# Gearing question - any engineers out there?



## Nick Sandmann (Oct 24, 2014)

I am hoping someone here can help point me to some books/resources on the math I need for the jig I want to build as I can't seem to find it myself on google(I may not be searching the right terms).

What I am trying to do is retro fit an old Craftsmen lathe I received for free. It has a motor that hangs off the bottom(it's dead) with a belt that hooks onto the lathe and spins the centers. 

I'm not really interested in replacing the motor right now(it's not the greatest lathe). My goal is to make a sled that will hold a portable router above the work piece when it's mounted between centers and have that sled move down the length of the lathe bed by turning a handle by hand. As I crank the gears it would spin the board mounted between the centers on the lathe and simultaneously move the sled(with the router) down the piece at a certain rate. Ideally I will be able to change out the gears to do different types of work(cutting spirals, cutting threads on wooden screws, etc...).

I've found plenty of information online on how to calculate gear ratios and the rate of travel gears moving in 1 direction. But where I'm hitting a wall is trying to calculate what RPM I would need to spin the work piece at and what rate of speed I would need to move the sled/router along it to achieve the desired rate(eg: fast moving sled for a spiral, probably a slow moving sled for screw threads, and anything in between.). I know the rate of feed to RPM ratio is going to be determined by the diameter of the piece on the lathe, but I have no clue what formulas would calculate all of this.

If anyone could point me to a website, books, magazine, or anything that I would need to calculate this, it would be greatly appreciated! I've taken college level Calculus, algebra, geometry, etc... so I'm not afraid of the math, but I think this is hitting on mechanical engineering that I don't have much experience with.

Also, any recommendations on where to find a good selection of gears for a project like this? I would prefer to just purchase some good metal or plastic gears than make wooden ones myself.


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## Alchymist (Jan 2, 2011)

Your gear ratios will be determined not by speed, but by the numerical relationship between the spindle and whatever mechanism you use to move the router. As an example - using an 8 TPI acme screw to drive the router carriage, it takes 8 turns of the shaft to move the router one inch. If you gear the spindle drive to the acme shaft at an 8 to 1 ratio, the router will move 1 inch for every revolution of the spindle. This will result in a 1" spiral. Driving the acme shaft at 16 to 1 would result in a 2" spiral, and so on. All this is independent of speed.

Machinery's Handbook - everything you want to know about gears.


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## Nick Sandmann (Oct 24, 2014)

Alchymist, thank you for the clarification, and that makes sense. So I guess in my example I would need to calculate what gears I need to make 1 revolution of the dowel on the lather as I would imagine the same gears would have more RPM's for a 1/2" dowel than if I had a 2" dowel in the lathe.

Thank you for the book reference. Just out of curiosity, do you know if the pocket guide covers gearing to the extent I would need for a project like this? Or would the full edition be better? Only asking because there's about a $100 difference between the pocket guide and the full edition of that book.


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## Alchymist (Jan 2, 2011)

Nick Sandmann said:


> Alchymist, thank you for the clarification, and that makes sense. So I guess in my example I would need to calculate what gears I need to make 1 revolution of the dowel on the lather as I would imagine the same gears would have more RPM's for a 1/2" dowel than if I had a 2" dowel in the lathe.
> 
> Diameter of the work piece in the lathe has no bearing on the gearing. It's strictly spindle turns vs Acme thread (or whatever advance device is used) turns. Spindle makes one revolution per so many turns of the advance mechanism, regardless of work diameter.
> 
> Thank you for the book reference. Just out of curiosity, do you know if the pocket guide covers gearing to the extent I would need for a project like this? Or would the full edition be better? Only asking because there's about a $100 difference between the pocket guide and the full edition of that book.


No idea what the pocket reference covers vs full edition. Best bet is picking up an earlier edition used - ebay or used book stores.


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## FrankC (Aug 24, 2012)

Here is a method from 1884, but you would need a spiral to build the jig.


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## Alchymist (Jan 2, 2011)

Here's a couple videos showing 2 methods:

https://www.youtube.com/watch?v=eJ7NYL0HnsQ

https://www.youtube.com/watch?v=X3aEw7eSAGg


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## Nick Sandmann (Oct 24, 2014)

Thank you for the videos. That top 1 is exactly what I am trying to do. It's not clear from the video if that setup has the gear setup I am looking to implement so I can change out the gear ratios or not, but that looks very similar to what I had in mind.


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## wizard of oz (Dec 17, 2013)

The top video is a "*Sears Router Crafter*". They come up on flee-bay from time to time. I had one, but never used it, so I sold it.


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## Nick Sandmann (Oct 24, 2014)

wizard of oz said:


> The top video is a "*Sears Router Crafter*". They come up on flee-bay from time to time. I had one, but never used it, so I sold it.


:thumbsup:

I already have one sitting in my garage. A co-workers father was giving it away, and it came with some old craftsman gouges, so I figured worst case I have some more gouges for my midi-lathe. It doesn't run, it looks like a mouse chewed through some of the wiring, and I'm thinking of building one of these contraptions with it rather than spending the time trying to get the motor running again.


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## dcauto (Nov 24, 2014)

For the amount of work it'll take to build a jig like that for yourself, it may be worth just buying a metalworking lathe for this. They're already set up with an ACME screw to turn threads (spirals) like this. You could swap out the screw for a different pitch if you wanted to make "shallower" spirals instead of having them as tight as a screw thread.

The fact that you have to replace this motor on the craftsman leads me to believe it would be a wash to just buy a cheap used metalworking lathe.

The screw on a lathe like this is geared directly to the motor (most of the time). So, you don't have to worry about setting the feed and speed independent of each other - set your spindle speed and the feed per revolution will be fixed (regardless of spindle speed) set according to your selection on the gearbox.

Check out South Bend for something like this: http://www.ebay.com/itm/MACHINIST-L...233?pt=LH_DefaultDomain_0&hash=item2c914190b9


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## Nick Sandmann (Oct 24, 2014)

dcauto said:


> For the amount of work it'll take to build a jig like that for yourself, it may be worth just buying a metalworking lathe for this. They're already set up with an ACME screw to turn threads (spirals) like this. You could swap out the screw for a different pitch if you wanted to make "shallower" spirals instead of having them as tight as a screw thread.
> 
> The fact that you have to replace this motor on the craftsman leads me to believe it would be a wash to just buy a cheap used metalworking lathe.
> 
> ...


That would be awesome, however well above what I am looking to spend. Should I decide to get into the screw, or wooden rope, making business, that might be a consideration, but my current plans are purely for personal work.

Also, I have no intention of screwing around trying to fix that motor either way. If I was going to to that I'd just use it as a lathe. There's no need for power to the lathe if I'm using a powered router to cut the board. The lathe is just handy to hold the work, and then I just need to rig up a lead screw, some gears, and a hand crank, which I am pretty sure I can come up with something for under $100(stay tuned!)


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## PhilBa (Jun 30, 2014)

I just pickup up a router crafter off CL. It's in primo shape and was $65. I see them from time to time on CL - usually less than $100. The benefit of the RC is that there is a small but active community. You will find a couple of threads on this and other WW forums (fora?) that pretty give you everything you need to know to get started. I'm just about to cut my first spiral but the Thanksgiving Honeydew list is interfering with that. Maybe this weekend...

I'm thinking that it might not be that hard to motorize the RC and use a feed screw instead of the wire and pulleys. If I use stepper motors and drive it from a microcontroller like an ardunio I think I can do a lot much more complex turnings. In that case, moding a lathe might result in a more solid machine. Not going to happen real soon, though...


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## Alchymist (Jan 2, 2011)

dcauto said:


> For the amount of work it'll take to build a jig like that for yourself, it may be worth just buying a metalworking lathe for this. They're already set up with an ACME screw to turn threads (spirals) like this. You could swap out the screw for a different pitch if you wanted to make "shallower" spirals instead of having them as tight as a screw thread.
> 
> The fact that you have to replace this motor on the craftsman leads me to believe it would be a wash to just buy a cheap used metalworking lathe.
> 
> ...


Modifying a metal lathe will be difficult, as it's designed to turn "spirals" from some 40 turns per inch to about 6 or 8, much too tight for what the OP wants to do. It looks like 1/2 or 1/4 turn per inch is what he's looking for, which is 10 or more times slower on the feed screw than original. Feed speeds on a metal lathe are fixed by gears, either a quick change gearbox, or individual gears that get swapped around. The size of the gears needed for the slow spirals would be of a size that would preclude mounting them. It would take a lot of cobbling to drive the feed screw at the right pitch. Changing the lead screw would prove problematic also, as the apron half nuts would have to be changed, and with that steep of a pitch probably wouldn't drive easily. Just too much mod to be practical.


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## hazimkazim (Nov 27, 2014)

I totally agree with Alchymist about that


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## dannyk (Dec 7, 2014)

ShopNotes magazine Vol.20 Issue 115 has a home built machine. It will let you vary the number of starts,and the length of the spirals.If you will study the plans you modify the plans to make both right and left hand spirals.


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## navy1 (Dec 21, 2014)

Machinery's Handbook is the bible for anything to do with mechanical reference, speeds, feeds, mathematics, and enough math to choke a horse. That handbook is used everywhere from the engineering department, maintenance and repair, design, etc. You can find used ones on eBay, or amazon. A handbook from the 1940's will have all that you will need. Another good one is Standard Handbook of Machine Design, by Shigley and Mischke. A great source of just about anything in the line of MRO, Maintenance Repair and Operations is McMaster Carr. They are on the internet, and I have purchased from them for almost 50 years. Very fast order filling and shipping. They have warehouses all over America, and I believe that they also have UPS terminals right there. They have gears of all kinds, in stock, and their catalog and online has lots of guidance in selecting what you need. Usually, if no one else has it, they do, and quantity is no problem.


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