55 tooth gear

[chucketn]

Inspired by Arnoldb's thread on hobbing, I am about to undertake making a 55 tooth gear for a lift chair. Not a model engine, so if I need to take this thread elseware say so.
I have never made a gear before so jump in there with corrections, advise, warnings, as appropriate. Here's the info I have. The gear is 55tooth and mates with the mod 1 change gears for my MicroMark lathe. It is 2.350 " in diameter, but the 55 tooth change gear is only 2.241 in diameter. Will that make a difference? I believe the pressure angle is 14.5 deg.
I will make the gear from 6061T1 aluminum, as thats what I have on hand. I just happen to have a bar end big enough and my 4x6 is cutting the blank now.
Other questions I have at this point is what diameter hob min or max do I need, as I will have to order drill rod for the correct diameter. I have 1/2" on hand. Will that work?

Chuck

[IanR]

Is the 2.35" diameter derived from a gear you want to duplicate? It would appear to be right for a 55 tooth wheel of 24.25DP or 1.05 mod, neither of which sound likely. The diameter of the change wheel is right for 1 mod.

[Bogstandard]

Chuck,

As far as I can make out, the working depth for Mod 1 is 2.0mm (0.0787").

Blank diameter should be 57.0mm (2.2441") for a 55 tooth gear using mod 1.

For cutter

1/2" = roughly 12.5mm, so if you make the cutter teeth 3mm deep, you would end up with a central core of just 6.5mm (a tiny bit over 1/4"). I would reckon that needs to be a little thicker to stop vibrations and flexing as it is cutting. 3/4" would be a lot better.

My commercial mod 1 hobs are around 1.5" diameter.

John

[chucketn]

I have the same program John. I realize the gear I want to make is not exact mod 1.
I guess my question is, if I duplicate the existing gear diameter, grind a tool to fit the tooth valley, make a hob and cut the gear, will it work? I think it will.
The origional gear is plastic of some type, like delrin.

Chuck

[arnoldb]

Hi Chuck.

Thank you for the mention - I'm really pleased that you found my thread inspiring.

I'll try to add some of the missing information you asked about, but first I want to add a caveat - I really hope it's not misunderstood - it's for everyone's safety.

You're proposing to make a spare part for a piece of medical/human assistance equipment.  I'm by no means a qualified mechanical engineer, so I cannot really comment on the suitability of material you selected, or whether this method of making the gear is suitable, but I do have reservations.  When someone's health or safety could be at stake, "adequate" takes on a whole new meaning.  Building small model engines or the tools to build them for fun is one thing, but when it comes to repairing or making parts for medical or human assistance devices, it might be better left to qualified people in those fields.  If you do choose to make parts for such devices, you are leaving yourself open to a whole world of litigation.  You'll have to make the choice on that; everybody's circumstances and even immediate needs differ, and you have to make that personal choice.  Somehow I'm not comfortable with an aluminium alloy gear for your application, and neither am I comfortable that the relatively crude method that I showed is adequate for your intended application.  It's good for hobby requirements any time, but when there's human safety involved, it might not be your best choice.

The measurements you shared makes sense for a module 1 gear; with the measurements from your lathe change wheel the closest coming in at module 0.9986.  If you took the diameter measurement over the gear, it having an odd number of teeth, you would have measured slightly below the true OD, and the 0.9986 module is very close to module 1.
The other gear - the one that you want to replicate - concerns me a bit though.  It calculates out to 1.0472 mod or 24.2553 DP, neither of which makes sense. 
Purely at a guess, I'd say it is module 1, but machined over size to add an additional safety factor in this application.  That would mean it has to be machined very accurately.

If the gear is truly a Module 1, it is more likely that the pressure angle would be 20^o, as metric gears normally use a pressure angle of 20^o while imperial DP based gears tend to use a 14.5^o PA.  That's not a hard and fast rule though, and they do get mixed up on the pressure angle.

Kind regards, Arnold

OK, so while I typed this up, 3 other replies had been added to the post.

Chuck, grinding up a tool to fit the tooth valley would save you having to make up a hob; you could just use it as a single-point cutter to cut all the spaces between the teeth  - no need to make up a hob then, unless you're feeling adventurous 

[chucketn]

I have actually lost contact with the folks that needed the gear. I have sent an e-mail to the last known address to see if they actually still need/want it.
Arnold, I understand the litgation possibilities, and I appreciate your concern, and bringing it up. I would actually require the user to sign a release before giving them the gear. I had offered to make the gear without charge, but at the time, I was not equiped, nor had the knowledge to make it.
Wouldn't 6061 aluminum be stronger than delrin?
I will still go through the process of making the gear, just for the practice, wether it gets used for it's intended purpose or not.

Chuck

[steamer]

GailNM has a spreadsheet that will give the dimensions for a single point flycutter.   It's based on Ivan Halls book.

Dave

[chucketn]

Anyone have a spreadsheet for determining the correct hole plate to use in indexing for gearcutting? Or can explain how to calculate it? I thought I had one but can't find it.


Chuck

[mklotz]

You can use my DIVHEAD program or...

N = number of divisions
G = rotary table gear ratio (relative to 1)
T = turns between adjacent index points

T = G/N

Example:

55 divisions on an RT with a 40:1 ratio

T = 40/55 = 8/11

You need to move the table crank 8/11 of a turn between divisions.  You need an 11 hole plate or any plate with an integer multiple of 11 holes...

8/11 = 16/22 = 24/33 = 32/44 = 40/55 = ...

[chucketn]

Thanks, Marv. I have a 90/1 RT, So, I calculate 1 turn, 21 holes on a 33 hole plate. Correct?

Chuck

[mklotz]

Thanks, Marv. I have a 90/1 RT, So, I calculate 1 turn, 21 holes on a 33 hole plate. Correct?

That'll work.