View Full Version : Gearing ? related to mill spindle speed.

09-14-2006, 09:50 PM
I am just about done cnc'ing my X3 mill and I would like to increase the max spindle speed as well. I see a fairly easy way to swap the belt driven pulleys on the primary side of this mill for a quick boost in spindle speed. But I just can't seem to figure out how much it would actually increase.

This is the the gearing information I can provide and hopefully somebody better at math can help me.

The motor is rated 4000rpm.
The motor uses a 16 tooth pulley belted to a 26 tooth pulley connected to a 2 speed gear box.
The gear box has 2 speed ranges and I did not count the teeth but in low speed gives a max spindle speed of 1000rpm and high range is 2000 rpm max.

If I swap the 26 tooth primary pulley to the motor and the 16 tooth to the other shaft to over drive the primary side, can anybody figure out my new max speed ranges in both high and low range?

Thanks for any help!

09-14-2006, 11:10 PM
Take the one which gives 1000rpm first:
First find the gear box ratio
4000*(16/26)*GR=1000 where GR is the gear ratio of the gear box
solve for GR:


Now interchange the two timing pulleys so:

4000*(26/16)*.406=2639 rpm

and the calculation for the other case is the same, just find the other gear box ratio substitute it for .406 in the last formula.
Actually it is simpler than this. Just note the high range is double the low range so double 2639 and get 5278.


09-14-2006, 11:19 PM
Your first ratio is simple 26/16= 1.625:1 reduction.That yields a speed of 2361.5rpm assuming a 4000 rpm motor.If you reverse that you then have a 1.625:1 increase,so assuming the same 4000 rpm motor would be 6500 rpm to the input of the gearbox.

So,if we extend that 1.625:1 increase to the lo/hi max rpm you get 1.625 x 1,000= 1,625 rpm/lo speed and 1.625 x 2,000=3250 rpm/hi speed.The ratio of the gearbox is meaningless.

It's late and I think that's right,unless I missed something:D

Edit: buy a optical tach and see what the lo/hi speeds really are,then do the calculations:D

09-15-2006, 12:11 AM
Sorry weird, I'm with KF...2640rpm low/5280rpm high. You have to use the 1.625 factor twice. It really is getting late though ;)

09-15-2006, 08:31 AM
Thanks for the replies guys. I think I will go ahead with this mod as it will give a much needed rpm boost for my needs.
I'll try and setup some sort of tach as well.:D


09-15-2006, 09:35 AM
I always regard these postings, answering questions, etc. as an educational exercise and as such it should be put in a step by step form which is easy to follow. In this case I could have used an algebraic expression for the gear ratio and sustituted it back in the equation thus camoflaging the actual gear ratio but it would have taken more mental gymnastics to understand so I chose to present it in a two step form which appeared to me to be easier to follow.


09-15-2006, 11:50 AM
I'd be very careful about speeding up a gear head mill. Especially a cheapo import.

Typically the gears in this kind of machinery are not precision. Most likely it's running about the safest max speed now.

I've noticed a bunch of the clone gear head lathes from China don't have as high a top speed as the "parent" they were cloned from. Can you guess why?

09-15-2006, 02:15 PM
It's a Sieg X3 bench mill. I don't believe it is a clone of any other mill really.
I know a few guys have done complete belt drive conversions to this mill and that will be an eventual project. I would think the spindle bearings can handle 5000rpm. I was thinking this would be a temporary way to increase the spindle speed.
But you may be right about the gears at that speed. It would not be good to have one break apart at high rpm that is for sure.


09-15-2006, 03:12 PM
"I've noticed a bunch of the clone gear head lathes from China don't have as high a top speed as the "parent" they were cloned from. Can you guess why?"

Dr-- I don't think it is because the gears (made from a solid piece and small in diameter) are in any danger of flying apart, but rather because it takes a very good *chuck* to stay together under the forces it sees at high RPM. Chucks have jaws that really want to become low earth orbiting objects when you get up around 2000 rpm. A post a while back calculated forces on a typical set of chuck jaws and they are huge. Increase the diameter and they get even bigger. The forces also increase as you grip a larger object, moving them farther from center.

I looked at a Clausing/Colchester downstairs that has a sticker on it warning you that changing chucks from the Burnerd chuck that was supplied may not be safe due to the high spindle speeds the lathe supports.

My chinese import lathe has a top speed of 2000 RPM...and I have no need or desire to use it


John Stevenson
09-15-2006, 06:25 PM
Ken is as close as possible not given all the facts.
Given that the quoted speeds of 1,000 and 2,000 are only approximate you have to use the motor revs [ again given a design licence] of 4,000 rpm
Primary motor drive on these early X3's is 16 to 26 as stated, so that's a 1.625:1 reduction.
low speed in the box is 15 to 30 [ 2.0 :1 ] and high is 23 to 22 [:0.957 :1 ]
Final drive is by a 28 to 34 gear reduction giving 1.214:1

So low is 4,000 / (1.625 x 2.0 x 1.214 ) = 1014 rpm
High is 4,000 / (1.625 x 0.957 x 1.214) = 2119 rpm.

Now if we reverse the 16 and 26 pullies we get a ratio of 0.615 so now putting this in the same formulae we get:-

Low is 4,000 / (0.615 x 2.0 x 1.214 ) = 2,678 rpm
And high is 4,000 / (0.615 x 0.957 x 1.214 ) =5,594 rpm.

The spindle bearings can stand these revs but at this speed noise and heat from the crude gearbox could become a problem.

What is probably a greater problem is the fact that because the top speed have increased so have the lower speeds and the DC controller can't put enough torque out to achieve the lower sppeds needed for a lot of operations.


09-15-2006, 08:49 PM
I looked around a found a couple pictures of what the inside of the box looks like,crude is a good discription.I don't think if I were going to increase the speed of the mill that I would use the original motor and gear train.A larger motor and belt drive would be much better IMHO.The pic I saw showed and open set next to the belt pulley and a closed set in the head with grease lube,not a good setup.

On the otherhand,on my little X2 I went to a belt drive and love it.Lots quieter too.

09-15-2006, 09:39 PM
On the other hand,on my little X2 I went to a belt drive and love it.Lots quieter too.
I built a cnc'd x2 and my own belt drive system for it. I run that thing at 4500 rpm 4 -5 hours at a crack and no issues. I just use a v belt. Should have used a timing belt though.

Well I just had to try it. All I had to do was turn a small spacer /cut a key way to give it a whirl.
I lubed up all the gears real good. In low speed no problem. Plenty of torque and not to noisy. I noticed a bit of extra heat in the spindle/quill after 15 minutes.
I then put it in high range., whoa...now things got interesting. The mill reached a certain point and then began to hum/resonate. It sounded like a model airplane. The gear train really did not get that noisy though.
Most of the "hum" noise was coming from the large sheet metal back enclosure as it vibrated, I could feel a low level high frequency vibration in the entire mill.
The head/quill after 15 minutes was almost to hot to keep your hands on and I ended the experiment.

IMHO the gear train is just not up to the task. The motor stayed at about the same temp as it always does.



John Stevenson
09-16-2006, 07:37 AM
I am in the process of converting an early X3 [ this is a test bed machine ] to belt drive and do away with the gears all together.
I have worked out that two speeds that mimic the overall reduction of the two original speeds will suffice.

I propose to use a Poly Vee belts as these can transmit more HP than normal Vee given the space restraints.

I don't like pure timing belt drive as it's too positive and has no way of slipping if something drastic occurs.

I also aim to experiment with an extra speed on a lighter belt to see if 6,00 to 7,00 is available for engraving work.
With no gears in mesh this will help on the heating effect but only physical tests will answer the question.

It will also mean new bottom and top covers as the belt run is outside of the existing ones.


09-16-2006, 12:05 PM
John, Please keep us updated with your progress.
Are you talking about using Gates 5mm polyflex belts? That is what they use on my import 9x lathe. Looking at the graph printed in SDP/SI booklet it shows a 5m polyflex running at 6000rpm will support around 3hp and exceeds GT2 5mm timing belts in both power and rpm capability.
The real nice thing about using those belts is that they use a non standard 60' wedge angle which makes pulley making very easy. I made a few pulleys for my lathe by plunging a standard 60' carbide insert with excellent results. You can also easily cut larger pulleys with a 60' drill mill. They run very smooth as well. At very low speeds they will slip. So a low speed timing belt might be an option and then 2 higher speeds using polyflex belts.
The spindle can always have a coolant system hooked up for running high speeds.

John Stevenson
09-16-2006, 06:22 PM
Steve, the poly vee is a multi ribbed belt often found as fan belt on cars and washing machines.
A 6 rib J series can handle about 2-1/4 HP so it's well within range and you can get good contact even on small pulleys.

You can plunge in with a vee tool to form the sheaths or cheat like I do :D and use an 11 tpi threading die out of a die box to form the whole lot in one go.



09-16-2006, 07:53 PM
The poly-vee or micro-vee either will work fine.The belt conversion I bought used the Gates 2l crossection (1/4"top width)vee-belts.I didn't think I would like it at first,but it will carry much more power than the mill motor can stand.

I also have used John's method for making the poly-vee sheaves and it does work great.Been meaning to figure the pitch needed to carry the M section belts,but never got around to it.

09-16-2006, 08:33 PM
geez there are so many different belts and similar names.
I have made the multi rib pulleys before for my strip car. 8 rib supercharger pulleys.I figure an easy 75hp was transmitted by the belt to spin the supercharger.
I like your method John. I doubt my little import lathe could plunge that die though!
wierdscience, I am also using the 1/4" 2l belt on my x2 mill. It is nice and smooth.My only complaint is it will slip to easy if doing any slow rpm work such as larger drilling in steel.

Well I should have a working belt drive on the x3 tomorrow or early next week.
I decided to go with a normal 40' wedge type belt. I will still keep the entire 2 speed x3 drive intact. To run the belt drive I will just need to remove the timing pulley from the motor and remove the timing belt. Slip on the v pulley to the motor which runs directly to the spindle and by passes the gear box and the belt length should be the same as needed for the timing belt so I won't have to move the motor to adjust tension.
For now the spindle pulley is 2.7" and the motor pulley is 2.3" for I think around 3300rpm.
I'll take some pics tomorrow if I get it going.

09-17-2006, 03:45 PM
well it is done and works great. The x3 with the belt drive is actually even quieter than my little x2 with a belt drive:D
The spindle pulley is cast iron and installed with high strength loctite.
The motor pulley is regular 6061 aluminum.
I just had an old belt from a drill press about the right size that worked to try it out. I'm going to try one of those link belts though. The pulleys are 3/8" wide and I cut them with a 36' angle.So just about any 3/8" belt will work.
I ran it for 45 minutes. The spindle got fairly warm but nothing extreme. I don't think you'll get much more rpm out of the spindle than 3500rpm without overheating.
With the covers off it is less than 5 minutes work to go from the factory drive to the direct belt drive.
This way I now have 3 speeds but I suspect I;ll be using the direct drive for 80% of what I do.
I just have to figure out a cover arrangement now.

09-17-2006, 08:07 PM
Looks good,how are you handling tensioning the belt?An idler on the slack side or slots in the mounting plates?

Also,one question about the x3,does it use the same motor as the x2?

09-17-2006, 11:51 PM
I picked up a new 22" 3L belt at the local hardware store. It is very quiet. I ran the belted x2 at the same time and it drowned out the x3.

I did not want to use a idler or tensioner. IMO they just add problems and the belt span is short enough to not need them.I used a auto tensioner on the x2 for a while but found it ran smoother and quieter without it.
I drilled/tapped 2 new holes in the head to be able to pivot the mounting plate to adjust belt tension.
I had to cut out a section of the heavy wall iron top cover to clear the belt.
The e-stop and a power and fault LED were relocated to the other side of the head inside the heavy iron top cover. They are no where near any moving parts and protected by sheet metal.I like having all the controls on the same side anyhow.
I will add studs to the top cover with hand knobs to be able to quickly remove the top cover if I want to switch back to the factory 2 speed gear box. I figure it will be a 10 minute job.
The directto spindle belt drive has no low speed torque. I can grab the spindle and trip the speed control quite easily at low rpm. The belt even at low speed grabs hard and tight. I could not get it to slip before the motor control kicked out.
I figure the usable range of speed will be about 1500-3400 which fits in nicely with the 1000 rpm max low and 2000 rpm max 2nd. I would have liked more rpm but I think this will do just fine.
Just waiting on my drive components now to hookup the steppers and I can begin tweaking the mill and see what it can do.
Oh, the x3 motor is quite a bit larger than the x2, rated at .8hp and 6.5 amps and 4000rpm. The x2 is 4.5 amp,6000rpm and an optimistic .8hp.