessary to get correct running clearance. Though this may be a compromise if the ways are well worn....

If you can get the correct metric cutter (or make one as a form tool to run on the shaper), it would probably make best economic sense & faster to do as others have mentioned: make a new gear and reverse the existing rack.

smt

tnmgcarbide · Jun 13, 2016

what size machine is this ? is this of dubious mfg. that is unmentionable here?
i know there's no parts list available for such a machine , should it exist ...
perhaps you could source these parts from a similar machine .

anyway, if you can't fix what you have , make something that will work .
how did the rack and pinion fail? never heard of this , unless the rest of the
machine was worn out and shot .

Impact-vector · Jun 13, 2016

The machine is a Storebro Bruk. Its Swedish but Ikea had no parts!

The lathe fell forward so its hard to tell if there was another issue. Some of the teeth are mangled but I could flip the rack if needed.

The pinion is the hardest part to find. Its 11 teeth and I dont have a vertical mill to cut them. Even finding the pitch and diameter seems hard.

I would love to keep the ratios etc but so far its a bit of a challange!

Thanks again for the help guys!
Any help identifying the pitch on the rack would be great. I think 4 TPI but couldnt tell and since its metric.. who knows!

stephen thomas · Jun 13, 2016

WAG: Is the OD about 26mm? Should be Mod 2. But you will probably have to make that, they stock seems to start at 12T.

Try Maryland Metrics.

smt

gadget73 · Jun 13, 2016

Is that gear pressed onto the shaft? Does it use the whole tooth? Sure looks like its worn only on the inner side. Maybe if it doesn't actually contact the whole tooth width you could flip the gear over and run on the un-damaged part.

Forrest Addy · Jun 13, 2016

The rack looks like its 1/4" circular pitch. Circular pitch rack and gearing is scarce; you'll have to make it. There's sure to be suitable diametral or Metric Module rack and pinion. Get out your calculator, work the math and beat the bushes for a supplier. Once you find a suitable rack and pinion, turn off the existing integral pinion ,cut a keyseat, and mount a replacement pinion.

So long as the backlash is acceptable you should have no problem other than dial readings and feed per revolution being off by some amount.

Since its a rack, the error will not be a ratio but a proportional translation. So long as the pinion pitch diameter is roughly the same the feed error per rev should be negligible. The dial error is another matter but a puck magnet on the back of a travel indicator used as a carriage stop will quantify critical movements within its range.

tnmgcarbide · Jun 13, 2016

ok, now i get it . not a chinese monster , but the forklift accident lathe
on the atlanta craigslist .the equipment hub.

L Vanice · Jun 13, 2016

tnmgcarbide said:
ok, now i get it . not a chinese monster , but the forklift accident lathe
on the atlanta craigslist .the equipment hub.

This one? But the seller said it was almost in good condition. Page Not Found - Your Ultimate Source For Buying and Selling Quality Metalworking, Woodworking and Industrial Machinery

"Lathe has been tested and is in working condition. When manually turning handwheel to move carriage you can feel a slight wobble at a certain point in the turn possibly from a bad tooth in gear wheel. It is minor and the lathe is still fully functional, but could effect the precision. We are glad to show under power. ..."

I had a 1969 Volvo that used all American standard (inch) fasteners on the Swedish components. Only the German-made components were metric. So I would not assume the machine is metric without looking at the dials and checking the leadscrew and feed screw pitches. The thread chart looks to me like the leadscrew is inch-based and the 127 tooth gear is an idler for inch threads and is used in the reduction train for cutting metric threads.

There is no dial on the carriage crank, as far as I can see from the pictures.

Larry

Bill D · Jun 13, 2016

I had a 1969 Volvo that used all American standard (inch) fasteners on the Swedish components. Only the German-made components were metric. So I would not assume the machine is metric without looking at the dials and checking the leadscrew and feed screw pitches. The thread chart looks to me like the leadscrew is inch-based and the 127 tooth gear is an idler for inch threads and is used in the reduction train for cutting metric threads.

I believe it was only a few years earlier that Sweeden switched which side of the road they drove on. Today they drive on the right . I believe they switched overnight around 1962. I am pretty sure they always drove from the left hand seat. I know my 1974 245 had the emergency brake handle on the left next to the door with a crank running under the seat to the middle where every other car had the lever.
Same deal with mostly SAE fitting.
Bill D.

Monarchist · Jun 13, 2016

Impact-vector said:
The pinion is the hardest part to find. Its 11 teeth and I dont have a vertical mill to cut them. Even finding the pitch and diameter seems hard.

Horizontal mills will do pinions just fine. End-mill OR milling-cutter OR fly-cutter.

The old one - even damaged - may be all the indexer/Dividing Head you need, too.

bILL

tnmgcarbide · Jun 13, 2016

looks like a nice machine , worth fixing . just 'cause some dummy tipped it over
and broke a few doo-gads ...

unless its' worn and beat , it might be a really great machine.

go for it . even if you must replace the rack and pinion with something else that's
close.... it will work.

johnoder · Jun 13, 2016

The rack looks like its 1/4" circular pitch

12 DP is .2612" See if TEN circular pitches add up to 2 39/64". If 12 DP, pinion may be about 1.083 OD

3t3d · Jun 13, 2016

Find a couple of dowel pins, and place them into the rack maybe 10 teeth apart. ( ten pitches)
Measure the distance of the dowel pins divide by ten, and Know the actual pitch of the rack.
Not so much guessing and speculation.

Making a few assumptions here about getting the correct number of teeth apart so that you are measuring over ten, not nine, not eleven, spaces.
Measuring the overall outside distance and subtract the diameter of one dowel pin..
The PA of the rack will not affect the center distance of the two dowel pins to measure the pitch.

Next you can measure the pressure angle. Being a rack, it will not be some magic involute shape, but a simple angle.

Hope this helps.

tnmgcarbide · Jun 13, 2016

my dad had a '69 volvo 142 . triumph engine , gurling brakes, whatever,. english,
metric , who knows......

Impact-vector · Jun 13, 2016

L Vanice said:
This one? But the seller said it was almost in good condition. Page Not Found - Your Ultimate Source For Buying and Selling Quality Metalworking, Woodworking and Industrial Machinery

"Lathe has been tested and is in working condition. When manually turning handwheel to move carriage you can feel a slight wobble at a certain point in the turn possibly from a bad tooth in gear wheel. It is minor and the lathe is still fully functional, but could effect the precision. We are glad to show under power. ..."

I had a 1969 Volvo that used all American standard (inch) fasteners on the Swedish components. Only the German-made components were metric. So I would not assume the machine is metric without looking at the dials and checking the leadscrew and feed screw pitches. The thread chart looks to me like the leadscrew is inch-based and the 127 tooth gear is an idler for inch threads and is used in the reduction train for cutting metric threads.

There is no dial on the carriage crank, as far as I can see from the pictures.

Larry

That's the one!

Needless to say I'm a little less happy than I thought I would be with the entire thing...

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EPAIII · Jun 14, 2016

If the pitch diameter of the gear is about the same, then the ratio should also be about the same. I would:

1. Buy a standard rack that is a bit thinner than the existing one to leave room for adjustment.

2. Buy a matching gear with a pitch diameter that is about the same as the existing one. If you must differ, and you probably will, then make your new gear's pitch diameter a little smaller than the existing one. Keep the DP as close to the original as possible, but don't worry too much about it.

3. Mount the gear and rack. Shim the rack to get the proper mesh with the gear.

Keep in mind that matching the pitch diameter is more important than the tooth count.

Tony Quiring · Jun 14, 2016

EPA beat me to it...

Gear shape does not matter but gear ratio does.

You should be able to measure full length of rack then count teeth and measure the pinion and measure diameter and or tooth count and come up with suitable gear ratio or other useful data to replace both with something else that results in same gear ratio or so close it does not matter.

Or swap ends and shift or make new pinion...

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