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Arnak
08-12-2005, 06:08 AM
Hi Folks,

Having made a start on gear making I wondered how do you make a rack?

Does anyone know of any books or video's etc on doing this?

Regards,

Arnak

John Stevenson
08-12-2005, 07:53 AM
Racks are not easy things to do.
Not in terms of the cutting, in this respect they are just like gears, feed to depth, cut and index on the Circular Pitch of the tooth form then repeat.

Where they are not easy to do is in the setup. Because racks are usually long they don't conform easily to the most handy axis on a machine.

On a vertical mill with the cutter mounted on a stub arbor the rack needs to be placed vertical. This limits you in hight, bed or quill travel and how much you can support on an angle plate.

On a horizontal mill or a vertical with horizontal attachment the rack needs to lie front to back or the Y axis as it's known.
Again this limits travel as the Y is usually the shortest axis and you have a further problem of the overarm or horizontal attachment hitting the work given that most gear cutters are less tha 4" in diameter.

On a lathe with the cutter between centres you are better off except that you only have a small footprint of the saddle to support the work on and may have to move either the work or the cutter along.

There are various attachments to fit both horizonatl mills and verticals that use compact gearing to turn the drive thru 90 degrees to let you use the longer X axis and also the get away from the attacment not clearing the work.

Here's a good example.

http://homepage.ntlworld.com/stevenson.engineers/lsteve/files/rack%20attachment.jpg

There is probably as much work in thinking about the operation as doing it.
This is one area where a bit of foresight and planning pays off.

John s.

Arnak
08-12-2005, 08:00 AM
Hi John,

Thanks for the reply.

I thought that must be the case. 8-((

It seems I'd have to come up with some sort of horizontal cutter system then to use my vertical mill...

Oh well, back to the drawing board for a bit then!

Thanks again,

Arnak

SGW
08-12-2005, 08:54 AM
Since the teeth on a rack have straight sides, I wonder if it would be possible to use a suitably-sized tapered end mill to cut the teeth.

The trick might be the "suitably-sized" part.

3jaw
08-12-2005, 09:17 AM
Try using a horizontal attachment on a vertical mill similar to the picture above.

Paul Gauthier
08-12-2005, 09:57 AM
Isn't it possible to have an end mill ground to the tooth form, if so one could rough with a smaller ball mill and finish with the special.

------------------
Paul G.

Arnak
08-12-2005, 10:17 AM
Hi,

Just a thought but couldn't you just make a singe toothed hob to do the cutting?

As per the DVD's by Jose F. Rodriguez.

Arnak

MechHead
08-12-2005, 11:11 AM
A rack is just a gear that has an infinite pitch line radius; ie, a straight line. Therefore, because it is going to (I'm assuming) mesh with an involute tooth form, the involute curve will also have an infinite radius, which means straight sided teeth.

dsergison
08-12-2005, 12:05 PM
looks like a job for a shaper

Arnak
08-12-2005, 12:23 PM
Hi,

Yes I'd agree with the straight sided teeth.

I still think that maybe a sigle tooth hob made on the lathe would do as a cutter.

Then hold the work verical in the mill and use the vertical index to get the correct position for the teeth.

That assumes that you haven't got a shaper. 8-((

Arnak

Mcgyver
08-12-2005, 12:37 PM
I'm not following you - how's a single tooth hob any different than the profile cutter for the rack? as per John's post the challenge isn't in the tooth form or cutting, its getting a set up that can get any distance out of.

The correct angle end mill ideas seems promising - anyone tried this or know if a cutter is available?

Paul Alciatore
08-12-2005, 01:32 PM
Yes, any cutter that cuts on it's periphery would require the rack to be mounted either vertically or near vertically on a mill drill. But this could be done with an angle plate. The hard part would be the feed as you would have to use the quill feed or mount an XY table on the angle plate. Then you would need a large diameter cutter for clearance. Not a pretty picture.

As an alternative, and if your mill drill has a tilting head, you could cut a rack with simple square ended end mills. First cut would be to full depth and the width of the root with the head at the normal 90 degrees. Then tiltthe head one way to the pressure angle and do one tooth face. Finally tilt the other way (or reverse the rack) and do the other face. A nice touch would be to use a cutter with a corner radius to leave a fillet at the root. You might also use a wider cutter to rough out to half depth first. This would leave less for the small diameter cutter(s) to remove. Probably better for brass than steel.

I also think you could make a special end mill with a tapered shape that matches the tooth form. Since a rack has straight faces on the teeth, this should be relatively easy.

Either of these two methods would allow the rack to be setup along the length of the table but you would have to feed with the cross feed so CNC or at least power feed there would be nice.

Paul A.

dsergison
08-12-2005, 01:49 PM
http://www.mech.uwa.edu.au/DANotes/gears/generation/generation.html

I just posted for a general reference. it's the best site I'vs seen on gear teeth.

Arnak
08-12-2005, 02:12 PM
Hi,

The comment on the single tooth hob was just that if you don't already own a suitable cutter or want to save money perhaps you could make one.

That's just my empty wallet talking. 8-((

Arnak

Carl
08-12-2005, 02:13 PM
If you have access to a lathe with a good size center to center dimension, maybe you could mount the cutter centered on a long arbor between centers and mount the rack stock on the cross slide at the proper height to give proper depth of cut.

Lock the carriage and take the first cut. With a dial indicator on the carriage, unlock it and use the indicator to move the carriage the correct distance for the next cut. Lock the carriage and take the second cut. Repeat as needed.

John Stevenson
08-12-2005, 06:07 PM
Arnak,

No problem if you wnat to make one. The profile for a rack cutter is straight sided with an angle of 14-1/2 degrees or 20 degrees depending on the type.

As someone else has pointed out you could use a tapered slot drill or end mill if you can get the correct taper and tip size.

This will be the stumbling block in getting those two to match. Tapered end mills are available off the shelf J&L do them from 1 degree up to about 12 in ones then the tend to go in 2's or 5's.
Only snag is with these the tips are about 3/32 or 1/8", OK for doing a 10DP rack but not a lot of good at myford size levels.

Again you can cheat and make a home made one.
You can even use a 'D'bit if you mill the bulk out first.

Here's one I did earlier http://bbs.homeshopmachinist.net//biggrin.gif

http://homepage.ntlworld.com/stevenson.engineers/lsteve/files/helical%20gear.jpg

This was a "get John out the sh*te job" brass helical that needed to be done whilst a new one was ordered [ in fact it's still runing from 3 years ago http://bbs.homeshopmachinist.net//biggrin.gif ]
Milled on a 4th axis on the CNC but besides that it proves that a simple 'D' cutter turned up with a form button will work.

John S.

Bond
08-12-2005, 11:25 PM
Just a thought but boats use a rack & pinion you can go to boat salvage.

Allan Waterfall
08-13-2005, 02:57 AM
<font face="Verdana, Arial" size="2">Originally posted by Carl:
If you have access to a lathe with a good size center to center dimension, maybe you could mount the cutter centered on a long arbor between centers and mount the rack stock on the cross slide at the proper height to give proper depth of cut.

Lock the carriage and take the first cut. With a dial indicator on the carriage, unlock it and use the indicator to move the carriage the correct distance for the next cut. Lock the carriage and take the second cut. Repeat as needed.</font>

Something like this?
http://bbs.homeshopmachinist.net//Forum1/HTML/010482.html

Allan

Arnak
08-13-2005, 05:14 AM
Hi Guys,

Thanks for all the great suggestions and brilliant photo's.

I should have enough info there to get started now!

Arnak

Al Messer
08-13-2005, 08:54 AM
Arnak, I'd vore for using the Shaper, if you have access to one.