Surface grinder question

How do surface grinders move the wheel (or table?) to give micron positioning diferences?
I would have thought that a straight thread wouldn't work. Is there some
other secret?
Ta
Peter Fairbrother
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 12/11/17 13:27, Peter Fairbrother wrote:

Peter,
trapezoidal thread, of 0.1" pitch and is driven through a 2:1 reduction bevel from a large 9" diameter handwheel with 50 graduations so each division is 0.0001" (2.54 microns).
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 12/11/17 17:37, David Billington wrote:


Correction the handwheel is marked 0 to 49 with tenth divisions between those so 500 marks on the wheel so even finer precision.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 12/11/17 19:37, David Billington wrote:


I'll get this right eventually, not been a great day for thinking. It is as I said originally each division is 0.0001" of an inch as 2 turns of the 500 division handwheel moves the head 0.1".
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

It tends to be an ACME or trapezoidal screw lifting the head. It's always under load from the considerable weight of the head, so backlash isn't a problem. Even though the screws and nuts can suffer significant wear over the years (especially if there isn't an automatic lubrication mechanism), the wear over short distances will be consistent, so a properly geared screw can produce very high precision movements. Silly as it might seem, the slight vibration that is inherent in running the machine ensures that any slack will be taken up.
Note that a heavy cut can lift the wheel-head within the 'play' of the screw and nut. This is one reason for doing a spark-out pass at the end of the job,
Mark Rand
--
RTFM

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.