# Motor Size for 18" lathe



## Duncan K (Jan 19, 2020)

I picked up an 18" McDougall lathe a while back with the intentions of cleaning it up and getting it useable again. Right now it is powered by a 1 HP motor and a 4 speed Dominion gear box which isn't even enough power to spin the cuck without some help to get it started. My plan is to switch to a 3 phase motor and VFD. I am assuming the limiting factor to how big of a motor i can run would be the belt slipping? Looking for some opinions on motor size for this lathe. I have a 5 HP right now, but also found a good deal on a 7.5 and 10 HP. I don't want to waste money buying a bigger motor if it's just going to slip the belt.


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## Janger (Jan 19, 2020)

Cool lathe.


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## Brent H (Jan 19, 2020)

Does the Dominion Gear box have any information on it - It may say what horse power it is rated for.  A 1 Hp on an 18" lathe is pretty funny.....We have a 13" here with 10 Hp - But it is a variable speed drive and not a geared drive head stock.  That makes a difference in what actually gets to your work with respect to torque.

I have to review some things for a better answer, but, I have my mill and lathe running 3 phase off a VFD but I only use the VFD for the 3 phase conversion (except the mill is running 50 Hz as per motor spec).  The design of the machines is to use the mechanical advantage of the pulleys and the gears to run properly.  Some speed difference using the VFD would be fine but I would select the closest gearing or pulley speed first and then "tweak" with the VFD if required.  The other functions of the VFD which I find way better than the speed control is the soft start and dynamic braking.


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## Brent H (Jan 19, 2020)

Doing a little searching a guy was driving a 16" with 3 Hp.  A 5 seems like it would be ok.    Other "new" 18" lathes seem to be around 10 Hp - So, I would throw on the 5 Hp and see how it performs.

If you have a big shaft or something you can turn between centers to represent your "largest"  lathe project - get er going and take some ampere readings and check the motor temp.  If the Amp draw is reasonable and the motor stays cool...You have found your motor


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## Duncan K (Jan 19, 2020)

My plan is to get rid of the gear box and run directly belt driven off the motor. I’ll make the pulley off the motor the same size as the the small step on spindle and use the VFD to control the speed and display the RPM. I will probably try out the 5 HP but buy a VFD rated for 10 in case I need to go bigger.


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## Brent H (Jan 19, 2020)

Good plan - what is the gear box ratio?


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## Duncan K (Jan 19, 2020)

It’s a four speed gearbox, I don’t remember the ratios. It’s the same as the one seen here

https://www.hobby-machinist.com/thr...earbox-dominion-oil-change-and-running.73797/


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## Johnwa (Jan 19, 2020)

There’s a short write up on http://www.lathes.co.uk/mcdougall/
Not much technical info but the author is looking for more.


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## YYCHM (Jan 19, 2020)

18"?  As in swing?  What's the bed length?


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## trlvn (Jan 20, 2020)

@Duncan K would you consider contributing some photos to the Vintage Machinery website?  They have some submissions of R. McDougall lathes but nothing with an 18 inch swing.

http://www.vintagemachinery.org/mfgindex/detail.aspx?id=370&tab=4

Craig


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## Duncan K (Jan 20, 2020)

YYCHobbyMachinist said:


> 18"?  As in swing?  What's the bed length?



Yes 18" swing, bed length is about 6 feet between centers give or take. Over all the lathe is 11 feet long


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## Dabbler (Jan 22, 2020)

3hp 3phase should be a great start.  5hp can be transmitted on flat belts 4" wide is also just fine.


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