Pm-1340gt Vs Pm-1440gt

[dogma]

I have been researching my first, possibly last, lathe purchase for a couple of months. I thought I had settled on a 1340GT and contacted Matt @ QMT last week. It was a fantastic pre-sales experience and Matt pointed me at this forum to research VFD conversion options. This led to the discovery of @mksj's proximity sensor setup and that we live in close proximity. @mksj was extremely generous and invited me over to view his setup yesterday (and endured a couple of hours of my naive questions -- for which I am extremely grateful). As I was leaving, Mark mentioned that the PM-1440GT was shipping soon... After viewing the 1440GT spec sheet, this set me to wondering if I need to better define my lathe requirements. I've made a simple spreadsheet attempting to compare the the two machines "by the numbers".

https://docs.google.com/spreadsheets/d/15LhsoYRhdKQnq4hONc795D3LZFY7TPZd-HZoRTGIYiM/edit?usp=sharing

The 1440GT has 34% mass and 50% more power in approximately the same size envelope for a 35-40% cost premium. However, there are some trade offs. The bore is larger but the distance through the headstock is also 1.75" longer. The 1340GT can cut a larger range of imperial threads and smaller imperial metric threads. The 1440GT has much faster feed rates and considerably and a whopping 2.25" greater clearance over the cross slide.

The extra weight, and hopefully rigidity, of the 1440GT sound attractive to me but I am firmly in the unknown-unknown phase of learning to run a lathe. I'm sure that either machine would exceed my needs but this is purchase that I may live with for decades -- I'd like to make the best choice among the available options. I don't have well defined, or really any, requirements for threading capability. Either machine is within my budget but going with the 1440GT would mean deferring all but very basic tooling for a month or two.

I am left trying to answer several questions:

• What range of threads is reasonable for a generic usage lathe?
  
• Is there any common use case for cutting a TPI > 56?
• What sort of job in a machines of this size class would use 3HP?
• Is it reasonable to assume that the 1440GT is more rigid under load?
  
• What would an example of a job that needs a 2" bore?

Any guidance would be greatly appreciated!

 

[Dan_S]

I am left trying to answer several questions:

• What range of threads is reasonable for a generic usage lathe?
  
• Is there any common use case for cutting a TPI > 56?
• What sort of job in a machines of this size class would use 3HP?
• Is it reasonable to assume that the 1440GT is more rigid under load?
  
• What would an example of a job that needs a 2" bore?

Any guidance would be greatly appreciated!

my opinion/thoughts are as follows:

• 4-56 TPI & .4-7mm are both more than enough to cover just about everything you will ever want to make. 4 tpi at 52rpm is still scary fast for threading. 56TPI is breakout out the magnifier territory. Above 40 tpi i just use a die.
• 3hp, is more about being able to take big cuts, than the actual size of the work piece. Lets say you need to take a 1" bar and turn a section of it down to 3/8". A 3Hp machine will let you do that in 3 or 4 cuts. On my 3/4 hp machine it would be more like 7 or 8 cuts.
• Generally larger/heavier machines are more rigid. However, to be 100% sure it pays to look at the size/configuration of the spindle bearings and the thickness of the spindle walls.
• A bigger bore is almost always better as long as spindle rigidity hasn't be sacrificed to get it. For example if you need to face a 1 foot long 2" bar to length on the 1340 you need to break out the steady rest, while you can just chuck it up through the head stock on the 1440.

 

[dogma]

3hp, is more about being able to take big cuts, than the actual size of the work piece. Lets say you need to take a 1" bar and turn a section of it down to 3/8". A 3Hp machine will let you do that in 3 or 4 cuts. On my 3/4 hp machine it would be more like 7 or 8 cuts.

Whoa - 0.2" cuts blow my mind! One day-dream project of mine would involve turning down a ~6" section of 1.75" 309/310 stainless to 0.75" and might evolve into a small production run of ~100 parts.

 

[tmarks11]

The extra weight, and hopefully rigidity, of the 1440GT sound attractive to me...

Just to make your decision tougher: Matt is working on getting a one piece cast iron base made the the 1340GT, which would be a HUGE! improvement in weight and rigidity.

So you might ask him about the ETA of that option.

http://www.hobby-machinist.com/threads/arrrrrgg.45537/#post-388535

 

[Dan_S]

Whoa - 0.2" cuts blow my mind! One day-dream project of mine would involve turning down a ~6" section of 1.75" 309/310 stainless to 0.75" and might evolve into a small production run of ~100 parts.

Keep in mind though, a .2" doc on a 6" bar requires a lot more power than a .2" doc on a 2" bar, because the MRR is much greater. MRR is really what what power buys you.

 

[Chipper5783]

Get the 14". Waiting a few months for certain tooling on a machine you'll keep for the rest of your life - not really an issue.

The 14" has a more useful slow speed (52 rpm is still too fast for many operations). Dan is absolutely correct. I have threaded 3 tpi at 45 rpm - scary stuff (I popped the drive key). One of my "some day" projects is to change my driven pulley - since I almost never use the top speeds. My "new" - to me, little lathe goes down to 30 rpm - it is awsome.

I have not used a variable speed lathe, but I can imagine that being able to drop another 50% of the speed would be very nice (there should still be pretty good torque, the gearing is in your favor).

The 13" (as well as being too fast at the bottom end) isn't really fast enough at the top end. I wouldn't use the 14" machinie's 2200 rpm very often, but something faster than 1600 rpm is occasionally nice to have.

Threading range? Don't sweat it. The range of 8 - 40 will cover 99.9% of conventional threading. Pretty well any lathe will cut whatever other thread you want - just change the end gears (and there will be a chart in the manual). I had to cut 3 tpi on my lathe, but my change box only went down to 4 tpi. The manual didn't have a chart - so I had one gear made to double the speed into the threading box (I set the box at "6" tpi to = 3 tpi).

Metric threading on an Imperial lathe. The data sheet is not giving you the whole story. Sure, there is a lever that flips to "Metric" and a chart that gives you the advance per rev - but you will find there are very few useful metric threads. Think about it, the box is designed to split a bunch of Imperial threads - just adding a 127 tooth gear is not going to magically give you a nice tidy range of metric threads on an Imperial lead screw. In order to get a decent selection of Metric threads using an Imperial box & lead screw requires the 127 tooth wheel and probably 2 additional end gearing ratios.

What sort of jobs to use 3 HP? I my world - less than 0.1%. I have a 3 & 5 HP lathe, and I almost always run it on the 3 HP setting, because it is quiter and starts easier - it has my 3 favorite speeds. I have maybe popped the overloads once or twice in 30+ years: A. I am not in that big a hurry. B. To use full power, you have to have a seriously solid set up - most work pieces are just not that solid.

The longer cross and compound slides are nice.
Some of the tooling on the 14" will be a little more expensive: D1-5 is bigger and not as common second hand. You should consider going up a size in the QCTP (the smaller one will work, but there are advantages to using one as large as the lathe can comfortably handle - don't get too crazy).

Sure the 14" will be more rigid, but the 13" will be fine. Occasionally you'll need to take one more pass.

The 2" spindle bore? Unless you have a specific job, I would put that as a nice to have. I have 1-1/2" spindle bore and it is fine - either there is lots of room, or it is way to small. I suggest it will be rare that having that extra half inch will be a game changer.

If you can swing the 14" - go for it.

 

[wrmiller]

Some folks need/want as big a machine as they can get. Some don't. I fall into the latter category. I have a 1340GT, modified with a VFD and it is plenty for me. Actually, I don't need a lathe this big, but went for it to get the step up in quality. On my 1340 I can spin over 2000 rpm or down under 100. I don't thread really coarse threads, so really low speeds are not something I need. The 1340GT has a two-step pulley on the motor and I spend 99% of my time in the high range. I only thread in inch and love the range of the Norton gearbox. If I ever do need to thread in metric I'll buy a tap or die.

I am waiting for Matt to get the cast iron stands in for the 1340GT, I will upgrade to those (Matt is saying a few months) and then I will be done with the modifications to this thing. If you have larger projects that require a bigger lathe, go for the 14. I would have considered it had it been available at the time I bought the 13. Probably.

 

[dave2176]

PM has some great machines. If I were looking, things that would be big include bed width as in 10" and longitudinal feed down to .001". The gears that give you 56 tpi are the same ones that limit the feed to .0026" although change gears probably would get you much finer. For example, my machine can do 112 tpi and a .0011" feed according to published info but change gears can get it to 288 tpi and closer to .0003" feed. The point being that there is ability in both those machines that is not typically published. Yes I love my D1-5, just seems more manly having 6 cams instead of 3 but I'm not sure it adds up to anything. Just recently I was wishing I had a 2" bore. I think I would love either of them. One thing to consider is that tooling will cost more than the machine by time you are through.
Enjoy the process. Dave

 

[dogma]

Get the 14". Waiting a few months for certain tooling on a machine you'll keep for the rest of your life - not really an issue.

I completely agree!

I have not used a variable speed lathe, but I can imagine that being able to drop another 50% of the speed would be very nice (there should still be pretty good torque, the gearing is in your favor).

The 13" (as well as being too fast at the bottom end) isn't really fast enough at the top end. I wouldn't use the 14" machinie's 2200 rpm very often, but something faster than 1600 rpm is occasionally nice to have.

Neither have I but which ever way I go, I'm planning to do a VFD retrofit immediately. One thing I just discovered is that 2k+ speeds require a steel chuck.

Thank you for your guidance.

 

[dogma]

If I were looking, things that would be big include bed width as in 10" and longitudinal feed down to .001".

What is the advantage of the a wider bed? Less deflection of the carriage? .001" seems like a very fine feed. Is that useful for a fine cleanup pass?