Specifying/making a rack

This topic has 14 replies, 10 voices, and was last updated 29 February 2016 at 14:19 by Ian Parkin.

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27 February 2016 at 13:43 #227235
Ian Parkin
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@ianparkin39383
How do I specify this rack

its off a large Fobco drill press and i'd like it to be double the length

Its 3/4 " wide and 1/2" deep

is this likely to be off the shelf anywhere?

is anyone on here able to make me a length

even 12 inches or so i can join it then to existing

I have a vertical mill is there a end mill made to cut those teeth profiles?
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27 February 2016 at 13:43 #8022
Ian Parkin
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@ianparkin39383
27 February 2016 at 14:05 #227237
Anonymous
You need to know the pressure angle and diametric pitch. Given that a rack has straight sided teeth, at the pressure angle, the PA is easy to measure. It is likely to be 14.5º or 20º. Assuming that the rack engages with a gear with integer diametric pitch the pitch of the rack will be irrational since pi is a factor going from diameter to circumference.

There may well be an appropriate rack available off the shelf. I doubt you'll find an endmill to create the rack directly. However, given that the teeth are straightsided, and if you're not worried about a radius at the bottom of the teeth, then you can cut the rack using an ordianry endmill or slot drill in three passes. One pass is full depth with the head vertical and the other two with the head set over at the pressure angle for each side.

Alternatively use a shaper or slotter with a ground HSS form tool.

The biggest issue will be keeping track of the irrational pitch of the teeth.

Andrew
27 February 2016 at 14:10 #227238
Brian Wood
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@brianwood45127
Hello Ian,

Better measurement will confirm it but at first sight it looks very close to 12 DP.

That has a theoretical tooth pitch of Pi divided by 12 which is 0.2618 inches [6.65 mm] Check it out accurately over say 10 pitch widths to be sure The matching gear dimensions will help also in confirmation; diameter in inches is T + 2 divided by DP, where T is the tooth count.

I imagine , if this is an old machine, the pressure angle will be 14.5 degrees—modern gears and racks are made to 20 degrees

I'm sure there will be suppliers if you Google for information, HPC gears in Chesterfield come to mind as a possible source http://www.hpc.co.uk

Regards

Brian

Edited By Brian Wood on 27/02/2016 14:13:39
27 February 2016 at 15:42 #227248
Ian Parkin
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@ianparkin39383
Thanks to Andrew and Brian

I'll try hpc on monday or have a go at making my own

Given that its purpose is to raise and lower the table on a drill press will a 20 pa work over a 14.5 pa

Ian
27 February 2016 at 16:15 #227254
Brian Wood
Participant
@brianwood45127
Hello Ian,

Indeed it will, that thought had just come to me over a cup of tea. The meshing will not be that critical and in any case it will be intermittent in nature and I imagine you lock the table to the column having moved it, in which case those little niceties don't matter at all,

Good luck with the search or better still the making it yourself

Brian
27 February 2016 at 16:38 #227257
ega
Participant
@ega
Ian Parkin:

If you do go in for a PA "mixed marriage" it might pay you to make the rack from something softer than the meshing gear – aluminium, say.

You could cut the long rack you want in your vertical mill using a shop-made D bit type cutter. Alternatively, you might be able to use the B&S type with the cutter on an arbor between centres on the lathe and the workpiece on the cross slide, re-indexing the work as necessary; Martin Cleeve made his racks this way.

Edited By ega on 27/02/2016 16:49:10

Edited By ega on 27/02/2016 16:50:01
27 February 2016 at 16:45 #227260
Tim Stevens
Participant
@timstevens64731
If you really wanted a last resort bodge, cut your best version in light alloy, and 'rectify' the tooth shapes by pressing the steel rack into it in a bench vice.

But don't tell my grandad I said this …

Cheers, Tim
27 February 2016 at 16:54 #227265
John Reese
Participant
@johnreese12848
Remember that the table adjusting rack acts as a column. Doubling its length could put you at risk of buckling failure. Use of aluminum with an elastic modulus about 1/3 that of steel increases the likelihood of buckling.
27 February 2016 at 17:15 #227271
ega
Participant
@ega
John Reese:

Good point about the danger of buckling. I'm not familiar with the construction of the Fobco: if the rack is not supported at intervals (not clear from OP photos) the danger would be greater but if he is using two or more lengths they would presumably have to be fastened to the adjoining structure – column or whatever – which would reduce the danger.
27 February 2016 at 18:10 #227286
Sam Longley 1
Participant
@samlongley1
Just checked my Sealey bench pillar drill. This has a rack for the table which is about 15 inches long although the rack part is 9 inches & same tooth pitch as yours. Other dims the same.

It is a loose fitting that is held in a ring at the base & another at the top. It rotates around the pillar as the table is moved from side to side

This would lead me to suggest that you look at a Sealey floor standing model rather than a bench one & you may be able to buy a long rack as a spare part. You may even find that Clarke do a similar thing
27 February 2016 at 21:02 #227316
Simon Williams 3
Participant
@simonwilliams3
Sam's post raises another alarm bell. If the rack skates round the column as you move the table in the horizontal plane, then this creates a significant lateral bending moment on the rack. The friction of the lower end acts in one direction, and the force to move it is applied higher up, so the material from which therack is made has to resist the resultant bending force. I'd worry about an aluminium construction folding and dropping the table. The same is true but to a much lesser extent of extending the rack in steel.

Hopefully the rack is supported by the column, in which case the lateral bending moment isn't ann issue. But that presumably means that the table can't swivel on the column

Can we have some pic's of the OP machine please?

Rgds Simon

Edited By Simon Williams 3 on 27/02/2016 21:03:28
27 February 2016 at 21:06 #227317
Neil Wyatt
Moderator
@neilwyatt
Posted by Simon Williams 3 on 27/02/2016 21:02:22:

Sam's post raises another alarm bell. If the rack skates round the column as you move the table in the horizontal plane, then this creates a significant lateral bending moment on the rack.

Or take the same approach as on my mill – extra counterbored screws at intervals along the rack. they don't need to be large – M3 should be fine and leave plenty of intact tooth either side.

Neil
28 February 2016 at 09:18 #227355
Ian S C
Participant
@iansc
Ian, is your rack screwed to the column or held in a ring top and bottom? If the former, and you want to go longer, think of counter boring another hoe about half way for another screw.

Few years back a friend put a cylinder head on his Chinese floor standing drill press, unclamped the table and the rack collapsed, replaced at the great cost of NZ $3.50, it's great what you can make out of used chewing gum.

Ian S C
29 February 2016 at 14:19 #227582
Ian Parkin
Participant
@ianparkin39383
The rack is carried at its bottom end on a large bearing that rotates around a locking collar

the rack then fits snuggly against the column untill it goes into the table casting collar its a good fit into this over about 4 inches

The rack i want to be about 18inches long I had apiece of stock the right size

Set it up in the mill but bottled out after the first cut with a 2mm end mill cutting in steps down to 6mm depth

This is going to take me ages to do !!! and cost a bit in broken cutters

Rang a local co to see if they could do it but they quoted of the top of his head £200 ish

I ordered a sealey rack off one of their floor standing pillar drills for £35 delivered

it may be easier to modify that !!!
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