Unimat 3 Mill head Modulus

[Pete SavageParticipant @petesavage40461]

Hi guys, I’ve just been fiddling with a quite well worn unimat 3 lathe I recently bought, and in trying to sort a few things out, I found something rather interesting/ peculiar about the rack and pinion Z axis.

The rack on the bearing carriage has a pitch of 3.14mm, which is a module of 1 (pi*mod) which I measured over 10 points and divided for better accuracy.

Now when I look at the handle (which I am 99% certain is the original) and offer up gear cutters to it, the closest fit is a 0.9 modulus cutter.

When the pinion is placed in the rack outside of the housing and rolled, it does not seem to move smoothly all the way.

This seems like quite an oversight by emco unless it is for a reason?

The reason I am going through it with a fine toothed comb is that I am looking to make a stepper motor attachment for it and the play that it has at the moment is not good enough.

I will try and add some pictures when I work it out.

I look forward to the discussion!

Pete

[Pete SavageParticipant @petesavage40461]

[Pete SavageParticipant @petesavage40461]

Here is the bearing carrier with the Mod 1 rack

This is the 0.9 cutter, and it may not look too conclusive from the image but the involute curves match pretty closely.

None of the involute curves come anywhere close to touching on this gear cutter, yet the gear does not look excessively worn.

[mahgniaParticipant @mahgnia]

Pete,

What's the OD of the pinion?

Should be 12mm if MOD1.

Andrew

[JasonBModerator @jasonb]

Involute cutters are a compromise on shape so if the pinion was hobbed then you won't get an exact match, also may be a modified form due to the small number of teeth.

[Pete SavageParticipant @petesavage40461]

OD is 12mm yes. I seem to remember something about a pinion cutter and a wheel cutter, presumably the former is for low tooth counts and the latter is for large tooth counts?

The mod 1 cutter has a 6 or a 9 opposite the "M 1.", whether this means a tooth number approximation or not I don't know.

Edited By Pete Savage on 02/01/2018 09:32:30

[mahgniaParticipant @mahgnia]

#1 cutter (out of 8 cutters in the involute cutter set, PA 20 degrees) is for 12-13 teeth, so the 10 tooth pinion cannot be formed accurately with an involute gear cutter. There probably is slight undercutting on the tooth form.

The 10 tooth pinion gear was most likely hobbed.

No1	12-13 teeth
No2	14-16 teeth
No3	17-20 teeth
No4	21-25 teeth
No5	26-34 teeth
No6	35-54 teeth
No7	55-134 teeth
No8	135 to rack

Andrew

Edited By mahgnia on 02/01/2018 10:06:42

Edited By mahgnia on 02/01/2018 10:14:57

[Michael GilliganParticipant @michaelgilligan61133]

Pete,

This calculator might be useful: **LINK**

http://www.otvinta.com/rack.html

As Jason has mentioned, the original pinion would have been a commercially convenient approximation.

… it would be interesting to compare this with the calculated profile.

MichaelG.

.

P.S. Unless you happen to have a decent 'profile projector' to hand, scanning the pinion on a flat-bed scanner, at [say] 600dpi would a useful start.

[Pete SavageParticipant @petesavage40461]

Posted by Michael Gilligan on 02/01/2018 10:21:40:

Unless you happen to have a decent 'profile projector' to hand, scanning the pinion on a flat-bed scanner, at [say] 600dpi would a useful start.

It's funny you should say that, I work at a testing laboratory which performs metallurgical analysis, and we have an ancient one downstairs! I will have to put a bit of paper behind the teeth for contrast though.

[Michael GilliganParticipant @michaelgilligan61133]

Pete,

I've just been playing with this: **LINK**

http://hessmer.org/gears/InvoluteSpurGearBuilder.html

I think it might be just what you need to design a better pinion.

MichaelG.

.

Edited By Michael Gilligan on 04/01/2018 00:25:49

[Julian Saunders 1Participant @juliansaunders1]

I made a new quill out of free cutting steel, 40mm to house tapered bearings, for the Uniamt, and like you, noticed the issue you describe, I used a horizontal cutter holder and ground the tip to the correct profile by eye and feel, I then used a Chester milling machine to hold the new quill whilst I cut the new rack, unfortunately I'm going to have to redo one day, as the Chinese seem to have used an imperial gear in THEIR quill rack, which approximates metric, but not anywhere near the accuracy I was working to, 0.001mm, over the length of the new quill (I made a new alloy vertical head to accomodate the new quill, which has more feed depth) . Moral, never trust a cheap Chinese fine vertical feed or quill !!

[Martin ConnellyParticipant @martinconnelly55370]

Are you planning to convert this to CNC. If so then a straight rack and pinion is unlikely to give a smooth linear motion, helical would help but would be hard to get acceptable mesh between the parts. As an alternative here is a link to a system that uses a ball screw that may be something that you could adapt to your needs.

**LINK**

Martin C

[alex h.Participant @alexh-2]

i done a cnc conversation.

i had the same problemes.

my solution:

grind a flycutter insert and cut the 10teeth gear or cut with modul 1 gearcutter no1 12 teeth gear. put your stepper to 1/8 microstepping and good.

but!

you can buy a dovetail-profil.

you need only a adapter to mount the dovetail profil column instead of the original. then you get more then 25mm way in Z.

[alex h.Participant @alexh-2]

ballscrews are ok but trapeziodal spindles with backlashnuts are cheaper and smaller.

dont forget the size of the unimat!

[Jouke van der VeenParticipant @joukevanderveen72935]

Would a ballscrew be more accurate (less backlash) than a trapezoidal spindle with a (spring loaded) anti-backlash nut?

[Jouke van der VeenParticipant @joukevanderveen72935]

Alex,

Did you remove your topic about twist of Unimat 3 support?

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