Mini Lathe Electronic Spindle Divider

[craynerd]

Evening everyone
I`m about to add a dividing setup to the back of my Cowells lathe to divide the headstock/spindle for wheel cutting. I will be using an electronic dividing system by Kwackers, Steven Ward, as I already have a few of these. I`ll log my progress on here.

My first query would be based on getting suitable gearing. I have already got a 4" rotary table converted with one of these drivers attached and have seen people use an expanded mandrel in the back of the tailstock fitted to the dividing head to drive (divide) the spindle. Even a 3" rotary table would be too small for a cowells. This then leaves me with two choice:

1. Use a timing belt setup. But with that I`d realistically only get a 1:20 reduction. This would be very easy to source the parts and by far the easiest method to fit and keep looking nice. I`m struggling with the maths of the reduction, stepper motor steps and resolution but I`m thinking it may not be accurate enough when you consider the rotary table is running on a 1:90 reduction and therefore quite a high precision in rotation per step.

2. Use a worm gear setup - mount the worm gear to the stepper motor and then the wheel to the back of the spindle (possibly even temporary where one of the normal drive gears would go for the feed!) This would give me equal resolution as I have on my vertex rotary table. The biggest issue with this is sourcing the worm gear! If you google "1M 90M Worm Gear" you get a mass of pictures of exactly what I want - but they are all sold on AliExpress! I can`t even find anything on ebay or amazon!  https://www.google.co.uk/search?q=1M+90T+worm&tbm=isch&source=lnms&sa=X&ved=0ahUKEwjemYnK9NXbAhXJVsAKHTwzBucQ_AUIRigB&biw=1504&bih=897

Any thoughts of which direction you would go if this was you doing it? However I get on, I`ll keep it logged here.

Chris

[tangler]

Chris,

HPC gears do the worm and wheel but you are looking at about 100UKP for the pair 

http://www.hpcgears.com/n/products/4.worms_wheels/worms_wheels.php

HTH,

Rod

[Mcgyver]

A worm usually implies a greater ratio and hence resolution, which you think of as good for dividing, but nowadays these geared steppers that have with a few thousand steps per revolution are so cheap, I think it go with that and a timing belt.  The worm is more difficult to make (or expensive to buy) and has back lash that needs a way to be taken up; a timing belt avoids that. 

[Firebird]

Hi

A cheap source of worm and gear is a car windscreen wiper motor. My mate Julian used one to drive a rotary table on his cnc mill.

Cheers

Rich

[craynerd]

Chris,

HPC gears do the worm and wheel but you are looking at about 100UKP for the pair 

http://www.hpcgears.com/n/products/4.worms_wheels/worms_wheels.php

HTH,

Rod

Rod, ouch! I’m not spending that unfortunately. Thanks for sourcing them though. They are £50 from AliExpress to be fair.

Rich, I’ll keep the suggestions of ripping one out of a windscreen motor in mind but don’t fancy then messing trying to correct bores with bushings and such...but good suggestion! Thank you.

McGovern - I like your suggestion of a geared stepper driver. I presume there is no difference in driving these than a normal stepper. Bearing in mind it will spin the spindle under little torque because I’m only dividing and won’t be cutting during this process!

I have to admit that I’ve been a bit quiet on the engineering front recently, went over to arceuro to look at what they had an they don’t seem to stock the steppers and drivers any more - just the other linking accessories (unless I’m looking wrong!). Have they stopped their cnc stock?

Any recommended links for geared steppers?

Chris

[Bluechip]

Steppery things ???

Try:

www.motioncontrolproducts.co.uk/sections/14/cnc-stepper-drive-kits/

www.cnc4you.co.uk

www.robotshop.com/uk/

Dave

[craynerd]

I was about to order and I’m Just tying myself in knots...your opinion again would be appreciated.

Steppers with planetary gears are cheap £50 but have backlash of <= 1.2 degrees! Anything with precision is £230+ but have backlash less than 20 arcminutes (0.3 degrees). My worry is that 1.2 degrees is a lot and although most of my dividing will be in one direction, I do occasionally step back to check the depth of cut. I really don’t want to spend £230 on a stepper motor!

Which got me doing some calculations.

I can get a 1:6 belt drive. 200 steps per rev, on microstepping is 400 steps per rev. Multiply this by my reduction 400x6 = 2400 steps per revolution.  360deg / 2400 = 0.15 deg per step.

Maybe I’m being dim and missing something but would this not be acceptable for basic dividing of up to say 200 divisions in a rovolution. I’m doubting myself because this is some way off the 36,000 steps per revolution that a dividing head gives you!!

Help appreciated before I part with my brass!

Chris

[Vixen]

Chris

Unfortunately you will get a lot of backlash with any inexpensive planetary gearbox. They may not be the answer to this application

You get NO backlash from a stepper. Most steppers, these days give 200 steps per rev. With microstepping you can get 400, 800, 1600 steps per rev ect. It all depends on how you set the drive electronics switches. Just a word of caution, 400 microsteps will only have half the torque of the normal stepper, 800 microsteps will have a quarter, etc etc. You could easily run out of torque to turn the worm gear, especially if you have set it "tight" to minimise it's backlash.

A dividing head with 200 divisions per revolution will be very limiting, you would only be able to cut gears with 10, 20 , 25,40, 50, 100 or 200 teeth. You need many more steps per revolution than that. More the better; but watch out for the torque reduction.

The good news is there is no need for a spindle lock, the worm gear will lock the spindle from rotation from the cutter action:



Mike

[craynerd]

Hi Mike,
Whilst I have now decided against using a worm gearing and going for belt drive, I don’t think torque is a concern as it is litterally just turning the headstock with no cutting going on.

Your comment on 200 steps per rev got me thinking....

The dividing controller I use allows 1-999 divisions. Like you suggested, the angle of the division must be divisible with the angle per step. So I put together a couple of spreadsheets, one showing the mighty 1:90 reduction which a fine 0.1 deg per step and 36000 steps per rev and my suggested 1:6 reduction with only 2400 steps per rev.





As you can see, even with the low resolution for between 1-250 cuts there are only 29 available true integers and accurate divisions. However, what blew me away was that with the huge 1:90 reduction and higher resolution, this only increases to 39 true divisions and integers!!

Yet the divider controller can do all divisions between 1-999 so it must be making approximations some how.

I feel like I’m justifying sticking to this lower 1:6 and much easier reduction to achieve.

I know I’m out of my depth and I expect someone will shoot a big hole through my calculations or argument. Feel free to do so....I’m appreciating the help!

Chris

[Vixen]

Hello Chris,

I have made a similar belt reduction dividing head by converting an old Emco 6 lathe bed. It allows me to hold work with interchangeable 3 jaw, 4 jaw, collet chucks or between centres.



I used a 3.6 : 1 belt reduction (3.6 being a subdivision of 360 degrees)

The stepper is a large hi torque 200 step motor bought from www.motioncontrolproducts.co.uk/sections/14/cnc-stepper-drive-kits/

My controller was set to 32 microsteps which gives 6400 microsteps per stepper motor revolution. That's a long way from your 400 microsteps/rev. Each of the microsteps therefore equates to an angular division of  0.015625 degrees. Some odd number or prime number reductions will be rounded to the nearest microstep, which is a very small error.

I could go to 64 microsteps for even higher angular resolution but the holding torque drops off. It's always a compromise : angular resolution  v holding torque

I drive the stepper with maximum holding current when stationary. On my set-up, the holding (breakout) torque was measured at 4 ft/pounds (5.42Nm). I have the fall back of being able to clamp the large pulley to the headstock with a large G clamp if the cutting forces try to move/rotate the spindle.

Mike

[craynerd]

That is excellent Vixen! Can I ask two questions:
1. What timing belt is this (I was looking at using T5) and where was it purchased?
2. What stepper driver are you using please? I believe some of the 'cheaper' ones do not handle micro stepping very well.

Thank you. You have solved my problem!

Chris

[Vixen]

Hello Chris,

Pleased that I was able to help. That's what we like to do on the MEM forum

Timing belts and pulleys are available from www.hpcgears.com or evil-bay. use the widest belt you can fit and a roller tensioner. HPC do a nice free catalog book

Stepper motors and stepper drivers are available from  www.motioncontrolproducts.co.uk. Use the highest torque steppers and highest current drivers you can.

Cheers

Mike