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kingcreaky
30-06-2017, 04:57 PM
Afternoon all.

Planning another build, this time plasma. This time i shall try rack and pinion with single shaft drive. something like this

22032

22033

22034

As much as this at the moment is the rough plan, as with most things I will make it up as I go along to a certain extent. Would welcome any feedback at this stage though.....


WHERE DO I BUY RACK AND PINION FROM? Seems expensive at Zapp?

m_c
30-06-2017, 05:09 PM
Looks good. A plasma cutter is something on my long term 'to build' list, as I do quite a few things where one would come in handy.

HPC gears do rack, but it's something I've never seen a noticeably cheap source for. I suspect it's because it's quite a pain to machine. Gears can be hobbed in whatever reasonable size with minimal effort, but rack takes a lot longer to machine.

Lee Roberts
30-06-2017, 09:22 PM
CNC4YOU has it also: https://www.cnc4you.co.uk/MOD1-Rack

Good luck, will look forward to seeing it come to life :-)

Boyan Silyavski
02-07-2017, 03:19 PM
Have you calculated the inertia of that shaft? At that span is like spinning a ball screw

What motors? Plasma needs to be fast

kingcreaky
07-08-2017, 09:31 PM
Thanks for replies;
Boyan: im hoping I wont have to worry about the inertia in the shaft, because, unlike a ballscrew I can place inline shaft support bearings to reduce the whip?

After shopping around, Zapp doesnt seem overly expensive when taking into account they do free postage at that type of money, and you do get good service and can ask stupid questions.

My linear rails have arrived (SBR25 for X) SBR20 for Y, and SBR16 for Z (please dont wince, for my application they will be fine)
im half way through building the frame (il apply pictures of the frame once done)

Before, I order the Racks, Im hoping to harness some brain power from you guys

Although this isnt going to be a commercial machine, I hope to make it well enough for it to be used a lot, and on a wide variety of thickness material, in order to achieve this, based on my limited research the maximum cutting speed I think i need to achieve is 6000mm a minute

Im hoping to get away with stepper motors (only because I have them and some 70v drives), and a old computer with parrelel port

(but servos and ess are not out of the question) I know how good they are from other machines...

MOD1 rack seems cheapest, assume I use a 22 tooth pinion, driven from, lets say a 200 input belt off a 100 tooth drive off a stepper motor. so essentially per stepper rev, Im achieving 44mm actual movement

i shall illustrate this as follows

a) 22/200/100 = 22 1:2 == 44mm
b) 22/100/400 = 22 1:4 == 88mm

assuming I have the servo drives set at 400 pulses per rev, my parrelel port will need to kick out 6000 / 44 = 136 * 400 (54,400) PULSES PER MINUTE which I think is achieveable from a < 1khz parrelel port kernal?

but im tired, so can somebody shed some light?

Boyan Silyavski
10-08-2017, 06:46 AM
I think a plasma cutter should be faster for thin material. Depends on your PC, but yes you could achieve even faster . i have managed at least.

also http://www.cncrouterparts.com/why-do-your-rp-units-use-a-belt-why-not-direct-drive-p-213.html

kingcreaky
10-12-2017, 06:17 PM
Been cracking on with this, as with most things I do, the design got tweeked from original drawings as I went along. Here is a video summarising where im at, and also details my present predicament. Welcome any feedback you can afford me

https://youtu.be/M1kGiRnGnx8

cropwell
10-12-2017, 07:01 PM
Hi Matt,

Maybe you could put a limit stop in to prevent the pinion disengaging from the rack and jumping, leaving the spring to keep the rack fully meshed at the end of a travel move.

Cheers,
Rob

Neale
10-12-2017, 08:42 PM
Essentially, I think you have a geometry issue here. On the gantry, the arm is positioned and pivoted so that the pinion moves roughly at 90deg to the rack. However, the X pinion arm is pivoted so that the pinion moves almost parallel to the rack. I know that it's not quite parallel, but it's close enough that even with a very strong spring, there is relatively little force acting to keep the pinion engaged. Would it be possible to use a fixed vertical plate going down from the gantry with a pivoted horizontal arm carrying the pinion? I can see that that might give belt tension issues, but that might be an easier problem to fix!

Lee Roberts
10-12-2017, 09:57 PM
Split the plate, fix the top one, pivot the bottom, adjust belt sizing accordingly.

cropwell
12-12-2017, 03:30 AM
Essentially, I think you have a geometry issue here. On the gantry, the arm is positioned and pivoted so that the pinion moves roughly at 90deg to the rack. However, the X pinion arm is pivoted so that the pinion moves almost parallel to the rack. I know that it's not quite parallel, but it's close enough that even with a very strong spring, there is relatively little force acting to keep the pinion engaged. Would it be possible to use a fixed vertical plate going down from the gantry with a pivoted horizontal arm carrying the pinion? I can see that that might give belt tension issues, but that might be an easier problem to fix!

The solution for belt tensioning would be to have two belts, one on the vertical arm and one on the horizontal, with two pulleys on a shaft at the pivot point.

kingcreaky
12-12-2017, 10:28 AM
Thanks guys, am going to fix the shoulder drops, then pivot the plate on the same axis as the rack. this will require more belts. Matt

Lee Roberts
13-12-2017, 06:09 PM
I was thinking split the plate, fix the top, pivot the bottom, use a bigger belt (probably) and design in a spring loaded cam follower feature that would auto adjust the timing belt tension as the pinion mechanism/plate adjusts in real time.

23403

kingcreaky
18-12-2017, 03:42 PM
Whats the software of choice for plasma? and can anybody steer me in the right direction ? :D