# Thread: Any validity to this pulley ratio to motor speed conjecture

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## Any validity to this pulley ratio to motor speed conjecture

Hi,

I am restoring an old drill press - interested in an estimate of the slowest speed config with pully - belt setup. Wondering if this logic makes any sense - motor speed is 1750rpm - using the diameter ratios of motor pully to drill pulley - is the estimate of drill bit speed approximated reasonably? i.e. motor pulley 2.75", drill pulley 6.5" - ratio of .42 - therefore drill speed .42 X 1750rpm ~ 735 rpm?

thx

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## Re: Any validity to this pulley ratio to motor speed conjecture

You have to convert the diameter to circumference first then do your ratio. And they need to be the theoretical diameter numbers not the actual OD's of the pulleys

2.75 in = 8.63 inches
6.5 inches = 20.42 inches

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## Re: Any validity to this pulley ratio to motor speed conjecture

Thx Ronsii - I thought about the linear travel (circumference) and then did the ratio - it appears to be the same at .42 - figured with desiring an "estimate" only would be OK - ultimately wanted to determine I'd be OK to drill out 1/4" and under thickness carbon steel pieces - on occasion - seems like I will be OK. Thx for your comments.

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## Re: Any validity to this pulley ratio to motor speed conjecture

Yeah, I just always find it easier to use linear/circumference numbers as a lot of times I will matchmark the pulley and belt then rev it an measure the actual travel just to see the real numbers and in the past I have inadvertently made some errors when doing it by radius/diameter

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## Re: Any validity to this pulley ratio to motor speed conjecture

Originally Posted by ronsii
You have to convert the diameter to circumference first then do your ratio. And they need to be the theoretical diameter numbers not the actual OD's of the pulleys

2.75 in = 8.63 inches
6.5 inches = 20.42 inches
You don't have to convert to circumference.

If you want "exact" numbers, you have to know the diameter of the pulleys "where the belt rides". Once you know that it's just diameter ratio, just like the OP calculated.
Last edited by John Bartley; 10-17-2020 at 09:27 AM.

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## Re: Any validity to this pulley ratio to motor speed conjecture

Originally Posted by John Bartley
You don't have to convert to circumference.

If you want "exact" numbers, you have to know the diameter of the pulleys "where the belt rides". Once you know that it's just diameter ratio, just like the OP calculated.
Yes, but that's why I mark/measure to get length... I suppose from there I could convert back to diameter but I still find working with length numbers safer

Although sometimes I just cheat and use an optical tach to see what's going on

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## Re: Any validity to this pulley ratio to motor speed conjecture

Originally Posted by John Bartley
You don't have to convert to circumference.

If you want "exact" numbers, you have to know the diameter of the pulleys "where the belt rides". Once you know that it's just diameter ratio, just like the OP calculated.
"Where the belt rides" is known as pitch diameter ie: the radius measured from shaft center thru half the thickness of the belt which of course is smaller than pulley OD. In the case of an adjustable pitch pulley the pitch diameter is infinite from minimum to maximum.

8. ## Re: Any validity to this pulley ratio to motor speed conjecture

Originally Posted by John Bartley
You don't have to convert to circumference.

If you want "exact" numbers, you have to know the diameter of the pulleys "where the belt rides". Once you know that it's just diameter ratio, just like the OP calculated.
Correct. Circumference is proportional to the diameter, which is why it makes no calculational consequence. In the end the pi's "cancel out" anyways when the division is carried out, leading to the same 0.42 number.

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## Re: Any validity to this pulley ratio to motor speed conjecture

Just guessing...translating to circumference might make it easier to relate cutting distance, since the cutting distance changes as the drill not diameter changes for a given rpm. Just thought about that after ronsiis post.

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## Re: Any validity to this pulley ratio to motor speed conjecture

Screw it. Get a VDF!

Now, in all seriousness. Your calculations are ok and Oscar's clafirication is spot on.

What drill press are we talking about that gives you a +700rpm's slowest speed? Waaaaaaaaaay to high for metalworking (in any reasonable drill diameter).

Mikel

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## Re: Any validity to this pulley ratio to motor speed conjecture

Originally Posted by Mikel_24
Screw it. Get a VDF!

Now, in all seriousness. Your calculations are ok and Oscar's clafirication is spot on.

What drill press are we talking about that gives you a +700rpm's slowest speed? Waaaaaaaaaay to high for metalworking (in any reasonable drill diameter).

Mikel
VFD !!! 3 phase baby !!! Infinite !!!

You'll still be using the pulleys even with a variable speed VFD. It is still going to need the ratio changed for different jobs.
Last edited by danielplace; 10-20-2020 at 10:51 AM.

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## Re: Any validity to this pulley ratio to motor speed conjecture

Originally Posted by danielplace
VFD !!! 3 phase baby !!! Infinite !!!

You'll still be using the pulleys even with a variable speed VFD. It is still going to need the ratio changed for different jobs.
The motor needs to be 3PH, sure. But you can get 1PH input / 3PH output VDF's which are life savers for those wanting to use industrial equipment from a residential outlet.

They also allow you to vary the speed of the motor withouth modifying the ratio in the pulleys (also it will help to do so, obviously).

This reminds me that I need to buy one and rebuild my belt grinder with a 3PH motor (already have it) and a VDF... when I do any kind of wood work the fixed speed is too much and I end up burning it in spots.

Mikel

13. ## Re: Any validity to this pulley ratio to motor speed conjecture

Originally Posted by danielplace
VFD !!! 3 phase baby !!! Infinite !!!

You'll still be using the pulleys even with a variable speed VFD. It is still going to need the ratio changed for different jobs.
There's a practical limit to the range you can run a motor on a VFD and you will lose horsepower as you slow it down.

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## Re: Any validity to this pulley ratio to motor speed conjecture

Originally Posted by Mikel_24
Screw it. Get a VDF!

Now, in all seriousness. Your calculations are ok and Oscar's clafirication is spot on.

What drill press are we talking about that gives you a +700rpm's slowest speed? Waaaaaaaaaay to high for metalworking (in any reasonable drill diameter).

Mikel
Mikel - interesting - I've been researching "on the line" - and since "on the line info" is always accurate - - I find a number of charts citing rpm speeds above 700rpm as "OK" for mild steel, smaller hole diameters and thinner plate - I'm not intending on drilling out a 2" hole in a 1" plate or anything - but likely will run out back and just have at it and see how it goes! (max diameter hole I'd likely ever deal with is about 1/2" on 1/8" or so mild steel - and be piloting first most likely....most 1/8" or 3/16" needs)

but maybe I look into a VFD to hook into nonetheless! This is apparently a 1980 or 81 Delta floor drill press I have been reconditioning.....wouldn't mind improving the setup with proper planning

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## Re: Any validity to this pulley ratio to motor speed conjecture

Originally Posted by jfk92
...- I find a number of charts citing rpm speeds above 700rpm as "OK" for mild steel, smaller hole diameters and thinner plate...
While I don't know how metal thickness has any effect on the recommended drilling speeds (It doesn't have anything to do with surface speed) I have to agree with you. The smaller the drill diameter and softer the material, the faster you can run it.

However there will be a time when you would like to run hole saws... and you will miss not being able to go below 150 rpms!

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