RPM and SFM calculator

[TorontoBuilder]

I know that the machtach is defunct, and that there is another product out there the Tachulator, which appears to be pretty basic but a healthy $120 USD for a completed unit.

Is there a need for another? For cheaper and with a greater depth of features? One where the core processor is swappable between machines?

What would your dream machining tachometer have as feaures?

My brother wants to have an optional lathe setting that will input the workpiece diameter directly from the DRO.

I want a hyper accurate revolution counter for when I wind copper magnet wire onto bobbins for making electric motors and generators.

I dont think I'll ever own a shaper, but it with forethought the tool can easily be made to calculate SFM based on rpm and stroke length, with a factor for the fraction of the revolution that is actually cutting metal. I worked this all out on another forum last year when someone was trying to design a calculator for their shaper based on an optical sensor.

so what would you want?

 

[TorontoBuilder]

i should have added that milling feed rate calculations would be a basic feature to be included or why bother.

 

[graham-xrf]

There is not much problem about revolution counters, nor with getting the output digitally. Every kind of trick tacho can be used, or now, one can get a reasonably low cost shaft encoder normally used for CNC or servo feedback. For single display outputs, there are low cost Arduino projects that get you there.

Efficient cutting, roughing, and getting very good finish all have "good" SFM setting ranges. The speed is also mediated by the tool type, material, and tool shape, but for most of us, we get what we need because there is that range. Thus from desired SFM, and a very simple calculation, one can settle on a reasonable good RPM. Using that RPM gives a SFM that won't stay constant as we turn down the diameter, but the "range" is adequate.

A CNC machine at speed has a programmed calculation that sets the speed demand every revolution, or perhaps more often than that. When facing, the effect is to see the chuck automatically speed up as the tool gets nearer the centre, all in an effort to maintain a constant SFM. It finds it's limit at some point, because the RPM required becomes infinite.

I have used a pure mechanical counter for coil bobbin winding, but again, the shaft encoder does it, even if you "unwind" and then "rewind", because they have two logic signal outputs 90° out of phase with each other, allowing to sense direction.

There is a fundamental difference between counting revolutions as turns, and counting revs per minute. The latter is a speed, so requiring a clock signal. All is reasonably OK if the thing is going fast enough, and the clock gate slow enough, to get a good number of count samples. The slower it goes, the less accurate it gets. The answer is to swap the signals, so that a high speed clock, (say 4 MHz or something) is gated by the slow shaft signal. It delivers reciprocal time. You can instantly get incredibly accurate speeds known at rates more than sufficient for any CNC need, even if the shaft is turning very slowly.

The gadget you ask for is, I think, already available as small electronic projects kits, but you can roll your own. My choice would be Arduino, Teensy, or Raspberry Pi. Any of these little boards can easily interface to encoders, and can do the calculation. The lookup table could accept your choice of input conditions, meaning cut depth, tool type, diameter, desired SFM, etc, and have it deliver the needed RPM. It becomes automatic if the RPM from the calculation is then used as the demand to a machine motor speed controller.

 

[mksj]

Some lathes have a feature for constant SFM as the diameter changes, I have also installed the MachTach and Tachulator on various lathes including my own, but almost never use the SFM feature. Mostly because you can gauge the cutting based on the chip size coming off and the feel of the lathe. If you were trying to keep constant SFM, you were far exceed the spindle RPM if using carbide inserts, might be OK with HSS. A constant SFM based on diameter would need to determine the material diameter and this would need to be set. Coil winding use to use mechanical counters, there is another forum member "B2" who is using his 1440GT for coil winding and has an incremental revolution counter posted below.

VFD conversion via solid state electronic components. PM1440GT, VFD, 3-phase

Hi Folks, I just posted a new thread on finding all the possible TPI and FEED rates on my PM1440GT as well as on other lathes. If you are interested look here: TPI Feeds X-Feeds: Generalized Excel file for your Lathe

www.hobby-machinist.com

 

[WobblyHand]

One could have an estimate of the diameter if one reads the X DRO output. My home made ELS can read the DROs. (My ELS touch display is the DRO readout, there is no separate DRO display.) However, I've not closed the loop with the VFD. At this point, it adds complexity that I do not want to add. My ELS was not designed to do CNC, although my chosen platform is capable of it.

 

[TorontoBuilder]

There is not much problem about revolution counters, nor with getting the output digitally. Every kind of trick tacho can be used, or now, one can get a reasonably low cost shaft encoder normally used for CNC or servo feedback. For single display outputs, there are low cost Arduino projects that get you there.

Efficient cutting, roughing, and getting very good finish all have "good" SFM setting ranges. The speed is also mediated by the tool type, material, and tool shape, but for most of us, we get what we need because there is that range. Thus from desired SFM, and a very simple calculation, one can settle on a reasonable good RPM. Using that RPM gives a SFM that won't stay constant as we turn down the diameter, but the "range" is adequate.

A CNC machine at speed has a programmed calculation that sets the speed demand every revolution, or perhaps more often than that. When facing, the effect is to see the chuck automatically speed up as the tool gets nearer the centre, all in an effort to maintain a constant SFM. It finds it's limit at some point, because the RPM required becomes infinite.

I have used a pure mechanical counter for coil bobbin winding, but again, the shaft encoder does it, even if you "unwind" and then "rewind", because they have two logic signal outputs 90° out of phase with each other, allowing to sense direction.

There is a fundamental difference between counting revolutions as turns, and counting revs per minute. The latter is a speed, so requiring a clock signal. All is reasonably OK if the thing is going fast enough, and the clock gate slow enough, to get a good number of count samples. The slower it goes, the less accurate it gets. The answer is to swap the signals, so that a high speed clock, (say 4 MHz or something) is gated by the slow shaft signal. It delivers reciprocal time. You can instantly get incredibly accurate speeds known at rates more than sufficient for any CNC need, even if the shaft is turning very slowly.

The gadget you ask for is, I think, already available as small electronic projects kits, but you can roll your own. My choice would be Arduino, Teensy, or Raspberry Pi. Any of these little boards can easily interface to encoders, and can do the calculation. The lookup table could accept your choice of input conditions, meaning cut depth, tool type, diameter, desired SFM, etc, and have it deliver the needed RPM. It becomes automatic if the RPM from the calculation is then used as the demand to a machine motor speed controller.

I know I can buy one... but I am working with a member who is electronics designer. For him this is a simple task and we will keep it rather simple with the exception of the millign calculations being included. This is just a preliminary project.

Then a future project will be feature rich ELS servo lathe with all the goodies.

 

[TorontoBuilder]

I should change the title of this thread... to something like "features for machining computers and motion controllers" because that is the end game, a DYI motion control to convert a hobbyist size machine between 12x20 and 14x40 into a smart lathe...

In soliciting recommendations for features to include in such devices I'm not speaking about generalities, nor am I speaking about non-functional performance goals like faster work, greater safety, less setup and simpler operation, I'm speaking about functional requirements that explain how the system must work.

@homebrewed gave an excellent example of the types of functional requirements users would like to see in a controller back at the end of 2019, I'm sure that these forums are filled with others just as good. Who will help me to curate and collate such information and to package it into a guide?

https://www.hobby-machinist.com/threads/an-els-procedural-question.81142/#post-701272

Tell me your specific machining tasks that a controller should perform as well as features to make operation easier and more universally accessible, such as a cell phone app to operate the smart lathe like features to be able to eliminate a built in GUI for users on a budget...

In my case, not being a trained machinist like my brother I want these specific machining abilities:

DRO integration to read x-y position;
Ability to return to any sub-datum x-y coordinate;
Inputs for standard machining features like relief cuts for threading and grinding run outs.
Reverse threading;
Multiple lead threading;
and the ability to create non-standard threads via inputs for lead, pitch, # starts, major and minor diameters, and taper for instance

That's for the most basic threading only machine. That of course is not what I really want.

On the advanced version I want cnc like features that do not require g-code, but rather only require simple user inputs to turn features such as internal and external tapers, simple internal and external curvatures like funnels, and inlet ducts.

On top of that I'd like the ability to mount a small milling spindle or tool post grinder on the cross slide to perform more complex tasks and use the lathe like a fourth axis mill chasing thread grinding and possibly work with cnc software packages via standard connection port.