Poor mans broach…

[Iain DownsParticipant @iaindowns78295]

The I'm in the process of trying to turn my CMD10 into a poor mans CNC machine. To be honest it's going to be more of a 2D power drive, given the 10 thou+backlash… (I have some biggish plates to trim)

Being a poor man (and from God's country), I'm trying to do this with bits I already have. a Raspberry Pi, some Nema 14 steppers, spare power supplies – and so on.

First try was to attach the stepper directly to the feed screw and see if it would turn. Ah no, too weak (though the cross feed is a bit stiff). So I've spent some money to buy some plastic gears (and I feel shame).

The big gear (delrin 80 teeth) goes on the 8mm feed screen which has a key (about 2mm). the small one (nylon 16 teeth) goes on the 5mm stepper drive.

So. I need to cut a keyway in the innards of the large gear. How to do this? If pressed I could attack it which a warding file, but that feels innacurate.

Equally on the 5mm, I don't want particular want to cut a keyway in the stepper, so I thought I'd use a grubscrew. Will that take in nylon or do I need to turn something metal to take the screw round the outside of the hub (13mm dia and 10mm long)?

Iain

[Iain DownsParticipant @iaindowns78295]

[Frances IoMParticipant @francesiom58905]

have a look at http://www.einfach-cnc.de/x1_version_1.html (in german but photos almost self explanatory) – suspect you will need to adopt a similar scheme belt drive – also suspect your stepper motors might be underpowered (+ also suggest sorting out the gib strips first)

Edited By Frances IoM on 23/02/2016 20:04:45

[Michael Cox 1Participant @michaelcox1]

Posted by Iain Downs on 23/02/2016 19:41:46:

So. I need to cut a keyway in the innards of the large gear. How to do this? If pressed I could attack it which a warding file, but that feels innacurate.

Hi Iain,

I made a small adjustable broach to make such keyways, see:

http://mikesworkshop.weebly.com/small-hole-slotting-tool.html

Mike

[Iain DownsParticipant @iaindowns78295]

Hi Frances IoM. I'll see about the motors soon enough. If they end up being strong enough, all is well. If not, NEMA 17 are quite cheap (but I may need to do something clever with my driver).

Mike – thanks for the link. I can make that. Er. I think

Iain

[Frances IoMParticipant @francesiom58905]

the simplest way of constructing a keyway without using a broach is to enlarge hole, make a liner to fit between new hole size + shaft but cut a slot in liner before expoxying it into the enlarged hole – however Delrin + nylon are I think almost impossible to glue – a simple ribbed belt over aluminium ‘gears’ as eg used in many sewing machines seems the easiest way to go

[IanTParticipant @iant]

I can tell you nothing about NEMA stepper motors or anything similar Iain.

For the "poor man's broach" I could simply suggest that you ask your friends if one of them has a Shaper?

However, that probably wouldn't be too helpful, so instead I will suggest that you hold your gear in the lathe's 4-jaw chuck. Centre the hole, lock the spindle and use a small parting-type tool (held sideways in a boring tool) to plane your keyway, moving the saddle up and down and putting the cut on with the cross-slide. In other words, the keyway will be cut towards you in the horizontal plane. It's a small one-off operation and won't damage your lathe if done carefully. You need to set the tool so it spans the lathe's centreline – such that it cuts evenly on either side (both above & below). Hope this is clear enough.

As for the smaller shaft/gear – I imagine that grinding small flats on the shaft and using grub screws at 90 degrees would work.

Regards,

IanT

[JasonBModerator @jasonb]

Just turn a boring bar on its side with a 2mm wide bit in the end and use the lathe carrage to plane it out a bit like this but square tool

Edited By JasonB on 24/02/2016 07:38:59

[MWParticipant @mw27036]

The above method is a well established way of doing it without special equipment as such, i'd go for that as the most economical but effective way of cutting a female keyway. If you want REALLY cheap then i suppose you could just file it but it aint gonna be pretty..

Ps. i have made keyways using the latter method before and i'd rather not have done, if i did have the right equipment but sometimes its more valuable to learn the lesson of not always being able to work in the best of circumstances and learn to work with that.

Michael W

Edited By Michael Walters on 24/02/2016 08:27:07

[Neil WyattModerator @neilwyatt]

I've made small keyways by turning a 'top hat' to fit the bore, then cutting a slot to match the required keyway. Use silver steel , harden but don't temper.

You can then use the 'top hat' as a filing guide – bearing in mind you can file the top hat away if you aren't careful with a diamond filel. I've made 1/16" keways in 5/16" bores this way.

 

Edited By Neil Wyatt on 24/02/2016 08:54:40

[Michael GilliganParticipant @michaelgilligan61133]

Straying slightly off-topic [but, I think the original question is answered]:

Here is an excellent article about making broaches; and it ends with a link to another, describing a simple pull-broaching machine … well worth reading.

MichaelG.

Edited By Michael Gilligan on 24/02/2016 09:31:21

[Tim StevensParticipant @timstevens64731]

An idea on the 'poor man's' principle – select a bar of round steel which fits the bore of the gear, a sliding fit, and a short length of silver steel, round or square, the width of the keyway or a bit over. Grind or file a notch about a third of the way through the bar about an inch from one end which we call end A. Drill a hole across the bar, starting from the notch face further from A, and sloping a few degrees – 10 say – towards the other end of the bar. This hole will take the tool, so drill and tap a small hole so that a grub screw will locate it. If the diameter is small, drill the grub screw hole in the centre of end A. Now hold your short tool bit in a pin vice (etc) and grind it to the shape of a parting tool, the width and depth of the keyway. Fit the tool into the hole, with about 0.5mm proud, and if any sticks right through grind that off to fit. With nylon or delrin (hostaform, etc), it should not be necessary to harden the tool, but do so if you can. Set the face so that it runs at right angles, or you might produce a spiral key-slot.

To cut your keyway, force the bar through the bore of the gear, adjust the tool a bit further out, and do the same again and again. The curls of material cut away should roll up in your filed (ground) notch. The easy way to do the pushing is with a bench vice.

Quicker to do than describe, almost.

Regards, Tim

[Iain DownsParticipant @iaindowns78295]

I ended up following Michael's (Cox) approach and made this.

It worked pleasing well and I've got my gear on the mill feed.

I could have gone on to see if my little motor would now turn it, but I think I could do with rebuilding and freeing up the table first. And also, having had a little success, I thought I would quit whilst I was ahead!

Iain

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