View Full Version : can I slow down a drill press??

02-07-2012, 07:29 PM
OK, for those that read the roll pin in the blind hole saga, here is my next question about the drill press. I want to slow it down a little. Its a Craftsman 15" floor model, 12 speed, 250-3100. I would like to get the low speed down to around 100-150 ish. There are three step pulleys. I think the model number is 113.213150, the book isnt here right now.
I used a caliper to measure the pulleys the best I could. Here are the measurements: Motor, 1.700 2.050 2.850 3.400
Idler, 1.780 2.480 3.230 3.580
Spindle, 3.800 4.125 4.800 5.460

Now, these are the outside diameters, Im not sure where to measure to obtain figures for figuring speeds. I have room to go to a larger step pulley for the motor and idler, but not on the spindle. Can someone explain how I should go about determining what pulleys I would need to hit my target range? Thanks again

02-07-2012, 07:33 PM
Maybe something like this?


02-07-2012, 07:55 PM
The pulleys represent a ratio, hence, it doesn't matter if you can accurately measure the pitch diameter. You just need to measure at the same point on each pulley. From there, you multiply your motor speed by the ratio. For example, 1750rpm motor x 4/5 (4" pulley diameter on motor, 5" pulley diameter on spindle) = 1400rpm. Another way to do it is to use a tachometer on the spindle itself. Record each rpm (or look it up in the speed chart on the drill) for each pulley selection. Spindle rpm / motor rpm = pulley ratio (as a decimal--not a fraction).

Hope that makes sense. Is it possible to add a third pulley between the motor and the spindle? That is likely the most versatile answer to your issue. It will allow slightly larger speed ranges. A number of drills in the past have offered "add-on" multi-speed attachments that amounted to such. They usually mounted in the hollow of the vertical post.

02-07-2012, 08:35 PM
I bought an electronic tach that uses the little pc of tape & a laser. well I bought 2, a larger H-F one for $20 on sale & a pocket size under $10 on ebay. both work well.
Also H-f sell a router speed control that should slow it down to a stop but I don't think it is good to run the motor real slow $20-coupon =15.99 not on sale. It might work well for tapping. Damn-it I could have saved the $35 I have in the mod 21/2HD 3HP + coolant 2500# semi-auto Chas Allen drill press tapping machine. Oh-Well;)

02-07-2012, 08:55 PM
Check the rpm of the motor- if it's currently 3450 or thereabouts, you could swap it for a 1725 rpm. That will put your low speed at about 125 rpm. Of course it will also drop your highest speed in half.

If you currently have the slower speed motor, you're faced with adding some kind of motor speed control, possibly killing the power, or rigging up another reduction pulley set, which will also rob some power and probably be a royal pain to add to the existing drill press.

If you did this, you would leave the spindle pulley alone, remove the motor pulley and mount it on an idler shaft, then make the height of the idler pulley to put the largest diameter one level above the smallest spindle pulley. You then add one pulley above the largest idler pulley, probably a smaller one, then mount a two step pulley on the motor, with the larger one above. That gives you a two speed swap from the motor, and leaves you with one less ratio change on the original pulleys. You lose the highest speed, but your next slower speed is down by only about a third or so.

As far as matching the belt lengths to the placement of the idler shaft, now might be the time to bite the bullet and go with a link belt system.

02-07-2012, 09:02 PM
He said it was 3 pulley. Most of these cheaper drill presses run way too fast to drill larger holes in steel. I see drill presses advertised that say they have a 1" or 1 1/4" capacity in steel. But,they run entirely too fast to not just burn up the bit.

You could put a 3 phase motor on it and get a small VFD for about $100.00.

Bob Fisher
02-07-2012, 09:12 PM
I don't think a router speed control will work on a drill press unless it has a universal type motor,most unusual in my experience. Even then, you would probably see a loss of torque. A mechanical reduction is most likely your best bet.Bob.

02-07-2012, 09:20 PM
It has 3 step pulleys, motor, idler, spindle. I have room to add larger step pulleys to the idler and motor, but not on the spindle. Thanks guys

Lew Hartswick
02-07-2012, 10:01 PM
You want SMALLER on the motor and LARGER on the idler.

The Artful Bodger
02-07-2012, 10:11 PM
One trick you can do to get a slight increase in V belt ratio is to use a smaller section belt on the same pulleys. The smaller section belt will run lower in the pulleys and the significance of this will be greater on the smaller pulley. So in the drill press application where the belts are reducing the speed the greater ratio will slow the output, but not by much!:rolleyes:

J. R. Williams
02-07-2012, 10:16 PM
I had the same unit many years ago and it had the same problem. I replaced the motor with a 1200 rpm motor and then it was a good drill press. Today I would install a three phase motor and a VFD.

02-07-2012, 11:00 PM
Mount another motor next to your original one, put a small pulley on the new motor in line with the big pulley on your original motor, throw on a belt and your good to go, just be careful you will have a lot more torque.

02-07-2012, 11:16 PM
What happens when you run the v-belt from the motor on the shaft to the largest pulley on the idler? Apply tape as needed if you're keyed.

Paul Alciatore
02-08-2012, 12:28 AM
The pulleys represent a ratio, hence, it doesn't matter if you can accurately measure the pitch diameter. You just need to measure at the same point on each pulley. From there, you multiply your motor speed by the ratio. For example, 1750rpm motor x 4/5 (4" pulley diameter on motor, 5" pulley diameter on spindle) = 1400rpm. ....

Arthur, this is not really true. It is true that the pulleys work via a ratio, but that ratio would change from minor diameter to pitch diameter to major diameter because you would be changing diameters by the SAME amount on each pulley, not by a proportional amount as a true ratio would require.

Consider a system with 1" and 2" pulleys for a 2:1 step down. These dimensions are for the effective pitch diameter. Now I know that the pitch diameter of a belt is not exactly at the half way point between the inside and outside surfaces of the belt and I am not making reference to any particular size of belt, but lets assume, for simplicity, that these pulleys have a major diameter that is 0.4" greater than the pitch diameter and a minor diameter that is 0.4" less. Here are the three, DIFFERENT ratios calculated by using these figures:

At the Minor Diameter:
Small pulley minor diameter = 1" - 0.4" = 0.6"
Large pulley minor diameter = 2" - 0.4" = 1.6"
Ratio = 1.6" / 0.6" = 2.666

At the Pitch Diameter:
Small pulley pitch diameter = 1"
Large pulley pitch diameter = 2"
Ratio = 2" / 1" = 2

At the Major Diameter:
Small pulley major diameter = 1" + 0.4" = 1.4"
Large pulley minor diameter = 2" + 0.4" = 2.4"
Ratio = 2.4" / 1.4" = 1.714

This is a range of ratios from 33.3% higher than the pitch ratio to 28.6% lower. That is not an insignificant deviation.

For accurate speed calculations, you MUST use the proper pitch diameters of the pulley/belt combination. As I said above, the pitch diameter of a Vee belt is not necessarily at the 50% point: it is generally at a higher point due to the construction of the belt. So do consult the manufacturer's specs. for the proper point for the particular belt you are using.

Finding this pitch diameter may not be particularly easy for imported machines, but super precision is not really needed. From a practical point of view, find a point where the ratios work out to those shown in the speed chart and work from there.

As for the OP's question, he could look for different pulleys that will fit. I also have an older, bench top drill press with three step pulleys and I have done some research on different pulleys, perhaps converting it to a five step set. It is not easy to find any. I have also considered adding single pulleys on top of the existing ones for a four step assembly. Although neither shaft extends far enough for this, they could be fastened to the existing pulleys with several bolts and perhaps two locating pins. This would probably require a new pulley cover.

02-08-2012, 12:39 AM
Your motor now is most likley 1725 RPM ...if you search around you may be able to find a 1/2 HP 1200 RPM motor that's designed for 120 VAC single phase. That's what I did with my belt drive Clausing 15 " drill press yeas ago. Mine only has two pulleys but at least they are five step. The lowest speed now is 260 RPM.
Joe B

02-08-2012, 12:59 AM
The OP has three pulleys on his Craftsman drill press. I has a 1959 Craftsman that came with only two pulleys. They made an accessory kit that added a third pulley to get the speed down to about 250 rpm from 600 or so on the lowest speed with a 1,750 rpm motor.

I missed two of the accessory pulleys on eBay so I decided to build my own. I did that a couple of years ago and it works nicely. If I ever want to go slower, I would goto a three phase motor with a VFD as GWilson mentioned. That would be more than I would care to spend since a motor would be $100 or so with shipping plus the $100 mentioned for a VFD. 250 rpm is plenty slow for the use I give the drill press. I have my Webb Mill for anything slower.

02-08-2012, 07:40 AM
Howsa bout an AC motorspeed controler? Make sure you get one that can handle the amperage of your drill.

They are on ebay, here is an example 15A speed controller that is not mine, just an example


02-08-2012, 07:45 AM
Consider that one of the biggest benifits of low RPM is higher torque, ensure you can have LOTS of tension to transmit that torque, and motor speed controllers are not the way to get higher torque.

My 2hp gearhead mill, in 100~200rpm mode.. Man, It just EATS steel with 1" drills like nobodys business.

02-08-2012, 08:26 AM
Thank you for all the information guys.....going to have to put some thought into this to see just how carried away I want to get with this cheap drill press. I keep my eyes open for larger heavy duty ones but haven't found one yet.

02-08-2012, 08:30 AM
Thank you for all the information guys.....going to have to put some thought into this to see just how carried away I want to get with this cheap drill press. I keep my eyes open for larger heavy duty ones but haven't found one yet.

Heavyer duty eh?


Heres mine :)

Best value (new) for the size/capacity IMO! Any larger is thousands more, and generaly won't have any more travel, just more rigidity.

02-08-2012, 10:30 AM
monkers, I looked at photo's of your drill press on some web sites and it looks a lot like the Central Tool drill press I have. The easiest way to reduce the rpm is to put a smaller step pulley on the motor. You will have to change the belt lengths when you do that.

You may also be able to change the size of the intermediate pulley to get it even slower.

My drill press has 3 - 4 step pulleys with a speed range of 155 to 4200 rpm. I would prefer it have a low speed of 50 rpm and a lower top end rpm. I have seldom used over 1000 rpm.

02-08-2012, 12:12 PM
Ggerg 1168
The link you have shown is for a brush motor not for an induction motor.
It would be no good for monkers motor.