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Quick Change / Change Gear Question

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cjones6108

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I have a Bolton 1337G lathe, which research indicates is essentially the same machine as a PM-1340 except that the Bolton has a "closed" gear box while the PM is "open." In practical terms, I think this mainly means there are not as many gear choices on the Bolton. No big surprise there. So I need to make a stud with a 28tpi thread, and much to my surprise, my brand-new, sooper-dooper lathe won't do it. o_O Seriously???

Here's a picture of the thread chart from the manual:
1532802713902.png

Yep -- 28tpi just isn't one of the options. Contrast this with the chart from the PM machine:
1532802832129.png
I'm guessing I could probably get the thread I need by using some different change gears. Kind of defeats the purpose of the quick-change gearbox, but OK. So then I start trying to figure out gear ratios and such, which was easy enough on my previous pre-quick-change lathe, and discover that the arithmetic doesn't really work anymore... :crushed:

So can anyone tell me what I might need (or offer some insight as to how I can figure out what I need) to accomplish what should be a fairly straightforward task?
 

Bob Korves

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#2
Is that the only threading chart in the Manual? Is there a threading chart on the lathe itself? If there is, please post a readable picture for us to see.
 

cjones6108

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Here are a couple of shots, one of the thread chart and one of the controls. Just so you know, I did send a note to the VP of the company regarding the major misspelled word emblazoned across the front of my machine. Inexcusable, so far as I'm concerned, but that's an issue for another day...

IMG_20180216_151613085.jpg
IMG_20180216_151452802.jpg
 

mksj

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I have a Bolton 1337G lathe, which research indicates is essentially the same machine as a PM-1340 except that the Bolton has a "closed" gear box while the PM is "open."
I am not sure how you made this comparison, other than they are both a lathe. Beyond that there is no comparison/equivalency between the two machines.

Reviewing your manual, there is no provision for cutting threads other than what is on the threading plate. Sorry, but it is an extremely limited gearbox design and typically there are many more change gears to achieve other thread pitches. In addition be aware that the in order to get the slower feeds that you must use the 30T input to 70T output gear combination. Based on the manual's description the leadscrew is metric as it uses the transposing gear to cut imperial threads which differs from the machine plate. They also state "Half nut must be kept engaged to leadscrew at all times, when the thread is being cut. Every time you are finished cutting, move tool back and reverse the motor", which usually applies when the threading dial and leadscrew are metric and you are cutting imperial threads (and visa versa) at least in the manual. Typically the half-nut is disengaged and you reengage based on the threading dial marks. It is unclear if this is an error in the manual and/or the machines comes with different leadscew and the manual is in error.
 

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BtoVin83

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A couple of caveats here, if you have the 70 tooth gear and the lead screw is imperial at 8 TPI I think you can cut the 28 TPI with the N gear = 70, QCGB input set at 30 and the lever position at III. If this is wrong I don't know who posted this.
 

cjones6108

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Looking at these numbers again, and adding the 70 gear into the N position, damn me if I don't think you just might be on to something. Easy enough to check it at any rate.
 

cjones6108

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OK, so it took me a bit to switch the machine over for threading, as that had not actually come up yet. I put the 30 gear in the gearbox input, as it seems all threading is done this way, and the 70 gear in the N position. Chucked up a scrap piece of rebar and turned it round, then tried the threading. My best guess was to put the gearbox in C-I, so ran a few passes and checked it with the thread gauge: It's right on 28tpi! Exactly what I need. Thanks for the suggestion.

My plan now is to see what I get with the other eight gearbox positions. The documentation is just totally deficient on this, which shouldn't come as any big surprise. Damn thing is written in friggin' Pidgin English with many many errors in spelling and grammar.

My recommendation, now that nobody's asked for it, if you ever have the opportunity to buy (or otherwise receive) a Bolton machine, PASS!! Wasn't going to be my choice either, long story how I got there, but I hope, never again. I have a Precision Matthews milling machine, which I have been very happy with.
 

P. Waller

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#8
A quick change gearbox changes the gears for you, in over 30 years of running lathes I have never once used a machine with change gears aside from the one metric to inch gear, I am surprised that any company even makes such a machine in the year 2018.
 

Creativechipper

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#9
Good on you, it's always great when it works out like that did for you.

I have a bunch of metal gears that came with my new lathe, 2018 model..lol

I have never done threading but intend to soon.
 

cjones6108

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So I spent most of the afternoon fooling with the threading to see if I could figure out what all the (undocumented) threads my machine would do. Didn't actually get many of them done, but the ones I did tended to come out kind of odd.

First problem, my piece of scrap rebar I was using was too flexible, and I couldn't get it steady enough, even with a follow-rest. Finally gave up on that and just worked down closer to the chuck.

Next, since I didn't know exactly what thread I was cutting at a given gearbox setting, I was careful to engage the half-nut at exactly the same spot on the thread dial every time. This worked OK some of the time, but on one in particular, which I was estimating should be somewhere close to 9tpi +/-, even when I engaged at exactly the right spot, I ended up with multiple thread starts. Might be great, if that's what you wanted, but it wasn't. My conclusion was that the thread in question was so odd that it wouldn't synchronize with the leadscrew no matter what. Started that one again, did just a single scratch and measured it: 8 threads per 31/32"; works out to 8.26tpi ... yes, I can see how synching with an 8tpi leadscrew might be a challenge there.

My 28tpi is actually more like 27-1/2 - 27=3/4, thereabouts. Probably close enough for the 3/8" or so length I need on the end of my stud. The rest I can make whatever I want. Other threads I cut today measured out at 41 and 33 tpi. Yeah, a bit off the beaten path.

So tomorrow I'm going to have another go at this, and I'm thinking I'll try it "metric" style; that is, not disengaging the half-nut. That should eliminate the spurious thread starts.
 
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