In the words of Al Read . . . . "You'll be lucky, I say,
You'll be lucky".
You could have one made of course but I doubt that you really want to
spend the amount of money necessary.
38 DP is going to be a 'Special' whoever you try.
If my calculations are correct then 38DP is 0.668 MOD so if you can get
away with 0.7 MOD . . . . and you want 20° PA . . . . . . and you can
make two passes . . . . . . ArcEuro have a 25mm dia Hob with 8mm bore
which would cut about 15mm wide.
Many thanks for the info, shows I wasn't fibbing about knowing little when
it comes to gear cutting. I'll check with my "man" about the alternatives,
and confirm the original info is correct. 2 passes is not a problem as the
job is to be on the CNC. As the job is an ongoing one, I may have to
consider having one made if the alternative is not suitable, but dread how
much that will be, all guesses welcome !!!
I had assumed that you were looking at a one off job or at most a few.
In the light of it being an on-going job you could try Dathan -
http://www.dathan.co.uk/ and get a quote - I haven't used them
(yet) but their on-line enquiry system allows you to enter all manner of
Bob you can't cut a rack with a hob.
A hob has spiral teeth and as it rotates it expects the blank to move
with it, Ok for a gear as it round and the start point will come round
again but a rack won't.
Imagine a worm and wheel, that's the action.
It could be done with a non spiral hob but chances of finding one of
these is as great as getting Gordon Brown to admit he's wrong.
I see from a later post you will be doing this on CNC so I's advise
getting a normal Brown and Sharp gear cutter either 38 DP, or 0.7 mod
in a No1 , that does from 135 teeth to a rack, cutting one tooth,
raise or lower the cutter pi / DP or in this case 83 thou and cutting
another tooth etc
Sounds long winded but not too bad with CNC
Depending on material another way is to grind a Vee shaped D bit up
with a included angle of 40 degree and use this vertically, again cut
one tooth and index along.
I hope you don't mind me adding onto this, for the sake of
clarification. I had to read it twice to understand.
The first paragraph of John's suggestion above has the stock that you
are cutting mounted _vertically_. Hence, as he says, the spacing
between adjacent teeth is 0.083" (raise or lower the
cutter/quill/knee). The depth of cut in this case would be put on
using either the X- or Y- axis. Depth of cut is normally denoted D+f
and should be noted on the cutter (should be about 0.057").
In the second paragraph, as in the photo, the stock is mounted
_horizontally_. Hence, you move the X- or Y-axis 0.083" for each
tooth, and use the Z-axis to put on the cut.
If you work on a 52% space and 48% tooth, the end of your D-bit needs
to be 0.021" (21 thou') diameter, again for a 0.057" (57 thou') depth
of cut. Hopefully John will chime in and check this calculation or
advise otherwise as these numbers are from my understanding of Ivan
Law's book, not from actually doing this.
I forgot on the first post to add that the hob had to be a "straight" one. I
was told to ask for one of them.
The samples we made were indeed done on the mill one tooth at a time,
The previous manufacturer of the item gang milled the teeth on the rack in
one pass, trouble is, you can see the error in the teeth, even I can see it,
which says something !! the many cutters that were ganged must have been
spaced wrong, or slightly different diameter, as flats on some parts of the
rack are more evident in places. I hasten to add that this job is not some
intricate mechanism. There are two racks in this product, with a pinion in
between them, so when the pinion is turned, it advances or retards the racks
The plan of a straight hob, or something similar, was to do it in one or two
passes, time being the essence in an ongoing production job........
Appologies Bob. I had considered this issue but with only a short time
available this morning (I had to be at a concert rehearsal) I didn't
think the issue through correctly. My thought process had the workpiece
mounted at the helix angle of the hob but in fact you would either have
to have the hob (and therefore the milling spindle) or the milling table
turned to the helix angle.
It might be worth talking to Dathan as I suggested in my second post.
They may well be able to provide an annular 'hob' - essentially a 1" +
wide milling cutter with the rack form or a multi N° 1 B&S Gear cutter.
This would circumvent all the valid issues JS raises.
Unless this is Bob's retirement job I'm guessing that the money Dathan
want for making a special cutter will not be acceptable.
38 Dp will be easy to cut in one pass even in steel but hard on
homemade cutters unless you have plenty of time to mess about making,
hardening and grinding form cutters, something I'm guessing Bob hasn't
Two ways I can see to address this cheaply and commercially, one is to
but enough new No 1 cutters that you can surface grind to thickness
and stack on an arbor. Downside to this is I can't find any 38DP or
-0.7 mod cutters new so they all match, forget Tracy and the like as
they will be all mismatched.
Other way is to buy two hobs from Arc, grind the ends so they match
the pitch and then grind all the rows of teeth off except one row so
you now have an annular hob or more to the point, flycutter.
Sounds drastic but it will work for Bob's application.
Now it seems that my customer had originally measured the rack wrong, and
its not 38 but a 32dp. So the search is on for a plain/straight tooth hob
32dp and 14.5 or 20 pa. with 18 teeth or more if possible. Less teeth would
be ok, but would require a second pass.
What about doing it with a sunderland cutter on a shaper or slotter ?
Not sure how easy cutters are to obtain though.
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Smallest I have heard of for a Sunderland is 24 DP.
HPC does a 400mm length of 32 DP rack in EN8 for about 25 quid.
That could make a cutter or you could cut it up have it hardened and
make a holder to use the rack as an insert ? or even multi inserts ?
If you are still in seardh mode, you could try the following for a
ready made rack, that you can modify..
They sell, according to their catalogue. in lenghts of 3" upwards.
Sorry no prices in their catalogue.
I have had no dealings with them except to collect a catalogue, so
neutral recommendation only.
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