By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by Allendale Jan 24th

Pillar drill motor size?

All Topics | Latest Posts

Search for:  in Thread Title in  
Twf18/05/2021 09:08:09
20 forum posts

I have a Boxford pillar drill which was originally fitted with a 0.5hp 3ph motor, but was converted to a single phase motor some years ago.

The motor currently fitted to it is a 0.5hp AEI, however at some point it appears to have been dropped and the spindle is slightly bent causing the pulley to wobble slightly when running, and resulting belt debris.

I have just acquired a very old but unused Brook Crompton motor of exactly the same dimensions and spindle size to replace the bent motor, however I have just realised that this new replacement is only 0.33hp, not 0.5hp as was originally fitted. Is this lower powered motor likely to cause me any problems in use (I drill mainly stainless steel plate up to 12mm diameter holes at slow speed using the drill back gear)

I assumed the Brook crompton motor being physically the same size or actually a little larger than the AEI was the same hp rating!


Circlip18/05/2021 09:31:25
1427 forum posts

Question worth asking, are you sure the shaft is bent? Seen so many good models let down with the flywheel exhibiting a distinct wobble. Worth checking the shaft with a "Clock".

Regards Ian.

noel shelley18/05/2021 09:56:21
1021 forum posts
19 photos

The main thing is what size holes in 12mm stainless ! I would say above 6mm hole you would be better to go to .75hp 1ph. Ian has a good point. Noel.

Circlip18/05/2021 10:02:34
1427 forum posts

12DIA holes Noel


SillyOldDuffer18/05/2021 10:26:00
7921 forum posts
1725 photos

Difficult to say how much the difference will be noticed because so much depends on how the tool is used, but drilling 12mm holes in stainless suggests bigger would be better.

Drilling mild-steel, a .33HP motor will take about 1½ times longer to remove the same amount of metal as 0.5HP. In theory 0.33HP will just take a bit longer to do the same job, which if often not a problem.

However, the effect of reduced power on Stainless steel is trickier to predict because many stainless alloys work-harden. Misery ensues when a tool fails to cut stainless due lack of power or torque! In consequence more skill is needed to cut work-hardening metal with a tiny motor than a big one, because big motors can power through minor problems like a slightly blunt drill or wrong rpm or feed-rate.

Discussing the power output of motors, it's useful to understand the power claimed on the motor plate isn't an absolute limit. Idling electric motors deliver much less than their rating, whilst heavily loaded electric motors can deliver much more than the plate rating. But not for long - the windings overheat and out comes magic smoke!

Thus the physical size of electric motors is a weak indicator of power output. Old motors are physically bigger than modern ones because the enamel used to insulate golden age magnet wire is much less heat proof than modern enamels. And to prevent micro-holes in the insulation, older enamel was applied thickly which reduces the motor's efficiency. Old motors have to be physically big to keep cool, whereas modern equivalents run considerably hotter and 10 to 20% more efficiently. Duty cycle is also a major influence on physical size: a motor delivering 1HP but only run for 30 minutes in every hour can be made much smaller than a motor designed to output 1HP continually.

My guess is the Boxford's original motor was rated for continuos hard work in a busy professional workshop. In comparison most home workshops are a holiday camp! There's a reasonable prospect a 0.33HP motor can occasionally be flogged hard enough in a home workshop to replace a 0.5HP unit, but bear in mind the drill's original 0.5HP motor was probably called on in the same way to deliver more power in bursts. A small motor isn't as capable as a big one.

Personally I'm nervous of re-using ancient electric motors of whatever power. Age, overheating and damp are all unkind to electrical insulation and the metalwork inside may be corroded too. Single-phase motors are extra vulnerable because their capacitors and centrifugal switches also degrade over time. As few of us have the facilities to do a proper insulation test there's much to be said in favour of upgrading to a new 3-phase motor and a VFD. 3-phase motors have better torque performance than single-phase motors too. No one regrets replacing single phase motors!


Nigel McBurney 118/05/2021 11:02:01
965 forum posts
3 photos

I/2 hp single phase seems to be the standard for half inch capacity pillar drills ,with a standard low spindle speed of around 500 rpm, As your machine has back gear and your using the back gear low speed range,I would try the 1/3 hp motor. Set the belt a little slack as if you stall a s/phase motor it will burn out very quickly as I once found out.

bernard towers18/05/2021 14:28:37
400 forum posts
102 photos

Drilling holes is about how sharp your drill just as much as horsepower and for stainless how slow can you get down to.?

bernard towers18/05/2021 14:28:37
400 forum posts
102 photos

Drilling holes is about how sharp your drill just as much as horsepower and for stainless how slow can you get down to.?

Twf18/05/2021 19:22:04
20 forum posts

Thankyou for the replies

The spindle is definitely bent on the 0.5hp motor - I removed the armature and set it up in lathe and it was over 1mm out, this combined with the Boxford’s very tall pulley causes a real wobble. With the motor on the bench and plugged in it really vibrates hence me swapping it out.

I have had to replace the capacitor in the 0.33 Brooks motor however once fitted, it runs true and is nice and smooth. I will give it a go and see how it fares - it hasn’t cost me anything other than £10 for the new capacitor.

The Boxford does 82rpm in lowest gear which is handy for drilling stainless.

Dave Wootton18/05/2021 20:01:10
234 forum posts
56 photos


I've got a Fobco Star drill 1/2" capacity, just looked at the motor plate and it has only got a 1/3 HP motor (which I know is the original fitted by the makers). I've had it for years and never noticed any lack of power. So unless you are some kind of drill demon I would think 1/3 HP motor would be fine on yours.


old mart18/05/2021 20:10:49
3524 forum posts
217 photos

You are lucky to have the backgear, that will more than make up for the slightly smaller motor.

All Topics | Latest Posts

Please login to post a reply.

Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Support Our Partners
rapid Direct
Eccentric July 5 2018
JD Metals
Eccentric Engineering
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest