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bob ward
07-28-2008, 10:57 PM
Is there an 'approved' method for remeshing the headstock spur gears on a 14 x 40 when changing them for threading and turning?

What I do at the moment is hold them in mesh just lightly by hand then tighten up the clamp. There is no fine adjustment.

I don't know whether the gears are properly in mesh or bottoming, and given the size of the teeth there will only be a thou or 3 between OK and not OK.

Is there a more scientific method or am I worrying too much?

Rich Carlstedt
07-28-2008, 11:05 PM
The old way of doing it was to place some cellophane from the outside
of a package of cigarettes, between the gears, and hold them while tightening the banjo stud.
It is important, that your total force be directed at the center of the gear face ( 90 degrees to the gear blank !)
and not infront or behind the gear
This thin film was about .001 thick and gave a .002 backlash to the setup.
any similar technique should work.


A word of caution, particularly on some foriegn made lathes.
What I have found is that some have NOT made a effort to keep the stud square to the banjo when tighening.
check your lathe . If when you tighten, the gear binds, your stud may not be square !
Rich

oldtiffie
07-29-2008, 12:12 AM
I agree Rich.

But I'd "spin" the meshing gears and find any "tight spot/s" and set the cellophane/cigarette paper etc. at the tight spot/s.

You should be able to "feel" the "back-lash" (even a "thou") after it is tightened. Test "spin" for "feel of mesh" as well.

Start at one end of each gear train - not in the middle if possible.

Paul Alciatore
07-29-2008, 12:12 AM
The cellophane thing sounds good. A couple of thicknesses of aluminum foil should also work and I always have a roll on hand in the shop. (I don't smoke.) The foil is about 7 tenths so a double thickness will provide about 0.003" backlash.

Another thing to remember with a "banjo" is it is usually best to work from the screw gear toward the spindle or stud gear, one gear at a time. This works well even with multiple (idler and/or compound) gear setups.

Forrest Addy
07-29-2008, 12:20 PM
In my incarnation as a gear cutter I set zillions of change gears for backlash shooting for 2-3 thou sometimes more in larger pitches. The gears were always oily and shrp edged, the tools slippery and the little black specs and smears on the doors and quadrants were blood from 40 years of fumble fingered people changing gears.

I ususally set backlash by eye looking at the oil bead oozing from the mesh as I rocked the train this way and that.You can set them by feeler if you're fussy but involute gears are not affected much if the B/L is a bit excessive.

There's a formula for setting backlash using diameteral pitch as the determinant but I can't recall what it is. Something like 1 / (P x 40) or circular pitch / 15 but I may have the constants wrong.

It's a good idea to roll the train by hand through several revolutions checking for stiff spots and hesitations. Gears run under power with insufficient backlash can impose large radial separation forceswhen eccentricities align. Better a little too much backlash than not enough.

lane
07-29-2008, 06:37 PM
As usual Forest has the answer. but I was taught to use a piece of brown paper bag like a grocery sack . Kind of hard to find any more.

bollie7
07-29-2008, 08:35 PM
Small tolerances make for quiet running,
Large tolerances prevent seize ups. LOL.

regards
bollie7

oldtiffie
07-29-2008, 09:32 PM
I can recall an article on this in the McMaster-Carr (Sp?) web site - I think.

There has to be back-lash - in every case.

For a detailed description see Machinery's Handbook. It is under "Backlash" in my copy of the 27th. Edition (page 2067)- see tables 1 and 2 - but a caution - do read the associated text.

It is very informative.

J. Randall
07-29-2008, 11:15 PM
Lane, if all you get at the grocery store is plastic, next time tell the clerk that you are bi-sackual and you might get some brown paper.
James

bob ward
08-12-2008, 06:54 AM
Thanks guys.

What I have done the last couple of times I've messed with the headstock gears - its a 3 gear train - is to tape a strip of shopping bag plastic (1 thou, 25um) around the circumference of the middle gear to give me clearance.

It works well without any tight spots in the gear train and the headstock runs nice and quiet.