Often it measured at different areas or positions the reading will change and some machines used a laser interferometer calibration check to create a compensation table to correct errors in true position every 1/4". it was normal for many cnc to have dro calibration checked and if off to be put in a special mode to tell it what it actually moved and the compensation factor was updated so 6.0000 movement will read 6.0000 We used to use a 6.0000 gage block and a dial indicator, zero to block against something, parallel to a travel axis, remove 6.0000 gage block and move to indicator zero to what gage block was against and see what dro says. Not for nothing but i learned a long time ago that a dro may read 0.0001" but that does not mean it will repeat 0.0001' or measure accurately. many machinist use long tools to avoid hitting the tool holder collet nut, trying to drill deep but if holes not that deep then use a short length split point drill, carbide is more rigid and flexes less than hss Use a short drill bit length for better accuracy.
#True position to one datum manual
this is mentioned in my cnc machine manual and there is a mode to try for better accuracy but is not on by default usually. i have often noticed circular interpolated holes not round as at higher feed rated many machines actual position drifts from programmed position. i have had scrapped parts because drill bit hit another internal hole partially and this really caused the drill to drift out of position.ĭrilled holes should be within 0.004" often within 0.002", i usually use a second operation using circular interpolation with an end mill or better yet a boring bar if higher accuracy is needed. i have scrapped parts assuming my drill bits were sharpened correctly without looking at them first.Ĥ) no through coolant but you are using coolant ?ĥ) are there interior holes already in the block that a drill bit entering a internal void will drift off position because off. i am always amazed when a machinist uses a long length drill bit and high feed rate and sometimes no spot drilling then seems surprised the drill walked off position.Ģ) thermal expansion both on the part being machined and the machine changing temperature can effect thingsģ) are you drill at right angle to surface? how good is your drill bits sharpened and are they sharp. The reason i ask is some do not if drill bit is short, for example screw machine length drill bit accuracy can be considerably better than using a longer drill bit. 1) are you using a spotting or center drill 1st.
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