Help identifying and duplicating this thread.

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Torch
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Help identifying and duplicating this thread.

Post by Torch »

I am doing some maintenance and tuning on my machine, cleaning and adjusting for wear. The brass cross-slide leadscrew nut is getting worn and backlash is now up to 0.049", so I figure it's time to replace it again. I previously purchased one from BusyBee (in Canada) and had the part number. I called them up, and long story short, they have a superseding part in stock -- for the low, low price of only $180! (previous version was around CDN$20 or $30, IIRC).

So I thought, hey, why don't I just make a couple? Step 1, identify the thread. So I removed the shaft and had a close look. Hmmm. Maybe not so simple after all!

The OD is 19.98mm by calipers. But the thread pitch is not metric -- it's 8TPI. A close look at the thread form on the optical comparator shows a strange thread profile:

Image

Flat topped but about a 0.030" radius at the root. And the flank seems to start rounding off just before the crest, as if it was a round thread that was truncated. Height from crest to root is about 0.050"

Included thread angle of the short flat sections between the rounding is 40° -- 18° on one side, 22° on the other.

Image
(sorry the focus is off a bit on that one. Tiny phone screen preview, now it's reassembled).

Since my only lathe is the one the part is for, unless I can find a commercial tap of the correct thread, the next step would be to duplicate it for a test piece. Not sure my lathe form bit grinding skills are up to the task of a compound curve, but I could try. Then I would have to either create a tap from O1 or an internal threading bit.

How would you approach this?

A possible alternative is to order from Grizzly in the US. Lord knows when it would arrive given current border disruptions, but the machine is still usable for the time being as it is. Interestingly, when I look up the equivalent part number on their website, it is also superseded -- by "Grizzly P9729724A - CROSSFEED SCREW W/NUT ASSEMBLY" -- that seems to include both nut and shaft. US$50 + shipping has to be cheaper than BB's price for the nut. However, the listing for the crossfeed shaft alone is US$70, which makes me question what is really going on. Unfortunately there is no photo of the new part(s).
Bob D.
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Re: Help identifying and duplicating this thread.

Post by Bob D. »

You might be looking at a lot of wear and not the original profile. Maybe look at the very end of the screw. If you make the screw and nut the only important thing is the TPI and you can make the profile anything you want.
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NP317
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Re: Help identifying and duplicating this thread.

Post by NP317 »

As for Grizzly, give them a phone call for help answering your parts questions.
They are historically very responsive to customers asking for tech help.
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Re: Help identifying and duplicating this thread.

Post by Rich_Carlstedt »

The pitch is based on your Cross-Slide Hand-wheel Dial Range , forget the Comparitor readings
"If" your dial reads .125 for a 360 revolution, then the pitch is indeed .125 (8 Pitch) and LH
Years ago I had to make a ( un-available) nut for a lathe and made the tool bit as best I could and bored out the brass stock and did
a rough thread cut, then took the lathe crosslide apart and gashed the end of the slides leadscrew, making it into a tap. Then I taped the brass
which took a lot of force, but work. When done, it will be tight on the end, but is perfect in the middle of the range

Rich

Oh, the round thread you have is from excessive wear !
Torch
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Re: Help identifying and duplicating this thread.

Post by Torch »

Well I won't deny that wear could play a role, the profile is consistent along the length. Plus, why would the wear affect the root of the thread so much more than the crest? I thought the whole point of the brass nut was to be sacrificial.

I sent an e-mail to the technical department at Grizzly. We'll see what they say.
hobgobbln
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Re: Help identifying and duplicating this thread.

Post by hobgobbln »

Torch wrote: Thu Feb 10, 2022 3:12 pm Plus, why would the wear affect the root of the thread so much more than the crest? I thought the whole point of the brass nut was to be sacrificial.
Out of curiosity I searched for "round thread form" and found a wiki page for "knuckle thread". It appears to be a different thread, but its very similar to yours. One of the uses for it says, "Other applications use a knuckle thread's rounded edges to reduce the stress on softer materials at a point of connection. Some linear actuators use a knuckle thread to reduce the wear of the steel leadscrew against a plastic sliding nut." Maybe they used that thread intentionally to make the nut last longer? You should check it out.

Griz
John Hasler
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Re: Help identifying and duplicating this thread.

Post by John Hasler »

Could it be severely worn stub acme?
Torch
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Re: Help identifying and duplicating this thread.

Post by Torch »

The thing that argues against acme is the 40° included angle.
Torch
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Re: Help identifying and duplicating this thread.

Post by Torch »

I'm having trouble finding exact specs on knuckle threads. Machinery's Handbook failed me! One site says they are rounded square threads, with a radius of 1/4 of the pitch in both root and on crest. Another says they have a 30° included angle. Looks like the crest of the external thread does not go all the way to the root of the internal thread, so where the humps meet must be 30° (kind of like the pressure angle of a spur gear)?
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Bill Shields
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Re: Help identifying and duplicating this thread.

Post by Bill Shields »

You are all assuming that these threads are a standard...and that the manufacturer did not intentionally create a thread that would all but force customers to return for parts.

Beyond the pitch...what part of these threads appears to be close to a 'standard'?

The 20 mm od seems ok ..pitch can be whatever is needed for a metric or imperial machine

Be thinks there is round peg in square hole thinking here.

I would cut to the chase and as recommended fit something standard and be done with it.
Too many things going on to bother listing them.
Torch
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Re: Help identifying and duplicating this thread.

Post by Torch »

Bill, I'm pretty sure you are on the right track. Not necessarily about forcing people to return for spare parts, I suspect that bit was just a lucky coincidence. I suspect the machine design was originally in metric and the Chinese were looking for an expedient way to make a version with inch dials for the North American market. Rather than change all the castings, bearings and bushings to accommodate lead screws with standardized threads, they simply came up with a custom thread, possibly by turning down a length of stock made in a different factory.

I say this with some justification. I bought my machine from the factory, knowing that one outfit made all the housebrand machines from Grizzly, BusyBee, Harbor Freight, etc. and wanting certain features not all available from one domestic source. I had a few conversations with people at the factory in the process. They were quite accommodating -- to a point. And that point was defined by government regulations.

They were quite constrained as to what they were allowed to produce, the approved designs they were allowed to use and the sources of materials and parts. EG: I ordered mine with 1-1/2hp DC motors. This particular motor could only be exported as part of a complete machine, but could not be exported as an individual spare part. Change gears could only be ordered in sets, sourced from another specific factory. All part of their planned central economy.
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Bill Shields
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Re: Help identifying and duplicating this thread.

Post by Bill Shields »

Gotta love PRC management style...

In reality...as long as you get a nut that matches a new thread -> and the thread out of the box is long enough...do you care?

Sure ..you are probably going to have to put it back together as is to machine the parts -> unless you have another lathe...but it is a short walk to get where you need to be -> not a marathon.

Are there any other wonky screws in the machine?

It is going to cost you $ one way or another...make it easy and save on valium, aspirin and Percocet.

Spend on good scotch and consider it a win when the machine is back together.
Too many things going on to bother listing them.
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