Optimal GH Class For a Toe-Stop Hole Tap

[Vertman]

Here's a question for skaters who are also machinists. I don't usually have any problems with the toe-stop hole threads on my own skates, since I am savvy enough to grease them whenever I swap my toe-stops or do a full rebuild, but I have seen some pretty nightmare-ish galling and full-on aluminum-to-aluminum friction bonding between skate plates and toe-stop stems.

I have taps which I have made by grinding slots into, or alternately by grinding flats onto, the axis of old toe-stop stems. These have generally been sufficient for cleaning threads. But I recently came across a case of a full friction weld between stem and plate. Luckily, the stem was softer than the plate, so that when the toe-stop was removed the plate was OK, but the threads were entirely filled in (and over) with aluminum from the stem. So I am faced with not just cleaning a thread, but rather with essentially cutting a new thread. So I want to procure a real (rather than home-made) tap.

Hence, my question: what is the best tap GH class for re-cutting toe-stop threads into a plate? For those who are interested but unfamiliar, a GH1 tap cuts no more than one-half mil (0.0005") over nominal. GH2 cuts no more than one mil over nominal. And so forth.

At what GH number do I start removing more material than should be prudently removed?

BTW, the hole is 5/8"-18 TPI. And I believe that it is an NF (fine), not NC (coarse), thread. If fine is not the case, please let me know that as well.

Thanks.

[Stevbrdly]

Hi Vertman,

Which plates are you running?

In my shop, the lowest thread we use is GH5, our customers want a slip fit on pretty much any part for ease of assembly. We make parts for Coca cola and Pepsi for their fountains, and vending machines.
We also make parts for automotive. they require us to use GH8 and 10 sized threads in Steel, which is stronger than aluminum, i understand.

as far as which you should go with, you could get away with any tap,
A quick look on amazon gives the *cheap* taps at around 8 dollars, and if you want one that is GH3, its going to cost you around $50. sure you are paying for the tolerance, but is that necessary in this application? the force is going to be shared by the aluminum insert in your plate ( or possibly cut threads in the plate if it is all aluminum) and the jam nut on your toe stop stem.

Clean threads, with a little bit of grease to keep from sticking, even at GH10 tolerance should hold just fine under normal skating load.

[Vertman]

The plate is an old SG Invader, so there is no jam nut to help snug the stem threads against the hole threads. But there is the pinch screw. And this is not my plate. As I mentioned, I always grease the hole and stem whenever I do any maintenance on my own skates.

In case anyone else is interested in purchasing a tap, there are Bendix Besly and Irwin Hanson taps, among others, new on eBay for about $15. They don't have any appearance of being counterfeit.

Maybe I am not correctly comprehending the GL/GH specification itself. My understanding is that a GH10 tap would produce a hole with a thread pitch diameter which is 5 mils over basic. If a stem that is exactly at basic is screwed into this hole, there will be 2.5 mils of radial or 5 mils of diametral play/slop between stem and hole.

Now, 5 mils is only a little more than a 10th of a millimeter. So we are talking small differences. But, my concern is that if a GH8 or GH10 tap cuts threads which have about the right "snug-ness" between stem and hole, then a GH3 tap might produce a hole which is a little too snug.

Alternately, if a GH3 tap cuts threads which yield about the right amount of snug-ness between stem and hole, then a GH8 tap might produce a hole which is too loose.

So which is optimal given that I don't want to cut away any more material from a plate than what is necessary to produce a hole that it is not overly difficult to thread a toe-stop stem into?

[Stevbrdly]

It sounds like you need to do a 3 wire measurement of the toe stop stem pitch diameter and find a tap that is a few thousandths over what the toe stop comes out to be.

Standard 3 wire measurements with .032 wire for 18tpi for 2A threads (this is a fairly standard for fitment between ID and OD threads)

Min: .6312 (this would be your GH1 tap, which should cut nominal)
Max .6357 (GH11 (this is considered oversize for this thread size))


Remember however, if the owner of the plate ever goes to use different toe stops, the threads may be tight or loose depending on the toe stop stem.


Hopefully this helps.

[Doc Sk8]

I have a early 70s Craftsman Chrome edge taps. The 5/8-18 UNF in my kit has done everything from chase the corrosion out of Ultimate plates to convert Metric Roll Line threads to standard. Never had any kind of issue. I would be willing to make a very large wager that you are well past any manufacturer in your thinking.. How do I know that?? Try gaging some ID or OD sk8 threads against nominal. I'm sure you will get my drift here.


Basically I rescue threads with cheap Chinese knock off cutters. No issues so far (and I have been doing this for about 20 years.. There are roller skates, not space craft. As Stevbrdly points out, there is far more variability in the production spec of various manufacturers, not to mention the inability of the operators to hold said spec over a run of parts.


Have fun but please, don't bust a gasket over thinking this stuff.

[Vertman]

Well, I think I'll get a GH3 or a GH5 and see how that goes.

As far as manufacturer variability (both plate manufacturers and toe-stop manufacturers), I would say that, from my experience, the accuracy and precision of the held tolerances must be at least fairly good. I currently have nine pairs of built up skates and two others that I've retired, and stops have always threaded into the holes snugly, not too loose or too tight.

My biggest complaint has been manufacturers leaving little slivers of metal in every hole when they cut the threads. I think every SG plate I've owned has had this problem.

I'm going to have to call hypocrisy on the notion that I'm over-thinking this tap class issue, since I've seen threads arguing the physics of 7mm versus 8mm axles or the use of bearing spacers that go on for up to ten pages of comments.

Over-thinking things is the guiding purpose of a skate forum. Otherwise you top posters would be out of a job.